- © 2013 UW Department of Geology and Geophysics
Linkage of paleontological and geological discoveries provides new opportunities to strengthen interpretations of paleogeographic evolution of the Rocky Mountains' deepest structural basin. We report discovery in the northeastern Hanna Basin (south-central Wyoming) of a lower first molar of Meniscoessus cf. M. robustus, an advanced form of multituberculate mammal known only from the North American Western Interior in Upper Cretaceous local faunas of the Lancian North American Land Mammal Age. It aids in dating patchy outcrops of the Ferris Formation, overlain and covered laterally by significantly younger, thrust-emplaced Hanna Formation. The specimen documents a member of the M. robustus species group, also recovered from more southwesterly strata of the Ferris Formation in the Hanna Basin. The fossil-bearing strata were deposited close to ancient sea level but are tectonically overturned and bounded above and below by what originally were north-vergent thrust faults. We present a new geologic map (scale 1:24,000) including two representative cross sections. Using an interpretive cross-sectional evolutionary model, we propose that the Hanna Basin, until late in Laramide orogenesis, had a markedly more extensive northern existence in the upland areas now occupied by the Freezeout Hills and southern Shirley Mountains. Local Laramide orogenic history in the mapped area is dominated to the north by development of at least 10 kilometers of Cretaceous–early Eocene structural relief across Archean granitic rocks. Those ancient rocks today form the NNE–SSW-oriented, axial core of the asymmetrical Shirley Mountains Anticline. Completion of the Shirley Mountains' uplift postdated deposition of almost the entire stratigraphic sequence now exposed along the northern Hanna Basin. North-vergent, out-of-the-basin thrust faults developed in response to crowding initiated by the much larger, south-vergent, basement-involved thrust complex known as the Shirley Fault. Those out-of-the-basin thrust faults had mostly bedding-parallel planes of displacement. But they commonly cut stratigraphically down-section during basin-margin deformation, thus placing younger strata of the hanging walls onto older strata of the footwalls. These thin-skinned, younger-on-older fault relationships today exhibit steeply basinward-dipping to overturned strata. The faulting led to greatly thinned stratigraphic sections when juxtaposed against basin-margin, mountainous uplifts expressing oppositely vergent, basement-involved thrust-fault systems. These kinds of down-section thrust faults probably will become recognized as common expressions of basin subdivision along steeply dipping, basin-margin strata throughout the Rocky Mountain province. Furthermore, several occurrences of this phenomenon appear to have been long-misinterpreted as depositional/erosional angular unconformities. Such recognition demands re-thinking of the areas' geologic histories. Complexities of erosional history constitute central parts of our evolutionary scenario. Locally derived clastic deposits within uppermost Cretaceous and Paleogene sequences of the northern Hanna Basin originated principally from north of the Shirley Mountains and other upland areas that today closely border the basin. Multiple source areas existed across deeply eroding, mountainous landscapes (the Granite Mountains) that existed during latest Cretaceous through Eocene time. Beginning in the latest Eocene and continuing late into the Miocene, dominantly airfall volcaniclastic materials from distant sources covered all but the high peaks of Wyoming. Late in the Miocene, however, the heavily eroded core of the Granite Mountains collapsed via extensional tectonics, allowing preservation of remnant volcaniclastic strata atop the Granite Mountains Graben.
- Ferris Formation
- Hanna Basin
- Hanna Formation
- Laramide orogeny
- out-of-the-basin thrusts
- Rocky Mountains
- Shirley Fault
- Shirley Mountains
Biostratigraphic Value of a Single Mammal Tooth
The Hanna Basin is in south-central Wyoming (Fig. 1). Only relatively small areas of its northernmost parts exhibit well-exposed strata. Most outcrops are dominated by coarse-grained rocks that rarely yield biostratigraphically useful fossils. In this paper we report discovery of a lower molar of Meniscoessus, a multituberculate mammal known only from the Western Interior in Upper Cretaceous strata carrying local faunas of the Lancian North American Land Mammal Age. The specimen is the first mammalian fossil reported from the northern Hanna Basin between southern flanks of the Shirley Mountains and the Medicine Bow River. It helps confirm identification of its host rock as the Ferris Formation, a named unit that is restricted in use to the greater Hanna Basin area. The Ferris Formation was deposited during latest Cretaceous through earliest Paleocene time (Eberle and Lillegraven, 1998a, b; Lillegraven and Eberle, 1999; Boyd and Lillegraven, 2011a, b).
Poor exposures and faulting make lateral tracing of the tooth-bearing unit to elsewhere in the basin impossible. About 300 meters south of the collection site a north-vergent, out-of-the-basin thrust has placed the significantly younger Hanna Formation directly onto the Ferris Formation. Also, both to the west and east of the tooth-bearing locality, strata of the Hanna Formation have been faulted down-section (i.e., across progressively older parts of the Ferris Fm.). The Meniscoessus-bearing, overturned Ferris strata thereby became isolated as a small patch of scattered outcrops (Fig. 2). Identification of the fossil as a member of the Meniscoessus robustus species group confirms its host rock to be correlative with fossiliferous, latest Cretaceous parts of the originally defined Ferris Formation as exposed 17 kilometers to the west-southwest. The temporally diagnostic molar therefore aids refinement of age of the northern basin's patchy outcrops.
Toward Better Understanding of Local Structural Evolution
That biostratigraphic refinement, in turn, aids understanding Laramide structural evolution in key parts of the northern Hanna Basin and its adjacent, fault-bounded mountainous uplifts. Thus our paper capitalizes on linking paleontological and geological discoveries in interpreting the timing and nature of paleogeographic evolution of the Rocky Mountains' deepest structural basin. The Phanerozoic sedimentary sequence attains a maximal thickness of almost 15 kilometers.
Specifically, we are attempting to answer questions such as: (1) how much of the Mesozoic and Paleogene stratigraphic section originally existing in the northern Hanna Basin adjacent to the incipient Shirley Mountains was lost due to the combination of out-of-the-basin thrusting and associated erosion?; (2) what evidence exists for constraining timing of late Laramide structural evolution of the southern Shirley Mountains and adjacent Hanna Basin?; and (3) how do discoveries from current mapping alter previous interpretations of structural evolution of the area? Answers to the preceding questions contribute to research in progress on more general aspects of late Laramide paleogeographic history across southeastern Wyoming.
For clarity in presentation, we divide this paper into two main sections. The sections appear sequentially as ‘Mammalian Paleontology’ and ‘Structural Geology.’ A ‘Conclusions’ section synthesizes the originally separated components.
- North American Land Mammal Age
- University of California Museum of Paleontology, Berkeley CA
- Collection of Fossil Vertebrates, Departmental Scientific Collections, Department of Geology and Geophysics, The University of Wyoming, Laramie WY
Lower-case letters (e.g., m1) designate teeth from lower dentitions, and upper-case letters (e.g., M2) designate teeth from upper dentitions.
5:4–5 Numbers of cusps of multituberculate lower molars in the buccal and lingual rows, respectively (variability among counts indicated by en-dashes).
All dental measurements are given in millimeters.
Class MAMMALIA Linneaus, 1758
Order MULTITUBERCULATA Cope, 1884
Suborder CIMOLODONTA McKenna, 1975
Comment. — Following the recommendation by Weil and Krause (2008), we abandon the long-standing practice of subdivision of the Cimolodonta into superfamilies.
