- UW Department of Geology and Geophysics
The eastern border of the Hanna Basin, Wyoming, is defined by Simpson Ridge anticline, a Laramide structure that separates the Hanna Basin from the more easterly Carbon Basin. New geologic mapping along with interpretation of well logs and seismic-reflection data suggest that this structural feature was created through a combination of thick- and thin-skinned deformation during the contractional Laramide orogeny. Major west-directed, basement-involved faulting occurs beneath Simpson Ridge, suggesting that this structural feature is not related to the eastvergent Elk Mountain anticline. Although major basement-involved thrusts underlie Simpson Ridge, the development of the anticline also involved thin-skinned, out-of-the-basin thrusting related to the deeply rooted basement faults by a trishear deformational zone. An unconformity between the Cretaceous–Paleocene Ferris Formation and Paleocene Hanna Formation marks initiation of fault-controlled uplift along the Simpson Ridge anticline. On the eastern side of Simpson Ridge, the Hanna Formation was faulted westward onto the Lewis Shale, thereby yielding a younger-on-older structural relationship. Palynological data suggest that the age of the Hanna Formation in the southwestern Carbon Basin is late Paleocene. These data thus provide a maximum age for later deformation along Simpson Ridge anticline and for the consequent definition of the western edge of Carbon Basin.
- basement-involved uplifts
- out-of-the-basin faulting
- Laramide orogeny
- seismic-reflection profiling
- Hanna Basin
- Carbon Basin
The purpose of this study is to investigate the structural and temporal development of the southeastern Hanna Basin. The basin is surrounded by structural highs that exhibit typical basement-involved, Laramide-style deformation. This thick-skinned style of deformation is commonly associated with the Late Cretaceous–early Tertiary Laramide orogeny and has been observed throughout the Rocky Mountain foreland. Recently, several studies have also recognized a thin-skinned component associated with Laramide deformation (e.g., Lillegraven and Snoke, 1996; Hitchens, 1999; among others). Although this thin-skinned component is genetically related to the basement-involved deformation, the overall style is markedly different. The present study helps define how local Laramide activity produced differing styles of deformation. Additionally, this study will determine when the local deformation occurred through detailed analysis of structural relationships combined with biostratigraphic data.
The Hanna Basin is located in south-central Wyoming in the “heart” of the area of Laramidestyle deformation. Figure 1 shows the distribution of Laramide-style deformation in the western United States. During the Late Cretaceous and early Tertiary much of the Rocky Mountain foreland became segmented due to the onset of Laramide uplifts and/or arches. Large, rapidly filling basins developed adjacent to many of the basement-involved uplifts/arches. Although the timing of completion of the Laramide orogeny is still debated, scientists agree that deformation in the southern part of the orogenic belt probably lasted longer than in the north, as late as 35–40 Ma in Colorado and New Mexico (Dickinson et al., 1988). That study suggested that Laramide activity ended around 50 Ma in most of Wyoming. Dating the initiation of the orogeny has proven more difficult; however, a Late Cretaceous age is generally accepted in the literature (Snoke, 1997).
Surrounded by Laramide uplifts, the Hanna Basin is an atypical Laramide basin due to its relative small size but great depth (Fig. 2). To the north of the basin are the Seminoe Mountains, Shirley Mountains, and Freezeout Hills; to the west is the Rawlins uplift; to the south are Elk and Sheephead mountains; and to the east is Simpson Ridge anticline. All of these structures expose Precambrian basement except for the Simpson Ridge anticline and Freezeout Hills. Simpson Ridge anticline plunges to the northeast and is oriented roughly north-northeast–south-southwest.
To the east of the Hanna Basin is the Carbon Basin. Simpson Ridge anticline serves as a distinct boundary between these two basins. To the west, the Hanna Basin is separated from the greater Green River Basin by the Rawlins uplift. It is probable that these three basins were once connected and later segmented due to the onset of Laramide orogenic uplift (Grimaldi et al., 2000). Figure 3 shows the areal distribution of the Hanna and Medicine Bow Formations surrounding the study area. Additionally, major Laramide faults are shown along with axial traces of anticlines and synclines.
The study area is located ∼15 miles (24 km) southwest of the village of Medicine Bow, Wyoming, and it covers approximately 50 mi2 (129 km2) along the eastern border of the Hanna Basin (Fig. 4, insert). A stretch of U.S. Highway 287 defines the northern border of the study area, whereas Interstate Highway 80 defines the southern border. No major roads transect the study area, although the original Lincoln Highway transected the study area from east to west. The old road passes through the abandoned mining town of Carbon, located just east of the field area.
The most prominent topographic feature in the field area is Simpson Ridge. This feature has been referred to as both Simpson Ridge or the Saddleback Hills (Hague, 1877; King, 1878; Veatch, 1907; Dobbin et al., 1929; Veronda, 1951; Ryan, 1977; Blackstone, 1993a). Perhaps much of the confusion is due to different labels on topographic maps. On the U.S. Geological Survey 15-minute Saddleback Hills quadrangle published in 1921, the topographic feature is labeled as Saddleback Hills. In contrast, the same feature is labeled as Simpson Ridge on the 1951 U.S. Geological Survey 7.5-minute Carbon quadrangle. In a study that pre-dates both maps, Veatch (1907) referred to the structure as Simpson Ridge anticline and labeled it as Simpson Ridge on his geologic map, which does not show topography. The earliest references to the structure that I could find were by King (1878) and Hague (1877). Both scientists were part of the expedition conducted by the Corps of Engineers to survey the Fortieth Parallel, and both refer to the structural and the topographic features as Simpson Ridge (although Simpson Ridge anticline was sometimes used to refer to the structural feature). Based on these early usages, I suggest that the name Saddleback Hills should be abandoned, and Simpson Ridge and Simpson Ridge anticline should be used for the topographic feature and structural feature, respectively.
