- UW Department of Geology and Geophysics
The Rock Springs uplift of Wyoming and the Douglas Creek arch of Colorado are intrabasinal, Laramide-age basement uplifts within the Rocky Mountain foreland, and are currently separated by the east–west-trending Uinta Mountains. The geometry, timing, and progressive development of these uplifts were investigated using a combined geophysical and geological approach. New gravity surveys were combined with existing regional data to provide a regional Bouguer gravity anomaly map of these two uplifts and the intervening Uinta uplift. The gravity data show a distinct and continuous north–south-striking gravity high along the trend of the two uplifts that crosses the east–west-trending Uinta uplift. The relatively constant amplitude (∼40 mGal) of the gravity anomaly indicates that the inferred basement relief is similar for both arches (∼4 km). Sedimentation patterns indicate that the Rock Springs uplift and Douglas Creek arch formed simultaneously in the Late Cretaceous.
The intrabasinal setting of the uplifts records aspects of foreland deformation that are overprinted or obscured in better-developed uplifts. On the local scale, neither the Rock Springs or Douglas Creek uplift apparently reactivates a pre-existing structure. On a regional scale, there is no change in structural style or timing of the two uplifts, despite their formation in different crustal provinces. The Rock Springs uplift occurs within the Archean Wyoming province north of the Cheyenne belt, whereas the Douglas Creek arch occurs in Proterozoic crust south of this boundary. Timing relations, available from the basinal stratigraphy, indicate the uplifts were initiated as broad arches in the Late Cretaceous before developing into more concentrated uplifts. Thus, large-scale folding, and not reactivation of pre-existing structures, may be the primary control on the initial pattern of north- to northwest-trending foreland deformation.
- basement-involved uplift
- Rocky Mountain foreland
- Laramide orogeny
- Bouguer gravity
- Uinta Mountains
The foreland area of the Cordillera in the United States displays block uplift (“Laramide-style”) deformation that occurred during Late Cretaceous–Paleogene time. These basement-involved block uplifts typify the structural style and have become the type example of “thick-skinned” foreland deformation (Lowell, 1983; Burchfiel et al., 1992; Schmidt et al., 1993; Snoke, 1993). The causal mechanism of the uplifts has been a source of controversy, because of the uncertain relation to plate-margin processes (e.g., Erslev, 2005). A chief controversy in the late 1970s focused on whether these structures formed in relation to horizontal contraction or to vertical tectonics, a debate which required significantly different interpretations of these structures at depth (Oldow et al., 1989; Brown, 1993; Snoke, 1993). D.L. Blackstone, Jr., in a series of publications, advocated horizontal contraction (Blackstone, 1980, 1991). Most recent publications support this viewpoint, based on interpretation of seismic data (Smithson et al., 1978) and structural restoration of cross sections (Erslev, 1991).
There are, nonetheless, several remaining controversies concerning Laramide structures, including uplift geometry, timing of deformation, number of phases of deformation, and the role of reactivation of basement structures. The different interpretations for these features are based on exposures of uplifts (Erslev, 1993), seismic and borehole data (e.g., Berg, 1962), and analog models (Sales, 1968). This ambiguity results because of the significant horizontal shortening recorded by the most prominent Laramide uplifts, which obscures the earlier history. Although several workers have tried to address this problem using structural restoration (trishear: Erslev, 1991; Bump, 2004), information on the initial geometry and timing is always indirect. In contrast, intrabasinal uplifts of lesser amplitude are surrounded by syndeformational strata that provide a record of the geometry and timing of the initial phases of uplift.
In this study, we investigate the structural history of the intrabasinal Rock Springs uplift and Douglas Creek arch. The Rock Springs uplift is in the center of the Greater Green River Basin and the Douglas Creek uplift is in the center of the greater Uinta Basin (defined here as the composite basin consisting of the Uinta and Piceance sub-basins) (Fig. 1). Both features are Laramide-style uplifts that occurred in the Late Cretaceous–Early Tertiary, and their structural development is less studied than uplifts with greater structural relief.
The intrabasinal setting provides three distinct advantages in the study of these structures. First, preservation of the Cretaceous and Tertiary sedimentary record above these structures provides detailed constraints on the uplift timing through tectono-stratigraphic relationships (e.g., Montgomery, 1996). Second, the uplifts and adjacent basins have been studied extensively during petroleum exploration, and thus abundant geophysical data are available (e.g., Kopper, 1962; Montgomery, 1996). Third, low structural relief allows us to observe the process of “thick-skinned” deformation at an incipient stage of development.
This study compares the Rock Springs uplift and the Douglas Creek arch in order to document the formation of “block-uplift” structures within basinal settings. The subsurface geometry of the two structures is compared using gravity models, based on industry seismic data. The timing and evolution of uplift of the two arches is investigated by means of two-dimensional seismic interpretation, observed stratigraphic relationships, and patterns of basin subsidence. The results indicate that the Rock Springs uplift and the Douglas Creek arch developed as a single continuous arch, with similar magnitudes of uplift and a coincidence in timing of uplift.
Rock Springs Uplift
The Rock Springs uplift is a north–south-trending, anticlinal structure in southwest Wyoming that formed in the Late Cretaceous–Early Tertiary (Fig. 1). The uplift is approximately 96 km (60 miles) long by 64 km (40 miles) wide (Roehler, 1976). The uplift lies in the middle of the greater Green River basin and separates the Green River sub-basin to the west from the Washakie and Sandwash sub-basins to the east.
Stratigraphy provides critical timing relationships for uplift (Fig. 2). The Rock Springs uplift exposes Upper Cretaceous and Tertiary formations, with the oldest rocks in the center of the arch and younger rocks on the flanks (Fig. 3). The Cretaceous Baxter Shale forms the eroded core of the Rock Springs uplift (Roehler, 1993, and is overlain by the more resistant Cretaceous Mesaverde Group. The Mesaverde Group, in this area, consists of–from oldest to youngest–the Blair Formation, Rock Springs Formation, Ericson Sandstone, and Almond Formation (Roehler, 1993). The upper part of the Mesaverde group (Lewis Shale, Fox Hills Sandstone, and Lance Formation) and the overlying Paleocene Fort Union Formation are present on the eastern flank of the Rock Springs uplift, but are missing on the western flank (Roehler, 1961; Roehler, 1993), where a paleosol formed in the Almond Formation (Kirschbaum and Nelson, 1988). Finally, the Eocene Wasatch Formation unconformably overlies the Upper Cretaceous formations (Roehler, 1993) on both flanks of the Rock Springs uplift.