CIMOLOMYIDAE Marsh, 1889
Meniscoessus Cope, 1882
Meniscoessus cf. M. robustus
Referred specimen.—UCMP 213000, an isolated left m1.
Locality.—UCMP locality V12013. The locality is on the southern slope of a hill forming part of the northern side of the valley of Dry Creek. It is near the northern border of SW ¼ of SW ¼, sec. 12, T. 24 N., R. 82 W. (measurement station 1256 on Fig. 4; base map ‘T E Ranch, Wyo.’ 7.5 minute topographic quadrangle, scale 1:24,000, 1953, photorevised 1982) in the northeastern part of the Hanna Basin, Carbon County, Wyoming. Approximate elevation 6850 feet. Grid references 0367995, 4657358 (1927 North American datum 1000-meter Universal Transverse Mercator grid, zone 13). Lillegraven collected the specimen on July 15, 1998. The locality is on private land within the 9V Ranch, co-owned by Keith and Pat Libbey.
Geology.—The tooth was discovered on the depositional undersurface of a structurally overturned, mostly fine-grained, well-indurated quartz sandstone of the Ferris Formation (Fig. 2B–C). Sideritic inclusions and probably extra-basinal stringers of tiny chert pebbles are common in this unit. The sandstone is heavily indurated. Scattered occurrences of isolated small pieces of large bones, most probably dinosaurian, have been found at nearby outcrops of Ferris Formation.
Age.—Latest Cretaceous, probably Lancian NALMA. UCMP 213000 represents a derived species of Meniscoessus, a member of the M. robustus species group of Flynn (1986, p. 20).
Description.—UCMP 213000 is a typical cimolodontan, left m1 with two rows of cusps of approximately equal length (Fig. 3). The cusp formula is 5:4. A slight indentation at mesiolingual corner of the crown received the posterior end of p4. Four cusps of the lingual row slightly increase in size distally. Mesial slopes of the first through third cusps are convex. Lingual and buccal crests from the apices of these cusps are oriented transversely. The distal lingual cusp lacks a lingual crest. Distal surfaces of the first through third cusps are flat (not concave) and oriented vertically.
The mesial cusp of the buccal row is conical in form and the smallest cusp on the crown. The second buccal cusp is higher, and its transverse ridges, particularly the lingual, are slightly curved but its distal face is flat. The third through the fifth buccal cusps are higher and equivalent in size to those of the lingual row. They differ in morphology of the crests from their apices, which are distinctly curved and concave distally. Unlike the lingual cusps, the distal faces of the third and fourth cusps are concave. Lingually oriented crests of the second through fourth buccal cusps extend through the central valley to reach the bases of the second through fourth lingual cusps. Otherwise, the opposing slopes of the buccal and lingual cusps are smooth and lack grooves or other ornamentation. The occlusal surface of the fifth lingual cusp is missing enamel distally.
The first two buccal cusps and first lingual cusp are hardly worn. Wear on the remaining cusps becomes more pronounced posteriorly. Small patches of enamel are missing from the buccal side of the crown. Thus its mesial and distal widths cannot be accurately determined (Table 1). In occlusal view, however, the crown does not appear to taper mesially. The base of the crown remains embedded in sandstone, so the number and morphology of roots, if preserved, cannot be determined.
Comments.—Meniscoessus seminoensis, known only from the Hanna Basin (from UW locality V-93006, ‘No Toad Left Unturned’; Lillegraven and Eberle, 1999, Fig. 4 and appendix 1), is a valid species. The principal bases for its recognition are the distinct morphology of p4 and the relative dimensions of p4, m1, and m2. In the diagnosis of the species it is noted that, in occlusal view, m1 tapers mesially.
In comparison to the m1 of Meniscoessus seminoensis (Table 1), UCMP 213000 is large (length = 9.455 mm, mesial width >5.14 mm, distal width > 4.60 mm). In a sample of m1s of M. robustus from the Lance Formation (UCMP locality V5711; see ‘type Lance Fm.’, Fig. 1A), the range of variation in length is 7.69 to 9.76 mm. Using that range of variation as a yardstick, UCMP 213000 would qualify as a large m1 of M. robustus.
Mesial and distal widths of m1s of Meniscoessus are difficult to determine because of small, irregular projections at the base of the crown, particularly at the ends of some transverse crests. Also, several small chips of enamel are missing from the buccal base of the crown of UCMP 213000. Recognizing that they are at best approximations, the ratios of mesial/distal widths were calculated. The ratios are: M. seminoensis = 0.92; UCMP 213000 = 1.12; and a sample of M. robustus from UCMP locality V5711 in the Lance Formation ranges from 0.92 to 1.02. This character favors reference of UCMP 213000 to M. robustus or, possibly, a new species.
As is true for Meniscoessus seminoensis, the cusp formula of UCMP 213000 is 5:4. Morphologically, its cusps resemble those of M. seminoensis in most aspects. The one possibly significant difference is in morphology of distal faces of the first through third lingual cusps. In M. seminoensis and M. robustus these faces are distinctly concave. In UCMP 213000 they are flat and vertically oriented. This difference might be of taxonomic significance but, given the small sample sizes, it is simply noted.
The m1 of Meniscoessus seminoensis, preserved in the dentary, was described as supported by three roots. In the sample of M. robustus from the Lance Formation (UCMP locality V5711) the roots of several isolated m1s are preserved. These teeth usually have large mesial and distal roots that extend the width of the crown. Between them are small, separate lingual and buccal roots.
The possibilities that UCMP 213000 represents a second individual of Meniscoessus seminoensis, a member of M. robustus, or a new species are viable working hypotheses. The small sample size and magnitude of differences in dimensions and morphology argue in favor of delaying a decision on identification of UCMP 213000 at the species level until more material becomes available. For the moment, we provisionally identify UCMP 213000 as Meniscoessus cf. M. robustus and consider it a member of the M. robustus species group (Flynn, 1986, p. 20). We interpret this species group as including: (1) M. robustus and the synonymous species M. borealis and M. greeni (see Archibald, 1982 and Eberle and Lillegraven, 1998a); (2) the questionably distinct M. conquistus; (3) M. seminoensis; and (4) now Meniscoessus cf. M. robustus.
Biostratigraphic Significance of Meniscoessus cf. M. robustus (UCMP 213000)
Lillegraven (1987) published the most recent cladistic analysis of the species of Meniscoessus. He recognized three clades: M. ferox, M. intermedius + M. major, and M. collomensis + the M. robustus species group. This species group was characterized by: (1) larger size of its molars; (2) fully crescentic molar cusps; and (3) hypertrophy of M2 relative to M1. The specimen of M. collomensis is from a northern Colorado fossil locality thought to have been formed during the ‘Edmontonian’ NALMA.
Almost all reported occurrences of members of the Meniscoessus robustus species group are in a series of Lancian NALMA local faunas extending from Saskatchewan southward into Wyoming. The only possibly older record of the species group is the reported occurrence of M. conquistus in the Scabby Butte Local Fauna from the St. Mary River Formation of Alberta, thought to be of ‘Edmontonian’ age (Sloan and Russell, 1974; Russell, 1975). Archibald (1982, p. 89) challenged this record that was based on an anterior upper premolar, a fragment of a molar, and a small (relative to M. robustus) worn M2. The first two fossils are uninformative for identifications at the species level. Archibald suggested that the molar represented a small species of Meniscoessus sensu lato. Very possibly it could represent a member of the M. major + M. intermedius clade. On the basis of uncertainties associated with identification of the Albertan specimens, we restrict the demonstrated range of the M. robustus species group to the Lancian NALMA. The often-repeated record of M. robustus in the Scabby Butte Local Fauna most probably stems from an error in compilation by Lillegraven and McKenna (1986, table 10).
