- UW Department of Geology and Geophysics
The Hanna Basin of south-central Wyoming has been considered anomalous among other Laramide depocenters of the Rocky Mountain region because of its combination of small size and great thickness of synorogenic strata. Most prior interpretations of the Hanna Basin have assumed a history of subsidence and sedimentary infilling discrete from surrounding basins. In contrast, we summarize new geological and paleontological information from the northeastern corner of the modern Hanna Basin suggesting that, prior to late Paleocene time, the Hanna Basin and nearby Carbon, Pass Creek, Laramie, and Shirley Basins were unified and depositionally continuous with the much larger Green River Basin to the west. Only late in the local expression of the Laramide orogeny (late Paleocene and early Eocene) did this “greater Green River Basin” become subdivided through completion of development of intrabasinal, basement-involved thrust faulting and associated anticlines. We view the present Hanna Basin as only a small, structurally defined remnant of an enormous, ponded basin that extended eastward during most of Paleocene time from the Wyoming–Idaho–Utah thrust belt to the newly uplifted Laramie Mountains. That basin was bounded on the south by the Uinta Mountains and combined Sierra Madre–Medicine Bow Mountains and on the north by the Gros Ventre Range, Wind River Mountains, and Sweetwater arch. When the original configuration of the unified greater Green River Basin is taken into account and combined with palinspastic removal of late Laramide faulting that defines its various margins, this basin at least rivaled dimensions of the Powder River Basin of Wyoming and Montana; the greater Green River Basin greatly exceeded the volume of sedimentary accumulation within the Powder River Basin. Rates of Paleogene basinal erosion associated with structural subdivision of eastern components of the greater Green River Basin were prodigious.
Within context of a measured section, we describe and interpret the lithologic nature, depositional settings, source areas, previous geographic extent, and deformational history of the Hanna Formation as seen today in the eastern Hanna Basin. Temporal control is linked to summaries of included mammalian fossils, freshwater molluscs, leaf macrofloras, and palynomorphic assemblages. Unexpectedly early occurrences of certain palynomorphic assemblages suggest that paleoecological controls on distributions of pollen-forming plants during Paleocene time were particularly important.
Deformation of strata in the eastern Hanna Basin chiefly reflects a complex interaction between a major, south-directed, low-angle, basement-involved thrust system and the thick and generally incompetent, mudstonerich Hanna Formation. We argue that out-of-the-basin faults, the result of spatial crowding in the basin, are kinematically linked to the basement-involved thrust system through a triangle-zone geometry. This trishear-style of deformation involved north-directed, out-of-the-basin, décollement-type thrusts and a synchronous south-directed, basement-involved, blind-thrust system. Map relationships along the northern margin of the basin indicate that many of the out-of-the-basin faults emplaced younger rocks onto older rocks, seemingly an anomalous structural relationship for contractional deformation. One explanation of these relationships involves synchrony of the: (1) cutting down section of north-directed, out-of-the-basin fault planes; with (2) upturning of basinal strata related to south-directed, basement-involved faulting along the northern margin of the Hanna Basin.
Finally, the inherited Precambrian structural grain of the Archean Wyoming province was instrumental in controlling the orientation of many of the Laramide-age, basement-involved structural features. Tectonic heredity, rather than changing stress orientations, probably accounts for the variation in orientations of structural features associated with the northern and eastern margins of the Hanna Basin.
- Green River Basin
- Hanna Basin
- Laramide orogeny
- Rocky Mountains
- structural geology
- vertebrate paleontology
The Hanna Basin of south-central Wyoming (Fig. 1) provides a splendid setting for geological study of structural interactions between an evolving basin and its adjacent mountains. The basin exhibits a prodigiously thick, Upper Cretaceous and Paleocene stratigraphic section (Fig. 2; Lillegraven and Snoke, 1996, figs. 5 and 8; Grimaldi et al., 2000, fig. 3). The section was deformed by Laramide-style (sensu Brown, 1993; Bump, 2003), basement uplifts during latest Cretaceous through early Eocene time. Basement-involved thrust faulting occurred not only along the basinal margins but within the deep basin itself (Hitchens, 1999; Kraatz, 2002). In some cases, basement-involved folds in adjacent mountains can be traced directly into the basin (Lillegraven and Snoke, 1996, fig. 4; Hitchens, 1999; Kraatz, 2002). Localized parts of the nonmarine strata bear fossils having biostratigraphic utility (e.g., Secord, 1998; Lillegraven and Eberle, 1999; Grimaldi et al., 2000; Burris, 2001; Dunn, 2002, 2003; Higgins, 2003a, 2003b). The fossils have allowed determination of relative ages of important depositional and tectonic events pertinent to the Laramide orogeny as evidenced in southeastern Wyoming.
Historically, the Hanna Basin has been considered as an anomaly within the Rocky Mountains because of the combination of its small areal size and great thickness of Upper Cretaceous and Paleocene sedimentary rocks (Blackstone, 1993a, 1993b). Virtually all prior interpretations of the basin have started with the assumption that, during most of its Laramide history, the Hanna Basin was an independent structural depression that had a history of subsidence and sedimentary infilling discrete from surrounding basins (e.g., Dobbin et al., 1929; Hansen, 1986; LeFebre, 1988; Dickinson et al., 1988). Dickinson et al. (1988) classified it technically as an “axial basin.” More detailed field research (e.g., Kraatz, 2002; Otteman, 2003; this paper) documents, however, that prior to late Paleocene time the Hanna Basin (including the adjacent Carbon Basin, immediately to the east) was merely the eastern extension of a previously much larger Green River Basin. Near the end of Paleocene time, the eastern Green River Basin became technically subdivided into what we now know as the Hanna Basin and Carbon Basin. These subdivisions, which today are delimited on the west by the Rawlins uplift (Barlow, 1953) and on the east by the Simpson Ridge anticline (Ryan, 1977), were effected through basement-involved thrust faults (Kraatz, 2002; Otteman, 2003) that had their origins within the depths of what was close to the eastern end of the original Green River Basin.
Because of those geologically late tectonic events within local history of the Laramide orogeny, virtually all of the great thickness of Upper Cretaceous and Paleocene strata within the Hanna Basin was affected by faulting associated with basinal subdivision. Additionally, uplifts of the Sweetwater arch (Love, 1970) to the north of the Hanna Basin, the Medicine Bow/Sierra Madre complex (Houston et al., 1968; Mears, 1998) to the south of the basin, and the “area of northeast-trending folds” (Blackstone 1993a; Figs. 1 and 3), bounding the northeastern part of the basin, reached their tectonic culminations in the late Paleocene and early Eocene. Laramide uplift of those complexes of enormous anticlinal structures also deformed virtually all of the stratigraphic sequences that were present along the northern, northeastern, and southern margins of the Hanna Basin. Thus contractional tectonics (Erslev, 1993; Bird, 1998; Tikoff and Maxson, 2001) on a grand scale affected all sides of the newly isolated Hanna Basin during locally expressed, late phases of Laramide orogenesis.
The present study concentrates on that structural history through detailed analysis of Upper Cretaceous and Paleocene strata that are well exposed in an area of badlands known as “The Breaks” (Como West, 1971 and Difficulty, 1961, USGS 7.5 minute topographic quadrangle maps) near the extreme northeastern corner of the Hanna Basin (Figs. 1, 3, 4A–B [insert], and 5–9). That specific area was selected for study because its fossiliferous, Paleocene strata provide especially instructive examples of the nature of structural responses of thick sections of largely incompetent mudrocks to immediately adjacent, basement-controlled anticlinal uplifts. The fossils provide constraints to dating the sequence of specific depositional and tectonic events. Much of the local deformation in the basinal rocks involved thin-skinned, “out-of-the-basin” thrusts that commonly emplaced younger strata upon older. We view these north-directed, out-of-the-basin thrusts as part of a triangle-zone geometry (e.g., Erslev, 1991), which involved a major south-directed, low-angle, chiefly blind-thrust system that developed in part synchronously with the out-of-the-basin thrusts (Fig. 10). This south-directed thrust system was long-lived but episodic, thus yielding complex overprinting relationships (e.g., folded folds in its hanging wall; i.e., Beer Mug anticline, see Lillegraven and Snoke, 1996, fig. 19a, b).
The main purposes of this paper are to provide: (1) description of gross lithologic features of the Paleocene–lowermost Eocene Hanna Formation of the northeastern Hanna Basin; (2) a detailed geologic map of The Breaks; (3) integration of invertebrate-, vertebrate-, and plant-based paleontological zonation of local Paleocene strata; (4) a stepwise, evolutionary series of interpretive maps and cross sections as graphical models to aid visualization of complexities of Laramide structural evolution in this corner of the Hanna Basin; and (5) interpretation of Paleogene geographic history of southeastern Wyoming.
LITHOLOGIC FRAMEWORK OF LOCAL HANNA FORMATION
The considerable sedimentary thickness and localized areas of intense deformation of the Hanna Formation in the northeastern corner of the Hanna Basin pose practical challenges to its description. A composite section for the formation as seen in The Breaks was provided by Lillegraven and Snoke (1996, fig. 13). That preliminary step was established in the field through use of a five-foot Jacob's staff and Brunton compass. Detailed lithologic notes were recorded on standard stripcharts, using foot-thick intervals (∼0.3 m) as the general limit of descriptive resolution. In the present paper, we provide summaries of additional lithologic detail.
Primary features of strata characterizing each of the 20 legs of the measured section (Fig. 11; legs 1–17 shown on Fig. 4B) are summarized in Appendix 1. Additionally, a few lithologic generalizations are provided within the following paragraphs, graphically aided by Figures 12 and 13. Those figures were developed by processing numerically coded lithologic data contained in a spreadsheet via a program entitled “Finley1,” as described by Eberle and Lillegraven (1998a, p. 10). The various legs of the measured section were located strictly on the basis of practicality, emphasizing the best combinations of rock exposures, minimal deformation, and unequivocal super-positional relationships.
Note in Figures 4B and 11 that fault contacts occur near each of the respective boundaries between legs 4–5, 5–6, and 9–10 of the composite section. Similarly, the upper part of leg 2 is itself cut by a fault. As discussed in more detail below, mapping relationships suggest that stratigraphic separations resulting from displacement along the lowest three of those four faults were minor. Therefore, we believe that the composite thickness of the Hanna Formation from its base upward into leg 9 as presented in Figure 11 is a reasonable representation of its original, compacted thickness to that level. Stratigraphic separation across the fault between legs 9 and 10, in contrast, was more consequential. Its details are a focus of discussion below. Importantly, despite localized complexities of deformation in The Breaks, we accept as true that the order of our enumeration of the measured section's legs within the Hanna Formation (i.e., numbers 1–20) reflects the relative sequence of their deposition.
Figure 12 illustrates the extent to which strata composing the 20 legs of the Hanna Formation along the measured section are covered from view by modern soil and vegetation. Exposure of rocks is almost total up to the higher levels of leg 15. Important parts of legs 15–18 are covered, and only minor parts of legs 19 and 20 exhibit exposed strata. Most aspects of the present paper, however, are restricted to information relevant to legs 1–17.
Figure 13A summarizes the distribution along the measured section of Hanna Formation composed of siltsized or finer clastic particles. Clearly, most of the exposed formation in the northeastern Hanna Basin is characterized by fine-grained rock (Figs. 6–9). As is true for most of the Laramide basins of the Rocky Mountain region, much of the fine-grained, clastic fraction of the basin fill was derived from erosion of thick Cretaceous strata, both from nearby and more distant tectonic uplifts. Direct evidence that this was also the case in the Hanna Basin comes from documentation of reworked fossils that are characteristic of local Cretaceous, or even Paleozoic, rock units. Reworked shark teeth and scales (Burris, 2001) and palynomorphs (Dunn, 2003) are common within fluvial deposits of the Hanna Formation in The Breaks section.
Figure 13B shows that the great bulk of fine-grained strata illustrated in Figure 13A consists of carbonaceous shale (Figs. 7A, D–E and 8A, C, and D), of which (especially in legs 14–18) some is highly carbonaceous (Fig. 9B). Figure 13C suggests that lignitic to bituminous coal is restricted principally to legs 5–6, 14–15, and 18–19. Unquestionably, however, the actual abundance of poorly indurated coal within this section is significantly higher than shown, because only part of the total measured section was trenched during description.
Iron carbonate (as siderite) is prominent as a cementing agent within sandstone throughout all of the locally exposed Hanna Formation. And as shown in Figure 13D, siderite also occurs in the form of thin concretionary zones from the base of the formation as far as the lower parts of leg 17. These concretionary beds usually are not extensive laterally. Commonly they are rich with woody plant debris, some contain poorly preserved remains of fossil molluscs, and many crop out as lateral equivalents of thinly laminated, unionid-bearing strata that we interpret as pond deposits.
Figure 13E shows that sandstone beds occur interspersed in the shaly sequences through all exposed parts of the local Hanna Formation (Figs. 8E and 9A and E). The least sandy parts of the section are in midlevels of the formation, especially within legs 9–14. Fine-grained sandstone dominates most sandy fades, usually followed by markedly lesser thicknesses of medium-grained beds; occurrences of coarse-grained sandstone within this section are remarkably limited. As determined in the field from hand specimens, basement-derived sand grains or conglomeratic pebbles do not exist within this local section below the 7,827 ft-(2,385.7 m-) stratigraphic level, which is within lower parts of leg 17 (Fig. 4B; Appendix 1). Quartz sandstone continues to dominate the section upward into leg 18. Above that, many of the sand beds are true arkoses, becoming rich with feldspar as well as muscovite and diverse grains of meta-morphic origin. Virtually all sandstones stratigraphically below the appearance of feldspar include rounded to well-rounded quartz grains. In contrast, starting within lower parts of leg 17, many quartz grains, especially the larger ones, exhibit greater angularity.
The above-described, late occurrence of basement-derived materials within the Paleocene section of The Breaks differs markedly from the situation observed only 12 miles (19.3 km) to the west-northwest in the northern Hanna Basin along southern flanks of the Shirley Mountains. As reported by Grimaldi et al. (2000, p. 183), detrital feldspar grains in that part of the basin first occur in sandstone fades within higher levels of the Upper Cretaceous Medicine Bow Formation. Occurrences of basement-derived materials then continue upward stratigraphically as obvious components of sandstone layers, not only through the remaining Cretaceous strata, but also through the entirety of Paleocene sequences (Lillegraven, unpublished data). Feldspathic debris in sandstone also is common throughout all of the uppermost Cretaceous and lower Paleocene stratigraphic column in more westerly parts of the Hanna Basin (Eberle and Lillegraven, 1998a).
Figure 13F emphasizes the comparative rarity of conglomeratic beds in the Hanna Formation in vicinity of The Breaks. Most beds of pebble conglomerate are thinner than 0.3 m, so fewer are shown on Figure 13F than one observes in the field. Most beds of cobble conglomerate, in contrast, exceed 0.3 m in thickness, so their representation on Figure 13F is closer to reality. Significant pulses of conglomeratic deposition are represented within this measured section only in legs 2 and 5, at the boundaries of legs 7–8 and 8–9, and within legs 12 and 13. Isolated granitic boulders do occur, however, just west of the transect for leg 20. The true cobble conglomerates (restricted to below leg 15) occur both as matrix- and clast-supported accumulations. Generally, the maximum diameter of individual clasts is around 20 cm; the greatest observed length of an individual clast is 44 cm. Most of the conglomeratic beds are poorly sorted, and they exhibit little regularity of depositional fabric. The typical “dumped-in” appearance of these beds makes determinations of paleocurrent flow directions difficult.
In terms of sources, virtually all conglomeratic clasts preserved in The Breaks are from the Lower Cretaceous Mowry Shale (grey to pure white siliceous shale), Cloverly Formation (grey to white orthoquartzite or quartzitic puddingstone), or the Pennsylvanian Tensleep Sandstone (white sugary sandstone). All three of these formations are exposed broadly in the southern Freezeout Hills, immediately north of The Breaks (Fig. 4A). Fractured grains from these three rock units also are seen in abundance within sandy units throughout the local Hanna Formation. Pulverized pieces of Mowry Shale occur ubiquitously, and the resulting particles commonly are seen in astonishing concentrations (Fig. 8E) within almost any bed containing grains coarser than siltstone. In heavily deformed zones (e.g., north wall of “The Great Tortilla,” Figs. 4B and 7D and E), individual cobbles often show secondarily cemented, multiply displaced fractures that clearly developed post-depositionally.
Signs of diagenetic alteration occur almost everywhere at outcrops of the local Hanna Formation. Virtually all conglomeratic beds, and many layers of sandstone, have been bleached to white or grey, showing considerable evidence of post-depositional leaching. Higgins (1999, 2003b) reported evidence for transit of uranium roll-fronts through discrete layers of the formation, possibly destroying much of a pre-existing record of fossil vertebrates through secondary dissolution of hydroxyapatite. Particularly in leg 7 (Fig. 7E), much of the full range of lithologic types (from carbonaceous shale to conglomerate) is heavily altered and has secondary sideritic overgrowths, resulting in a distinctly “cooked” appearance within the general section.
Despite the localized effects of tectonism and diagenesis in vicinity of The Breaks, evidences of Paleocene life abound throughout the Hanna Formation. These include burrows (Fig. 6D), freshwater stromatolites (Fig. 9C), trackways, coprolites, feeding traces of a considerable variety, leaf impressions and preserved leaf tissues (Fig. 8D–E), flower impressions, intact pollen, seeds, tree stumps (Fig. 9D), fallen logs, locally abundant ancient resins (Grimaldi et al., 2000), freshwater molluscs, and vertebrates (fishes through mammals and birds; Fig. 8B). Some of these fossils help in interpreting depositional environments of the strata that now encase them.
Depositional History of Hanna Formation
Diverse aspects of the latest Cretaceous and Paleocene Laramide tectonic and depositional evolution of nearby parts of the Hanna Basin outside of The Breaks were reviewed by Gill et al. (1970), Eberle and Lillegraven (1998a), Secord (1998), Lillegraven and Eberle (1999), and Kraatz (2002). Lillegraven (1994) and Lillegraven and Snoke (1996) presented overviews of Laramide history of the northeastern Hanna Basin, parts of which were dedicated specifically to vicinity of The Breaks. The present paper focuses more closely on localized deposition of the Hanna Formation in The Breaks, its later deformation, and the implications of these localized events upon broader paleogeographic interpretations of central Wyoming prior to the culmination of the Laramide orogeny.
Although Paleocene strata of the Hanna Basin were deposited very close to sea level of that time, we have recognized no rocks of the Hanna Formation within The Breaks section that were deposited under marine conditions. Minor tongues of marine and marine-influenced strata have been reported, however, within the Ferris and Hanna Formations short distances to the southwest (Wroblewski, 2002) and also may occur in the Carbon Basin (Lillegraven, unpublished data) to the south, as well as in various Paleocene levels of the underlying Ferris Formation in more westerly parts of the Hanna Basin (Dunn, 2003).