The Rock Springs uplift is currently characterized by its asymmetric (west-vergent) antiformal shape and doubly-plunging geometry. Seismic data suggest that a high-angle, west-vergent reverse fault occurs under the steeper western flank of the anticline and that basement is involved in the uplift (Bradley, 1964; Garing and Tainter, 1985; Montgomery, 1996). The uplift displays ∼4.5 km of structural relief relative to the surrounding basins (Montgomery, 1996). A series of east-west–east-northeast-trending normal faults occurs within the Cretaceous section. Movement on these faults occurred during deposition of the Campanian Ericson Sandstone (Love and Christiansen, 1985; Martinsen et al., 1999).
There is no indication of pre-Laramide deformation associated with the Rock Springs uplift (Garing and Tainter, 1985; Montgomery, 1996). The major Phanerozoic orogeny that may have affected the region prior to the Laramide orogeny was the Pennsylvanian–Permian Ancestral Rockies deformation. The Ancestral Rocky Mountains are a complex of broad northwest–southeast-trending uplifts that occupied approximately the same region of the Central Rocky Mountains as the southern Laramide uplifts (Kluth and Coney, 1981; Miller et al., 1992). Three key observations suggest that the Rock Springs uplift neither coincides with earlier structures nor represents reactivation. First, the Rock Springs uplift trends north–south, and not parallel to the dominant northwest–southeast trend of the Ancestral Rockies. East–west-oriented structures attributed to Precambrian tectonic events, such as the Cheyenne belt, Wamsutter arch, and the gravity-determined Blue Rim – Red Desert structure (Bankey and Merewether, 1990), also do not correlate to the trend of the Rock Springs uplift. Second, no differences in Paleozoic stratal thicknesses are reported across the uplift from seismic and borehole data. Rather, the entire Paleozoic and Mesozoic sequence is uniformly folded above blind reverse faults that were developed during the Laramide orogeny (Montgomery, 1996). Third, the Archean Wyoming province, in which the Rock Springs uplift is found (Roehler, 1976), is less affected by the Ancestral Rockies uplift than by the Proterozoic terranes south of the Cheyenne belt (Miller et al., 1992).
Douglas Creek Arch
The Douglas Creek arch is a north–south-trending, anticlinal structure centered in northwest Colorado (Fig. 1; Kopper, 1962; Tweto, 1975; Gries, 1983; Rowley et al., 1985). The arch exposes Upper Cretaceous rocks in its core and Paleocene to Eocene rocks along the flanks. The uplift lies south of the projection of the Cheyenne belt, and thus the affected basement rocks consist of the Proterozoic terranes of the Colorado Plateau. This inference is consistent with the age of exposed Precambrian rocks both north and south of the Douglas Creek arch, in the Owiyukuts complex and the Uncompaghre uplift, respectively (Sears et al., 1982; Bradley, 1995). The Douglas Creek arch ends southward into the Uncompaghre uplift (Stone, 1977) and northward into the Rangely oil field and the exposed Paleozoic section in Dinosaur National Park. The uplift separates the Uinta Basin to the west from the Piceance Creek Basin to the east.
Upper Cretaceous and Tertiary formations are exposed from the center of the Douglas Creek arch across its flanks. Although the depositional pattern is similar to that of the Rock Springs uplift, the strata are both less exposed and less studied. The Mesaverde Group is exposed in the center of the arch, whereas Paleocene and early Eocene stratigraphic units (Fort Union and Wasatch Formations) are exposed on the flanks (Fig. 2). Two unconformities are locally observed along the crest of the arch. The oldest corresponds to a widespread Cretaceous–Tertiary unconformity, in which the Mesaverde Formation and the overlying Paleocene Fort Union Formation are juxtaposed along a subtle angular unconformity (Roehler, 1972). The second unconformity is between the Paleocene and the Eocene, juxtaposing either the Fort Union Formation or the late Paleocene – early Eocene Wasatch Formation against the Eocene Douglas Creek Member of the Green River Formation.
Isopach mapping and stratigraphic studies in the Douglas Creek arch indicate that the arch was formed during the Late Cretaceous (Campanian) through the Eocene (Kopper, 1962; Tweto, 1975; Johnson and Finn, 1986). The bedding dips are moderate (∼ 5°) in both Cretaceous and Tertiary strata, although they are slightly steeper on the E-limb (Roehler, 1972; Tweto, 1979). By analogy with better-studied Laramide uplifts, the increased dip of strata on the east side of the arch suggests that the eastern limb is underlain by an east-vergent, blind reverse fault. A well-developed series of northeast-trending normal faults occurs within the core of the antiform, with the end of fault movement constrained by lack of fault continuation upward into the Mesaverde Formation (Johnson and Finn, 1986). Final movement occurred prior to the middle Eocene Mahogany zone of the Parachute Creek Member of the greater Uinta Basin (Fig. 2).
The Douglas Creek arch does not follow any known, pre-existing structure. Although the Pennsylvanian–Permian Ancestral Rockies deformation did affect Proterozoic terranes south of the Cheyenne belt, there is no evidence for thinned or thickened deposition of Pennsylvanian strata over the arch. The trend of the Douglas Creek arch is not parallel to the east–west-oriented Cheyenne belt, the northwest-oriented Uncompaghre uplift, or the inferred northwst-oriented sutures of the Proterozoic terranes south of the Cheyenne belt (Karlstrom and Bowring, 1993).
The Uinta uplift trends east–west and is located south of the Wyoming border in Colorado and Utah (Figs. 1, 4: Ritzma, 1969; Hansen, 1986; Bradley and Bruhn, 1988; Stone, 1993b; Bradley, 1995). It is 48 km (30 miles) wide by 250 km (155 miles) long, extending from the Cordilleran (Sevier) thrust belt on the west to the Axial Basin arch near the Utah/Colorado border on the east. The uplift changes orientation slightly on its eastern end as it merges with the Grand Hogback and White River uplift, from its dominant east–west trend to an east-southeast orientation (Stone, 1986; Bradley, 1995; Gregson and Chure, 2000).
Rocks exposed in the Uinta uplift consist dominantly of Uinta Mountain Group strata, a sequence of Proterozoic quartzite and micaceous shale and siltstone. Flanking the Uinta Mountain Group are ∼1 km of Paleozoic strata and ∼3 km of Mesozoic strata (Hansen et al., 1983; Rowley et al., 1985; Gregson and Chure, 2000). Near the Colorado/Utah border, basement rocks below the Uinta Mountain group are exposed. These are the Proterozoic gneisses of the Owiyukuts complex, which occur north of the Douglas Creek arch in fault-bounded blocks (Sears et al., 1982; Bradley, 1995).