Although expeditions in the late nineteenth century first surveyed and mapped the area, Veatch (1907) was the first to compile a geologic map that covered Simpson Ridge and much of the surrounding region. That work was greatly improved when Dobbin et al. (1929) conducted detailed mapping of the area in search of coal resources. Prior to the present study, the map produced by Dobbin et al. (1929) remained the most detailed of the area. Unfortunately, none of these early studies dealt specifically with the structural development of Simpson Ridge anticline. Veronda (1951) was the first to discuss this subject. In addition to new geologic mapping along Simpson Ridge, Veronda (1951) suggested that considerable thrust movement occurred along the eastern flank of Simpson Ridge based on the appearance of overturned strata within the Lewis Shale.
Ryan (1977) conducted the first study that attempted to closely constrain the age of uplift along Simpson Ridge anticline. Based largely on sandstone crossbedding data from the Ferris and Hanna Formations, he argued that Simpson Ridge existed as a topographically significant feature throughout deposition of both formations. Additionally, Ryan (1977) suggested that Simpson Ridge anticline is a northerly extension of the Elk Mountain structure. In a study by Hansen (1986), Simpson Ridge anticline was assigned an age of Middle Eocene. In contrast, two studies completed at the University of Wyoming suggested different dates for the uplift of Simpson Ridge anticline. LeFebre (1988) argued that the anticline was slightly active during deposition of the Ferris Formation, but that significant activity did not occur until deposition of the Hanna Formation. Similarly, McElhaney (1988) argued that Simpson Ridge became active during the late Paleocene.
He also considered Simpson Ridge anticline as genetically related to Elk Mountain. Blackstone (1993a) was the first to produce detailed geologic cross-sections for Simpson Ridge anticline, proposing three possible interpretations. One interpretation suggests no basement faulting, one shows an eastward-dipping basement fault with a backthrust, and one shows a westward-dipping normal fault that offsets basement rock. In that study the interpretation showing an eastward-dipping basement fault was preferred by the Blackstone (1993a, his Fig. 4 and “B” model).
Secord (1998) conducted the most recent study of Simpson Ridge during work in the Carbon Basin. Although his work was largely biostratigraphic, he did make some conclusions regarding the timing of uplift along Simpson Ridge anticline. He argued that uplift of Simpson Ridge occurred during or after deposition of the Ferris Formation, and that activity had ceased prior to deposition of the Hanna Formation. Secord's study resulted in the collection of a significant Torrejonian–Tiffanian fauna. However, this paleontological collection did not yield age-diagnostic fossils that would allow him to date the lower Ferris Formation, which would be important in providing a maximum age constraint for uplift of Simpson Ridge. It is important to note that other than geologic mapping by Ryan (1977) and paleontological collecting by Secord (1998), none of these studies involved detailed fieldwork around Simpson Ridge.
I conducted fieldwork during the summers of 1999 and 2000; a total of four months was spent in the field. All geologic mapping was done at the scale of 1:24,000, using U.S. Geological Survey 7.5- minute topographic quadrangles as the base. Subsurface interpretations were facilitated by use of six seismic-reflection profiles acquired through the generosity of Union Pacific Resources and well data acquired through the Geological Survey of Wyoming. At various times during the field seasons, various people joined me in the field to prospect for scientifically important vertebrate fossils. Additionally, I collected seven samples of pollen from four localities within the Hanna Formation to help constrain its stratigraphic age. Although an outside laboratory prepared the pollen samples, I identified the grains.
The Hanna Basin is geographically small relative to other Laramide basins, yet it is one of the deepest. Blackstone (1993b) showed that the elevation of Precambrian basement in the basin was approximately 30,000 feet (9.1 km) below mean sea level in the center of the basin. Lillegraven and Snoke (1996) estimated a composite thickness of almost 43,000 feet (13.1 km) of compacted strata in the northern Hanna Basin. Only lithologic units observed in the study area are discussed in this section. Readers are referred to Gill et al. (1970) and Van Ingen (1978) for more detailed descriptions of Mesozoic and Paleozoic–lower Mesozoic units, respectively. Table 1 is a summary of local stratigraphy based on other workers' measured sections, field observations, and well data.
The main topographic expression of Simpson Ridge involves the Mesaverde Group (Fig. 5). Locally, this group is divided into four formations, including the Haystack Mountains Formation (at the base), Allen Ridge Formation, Pine Ridge Sandstone, and Almond Formation (at the top). Although these formations previously were mapped as a combined unit in this area (Dobbin et al., 1929), I considered them separately.
The Haystack Mountains Formation conformably overlies the Steele Shale. Thick units of marine sandstone are the dominant rock type of the formation. However, shale units occur low in the formation and are probably lenses of Steele Shale (Gill et al., 1970). The Haystack Mountains Formation is exposed along much of the axis of Simpson Ridge anticline. Excellent outcrop can be seen in section 17, T. 21N., R. 80 W. The Allen Ridge Formation is largely nonmarine. However, marine, fluvial, and brackish-water units are also present within the formation. Fluviatile sandstone and shale dominate the lowest unit, accounting for most of the total thickness of the formation (Gill et al., 1970).