Donohue et al. (2013) described and analyzed the relatively large sample of multituberculates from the Lancian Black Butte Station locality, which is approximately 200 km west-southwest of the locality in the Hanna Basin at which UCMP 213000 was discovered (Fig. 1A). Meniscoessus robustus is represented in this collection by a complete m1, a fragment of an m2, and two worn and damaged P4s. Resembling UCMP 213000, the complete m1 (UCMP 197615) has a cusp formula of 5:4. UCMP 197615 differs in length and width of its crown (7.82 mm and 3.96 mm, respectively), which fall toward small ends of the ranges of variation of a sample of M. robustus from the type Lance Formation (Table 1). In occlusal view, UCMP 197615 lacks the mesial tapering of the crown of m1 characteristic of M. seminoensis. None of the other teeth from Black Butte Station appears to be referable to M. seminoensis.
Exposures of the Ferris Formation west and north of Seminoe Reservoir on the North Platte River have yielded extensive collections of Lancian and Puercan mammals (Eberle and Lillegraven, 1998a, b; Lillegraven and Eberle, 1999). Among them is the Lancian Meniscoessus seminoensis. UCMP 213000, here identified as M. cf. M. robustus, is the first and only mammalian tooth to be discovered and described from the Ferris Formation in the Hanna or Carbon basins east of the North Platte River. Some 200 km to the west, M. robustus is a member of the Lancian Black Butte Station Local Fauna. These three occurrences of Meniscoessus are at approximately the same southern latitude. The duration of the Lancian NALMA was at least 1.8 million years (Wilson, 2005). At the species level, the taxonomic diversity of these occurrences probably refects some combination of differences in age within the Lancian and paleoenvironmental settings.
Development of a Geologic Map
Confirmation of key formational identifications has aided development of a geologic map (Fig. 4; scale 1:24,000) covering this highly deformed part of the eastern Hanna Basin. Almost all of the extraordinary folding and faulting within the mapped area represents contractional tectonics that occurred late in the ‘Laramide’ episode (Snoke, 1993) of continental orogenesis. It persisted well into the Eocene epoch. Most faults, folded structures, and measured attitudes of strata indicated on Figure 4 appear for the first time here on any map.
The geographic area covered in Figure 4 is extracted from the most northwesterly of three geologic maps nearing completion by Lillegraven (see inset, digital-elevation model in Fig. 5). The map was constructed by Lillegraven using Adobe Illustrator (Creative Suite 5) software. Data were gathered using standard field methodologies involving lateral tracing of distinctive stratigraphic horizons, measuring bed attitudes with a Brunton pocket transit, and effecting local excavations to determine natures of diverse stratigraphic contacts. One may request from Lillegraven brief lithologic descriptions from any given measurement station (by station number).
Development of Interpretive Geologic Cross Sections and Stratigraphic Columns
The geologic map (Fig. 4) shows straight-line transects for two interpretive cross sections (presented in Fig. 5, R′–S′ and T′–U′, upper right-hand part of figure). The interpretive cross sections are shallow, arbitrarily extending downward only to 4,500 feet above present sea level. The small yellow star in cross section T′–U′ shows the approximate stratigraphic horizon that yielded the Meniscoessus molar (UCMP 213000) described above. We also developed composite stratigraphic columns in Figure 5 from measurements taken from the two cross sections. See caption for Figure 5 for an explanation of estimated losses of original stratigraphic thicknesses resulting from faulting.
Development of Model for Late Laramide Evolution of Northern Hanna Basin
We present as Figure 6 a two-step, interpretive cross-sectional model for late Laramide evolution of the northern Hanna Basin. It involves the entire Phanerozoic section down to Archean basement rocks. The model is constrained by measurements derived from cross section R′–S′ (of Figs. 4 and 5). The model presents what we consider to be minimal estimates of tectonically and erosionally lost strata needed to explain the remnants observed today along the transect of that cross section. We provide further explanation in the caption to Figure 6.
Introduction to Essential Geological Relationships
The local geological picture is dominated by occurrence of at least 10 kilometers of Late Cretaceous, Paleocene, and early Eocene elevation (structural relief) of granitic rocks forming the core of the highly asymmetrical, Shirley Mountains Anticline (Fig. 4; Lillegraven and Snoke, 1996, figs. 4 and 26). The axis of that anticlinal fold, dominated by effects of multiple phases of pervasive faulting and folding upon granitic basement rocks of Archean age, trends roughly NNE–SSW (Lillegraven and Snoke, 1996, Fig. 4). The fold's southwestern flank (in sec. 6, T. 24 N., R. 82 W. and northward) exhibits markedly overturned, faulted, and attenuated upper Paleozoic and Mesozoic strata (Bergh and Snoke, 1992; Lillegraven and Snoke, 1996, figs. 4, 26, 31, and 33). The anticline's southeastern flank (in sec. 2, T. 24 N., R. 82 W. and eastward) exhibits a remarkable sequence of generally well-exposed, less-deformed, more shallowly east-dipping Paleozoic through Lower Cretaceous strata. The Shirley Mountains' anticlinal axis is transversely terminated (essentially east–west) to the south by the exposed Shirley Fault. It is a basement-involved thrust fault having a plane that dips steeply northward (see Bergh and Snoke, 1992, Fig. 2). The exposed trace of the Shirley Fault sharply cuts the Cretaceous (and possibly older, covered units) stratigraphic sequence of the northernmost Hanna Basin and structurally defines the basin's post-Laramide northern extreme. As discussed below in relation to Figure 7, however, we suggest that the Hanna Basin, until late in Laramide orogenesis, had a more extensive northern existence in the areas now occupied by the Freezeout Hills and southern Shirley Mountains (Fig. 1B).
Observed dips of pre-Oligocene strata indicate that completion of tectonic uplift to the landscape's surface of granitic masses forming the core of the Shirley Mountains postdated deposition of almost the entire stratigraphic sequence now exposed along this part of the northern Hanna Basin. The only known exception is presumedly post-orogenic, rhyolitic ash-flows and volcanically derived reworked strata comprising Chalk Bluff (Fig. 4, in sec. 11, T. 24 N., R. 82 W.; its radioisotopic age is under study by K. R. Chamberlain, M. T. Heizler, J. M. Cottle, and Lillegraven). In other words, sedimentary accumulation in an originally more extensive northern Hanna Basin, even as late as the early Eocene, persisted well north of the basin's present, sharply defined tectonic boundary. Reflecting structural completion of the Shirley Mountains Anticline, originally flat-lying strata as young as early Paleocene (post-Puercan) of the northern Hanna Basin south of the Shirley Fault became steeply bent into nearly vertical or even dramatically overturned orientations.