Unlike the Rock Springs and Douglas Creek structures, the Uinta uplift represents reactivation of a pre-existing structure. The Uinta uplift coincides with an east-trending Middle to Upper Proterozoic sedimentary trough that accommodated deposition of 7 to 10 km of Uinta Mountain Group sediments (Hansen, 1957, 1986; Stone, 1993b). These strata were subsequently tilted northward and beveled by erosion prior to deposition of the Cambrian Lodore Formation (Gregson and Chure, 2000). The Uinta area was tectonically quiescent from Cambrian through part of Cretaceous time (Hansen, 1965; Stone, 1986). The main uplift of the Uinta Mountains occurred during the Cretaceous and Early Tertiary, overlapping with both the Sevier and Laramide orogenies (Bradley and Bruhn, 1988; Bryant and Nichols, 1988; Gregson, 1994; Paulsen and Marshak, 1999; Johnston and Yin, 2001).
The Uinta uplift is essentially an east–west-trending, large-scale arch with outward-vergent thrust faults (Fig. 4; e.g., Gries, 1983; Hansen, 1986; Bruhn et al., 1986; Bradley and Bruhn, 1988; Stone, 1993b). Major thrust faults occur on both the north and south sides of the range (e.g., Bradley, 1995). Important for this study, the Uinta uplift exhibits along-strike variations in exhumation along its east–west trend. Two domal closures are inferred within the trend of the arch, where the east–west-oriented Uinta Mountains intersect other north–south-oriented foreland structures (Fig. 4; Ritzma, 1969). The westernmost dome forms at the intersection of the aligned north–south-oriented Moxa, Church Buttes, and Bridger Lake structures, and the easternmost dome occurs along the north–south-oriented Rock Springs and Douglas Creek structures (Hansen, 1965; Ritzma, 1969; Gries, 1983). These domal structures were formed by exhumation of deeper levels of the stratigraphic succession of the Uinta Mountain group. The Owiyukuts complex is also uplifted along the easternmost dome, along the trend of the Rock Springs and Douglas Creek structures.
The Uinta uplift also records a significant sinistral strike-slip component, as constrained by field observation (Gregson and Erslev, 1997; Johnston and Yin, 2001) and consistent with paleomagnetic data from the Colorado plateau (e.g., Livaccari, 1991; Wawrzyniec et al., 2002). The sinistral component of offset is thought to have occurred in the Eocene (Johnston and Yin, 2001), the same timing as for sinistral motion on other east–west-trending features in the region (e.g., Tikoff and Maxson, 2001). Clockwise rotation of the Colorado plateau and right-lateral slip on the features near the Colorado Front Range also require 8–12 km of left-lateral slip on the Uinta trend (T. Wawrzyniec, personal communication, 2005). Clockwise rotation of the Colorado Plateau is one mechanism for left-lateral slip on the Uinta trend, although differential east-northeast–west-southwest shortening between the Wyoming province and the Colorado Plateau could also cause the same effect. Farther east along the Cheyenne zone, where it trends northeast–southwest, dextral deformation is inferred to occur within the Quealy wrench duplex (Stone, 1995). How this dextral offset interacts with the sinistral offset further west is unknown.
Synorogenic stratal units record two episodes, latest Cretaceous and Eocene, of the uplift of the Uinta Mountains. Uplift and erosion of 2.3–3.4 km of strata occurred near Clay Basin between the end of deposition of the Upper Cretaceous Ericson Sandstone and subsequent deposition of the Paleocene Fort Union Formation (Bradley, 1995). Moreover, strata of Campanian Ericson Sandstone are overturned, dipping 50° to 80° to the south. The overlying Paleocene Fort Union Formation strata dip 10° to 25° to the north and are upright (Bradley, 1995). The dip of the Paleocene strata and the lack of dip on Oligocene strata indicate that the second phase of deformation occurred in the Eocene.
Cheyenne Belt and Basement Exposures
The field area is crosscut by the projection of the east–northeast-trending Cheyenne belt in the subsurface; this projection is thought to occur at the northern boundary of the Uinta uplift (Fig. 1) and forms the boundary between the Proterozoic terranes of the Colorado Plateau and the Archean Wyoming Province. The Cheyenne belt has been interpreted to extend, from west to east through the Uinta Mountains, Cherokee arch, Sierra Madre Mountains, Laramie Range, Hartville uplift, and the southeastern flank of the Black Hills (e.g., Stone, 1969; Hills and Houston, 1979; Allmendinger et al., 1982; Brewer et al., 1982; Sears et al., 1982; Johnson et al., 1984; Bryant, 1985; Houston, 1993; Stone, 1993a). Presently, a northeast-striking series of fault blocks separated by zones of intense mylonitization occurs along the Cheyenne trend (Bryant and Nichols, 1988; Houston, 1993). The Cheyenne belt is interpreted to have formed in the early Paleoproterozoic, when a Proterozoic oceanic arc terrane was accreted to the southern margin of the Wyoming province during an oblique collision (e.g., Snoke, 1993). Alternatively, a transform origin has been suggested (Warner, 1978). The last episode of accretion culminated in penetrative deformation and metamorphism about 1700 Ma (Hoffman et al., 1988; Burchfiel et al., 1992).
Geophysical analysis of the Cheyenne belt indicates a change in crustal thickness from the Archean Wyoming province to the Colorado Plateau (Prodehl and Lipman, 1989; Sheehan et al., 1995; Lerner-Lam et al., 1998; Snelson et al., 1998; Crosswhite and Humphreys, 2003). The crustal thickness under the Colorado Plateau gradually increases from south to north, reaching ∼45 km adjacent to the Cheyenne belt (e.g., Snelson et al., 1998). A narrow zone of thick crust (∼ 50 km) occurs below the suture zone between the Archean and Proterozoic crusts, perhaps amplified by Laramide shortening related to the Uinta uplift (Snelson et al., 1998). Immediately north of the Cheyenne belt, the Archean crust is thin (∼ 40 km), but it thickens to ∼ 50 km north of the Wind River uplift.