The Pine Ridge Sandstone unconformably overlies the Allen Ridge Formation (Gill et al., 1970). This relationship is unclear in the field area due to poor exposure; however, previous studies have documented the unconformity through faunal analysis. The Pine Ridge Sandstone is composed primarily of thickly bedded, pale yellowish-gray marine sandstone that commonly weathers white. Two prominent white sandstone beds, which mark the lower and upper boundary of this formation (Fig. 6), were recognized in the study area.
The Almond Formation is a sequence of sandstone, shale, and minor coal. This formation is the primary ridge-forming unit of Simpson Ridge. Exposure of the contact between Lewis Shale and Almond Formation is poor throughout much of the study area; however, there is an exceptional exposure of this contact at the northern tip of Simpson Ridge (Fig. 7). A distinct coal bed is present approximately 20 feet (6.1 m) below the contact of the Almond Formation and Lewis Shale. Above the contact, carbonaceous shales are capped by brownish sandstone containing iron concretions.
The Lewis Shale is not resistant, and commonly forms the valley surrounding Simpson Ridge. In some areas, sands from the Fox Hills Formation intertongue with shales in the upper part of the Lewis Shale. The contact between the Lewis Shale and Fox Hills Sandstone is generally placed at the transition between marine shales of the Lewis Shale and shallow-water marine sandstones of the Fox Hills Formation (Gill et al., 1970).
The Medicine Bow Formation is continental in origin and associated with eastward regression of the Cretaceous sea (Bowen, 1918). Rock types of this unit vary widely and include shale, coal, and sandstone. Cross-bedded sandstone and intermittent coal layers dominate the lower part of the unit.
The Ferris Formation is a nonmarine unit that spans the Cretaceous–Tertiary boundary. At the base of the formation is approximately 1,000 feet (305 m) of dark conglomerate containing chiefly chert and quartzite pebbles with other minor constituents. These pebbles are not composed of local material, and probably were derived from distant sources (Bowen, 1918; Gill et al., 1970). The upper part of the formation contains numerous coal beds and carbonaceous shales among tan sandstones. The thick, dark conglomerate seen in the lower parts of the formation are not present in the upper part of the formation. Although thick conglomerate occurs at the base of this unit, the Ferris Formation conformably overlies the Medicine Bow Formation. At the top of the formation is a distinct angular unconformity with the overlying Hanna Formation. A thickness of 4,000 feet (1,219 m) has been used in this study for the Ferris Formation.
The Hanna Formation consists of conglomerate, sandstone, coal, and shale. The formation unconformably overlies the Ferris and Medicine Bow Formations and Lewis Shale in the study area. In certain other parts of the Hanna Basin, the formation may conformably overlie the Ferris Formation (Gill et al., 1970; Lillegraven, 1994). On the eastern flank of Simpson Ridge, the Hanna Formation has been faulted onto the Lewis Shale. Conglomerate and sandstone are the dominant rock types near the base of the formation (Fig. 8); fine-grained rocks dominate much of the remainder of the formation (Grimaldi et al., 2000, their Fig. 15). Pebbles within the conglomerates are similar to those of the Ferris Formation; however, a larger feldspathic component is present along with clasts of Mowry Shale (Bowen, 1918; Gill et al., 1970). Much of the unit is dominated by coal and shale as is evident by the numerous coal mines in the area. The total thickness of the Hanna Formation in the study area is uncertain due to the erosional surface that truncates the top of the formation. However, previous studies have measured the thickness of the unit in excess of 11,000 feet (3,353 m) (Gill et al., 1970; Lillegraven and Snoke, 1996).
Simpson Ridge Anticline
Simpson Ridge anticline trends roughly N25°E and plunges gently to the northeast (Fig. 4). Although previous studies have characterized the structure as highly asymmetrical, my field observations and study of seismic-reflection data suggest that the structure is generally symmetrical. Both limbs of the anticline dip away from the axis at approximately 55°–60°. Simpson Ridge is the dominant topographic feature of the study area, with a relief of over 600 feet (183 m). Most of the topographic relief is expressed within the Mesaverde Group (Fig. 5). Simpson Ridge is composed of two parallel prominent ridges with a broader ridge of less relief located between them. The Almond Formation forms the two steeper ridges, and the Allen Ridge and Haystack Mountains Formations form the broader ridge.
Hi Allen Ridge is located in the northwest part of the study area. Strata on this feature dip approximately 45°–50° to the northwest and are structurally related to Simpson Ridge anticline. Composed of Ferris Formation, it shows a relief of more than 500 feet (152 m), but covers significantly less area than Simpson Ridge. About 4 miles (6.4 km) north of Hi Allen Ridge is a ridge of Hanna Formation. This feature is also part of Simpson Ridge anticline, suggesting that the structural feature is significantly larger than the topographic ridge. Dr. Jason A. Lillegraven is studying this area. Several areas along Simpson Ridge show minor faulting. In the northern part of section 4, T. 21 N., R. 80 W. several minor faults can be seen near the axis of Simpson Ridge anticline. Offset is minor along these faults, suggesting that they are associated with folding along the axis. Similarly, minor faults are seen in the south-central part of section 9, T. 21 N., R. 80 W. that also probably are related to folding along the axis. A slightly larger fault transects Simpson Ridge in sections 9, 16, 17, and 18, T. 21 N., R. 80 W. (Fig. 9). Although the offset of this fault, with the “up” block on the north side, may be as much as 200 feet (61 m), it does not show up on seismic-reflection profile “B”, suggesting it is not a deeply rooted feature. Also, offset of the Pine Ridge Sandstone on both sides of the ridge suggests that there is a strike-slip component to the fault. The surface trace of this fault is lost in the Lewis Shale on either side of Simpson Ridge.