The Sweetwater arch (Blackstone, 1993b) is a mountainous, Laramide anticlinal feature that plunges to the southeast and borders the northern flanks of the Hanna Basin and eastern half of the greater Green River Basin (Lillegraven, 1993, fig. 1; Fig. 1). Diverse regional geologic evidence has shown that the Sweetwater arch began its basement-involved process of uplift late in Cretaceous time (Love, 1970; Lillegraven and Ostresh, 1988; Grimaldi et al., 2000). Elevation of the Sweetwater arch disrupted the vast, topographically monotonous plain that previously had bordered the western shorelines of the Cretaceous Western Interior Seaway in what is now central Wyoming (Lillegraven and Ostresh, 1990). That new uplift, which itself became complexly subdivided by mostly Paleocene tectonism, also led to origin of the northern border of what is now the Hanna Basin.
The southeastern extremity of the greater Sweetwater arch is known today as the Freezeout Hills (Figs. 1, 3, and 4A). Modern erosion on the Freezeout Hills has yet to breach the Phanerozoic sedimentary cover down to Precambrian basement rocks. Immediately adjacent on the west, however, is another tectonic subdivision of the Sweetwater arch, known as the Shirley Mountains. Structurally, it is a north–south-trending, asymmetric anticline with a broadly exposed core of Precambrian granitic and metamorphic rocks. The core of the Shirley Mountains, as documented later in this paper, became exposed by erosion during late Paleocene time. The Freezeout Hills, Shirley Mountains, and other, much larger parts of the Sweetwater arch to the north and west of the Shirley Mountains (i.e., the Granite Mountains of Love, 1970) were partial source areas during deposition of the Hanna Formation in The Breaks.
When the Hanna Formation was being deposited, the Hanna Basin was broadly open to the east. New mapping of remnants of the Hanna Formation along the eastern extremity of the Hanna Basin shows that the basin, as a structural and depositional feature, must have extended far to the east of its present, erosionally defined limit through most of Paleocene time. The Laramide anticlinal features just to the east of the modern Hanna Basin (i.e., the “area of northeast-trending folds” of Fig. 1; Blackstone, 1993a; Fig. 3) did not form until after deposition of most of the Hanna Formation; their late uplift figured importantly in completion of deformation of the Hanna Formation in vicinity of The Breaks. The same is true for the history of uplift of Simpson Ridge anticline (Kraatz, 2002), which separates the southeastern corner of the Hanna Basin from the Carbon Basin to the east. The eastern extremity of today's Hanna Basin (Figs. 1, 3, and 5), therefore, attained its configuration no earlier than very late Paleocene. At that time, strong anticlinal development in the “area of northeast-trending folds” dramatically contracted the Hanna Basin's northern, eastern, and southern margins and induced prodigious rates of erosion of the eastern basinal strata.
Veatch (1907), Bowen (1918), and Dobbin et al. (1929) showed that localized Upper Cretaceous strata on the southern flank of the Freezeout Hills were deeply beveled by erosion prior to deposition of the Paleocene Hanna Formation. That erosion resulted from initial uplift of the southeastern corner of the Sweetwater arch. Specifically, the erosion removed part of the stratigraphic cover on the south flank of Freezeout Mountain anticline (Maravich, 1941, fig. 2; Lillegraven and Snoke, 1996, fig. 7; Fig. 4A).
Figure 14 provides a semi-diagrammatic, computer-generated model to help visualize the original nature and further evolution of this localized but important erosional surface during Paleocene time. The stratigraphically highest level of this beveled surface is in the Ferris Formation (Fig. 14B). The erosion cut down-section through the Freezeout Mountain anticline (Lillegraven and Snoke, 1996, figs. 8–11). When it reached the upper Steele Shale, deposition of the Hanna Formation began (Fig. 14C). The unconformable contact of the Hanna Formation and Steele Shale is visible in many outcrops at The Breaks (Fig. 4). The process of erosion through Upper Cretaceous strata involved localized removal of close to 20,000 ft (∼6 km) of section (see thicknesses in Fig. 2). As shown in Figure 14B–C, however, the Freezeout Mountain anticline became eroded even more deeply northward, exposing sedimentary rocks within the core of the fold down to the Pennsylvanian Tensleep Sandstone.
At the northern margin of the Hanna Basin, the Hanna Formation gradually accumulated upon this newly formed erosional surface (Figs. 6A and 14C), progressively onlapping and burying it. Today, the deeply eroded remnants of the Hanna Formation form an angular unconformity upon the Cretaceous sequence. Beveling of exposed parts of the erosional surface continued during the Paleocene until the Freezeout Mountain anticline became covered by the accumulating strata of the Hanna Formation. Eventually, accumulation of the Hanna Formation completely buried the entire Freezeout Hills.
The lowest part of the Hanna Formation (i.e., leg 1) was deposited in angular unconformity upon the Steele Shale at the Hanna Basin's north margin (Lillegraven and Snoke, 1996, fig. 10). As progressively younger strata of the Hanna Formation continued to accumulate (Fig. 14C), close lateral equivalents to the east of legs 2–5 were deposited on that same erosional surface, onlapping progressively higher stratigraphic units of the Cretaceous sequence (Fig. 4B). Starting with its base on the Steele Shale, the accumulating Hanna Formation encroached upon the Haystack Mountains Formation (Lillegraven and Snoke, 1996, fig. 11), and then sequentially onto the Allen Ridge Formation, Pine Ridge Sandstone, Almond Formation, and base of the Lewis Shale (Lillegraven and Snoke, 1996, fig. 9; Figs. 4B and 7E; terminology of Lewis Sh. sensu Lillegraven and McKenna, 1986, p. 9). Wherever the contact between the Hanna Formation and older strata can be observed clearly, the underlying surface is virtually unweathered. Staining of rocks below the ancient erosional surface is minor and may have developed during Quaternary time through oxidation of disseminated iron.
All existing outcrops representing legs 6–9 of the Hanna Formation are in fault contact with underlying Cretaceous rocks. Map relationships (Fig. 4B), however, allow us to infer that close lateral equivalents just to the east of these Paleocene strata were deposited sequentially upon progressively younger Cretaceous units in the manner shown in Figure 14C. That is, the Hanna Formation was deposited first upon higher parts of Lewis Shale, then onto the Medicine Bow Formation, and finally upon the Ferris Formation. In contrast, strata equivalent to legs 10–18 of the measured section (Figs. 7–8; south of a major fault shown in Fig. 4B) are stratigraphically higher than any shown diagrammatically in Figure 14 C and were deposited well away from the erosional surface that formed the immediate basinal margin. Thus those higher parts of the stratigraphic column accumulated upon components of the Hanna Formation represented in more central parts of the basin and were transported to the northeast by out-of-the-basin faulting.
Generalized Depositional Settings of Hanna Formation
As discussed above and summarized in Appendix 1, the Hanna Formation in The Breaks is dominated by fine-grained rocks (Fig. 13A), mainly carbonaceous shale (Figs. 6–9 and 13B). In places, these carbonaceous units grade progressively into lignitic to bituminous coal (Fig. 13B–C). Most occurrences of these dark fades, whether they developed along the basinal margin (i.e., legs 1–9 of measured section) or more centrally within the basin (i.e., legs 10–18), are rich with evidence of ancient vegetation, strongly suggesting well-watered, generally paludal depositional settings. Laterally discontinuous, mollusc-bearing pond deposits, usually composed of fine-grained sandstone beds, commonly are interspersed among the darker, carbonaceous shales.
These generally well-watered conditions were taken to the extreme during deposition of the upper part of leg 14 through leg 18. Together, they contain the lower and upper lacustrine units (Figs. 8A and D, 9A–C, and 11) and an intervening, complex sequence of interbedded fluvial, paludal, and lacustrine facies. Relationships within the measured section clearly show that the lower and upper lacustrine units each began with progressive flooding of well-established coal swamps (Fig. 9B). The geographic extent of these lake systems is unknown, because the lacustrine facies extend across preserved parts of the Hanna Formation all the way to their present-day, erosional edges. Judging by the combined thicknesses of the lacustrine facies and the considerable thicknesses of some of their individual units (> 40 ft [> 12 m]), however, these were major bodies of water that at times probably covered most of the area of the Hanna Basin and extended well beyond the limits of the modern basin.
The high diversity of lithologic types represented within the lacustrine facies and the close spacing of recurrent facies suggest that the lakes experienced frequent and dramatic variations in depth and shoreline position. The generally abundant and diverse fossil record within the lacustrine facies also suggests that the lakes usually were richly oxygenated and teeming with animal life. Contrariwise, the lake bottoms on occasion became inhospitable settings for benthonic animals, thus allowing rich and comparatively undisturbed accumulations of leaves and other plant parts that floated from shore, sank to the bottom, and became preserved.
Despite this general picture of well-watered paleoen-vironmental settings, the Hanna Formation in The Breaks also contains beds at many levels showing direct evidence for extended seasonal dryness and consequent deeply placed water tables. This was first recognized in The Breaks by Hasiotis and Honey (2000, fig. 9A–D), through their paleoecologically oriented examination of crayfish burrows similar to those found commonly in non-paludal, Paleocene sections in various Laramide basins of Colorado and Wyoming. Their main points were that (p. 134): (1) “The presence of crayfish burrows suggests infrequent overbank deposition with seasonally shallow to deep water tables and intermittent pedogenesis of graded sediments, which were mixed by crayfish activity"; and (2) “The lack of crayfish burrows and the preservation of carbonaceous material in these deposits suggest annually high water tables and standing water…”.
Ancient crayfish burrows (Fig. 6D) are especially densely packed and obvious within legs 1–4 of the Hanna Formation in The Breaks, where they occur exclusively in layers of siltstone to fine-grained sandstone. The dense packing of burrows reflects prolonged bioturbation, which seems to have destroyed other sedimentological evidence for dryness (such as mudcracks and elongated root traces). Such burrows have not been observed in legs 5–7 of the measured section. They are well developed again in legs 8 and 9, and one occurrence in the latter is in carbonaceous shale. Crayfish burrows also occur fairly commonly in legs 10–14 of the measured section, although their preservation in these more basin-center facies usually is much more cryptic than in lower legs of the measured section that were deposited at the basinal margin. Some occurrences of crayfish burrows in leg 14 are within beds of slightly carbonaceous shale. They have not been observed in upper parts of leg 14 that are characterized by highly carbonaceous facies. Similarly, crayfish burrows do not occur either in the lower or upper lacustrine units. The highest of such burrows observed in The Breaks section occurs at the 7,598 ft- (2,315.9 m-) level within leg 17, in the largely fluviatile sequence between the two lacustrine units.
It appears, therefore, that even though the general landscape in the northeastern corner of the Hanna Basin during deposition of the Hanna Formation was generally well-watered to swampy or even lacustrine, there were many intervals characterized by strong seasonal dryness and resulting deep water tables. Local outcrops of the Hanna Formation show little in the way of erosional cutting and filling, and evidence for anything beyond the most preliminary stages of pedogenesis was not observed. Thin sandstone beds are common within the usually much more massively represented sets of fine-grained facies (Figs. 8B and E and 13E). The sandstones tend to occur as thin, laterally discontinuous sheets that show little pre-depositional erosion into underlying strata. Clearly, a rapidly aggrading depositional regime dominated accumulation of the local Hanna Formation. As might be predicted on the basis of their greater distance from the northern margin of the basin and the basin's structural axis, legs 10–17 exhibit lesser amounts of sandy facies and greater spacings between individual beds of sandstone than exist in legs 1–9. Nevertheless, beds of conglomerate, which generally make up only a small fraction of the total volume of the Hanna Formation, are well developed in upper parts of leg 12 (Fig. 7A) into the lower part of leg 14.
Changing Source Materials
Compositionally, the rocks constituting most of the Hanna Formation in The Breaks show little variety. Volumetrically, carbonaceous shale is dominant. Rounded to well-rounded quartz grains dominate the sandy facies from leg 1 into leg 18 of the measured section, accompanied by ubiquitous, comminuted soft-rock debris (most obviously, Mowry Shale). Feldspathic grains seem not to enter the sandy facies at all until deposition of the lower part of leg 17. That same level also marks the first appearance of grains of angular quartz, flakes of muscovite, and consequential amounts of diverse metamorphic minerals. Pebbly stringers throughout the entire measured section are composed principally of clasts from the Mowry Shale, Cleverly Formation, and Tensleep Sandstone. Cobble conglomerates, although restricted in distribution to below the middle of leg 14, are derived almost exclusively of clasts from those same three sources.
Lithologic composition does undergo progressive change upward, however, beginning in a clear fashion within leg 18 of the measured section. Sandstone beds become markedly more abundant at that level, and they become progressively coarser-grained and more arkosic upward. Certain sandstone beds near the highest levels of the formation are composed almost exclusively of coarse-grained, angular, potassium-feldspar-dominated debris from granitic sources (Fig. 9E). Although increased detrital contributions from Precambrian rocks are heralded within the lower half of leg 17 of the measured section, the transition does not become obvious until within leg 18. In any case, our evidence suggests that deposition of parts of the Hanna Formation in The Breaks below lower parts of leg 17 (upper Paleocene) did not involve a source area having broad exposures of Precambrian basement.
Much of the muddy fraction of sediments in the local Hanna Formation could have been brought into this area by river systems draining western parts of the greater Green River Basin. All elastics in legs 1–16 could have been derived from Phanerozoic strata in the Freezeout Hills (Figs. 4A and 14B–C). In contrast, a shift in source area toward exposed basement rocks of the Shirley Mountains (and/or the much more extensive and more northerly parts of the Sweetwater arch; Fig. 1) began during deposition of lower levels of leg 17 of the Hanna Formation. The change becomes lithologically obvious within leg 18. The transition in source areas may reflect the complete burial of the Freezeout Hills by progressive accumulation of the Hanna Formation (Fig. 15). The evidence for continuing input of well-indurated clasts from the Mowry Shale, Cloverly Formation, and Tensleep Formation during deposition of the highest levels of the Hanna Formation does not preclude the concept of burial of the Freezeout Hills, because all of those rock units are broadly exposed along most of the length of the northern flank of the Hanna Basin (Fig. 4A).
BIOSTRATIGRAPHICALLY USEFUL COMPONENTS OF HANNA FORMATION
Knowledge about the age of the Hanna Formation in all of its parts in vicinity of The Breaks is far from complete, but we do have a few key constraints for specific levels within the measured section. We have yet to recognize isotopically datable minerals within the Hanna Formation. Therefore, information about its age is restricted to biostratigraphic data linked to a reasonably well-ordered coal stratigraphy (Fig. 5). In terms of ages that can be correlated to other basins and to the global geologic time scale, fossil vertebrates (especially mammals) presently are the most reliable. Additional attempts at biostratigraphic study have focused on freshwater molluscs, leaf macrofloras, and palynofloras. We summarize the status of each of these approaches within the following sections.
Vertebrate-based, biostratigraphic studies useful in determining ages of Paleogene strata in the Hanna Basin are limited to the following parts of the section: (1) upper parts of the Ferris Formation that underlie the Hanna Formation in west-central parts of the Hanna Basin (Fig. 2; see black spot marked “1” in Fig. 5); (2) stratigraphic equivalents of legs 11 and 12 of the measured section within The Breaks (Figs. 4B and 11); and (3) near the highest levels of the Hanna Formation (in leg 19 of measured section; Fig. 11) at the northwestern margin of The Breaks. Additionally, several levels of mammal-bearing strata have been studied within the Hanna Formation of the northern Carbon Basin. All of these fossils have been linked to standard schemes of North American, mammal-based biostratigraphy (e.g., Wood et al., 1941; Lofgren et al., 2004). Those linkages, in turn, provide correlation with global geochronology. The following paragraphs summarize essential biostratigraphic conclusions from each of those studies.
Ferris Formation of West-central Hanna Basin
Diverse and locally abundant fossils of nonmarine vertebrates were used by Lillegraven and Eberle (1999) to document the stratigraphic position of the Cretaceous–Paleocene boundary within the type Ferris Formation. Above that stratigraphic level, within upper (but not uppermost) parts of the Ferris Formation, Eberle and Lillegraven (1998a, 1998b) collected and identified mammalian assemblages characteristic of all three interval zones (i.e., Pul, Pu2, and Pu3) of the Puercan North American Land Mammal “Age” (NALMA). Those interval zones were defined originally in New Mexico, and the unusually thick column of rocks of the Ferris Formation permitted its mapped zonation on the basis of superposed assemblages of fossil mammals. The Puercan NALMA represents roughly the first million and a half years of Paleocene time (minimally involving 65–64 Ma).
Strata of the type Ferris Formation above the level documented as Interval Zone Pu3 exceed 2,500 ft (762 m) in thickness. Unfortunately, only a few, temporally undiagnostic mammalian teeth have been discovered within this comparatively coal-rich, uppermost component of the formation. The landscape in which it occurs constitutes the “Ferris coalfield” (as used by Ellis et al., 1999; Fig. 5), and it is the source of most of the coal mined commercially from the Ferris Formation. The upper part of the Ferris Formation is assumed to represent early Torrejonian time on the bases of: (1) well-studied mammalian assemblages below and above it; and (2) macrofloral and palynomorphic associations (summarized below).
Legs 11 and 12 of Hanna Formation in The Breaks
Despite the prodigious thickness of the Hanna Formation in The Breaks, only a minor part just below its mid-level, approximately 1,640 ft- (500 m-) thick, has yielded rich accumulations of temporally diagnostic, fossil mammals. Not a single bone fragment has been discovered in the Hanna Formation from below that interval, but within it fossils occur at many localities, scattered through most of the length of the badlands that constitute The Breaks. Higgins (2000, 2003b) referred to this laterally extensive band of fossiliferous strata as the “Vertebrate Fossil Bearing Zone” (or “VFBZ”). It represents the stratigraphic equivalents of the upper two-thirds of leg 11 and virtually all of leg 12 of the measured section (Figs. 4B, 8B and C, and 11).
The lowest recorded locality in the VFBZ is at about the 3,670 ft- (1,118.7 m-) level of the measured section, occurring northwest of its actual transect. The taxonomically diverse mammalian assemblages of the VFBZ were studied by Higgins (2000, 2003a, 2003b), who recognized that the animals lived during late parts of the earlier half of Paleocene time. Specifically, applying terminology for biochrons devised by Archibald et al. (1987) and updated by Lofgren et al. (2004), the mammalian assemblages of the VFBZ represent the last interval zone (To3) of the Torrejonian NALMA through the first three interval zones (Til–3) of the Tiffanian NALMA.
Additionally, in part because of the combination of extraordinary thickness and relatively rapid deposition of the Hanna Formation, a paleontologically unique interval (represented by strata roughly 164 ft- [50 m-] thick) within the VFBZ was discovered in The Breaks. It holds species known elsewhere exclusively from strata of Torrejonian or exclusively of Tiffanian age. Higgins referred to this band of strata as the “overlap zone,” and the temporal interval it represents has been recognized only in The Breaks. The Torrejonian–Tiffanian NALMA boundary (represented by the breadth of the overlap zone) typically is set at about 61 Ma, and the total time range of To3 through Ti3 usually is considered as 62–58 Ma (Lofgren et al., 2004). Thus, between three and four million years transpired during deposition of stratigraphic equivalents of legs 11–12 of the measured section. On the basis of superposed assemblages of fossil mammals, it has proven possible to subdivide that part of the Hanna Formation into five recognizable biostratigraphic units (Higgins, 2003b, fig. 10).