GRAVITY FIELD INVESTIGATIONS
Field Gravity Survey
The gravity results (5,993 stations) from an area of ∼80,000 km2 in southwestern Wyoming, northeastern Utah, and northwestern Colorado are presented in a United States Geological Survey report (Bankey and Merewether, 1990). Three new gravity surveys were conducted in the region to further delineate the regional structure (Fig. 5). The first was a detailed, east–west transect across the Rock Springs uplift in Wyoming. The second was a regional-scale survey on the Douglas Creek arch. The third was a regional-scale survey on the eastern end of the Uinta uplift, near the town of Maybell, Colorado. These later surveys were necessary to fill the gaps of the regional gravity compilation of Bankey and Merewether (1990).
In all three new surveys, gravity measurements were taken with a Lacoste & Romberg Model G gravimeter. The baselines were established with the local gravity base stations set by the United States Geological Survey or the Utah Geological Survey. Accurate locations and elevations of the gravity stations were obtained using a GPS (Leica 200) system run in differential mode. The horizontal location accuracy of this system is +/− 3 cm and generally better than +/− 1 cm, with vertical elevation uncertainties approximately twice those values (Mederos, 2004).
Both pre-existing and newly acquired gravity data are relative to the IGNS-71 datum (Morelli, 1974) and were reduced to the Bouguer anomaly using the 1967 gravity formula (International Association of Geodesy and Geophysics, 1971) with a reduction density of 2.67 g/cm3. Terrain corrections were made radially from each station to a distance of 166.7 km using the computer method of Plouff (1977). The estimated accuracy is ± 1 mGal. A total of 6,118 gravity readings, combining the new and existing gravity data, were contoured at an interval of 5 mGals.
Bouguer Anomaly Map
The study area shows Bouguer gravity anomalies that range from −280 to −180 mGals. Basins correspond to low gravity anomalies (−235 to −280) caused by low-density sedimentary rocks, and uplifts correspond to high gravity anomalies (−180 to −235) caused by denser basement rocks that were uplifted (Fig. 5). The basins in the map area are the Green River, Uinta, Great Divide, Washakie, Sand Wash, and Piceance. These basins are divided by north–south-oriented uplifts (Rock Springs uplift, Douglas Creek arch) and by the east–west- oriented uplifts (Uinta uplift, Cherokee Ridge arch) (Bankey and Merewether, 1990). The Bouguer gravity anomaly map also shows subtle east–west-trending features north and south of the Cheyenne belt that are sub-parallel and possibly reflect Precambrian structures (Bankey and Merewether, 1990). These features are the Blue Rim–Red Desert gravity anomaly at the northern end of the Rock Springs uplift and an unnamed anomaly at the southern end of the Douglas Creek arch.
The Rock Springs uplift produces a 35 mGal high anomaly with values that range from −235 to −200 mGals (Fig. 5). The Rock Springs uplift gravity anomaly is elongated and slightly arcuate to the west. It is also slightly asymmetrical with a steeper gradient on the western side of the uplift. The north end of the Rock Springs uplift anomaly is contiguous with, and perpendicular to, the Blue Rim – Red Desert anomaly. The Blue Rim – Red Desert produces a 10 to 20 mGal high anomaly oriented east–west (Bankey and Merewether, 1990).
The Douglas Creek arch produces a 49 mGal high anomaly, with Bouguer values that vary from −235 to −184 mGals. The gravity anomaly associated with the Douglas Creek arch is elongated in the north–south direction with the northern end slightly arcuate to the east (Fig. 5). It is asymmetrical with a steeper gradient on the eastern side of the uplift.
The Uinta Mountains produce a high anomaly that ranges from 30 to 50 mGal (Fig. 5). The new gravity data show that the maximum amplitude of the Uinta uplift anomaly is located between 90°50′ and 90° 00′ W longitude. This location correlates exactly with the easternmost of the two domal structures in the Uinta Mountains proposed by Ritzma (1969; following Hansen, 1965). A minor gravity high anomaly continues to the east from this maximum, underneath Cherokee Ridge arch. This 30 to 50 mGal high anomaly represents the easternmost subsurface extension of the Uinta uplift along the Cheyenne belt. This result is somewhat unexpected, because the trend of the exposed Uinta Mountain Group rocks changes orientation to the southeast while the gravity anomaly does not.
The more prominent gravity feature is a north–south-trending anomaly that encompasses the maximum anomaly in the Uinta uplift and continues north to the Rock Springs uplift and south to the Douglas Creek arch. This gravity feature crosses the Cheyenne belt, although its trend is not perfectly linear. The Rock Springs uplift anomaly is arcuate to the west while the Douglas Creek arch, together with the eastern end of the Uinta Mountain anomaly, is arcuate to the east. The trend also has a broad, sinistral offset (∼ 24 km) in the Uinta uplift such that the Douglas Creek arch is displaced eastward relative to the Rock Springs uplift.
Construction of gravity models requires knowledge of the density of the modeled geological units. Hand samples of igneous and metamorphic basement rocks were collected from the nearest exposures in order to determine density of these units. Because the field area occurs on both sides of the Cheyenne belt, samples were collected north of the belt for the Wyoming province and south of the belt for the Proterozoic crust. Density was determined using a pychtnometer on multiple rock types and samples. While neither dataset is stastically significant enough to characterize the region, they do provide constraints on the modeled basement density. The methodology is given in Mederos (2004) and the data given in Appendix 1.
The Wyoming Archean province is exposed in the cores of the mountain ranges in Wyoming and Montana (Hedge et al., 1986). The Wind River Range, the closest range to the Rock Springs uplift and located to the north-northwest, was sampled for density measurements. The Precambrian crystalline rocks exposed in the Wind River Mountains consist mostly of felsic gneisses, tonalitic to granodioritic plutons, and metasedimentary rocks (Steidtmann and Middleton, 1991). A suite of eight measured granitoids and amphibolites provides an average density of 2.81 g/cm3.
The Owiyukuts complex is the oldest unit exposed in the Uinta arch along the Colorado/Utah border (Canyon of Beaver Creek, Mountain Home and Bender Mountain) (Hansen, 1965; Graff et al., 1980; Sears et al., 1982; Bradley, 1995). Although within the Uinta uplift, the Owiyukuts complex is part of the Proterozoic terranes that constitute the Colorado Plateau. A Paleoproterozoic age was assigned to these rocks based on Rb-Sr whole rock isochron dates (Sears et al., 1982). Outcrops of the Owiyukuts complex are the nearest exposures of Precambrian crystalline rocks to the Douglas Creek arch and are directly on strike with this feature. These rocks are mainly potassium feldspar-rich granitic gneisses that alternate with quartzo-feldspathic gneisses, garnet gneisses, biotite gneisses and garnet amphibolites (Graff et al., 1980). The average density is 2.74 g/cm3, based on twelve samples and a wide variety of rock types.