On the west side of Simpson Ridge, several faults appear in the Ferris Formation in sections 1 and 2, T. 21 N., R. 81 W. and sections 25 and 36, T. 22 N., R. 81 W. Although these faults previously were described as normal (Dobbin et al., 1929), there exist insufficient data to determine whether they are normal or reverse faults. The faults have been labeled as high-angle faults and the “up” and “down” blocks have been marked on the map (Fig. 4).
On the eastern flank of Simpson Ridge, the Hanna Formation overlies the Lewis Shale. Previous studies referred to this as an unconformable depositional contact (Dobbin et al., 1929; Veronda, 1951; Ryan, 1977; Blackstone, 1993a; Secord, 1998). This relationship can be seen clearly in section 27, T. 22 N., R. 80 W., where the Hanna Formation rests unconformably on the Medicine Bow Formation (Fig. 10). However, my detailed field investigations have shown that the contact between the Lewis Shale and Hanna Formation directly to the east of Simpson Ridge is a thrust fault contact. Veronda (1951) mapped steeply dipping to overturned Lewis Shale near this contact. Blackstone (1993a) used that observation to suggest that there is a major west-dipping thrust near the area. Figure 11 shows the nature of the contact between the Lewis Shale and Hanna Formation in section 21, T. 21 N., R. 80 W. The photographs show that strata dipping steeply to the west near the contact, previously interpreted as overturned Lewis Shale, are actually part of the Hanna Formation. The figure also shows a rollover structure within the Hanna Formation, clarifying that strata dipping gently to the east is in the limb of a small anticline. Additionally, some of the steeply dipping strata previously interpreted as overturned Lewis Shale are actually Hanna Formation that form the other limb of the small anticline. The lower photograph shows that stratigraphic “up” is to the west in the steeply dipping strata, an opposite direction than what would be expected if the strata were composed of Lewis Shale. A cartoon cross-section summarizes this relationship. As indicated on Figure 4, I have mapped this contact as a thrust fault in which the Hanna Formation has been thrust westward, out of the Carbon Basin, onto Lewis Shale of Simpson Ridge.
On the western flank of Simpson Ridge, I have found a few areas of overturned strata within the Ferris and Medicine Bow Formations. Specifically, overturned strata were found in section 7, T. 21 N., R. 80 W. and section 19, T. 22 N., R. 80 W. These features suggest that some sort of faulting or folding exists in cryptic form along the western side of Simpson Ridge.
Well and Seismic-reflection Data
Although numerous wells penetrate Simpson Ridge anticline, few are found far from the axis of the anticline or outside of sections 17 and 20, T. 21 N., R. 80 W. Seven wells penetrate Simpson Ridge anticline that I deemed scientifically important to this study: 1 UPPR-Chace, #1 Union Pacific, 1 Simpson Ridge, 22H-20 UPPR, 1 “A” U.P.R.R., 1 UPRR Chace (a different well from the other of the same name), and 16-1 Ramsey. The first six wells are found in sections 17 and 20, T. 21 N., R. 80 W., and were used for cross-section B–B′ (Fig. 12).
The deepest of these wells is 1 UPPR-Chace (sec. 17), which reaches a depth of 13,038 feet (3,974 m) below the surface and ends in the Tensleep Sandstone. Well 1 Simpson Ridge reaches a depth of 10,514 feet (3,205 m) below the surface and ends in the Frontier Formation. Steeply dipping strata in the well correlate to Steele Shale within Simpson Ridge anticline. Additionally, a drastic dip change is seen in the well that correlates to the crossing of the eastward-dipping backthrust seen in cross-section B–B′ Well #1 Union Pacific was drilled to a depth of 5,619 feet (1,713 m) below the surface into the Steele Shale. Dips in the Steele Shale match those expected from crosssection B–B′. Wells 1 UPPR-Chace (section 20), 22H-20 UPRR, and 1 “A” U.P.R.R. are all relatively shallow (equal to or less than 1,500 feet [457 m] below the surface) and stop within the Steele Shale. All three helped to constrain the contact between the Steele Shale and Haystack Mountains Formation.
I acquired six seismic-reflection profiles from Union Pacific Resources that transect or parallel Simpson Ridge. Because some lines were of poor quality, only three significantly added to geologic understanding of this area, two of which are presented in this study. One transects the southern part of Simpson Ridge anticline and helps in interpretations of cross-section A–A′ (Fig. 13). Figure 14 shows this seismic-reflection line both uninterpreted and interpreted by the author. A good basement reflection exists along much of the line. The northwest part of the line shows several southeastward-dipping, basement-rooted faults. These faults seem to form a type of triangle zone in which significant bedding-parallel displacement occurs within the Steele Shale. Two major thrusts are shown underneath Simpson Ridge anticline. Although clear in seismic data, these faults have been mapped as blind thrusts because they are impossible to trace on the surface. However, some overturned bedding is observed in the Ferris and Medicine Bow Formations on the west side of Simpson Ridge that may be associated with the western blind thrust. The northern extent of these faults is defined by these beds along with seismic-reflection evidence not shown here. One reason that the faults may not be expressed at the surface is that offset may dissipate into a broad zone of folding and faulting as the faults near the surface. In turn, much of the deformation may be distributed throughout the Lewis Shale and Medicine Bow Formation. Nonetheless, these faults have been projected to the surface in Figure 4 based on seismic-reflection data. Additionally, several minor faults are seen within the fold of Simpson Ridge anticline that probably sole into the Steele Shale. The seismicreflection data suggest that overall tectonic transport in the section was due to thrusting directed to the northwest.