Near Top of Hanna Formation at Margin of The Breaks
Scattered vertebrate fossils do occur within the upper half of the Hanna Formation in vicinity of The Breaks. Within numerous levels of the two lacustrine units, for example, disarticulated bones, teeth, and scales of freshwater fishes are locally abundant. Similarly, several layers are known to bear remains of terrestrial or semi-aquatic lower vertebrates. To date, however, no temporally diagnostic mammalian fossils have been discovered from strata equivalent to the top of leg 12 of the measured section to near the top of leg 19. But at that latter level, many bone fragments of a large mammal (probably a pantodont) and a few teeth of the primitive horse Hyracotherium grangeri were discovered and collected. As discussed by Lillegraven (1994), the latter is characteristic of early phases of the Wasatchian NALMA (c. 55 Ma), now considered to represent a very early part of the Eocene Epoch.
Those specimens occur only a few hundred feet stratigraphically below the top of the Hanna Formation. The highest levels of that rock unit are cut by a modern surface of erosion, so the original total thickness of the Hanna Formation remains unknown. These few fossils are particularly important, however, in confirming the early Eocene age of upper parts of the Hanna Formation, an interpretation made long ago on the basis of fossil leaves and pollen. Also, the horse and pantodont fossils show that the Hanna Formation from the top of the VFBZ into leg 19 must have been deposited late in the Paleocene (presumably involving late Tiffanian and Clarkforkian NALMAs) and during the earliest Eocene. Huge uncertainties exist, however, in placement of the various temporal boundaries of interest within that body of strata – which comprises more than half the total remaining thickness of the Hanna Formation. In contrast to the suggestion by Knight (1951), we recognize no unconformity in the northern Hanna Basin separating the Hanna Formation from an overlying, unnamed (“Wind River”) rock unit.
Hanna Formation in Northern Carbon Basin
Secord (1996, 1998) described three mammalian assemblages (collected from numerous localities) that occur in Paleocene strata mapped as Hanna Formation in the northern Carbon Basin. Although these fossils are not especially relevant to the geological emphasis of the present paper, they do add importantly to the overall taxonomic documentation of mammalian assemblages in the general area. Two of the assemblages (Grayson Ridge and Halfway Hill faunas), although geographically separated by several miles, include vertebrates that were approximately contemporaneous with those preserved in the VFBZ of The Breaks. The case for these two assemblages in the Carbon Basin faunas is less clear than the faunally richer section in The Breaks. It can only be said that the included mammals represent either late Torrejonian or early Tiffanian time.
The third assemblage (Sand Creek fauna of Secord, 1998), however, is younger than most of the fossils preserved in the VFBZ, representing an indeterminate part of middle to late Tiffanian time. Secord suggested a reasonable range of Ti3–Ti5 for the assemblage, but available fossils do not permit specific reference to any one of those biochrons. In any case, taphonomic conditions in the Carbon Basin did allow preservation of vertebrate fossils in fluvial strata during part of an interval (reasonable range of 59–56 Ma) that is not represented by fossils in the Hanna Basin (Higgins, 1999). Such a difference in preservation of hydroxyapatite-based fossils would seem to reflect difference in lithologic composition, taphonomic conditions, or the primary influence of geographically localized groundwater chemistry rather than more general influences of climatic conditions.
Hanna Basin Coal Stratigraphy
The occurrence of coal provides no independent information of biostratigraphic value. When individual coal beds are geographically extensive, however, their positions within the stratigraphic column can be recognized (from outcrop or subsurface data), thereby providing important references to relative ages. That is very much the case in the Hanna and Carbon Basins, even though most of the individual coal beds variously bifurcate and merge. Dobbin et al. (1929) produced an extraordinarily detailed report (with an associated geologic map, pl. 27) on the occurrences of individual coal beds within Upper Cretaceous and Paleocene rock units of the Hanna/Carbon Basin. Their system of enumeration of the beds remains in use today by the mining and scientific communities (e.g., Glass and Roberts, 1980; Flores et al., 1999a, 1999b). Also, macrofloral and palynological remains commonly are specifically associated with highly carbonaceous layers. In this section, we focus on coal stratigraphy of the Hanna Formation within the Hanna coalfield (northeastern part of Fig. 5).
Dobbin et al. (1929, pl. 8) applied the numbers 67 through 89, in ascending numerical and stratigraphic order, to the most important coal beds within the Hanna Formation of the eastern Hanna Basin. Additionally, they specially named a few other significant coals (e.g., Brooks and Hanna No. 1–5). The occurrences of coals 77 through 87 are plotted on Figures 4B and 11 as they appear on the transect of our measured section; coal beds 77–87 occur within legs 14–17. Dobbin et al. (1929) did not identify their enumerated coals below number 77 within badland areas of The Breaks. Neither did we attempt to do this, although significant coal beds do occur much lower in the section within The Breaks. For example, Dobbin et al. (1929, p. 82) said: “Other coal beds occur near the base of the Hanna formation in sec. 15 [T. 23 N., R. 80 W.] and vicinity, but are too badly squeezed, crushed, and folded to be of economic value.” Indeed, as we discuss below in relation to faulting within lower parts of the Hanna Formation, such coaly units are among the weakest rocks in the section and in large part probably determined where the faults were initiated (Fig. 7C).
As a convention for simplicity of discussion, Flores et al. (1999a, fig. HS-3) lumped stratigraphically associated coal beds into “bundles.” Their bundle H4 includes coals 77–81, and their bundle H5 includes coals 82–89 (Fig. 11). Bundle H4 reflects the cycle (involving parts of legs 14–17 of our measured section) from paludal to lacustrine, and then back to fluvial conditions, that is related to the lower lacustrine unit of our terminology. Similarly, bundle H5 reflects the same sort of cycle (involving most of legs 17 and 18) that is related to our upper lacustrine unit.
Notice in Figure 5 that the distribution of coal beds 77–89 in map view is reflective of two synclines (i.e., Hanna syncline and the defining syncline of northeastern Hanna Basin). Although their axes are essentially at right angles to one another, both reflect responses to basinal crowding related to Laramide uplift of surrounding anticlines. The Hanna syncline is principally related to development of Simpson Ridge anticline (Kraatz, 2002). The defining syncline of northeastern Hanna Basin, in contrast, had a much more complex history, reflecting combined influences of the Freezeout Mountain, Flat Top, and Simpson Ridge anticlines (Fig. 3).
Kirschner's (1984) well-documented master's thesis deals with assemblages of freshwater molluscs that he collected from the Ferris and Hanna Formations of the Hanna Basin. Although unpublished, so far as we know it is the only work on such fossils from the basin in which taxonomic identifications and locality data can be linked to the stratigraphic positions of actual specimens. For the Hanna Formation, Kirschner studied collections from six sites within the upper half of the formation, essentially representing three stratigraphic levels (each marked “M” in Fig. 11). One locality is in the upper part of the lower lacustrine unit, above the 80 coal. Four sites were sampled in upper parts of the intervening fluvial sequence between the lower and upper lacustrine units, above the 82 coal. A final site is in the upper lacustrine unit, probably below the 88 coal.
Of the 25 species-level taxa of freshwater molluscs reported by Kirschner (1984), about half occurred both in the Ferris and Hanna Formations. Six of those species were restricted to the Ferris Formation, and seven were recovered only from the Hanna Formation. Most of the species reported, as known from studies elsewhere, were long-lived throughout the early Paleogene. Thus, although the specimens from the Hanna Basin provided useful paleoenvironmental information, they were not effective tools for biostratigraphic correlation. There exists no obvious break in distributions of molluscan species within the sampled parts of the Hanna Formation. As discussed below, the strata sampled cross the level of first occurrence of Platycarya, a palynomorph that has been used regionally to indicate advent of Eocene time.
Numerous collections of fossil macrofloras from Cretaceous and Paleocene strata have been made from vicinity of the Hanna Basin. Nevertheless, very little of that material has been described in publications, and even less has been documented in relation to a measured section. There also exist many published mentions of rich accumulations of fossil leaves from the area (e.g., Bowen, 1918; Dobbin et al., 1929; Gill et al., 1970; Glass and Roberts, 1980). Typically, however, these references contain neither illustrations of specimens nor their descriptions.
Relatively minor taxonomic references to Upper Cretaceous macrofloral elements from the Hanna Basin have been published by Knowlton (1922), Dorf (1938), Johnson (1996), and Grimaldi et al. (2000). Of those, only specimens reported by Grimaldi's team are placed within the context of detailed stratigraphy. The stratigraphic position of an uppermost Cretaceous macroflora in the Ferris Formation was reported by Eberle and Lillegraven (1998a, fig. 2), but they did not specify its taxonomic composition.
Brown (1962), in context of his general study of Paleocene floras from the Rocky Mountains and High Plains, described important fossils from a series of localities in the Hanna Basin. Locality data presented by Brown (1962, p. 30–36) are generalized, and the collecting efforts focused on Paleocene components of the Ferris Formation. None of the localities was linked to detailed stratigraphic information then available. Eberle and Lillegraven (1998a) documented the stratigraphic occurrence of a rich floral site in strata of the Ferris Formation, dated as late Puercan on the basis of fossil mammals. Subsequently, the contained flora from that locality was studied by Dunn (2003).
Grimaldi et al. (2000) reported occurrences of amber from Upper Cretaceous and Paleocene sites at multiple levels within the Hanna Basin. On the basis of chemical compositions of the Paleocene amber, they made reference to the broad kinds of tree groups that probably formed the original resins. Similarly, Teerman et al. (1987) studied the physical properties of resinite in coals from the Hanna Formation, which presumably reflect composition of the botanical sources for the coal. Their research did not, however, directly approach issues of taxonomic interest.
In an unpublished master's thesis, Dunn (2003) provided the only verifiable summary of Paleocene macrofioras (including leaves and assorted flowers, seeds, catkins, and cones) of the Hanna Basin. Her work used our measured sections and unpublished geologic base maps (e.g., fig. 2.2, with its source incorrectly identified). Wherever possible, Dunn linked her sampling to our mammal-based zonation. She developed 32 collections (Dunn, 2003, table 4.3) in: (1) Cretaceous?–Paleocene parts of the Ferris Formation on both sides of Seminoe Reservoir (Fig. 5, areas marked 1 and 2); and (2) through roughly the lower half of the Hanna Formation in The Breaks (spanning legs 1–15 of the measured section, Figs. 4B and 11). Her Paleocene samples included strata deposited from late Puercan into early Tiffanian time, thus covering essentially the earlier half of the epoch.
Dunn's work combined: (1) identifications (based upon morphotypic analyses of leaves and reproductive structures, with formalized taxonomy de-emphasized) from each site; (2) data from quantitative taxonomic censusing of leaf occurrences at 10 localities; (3) palynological sampling and analysis from 21 productive sites (including all censused macrofloral localities, in addition to other leaf- or vertebrate-bearing levels; 3 localities are in uppermost Cretaceous strata); and (4) approximate stratigraphic placement of most samples within measured sections. She summarized first- and last-occurrence data for each leaf morphotype (119, plus reproductive structures totaling 161 morphotypes) through the entire section (Dunn, 2003, fig. 5.2) and provided tentative estimates of ancient mean annual temperatures and precipitation via leaf-margin and leaf-area analyses, respectively. Representative leaf-morpho-typic associations of specimens are housed within collections at the Denver Museum of Nature & Science.
The macrofloral investigations conducted by Dunn (2003) in the Hanna Basin represent initial steps in paleobotanical documentation of the local section. Potentially, further work in this thick, plant-rich, Paleocene sequence could enhance the biostratigraphic utility of leaf fossils. Although Dunn placed appropriate emphasis on that aspect of her research, she used vertebrate-based biostratigraphy as the primary basis for temporal subdivision of the stratigraphically superposed, leaf-bearing samples. Inspection of the first- and last-occurrence data plotted in her figure 5.2 (“Stratigraphic ranges of Hanna Basin morphotypes …”) shows general coherence and regularity of evolutionary replacements. The general incompleteness of taxonomic sampling from the Ferris and Hanna Formations, however, is shown by the following quotation (Dunn, 2003, p. 56): “Of the morphotypes found in the two formations, 88 (55%) occur at only one locality, with 57 (35%) occurring only as single specimens.”
Despite the incomplete nature of taxonomic sampling, Dunn's (2003, fig. 5.2) summary of morphotypic occurrences generally agrees with predictable levels of macrofloral similarity through the stratigraphic section as based upon superpositional relationships alone. For example, taxonomic similarities compared among samples from the Ferris Formation exceed macrofloral similarities when compared to collections from the Hanna Formation, and vice versa. Indeed, only eight morphotypes (of 161 total) have been recorded in both formations (Dunn, 2003, p. 125). Also following expectations based upon stratigraphic superposition, macrofloral elements from lowest parts of the Hanna Formation show the greatest similarities when compared to morphotypes of fossil plants from the underlying Ferris Formation. Of the eight macrofloral morphotypes held in common between the formations, we note from inspection of Dunn's (2003) figure 5.2 that five remain unknown above leg 4 in the measured section of the Hanna Formation. Contrariwise, only three morphotypes from the stratigraphically highest, well-sampled localities in the Hanna Formation (in leg 15) also occur in the Ferris Formation, and one of the three has a virtually ubiquitous Paleocene distribution as sampled within the Hanna Basin.
As discussed by Nichols (2003, p. 115–116), paly-nostratigraphic research in the Hanna Basin has been of a reconnaissance nature despite the fact that “… because of its rich vertebrate paleontological record…, it may prove to be one of the best places within the Rocky Mountains and Great Plains region to correlate vertebrate and palynological biostratigraphies.” Nichols (in Eberle and Lillegraven, 1998a, p. 17–18) listed palynomorphic occurrences within his own collections made below and above the Cretaceous–Tertiary boundary (recognized in the Ferris Formation via occurrences of fossil vertebrates). Grimaldi et al. (2000, fig. 13) illustrated a few, unidentified pollen grains or spores found as inclusions within amber from upper parts of the Hanna Formation. Until very recently, these were the only occurrences of palynomorphs that were intentionally linked with strata dated on the basis of vertebrate fossils.
Historically, the oldest palynostratigraphic applications in the Hanna Basin were provided by Gill et al. (1970) from the Hanna Formation in the northern part of the Hanna syncline (Fig. 5). The geographic coordinates they reported (p. 47) indicate sampling of a stratigraphic level close to the 83 coal. Gill et al. (1970) cited an assemblage of palynomorphs (identified by R. H. Tschudy) that included “… specimens of Tiliapollenites and is of late Paleocene or early Eocene age.” From a site farther north (north of the Medicine Bow River), Tschudy (in Gill et al., 1970, p. 47) reported an assemblage that “… includes the genera Carya, Jugulanst Tiliapollenites, and Platycarya, and is of Eocene age.” The reported geographic coordinates indicate sampling of a stratigraphic level close to the 88 coal (Fig. 5, just north of the small anticline shown at the river). Harrington (2001), studying pollen assemblages from the northern Bighorn Basin, suggested that the presence of Platycarya indicates strata of Wasatchian (early Eocene) age. On the assumption that such a generalization also holds in more distant geographic areas, the Paleocene–Eocene boundary in the Hanna Basin would be placed within leg 18 of our measured section, just above the middle of the upper lacustrine unit (Fig. 11).
Gill et al. (1970) also reported palynomorphs in the Hanna Formation near the northeastern part of the basin on the edge of The Breaks. They designated the locality as follows (p. 48): “USGS D4089. NW1/4 sec. 10, T. 23 N., R. 80 W., Carbon County. Lower part of Hanna Formation.” That information, in conjunction with an associated photograph (Gill et al., 1970, fig. 15) of the area, confirms that the strata sampled correspond to leg 6 of our measured section (Figs. 4B, 7E, 11, and 16A). Tschudy (in Gill et al., 1970, p. 48) said of the contents: “Sparse assemblage, but the presence of Momipites tenuipolus Anderson and the absence of Cretaceous and Eocene species indicate that this sample is of late Paleocene age.”
The specificity of age determination as “late Paleocene” for the pollen sample from locality USGS D4089 must have been based solely upon presence of Momipites tenuipolus. In terms of the present status of regional palynostratigraphy (i.e., Nichols, 2003), however, the known range of that species in other Rocky Mountain basins is from the uppermost Cretaceous (Leffingwell, 1970) into the “Momipites actinus–Aquilapollenites spinulosus Interval Biozone” (the former palynomorphic “Zone P3” of Nichols and Ott, 1978). For the Hanna Basin itself, Nichols (2003, table 3) shows occurrences of M. tenuipolus only in “Zones P1 and P2” (of the older terminology from Nichols and Ott, 1978). In other basins, however, the species persisted into Zone P3 (Nichols, 2003, tables 4–7) but not higher.
Within the palynostratigraphic terminology presently used by Nichols (2003, fig. 9), the following three (new, formally named) “interval biozones” correspond, respectively, to Zones P1, P2, and P3 of Nichols and Ott (1978): (1) Momipites inaequalis–Discoidites parvistriatus Interval Biozone; (2) Momipites wyomingensis–Kurtzipites trispissatus Interval Biozone; and (3) Momipites actinus–Aquilapollenites spinulosus Interval Biozone. Because the older, “P Zone” designations are less cumbersome in writing than the new interval-biozone terminology, we use them in the following text, as does Nichols in his recent work (2003, fig. 9). In terms of stratigraphic zonation, Nichols (2003) accepted the equivalency of Zone P3 with the Torrejonian NALMA, which transpired wholly within the earlier half of Paleocene time.
Flores et al. (1999a, fig. HS-3) summarized then-available palynomorphic data for Upper Cretaceous and Paleocene strata in the Hanna and Carbon Basins in the form of stratigraphic columns linked to coal stratigraphy. They used the system of “P Zones” established by Nichols and Ott (1978) and plotted them in association with certain of the numbered coals recognized by Dobbin et al. (1929). Although no species lists or locality data were provided, Flores et al. (1999a, fig. HS-3) showed the occurrence of pollen characteristic of Zone P2 in the 65 coal, about 505 ft (154 m) below the top of the Ferris Formation. Similarly, they showed presence of pollen characteristic of Zone P4 in their coal bundle H2 (which combines the 70–72 coals of Dobbin et al., 1929), occurring about 2,010 ft (613 m) above the base of the Hanna Formation.
Flores et al. (1999a, p. HS-12) also stated that palynomorph Zone P3 had not been found, either in the Ferris or Hanna Formations. Oddly, this differs from what was shown for the Hanna and Carbon Basins in the companion paper by Nichols (1999, fig. HB-3) in the same Professional Paper. Nichols (2003, fig. 5) has corrected that apparent conflict by showing occurrences of pollen (taxa listed in his table 3) characteristic of Zone P3 in the Hanna Basin. The palynomorphically recognized Zone P3 is shown to exist uninterruptedly (within a stratigraphic column labelled as a “composite section”) from about 591 ft (180 m) below the top of the Ferris Formation to about 3,051 ft (930 m) above the base of the Hanna Formation. Because the composite section provided as figure 5 by Nichols (2003) does not indicate lithologic features, coal stratigraphy, or locality information, the equivalencies remain uncertain between his zonation and our measured section of the Hanna Formation in The Breaks. His column does, however, emphasize palynomorphic recognizability of Zone P3 within lower levels of the Hanna Formation. That information is compatible with: (1) occurrence of Momipites tenuipolus in leg 6 of our measured section in The Breaks, as reported by Gill et al. (1970) and as discussed above; and (2) new information from The Breaks (Dunn, 2003).