GRAVITY DATA CONSTRAINING UPLIFT GEOMETRY
Methods and Constraints
To compare the geometry of the Rock Springs uplift and Douglas Creek arch, two-dimensional forward and inverse modeling was performed along transects A–A′ and C–C′ (Fig. 5). The gravity modeling was constrained by rock density and stratigraphic information. For the Rock Springs uplift, 2.81 g/cm3 was the value used for basement rocks and multiple different density values were used for sedimentary rocks (Wollard, 1962). For stratigraphic constraints, we used well-log data and and proprietary two-dimensional, reflection seismic lines (Mederos, 2004). The Rock Springs area has significantly more abundant seismic and well-log data, which better constrain the gravity models. This crucial information provided the depth of the basement, thickness of stratigraphic units, and their subsurface geometry. The composite seismic line analyzed by Garing and Tainter (1985) traverses southwestern Wyoming and crosses the Rock Springs uplift.
Equivalent data are scarce on the Douglas Creek arch. Data for Paleozoic, Mesozoic, and Tertiary thickness in the Piceance and Uinta Basins were collected from Osmond (1965), Cole (1975), Johnson (1985, 1989), Johnson and Finn (1986), and Gregson and Chure (2000), in addition to available borehole data. A basement density of 2.74 g/ cm3 was used, based on our density measurements.
The regional forward models were generated using WingLink Geophysical Interpretation Software (Geosystem SRL). Both cross sections were also constrained by surface geologic data. Two-dimensional inversion modeling on transects A–A′, B–B′ and C–C′ was conducted using the Program GMODEL from Lacoste & Romberg, following the method of Talwani (1973).
A forward model for the Rock Springs uplift is shown in Figure 6A. The A–A′ profile has an east–west orientation, a 6.2:1 vertical exaggeration, and crosses the eastern Green River Basin, the Rock Springs uplift and the western part of Washakie Basin (Fig. 1). A positive anomaly of 33 mGals occurs from the westernmost inflexion of the curve at −235 mGals to the highest value of ∼ −202 mGal. The modeled uplift is asymmetric with a slightly steeper flank on the west side of the uplift. The amount of uplift, using a base value of −235 mGal, is approximately 3 km on both sides of the arch. The model accommodates the possibility of a blind reverse fault beneath the western flank of the uplift (Fig. 6A), an interpretation supported by the seismic profile from Garing and Tainter (1985).
Thickness of the units along the gravity profile is tied to formation thicknesses measured in key wells (Mederos, 2004). To compensate for the observed gravity, a proposed thickening of the uppermost Upper Cretaceous units (K1) toward the basins was necessary. The thickening of the uppermost Upper Cretaceous units is also reported by Garing and Tainter (1985) based on seismic data. This thickening is more conspicuous west of the blind thrust that defines the western side of the uplift. The modeled gravity also accommodates a dramatic thinning of the Tertiary strata that must occur on both flanks of the structure.
The forward model for the Douglas Creek arch is shown in Figure 6B. The C–C′ profile has an east–west orientation and it crosses the eastern Uinta Basin, the Douglas Creek arch, and the western part of Piceance Basin (Fig. 5). The profile shows a gravity high Bouguer anomaly that goes from the basins (−255 mGals) to the top of the uplift (∼ −200 mGal). The uplift is asymmetric and verges to the east. The vertical relief across the uplift is approximately 3.5 km on the western flank and 4.5 km on the steep eastern flank.
Similar to the Rock Springs forward model, the Douglas Creek model does not show changes in thickness of the Paleozoic (Pz) and Triassic-Jurassic (Tr-Jr) units (based on Gregson and Chure, 2000). The model for the Douglas Creek arch also requires thinning of the uppermost Upper Cretaceous units toward the arch. The thinning of these units is more conspicuous on the eastern flank. However, no well data were available to accurately constrain unit thicknesses. The Douglas Creek arch forward model also requires a dramatic thinning of the Tertiary strata on both flanks of the structure. In order to match the calculated and the observed gravity data, the Tertiary strata in the Uinta Basin west of the uplift must be thicker than the equivalent section in the Piceance Basin east of the uplift.
Two-dimensional Inversion Models
The two-dimensional profiles shown on Figure 7 reflect the geometry of the basement along three east–west traverses (A–A′, B–B′ and C–C′) in the Rock Springs uplift, the Uinta Mountain uplift and the Douglas Creek arch respectively. The inversion technique is particularly useful in this study because the approach is more objective than the forward modeling. The disadvantage of this technique is that it requires simplistic density contrasts and the detailed density measurements of the sediments are not utilized. However, the results obtained from applying two-dimensional inversion corroborate some observations from the Bouguer anomaly map and the forward gravity models.
Transect A–A′ (see Figure 5 for location) indicates that the Archean basement underneath the Rock Springs structure is arched and asymmetric, with a steeper western flank. The crest of the arch is slightly concave and irregular, possibly resulting from faulting in the hinge. The amount of uplift of the Precambrian rocks is approximately 4 km on both flanks. The Douglas Creek Arch (transect C–C′) is higher, narrower and more asymmetric than the Rock Springs uplift. Here, the total uplift of the basement is approximately 4.5 km on the western side and approximately 5 km on the eastern side (see dashed lines on Figure 7). Note that greater uplift is required on the Douglas Creek arch than the Rock Springs uplift, despite the similar gravity anomalies, because of the less dense basement of the Proterozoic terranes (2.74 g/cm3) relative to the Archean basement (2.81 g/cm3).
A two-dimensional model was also constructed for the Uinta Mountain uplift (Fig. 7B). Transect B–B′ is located along the Uinta uplift in an area where basement rocks are not exposed. The inversion shows that the basement is higher toward the eastern edge of the anomaly. The maximum gravity anomaly is located near Clay Basin, where the maximum uplift along the Uinta thrust has been reported (e.g., Bradley, 1995).
Summary of Gravity Results
Observations from the Bouguer anomaly map, the two-dimensional forward models, and the two-dimensional inverse models indicate that the Rock Springs uplift and Douglas Creek arch form a continuous linear gravity anomaly with similar gravity magnitude and similar gravity gradient. In spite of their similarities, the uplifts are not identical. Douglas Creek arch has a larger gravity signal that correlates to a model of slightly larger basement relief, due to the lower density of its basement rocks. The Rock Springs uplift anomaly is slightly arcuate to the west while the Douglas Creek arch together with the eastern end of the Uinta Mountains is arcuate to the east.