Seismic-reflection profile “B”, which correlates to cross-section B–B′, shows a similar fault arrangement under Simpson Ridge to that seen in seismicreflection line “A.” My interpretation of seismic-reflection profile “B” is found in Figure 15 along with an uninterpreted line. This profile also shows that tectonic transport in the section was due to thrusting directed to the northwest. The eastward-dipping backthrust is identified on the geologic map as a blind thrust. An abrupt change of dip can be seen in the northwestern part of section 12, T. 21 N., R. 80 W., along with the previously mentioned overturned strata in the Medicine Bow and Ferris Formations on the western flank of Simpson Ridge. The large northwest-directed, basement-involved fault soles into the Steele Shale. Significant thickening of the section can be seen within the Steele Shale. Thickening in the section is also evident in the southeastern part of the seismic-reflection line.
Cross-sections through Simpson Ridge Anticline
Cross-section A–A′ is oriented northwest—southeast along the southern part of the study area (Fig 4). Note that cross-section A–A′ extends out of the study area (i.e., Fig. 4) to the northwest so that important additional structural features could be included in this cross-section. The northwestern part of the cross-section shows significant basement faults in the subsurface. These faults are interpreted to die out in the Steele Shale through bedding-parallel slip as well as to transfer slip into a thin-skinned fault system, rooted in the Steele Shale (Fig. 13). This fault system is chiefly directed to the southeast but also involves backthrust components. The main thin-skinned fault approaches the present surface in the Lewis Shale on the eastern flank of Simpson Ridge. A backthrust comes off this fault and nears the surface on the western flank of Simpson Ridge in Lewis Shale. To compensate for the triangle zone created in the Steele Shale, significant thickening of the section is shown in cross-section A–A′, as I inferred from seismic-reflection profile “A.”
Cross-section B–B′ shows a basement-involved thrust dipping to the southeast as suggested by seismic-reflection profile “B.” The geometry is similar to that of cross-section A–A′. That is, a basement-involved fault tips into the Steele Shale creating a triangle zone from which a thin-skinned fault system is directed to the southeast, emerging on the eastern flank of Simpson Ridge. One notable difference in cross-section B–B′ is seen in its extreme southeastern part where the Hanna Formation has been thrust onto the Lewis Shale. There is little evidence of this shallow-level faulting in the seismicreflection data. However, field observations strongly suggest that the Hanna Formation has been thrust onto the Lewis Shale (e.g., Fig. 11).
Bloody Lake Anticline
In the southwestern corner of the field area is the northern part of Bloody Lake anticline. This anticline is oriented in a similar direction as Simpson Ridge anticline and also plunges to the north. However, rather than exhibiting a gentle plunge as in Simpson Ridge, this structure shows little plunge then abruptly plunges at 55° or more. The feature is not seen in cross-section B–B′ two and a half miles to the north. The topographic relief associated with this feature is about 500 feet (152 m).
Well and Seismic-reflection Data
Although several wells have been drilled in this structure south of the study area, I found no wells in close enough proximity to be used for subsurface interpretations. Seismic-reflection profile “A” was the only line that crossed this structure (Fig. 14). The anticline is part of the hanging wall of a fault that splays off the major thin-skinned fault that soles in the Steele Shale. The splay dips to the northwest, and I interpret little throw associated with this fault. The anticline is obvious in seismic-reflection profile “A.” Nevertheless, no evidence of the structure can be seen in seismic line “B,” located just two miles to the north (Fig. 15).
Cross-section through Bloody Lake Anticline
Cross-section A–A′ crosses Bloody Lake anticline. The geometry of Bloody Lake anticline is similar to that of Simpson Ridge except that there is no backthrust off the main fault. Below the anticline, significant thickening of the section can be seen in the Steele Shale, as supported by seismic line “A” (Fig. 14).
This section is devoted to interpretations of the structural features described in the preceding pages. Special attention will be given to description of observed local deformational styles. Both basement-involved and thin-skinned styles will be discussed in regard to Simpson Ridge anticline. Additionally, the timing of local tectonic events will be proposed. Finally, this section will discuss how structural features such as Simpson Ridge anticline, Bloody Lake anticline, Elk Mountain, Hanna Basin, and Carbon Basin are related to one another.
Thick- and Thin-skinned Deformation
Seismic-reflection data from this study clearly show that thick-skinned, basement-involved faulting is a major component of the overall deformational style. However, no detailed analysis of basement rock was possible due to lack of Precambrian exposures at the surface. Nearby studies in the area of Sheephead Mountain suggest that basement deformation was accommodated by cataclastic deformation, chiefly involving microfaulting and/or fracturing (Chase et al., 1993; Hitchens, 1999).