Dunn (2002, 2003), in conjunction with her biostratigraphic research on macrofloral changes through early Paleocene time in the Hanna Basin, also collected samples for palynostratigraphic analyses (Fig. 16A). She sampled 21 productive sites, three of which (Lillegraven and Eberle, 1999, fig. 3, lower inset) are from strata deposited during latest Cretaceous (Lancian) time. Aside from the sampling by Nichols (in Eberle and Lillegraven, 1998a) from strata straddling the Cretaceous–Tertiary boundary in the Ferris Formation, her study represents the first attempt in the Hanna Basin at linking occurrences of palynomorphic species to strata zoned on the basis of fossil mammals. Dunn (2003, table 3.1) interpreted the samples following biostratigraphic conventions established by Nichols and Ott (1978) and Nichols (2003). Her identifications of the pollen grains were verified by Dr. Douglas J. Nichols (U.S. Geological Survey, Denver).
Dunn's (2003) palynomorphic samples from Paleocene strata included the following:
four samples from measured section in type Ferris Formation (Fig. 5, labelled area “1”; “Pu2–Pu3” localities in Fig. 16A) as established by Eberle and Lillegraven (1998a, 1998b) and Lillegraven and Eberle (1999) — representing late Puercan NALMA (interval zones Pu2 and Pu3) and palynomorphic Zone P2 (of Nichols and Ott, 1978);
one sample from peninsula south of confluence of North Platte and Medicine Bow Rivers (north of area shown in Fig. 5; “Shady Oasis” locality in Fig. 16A), not associated with a measured section — probably representing Puercan NALMA (but relationships to type Ferris Formation uncertain) and palynomorphic Zone P2;
two samples from short measured section established by Dunn in parts of type Ferris Formation at Pats Bottom, immediately east of North Platte River (Fig. 5, labelled area “2”; “Seminoe Beach” and “Este Lado” localities in Fig. 16A) — probably representing Torrejonian NALMA (although diagnostic mammalian fossils not yet found) and palynomorphic Zones P2–3, respectively;
one sample from vicinity of Pats Bottom in upper parts of Ferris Formation (Fig. 5, labeled area “2,” but detailed placement uncertain; “KJ0134b” in Fig. 16A) — probably representing Torrejonian NALMA (although diagnostic mammalian fossils not yet found) and palynomorphic Zone P3; and
10 samples from strata equivalent to parts of legs 1, 4, 11, 12, and 15 (essentially up to just below level of 79 coal) of our measured section (Figs. 4B and 11; all localities in Hanna Formation in Fig. 16A) — representing Torrejonian and early Tiffanian NALMAs (the latter including interval zones Ti1–Ti3, as discussed earlier) and identified, in increasing stratigraphic position, as palynomorphic Zones P5, P3, and P4.
Dunn did not sample the upper half of the Hanna Formation in the vicinity of The Breaks for palynomorphs.
Comparisons of stratigraphic occurrences of the species of palynomorphs reported by Dunn (2003; Fig. 16A) in the Hanna Basin with other assemblages recognized across the North American western interior (Nichols, 2003) suggest consistency with expectations in most respects. Nevertheless, the co-occurrences of four species of Caryapollenites at two localities in lower parts of the Hanna Formation in The Breaks (EBP04 and EBP03, shaded in Fig. 16A) are surprising. The following list of unexpected species presents (in parentheses) the total ranges of their recognized occurrences within other basins (using the abbreviated designation of zones previously favored by Nichols and Ott, 1978; also see alternative scheme of zonation provided by Pocknall and Nichols, 1996). The species are: C. wodehousei (P4–P6); C. veripites (P5–P6); C. inelegans (P5–P6); and C. imparalis (P4–P6).
Distribution elsewhere in the western interior of those species of Caryapollenites would suggest, as a simplest interpretation, that localities EBP04 and EBP03 of the lower Hanna Formation should be incorporated within the Caryapollenites veripites–Pistillipollenites mcgregorii Interval Biozone (of Nichols, 2003; equivalent to Zone P5 in terminology of Nichols and Ott, 1978). Indeed, the first occurrence of C. veripites is used by Nichols (2003) as the definition for advent of that biozone. The surprise comes upon examination of characteristic species of palynomorphs found stratigraphically above and below sites EBP04 and EBP04, all of which seem to warrant incorporation within Zone P3. Locality EBP04 is in strata equivalent to leg 1 of our measured section in the Hanna Formation (Figs. 4B and 11; Lillegraven and Snoke, 1996, fig. 10), and EBP03 is in leg 4 (Fig. 6A, arrow). As discussed earlier, we consider beds at those levels to have been deposited early in the depositional history of the more than two-mile-thick (Fig. 11) Hanna Formation. Figure 16B is a photograph of the approximate location of sample EBP04, in strata at the base of that formation.
The original construct of “P Zones” (Nichols and Ott, 1978) was based on the hypothesis that evolutionary lineages comprised by species of Momipites led to descendant species of Caryapollenites. As formalized by Nichols (2003, p. 126) “… the palynostratigraphic biozones of the Rocky Mountains and Great Plains are intended to be applicable throughout the region.” This implies that the biozones represent, with some anticipated variation from basin to basin, virtually synchronous intervals of geologic time. On that basis, palynomorphic biozones P1–6 have been used by various workers across the western interior as tools for temporal correlation of Paleocene nonmarine strata. The intentional linkage in the Hanna Basin of collections of palynomorphs and fossil mammals within a framework of detailed physical stratigraphy provides an opportunity to test the utility of established, pollen-based interval zones in temporal correlation.
If the system of palynomorph-based interval biozones suggested by Nichols (2003) truly reflects recognizable morphological steps within evolutionary lineages of closely related species, one would expect to find stratigraphic occurrences of pollen assemblages characteristic of Zone P5 only above those characteristic of Zone P4. Instead, within the Hanna Formation in The Breaks, Dunn (2003) recognized pollen-based assemblages characteristic of Zone P5 (Figs. 11 and 16A, shaded band) sandwiched between older and younger strata characteristic of Zone P3.
Specifically, Dunn reported pollen from four species of Caryapollenites in rocks stratigraphically lower than the Torrejonian–Tiffanian boundary; in their co-occurrence, these four species are best included within biozone P5 of Nichols and Ott (1978) and Nichols (2003). Dunn's identifications of the pollen grains were confirmed by Dr. Douglas J. Nichols (in Dunn, 2003, appendix E). As stated by Nichols (2003, p. 129): “Vertebrate biostratigraphic data from the Williston Basin (Hartman and Kihm, 1996) indicate that this biozone [i.e., P5] is equivalent to the Tiffanian NALMA.” Dunn (2003, p. 127 and figs. 6.5–6.6) noted a parallel situation as expressed by macrofloral components of leaf locality DMNH 2644 (“Emily's First”). That sample, although existing stratigraphically at the “overlap zone” between the Torrejonian and Tiffanian NALMAs of the Hanna Formation (Fig. 11), shows greatest floral similarities with much lower localities of the Ferris Formation. We suggest that paleoecological controls on the distributions of land plants during Paleocene time may have been important enough to confound their use in purposes of detailed biostratigraphy.
Alternatively, perhaps the source of stratigraphic inconsistencies of palynomorphic occurrences is not related to flaws in the evolutionary hypotheses underlying Nichols' (2003) system of palynostratigraphic zonation. Rather, because of undetected structural complexities in The Breaks, we may have fundamentally misinterpreted the superpositional relationships of local strata composing the Hanna Formation. If so, it is possible that what we, and all prior students of the area, have considered as the oldest parts of the Hanna Formation (i.e., legs 1–4 of the measured section, bearing fossil pollen attributable to Zone P5; Fig. 16B) belong instead to much higher parts of that formation, and therefore are as young as the late Tiffanian. That alternative requires postulating sequences of faulting that allow legs 1–4 of our measured section to fit into strata that originally were at (or more probably above) a level in the Hanna Formation comparable to the lower lacustrine unit (legs 15–17 of measured section; Figs. 9A and 11).
To explain the seemingly anomalous occurrences of P5 palynomorphic associations, Dunn (2003, p. 99) suggested that “… there is a previously unrecognized fault occurring in the section that places older strata on younger strata.” However, she neither mapped such a fault nor provided an explanatory model of how it might work, and we have not been able to fully grasp what was intended by those words. Stratigraphic relationships shown in Figure 16B indicate that the Hanna Formation (Paleocene rocks that bear pollen characteristic of Zone P5) was deposited directly on older strata (Cretaceous) of the Steele Shale. There exists no evidence in support of consequential faulting between the local top of the Steele Shale and the palynomorph-bearing locality near the base of the Hanna Formation.
As considered in NACSN (2001, p. 372), “Bio-stratigraphic units are distinct from all other kinds of stratigraphic units because organic evolution has produced a unique and nonrepeated sequence of fossils in the stratigraphic record” (emphasis ours). The unexpected appearance of pollen grains characteristic of Zone P5 amidst older and younger strata bearing assemblages characteristic of Zone P3 therefore leads to concern about the reliability of the prevailing scheme of palynomorphic zonation used for temporal correlation of Paleocene strata among basins of the North American western interior. Dunn's postulate of an unrecognized fault notwithstanding, we believe that the local base of the Hanna Formation (i.e., leg 1 of the measured section; Figs. 4, 11, and 16B) has been successfully identified. Furthermore, we accept the premise that lower parts of the Hanna Formation (e.g., legs 1–4 of the measured section) are older than its stratigraphically higher parts (e.g., legs 5–20). Local geological and paleontological relationships therefore raise the specter that the basic evolutionary assumptions underlying definitions of the scheme of Paleocene palynomorphic zonation established by Nichols and Ott (1978) and formalized by Nichols (2003) are fundamentally flawed. McKenna and Lillegraven (in review) pursue related issues in more detail.
We wish to provide a means by which the basis of our viewpoint can be independently tested. To that end, following a general discussion of structural aspects of The Breaks, we present a pair of stepwise, graphical models that reasonably reflect the Laramide structural history of the Hanna Basin's northeastern corner. All aspects of the models are based upon our own field observations and measurements. We provide the graphical models to aid other workers in: (1) understanding our vision of the complex depositional and structural relationships observable in The Breaks; (2) testing the concept that the relative depositional sequence in the local Hanna Formation is reflected appropriately in our system of enumeration for the legs of the composite measured section (i.e., legs 1 through 20 sequentially represent oldest to youngest components); and (3) evaluating efficacy of the most widely used scheme of palynomorphic zonation as a tool to temporal correlation of Paleocene non-marine sequences throughout the North American western interior.
STRUCTURAL FEATURES WITHIN THE BREAKS
Introduction to Breaks Fault System
Lillegraven and Snoke (1996) provided a synopsis of the major structural features exhibited by the Hanna Formation in the northeastern corner of the Hanna Basin. They coined the term “Breaks fault system” for the multiplicity of faults that cut the Hanna Formation and underlying Upper Cretaceous strata in the vicinity of The Breaks. As suggested above, this system of out-of-the-basin faults is part of a triangle-zone fault geometry (Fig. 10) related to a south-directed, low-angle, basement-involved fault system that forms the southeastern margin of the Sweetwater arch. We have no real data on the dip of the basement-controlled master fault, but we visualize something on the order of 15°–20°, dipping north-northeast. For discussion purposes, we divide the Breaks fault system into two phases of development, here referred to as the “older” and “younger” components. Although real temporal differences appear to exist between these phases of deformation, they probably overlapped somewhat in their times of development.
Older Component of Breaks Fault System
The generally earlier phase of deformation of the Breaks fault system is related to the basement-involved, Laramide uplift of the Sweetwater arch (specifically, the Freezeout Hills and Shirley Mountains as its southeastern components; Figs. 3, 4B, and 5). Although this process had its origin in Late Cretaceous time, the greatest uplift occurred during the latest Paleocene and/or early Eocene. The structure contour map focusing on the top of Precambrian rocks for Wyoming (Blackstone, 1993b) and the seismic-reflection profile from Kaplan and Skeen (1985) highlight the magnitude of differential uplift between the Sweetwater arch and Hanna Basin. This structural relief is clearly greater than 40,000 feet (>12 km), and unpublished seismic-reflection data suggest even a greater amount (Richard C. Hook, personal communication to Snoke, 2003).
Significant low-angle thrust displacement of Archean basement rocks related to the south-southwest-directed blind thrust-fault system crowded Paleocene and older strata of the northern Hanna Basin. The thick sedimentary sequence along the northern flank of the basin responded as a passive element not only by steep, southward tilting or overturning, but also by the development of out-of-the-basin thrusts that verged northeast, toward the rising mountains (i.e., Freezeout Hills). The “Dragonfly fault” (new name), “Owl Ridge fault” (of Lillegraven and Snoke, 1996, fig. 14), and a series of similar but unnamed faults shown on Figure 4B are examples of this style of structural response. All of these faults in The Breaks have map traces that trend northwest–southeast, and we interpret them as thin-skinned décollements, probably contained entirely within Paleocene and uppermost Cretaceous strata. Photographs of the surface exposure of the Dragonfly fault were provided by Lillegraven and Snoke (1996, fig. 21 and cover photograph for that publication). More distant views are provided here in Figures 6A–B and E, 7A–B, and 8A.
Footwall synclines commonly accompany this series of faults, usually occurring only short distances to the north or northeast of the fault traces (Figs. 4B–C and 6B). These minor synclines vary greatly in tightness of folding and resulting width of expression in outcrop. In the southeastern corner of section 4, T. 23 N., R. 80 W., for example, the footwall syncline is extremely tight, and different parts of the original upper surface of a single stratum can be seen in physical apposition. In contrast, other synclines are expressed as a broad fold, as seen just northwest of the center of section 10 in that same township (structure labelled “The Great Tortilla” on Fig. 4B; Fig. 7D).
No evidence of a footwall syncline accompanies the Owl Ridge fault. One is well developed, however, along most of the length of the Dragonfly fault. The bends and kinks in the syncline closely parallel those of the main fault trace (Fig. 4B), suggesting that both were deformed by a process that mostly post-dated the major displacement along the Dragonfly fault.
The practice of mapping traces of individual faults within this mudstonerich stratigraphic regime presents special problems. The faults usually do not occur as sharply defined breaks within the carbonaceous shale that dominates the section. Sandstone bodies are volumetri-cally minor, and they tend to be discontinuous laterally. Fault traces, therefore, generally bypass comparatively well-indurated layers of sandstone that otherwise would exhibit unequivocal evidence of disruption. Evidence of small-scale, inter- and intrastratal slippage within strata of The Breaks is common (Fig. 7C). Because of this combination of lithologic characteristics, local faulting tends to be dispersed through a broad zone rather than concentrated along a narrow trace. As a result, most of the lines representing fault traces in Figure 4B should be viewed as average “best fits” for positions of the faults. That point will be reinforced below, in relation to the two areas of light-blue patterning shown in Figure 17D. Those areas include variable-width bands of strata that parallel what we consider to be the “main traces” of the faults. The two areas also exhibit unusually common examples of locally chaotic deformation (Figs. 6E and 7B). The outer boundaries of those semi-chaotic areas are quite arbitrary, and they would surely be drawn differently in detail by anyone else attempting to map them.
Younger Component of Breaks Fault System
The younger episode of shortening related to the Breaks fault system was the basement-involved, structural development of the Flat Top anticline immediately east of the presently recognized Hanna Basin (Fig. 3). As shown in the southeastern part of Figure 4A, the strike of Upper Cretaceous strata (Haystack Mountains through Medicine Bow Formations) bordering the northeastern badlands in The Breaks bends markedly southward. From that bend southward, all exposed Cretaceous strata comprise part of the west-plunging nose of Flat Top anticline (Blevens, 1984). As indicated in Figure 4A by detachment of the “marker ss. in Steele Sh.” from the stratigraphically overlying formations of the Mesaverde Group in the northwestern quarter of T. 23 N., R. 79 W., late Laramide uplift of Flat Top anticline involved at least two miles (3.2 km) of westward translation relative to the eastern Hanna Basin. That deformation led to a second, generally younger, set of faults also included in the Breaks fault system.
Faults related to this younger deformational episode seem to be bedding-plane décollements. However, because in this case they follow bedding-planes within the steeply south-dipping and more competent Cretaceous strata, the faults appear in outcrop (e.g., Lillegraven and Snoke, 1996, fig. 15; Fig. 6C) as sharply defined, high-angle planes that generally strike eastwest. In map view (Fig. 4B), the faults show dextral separation implying some strike-slip component. In reality, they probably are oblique-slip faults that had significant out-of-the-basin vergence to the northwest.
Members of this second set of fault traces merge westward with the Dragonfly fault at multiple points (Fig. 4B). Those mergings resulted in complex, secondary translations and kinking of what originally was a straighter course for the Dragonfly fault and its related footwall syncline. Such relationships are the primary evidence for the concept that uplift of Flat Top anticline postdated much of the uplift of the Freezeout Hills (Fig. 3). The mergings of individual elements of these two strain systems also resulted in locally chaotic distortions of bedding along the trace of the Dragonfly fault, as discussed below in relation to the areas shown in light blue in Figure 17D. Footwall synclines in this set of faults are not well developed or perhaps did not develop at all, probably because of the generally much higher degree of induration of the Upper Cretaceous strata among which the main planes of these faults pass.
Additional mapping (Lillegraven, unpublished data) of the Hanna Formation along the extreme eastern margin of the present Hanna Basin shows important, out-of-the-basin thrusting of the thick column of Paleocene strata eastward onto the lower parts of the Ferris and upper parts of the Medicine Bow Formations. Again, such relationships almost certainly reflect passive responses by the Hanna Formation to late (i.e., after deposition of the Hanna Formation), basement-involved uplift of Flat Top anticline.
Also intimately involved in this story is the evolution of Simpson Ridge anticline. It, too, has been shown by Kraatz (2002) to represent uplift that post-dates deposition of the Ferris and Hanna Formations (contra Ryan, 1977). The prominent synclinal structure labelled on Figures 3, 4B–C, 5, 17D, and 18C as “defining syncline of northeastern Hanna Basin” reflects, in part, uplift of Simpson Ridge anticline. Thus the complex structural history of the Hanna Formation in vicinity of The Breaks has resulted from a three-way impingement upon those beds from basement-involved, Laramide uplift of the Freezeout Mountain, Flat Top, and Simpson Ridge anticlines. Contraction of the original landscape in association with each of the three was on the scale of kilometers.
Generalizations About Stratigraphic Separations Along Breaks Fault System
We present here a few additional general observations about fault relationships within the Breaks fault system (Fig. 4B). In all consequential faults of the system the footwalls are to the north or northeast, and the hanging walls are to the south or southwest. In the Owl Ridge and Dragonfly faults, the hanging walls involve essentially all the remainder of the Hanna Basin. All major traces within the Breaks fault system, therefore, reflect out-of-the-basin displacement involving northward translation. Fault traces observable in the field developed for the most part at depths of several kilometers; their existing surficial expressions were caused by prodigious, post-tectonic erosion.