Note on Figure 4 and in cross section R′–S′ of Figure 5 that steep to overturned southward dips in basinal strata exist as much as six kilometers southward from the exposed Shirley Fault. Notice, too, that strata of this part of the northern Hanna Basin are complexly cut by numerous thrust faults, almost all of which exhibit out-of-the-basin (in this case, originally north-vergent) displacements. Northward vergence of the relatively shallow (‘thin-skinned’), out-of-the-basin Laramide thrusting opposed south-directed, basement-involved, deep-crustal thrusting and folding of the Shirley Mountains block (see SW ¼ of sec. 2, T. 24 N., R. 82 W.). As considered in Figure 6, the out-of-the-basin thrusts probably developed in passive response to spatial crowding imposed by the enormously larger, exposed Shirley Fault. We postulate existence of a basement-involved thrust complex linked at depth within a ‘trishear triangle zone’ (sensu Erslev, 1991) between the exposed Shirley Fault and an associated blind thrust or more broadly distributed zone of penetrative shear.
Figure 6 develops a hypothesis in which cross-sectional stratigraphic relationships along transect R′–S′ observable through field-based mapping today (cross section B; shallow parts of section derived from Fig. 5) are projected to depths involving basement rocks. Cross section A interpretively modifies cross section R′–S′ and adjacent parts of the ancestral Shirley Mountains block into configurations they might have assumed late in the early Eocene, well before completion of local Laramide orogenesis. Although this transitional stage was intentionally visualized as the simplest of many possible alternatives, there exist no publicly accessible seismic- or drilling-based data with which to test the hypothesis. The semi-transparent presentation of rock-unit colors in cross section A suggests the magnitude of subsequent erosion necessary to attain the condition seen today (in cross section B). The small yellow stars shown in both cross sections (and Figs. 4 and 5) suggest approximately correlative stratigraphic positions from which the molar of Meniscoessus (UCMP 213000) was recovered (in transect T′–U′).
Readers should not assume that we claim accuracy or ‘truth’ through stratigraphic and structural relationships presented in Figure 6. We do suggest, nevertheless, that the presented relationships of depth and erosion are defensibly reasonable. The complexity of faulting, depth relationships, and extensiveness of post-Laramide erosion all would be expected to exceed what is shown in Figure 6, both in cross sections B and A.
Existence of complex, stratigraphic shuffling via out-of-the-basin, mostly bedding-parallel thrust faulting has been appreciated for decades within this restricted part of the northern Hanna Basin (e.g., Blackstone, 1983 and Lillegraven and Snoke, 1996, p. 37–42). But we present here, for the first time, a geologic map and representative cross sections scaled at 1:24,000 with much additional detail of positions of fault traces and orientations of bedding planes. The cross sections contain adequate detail to allow field testing of our hypothesis that the thrust faults, although having mostly bedding-parallel planes, regularly and preferentially cut stratigraphically down-section. Thereby, younger strata of the hanging walls have become emplaced onto older strata of the footwalls (also see Lillegraven et al., 2004, figs. 17 and 18 and Lillegraven, 2009, compare figs. 29 and 30).
The following elements of this paper highlight relevant details to be seen on Figures 4–6. To aid in gaining a summarized view, Figure 7 provides, in cartoon format, key tectonic, depositional, and erosional events that extend from early in the Laramide orogeny to present time.
Transect T′–U′ holds the collecting site for the Meniscoessus molar (UCMP 213000). Due to prominent faulting, however, that transect's stratigraphic completeness through parts of the column most relevant to this research is much less than along transect R′–S′. For comparisons of represented rock units, see the mid-region of Figure 5.
Notice in south-central parts of section 2, T. 24 N., R. 82 W. that, with a single step, one could move across the exposed Shirley Fault from sandstones of the Upper Cretaceous Medicine Bow Formation (deposited essentially at sea level) to Archean granite (crystallized 10–15 km below ancient Earth's surface). At that structural discontinuity, one may observe the effects of opposing directions of Laramide thrust faulting. South-directed faulting involves a massive and deep-seated granitic hanging wall above the basement-involved, exposed Shirley Fault. Originally north-directed, thin-skinned, out-of-the-basin thrust faulting, in contrast, exhibits secondary deformation in being steeply south-dipping or overturned. Rocks below that junction of oppositely vergent tectonism (shown in Fig. 6B) would seem to represent a ‘trishear triangle zone’ as concieved by Erslev (1991). The gross shape of the triangle zone and its individual components of rock-unit thicknesses, however, differ markedly from what is shown as the much earlier stage of deformation in Figure 6A.
That kind of evolution within the substance of the triangle zone would seem to be dependent upon existence of a deep-seated splay from the main basement-involved thrust that extends into the basin's deep stratigraphic levels (e.g., see Lillegraven et al., 2004, Fig. 10). Alternatively, rather than a discrete blind thrust involving brittle rocks, a deep zone of ductile and more broadly distributed penetrative shear could allow similar deformation within the triangle zone. In Figure 7, we employ the graphic convention of a discrete line to represent either phenomenon. Do recognize, however, that in the Hanna Basin no actual seismic-, well-, or outcrop-based evidence confirms the reality of this hypothetical construct.
The landscape nearby transect R′–S′ north of Horseshoe Ridge (northern parts of secs. 8–9, T. 24 N., R. 82 W.) to the exposed Shirley Fault is almost devoid of useful outcrops. Thus the many dashed lines and queries on Figure 4 suggesting uncertainties in positions of formational boundaries are deserved. Attitudinal measurements taken at two well-separated stations west of that transect, however, suggest the entire stratigraphic sequence is overturned. Northernmost involved strata also would be expected to have been severely sheared during uplift of the Shirley Mountains block.
Farther south along transect R′–S′, involving adjacent halves of sections 8 and 9 (of T. 24 N., R. 82 W.), is a most unusual, overturned syncline. With its outcrop area named ‘Card-tricks Hill’ and described by Lillegraven and Snoke (1996, p. 38–41, figs. 28–30), basal parts of the Medicine Bow Formation are severely shattered throughout. Despite the ubiquity of disruption of contained bedding planes, the hill can be seen to retain multiple, very tight folds that we interpret as ‘off-scrapings’ from a rolled-over sequence at the base of a hanging wall of an initially north-vergent, out-of-the-basin thrust. The thrust itself, which cut stratigraphically down-section, served to place lower components of the Medicine Bow Formation onto stratigraphically still-lower parts of the underlying Lewis Shale (see Lillegraven and Snoke, 1996, figs. 28–30C). Fold geometries seen throughout this part of the northern basin require existence of major magnitudes and great complexity of strain for their explanation.
The amount of landscape shortening resulting from that kind of north-vergent, out-of-the-basin thrusting is considerably greater along the more easterly transect, T′–U′. As interpreted in central parts of Figure 5, all of the Mesaverde Group and Lewis Shale, as well as most of the Medicine Bow Formation, were tectonically cut away, resulting in faulted placement of upper parts of the Medicine Bow Formation directly upon lower components of the Steele Shale. The most basic result was placement of younger strata onto markedly older strata. Only a relatively thin remnant of the Meniscoessus-molar-bearing Ferris Formation (sequestered between similar out-of-the-basin faults at top and bottom) is preserved along transect T′–U′.