All of the seismic-reflection profiles and geologic cross-sections summarized in this study suggest that overall basement faulting was largely due to a westnorthwest–east-southeast shortening field. However, changes in axial trends also suggest that some of the shortening may have been oblique to this orientation. Seismic-reflection line “A” shows that at least three major basement-involved thrusts exist under Simpson Ridge and Bloody Lake anticlines (Fig. 14). The structural relief in this line exceeds 12,000 feet (3,658 m). This interpretation is significantly different than that by LeFebre (1988). In that study, a cross-section through Simpson Ridge and the Hanna Basin shows a structural relief of over 16,000 feet (4,877 m) between Simpson Ridge and the Hanna Basin. However, LeFebre interpreted the relief as a result of a major normal fault located approximately 3.7 miles (6.0 km) to the west of Simpson Ridge. Although that fault was interpreted through an unpublished seismic line, none of the data from the present study supports existence of such a fault. Additionally, LeFebre (1988) interpreted a major westward-dipping basement fault underlying Simpson Ridge. Existence of that fault also is not supported by data used in the present study.
The basement-involved thrusts in this study produce geometries similar to the trishear model by Erslev (1991). Within the study area a basement thrust tips into a décollement of Steele Shale and, due to synclinal crowding, a backthrust is created. This generalized geometry is supported by seismicreflection data in which a significant thickening of the section in the Steele Shale appears, as would be expected by the trishear model. A geometry similar to that presented in the present study and by Erslev (1991) also has been proposed for the structural evolution of nearby Sheephead Mountain and environs (Hitchens, 1999).
Although basement-involved, thick-skinned deformation traditionally has been associated with the contractional Laramide orogeny, thin-skinned deformation has been increasingly recognized along various margins of the Hanna Basin (Taylor, 1996; Lillegraven and Snoke, 1996; Taft, 1997; Hitchens, 1999). Referred to as “out-of-the basin faulting,” this style of deformation thrusts strata out of a basin as a result of synclinal crowding during basement uplift along the margins of the basin. This structural style is a component of Erslev's (1991) trishear model and is an important element in the structural evolution of the study area. As seen in cross-sections A–A′ and B–B′ (Figs. 12 and 13), basement-involved thrusts tip into the Steele Shale, which created an oppositely directed thin-skinned fault system which, in turn, also includes some backthrust elements.
Timing of Local Deformation
Although several studies have proposed times for uplift of Simpson Ridge, ages vary from the late Cretaceous to middle Eocene (Ryan, 1977; LeFebre, 1988; Hansen, 1986; McElhaney, 1988; Secord, 1998). Much of the work regarding the timing of uplift has focused on the Ferris and Hanna Formations. Several studies (e.g., Ryan, 1977) have suggested that development of Simpson Ridge anticline was active during deposition of the Ferris Formation. Ryan's interpretation was based on paleocurrent measurements within the formation suggesting that streams were deflected by the evolving topographic uplift. However, Ryan's data lack the stratigraphic and geographic documentation necessary to closely constrain the age of the uplift. For example, Ryan (1977) did not describe the sources of measurements taken in the Ferris or Hanna Formations. Additionally, although a topographic feature may have been influencing paleocurrents, this is not direct evidence for a fault-produced structural feature or for magnitude of such a feature.
In the most recent study, Secord (1998) suggested that Simpson Ridge was active during deposition of the Ferris Formation as based on an unconformity seen within the unit. However, I found no evidence for a significant change in dip within the local Ferris Formation. Secord (1998) suggested that the change in dip is most evident on the southern end of Hi Allen Ridge. I did confirm that significant changes in dips of the Ferris Formation do exist in that area. However, I interpret these variations as post-depositionally deformed rather than controlled by syndepositional uplift. Detailed mapping and seismic-reflection data show that a major backthrust emerges at the southern end of Hi Allen Ridge. In short, I have observed no evidence for significant uplift along Simpson Ridge during deposition of the Ferris Formation.
The unconformity at the base of the Hanna Formation marks initiation of uplift along Simpson Ridge. Although that unconformity is unrecognized in much of the Hanna Basin (Hansen, 1986; Lillegraven and Snoke, 1996), it is apparent around Simpson Ridge. The Ferris–Hanna unconformity has led to confusion. This unconformity was originally described by Bowen (1918), but it has been difficult to locate in much of the basin. A careful reading of Bowen (1918) shows that the part of the section in which the unconformity was first recognized was on the eastern side of the Hanna Basin, near Simpson Ridge. The unconformity can be seen clearly on seismic-reflection line “B,” although that portion of the line is not shown in Figure 15. The unconformity between the Hanna and Ferris Formations resulted from uplift of Simpson Ridge anticline, as based on appearance of the unconformity only near that structure. This suggests that there was significant uplift of Simpson Ridge anticline after deposition of the Ferris Formation but before deposition of the Hanna Formation.
Structural measurements in the Hanna Formation show that the unit dips no less than 15°–20° away from Simpson Ridge around all its sides. All previous studies suggested that the Hanna Formation on the eastern side of Simpson Ridge unconformably overlies the Lewis Shale. However, new mapping of this area shows that the Hanna Formation was faulted onto the Lewis Shale, although the horizontal displacement associated with this faulting is likely minimal. In this area, dips within Hanna Formation may be in excess of 65° to the east; in some local spots it includes overturned strata. It is uncertain if some of these steeper dips are results of post-depositional uplift. It is clear, however, that some of these steep dips are the result of fault displacement. Regardless of what mechanism caused steep to overturned dips in the Hanna Formation on the eastern side of Simpson Ridge, it is evident that the Hanna Formation was deposited prior to final uplift along Simpson Ridge anticline.