Detailed field mapping in The Breaks (Fig. 4B) shows that recognizable structural separation related to the Owl Ridge fault begins just east of the center of section 5, T. 23 N., R. 80 W. and becomes progressively more important as its trace is followed to the northwest. On the hanging wall, eastward deflection of beds of the lower lacustrine unit (“Owl Ridge”; Fig. 8A) suggest rotation of the hanging wall with the pivot-point close to the point of origin of the fault. Presumably related to that rotation, the hanging wall of the Owl Ridge fault overrode the hanging wall of the Dragonfly fault at the north end of The Breaks (just northeast of the Medicine Bow River). It may well be that the planes of those two faults merge at about that point.
Whatever the details might be at depth, map relationships (Fig. 4B) clearly show that throw along the hanging wall of the Owl Ridge fault puts younger strata onto progressively older strata as that fault is traced to the northwest. Specifically, the stratigraphic equivalents of legs 12–14 (of the measured section seen to the southeast) were thrust upon strata equivalent to legs 11 and 12. The stratigraphic separation toward the most northwesterly part of the map area in Figure 4B becomes even greater, with placement of the hanging wall of the Owl Ridge fault directly upon greatly deformed beds within upper parts of the Steele Shale. Many other examples of this kind of younger-on-older, out-of-the-basin faulting relationship are recognizable along the northern (Lillegraven and Snoke, 1996) and eastern flanks of the Hanna Basin (Fig. 3; Lillegraven, unpublished data), as well as on western flanks of the Carbon Basin (Kraatz, 2002).
In a similar fashion, the main trace of the Dragonfly fault, as it is traced progressively northwestward through sections 10, 9, and 4 of T. 23 N., R. 80 W., cuts progressively down-section through the Hanna Formation. Hanging-wall strata generally equivalent to leg 10 (of the measured section) at the southeastern part of the trace are positioned upon strata in the footwall equivalent to leg 9. When the fault is traced to the northwest, however, strata attributable to leg 10 on the hanging wall become positioned onto sequentially older strata (Fig. 4B), continuing down-section to rocks stratigraphically equivalent to leg 2 of the measured section. The younger-on-older, out-of-the-basin relationships exhibited by the Dragonfly fault become even more obvious still further to the northwest. In section 32 of T. 24 N., R. 80 W., for example, stratigraphic equivalents of leg 10 within the Hanna Formation were thrust directly onto Cretaceous strata of the Steele Shale. Note also in Figure 4B that the thin, black dashed lines used to trace lateral equivalents of the leg boundaries of the measured section generally converge irregularly to the north-northwest. Probably this convergence reflects progressively more extensive, east-verging interstratal sliding along bedding planes that are not recognizable in outcrops as discrete faults.
Field relationships along the present northeastern margin of the Hanna Basin (south of the southeastern corner of the map in Fig. 4B) strongly suggest that much of the length of the Dragonfly fault within The Breaks is intimately associated with heavily carbonaceous to lignitic strata. This lithologic-structural relationship is not obvious from outcrop data presented in Figure 13B or C. Nevertheless, significant coals (designated as lenses 1191–1204 by Dobbin et al., 1929, pl. 27) at about that stratigraphic level crop out in sections 10 and 14–15 of T. 23 N., R. 80 W., just southeast of The Breaks. Those coaly strata are highly deformed and laterally discontinuous because of extensive faulting. These coal beds at the northeastern margin of the Hanna Basin may in part be equivalent to the 68–69 coals of lower parts of the Hanna Formation in more central areas of the basin (sees. 15, 21–22, T. 23 N., R. 83 W.). Whatever the basinwide correlations might be, these unusually carbonaceous units in lower levels of the Hanna Formation in The Breaks represent some of the weakest rocks within a thick, generally incompetent section, and probably they served as surfaces for initial fractures when components of the Dragonfly fault were initiated. As pointed out by a reviewer of this paper, fluid over-pressurization at depth due to volume loss during the process of coalification may well have been a factor in early establishment of the faulting.
Possible Precambrian Basement Control on Orientation of Superposed Laramide Structural Features
Structural features surrounding the greater Hanna Basin that were formed during the Laramide orogeny were superposed upon a complex, Precambrian basement template. The template included shear zones, dike swarms, intrusive boundaries, fabric variations, and other fundamental anisotropies that appear to have had profound effects on the orientation of various Laramide structural features (Bekkar, 1973). Brown (1993, p. 320–321) summarized basic observations on relationships between Laramide, basement-rooted faults and structural features in Precambrian basement-rock complexes. In Brown's figure 2, he noted the subparallel alignment between northeast-striking, subvertical faults in western parts of the Shirley Mountains and Precambrian mafic dikes intruded into Archean granitic rocks. This is one local example of the influence of Precambrian geology on the orientation of Laramide features. Additional examples (with generalized geographic positions for each indicated on Fig. 15, p. 28) near the area of the present study include the:
trend in common between the Como Bluff anticline (Fig. 3) and the Paleoproterozoic Laramie Peak shear system of the central Laramie Mountains (Fig. 15, item 1; Chamberlain et al., 1993; P. G. Resor and A. W. Snoke, unpublished data);
western termination of the “area of northeast-trending folds” (Fig. 1) by an inferred, northwest-striking Precambrian fault/shear zone (Fig. 15, item 2; Bekkar, 1973; Love and Christiansen, 1985; Rocky Mountain Map Company, 1992b);
localization of the high-angle (∼75°) Shirley fault and blind Shirley Mountains–Freezeout Hills thrust-fault system (Fig. 15, item 4; Lillegraven and Snoke, 1996, figs. 4 and 22–25) along the southeastern margin of the Sweetwater arch (i.e., along an inferred, west-northwest-striking Precambrian fault/shear zone);
opposition in sense of displacement between the Elk Mountain uplift (east-directed) and Simpson Ridge anticline (west-directed) across an inferred, approximately east-west-striking Precambrian fault/shear zone (Fig. 15, item 5; Kraatz, 2002);
abrupt change in strike of the west-directed, thrust-fault system associated with the Rawlins uplift from north-northwest-striking to approximately east–west-striking (Fig. 15, item 6a; Barlow, 1953; Rocky Mountain Map Company, 1992a; Otteman, 2003) and the localization of the Grenville dome (Fig. 15, item 6b) along the latter trend (which is the same east–west-striking, Precambrian fault/shear zone inferred in item 5; also see inferred left-lateral strike-slip fault system connecting these areas in Fig. 19); and
northerly change in strike of the Arlington thrust/reverse fault near the northeastern end of the Medicine Bow Mountains (Fig. 15, item 7) from approximately north–south to northwest–southeast (i.e., subparallel to the Rock Creek ductile deformation zone of Nyberg, 2001).
STRUCTURAL MODELS FOR LARAMIDE EVOLUTION OF THE BREAKS
The late Laramide deformation in vicinity of The Breaks is highly complex. To help in visualization of development of the present landscape, we provide, and attempt to link, two sets of stepwise, semi-diagrammatic restorations. This is done byway of a four-step, map-based model (Fig. 17A–D) in combination with a three-step, cross section-based model (Fig. 18 A–C).
The geographic area covered in Figure 17 approximates that of the geologic map (Fig. 4B). The cross-sectional model (Fig. 18C) has, as its final step, the geologic cross section A–A′ shown in Figure 4C (see line of section in Figs. 4B and 17D).
Figure 17A is a simplified version of the modern strati–graphic relationships shown in Figure 4B, the geologic map. For purposes of orientation during discussion, the informal name “Tepee-Ring Ridge” (shown on Fig. 17A; name coined by Lillegraven and Snoke, 1996, p. 2) is applied to the topographic upland in the southwestern quarter of section 3, T. 23 N., R. 80 W. (Figs. 6A–B and 7E). The badlands to the west and south of the ridge, dominated by Paleocene parts of the Hanna Formation, constitute “The Breaks.” The general trend in dips of the Hanna Formation across the map area (of Figs. 4B and 17A) is steeply to the southwest, then reversing (with much shallower attitudes) at the synclinal axis (“defining syncline of northeastern Hanna Basin” in Figs. 3, 5, and 17D) in the western part of the area. Orientations of successive legs in the measured section of the Hanna Formation shown in Figures 4B and 17A reflect the general southwesterly dip of strata within The Breaks.
In Figure 17A, we limit data for rock attitudes to a series of closely spaced couplets between the Hanna Formation and the various underlying, Cretaceous rock units. Without exception, the couplets close to legs 1–5 of the measured section represent deposition of the Hanna Formation upon the angular unconformity described earlier in this paper. The difference in dips within those couplets varies from about 9° to 67°, averaging about 38°. For the moment ignoring local details of postdepositional faulting, refer back to Figure 14C. Note that the depositional contacts (from northwest to southeast) between successively higher parts of the Hanna Formation and underlying rocks climb, in correct stratigraphic sequence, onto progressively younger Cretaceous units. Within the map area (Fig. 4B), the stratigraphically lowest depositional contact of the Hanna Formation is on upper levels of the Steele Shale (at the base of leg 1; Fig. 14C) and the youngest depositional contact is on lower parts of the Lewis Shale (leg 5; Fig. 7E). Although contacts of the Hanna Formation close to legs 6–9 in the map area are with the still younger, uppermost Cretaceous Medicine Bow and Ferris Formations, those relationships are tectonic in nature (i.e., strata of the Hanna Formation were thrust upon the Cretaceous rocks via deformation along the Breaks fault system).
The fundamental point of Figure 17A is to show that lower parts of the Hanna Formation were deposited during Paleocene time on an erosional surface cut onto the southeastern flank of Freezeout Mountain anticline (Figs. 4A and 14B–C). That anticline defines the southeastern margin of the Freezeout Hills, which in turn comprises the southeasterly plunging nose of the much larger Sweetwater arch (Fig. 1). Thus it is clear that the Freezeout Hills did exist as a topographic feature during the time of deposition of the Hanna Formation. Elements of sedimentary cover on Freezeout Mountain anticline lost to erosion (as seen in the center of sec. 4, T. 23 N., R. 80 W.) prior to deposition of the Hanna Formation included: (1) top parts of the Steele Shale; (2) all four formations of the Mesaverde Group; (3) Lewis Shale; (4) Medicine Bow Formation; and (5) Ferris Formation (Fig. 14B–C). Added together, at least 20,000 ft (c. 6 km) of strata were removed erosionally from that part of the southeastern flank of Freezeout Mountain anticline before the local base of the Hanna Formation was deposited. In contrast to the situation seen along the southern flanks of the Carbon Basin, there is no evidence from the northern or southern (Chadeayne, 1966; Hitchens, 1999) margins of the Hanna Basin that erosion prior to deposition of the Hanna Formation cut anywhere to depths below upper levels of the Steele Shale. South of the Shirley Mountains (Lillegraven, unpublished data) and toward the center of the Hanna Basin, deposition of the Hanna upon the Ferris Formation probably was wholly conformable.
Despite the great magnitude of pre–Hanna erosion observed in northern parts of The Breaks, comparisons of dips between the Hanna Formation and underlying rock units in that vicinity suggest that the Freezeout Hills of early Paleocene time were much more subdued in topographic expression than is the case today (Fig. 14B). Generally minor amounts of conglomeratic strata within the lower half of the Hanna Formation observed in The Breaks suggest their derivation from eroding Mesozoic and Paleozoic rock units (most obviously, from the comparatively indurated Mowry Shale, Cloverly Formation, and Tensleep Formation) exposed along the southern flank of the Freezeout Hills. Fossilized teeth of marine sharks and rays characteristic of the Frontier Formation and other, younger Cretaceous units also are found in localized abundance as reworked clasts (in association with terrestrial Paleocene mammalian fossils) in the lower half of the Hanna Formation of The Breaks (Burris, 2001). These highly resistant elasmobranch fossils most probably were derived from erosion of thick units of generally soft, Cretaceous marine strata on Paleocene southern slopes of the Freezeout Hills. Erosion of the soft marine shales also probably contributed greatly to the mudstonerich Hanna Formation, although the sources of the clay are not so easily identified as the coarser clastic elements.
Figure 17B presents a palinspastic reconstruction of the southeastern flank of Freezeout Mountain anticline and general vicinity of The Breaks as it would have appeared in early Paleocene time. The cross section shown in Figure 18A corresponds to the map view in Figure 17B. Both of these interpretations restore the paleo-landscape to conditions prior to deformation related to the Breaks fault system (Fig. 14C). Note that the southwest-dipping attitudes of the Hanna Formation shown in Figure 17A have been rotated in Figure 17B back to those expected during deposition (i.e., essentially to horizontal). The palinspastically rotated dips indicated for the underlying Cretaceous units, therefore, are at much-reduced angles from those seen today (Fig. 6A), again reflecting the more subdued topography that characterized Freezeout Mountain anticline early in Paleocene time.
Because of the more flat-flying nature of strata in the Hanna Formation during most of Paleocene time, estimated relative geographic positions of the numbered legs of the measured section are indicated by blue spots (rather than as linear transects) on Figures 17B–C. The series of contacts between the Hanna Formation and underlying Cretaceous strata shown in Figures 17A–B is time-transgressive upward through the section as the contacts are traced southeastwardly on the maps (Fig. 14C). In all probability, strata of the Hanna Formation during Paleocene time extended far to the north of the distribution shown on 17B, being deposited progressively higher onto flanks of the Freezeout Hills. Those relationships of progressive onlap through the Paleocene are implied at the northeastern end of the cross section in Figure 18A. Indeed, as mentioned earlier, the Freezeout Hills were onlapped and probably became completely covered by the Hanna Formation (Fig. 15) by the time its upper parts were deposited in the Hanna Basin.
The cross section shown in Figure 18B represents an intermediate stage of tectonism, younger than that illustrated in Figure 17B. Figure 18B illustrates an early step in the process of passive, northeasterly displacement of the hanging wall of the Dragonfly fault (with displacement indicated in map view by the four heavy black arrows in Fig. 17B) toward its final position.
At several places in vicinity of today's Tepee-Ring Ridge one can see relationships in which outcrops of the Hanna Formation are overlain by slivers of Cretaceous strata that appear to have been translated westward from their original positions (Figs. 4 and 17A and D; photograph in Lillegraven and Snoke, 1996, fig. 15; Fig. 6A and C). These tectonic overlaps are explainable by out-of-the-basin displacements on splays of thrusts that had strongly oblique components of throw to the northwest. The expected appearance in map view of such oblique displacement (following modern erosional beveling) is that of right-lateral separation.
Figure 17B illustrates palinspastic removal of those faulting relationships (Fig. 14C) along ancient Tepee-Ring Ridge. Five short, yellow bars are used to approximate the original (i.e., pre-faulting) spatial relationships among the various Cretaceous units and the depositionally overlying Hanna Formation. Each of the yellow bars is placed on the map where they would be cut later by faulting (as recognizable in the field today). In all probability, the total amount of separation on each of the individual faults was significantly more than is represented on the restored map (Fig. 17B); this conservative restoration is based upon minimal separations that can be directly observed on the modern outcrop (see Figs. 4B and 17D). This oblique-slip faulting, as recognized at the northeastern corner of the Hanna Basin, probably was caused by components of westward vergence associated with structural evolution of Flat Top anticline, just east of The Breaks (Figs. 3 and 4A).
Both the Owl Ridge fault and Dragonfly fault increase their amounts of structural separation as they are followed to the northwest. Note on Figure 4B that the map traces of those two faults progressively converge to the northwest (also see Fig. 8A). That is, strata on the hanging wall of the Owl Ridge fault were thrust onto the hanging wall and (still further north) upon the very trace of the Dragonfly fault (Fig. 4, SW corner of sec. 29, T. 24 N., R. 80 W.). Late components of displacement on these two faults probably occurred in part synchronously with the above-mentioned, oblique-slip faults. Additionally, the mapped trace of the Dragonfly fault itself, along with much of the localized mudstonerich, generally incompetent strata on either side of its course (Fig. 4B and the two light-blue areas in Fig. 17D; Figs. 6E and 7B–C), were secondarily deformed. The added deformation came from younger faults that variously intersected and probably merged with the originally straighter track of the Dragonfly fault.
Figure 17C has two key purposes. The first is to show virtual completion of displacement along the Owl Ridge and Dragonfly faults. That out-of-the-basin displacement probably reflected continuing southward vergence of the Sweetwater arch via blind, reverse faults at depth (Kaplan and Skeen, 1985, fig. 3; Lillegraven and Snoke, 1996), thus crowding strata along the northern margin of the Hanna Basin. For purposes of graphical simplification, those two faults are shown in Figure 17C as having completed their displacements prior to initiation of the oblique-slip faulting that strongly deformed strata in the vicinity of Tepee-Ring Ridge (note the continued, unbroken representation of the five short, yellow bars that cross the Hanna Formation–Cretaceous contacts). As mentioned above, however, some synchrony probably did exist between the displacements of the older and younger basic components of the Breaks fault system.
The second purpose of Figure 17C is to introduce another key point relative to completion of Laramide deformation in the northeastern corner of the Hanna Basin. Specifically, this involves a point of future rotation (estimated at about 33°) about a vertical axis as shown on Figure 17C in the southeastern part of the map (in sec. 11, T. 23 N., R. 80 W.). Much of the light-blue area in adjacent section 10 of Figure 17D (which represents today's landscape) includes the area of most extensive folding, faulting, and ductile deformation (Taft, 1997) seen in The Breaks (Fig. 7C). Structural relationships between the Hanna Formation and Lewis Shale in the eastern part of that heavily deformed area are uncertain because of poor exposures. Probably, however, part of the Hanna Formation over-rode the Lewis Formation to the east via another out-of-the-basin fault related to uplift of Flat Top anticline.
Figure 17D is the same map shown in Figure 17A, except that it superimposes a summary of the structural features presented in more detail in Figure 4B. Additionally, Figure 17D shows two generalized areas (in light blue) of unusually strong ductile deformation within the mudstone-rich Hanna Formation. Structural features exposed in the more southeasterly blue area were studied in detail by Taft (1997). Note that the course of the measured section through the Hanna Formation involving legs 1–9 was intentionally chosen in the field to: (1) insure correct superpositional sequence of the successively numbered legs; while (2) simultaneously minimizing traverse of the described section through markedly deformed strata.
Nevertheless, leg 2 and the respective boundaries between legs 4–5, 5–6, and 9–10 of the measured section unavoidably cross faults, for which the stratigraphic separations remain unknown quantitatively. Because of the younger-on-older style of faulting characteristic of this area, however, it is most improbable that stratigraphic section has been repeated across any of these faults. Conversely, it is highly probable that some parts of the stratigraphic section are missing across each fault. The total original thickness of the Hanna Formation in the northeastern corner (and structural axis) of the Hanna Basin, therefore, probably was somewhat greater than the thickness reported in our composite measured section (Fig. 11) and Appendix 1. Indeed, combining Paleocene parts of the Ferris Formation with the Hanna Formation, the Hanna Basin probably contained among the thickest sections of strata representing Paleocene time known within the Rocky Mountain region. Only the Goler Formation of California, in the southwestern corner of the Great Basin (Cox, 1987), would seem to be comparable across the North American continent within principally nonmarine settings.