The gravity data suggest that both the Rock Springs uplift and the Douglas Creek arch are continuous at basement depth, and consequently may be the same structure. The hypothesis that they are the same structure predicts that they must have formed at the same time. This section reviews the constraints on the timing of uplift, determined from stratigraphic relations, to determine if both uplifts could have formed simultaneously.
Rock Springs Uplift
The Rock Springs structure underwent several phases of uplift during the Late Cretaceous that had significant impact on depositional styles within the Sevier foreland basin (Roehler, 1993; Devlin et al., 1993; Martinsen et al., 1999). The initial uplift is interpreted to have occurred during deposition of the Ericson Formation, as evidenced by unconformities (Devlin et al., 1993; Martinsen et al., 1999) and fluctuations in sediment supply (Martinsen et al., 1999). The Cretaceous/Tertiary boundary on the western flank of the Rock Springs uplift is defined by a slight angular unconformity between the Almond Formation (Campanian) and the Fort Union Formation (Paleocene). The Maastrichtian Lewis Shale, Fox Hills Sandstone, and Lance Formation are missing on the west flank of the Rock Springs uplift (Roehler, 1961), but present on the east limb. These data indicate that either the formations were never deposited in this location, because it was a local topographic high, or that the sediments were deposited and subsequently removed. Either interpretation requires uplift of the western flank relative to the eastern flank. McMillen and Winn (1991) established that the upper part of the Almond Formation, the Lewis Shale, the Fox Hill Sandstone and the lower part of the Lance Formation form an unconformity-bounded sequence that does not correlate to the eustatically controlled systems tracts of Haq et al. (1987), implying tectonic control of sedimentation related to the Laramide orogeny.
Structural movements on the Rock Springs uplift were indicated on seismic data by scouring, onlap and thinning of reflections over the current uplift (Mederos, 2004). The timing of the movements was constrained with well data. Two main episodes of uplifting occurred in the Rock Springs uplift. The first episode started during the Campanian as evidenced by scouring of the Ericson Sandstone and onlaps on top of the Almond Formation. The maximum uplift, in the Maastrichtian, resulted in obvious thinning of the Lance Formation over the crest of the arch. The second and final pulse occurred during the late Paleocene–early Eocene. Truncation and onlaps over the top of the Fort Union Formation (Paleocene) and dramatic thinning of Early Eocene units occurred during that time. On the seismic profile presented by Garing and Tainter (1985), the Rock Springs uplift displays 4.5 km of structural relief relative to the surrounding basins and contains a blind thrust located on its western flank.
Douglas Creek Arch
The Douglas Creek arch is less well studied than the Rock Springs uplift, and the stratigraphic relations on the top of the structure are more poorly exposed. A series of geological maps from the United States Geological Survey provides the best data from the crestal area, which is not covered by Eocene strata (e.g., Roehler, 1972). Two unconformities are observed, the first between the Late Cretaceous and earliest Tertiary strata and the second between the Paleocene and Eocene strata (Mederos, 2004). The older unconformity juxtaposes the Cretaceous Mesaverde Formation and the Paleocene rocks of the Fort Union Formation along a subtle angular unconformity. The second unconformity juxtaposes the Late Paleocene – early Eocene Wasatch Formation and the Eocene Douglas Creek Member of the Green River Formation. This unconformity is of a larger magnitude in the Brushy Point area, located in the center of the Douglas Creek arch. In this location, the Douglas Creek Member of the Green River Formation directly overlies the Fort Union Formation. The Wasatch Formation is missing in this area due to erosion or non-deposition.
The timing of basin evolution is best illustrated through the use of isopach maps. Johnson and Finn (1986), using detailed basin stratigraphy, displayed the evolution of the Douglas Creek arch through four isopach maps of different age intervals: 1) Cenomanian–Campanian, 2) Campanian–K/T unconformity, 3) K/T unconformity– early Eocene, and 4) middle Eocene. The isopach maps (Fig. 8 A–D) show that the crest of the Douglas Creek arch migrated eastward through time as uplift progressively localized into the present arch structure. The initiation of arch-like uplifting is evident on the Campanian through the Cretaceous–Tertiary unconformity isopach. Thinning of these strata, particularly in Uintah County (Utah), indicates shoaling of a broad region to the west of the present Douglas Creek arch. An originally broad arch narrowed during the Paleocene, dividing the greater Uinta basin into the Uinta and Piceance Creek sub-basins. The isopach map of the middle Eocene (Figure 8D) shows thinning of Eocene units toward the arch. Finally, from early through middle Eocene time, the strata of the Green River Formation filled both basins with lake deposits and subsequently buried the arch (Johnson and Finn, 1986).
The timing of uplift of the Uinta Mountains is difficult to determine in as precise a manner as the Rock Springs or Douglas Creek structures, as there are no relevant stratigraphic relations exposed around the uplift. It is generally agreed that most of the uplift occurred during the Laramide orogeny during Eocene time (e.g., Gries, 1983; Hansen, 1986; Bradley and Bruhn, 1988). Bradley (1995), however, indicated that early (pre-Eocene) uplift of the Uinta Mountains occurred in two areas: 1) on the North Flank thrust at the western end of the range; and 2) on the Uinta thrust near the Utah–Colorado border. These areas correspond to the two domal structures of Ritzma (1969; following Hansen, 1965). The eastern domal feature is located along the gravity high that corresponds to the Rock Springs–Douglas Creek structure (Fig. 4). In order to explain sedimentation patterns in the Paleocene Currant Creek formation, Bruhn et al. (1986) also indicate that the Uinta Mountains were locally uplifted by Paleocene time.
An indirect method of determining if there was localized Late Cretaceous uplift of the eastern Uinta uplift is through study of Eocene reverse faulting. The maximum contraction of the Uinta thrust occurred in the vicinity of Clay Basin, located at the Rock Springs–Douglas Creek arch intersection (Hansen, 1965). The amount of north-vergent thrusting decreases both eastward and westward from the eastern dome. The same observation could be used qualitatively for the south side of the Uinta uplift. These reverse faults in Dinosaur National Monument (on the trend of the Douglas Creek arch) continue farther south (basinward) than south-vergent reverse faults located east or west of the Monument, on the south side of the Uinta Mountains.