Recently, paleontological discoveries have led to excellent vertebrate localities in the Hanna and Carbon Basins (Wroblewski, 1997; Secord, 1998; Eberle and Lillegraven, 1998a, 1998b; Lillegraven and Eberle, 1999; Higgins, 2000; Burris, 2001). One primary goal of the present project was to discover age-diagnostic mammalian fossils to help date syntectonic strata near Simpson Ridge. Unfortunately, no such fossils were found in the Ferris or Hanna Formations near Simpson Ridge beyond those reported by Secord (1998). Fortunately, other studies have used ancient pollen to date the Hanna Formation in nearby areas (Gill et al., 1970; Weichman, 1988). I recovered seven pollen-bearing samples from four localities within the Hanna Formation on the eastern side of Simpson Ridge. One of those samples yielded age-diagnostic pollen grains. The locality is in the southeastern corner of section 27, T. 22 N., R. 80 W. I counted over 250 grains from one slide that produced species of Momipites and Carypollenites. The dominant species within the sample was C. veripites, indicative of Paleocene pollen zone P6 of Nichols and Ott (1978). This suggests the age of the Hanna Formation at this locality is latest Paleocene, although it is probable that P6 is equivalent to the Clarkforkian (D. J. Nichols, personal communication, 2001). These dates suggest that some uplift along Simpson Ridge anticline postdates the latest Paleocene (i.e., Clarkforkian).
In summary, initiation of deformation probably occurred after deposition of the Ferris Formation but before deposition of the Hanna Formation. Additional or continued deformation occurred after deposition of the basal Hanna Formation, probably during the latest Paleocene or early Eocene. This is congruent with other studies that have proposed multiple phases of deformation around the Hanna Basin (Bergh and Snoke, 1992; Lillegraven and Snoke, 1996; Taft, 1997; Secord, 1998; Hitchens, 1999).
RELATIONSHIPS AMONG STRUCTURAL FEATURES
Simpson Ridge Anticline and Bloody Lake Anticline
Simpson Ridge and Bloody Lake anticlines are structural expressions of the same contractional event. As comparatively shown in cross-sections A–A′ and B–B′, both structures exhibit similar geometry. Specifically, both anticlines are the result of thin-skinned, out-of-the-basin thrusts that sole into the Steele Shale and are genetically related, by a trishear deformational zone, to eastward-dipping basement faults. Although Bloody Lake anticline is prominent in seismic line “A,” it is not present just two miles to the north in seismic line “B” (Figs. 14 and 15). Most probably, folding and faulting along Bloody Lake anticline accommodated shortening in the southern part of the study area. As Bloody Lake anticline quickly plunges and disappears, folding and faulting of Simpson Ridge anticline accommodates the shortening to the north of Bloody Lake anticline. Along the topographic feature, the axis of Simpson Ridge anticline remains horizontal for over four miles (6.4 km). This is reflected in the Haystack Mountains and Allen Ridge Formations (Fig. 4). Then, on the northern end of Simpson Ridge, the axis begins to slightly plunge and trend more northerly. Where the axis remains horizontal may reflect the presence of the underlying basement fault. The trend may change northward due to differential movement in the area to the north and the absence of the basement fault underlying Simpson Ridge. Figure 3 shows the changing trend of the axis of Simpson Ridge anticline and the differing orientations seen in tectonic structures to the north.
Simpson Ridge Anticline, Bloody Lake Anticline, and Elk Mountain
Several studies have suggested that Simpson Ridge anticline is a northern extension of Elk Mountain (Ryan, 1977; McElhaney, 1988). Data from the present study, however, show that Simpson Ridge and Bloody Lake anticlines are not structurally related to Elk Mountain anticline. Elk Mountain has been the focus of several detailed studies that disagree on the fundamental geometry of the structure (Beckwith, 1941; Houston et al., 1968; McClurg and Mathews, 1978; Blackstone, 1980). Although one study proposes that the major Elk Mountain fault was a vertical normal fault (McClurg and Mathews, 1978), all the others agree that Elk Mountain was elevated to the east by a major basement-involved, westward-dipping thrust. This eastward displacement is opposite the fault displacement inferred to underlie Simpson Ridge and Bloody Lake anticlines. Moreover, regardless of how the detailed structures may be resolved under Simpson Ridge and Bloody Lake anticlines, it is clear, based on seismic-reflection data, that the overall basement-involved faulting was along eastward-dipping faults. It is unclear why such a significant change occurred in the structural geometry between Elk Mountain and the study area, across such a short distance. Alternatively, Elk Mountain anticline may exhibit underlying structural geometry similar to that of Simpson Ridge and Bloody Lake anticlines, although basement rocks are exposed at the surface in the core of the Elk Mountain structure, suggesting the exhumation of a deeper structural level. Regardless of which hypothesis is preferred, additional mapping and subsurface data analysis south of the study area would be needed to resolve what controlled the change between these two areas. Unfortunately, little useful outcrop exists between Simpson Ridge and Elk Mountain anticlines.
Hanna and Carbon Basins
The Hanna and Carbon Basins did not become distinct structural features until late Paleocene time. During deposition of the Medicine Bow and Ferris Formations, there existed an unobstructed path for drainages through this area. However, upon uplift of Simpson Ridge after deposition of the Ferris Formation, a significant topographic feature developed that separated the two basins. This suggests that the Carbon Basin, as true for the Hanna Basin, was very deep until the middle Paleocene. The Carbon Basin may have had over 28,000 feet (8,534 km) of Cretaceous–Paleocene strata in its western part (i.e., involving a complete section up through the Ferris Formation). Seismic-reflection data show that most strata in the western Carbon Basin are nearly horizontal even though synclinal folding is evident in seismic-reflection profiles (Figs. 14 and 15).