The cross section presented in Figure 18C represents the situation today at section A–A′ as seen in map view in Figures 4B and 17D. The trio of solid dots placed on each of the successive steps illustrated by Figure 18A–C represents relative spatial positions of the same stratigraphic level as the system evolved. The spots are placed on the equivalent of leg 2 within the measured section. That stratigraphic level is seen today on the footwall syncline in section A–A′ (Fig. 4C) and just northeast of the trace of the Dragonfly fault in Figure 4B (i.e., west-central part of sec. 4, T. 23 N., R. 80W.).
Each cross section in Figure 18A–C is accompanied by a set of three grey straight lines projecting from the trios of closed, reference spots. Each convergence defines a geometric point of rotation that can be used to describe uplift of Freezeout Mountain anticline relative to the northern Hanna Basin during late phases of the Laramide orogeny. The geometric point of rotation that functions properly for this particular cross section (Fig. 18A–C) is about 21,280 ft (6.486 km) below the present surface of the landscape (measured vertically from the axis of the “defining syncline of northeastern Hanna Basin”). Laterally, the geometric point of rotation is approximately 13,735 ft (4.186 km) southwest of the axis of the syncline. Geographic placement of the rotational point was determined graphically by palinspastically rotating the base of the Hanna Formation back to near horizontality from the point at which it occurs today on the Steele Shale along cross section A–A′ (Figs. 4C, 14C, and 16B).
Measurements taken from the cross section presented as Figure 18C show that (using the geometric point of rotation described above) the reference point (indicated by closed circles on Fig. 18A–C) had the following components of displacement from depositional (Fig. 18A) to present (Fig. 18C) conditions: (1) horizontal, 6,040 ft (1.841 km) to southwest; (2) vertical, 9,055 ft (2.760 km) toward zenith; and (3) resultant vector, 10,880 ft (3.316 km) translating upward and toward the southwest. All of these values are compatible with magnitudes of displacement as interpreted by Kaplan and Skeen (1985) from the north–south seismic line traversing the northern Hanna Basin some 17 miles (27.4 km) to the northwest of the present cross section.
Additionally, a pair of solid stars is placed on each cross section in Figure 18 immediately above and below the modelled traces of the Dragonfly fault. The relative positions of the paired stars reflect the amount of bedding-plane translation of the hanging wall relative to the footwall (we assumed the latter to have remained fixed in position relative to the synclinal axis). The mostly horizontal component of displacement of the reference stars along the plane of the fault from initial to present conditions was assumed to equal the purely horizontal component of displacement of the closed circles. That latter distance was used to construct the eventual extent of northeasterly closure of the fault's hanging wall as represented in map view on Figure 17B (at the coordinates of cross section A–A′, plotted in Fig. 17D).
One particularly interesting concept arising from this discussion of out-of-the-basin thrusts is that younger strata are observed to have been faulted onto older strata. The normal situation within thrust belts (Royse, 1993) is that older rocks are thrust upon younger. But as considered briefly by Lillegraven and Snoke (1993, p. 37–42), examples of younger-on-older faulting abound along the northern flank of the Hanna Basin. They stated and queried (p. 40): “Additionally, and somewhat puzzlingly, strata showing younger-on-older fault relationships commonly exhibit closely similar attitudes above and below the fault-planes. Is there a plausible explanation for such relationships that does not involve stratigraphic down-ramping during the process of faulting?” The developmental cross sections presented in Figure 18 approach that concept.
Part B of Figure 18 attempts to model early phases of passive translation of the Dragonfly fault's hanging wall. Presumably, the fault formed, and continued its development, as the result of progressive crowding along the northern basinal margin due to late Laramide uplift of the Freezeout Mountain anticline (Fig. 3). Note that near the midlength of Figure 18B, the plane of the newly formed fault projected to the northeast, across bedding planes; its original nature was that of a décollement, forming parallel to bedding planes. Both in parts B and C of Figure 18, the northeasterly parts of the fault are modelled as an upramp (relative to Earth's surface) thrust fault that cut through roughly 2,500 ft (762 m) of stratigraphic section. As seen northeast of the synclinal axis, the fault's plane became progressively steeper (relative to Earth's surface) through time between cross sections B and C.
Notice, however, that the Dragonfly fault as modelled in Figure 18A–C exhibits a stratigraphic downramp within the Hanna Formation. That accounts in a simple fashion for the younger-on-older stratigraphic relationships that are seen within The Breaks. Specific to cross section A–A′ of Figures 4B–C and 17D, the stratigraphic downramp explains the superposition (observed at the fault's trace) of beds comparable to leg 10 (of the measured section) upon those comparable to leg 2. As implied above, we estimate that at least 2,500 ft (762) of section are missing across the Dragonfly fault where it is exposed along cross section A–A′. Other examples of similar younger-on-older relationships are common within The Breaks and at many other locations along the margin of the Hanna Basin. Such occurrences are exhibited at greatly varying scales. Probably, although not commonly recognized, they also occur in many other tectonically crowded, basin-margin settings.
We make no claim that the interpretive models presented in Figures 17 and 18 are wholly accurate – or even that they fully reflect the degree of structural intensity impacting the northeastern corner of the Hanna Basin during culmination of the Laramide orogeny. However, all aspects of both models were constructed on the basis of measurements gained through detailed, verifiable field observations (Fig. 4B). For that reason, we suggest that our stepwise models will provide valuable conceptual foundations for their own testing as further observations are made and as additional research questions arise within the area.
The present study has focused on detailed investigation of a minuscule part of the northeastern corner of what is today a relatively tiny Laramide basin. That was done, however, because evidence uniquely available from the vicinity of The Breaks provides opportunity for paleo-geographic insight that is applicable well beyond limits of the Hanna Basin. For example, the information presented here raises a fundamental question: Where were the original northern and eastern limits of the Hanna Basin? To approach that question, we consider three interrelated issues. First, what is the nature of the contact between the Hanna and underlying Ferris Formations as they occur along what is presently viewed as the eastern margin of the Hanna Basin? Second, to what extent does the Hanna Formation become thinner in the Hanna Basin as it is traced toward its present, eastern erosional limit? And finally, when did the great anticlines constituting the “area of northeast-trending folds” (Figs. 1 and 3) come into existence?
Hanna–Ferris Formational Contacts Along Eastern Margin of Basin
Full documentation of the Hanna Basin's present eastern margin awaits completion of detailed mapping (in progress by Lillegraven). Nevertheless, enough is known to state categorically that the Hanna Formation, from just east of The Breaks southward onto Simpson Ridge, is nowhere seen in depositional contact with the underlying Ferris Formation. The entire modern eastern border of the Hanna Basin (Fig. 3) is a complex zone of anastomosing, east-verging, out-of-the-basin thrusts that developed within weak, coaly, lower parts of the Hanna Formation. Specifically, strata representing eastern equivalents of legs 1–10 of our measured section in The Breaks constitute the broad zone through which these irregularly merging thrust planes developed. The map trace shown in simplified form as a single fault in the southwestern part of Figure 3 is, in essence, a southeasterly continuation of the Breaks fault system.
Thus, as also is true for the older component of the Breaks fault system where originally described, faulting relationships at the eastern border of the modern Hanna Basin have emplaced younger strata (Hanna Formation, equivalent of legs 10 and higher) onto older strata (Ferris and Medicine Bow Formations; Fig. 4A–B).
Thickness Trends of Hanna Formation Toward Eastern Margin of Basin
Mapping of the Hanna Formation south of the area shown in Figure 4B indicates no significant thinning of its observable section (i.e., equivalents of legs 10 and higher) to the east. The conclusion seems unavoidable that the original eastern boundary of the Hanna Basin (as an eastern part of the greater Green River Basin) must have been far to the east of its present margin (as delimited by the simplified fault trace in Fig. 3). Reasonable estimates of the basin's original eastern limits are discussed in the following section.
Timing of Uplift of “Area of Northeast-Trending Folds”
Immediately east of the modern Hanna Basin is a broad, grassland-dominated landscape of generally low topography (Figs. 1 and 3). Most strata cropping out in this area are of Mesozoic age (Love and Christiansen, 1985), poorly indurated, and were deposited primarily under marine conditions. The presently low topographic relief is deceptive because it masks the occurrence of five large anticlinal folds having hinge lines that are aligned en echelon, each showing northeast–southwest orientation (Blackstone, 1993a, fig. 2). Each anticline is asymmetric, with steep to overturned strata on its northwestern flank, and commonly these structures are complicated by well-developed, northwesterly directed thrust faults (e.g., southeastern corner of Fig. 4A). Such orientations for major Laramide structural features are uncommon in the central Rocky Mountain region, where hinge lines of most folds are aligned north–south or northwest–southeast.
Each of the anticlines within the area of northeast-trending folds reflects faulting in the Precambrian basement. Furthermore, in at least one case (i.e., Como Bluff anticline of Dunbar, 1944), the controlling fault of Laramide age can be traced northeastward, into a prominent Paleoproterozoic shear zone developed in Archean rocks and exposed in the central Laramie Mountains (Chamberlain et al., 1993; Resor and Snoke, unpublished data). The structural grain of Precambrian rocks in the central Laramie Mountains is dominated by a northeast–southwest orientation characterized by mylonitic shear zones, mafic dikes, and contacts between massive and gneissic granitic rocks. Thus, it is reasonable to assume that the unusual orientation exhibited by the juxtaposed area of northeast-trending folds is a byproduct of Laramide structural reactivation reflecting underlying fabrics of the Precambrian basement.
Interestingly, as recognized through compilation of subsurface data, each of the northeast-trending folds is terminated abruptly at its southwestern end where it abuts against a prominent, northwest-striking zone of faulted anticlinal features (Bekkar, 1973; Love and Christiansen, 1985; Rocky Mountain Map Company, 1992b). Those unexpected relationships suggest that a template formed by the Precambrian basement allowed partitioning of deformational strain (sensu Varga, 1993) into distinct, mappable domains in this part of the fractured Wyoming foreland. Although northwesterly striking Precambrian features are uncommon in southeastern Wyoming, Nyberg (2001) mapped a northwest-striking zone of Precambrian ductile deformation at the northeastern end of the Medicine Bow Mountains. Again, that ancient zone of deformation appears to account for the northerly change in strike of the Arlington fault (of Laramide origin) from approximately north–south to northwest–southeast.
Asymmetrical, western parts of the most northerly two anticlines in the area of northeast-trending folds (i.e., Flat Top and Como Bluff anticlines; Blevens, 1984 and Dunbar, 1944, respectively) are shown in Figure 3. Note that Freezeout Mountain anticline, which forms the southeastern flank of the Freezeout Hills and the plunging nose of the Sweetwater arch (Figs. 1 and 4A), shares the same orientation, degree of asymmetry, and nature of faulting. Except as they occur in the northeastern Hanna Basin, neither Freezeout Mountain anticline nor any of the other anticlines in the area of northeast-trending folds include covering strata of Paleocene or early Eocene age. Across most of the landscape in which these anticlines occur, the Upper Cretaceous Steele Shale is the youngest exposed rock unit. Attempting to narrow the timing of deformation of this area to a resolution exceeding simply “Laramide” has been difficult.
But geological relationships observable in the northeastern corner of the Hanna Basin, in vicinity of The Breaks (Figs. 3 and 4A–B), contribute directly and importantly to interpretation of relative ages of uplift of anticlinal features to the north and east. Recall the earlier discussions about the two, temporally overlapping phases of development of the Breaks fault system. The earlier phase, involving passive development of the Dragonfly, Owl Ridge, and related but unnamed, smaller faults (Fig. 4A–B), seems to have been formed principally in relation to late Laramide uplift of Freezeout Mountain anticline — after deposition of most, or all, of the Hanna Formation. In contrast, the later phase was more directly linked to uplift and northwestward fault vergence of Flat Top anticline.
Note from comparison of Figures 3 and 4A that the entirety of the landscape east of outcrops of the Hanna Formation in The Breaks is structurally part of Flat Top anticline. Indeed, immediately east of the complex zone of thrust faulting symbolized by one trace on Figure 3, intact stratigraphy of Flat Top anticline includes strata as high as uppermost Cretaceous parts of the Ferris Formation. Lower parts of the Paleocene Hanna Formation of the eastern Hanna Basin, therefore, were passively thrust onto Upper Cretaceous strata of the western nose of Flat Top anticline as the latter became uplifted.
Because all geological evidence in today's eastern Hanna Basin suggests that virtually the total thickness of the Hanna Formation existed prior to that faulting, in all probability the uplift of Flat Top anticline did not occur until very late in the Paleocene or, perhaps more probably, in earliest Eocene time. Also, we suggest that prior to the folding of Flat Top anticline, the original eastern structural and depositional margin of the Hanna/Carbon Basin (as the eastern extreme of the greater Green River Basin) extended far east of the fault system shown in Figure 3. The original basin probably extended all the way to the western flanks of the newly emerged extension of the Colorado Front Range known as the Laramie Mountains (Figs. 1 and 15). If true, the Hanna Formation as recognized in the Laramie Basin (Blackstone, 1975; Love and Christiansen, 1985) would have been depositionally continuous with the Hanna Formation of the easternmost Greater Green River Basin (Fig. 15).
As emphasized by Lillegraven and Ostresh (1988), the timing of uplift of the Laramie Mountains is especially difficult to interpret because of the prodigious amount of pre-Wasatchian erosion that the range underwent through its entire north–south length. The very fact that erosion had broadly exposed the Precambrian core of the Laramie Mountains by earliest Eocene time, however, is powerful evidence that the range served as a barrier to the generally east-flowing river systems that dominated the Green River Basin through Lancian (latest Cretaceous) and some uncertain part of Paleocene time (Blackstone, 1975, fig. 7; Lillegraven and Ostresh, 1988, figs. 4–9; Kirschbaum et al., 1994, fig. 7). Exactly when during Paleocene time the Laramie Mountains became elevated enough to divert the eastward drainages, however, remains unknown. Lillegraven and Ostresh (1988, fig. 5 and table 2) raised the possibility that a northward diversion of the rivers (through the Shirley Basin) may have occurred as early as Torrejonian time, thereby shunting waters from the ancient Green River drainage into important lake systems that were then developing in the eastern Wind River (Lillegraven, in press; McKenna and Lillegraven, in review) and western Powder River Basins.
Lillegraven (1994) also suggested that the continuing uplift of the Laramie Mountains during earliest Eocene time led to a full reversal of Paleocene drainage patterns through the vicinity of the Hanna Basin. That is, the previous eastward drainage pattern switched to a west-draining pattern, thereby contributing to the origins of Lakes Gosiute and Uinta during the Wasatchian. The timing of uplift in the area of northeast-trending folds as discussed here is consonant with that interpretation.
How Extensive Was the Hanna Formation by Late Paleocene Time?
In the previous section, we suggested that most parts of the Hanna Formation originally extended eastward from present vicinity of the Hanna/Carbon Basin (across what is now referred to as the “area of northeast-trending folds”) to the Laramie Mountains, thus being confluent with the Hanna Formation as recognized in the Laramie Basin. Original distribution of late Paleocene elements of the Hanna Formation becomes even more impressive when one considers paleogeographic implications of the idea discussed in earlier parts of this paper that lateral equivalents of the formation above leg 16 of our measured section in The Breaks probably covered all of the Freezeout Hills (Fig. 15).
Figure 15 presents a paleogeographic reconstruction of much of southeastern Wyoming as it might have appeared during late phases of deposition of the Hanna Formation in the Hanna Basin. The map emphasizes the depositional continuity during the late Paleocene among what today are structurally and/or erosionally isolated, small basinal elements (i.e., Hanna, Carbon, Pass Creek, Laramie, and Shirley Basins). We suggest that all of these elements, prior to the late Laramide culmination of local tectonism, comprised a single structural and depositional basin that was merely an eastern extension of the greater Green River Basin. Clearly, because of later erosion of edges of remnant strata, most of the boundaries shown on Figure 15 between the Paleocene basinal strata and adjacent uplifts must be estimates. Nevertheless, small patches of critically placed Paleocene strata such as seen in the Pass Creek Basin (Gries, 1964), western Laramie Basin (Blackstone, 1975), and Hanna Basin provide important constraints to paleogeographic integration.
However, the expansive basinal landscape restored in Figure 15 must have been short lived. Basement-involved faulting in the latest Paleocene and early Eocene led to elevation of the: (1) Rawlins uplift (Barlow, 1953; Otteman, 2003); (2) Simpson Ridge anticline (Kraatz, 2002); (3) Elk Mountain anticline and its close relatives (Beckwith, 1941; Chadeayne, 1966; Mears, 1998; Hitchens, 1999); and (4) area of northeast-trending folds (this paper). Those uplifts subdivided the previously depositionally continuous southeastern Wyoming basin into much smaller components. The thick Paleocene strata were disrupted by extensive out-of-the-basin thrusts and the rate of erosion across most of this area increased dramatically. For example, detailed mapping by Harshman (1968, 1972) shows that virtually all Paleocene and Upper Cretaceous strata (down to the Steele Shale) in vicinity of the Shirley Basin had been removed prior to deposition of the Wasatchian Wind River Formation. That erosion, as probably also was true immediately to the south in the area of northeast-trending folds, was related to latest Paleocene and/or earliest Eocene contractional elevation of the Laramie Mountains.
How Should the Hanna Basin be Classified Tectonically?
Dickinson et al. (1988) provided a useful, paleogeographically based summary of Laramide basinal evolution through the central Rocky Mountains. In this region, they recognized and characterized three geographic groups of Paleogene depocenters: (1) “perimeter basins” (present in eastern parts of the Laramide province [sensu Hamilton, 1981 and Brown, 1988], leading toward the Great Plains); (2) “axial basins” (comparatively small features, aligned on a north–south, intramontane trend now occupied by the mostly Neogene Rio Grande rift system); and (3) “ponded basins” (present in western parts of the Laramide province, closer to the overthrust belt). Dickinson et al. (1988, fig. 2) considered the Hanna, Laramie, and Shirley Basins to fall within their category of “axial basins.” They did not consider the still-smaller Carbon or Pass Creek Basins (Figs. 1 and 15). The basinal classification by Dickinson et al. (1988) in many respects paralleled the earlier attempt by Chapin and Gather (1983) and Cather and Chapin (1990) but, as considered below, their approaches diverged in interpretation of the small basins of southeastern Wyoming.
In addition to the geographical generalizations summarized above, Dickinson et al. (1988) characterized perimeter, axial, and ponded basins using developmental and tectonic criteria. The following are especially relevant quotations from their characterization of axial basins (Dickinson et al., 1988, p. 1026): (1) “Axial basins are relatively small in area, but high structural relief marks nearly all basin flanks, and conglomeratic fades are prevalent….”; (2) “Nonmarine Cretaceous strata are rare in the axial basins, and thin where present”; and (3) “Fluvial drainages from axial basins typically led into larger perimeter basins located nearby.”