As interpreted on Figure 4, the surface trace of the eastern extreme of the exposed, south-vergent, basement-involved Shirley Fault strikes due eastward from the south end of outcrops of the Pennsylvanian Tensleep Sandstone immediately north of Chalk Bluff (near south-central base of SE ¼ sec. 2, T. 24 N., R. 82 W.). The plane of that thrust dips northward at about 75° (see cover photo and Lillegraven and Snoke, 1996, Fig. 23). We raise the issue of the fault's position because previous mapping commonly shows the Shirley Fault projecting across the land surface southeastward from north of Chalk Bluff through section 18 of T. 24 N., R. 81 W. and beyond (e.g., Knight, 1951, Fig. 3; Blackstone, 1993a, Fig. 3; and Lillegraven, 1994, Fig. 3). Recent field searches there, however, provide no hint to existence of south-vergent faulting along that more southeasterly, previously assumed course.
The cross sections presented following transects R′–S′ and T′–U′ are notable in the highly variable orientations of included strata. The sequence of steeply tilted to overturned strata continues southward for nearly four miles from the exposed Shirley Fault. Nevertheless, as proposed through introduction of the blue bars in middle parts of Figure 5, much of the original stratigraphic content along both transects has been excised by several out-of-the-basin faults. All faults illustrated share the trait of having cut markedly down-section stratigraphically. Much more of the original rock content was lost by that means along transect T′–U′ than along transect R′–S′. The importance of the Meniscoessus molar (UCMP 213000) as a biostratigraphic marker is best appreciated when its insular position within a sea of expected yet missing younger and older strata is recognized.
Starting with the relatively shallow (mostly less than mile-deep), interpretive cross sections presented in Figure 5, we attempted to develop explanatory models for geologically earlier parts of their structural evolution at deeper levels. Figure 6 considers the conformation of cross section R′–S′ extended to depths of almost 14 kilometers, to slightly below contact with the Archean basement. Because transect R′–S′ is the more complete section, we chose it for use in this exercise in preference to transect T′–U′. Figure 7 explores a five-step, structural evolutionary scenario across a 160 kilometer-long transect connecting the southeastern reentrant of the Wind River Basin with the Hanna Basin.
To our knowledge, the only publicly available seismic-reflection profiles for the northern Hanna Basin were by Kaplan and Skeen (1985) and Sacrison (1978, Fig. 16). Their levels of detail, however, are inadequate for effective use toward present purposes. Similarly, we know of no well logs from this part of the basin deep enough to constrain depths or orientations of stratigraphic markers. Our approach in constructing Figure 6 was to start with cross section B, representing Holocene time. We emphasized visualization of minimally complex, cross-sectional models that would be compatible with newly available mensural data from our field investigations. Such information includes formational thicknesses and orientations of stratigraphic units as they occur exposed in diverse parts of the basin. Construction of cross section A was more difficult in that it attempts to represent an interval late in the early Eocene. That interval was well before local termination of deformation related to Laramide orogenesis. Even though development of section A was the more challenging, we applied the same philosophical approach to modeling through emphasis on use of constraining measurements gained from original fieldwork.
Principal modifications from cross section B that we built into cross section A of Figure 6 include: (1) shallower southward dips of the entire Phanerozoic column; (2) retro-deformational northward repositioning of Archean basement of the future Shirley Mountains perpendicular to the Shirley Fault's main surface trace; (3) shallowing of northward dip of the exposed Shirley Fault's plane; (4) recognition of components of the Phanerozoic sedimentary cover (including possibility of some unknown thickness of Hanna Fm.) that had to have been eroded from the progressively elevating granitic nose of the Shirley Mountains block prior to its attainment of the modern configuration; and (5) visualizing less complete stages of north-vergent, out-of-the-basin thrusting than are suggested in cross section B. Although we defend the reality of each of those elements of alteration in the evolution of cross section A to B, their actual magnitudes and orientations certainly would have differed from our conceptual model in light of its intentionally conservative basis. We initially attempted to construct a still-earlier evolutionary stage in which the stratigraphic pile in cross section A more closely approached its original depositionally horizontal orientation. But we found the degrees of probable variation for each of the above-listed variables to be too daunting. In short, any proposed picture, prior to gaining reliable evidence for provision of reasonable constraints, would need to be viewed as guesswork.
Notice in cross section A of Figure 6 that the upper third of our diagram exhibits reduced opacity in colors of the rock units. The pattern of greater transparency denotes parts of the rock assemblages that had to have been eroded away prior to Holocene time (as shown in cross section B). Most of that erosion would have been through subaerial, fluvial processes. Also, the principal erosion occurred during late stages of the Laramide orogeny, with the remainder having occurred from the latest Miocene into the present day. Deposition of thick layers of airfall volcanic ash from distant sources greatly reduced erosion of Laramide and older strata across Wyoming basins beginning in the latest Eocene and continuing until late in Miocene time (Fig. 7D). Remnants of that post-Laramide, latest Eocene–Miocene volcanic inundation exist in vicinity of the northern Hanna Basin as Chalk Bluff (Fig. 4, sec. 11, T. 24 N., R. 82 W.) itself and as plastered onto scattered patches across southern flanks of the Shirley Mountains (see Lillegraven and Snoke, 1996, Fig. 4, sec. 35, T. 25 N., R. 82 W.).
Discussion of More Specific Elements of Northern Hanna Basin
Original Northern Extent of Hanna Basin
We now return to a closer focus on several aspects of the Hanna Basin's northern margin. First, we suggest that sources of erosional detritus contributing to accumulation of known Paleocene and early Eocene clastic strata in the Hanna Basin were not dominantly the uplands now adjacent to the basin's northern border (Burris, 2001; Lillegraven et al., 2004, Fig. 14). Instead, most of the locally derived clastic debris within the basin came from multiple sources to the north of those uplands, from deeply eroding, mountainous landscapes that existed during latest Cretaceous through Eocene time (see Fig. 7B and C). We are referring to the former mountain range designated by Love (1970) as the ‘Granite Mountains’ (the core of what many other authors refer to as the ‘Sweetwater Arch’; Fig. 1B). At least southern parts of the present Shirley Mountains (Fig. 7C) and probably all of the Freezeout Hills had become covered by strata continuous to the south with now-eroded and markedly up-turned edges of the basin's Paleocene through early Eocene section. Lillegraven et al. (2004, p. 48–52 and Fig. 15) described and graphically illustrated their view of the original (Laramide) dimensions and paleogeographic/stratigraphic relationships of the northern Hanna Basin. Subsequently, those largely beveled-to-the-core highlands became mostly buried under Neogene sedimentary/volcaniclastic blankets, followed by extensional collapse as the Granite Mountains Graben (Fig. 7D and E; Love, 1970, pls. 9 and 10J).
Previously Unrecognized Structures and Their Salient Features
Almost all of the northeastern Hanna Basin's fault traces and fold structures presented in Figure 4 are new to this publication. Discovery and biostratigraphic analysis of the molar of Meniscoessus cf. M. robustus (UCMP 213000) helped a great deal in confirming field-based identifications of locally poorly exposed units. Recognition of Lancian age of the site itself provided an important constraint to biochronology of the local section. With exception of a few points along southeastern extremes of the Shirley Mountains, all stratigraphic attitudinal data are new.
The geologic map (Fig. 4) is dominated by juxtaposition of the deeply rooted, south-vergent thrusts of the exposed Shirley Fault against the largely bedding-parallel, north-vergent, out-of-the-basin thrusts newly traced along the northern Hanna Basin. Easternmost surface expression of the exposed Shirley Fault is now known to be immediately south of the Smith Creek Anticline, near the southeastern corner of section 1, T. 24 N., R. 82 W. No field-based evidence suggests validity of its oft-plotted (e.g., Blackstone, 1993b) southeasterly projection for several miles southeast of Chalk Bluff.
