- © 2011 UW Department of Geology and Geophysics
The Eocene Green River Formation in Fossil Basin, Wyoming provides a detailed record of the paleoecology and depositional history of ancient Fossil Lake. Fossil Lake was one of three Eocene lakes that formed an extensive lake system in Wyoming, Utah, and Colorado. It began as a flood-plain lake in the southern part of Fossil Basin and expanded northward as the lake evolved. Fossil Lake went through the major stages of lake evolution, including the overfilled (Road Hollow Member), balanced-fill (Fossil Butte Member), and underfilled (Angelo Member) stages. These stages are represented in the sedimentary record by a complete suite of lake-margin to lake-center facies.
This study establishes the Road Hollow Member of the Green River Formation as representing the earliest stage of lake evolution in Fossil Lake. We also revise the boundaries for the Fossil Butte and Angelo Members of the Green River Formation, which clearly delineate the latest two stages of lake evolution. These revisions not only describe and add a previously unrecognized and thick sequence of lacustrine rocks in Fossil Basin, but help us to better understand the depositional systems that existed during each stage of lake evolution.
- Angelo Member
- balanced-fill lake basin
- Fossil Basin
- Fossil Butte Member
- Green River Formation
- overfilled lake basin
- Road Hollow Member
- underfilled lake basin
GREEN RIVER FORMATION IN FOSSIL BASIN
The Eocene Green River Formation in Fossil Basin, Wyoming is one of the world's most recognized lake deposits. The lacustrine deposits in Fossil Basin include a complete suite of lake-margin to lake-center facies. These include prograding sandstone deltas (Petersen, 1987), nearshore bioturbated micrite, and relatively deep water “oil shale,” or kerogen-rich micrite (Buchheim and Eugster, 1998). Over 120 m of lacustrine sediments were deposited in the elongated north–south basin, covering an area of 1500 km2 (Fig. 1). Fossil Lake was one of three Eocene lakes that formed an extensive, great-lakes system in Wyoming, Utah, and Colorado. Fossil Lake represents a microcosm of these larger lakes and provides a critical record of lake-margin to lake-center facies transitions that are better exposed and more accessible than those of Lake Gosiute and Lake Uinta. A rich fossil fauna and flora includes fish, amphibians, crocodiles, turtles, birds, ostracods, gastropods, bivalves, insects, trace fossils (burrows), and a variety of plants and rare mammals (Grande and Buchheim, 1994).
This study examines the thickest part of the Green River Formation in Fossil Basin that was overlooked in previous investigations. Previous geologic mapping studies (Oriel and Tracey, 1970; Rubey et al., 1975) focused on exposures north of Clear Creek (CC on Fig. 1) where the Road Hollow Member is thinner and not as well developed. The Road Hollow Member is a unique sedimentary sequence that provides a detailed history of a basin in transition from a synorogenic, fluvial-dominated basin to a lacustrine- dominated basin in which subsidence and accommodation space gradually exceeded basin fill processes. Biaggi (1989) and Buchheim (1994a, b) recognized the significance of this sequence and informally referred to this sequence as the “lower unit.” In this study, the “lower unit” of Buchheim (1994a, b) is elevated to member status and renamed as the Road Hollow Member, and the Fossil Butte and Angelo member boundaries are revised and clarified.
Previous Stratigraphic Work
Veatch (1907) published the first general geologic map of Fossil Basin. Oriel and Tracey (1970) followed over 60 years later with a detailed geologic and stratigraphic study. They described the type sections and divided the Green River Formation into the Fossil Butte and Angelo Members (Fig. 2). Recent authors have followed Oriel and Tracey's (1970) Green River Formation nomenclature in their geologic mapping of Fossil Basin. Rubey et al. (1975) published two geologic maps of the northern half of Fossil Basin (Kemmerer and Sage 15-minute quadrangles) and more recently, M'Gonigle and Dover (1992) published the Kemmerer 30′ × 60′ quadrangle geologic map followed by the Evanston 30′ × 60′ quadrangle (Dover and M'Gonigle, 1993).
The Wasatch and Green River Formations represent the alluvial, fluvial, and lacustrine sediments deposited in Fossil Basin during the Eocene. The Wasatch Formation underlies, interfingers with, grades into, and overlies the Green River Formation. A mudstone tongue of the Wasatch Formation divides the Fossil Butte and Angelo Members in the extreme northern end of Fossil Basin. In the southern end of the basin, Oriel and Tracey (1970) divided the two members by the occurrence of a bed containing algal-encrusted logs. However, the time-transgressive nature of the contact and abundance of other beds containing algal-encrusted logs along strand lines in the southern end of the basin undermine the utility of this approach. In some areas, four distinct beds contain algal-encrusted logs. At other sites, these beds appear diachronous or time-transgressive relative to isochronous air-fall tuff beds.
Buchheim (1994a, b) divided the Green River Formation of Fossil Basin into the lower, middle, and upper units based on lithologic characteristics (Fig. 2). The lower unit was equivalent to the proposed Road Hollow Member. The middle unit was equivalent to the lower and middle part of the Fossil Butte Member. The upper unit was equivalent to the uppermost part of Oriel and Tracey's (1970) Fossil Butte Member and Angelo Member.
Lithofacies common to all members of the Green River Formation in Fossil Basin are summarized below, including: kerogen-poor bioturbated micrite (KPBM), partly bioturbated laminated micrite (PBLM), kerogen-poor laminated micrite (KPLM), kerogen-rich laminated micrite (KRLM), kerogen-rich laminated dolomicrite (KRLD), kerogen-poor massive dolomicrite (KPMD), kerogen-rich chemoturbated dolomicrite (KRCD), mudstone, sandstone, tuff, Magadi-type chert (MTC), stromatolites, oncolites, and tufa (Buchheim and Eugster, 1998 for original descriptions).