The present-day Hanna Basin is small, although that was not the case until very late in the Laramide orogeny when it became isolated from what are now known as the Green River, Carbon, Pass Creek, Laramie, and Shirley Basins. There does exist great structural relief along the Hanna Basin's northern and southern margins, but its present boundaries on the west and east are the results of basement-controlled, late Laramide faulting during the process of basinal fragmentation. Conglomeratic facies within the sedimentary column of the Ferris (Eberle and Lillegraven, 1998a, figs. 5–6) and Hanna (Fig. 13F) Formations are uncommon except locally. For example, conglomerates occur in very late Paleocene parts of the Hanna Formation in the southern Carbon/Pass Creek Basin and in the western Laramie Basin on the northern and northeastern flanks, respectively, of the Medicine Bow Mountains. The nonmarine Cretaceous strata of the Hanna Basin would not be characterized as “thin” (Fig. 2), as the combined thickness of nonmarine facies of the Mesaverde Group, Medicine Bow Formation, and Ferris (Cretaceous parts) Formation total approximately 8,500 ft (c. 2.6 km). Except perhaps early in the Paleocene, prior to uplift of the Laramie Mountains (Lillegraven and Ostresh, 1988, fig. 5), the Hanna Basin probably did not drain into a nearby perimeter basin. Thus, in light of all these differences from expectation, the Hanna Basin fails to meet the criteria that are said to characterize an axial basin.
Also, the Hanna Basin most definitely does not fit the characteristics of a perimeter basin as defined by Dickinson et al. (1988). They stated (p. 1026), “Perimeter basins were relatively broad bowls with gentle structural relief along basin flanks that merged with the stable continental craton and received mainly stream deposits with local paludal or lacustrine facies ….” Perimeter basins also are geographically much larger features than axial basins. Finally, Laramide sedimentary fill in perimeter basins generally is less than two kilometers in thickness, whereas thickness of the Laramide section in the Hanna Basin (i.e., Niobrara Formation through Hanna Formation; Fig. 2) exceeds 11 kilometers.
Paleogene ponded basins of the central Rockies, according to Dickinson et al. (1988, p. 1026), share the following relevant features: (1) “… fluvial drainages were blocked at times to form large freshwater or saline lakes. Consequently, depocenters contain thick sequences of dark, organic-rich lacustrine shale and associated calcareous strata deposited in perennial lakes, and some ponded basins also contain widespread saline deposits that record ephemeral lacustrine phases”; (2) “Lacustrine facies accumulated mainly between mid-Paleocene and mid-Eocene time …”; (3) “Nonmarine basin fills are generally 3,000– 5,000 m thick”; and (4) “Locally abrupt structural relief is present along basin flanks adjacent to prominent nearby uplifts that are fault-bounded.”
Except for the fact that in the Hanna Basin the total thickness of nonmarine basin fill exceeds seven kilometers (Fig. 2), the dominance of carbonaceous shale (deposited in generally watery settings) and lake beds fits most closely the stated lithologic characteristics of ponded basins. It should be kept in mind, however, that even the great sedimentary thickness observed in the northeastern Hanna Basin is matched by the column of subsurface Paleocene strata along the southwestern flank of the Wind River Mountains in the northern Green River Basin (Law, 1981; Lillegraven, 1993, fig. 4Q). Indeed, in this paper we interpret the Hanna, Carbon, Pass Creek, Laramie, and Shirley Basins as small, remnant, late Laramide tectonic subdivisions of the eastern end of the previously much larger ponded depocenter known as the greater Green River Basin (Figs. 15 and 19). Chapin and Gather (1983, fig. 1) considered the Hanna, Laramie, and Shirley Basins as miniature “Green River-type” basins, which interpretively are the equivalents of “ponded basins” as viewed by Dickinson et al. (1988).
In light of the above analysis, we suggest that although the concept of Laramide axial basins (sensu Dickinson et al., 1988) remains valid for New Mexico and Colorado, it is not applicable to any of the small Laramide basins of southern Wyoming. In our view of the geographic extent of the ponded, greater Green River Basin prior to latest Paleocene time (Figs. 15 and 19), it dwarfed most Laramide depocenters along the entire Rocky Mountain chain. Furthermore, prior to its late Laramide subdivision and profound marginal remodeling by basement-involved thrust faulting, the total area of the greater Green River Basin would have rivaled, or probably even exceeded, that of the Powder River Basin of Wyoming and Montana. And as a Paleocene depocenter, the sedimentary accumulation within the ponded greater Green River Basin would have greatly exceeded that of the shallower Powder River Basin, a perimeter basin.
SUMMARY AND CONCLUSIONS
The modern Hanna Basin of south-central Wyoming, in comparison with other Laramide depocenters of the Rocky Mountain region, is geographically minuscule. Nevertheless, when Paleocene parts of its Ferris Formation are combined with the overlying Hanna Formation, the Hanna Basin contains one of the thickest total sections of strata representing Paleocene time within the Rockies. In relation to the combination of its small size and thick Laramide stratigraphic section, the Hanna Basin was classified tectonically by others as an “axial basin.”
The generally well-exposed Hanna Formation, as seen in the northeastern corner of the Hanna Basin (in the area of badlands known as “The Breaks”), was deposited from late in the early Paleocene (Torrejonian North American Land Mammal “Age”) through earliest Eocene (early Wasatchian NALMA) time. Primary age controls for the formation are provided through study of mammalian fossils, but they are supplemented by temporally less diagnostic assemblages of freshwater molluscs, leaf macrofloras, and palynomorphs, all linked stratigraphically within a common measured section. Geological relationships observed within the Hanna Formation in vicinity of The Breaks, combined with its rich fossil record of animals and plants, allow new insight into the original magnitude of the Hanna Basin as well as other aspects relevant to paleogeographic development of Wyoming during the Laramide orogeny.
Oldest exposed parts of the Hanna Formation in The Breaks were deposited upon an erosional surface on the southeastern flank of Freezeout Mountain anticline that exposed the Steele Shale at the northern basin margin; erosion of the anticline was still deeper to the north. As the Hanna Formation accumulated, it sequentially buried progressively younger Upper Cretaceous formations on the Freezeout Mountain anticline across that erosion surface, and eventually the Hanna Formation covered the anticline.
The Hanna Formation in vicinity of The Breaks is a remarkably thick, paleontologically diverse rock unit that was deposited near ancient sea level under usually wellwatered but generally nonmarine conditions. No marine strata were recognized in that localized area. Fine-grained strata (especially carbonaceous shale) dominate the section, conglomerate is relatively scarce, and sandstone beds have grains that are rounded to well-rounded and quartz-dominated except near the top of the formation. Prominent within the upper half of the local Hanna Formation are beds deposited in coal swamps that grade upward into widespread lacustrine sequences; the lakes were well oxygenated and teemed with animal life through most of their existence. Uppermost parts of the Hanna Formation are dominated by angular, coarse-grained arkose with abundant muscovite and diverse, basement-derived metamorphic minerals and occasional granitic boulders.
In contrast to most other marginal areas inside the Hanna Basin, only the uppermost third of the Hanna Formation in vicinity of The Breaks exhibits evidence for broad exposure of Precambrian basement rocks within its source areas. Elsewhere around the Hanna Basin feldspathic input from exposed Precambrian basement begins within Upper Cretaceous rock units and persists throughout the accumulated Paleocene section. The eroding southeastern nose of the Sweetwater arch (Freezeout Hills) served as a principal source area for the Hanna Formation seen in The Breaks until the hills themselves became buried by progressive depositional onlap of that formation. Thereafter, the breached Precambrian core of the Sweetwater arch to the northwest of the Freezeout Hills became a primary source area contributing locally to the highest beds of the Hanna Formation.
Most of the Hanna Formation as seen in The Breaks aggraded rapidly, showing little evidence of channel cutting, secondary infilling, or any but the most rudimentary stages of pedogenesis. Widespread and obvious development of crayfish burrows throughout the lower half of the formation, however, suggests strong intervals of seasonal or more prolonged dryness as exhibited by markedly lowered, ancient water tables.
Taxonomic associations of palynomorphs, sampled in superpositional order within the Ferris and Hanna Formations of the Hanna Basin, concur for the most part with sequences of assemblage changes reported from other Laramide basins in the Rocky Mountain region. Exceptions, however, occur in samples from lower parts of the Hanna Formation in The Breaks as collected from strata that can be no younger than late early Paleocene (Torrejonian NALMA). Those samples (collected and studied by Dunn, 2003) exhibit palynomorphic assemblages characteristic of the Caryapollenites veripites–Pistillipollenites mcgregorii Interval Biozone (of Nichols, 2003; “Zone P5” of older terminology), which elsewhere has been recorded only from strata deposited during late Paleocene time (Tiffanian NALMA or younger). In The Breaks, assemblages characteristic of “Zone P5” have been recorded amongst sub- and superjacent strata characteristic of the Momipites actinus–Aquilapollenites spinulosus Interval Biozone (“Zone P3” of older terminology), which is characteristic elsewhere of strata deposited early in the Paleocene. The unexpectedly early occurrences of “P5” palynomorphic assemblages raises the spectre that evolutionary assumptions underlying definition of the existing system of biostratigraphic zonation based on fossil pollen are fundamentally flawed. Localized paleoecological controls on distributions of pollen-forming plants during Paleocene time may have been more important than previously recognized.
Deformation of the Hanna Formation in vicinity of The Breaks reflects complex influences from uplifts of Freezeout Mountain anticline (to the north), Flat Top anticline (to the east), and Simpson Ridge anticline (to the southeast). Of these three folds, the Freezeout Mountain anticline was the first to show initial development, which occurred prior to deposition of the Hanna Formation. Flat Top and Simpson Ridge anticlines had their initial uplifts during very late phases of deposition of the Hanna Formation. All three anticlines underwent their most prominent folding late in the Laramide orogeny, after deposition of virtually all of the Hanna Formation.
Synclines and widespread faulting within the thick and generally incompetent, mudstonerich Hanna Formation in the northeastern Hanna Basin developed as the result of spatial crowding from convergence by the surrounding anticlines. This deformation occurred in passive response to synchronous, contractional, blind thrust faulting and associated folding within the flanking Sweetwater arch (locally, the Shirley Mountains and Freezeout Hills), Flat Top anticline, and Simpson Ridge anticline. Shortening of the Laramide landscape was on the order of kilometers in response to development of each of the surrounding folds. In all probability, ancient discontinuities in the Precambrian basement rocks formed a template of weaknesses that strongly influenced orientation of local Laramide structural features. Rather than proposing variations in principal stress axes through the Laramide orogeny, we suggest that variability in the timing and orientations of folds and fault zones along the northern and eastern margins of the Hanna Basin reflected primary control by the template of basement discontinuities.
Major faults that formed during the Laramide orogeny within the Hanna Formation of The Breaks (as components of the “Breaks fault system”) are thin-skinned décollements having northwest–southeast map traces. Most of the faults exhibit out-of-the-basin senses of displacement, and they occurred most importantly in response to basement-involved, late uplift of Freezeout Mountain anticline (as an “older” component of faulting, resulting in vergence to the northeast) and initial uplift of Flat Top anticline (as a “younger” component of faulting, resulting in vergence to the northwest). The “older” and “younger” intervals of faulting probably were in part synchronous. The older component commonly is associated with footwall synclines that are set closely adjacent to traces of the faults. The combined fault traces and nearby footwall synclines were secondarily deformed as a common package, in response to the somewhat younger uplift of Flat Top anticline. Traces of the younger faulting within The Breaks merge with the older (as seen in map view), thereby forming localized, especially chaotic zones of deformation.
Laramide fault traces observable today in vicinity of The Breaks probably developed at depths of several kilometers. Their existing expressions on the modern landscape represent secondary exposures caused by prodigious, post tectonic erosion, both Paleogene and Neogene.
Map relationships observed in The Breaks clearly show that throw along the hanging walls of the most important out-of-the-basin faults has put younger strata onto progressively older strata as the faults are traced to the northwest. Many other examples of this kind of younger-on-older, out-of-the-basin faulting relationship are recognizable at greatly varying scales along the northern and eastern margins of the Hanna Basin and western flanks of the Carbon Basin. Such relationships of faulting demand existence of fault planes that cut stratigraphically downward in the direction of displacement. This initially unexpected configuration seems to have occurred when the out-of-the-basin faults propagated through strata that became steeply upturned by an oppositely directed, more fundamental (basement-involved in this case) fault system. Although not commonly recognized, this kind of younger-on-older, out-of-the-basin faulting relationship probably occurs in many other tectonically crowded, basin-margin settings.
Cross-sectional modeling of relationships within the Hanna Formation across the largest out-of-the-basin fault in vicinity of The Breaks (the Dragonfly fault) suggests minimal displacement of the footwall (i.e., southwestern end of Freezeout Mountain anticline) of the following magnitudes: horizontal vector >1.8 km to southwest; vertical vector >2.7 km toward zenith; and resultant vector >3.3 km translating upward and toward the southwest. These estimates are compatible with magnitudes of displacement as interpreted by others from seismic-reflection data along the north-central margin of the Hanna Basin.
The Hanna Formation along the entire eastern boundary of the modern Hanna Basin (from just east of The Breaks southward onto the crest of Simpson Ridge anticline) is nowhere in depositional contact with the underlying Ferris Formation. The length of the modern eastern border of the Hanna Basin is characterized by a complex zone of anastomosing, east-verging, out-of-the-basin thrusts that developed within weak, coaly, lower (but not basal) parts of the Hanna Formation. In essence, the map trace of this zone of faulting represents a southeasterly continuation of the Breaks fault system. Younger strata of the Hanna Formation (late Torrejonian NALMA) were thrust upon older strata of the Ferris Formation (probably of Cretaceous age). Roughly the lower quarter of the Hanna Formation was cut out by displacement along that fault system.
Because the upper three-quarters of the Hanna Formation as observed at the eastern margin of the modern Hanna Basin exhibits no consequential thinning from its more westerly exposures, the conclusion seems unavoidable that the original eastern boundary of the Hanna Basin was far to the east of its presently recognized border. The entirety of the modern landscape east of outcrops of the Hanna Formation in The Breaks is structurally part of Flat Top anticline. Because all geological evidence in today's eastern Hanna Basin suggests that virtually the total thickness of the Hanna Formation existed prior to faulting at that basinal margin, in all probability the uplift of Flat Top anticline (and probably all four of the other en echelon anticlines within the “area of northeast-trending folds”) did not occur until very late in the Paleocene or, perhaps more probably, in earliest Eocene time.
We suggest that prior to folding of Flat Top anticline, the original eastern structural and depositional margin of the Hanna/Carbon Basin extended all the way to western flanks of the newly emerged Laramie Mountains. That is, the eastern extreme of what we think of today as the Hanna Basin attained its present configuration no earlier than very late in the Paleocene. Under this interpretation, the areas recognized as the modern-day Laramie, Carbon, Shirley, and Pass Creek Basins would have constituted a unified depocenter that was confluent with the Hanna Basin and points west (i.e., all the way to the Wyoming–Idaho–Utah thrust belt). Additionally, by late Paleocene time, deposition of the Hanna Formation in vicinity of The Breaks must have overtopped the Freezeout Hills immediately to the north, thus becoming confluent with Paleocene strata of the Fort Union Formation in the southeastern Wind River Basin. By the late Paleocene, generally east-flowing drainages through the greater Green River Basin were diverted northward, by uplift of the Laramie Mountains, into the Wind River and Powder River Basins.
The combined Hanna, Carbon, Pass Creek, Laramie, and Shirley Basins during most of Paleocene time were unified, eastern components of a huge “ponded basin” (i.e., the greater Green River Basin). It became structurally subdivided into the presently expressed, small basins during the late Paleocene by basement-involved origins of the Rawlins uplift (on the west) and Flat Top and Simpson Ridge anticlines (on the east and southeast, respectively). Late in the Paleocene, the original, generally eastward drainages coursing through vicinity of the Hanna Basin became reversed to west-draining, thus contributing to initial development of Lake Gosiute in the Green River Basin during early Eocene time. Rates of Paleogene basinal erosion associated with late Laramide structural subdivision of eastern components of the greater Green River Basin were prodigious.
When the original, unified configuration of the eastern greater Green River Basin is taken into account and combined with palinspastic removal of late Laramide faulting that defines margins of the more westerly, main parts of the Green River Basin, one can see that this ponded basin rivaled, or perhaps exceeded, dimensions of the Powder River Basin of Wyoming and Montana. And as a Paleocene depocenter, the volume of sedimentary accumulation within the ponded greater Green River Basin would have greatly exceeded that of the much shallower Powder River Basin.
Although the concept of Laramide “axial basins” remains valid for New Mexico and Colorado, it is not applicable to any of the small Laramide basins of southern Wyoming.
Appendix 1. Lithologic summary of legs of measured section (Figs. 4 and 11–13) as seen in vicinity of “The Breaks.” Following standard geologic convention, the sequence of characterization is arranged to mimic ascending stratigraphic order. Measurements are cumulative upward, starting at base of leg 1. Numbered coal beds are from Dobbin et al., 1929
Leg20 – 11,110–11,645 ft (3,386.3–3,549.4 m); thickness of unit = 535ft (163.1 m)
Few useful exposures exist along leg 20. Minor outcrops involve complexly channeled beds of poorly sorted, coarse-grained arkose with pebbly stringers alternating with finer-grained strata of unknown composition. To west of line of transect, trucksized blocks of red granite exist as lag on weathered surface and, in at least two specific areas, within coarse sandstone beds of uppermost Hanna Formation.
Leg 19 – 9,716–11,110 ft (2,961.4–3,386.3 m); thickness of unit = 1,394ft (424.9 m)
Great majority of strata through legs 19 and 20 covered from view. Exposed parts of leg 19 composed of alternations of brown, bone-bearing (fish, turtles, and early Wasatchian mammals —Hyracotherium grangeri and a pantodont) mudstone, carbonaceous to very carbonaceous shale, and coarse- to very coarse-grained, occasionally pebbly arkose. One arkosic layer yielded fragments of large coprolites.
Leg 18 – 9,100–9,7 16ft (2,773.7–2,961.4 m); thickness of unit = 616ft (187.7m)
Top of “upper lacustrine unit” somewhat arbitrarily defined as top of leg 18 (recognizing that although most strata within this named unit were deposited under very watery conditions, much of the unit represents paludal rather than true lacustrine paleoenvironmental settings).
Bulk of leg 18 (constituting most of “upper lacustrine unit”) dominated by complex sets of carbonaceous to very carbonaceous shale and lignitic beds alternating with mostly medium- to coarse-grained, quartz-dominated to genuinely arkosic, often muscovite-rich sandstone layers, sometimes with pebbly stringers. Carbonaceous, fine-grained layers typically rich with plant debris, well-preserved leaves, stringers of amber, and sometimes lignitized wood, tree stumps (diameters up to 5 ft [1.5 m]), horizontal logs, and roots. Sandstone beds commonly with sideritic cementation, ripple cross-bedding, sometimes with delta-like foresets, abundant burrowing, fossils molluscs (molds, casts, and well-preserved, nacreous shells as coquina), and fish scales and bones. Occasional interbeds of thinly laminated silty shale with abundant fish bones and scales, nepionic molluscs, and abundant trace fossils. Includes numbered coals: 88 at 9,250 ft (2,819 m); and 89 at 9,695 ft (2,955 m).