Notice the area of the northern Hanna Basin (on Fig. 4) abutted against southwesternmost outcrops of basement rocks comprising the Shirley Mountains Anticline. That area exhibits a paucity of attitudinal data combined with a large number of queries (?) due to lack of exposures. Upon massive excavation, however, we would expect to see extraordinary levels of shearing of those soft sedimentry units as results of uplift along the exposed Shirley Fault.
Almost universally as interpreted across this map, the north-vergent, out-of-the-basin thrusting resulted in placement of (younger) hanging-wall strata onto (older) footwall strata. Preparation of geologic maps by Lillegraven that cover margins of the entire eastern half of the Hanna Basin indicate that this ‘younger-on-older’ faulting relationship is the general rule. Geometry and mechanics of how such unexpected relationships might have come to be were approached by Lillegraven et al. (2004, p. 37–43 and figs. 17–18) and Lillegraven (2009, figs. 9–11 and 29–30). Achieving a ‘younger-on-older’ relationship requires the plane of the thrust to cut stratigraphically down-section. Because of constraints related to physical crowding, that situation becomes mechanically probable only when affected strata (including the plane of the fault itself) are tilted uphill in the direction of thrust vergence. The hanging wall under those conditions usually would result in a shallower dip than the footwall along the zone of stratigraphic downcutting of the thrust. Both proximally and distally to that zone of the fault's stratigraphic downcutting, however, translation along the fault would be bedding-parallel and thus unlikely to be recognized at all during the process of routine field mapping.
We predict that these out-of-the-basin, younger-on-older thrust relationships — juxtaposed against oppositely vergent, basement-involved thrust-fault systems — eventually will be recognized as common expressions along basin margins throughout the Rocky Mountain province. Even now, some are documented in the northwestern Bighorn Basin (Lillegraven, 2009), and others have been recognized through excavations in central Wyoming's eastern Wind River Basin (against western edges of the Casper Arch; Lillegraven, personal observations) as well as along the southeastern border of southern Wyoming's Great Divide Basin (against western flanks of the Rawlins Uplift; unpublished studies by the late James G. Honey).
Furthermore, we propose that several occurrences of this phenomenon have been long misinterpreted as representing depositional angular unconformities (e.g., see Lillegraven, 2004, Fig. 5 and Lillegraven, 2009, Fig. 19A). Along the segments of discordant stratigraphic attitudes, the two very different phenomena would look just the same under usual field conditions involving substantial outcrop masking by slopewash. Differentiating fault-originated discordance from deposition on angularly unconformable surfaces depends upon thorough excavation and study of the intervening contact (see Lillegraven, 2009, p. 48–57, 68–71, and table 1). Following excavation, close examination is required to recognize presence/absence of weathering zones, fault imbrications, gouge zones, fault breccias, slickensides, or other clues.
Stratigraphically downcutting thrust planes are seen to pay little attention to formational boundaries. So, in addition to landscape shortening (expected through all forms of thrust faulting), entire packages of formations may be excised from the resulting rock record (e.g., in Fig. 5, cross section T′–U′, note absence of Lewis Sh. and all of the normally underlying Mesaverde Gp.). Our presentation in the middle area of Figure 5 (developed using interpretation of losses of section resulting from younger-on-older thrusting) intends to emphasize distinctions between: (1) end results seen following development of multiple faults (possibly from but a single, prolonged event of contractional stress); and (2) oft-repeated cycles of subsequent deposition onto new erosional surfaces (i.e., progressive unconformities). Needless to say, interpretations of geologic history as compared between those two scenarios differ profoundly.
Attempting to Visualize Evolution of Entire Phanerozoic Column
In this part of south-central Wyoming, very little relevant seismic or deep-well data are available. Thus little useful information is at hand about present configurations of strata or basement rocks within deeper parts of the Hanna Basin. Nonetheless, the very nature of modern surface geology does provide important constraints (and insight to aid in recognition of virtual impossibilities) on deeper elements of the picture. Indeed, establishing reasonable limits on modern configurations of the deep basin using data gained from the basin's margins we consider to be a highly worthwhile exercise. We applied that approach to development of Figure 6B. Once established, we then attempted, as a hypothesis to guide future testing, to visualize conditions representing late early-Eocene time (Fig. 6A). However, because of the unconstrained number and range of possible variables, we saw little merit in presentation of detailed, still-older, hypothetical configurations.
Pursuit of the kind of exercise presented in Figure 6 has the advantage of forcing consideration of the magnitude of erosion (between then and now) required to explain the observed nature of today's rock sequence. The probable extent of Laramide and post-Laramide erosion (as proposed in the partially transparent parts of the cross section) is a central component of Figure 6A. The vast loss of strata from the Hanna Formation itself (and continuing stratigraphically downward to the very base of the Phanerozoic column) essentially demands earlier continuation of the Hanna Basin section northward, across the presently exposed transect of the Shirley Fault, onto what today is the deeply eroded granitic mass of the southern Shirley Mountains. The Late Cretaceous tooth of Meniscoessus cf. M. robustus (UCMP 213000, preserved from a once air-breathing mammal) was collected on the present land's surface (at yellow star on Fig. 6B). During latter parts of early Eocene time, however, the already long-fossilized tooth probably lay buried at a depth of more than five kilometers (at yellow star in Fig. 6A).
Figures 6A and 7 suggest that the plane of the exposed Shirley Fault had a significantly shallower northward dip in Eocene time than today. The modern dip is directly measurable in the field at about 75° N. Figures 6 and 7 also suggest that the exposed Shirley Fault itself must represent a relatively minor splay emerging from an essential but as yet scientifically undocumented, comparatively enormous, shallowly north-dipping blind thrust or perhaps a broader, more ductile zone of penetrative shear. That currently hypothetical level of displacement was at a depth of at least 13 kilometers below the modern surface, and it had a southward vergence, trending into the evolving Hanna Basin (see Lillegraven et al., 2004, Fig. 10, following the ‘trishear fault-propagation’ model of Erslev, 1991). The triangular wedge of basement rocks sequestered between the hypothesized blind thrust and the exposed Shirley Fault (Figs. 6A and 7B–E) must, itself, have been pervasively penetrated by faulting and tight folding. As a result, it became progressively molded in cross-sectional shape through time (especially obvious in Fig. 7B–D). The deformation probably extended upward into the sedimentary column to make present geologic configurations (Figs. 6B and 7D–E) possible. Hints to the reality of such relatively small-offset, folded-basement deformation are presented in the pioneering seismic-reflection images developed by Kaplan and Skeen (1985) and Sacrison (1978, Fig. 16).
Ancient Sea Levels
How do the above-estimated elevations relative to the surface of the land (ancient and modern) compare to elevations relative to sea levels of the time? Meniscoessus unquestionably was a dry-land mammal. Sound geological evidence exists, however, to show that during Lancian time accumulation of the Ferris Formation in what is now the Hanna Basin occurred very close to sea level. Compensatory crustal subsidence served to approximately match rates of deposition of the > 2.5 km-thick Ferris Formation. Definite shallow marine beds exist within most of the underlying Medicine Bow Formation across the southeastern two-thirds of the Hanna Basin. Marine conditions also persisted across western parts of the Great Plains through the remainder of Cretaceous time, continuing through the first five million years of the Paleocene (Johnson, 2002).