If the thrusting is related in a direct way to a critical taper model of thrust formation, which is often assumed regionally (e.g., DeCelles and Mitra, 1995), the domal areas had higher topographic relief than elsewhere along the trend of the Uinta uplift. To explain this observation in terms of timing, either 1) the domal areas were already uplifted prior to Eocene uplift or 2) there were significant along-strike variations in amount of Eocene reverse faulting that happened to correspond to the trend of Late Cretaceous structures. The former scenario appears more likely and the prior uplift must have occurred in the Late Cretaceous, given the lack of stratigraphic thickness changes in the Paleozoic and early Mesozoic sections. Thus, the data suggest that uplift at the intersection of the Rock Springs–Douglas Creek arch and the Uinta uplift occurred in the Late Cretaceous.
Summary of Timing Relations
A comparison of the timing relations indicates the following: 1) The Mesozoic–Tertiary periods of uplift and quiescence of the Rock Springs uplift and Douglas Creek arch are identical and these structures formed simultaneously; 2) The stratigraphic and structural record in the basins indicates that during the Late Cretaceous there was local uplift of the east–west Uinta uplift, along the trend of the Rock Springs uplift and Douglas Creek arch.
A Continuous Rock Springs – Douglas Creek Arch
The continuity of the basement structure through the intervening Uinta uplift, suggested by the regional Bouguer anomaly map, is interpreted to indicate that the Rock Springs uplift and Douglas Creek arch are two parts of the same structure. The similarity of basement geometry, upper Cretaceous stratigraphic relations, and timing of uplift are compelling supporting evidence for this interpretation. We summarize these results below before discussing implications for foreland deformation in the Laramide orogeny.
The basement rocks of the Rock Springs uplift and the Douglas Creek arch, together with those of the eastern part of the Uinta uplift, form a distinctive north-south alignment of gravity anomaly highs that range from −184 to ∼ −235 mGals. The trend is approximately linear, except for a sinistral bend near the Uinta uplift that displaces the Douglas Creek arch eastward relative to the Rock Springs uplift. The continuity of this structure was hypothesized previously (e.g., Gries, 1983). The continuity of the gravity signal is consistent with the hypothesis that the Rock Springs uplift and Douglas Creek arch initially developed as a single arch.
There are two lines of geological evidence suggesting a continuous Rock Springs-Douglas Creek arch across the Uinta uplift during the Late Cretaceous. First, the Uinta uplift contains two separate structural culminations along its east–west extent expressed in the Proterozoic outrcrop. The eastern culmination coincides with the alignment formed by the Rock Springs–Douglas Creek arch, which contains the only exposures of basement (the fault-bounded Owiyukuts complex) within the Uinta uplift (Fig. 4; Graff et al., 1980; Sears et al., 1982; Bradley, 1995). The western culmination coincides with the Moxa-Church Buttes–Bridger Lake structural element (Ritzma, 1969). Second, as discussed in the previous section, both culminations were probably topographic highs during Eocene tectonism.
The amplitudes of the gravity anomaly and the basement relief observed in the forward and inversion gravity models are similar for both arches. The Rock Springs uplift produces a 35 mGal high gravity anomaly elongated in the north-south direction, for which gravity modeling indicates 3.5 km of uplift of the Archean basement. The Douglas Creek arch produces a slightly larger anomaly (49 mGal) that is also elongated in the north–south direction. The uplifting undergone by the Proterozoic basement underneath the Douglas Creek area is approximately 4.5 km relative to the surrounding basins. The apex of basement beneath both uplifts is approximately 3 km below the present surface.
Eocene-age Tectonism: Sinistral Offset and Uplift of the Uinta Mountains
The gravity data represent the present-day crustal geometry. Thus, it is necessary to distinguish between the Late Cretaceous and Eocene deformation in the area. We hypothesize that the Rock Springs – Douglas Creek arch formed during the Campanian as a single linear structure, which was offset sinistrally and progressively deformed into an arcuate shape in the Eocene.
A sinistral offset near the Uinta uplift displaces the Douglas Creek arch eastward relative to the Rock Springs uplift (Fig. 9). This offset presumably occurred during the latest uplift of the Uinta Mountains. Local studies indicate sinistral strike-slip movement during the Tertiary (e.g., Johnston and Yin, 2001). Similar timing for sinistral movement on the east–west-trending, North Owl Creek fault in southern Wyoming is reported by Sundell (1990) and Paylor and Yin (1993).
Both arches show a basement geometry that is asymmetrical, with their steepest gradient and vergence facing in opposite directions (Fig. 9). The Rock Springs uplift verges westward on a west-dipping reverse fault. The Douglas Creek arch, in contrast, verges to the east, as determined by the gravity data and sedimentological patterns (Johnson and Finn, 1986). The best evidence for timing of the eastward vergence comes from the sedimentological record in the adjacent basins. In contrast to the more regional homogeneity of the Cretaceous strata, the Rock Springs uplift and the Douglas Creek arch exerted some tectonic control over the sedimentation pattern of the Eocene Green River Formation, specifically for the evaporite deposits. Evaporite deposits (Wilkins Peak Formation) of the Green River Basin (Roehler, 1993) are found immediately west of the Rock Springs uplift. If subsidence occurred as a result of thrust loading, the sedimentation pattern is consistent with the westward vergence of the Rock Springs uplift during the Eocene. In contrast, similar evaporite deposits occur immediately eastward of the Douglas Creek arch in the eastern part of the Piceance Basin, consistent with the eastward vergence of that structure. On the opposite side of both structures (the Washakie Basin east of the Rock Springs uplift and Uinta Basin west of the Douglas Creek arch), no evaporites are present. This pattern suggests that thrusting and formation of arcuate map patterns for the uplifts occurred during Eocene deposition (Fig. 9). A more complicated interaction of sedimentation and tectonics is likely: The uplifts could have also acted as barriers (lake spills) that restricted freshwater flow to the Green River and Piceance Basins during the middle Eocene (A. Carroll and M. Smith, personal communication, 2003).
Implications for Basement-involved Uplifts
The results allow us to draw some implications for the nature and evolution of basement-involved uplifts in the Laramide foreland. There are three unique aspects of the Rock Springs–Douglas Creek uplift: 1) The syn-depositional basinal sedimentation provides information on timing and evolution of arch uplift; 2) The features are not as fully developed as other Laramide-style block uplifts that expose Precambrian cores, allowing one to study the early phases of development of these structures; 3) A continuous structural feature occurs within the Archean lithosphere, Proterozoic lithosphere, and the boundary zone between the two (Cheyenne belt and Uinta Mountains). The intrabasinal nature of the uplifts allows us to provide several other constraints on Laramide uplifts, outlined below. We hypothesize that these results apply to other, better known Laramide uplifts in the Rocky Mountain foreland, whose record was removed or buried by larger amounts of shortening.