Leg 17 – 7,410–9,100 ft (2,258.6–2,773.7 m); thickness of unit = 1,690ft (515.1 m)
Thickest leg of measured section and most diverse segment in terms of shifting paleoenvironmental representation. Thus, for purposes of description, divided here into three paleoenvironmentally recognized subcomponents (in ascending stratigraphic order) that should not be considered paleoecologically exclusive entities: (1) upper parts of lower lacustrine unit; (2) intervening fluvial unit; and (3) lower parts of upper lacustrine unit. Several levels within leg 17 poorly exposed (Fig. 12). Carbonaceous to very carbonaceous shale common throughout all three units, and each unit holds invertebrate-bearing strata probably deposited within lacustrine settings.
3. Lower parts of upper lacustrine unit (8,650–9,100 ft [2,636.5–2,773.7 m]).—Dominated by several cycles of carbonaceous shale grading upward into very carbonaceous shale (with lignitic stringers) and then into fish-bearing (disarticulated scales, bones, and teeth) paper shale and, less commonly, discontinuous masses of freshwater limestone. Lacustrine shale also contains localized concentrations of nacreous gastropods and unionid pelecypods, nepionic molluscs, occasional ostracod-rich layers, small coprolites, diverse worm-burrow traces, flattened, concentrically structured, bedding plane-restricted stromatolitic features (up to >20 ft [6.1 m] in diameter), and well-preserved leaf impressions or fossilized, cuticle-bearing leaves. Lowest feldspar-bearing sandstone in upper lacustrine unit occurs at 8,750 ft- (2,667.0 m-) level; feldspathic components initially minor but become progressively more common and abundant up-section to top of Hanna Formation. Siltstone and sandstone interbeds usually thinly laminated, commonly with ripple cross-bedding, soft-sediment deformation, and traces of muscovite. Sandstone beds usually medium-grained, but with strong pulses of coarse-grained fades containing grains of potassium feldspar and angular quartz. Includes numbered coals: 85 at 8,675 ft (2,644 m); 86 at 8,880 ft (2,707 m); and 87 at 8,965 ft (2,733 m).
2. Intervening fluvial unit (7,535–8,650 ft [2,296.7–2,636.5 m]).—Dominated at most levels by carbonaceous shale, with increasing proportions of very carbonaceous shale up-section, commonly with poorly preserved, horizontal logs and petrified wood. Highest-recognized crayfish burrows in entire measured section occur at 7,598 ft- (2,315.9 m-) level. Sideritic concretionary zones common in lower parts of unit, but highest observed in entire measured section occurs at 7,652 ft- (2,332.3 m-) level. Unit contains somewhat higher proportions of sandy fades than in most levels within legs 10–16. Most sandstone is fine- to medium-grained, commonly with sideritic cementation. All sandstone bodies from leg 1 to within this fluvial unit are composed virtually entirely of rounded to well-rounded quartz grains (with common but lesser particles of comminuted Mowry Shale, carbonaceous detritus, and other minor soft-rock sources), devoid of observed feldspathic or other granitic-derived grains. Compositional change to come in sandstone layers higher in section is heralded at 7,827 ft- (2,385.7 m-) level (plotted on Fig. 4) with appearance of a thin but laterally extensive, poorly sorted, coarse-grained pebbly sandstone bearing various feldspathic and metamorphic minerals, occasional flakes of muscovite, and pebbles of potasium feldspar. This thin sandstone (the only layer in leg 17 recognized to contain feldspathic components) is not “arkosic,” but it does mark lowest recognized occurrence in measured section of basement-derived detrital materials within sandy fades. Quartz grains in this sandstone layer are also more angular than in lower parts of measured section. Includes numbered coals: 81 at 7,725 ft (2,355 m); 82 at 8,095 ft (2,467 m); 83 at 8,390 ft (2,557 m); and 84 at 8,465 ft (2,580 m).
1. Upper levels of lower lacustrine unit (7,410– 7,535 ft [2,258.6– 2,296.7 m]).—Segment involves two successive cycles having base of laterally extensive, thin-bedded, fine- to medium-grained, quartz sandstone grading upward first to carbonaceous/very carbonaceous shale and then into soft mudstone or paper shale, locally rich with disarticulated bony remains of small fishes. A thin siltstone layer with extraordinarily well-preserved cone-in-cone structures occurs at 7,467 ft- (2,275.9 m-) level.
Leg 16 – 7,035–7,410 ft (2, 144.3–2,258.6 m); thickness of unit = 375ft (114.3 m)
Upper levels with thick, generally marginally exposed carbonaceous shale beds, locally lignitic. Almost all other parts of exposed section characterized by thinly laminated bedding of considerable lithologic variety and degree of induration. Many alternations of rippled siltstone to fine-grained quartz sandstone, occasional pulses of thinly laminated, medium-grained quartz sandstone, and poorly indurated mudstone with disarticulated fish parts (bony scales, internal skeletal elements, and teeth), rare and poorly preserved turtle bones, dense occurrences of nepionic molluscs, and layers rich with plant debris, including seed impressions. Top of leg marked by highly indurated (sideritic cementation), laterally persistent, obviously ripple-marked, fine-grained quartz sandstone, about 7 ft- (2.1 m-) thick. Includes numbered coal 80 at 7,295 ft (2,224 m).
Much of lower part not exposed.
Leg 15 – 6,66 1-7,035 ft (2,030.3–2,144.3 m); thickness of unit = 374ft (114 m)
Base has thick units of muddy, very carbonaceous shale grading into thinner layers of true lignite, alternating with thin beds of leaf-bearing siltstone, sideritic concretionary zones, and generally leached, fine- to medium-grained quartz sandstone. Definite lake beds begin at 6,865-ft (2,092.5-m) level, indicated by alternating layers of sideritic mudstone, siltstone, and fine-grained, ripple-bedded quartz sandstone, all commonly with indications of soft-sediment deformation and diverse trace fossils of invertebrates. Much of upper part of leg 15 and lower part of leg 16 covered (Fig. 12) from view, and that is the general case throughout area of study, probably reflecting poorly indurated underlying sediments. Although not all was deposited in lake waters, for descriptive purposes the “lower lacustrine unit” (of Lillegraven and Snoke, 1996 and as used in present paper) begins at 6,865 ft- (2.092 m-) level. Includes numbered coals: 78 at 6,710 ft (2,045 m); and 79 at 7,000 ft (2,134 m).
Leg 14 – 5,912–6,661 ft (1,802.0–2,030.3 m); thickness of unit = 749ft (228.3 m)
Dominated by carbonaceous shale, becoming progressively very carbonaceous near top, with advent of muddy lignite below base of leg 15. Extensively crayfish-burrowed at many levels, sometimes well preserved, and occasionally with burrow cross sections much narrower than found in lower legs of transect. Sideritic concretionary zones increase in frequency of occurrence upward and commonly are rich with plant debris. Many siltstone to fine sandstone layers have thin partings rich with fossil leaves, not uncommonly preserving intact cuticular tissues. Sandy layers generally thin, widely separated, and laterally discontinuous. One bed, however, reaches 12 ft (3.7 m) in thickness as a quartz sandstone, fine- to coarse-grained, occasionally with pebbly stringers, and generally rich with detrital carbonaceous debris. Conglomeratic beds only in lower part of section (cobbles 20 cm maximum diameter), and these are highest occurrences of significant conglomerate within Breaks section (aside from huge, scattered blocks of granite found in leg 20). Includes numbered coal 77 at 6,600 ft (2,012 m).
Leg 13 – 5,375–5,912 ft (1,638.3–1,802.0 m); thickness of unit = 537ft (163.7m)
Dominated by carbonaceous shale, occasionally with poorly preserved crayfish burrows. Sandy layers thin, widely separated, and composed mainly of fine- to medium-grained quartz sand, usually cemented heavily with siderite. Laterally discontinuous, well-separated, and thin units of pebble and clast-supported cobble conglomerate (20 cm maximum diameter).
Leg 12 – 4,435–5,375 ft (1,351.8–1,638.3 m); thickness of unit = 940ft (286.5 m)
Upper half, although continuing to be dominated by fine-grained strata (mainly carbonaceous shale), records initiation of pulses of clast-supported, pebble and cobble conglomerate (19 cm maximum diameter) starting at about 4,905-ft (1,495.0-m) level. Crayfish burrows much more ubiquitous and better preserved than in lower half. Capped by major unit of clast-supported cobble conglomerate (26 cm maximum diameter). Highest recorded vertebrate fossils (of Higgins', 2003b, “Fossil Vertebrate-Bearing Zone”) occur at about 4,860-ft (1,481.3-m) level. Fragmentary fossil leaves and detrital plant debris common in sandstone layers.
Lower half continues mudrock dominance and general lithologic characteristics of leg 11, but with even less frequent representation by thin layers of quartz sandstone. Crayfish burrows, although present as in leg 11, continue poor preservation.
Leg 11 – 3,491–4,435 ft (1,064.1–1,351.8 m); thickness of unit = 944ft (287.7m)
Highly similar to lithologic features in upper parts of leg 10 with even lesser occurrences of sandstone. Progressive but slight decrease up-section in occurrences of sideritic concretionary zones. Lowest known occurrences of fossil vertebrates in local Hanna Formation are northwest of 3,670-ft (1,118.6-m) level of transect; this represents base of Higgins' (2003b) “Fossil Vertebrate-Bearing Zone.” Occasional occurrences of generally poorly preserved crayfish burrows in mudrocks, especially in middle third of section. Although not occurring directly on measured transect, almost all sites bearing fossil vertebrates occur in pebble-, plant debris-, and rip-up clast-rich, laterally discontinuous channel deposits composed mainly of fine- to medium-grained quartz sandstone. One thin bed of cobble conglomerate (13 cm maximum diameter), rare in this generally mudrock section, occurs at about 2,775-ft (845.8-m) level just northwest of measured transect.
Leg 10 – 2,545–3,491ft (775.7–1,064.1 m); thickness of unit = 946ft (288.4 m)
Massive, monotonous section of carbonaceous shale (slight progressive increase in carbonaceous content up-section) interspersed with widely spaced, thin layers of sideritic concretions (increasing somewhat in frequency of occurrence up-section) and fine-grained, occasionally leaf-bearing, quartz sandstone usually containing stringers of carbonaceous detritus.
Base of leg 10 defined by major fault ("Dragonfly fault” of this paper).
Leg 9 – 2,352–2,545 ft (716.9–775.7 m); thickness of unit = 193ft (58.8 m)
Top defined by axis of syncline (within folded core of “The Great Tortilla”).
Thick, basal cobble conglomerate (20 cm maximum diameter), shifting upward to dominance of carbonaceous shale (commonly with crayfish burrows) interbedded with widely spaced, thin beds of fine-grained quartz sandstone, commonly sideritized.
As for legs 6, 7, and 8, all remaining outcrops of Hanna Formation related to its leg 9 are in fault contact with older strata. Field relations southeast of area covered by Figure 4, however, suggest part of the margin of leg 9 was deposited on an erosional surface cut into largely marine fades of Upper Cretaceous Medicine Bow Formation.
Leg 8 – 2,180–2,352 ft (664.5–716.9 m); thickness of unit = 172ft (52.4 m)
Thin, matrix-supported cobble conglomerate (15 cm maximum diameter) at base overlain by alternating sets of carbonaceous shale, fine-grained quartz sandstone, thin zones of sideritic concretions, and occasional crayfish-burrowed siltstone. Upper part with clast- to matrix-supported beds of cobble conglomerate (20 cm maximum diameter) and thin pebble conglomerate.
As for legs 6 and 7, all remaining outcrops of Hanna Formation related to its leg 8 are in fault contact with older strata. Field relations, however, suggest deposition on an erosional surface cut into upper levels of marine, Upper Cretaceous Lewis Shale.
Leg 7 – 1,571–2,180 ft (478.8–664.5 m); thickness of unit = 609ft (185.7m)
Highly similar to lithologic features of leg 6 except for greater proportion of very carbonaceous shale, somewhat higher proportions of sideritized zones (concretions and staining of sandstone), and individually thinner, commonly more closely spaced interbeds of quartz sandstone, which are almost entirely fine-grained. Diagenetic alteration apparently high, with many individual beds having a “cooked” appearance. Much indication of small-scale, intrastratal deformation along measured transect, becoming more pervasive and obvious to east of line of description. No evidence of crayfish-burrowing observed in leg 7.
As for leg 6, all remaining outcrops of Hanna Formation related to its leg 7 are in fault contact with older strata. Field relations, however, suggest deposition on an erosional surface cut into upper levels of marine Upper Cretaceous Lewis Shale.
Leg 6 – 912–1,571 ft (278.0–478.8 m); thickness of unit = 659ft (200.8 m)
Top defined by major fault (main trace of “Dragonfly fault” of this paper).
Composed predominantly of fine-grained units of carbonaceous to very carbonaceous shale, locally with thin lignitic layers. Contains many interbeds of white, mainly fine-grained, thin-bedded quartz sandstone, commonly partly sideritized, usually abundant with detrital flakes of richly carbonaceous materials or plant debris. Regular occurrences of thin zones of sideritic concretions. No evidence of crayfish-burrowing observed in leg 6.
Although all remaining outcrops of Hanna Formation related to its leg 6 are in fault contact with older strata, field relations strongly suggest that it was deposited on an erosional surface cut into middle parts of marine, Upper Cretaceous Lewis Shale.
Leg 5 – 590–912 ft (179.8–278.0 m); thickness of unit = 322ft (98.2 m)
Top defined by overlying fault within heavily deformed beds of very carbonaceous shale.
Generally fining-upward series, with thick, clast-supported cobble conglomerate at base (modal diameter c. 11 cm, maximum 44 cm) showing no obvious imbrications or regularity of bedding. Overlain by alternating sets of carbonaceous shale and mostly fine-grained, quartz sandstone, commonly with thin stringers of pebbles and rip-up clasts. No evidence of crayfish-burrowing observed in leg 5.
Leg 5 of Hanna Formation was deposited on an erosional surface cut into deltaic fades of Upper Cretaceous Almond Formation and marine fades in base of Upper Cretaceous Lewis Shale (Lillegraven, 1993, frontispiece).
Leg 4 – 440–590 ft (134.1–179.8 m); thickness of unit = 150ft (45.7m)
Top defined by syncline related to overlying fault.
Abundant carbonaceous shale (commonly crayfish-burrowed; Hasiotis and Honey, 2000, Fig. 9A–D), occasionally very carbonaceous, alternating with beds of sideritic concretions and sideritized sandstone, commonly crayfish-burrowed siltstone to fine-grained quartz sandstone, and stray stringers of pebbles and small cobbles. Several levels of fine-grained sandstone hold rich concentrations of leaf impressions.
To east of measured transect, leg 4 of Hanna Formation was deposited on an erosional surface cut into upper levels of fluvial fades of Upper Cretaceous Allen Ridge Formation, its overlying, “unnamed marine tongue” (of Gill et al., 1970), and mostly fluvial fades of Upper Cretaceous Pine Ridge Sandstone.
Leg 3 – 285–440 ft (86.9–134.1 m); thickness of unit = 155ft (47.2 m)
Alternating sets of carbonaceous shale, siltstone (occasionally crayfish-burrowed), discontinuous zones of sideritic concretions, mostly medium-grained, complexly channeled, dirty quartz sandstone with many stringers of pebbles, mudstone interbeds, petrified wood, plant debris and occasional leaf impressions, and matrix-supported cobble conglomerate (18 cm maximum diameter).
All of leg 3 was deposited on an erosional surface cut into nonmarine fades of Upper Cretaceous Allen Ridge Formation.
Leg 2 – 54–285 ft (16.5–86.9 m); thickness of unit = 231ft (70.4 m)
Alternating sets of carbonaceous to (relatively minor) very-carbonaceous shale, sideritic concretions, sideritized sandstone, crayfish-burrowed siltstone and fine-grained sandstone, mostly medium- with some coarse-grained quartz sandstone, and lowest cobble conglomerate (clast-supported, clasts up to 24 cm long, commonly multiply fractured) in Breaks section. Cut by fault at about 200 ft- (61.0 m-) level (Fig. 4).
Main part of leg 2 to east of line of measurement was deposited on a near-shore marine fades of Upper Cretaceous Haystack Mountains Formation, but Hanna Formation above fault (Fig. 4) at about 200 ft- (61.0 m-) level was deposited on a nonmarine facies of Upper Cretaceous Allen Ridge Formation.
Leg 1 – 0–54 ft (0–16.5 m); thickness of unit = 54ft (16.5 m)
Alternating sets of carbonaceous shale, crayfish-burrowed siltstone, and mostly fine-grained quartz sandstone, some with pebbly stringers. Occasional tree stumps in standing position.
Local base of Hanna Formation was deposited on erosional unconformity cut into slightly weathered, sandy, trace-fossil-rich, upper parts of marine, Upper Cretaceous Steele Shale (Lillegraven and Snoke, 1996, fig. 10).
Primary funding for this project came from National Science Foundation research grants EAR9506462 and EAR9909354. We thank the following individuals for their various contributions in terms of constructive criticism, help with fieldwork, technical assistance, scientific insight, advice, land access, and moral support for this project: Donald L. Blackstone, Jr.; Donald W. Boyd; John H. Burris; Michael L. Cassiliano; Steven M. Cather; Jean-Pierre Cavigelli; Claudette Cohen; Burton S. Davis, Jr.; Regan E. Dunn; Jaelyn J. Eberle; family of Bill and Marylou Ellis; Eric A. Erslev; David A. Grimaldi; Warren Hamilton; Pennilyn Higgins; Brian R. Hitchens; James G. Honey; Richard C. Hook; Daniella Kalthoff; Charles F. Kluth; Brian P. Kraatz; George B. LeFebre; Linda E. Lillegraven; the late J. David Love; Priscilla McKenna; Brainerd Mears, Jr.; Roland E. Miller, III; Douglas J. Nichols; Aaron S. Otteman; family of Burton and Kay-Lynn Palm; family of Casey and Nellie Palm; Carl Pirner; Robert G. Raynolds; Cory M. Redman; Mark Rogers; Frank Royse, Jr.; Ross Secord; Kendall L. Taft; David A. Taylor; Basil Tikoff; Kelli C. Trujillo; and Donald U. Wise. Douglas Boyer, Steven M. Cather, and David W. Krause served as stern (and eminently helpful) external reviewers. Donald W. Boyd provided extraordinary editorial attention to the final manuscript.
NOTE ADDED TO PROOF
The following article, of which we were unaware prior to its publication, became available for use in February of 2004:
Wroblewski, A. F.-J., 2003, Tectonic redirection of Paleocene fluvial drainage systems and lacustrine flooding in the Hanna Basin area, south-central Wyoming, in Raynolds, R. G., and Flores, R. M., eds., Cenozoic systems of the Rocky Mountain region: Denver, The Rocky Mountain Section SEPM (Society for Sedimentary Geology), p. 227–252.
Its interpretations of the Laramide geologic history of the Hanna-Carbon Basins differ fundamentally from those presented here.
Jason A. Lillegraven, Arthur W. Snoke, Malcolm C. McKenna
- Received November 17, 2003.
- Revision received February 26, 2004.
- Accepted February 26, 2004.