Boyd and Lillegraven (2011a, b) reviewed what is known about the history of latest Cretaceous–early Paleocene seaway persistence in vicinity of the Hanna Basin. They also pursued possibilities for direct marine connections between the Hanna Basin and lingering marine waters across the western Great Plains. Those authors concluded that sound evidence for tidal influences upon ancient river systems preserved in Paleocene parts of the Ferris and lower Hanna Formations is common. Genuine sub-tidal marine inundations of the Hanna Basin were uncommon, however, and such events were geologically ephemeral. Nevertheless, the possibility for marine inundations did persist in vicinity of the Hanna Basin significantly longer than was considered by Lillegraven and Ostresh (1990).
Spreite burrows referable to the typically sub-tidal ichnogenus Rhizocorallium do occur on a small peninsula of land (‘Pat's Bottom’) projecting into the northern Hanna Basin's Seminoe Reservoir. Host strata are within the uppermost third of the originally mapped (by Dobbin et al., 1929, pl. 27) Ferris Formation and overlie very late parts of the Puercan NALMA (Eberle and Lillegraven, 1998a; early Paleocene, ca. 64.52 ± 0.02 Ma, Clemens and Wilson, 2009, Fig. 2, p. 112). As judged by Boyd and Lillegraven (2011a, p. 64): “This top third of the formation could be latest Puercan and/or early Torrejonian in age, recognizing perhaps two million years of temporal uncertainty.” Without question, however, this brief pulse of marine transgression into the Hanna Basin is recorded from strata more than 700 m stratigraphically higher than the paleontologically recognized, local Cretaceous–Tertiary boundary (above which remains of Meniscoessus have not been discovered).
What's in a Formation?
Boyd and Lillegraven (2011a, p. 46–48 and 2011b) summarized the history of nomenclature relevant to original designations of the Ferris and Hanna Formations. Their summary included recognition of the transect along which geographic extent of the Ferris Formation was originally established. The plotted contact between the Ferris and overlying Hanna Formations (near the western edge of sec. 28, T. 23 N., R. 83 W.; Dobbin et al., 1929, pl. 27) is at least 640 m stratigraphically above the Rhizocorallium-bearing mudstones on Pat's Bottom. The beds from slightly west of Pat's Bottom to the top of the Ferris Formation are complexly channeled, exhibiting repetitive cut-and-fill sequences. Thus the depositional regime differed markedly from the lower two-thirds of the type Ferris Formation, which exhibit a striking, consistently aggradational history (Lillegraven and Eberle, 1999, p. 693). Almost certainly, therefore, the Ferris–Hanna formational contact would have developed in its type area well into early Paleocene time. The formational contact probably developed during early parts of the Torrejonian NALMA, which began at roughly 64.5 Ma (see: Lofgren et al., 2004, p. 46, Fig. 3-2; and Clemens and Wilson, 2009, p. 112, Fig. 2). We express significance of that assertion in the following paragraphs.
Hajek (2009) initiated a process-oriented, stratigraphic research project in the northeastern Hanna Basin. Her study involved cluster analyses of fluvial architecture observed among nearly vertical, reasonably well-exposed bodies of channel sandstone (and their lateral avulsion sequences), all enveloped within fine-grained overbank deposits. Part of the eastern half of Hajek's study area overlaps the western border of our geologic map (Fig. 4). Data reported from her study are confined within a west–east-elongated rectangle (with dimensions of ca. 1700 × 450 m) having the following corner points: NW corner = 42° 2′ 37″ N, 106° 41′ 52″ W; and SE corner = 42° 2′ 23″ N, 106° 40′ 38″ W (Hajek, 2009, appendix A, p. 145). On the ‘Schneider Ridge, Wyo.’ 7.5-minute topographic quadrangle, those coordinates occupy: slivers of the southern edges of sections 18 and 13, respectively, of T. 24 N., R. 82 and 83 W. along with >300 m of the northern borders of adjacent sections 19 and 24. Hajek (2009, p. 1 of abstract) identified the rock unit under study as “Ferris Formation (Upper Cretaceous/Paleogene…)”.
That doctoral dissertation subsequently led to a series of co-authored publications (e.g., Hajek et al., 2010, 2012, and Wang et al., 2011). Each of those papers attempted to refine spatial organization of the channel bodies to determine either their randomness or tendencies toward clustering. Also, each paper accepted identification of the strata under investigation as ‘Ferris Formation’ under the following justification (Hajek et al., 2012, p. 1899–1900):
“Based simply on the stratigraphic thickness of the unit in the study area compared to that of the well-defined biostratigraphic zones of Eberle and Lillegraven (1998a, 1998b), located ∼18 km southwest of this study area, the interval of Ferris Formation focused on in this study (∼1000 m above the Medicine Bow–Ferris contact) may lie within the Puercan biozone PU3 (ca. 64 Ma).”
For reasons discussed earlier in our paper in relation to loss of section coming from younger-on-older, out-of-the-basin thrusting, we urge caution in using simple stratigraphic thicknesses as a tool for correlation. Our caution would be especially strong within tectonically complex geologic settings such as the northern Hanna Basin.
We challenge assignment of those strata to the Ferris Formation. Instead, for two additional reasons, we suggest that use of ‘Hanna Formation’ would be more appropriate. First, notice on the 1929 “Geologic Map and Sections of the Hanna and Carbon basins, Wyoming” presented by Dobbin et al. that the east–west-trending Ferris–Hanna formational boundary was drawn (applying a solid contact line) along the southernmost one-sixth of sections 18 and 13, respectively, of T. 24 N., R. 82 and 83 W. All of Hajek's data, therefore, derived from basal strata of the Hanna Formation as it was originally mapped.
Secondly, with the use of color aerial photographs, we traced the laterally changing strike of outcrop sequences from Hajek's study area first to the southwest and then southwardly. That led to the above-mentioned formational boundary mapped by Dobbin et al. (1929, pl. 27) near the western border of section 28, T. 23 N., R. 83 W. For those reasons, we had followed the original spatial positioning of the Ferris–Hanna contact during development of Figure 4.
In the northeastern Hanna Basin the rock characteristics stratigraphically both below and above Hajek's study area conform closely with lithologic data provided by Hajek et al. (2012, p. 1910) in their figure 10. Thus the richly arkosic sandstone bodies that dominate the landscape both below and above their study area do not lend themselves to confident recognition of a formational distinction. Geologic ‘formations,’ of course, are defined and distinguished using lithologic criteria. So, from one perspective, it is almost irrelevant in this part of the northeastern Hanna Basin whether the name ‘Ferris’ or ‘Hanna’ is applied to the local outcrops. Any distinctions in lithologic composition in the ‘type’ area are not recognizable in this more northerly part of the basin.