Simpson Ridge and Bloody Lake anticlines are basement-involved Laramide structures. Seismicreflection data show that several major eastward-dipping basement faults exist under these structures. These basement faults tipped into the Steele Shale and produced bedding-parallel slip in that formation. In addition, thin-skinned, out-of-the-basin fault systems are present. The basement-involved thrusts created a zone of trishear, thus creating a local thin-skinned thrust system that soled into a décollement zone in the Steele Shale and propagated eastward toward the surface on the eastern flanks of Simpson Ridge and Bloody Lake anticlines. Backthrusts that are components of the thin-skinned fault system exist underneath these anticlines and approach the surface on the western flanks of both anticlines. Simpson Ridge and Bloody Lake anticlines thus share a similar structural geometry and consequently a similar kinematic history.
No evidence in the study area suggests that Simpson Ridge anticline was active prior to or during deposition of the Ferris Formation. Although several conglomerate layers occur low in the Ferris Formation, these coarse clastic rocks are not indicative of an unconformity nor are they composed of locally derived clasts.
A distinct unconformity exists between the Ferris and Hanna Formations in the eastern Hanna Basin that marks initiation of uplift along Simpson Ridge anticline. Unfortunately, no age-diagnostic fossils were discovered that would help to date this part of the section or the unconformity. Nevertheless, significant uplift occurred along Simpson Ridge anticline prior to deposition of the Hanna Formation, after deposition of the Ferris Formation.
Additional uplift occurred sometime after, and possibly during, deposition of the lower Hanna Formation. The contact between the Hanna Formation and Lewis Shale on the eastern flank of Simpson Ridge indicates that the Hanna Formation of the Carbon Basin was thrust onto Simpson Ridge anticline. Pollen samples from this area show a late Paleocene age for this part of the lower Hanna Formation, giving a maximum age for this later phase of tectonic activity. Additionally, the Hanna Formation dips eastward at a minimum of 15°–20° away from Simpson Ridge, further suggesting that the anticline was active during and/or after deposition of the lower Hanna Formation.
Overall deformational style in the study area is similar to that found in other areas of the Hanna Basin. Out-of-the-basin thrusting has become increasingly recognized in the region and is now also hypothesized to have played an important role in the development of Simpson Ridge and Bloody Lake anticlines.
This study shows that Simpson Ridge and Bloody Lake anticlines are not structurally related to the Elk Mountain anticline. Oppositely directed basement-involved thrust faults in the two areas suggest that a significant change in structural geometry exists between them. Additional research in the region south of the study area is needed to determine what caused this change in the relative direction and rooting of basement-involved faulting in this part of the greater Hanna Basin region.
Simpson Ridge and Bloody Lake anticlines share a similar kinematic and temporal history, because they are both expressions of the same shortening event. However, it is unclear whether some shortening was oblique to the structures due to varying trends in hinge axes (Fig. 3). Indeed, Simpson Ridge anticline is situated in a region in which distinct changes are recognized in the orientations of Laramide structures (i.e., form a more north–south orientation to the south to a more east–west orientation to the north and west of the study areas). Simpson Ridge anticline does not seem to closely fit either of these two recognized expressions of the Laramide orogeny. The formation of Simpson Ridge may be due to influences from both of these deformational regimes or may be due to a wholly unique event.
Finally, the Hanna and Carbon Basins existed as a unified, deep Laramide basin until at least the middle Paleocene, through deposition of the Ferris Formation. At that time, uplift began along Simpson Ridge anticline and subdivided the landscape into two distinct basins. For much of the Paleocene, therefore, no significant topography existed in the area that could have greatly influenced patterns of enormous fluvial systems that drained eastern components of the greater Green River Basin.
This paper was derived from a M.S. thesis completed at the University of Wyoming. The author thanks Dr. Jason A. Lillegraven, who supervised the thesis and showed endless encouragement throughout all its stages. Dr. Arthur W. Snoke served on the thesis committee and provided many helpful insights into the Hanna Basin and Laramide orogeny. Dr. Steven Buskirk also served on the committee and provided a helpful review of the thesis manuscript. Drs. Donald U. Wise and Steven Cather improved the manuscript through their careful technical reviews. Numerous landowners gave permission to conduct fieldwork on their land, including, Burt and Kay-Lynn Palm, Casey Palm, Darlene Herman, Rolly Bowen, and Bob Johnson. Pete and Linda Groth deserve special thanks for allowing me to stay at their rebuilt, one-room log cabin for the duration of the fieldwork. Union Pacific Resources generously supplied the seismic-reflection data, and Russ Harms of Global Geolab Ltd. prepared the pollen samples at no charge. Many people helped me in the field to look for vertebrate fossils, namely, Brent H. Breithaupt, John H. Burris, Michael L. Cassiliano, Regan E. Dunn, John R. Foster, Jason A. Lillegraven, Ilsa Lund, Justin Palm, Jessica W. Scott, Michael W. Webb, and Doris Weller. Financial support for this project was made possible by NSF grants EAR-9506462 and EAR-9909354 awarded to Drs. Jason A. Lillegraven and Arthur W. Snoke and through the Department of Geology and Geophysics, University of Wyoming. Finally, my wife Ilsa Lund deserves endless thanks for her patience and support throughout this entire project. This is University of California Museum of Paleontology publication number 1775.
↵* Present address: Department of Integrative Biology, University of California, Berkeley, CA 94720, U.S.A.
- Received December 3, 2001.
- Revision received April 19, 2002.
- Accepted April 25, 2002.