Laramide Structures Initiate as Arches
The simplest model for the continuous Rock Springs–Douglas Creek structure is that it initiated in the Late Cretaceous as a wide feature west of its present location. The Douglas Creek arch is better understood in this context, because of the study of Johnson and Finn (1986), although stratigraphic relations are consistent with the same interpretation for the Rock Springs uplift (Fig. 3). One caveat is that formation of the Rock Springs–Douglas Creek arch best describes the early (Late Cretaceous) part of the Laramide orogeny, and may not be applicable to younger (Tertiary) periods of Laramide tectonism.
The concept of arches is not a new one in the Rocky Mountains. The general pattern of deformation for the eastern Rocky Mountains area is an arched structure with a central domal core that exposed Precambrian basement flanked by outward-verging thrusts (Keefer and Love, 1963; Gries, 1983; Erslev, 1993). This geometry was first clearly elucidated by Berg (1962) and was modeled physically, using analog materials, by Sales (1968).
Laramide Structures Do Not Always Reactivate Basement Structures
Basement structure and the role of basement heterogeneities are commonly invoked in the formation of Laramide block uplifts. The lack of pre-existing structures, however, along the trend of the Rock Springs and Douglas Creek uplifts indicates that block-uplift style foreland deformation can occur without pre-existing features. An initially broad (>100 km) uplift is most consistent with an uplift formed by large-scale folding. Broad uplift is not consistent with reactivation of a basement-controlled fault. In fact, the reverse fault on the west flank of the Rock Springs structure and interpreted reverse fault on the east flank of the Douglas Creek arch are arguably younger (Eocene) features. Thus, the fundamental nature of Laramide uplifts does not require reactivation of deep-seated basement faults.
Late Cretaceous Laramide Shortening Is Oriented East–West
A major debate exists concerning the orientation of the shortening (or the interpretation of stress direction) during the Laramide orogeny. Gries (1983), for example, indicates that shortening is oriented east–west during the Late Cretaceous Laramide orogeny, and rotates to northeast–southwest and north–south during later stages of tectonism. Bird (1998), among others, argues for a constant northeast–southwest orientation throughout the Laramide orogeny. The difficulty is that the shortening direction is commonly inferred from uplifts that reactivate basement faults, and these do not provide a robust measure of shortening direction.
The Rock Springs–Douglas Creek arch is an ideal place to evaluate shortening, as 1) It apparently does not reactivate a pre-existing feature; 2) It develops as such a large structure (100 km east–west by ∼250 km north–south) that it provides regional constraint; and 3) The feature forms by arching (folding), which provides a robust measure of shortening. The north–south orientation of the arch indicates east–west shortening. At present, there is no evidence for strike-slip or oblique slip faulting in the core of the Rock Springs–Douglas Creek arch, suggesting that a large wrench component is unlikely.
The Control of Laramide Tectonism Is Lithospheric, Not Crustal
The Rock Springs uplift and the Douglas Creek arch lie on different sides of the Cheyenne belt, which separates Archean crust on the north from Proterozoic crust on the south (e.g., Sears et al., 1982; Johnson et al., 1984; Karlstrom and Bowring, 1993). Significant variations exist in crustal structure, thickness, and composition across the Cheyenne belt (Prodehl and Lipman, 1989; Sheehan et al., 1995; Snelson et al., 1998; Crosswhite and Humphreys, 2003). The Proterozoic crust below the Douglas Creek structure is regionally thicker relative to the adjacent Archean Wyoming province below the Rock Springs structure (Snelson et al., 1998). The presence of a through-going Rock Springs–Douglas Creek arch in both crustal types requires that block-uplift style deformation cannot result solely from reactivation of crustal heterogeneities.
This inference suggests that the geometry of the foreland deformation of the Laramide orogeny, on a large scale, may be controlled by lithospheric rather than solely crustal features. Crustal features do have obvious local effects, but may not control the pattern of foreland deformation on an orogenic scale. This inference is most consistent with a lithospheric buckling mechanism for Laramide tectonism (e.g., Tikoff and Maxson, 2001).
The following conclusions can be drawn regarding the geologic evolution of the Rock Springs uplift and the Douglas Creek arch during the Laramide orogeny:
The Bouguer anomaly data indicate that the Rock Springs uplift and the Douglas Creek arch form a continuous linear gravity anomaly across the Uinta Mountains. The gravity anomaly is corroborated geologically, because areas of uplift occur along much of its trend. This linearity suggests that the Rock Springs uplift and Douglas Creek arch were probably the same Cretaceous-age structure, as further evidenced by similar gravity anomaly magnitudes. The continuous arch was later reactivated and sinistrally offset (∼24 km) in the Eocene along the Uinta Mountains fault system.
The Rock Springs uplift and Douglas Creek arch have similar timing. There were two main deformational episodes for both structures. The first episode occurred in the Late Cretaceous, and the second episode occurred in the late Paleocene–early Eocene.
The Late Cretaceous uplift of the Uinta Mountains was controlled locally by the north–south alignment formed by the Rock Springs uplift and the Douglas Creek arch. The structural culmination, which records the maximum uplift on the Uinta thrust fault, exposes the Owiyukuts basement complex of the Uinta Mountains near the Utah/Colorado border.
The north–south gravity anomaly alignment (formed along the Rock Springs uplift, Douglas Creek arch, and the eastern part of the Uinta uplift) cross-cuts the Archean/Proterozoic boundary along the Cheyenne belt. This observation suggests that while crustal domains controlled local features in the Laramide orogeny, reactivation of crustal structures may not have controlled the large-scale deformational patterns within the Rocky Mountain foreland.
We would like to thank Alan Carroll, Eric Horsman, and Mike Smith for comments on the manuscript that led to significant improvements. Anadarko and BP are thanked for providing seismic lines. We thank the internal USGS reviewers Richard Saltus and Brad Van Gosen and editor Cathy Ager, as well as RMG reviewers E. Erslev and D. Stone. Art Snoke is thanked for his editorial help. We acknowledge and appreciate the role of Julie Maxson, who originally got BT interested in working in this area. The work was funded by a Packard grant to BT.
- Received March 23, 2005.
- Revision received September 27, 2005.
- Accepted October 14, 2005.