The general stratigraphic levels of particular outcrops, however, truly do matter. Any stratigraphic level is directly related to deposition during a specific geologic time. Bed-tracing laterally in this case clearly shows that the Hajek study section is not a stratigraphic equivalent of the Puercan (PU3) section documented by Eberle and Lillegraven (1998a). Rather, the Hajek study section was deposited a good deal more recently within Paleocene time, most probably during the Torrejonian NALMA. That presumed age, although not yet confirmed in this immediate part of the Hanna Basin by study of biostratigraphically diagnostic mammalian fossils, would post-date the youngest known, Rhizocorallium-indicated marine incursion into the Hanna Basin (documented by Boyd and Lillegraven, 2011a, b) by the better part of a million years. More specifically, the Hajek section appears to be correlative with strata in the lowest 1.3 kilometers of the complexly deformed Hanna Formation in ‘The Breaks’ (Fig. 1B; a badlands area 27 km to ESE) as studied by Higgins (2003, figs. 2–6 and 9).
1. Discovery and collection of specimen UCMP 213000 (isolated lower first molar; Mammalia, Multituberculata, Cimolomyidae) represents the first and only mammalian tooth recorded in publications about the Ferris Formation in the Hanna or Carbon basins east of the North Platte River. Species-level identification remains uncertain, but Meniscoessus cf. M. robustus is a member of the morphologically derived M. robustus species group of Flynn (1986). The specimen is biostratigraphically useful. It confirms, for purposes of detailed mapping, the correlative nature of its host strata with latest Cretaceous (Lancian NALMA) parts of the Ferris Formation as seen roughly 17 kilometers to the west-southwest in its originally recognized, much more fossiliferous section. The tooth-bearing strata are tectonically overturned and bounded below and above by originally north-vergent thrust faults. Deposition of the Ferris Formation occurred very close to latest Cretaceous and earliest Paleocene sea level.
2. The Hanna Basin, until late in Laramide orogenesis, had a more extensive northern existence within areas now occupied by the Freezeout Hills and southern Shirley Mountains. Sedimentary accumulation in an originally more expansive northern Hanna Basin, even as late as the early Eocene, persisted well north of the basin's present, tectonically sharply defined northern boundary. Those strata were continuous to the south with now-eroded and markedly up-turned to overturned edges of the northern basin's Paleocene through early Eocene section.
3. Sources of erosional debris contributing to accumulation of known Paleocene and early Eocene clastic strata in the northern Hanna Basin would have derived only in a minor way from the scouring of uplands now immediately adjacent to the basin's northern border. Most of the locally derived clastic deposits in the northern basin would have come from multiple sources to the north of those uplands, from deeply eroding, mountainous landscapes (the Granite Mountains) that existed during latest Cretaceous through Eocene time.
4. 10 to 15 kilometers of basement-involved, structural relief was required to reach the Paleogene landscape's surface as developed from thrust faulting. South-vergent uplift of granitic masses of the Shirley Mountains' core was not completed until after deposition of almost the entire stratigraphic sequence now exposed along the northern Hanna Basin.
5. North-vergent, thin-skinned, out-of-the-basin thrusts developed in passive response to spatial crowding initiated by south-vergence of the enormously larger, basement-involved thrust complex known as the Shirley Fault. The structurally detached faulting that exists within the severely deformed stratigraphic column of the northern Hanna Basin is laterally extensive. Also, the basin-margin strata behaved more-or-less independently from the uplifted mountains north of the exposed Shirley Fault. Therefore, one observes little continuity between thin-skinned structural features in the northern basin's strata and the dramatic, basement-involved anticlinal folding directly to the north.
6. The observed out-of-the-basin thrust faults, although having mostly bedding-parallel planes of relative displacement, regularly and preferentially cut stratigraphically down-section, thereby placing younger strata of the hanging walls onto older strata of the footwalls. The faulting thereby led to greatly thinned stratigraphic sections when juxtaposed against basin-margin, oppositely vergent mountainous uplifts. This ‘younger-on-older’ faulting is the general rule all around the eastern Hanna Basin. We suggest it to be a little-recognized but common phenomenon throughout the Rocky Mountain province. Several other occurrences distributed across Wyoming may well have been long-misinterpreted as erosional/depositional angular unconformities. Wherever that has been the case, profound re-thinking of the areas' geological histories would be required.
7. Prodigious amounts of basin-margin erosion occurred along the northern Hanna Basin during late stages of the Laramide orogeny itself. The erosion affected the entire, strongly upturned Phanerozoic stratigraphic column. Following effective burial under latest Eocene through Miocene volcaniclastic airfall materials of extra-basinal origins, the remainder of erosion occurred from latest Miocene time to the present.
8. We agree that the total duration of Laramide orogenesis extended from Late Cretaceous well into Eocene time (ca. 70–80 possibly through 40 Ma). However, the most important basement-involved deformation across the Wyoming Province in terms of basin subdivision occurred very late within that interval.
DIGITAL ATTACHMENTS (as pdf files):
Figure 4 (geologic map). Available for download at https://geobookstore.uwyo.edu/sites/default/files/download/rmg-48-2/clemens-lillegraven-fig-4.pdf. Also available at rmg.geoscienceworld.org in the Fall, 2013 issue (see URL for the present article in RMG's v. 49, no. 1 when published (Spring, 2014).
Figure 5 (two geologic cross sections and stratigraphic columns). Available for download at https://geobookstore.uwyo.edu/sites/default/files/download/rmg-48-2/clemens-lillegraven-fig-5.pdf. Also available at rmg.geoscienceworld.org in the Fall, 2013 issue (see URL for the present article in RMG's v. 49, no. 1 when published (Spring, 2014).
Figure 6 (cross-sectional model of structural evolution). Available for download at https://geobookstore.uwyo.edu/sites/default/files/download/rmg-48-2/clemens-lillegraven-fig-6.pdf. Also available at rmg.geoscienceworld.org in the Fall, 2013 issue (see URL for the present article in RMG's v. 49, no. 1 when published (Spring, 2014).
Figure 7 (right page only—interpretive scenario of geological evolution). Available for download at https://geobookstore.uwyo.edu/sites/default/files/download/rmg-48-2/clemens-lillegraven-fig-7-right.pdf. Also available at rmg.geoscienceworld.org in the Fall, 2013 issue (see URL for the present article in RMG's v. 49, no. 1 when published (Spring, 2014).
We thank Matt Smith for initial exposure of the specimen from its encasing rock and partial preparation of the tooth. We also thank Patricia Holroyd who aided in the specimen's curation and David Strauss who provided its photographic documentation. Prof. Arthur W. Snoke helped in many ways in terms of contributions to field work, geologic discussions, and structural insights. Special recognition goes to Pat and Keith Libbey, co-owners of the 9V Ranch. They generously allowed unfettered access to their lands and granted permission to collect fossils on them. Much of Lillegraven's field work for this project was supported by National Science Foundation research grants EAR 9506462 and EAR 9909354. Facilities provided through the University of Wyoming/National Park Service Research Station at the AMK Ranch in Grand Teton National Park greatly facilitated compilation of field data. Dr. Michael O. Woodburne and an anonymous individual provided beneficial technical reviews. The latter reviewer was especially helpful in suggesting specific means toward more successful reorganization of our presentation. And we owe profound thanks to Dr. Thomas A. Hauge, who provided a closely focused, insight-rich review through which our publication benefited greatly. Most importantly of all, Mrs. Dorothy Clemens and Mrs. Linda Lillegraven have exhibited impressive powers of patience through all stages of this project's evolution. This is University of California Museum of Paleontology Contribution Number 2046.
- Received April 30, 2013.
- Revision received September 11, 2013.
- Accepted October 11, 2013.