Siliciclastic lithofacies include mudstone, siltstone, and sandstone. The siliciclastic rocks are derived from volcaniclastics and older sedimentary rocks in the uplands surrounding the basin. They are concentrated around the margins of the basin and alternate with carbonate rocks throughout most of the Road Hollow Member.
Kerogen-poor bioturbated micrite (KPBM and BM) ranges from completely bioturbated to chaotic mixtures of micrite-matrix-supported intraclasts of the originally laminated sediments (pseudobreccia). Total organic-carbon content ranges from 0 to 2 percent (Buchheim and Eugster, 1998). Horizontal and vertical burrows range from 5 to 20 mm in diameter and some exhibit meniscus fillings. The predominant mineral assemblage consists of calcite with minor amounts of dolomite, amorphous kerogen, and siliciclastic material. The amount of bioturbation decreases basinward as these facies grade into partly bioturbated laminated micrite.
Partly bioturbated laminated micrite (PBLM) consists of alternating laminations of clay-sized calcimicrite and subordinate amounts of amorphous kerogen. The laminations average 0.07- to >1.0-mm thick and are typically disrupted by desiccation cracks and burrows. Invertebrates and fish in various states of preservation are common (Grande and Buchheim, 1994). Calcite, aragonite, and subordinate amounts of Mg-rich calcite, dolomite, and silicate minerals comprise the mineralogy of PBLM facies. PBLM facies grade basinward into KPLM.
Kerogen-poor laminated micrite (KPLM) is composed of alternating calcite and kerogen laminae. The light (calcite-rich) and dark (kerogen-rich) laminae couplets average 0.14 mm thick. The mineralogy of KPLM is similar to that of PBLM. In addition, desiccation cracks and siliciclastic and volcaniclastic detritus occasionally occur in this lithotype. KPLM grades basinward into KRLM.
Kerogen-rich laminated micrite (KRLM) is essentially the same as KPLM, except that kerogen content is significantly higher and the laminae are thinner. Fossil fish are abundant in some units of KRLM and tend to be better preserved than in other lithologies.
Kerogen-rich laminated dolomicrite (KRLD), similar to KRLM, has a high kerogen content (total organic carbon content ranges from ≥2 to 14 percent; Buchheim and Eugster, 1998). The dominant mineralogy is dolomite, with calcite and siliciclastic content ranging from 0 to 60 percent. Calcite pseudomorphs after saline minerals are common; they replace halite, shortite, nahcolite, and trona. These facies grade shoreward into facies containing increasing amounts of calcite.
Kerogen-rich chemoturbated dolomicrite (KRCD) is identical to KRLD except that bladed trona crystals disrupt the original laminae.
Kerogen-poor massive dolomicrite (KPMD) ranges from completely structureless to partly bioturbated. These facies consist of dolomicrite with little or no calcimicrite or kerogen. Loading and soft-sediment deformation structures are common.
Magadi-type chert (MTC) facies are a complex mixture of dolomite, evaporites, and microcrystalline chert. The chert occurs in layers of irregular blebs. The chert in the upper unit is similar to chert deposited in the Green River Basin and recent chert deposits in Lake Magadi of the East African rift system (Eugster, 1969; Eugster and Surdam, 1973). Magadi-type chert is restricted to the north-central parts of Fossil Basin.
Tuff beds are common, but are typically thin, ranging from several mm to a few cm in thickness. The thickest tuff bed, the k-spar tuff, is 8–25-cm thick depending on location. In the northwestern part of the basin it is 25-cm thick, but thins to 8 cm, 35 km to the south in the Little Muddy Creek area. The k-spar tuff is composed of authigenic k-feldspar, but grades laterally toward the lake margins into zeolite minerals and clay (Buchheim and Eugster, 1998).
Stromatolites and tufa are common in the Angelo Member and rare in the Road Hollow and Fossil Butte Members. The tufa is typically porous and forms cylinders with internal wood impressions. It often occurs as thin coatings over small twigs with up to 30 cm thick coatings covering logs. The logs are typically decayed away, leaving wood impressions in the preserved tufa covering. At some localities the stromatolites and tufa are associated with caddisfly larval cases and beetle-bored ichnofossils.
Oncolites are abundant at a number of locations along the margin of the basin and along the shorelines of apparent islands. The oncolites are dense and well laminated. The oncolite nuclei include gastropods, bivalves, fossil wood fragments, and flat cobbles.
Stratigraphic Revision and Relationships
Stratigraphic revision of the Green River Formation requires definition and clarification of the contacts of the Road Hollow, Fossil Butte, and Angelo Members in Fossil Basin. This information is summarized in Table 1.
The Wasatch Formation–Road Hollow Member contact is marked by a lithologic change from carbonate to siliciclastic (see description of the Road Hollow Member section for details). This contact becomes more complicated toward the south as the fluvial characteristics of the interfingering units become dominant.
The base of the Fossil Butte Member occurs 2 m below the lower sandwich bed (a distinct KRLM about 15-cm thick “sandwiched” between two, 1–3-cm thick tuff beds) at the base of the lower oil shale (a KRLM that is about 20–30-cm thick and occurs throughout the basin; Fig. 3, Table 1). The base of the Fossil Butte Member forms cliffs near the basin center and becomes slope-forming toward the margins. In the southern part of the basin, the Fossil Butte Member overlies the Sandstone Tongue of the Wasatch Formation. The Sandstone Tongue usually is apparent in the slope as outcrops, cliffs, or ledges of dark-brown sandstone. In the northern part of the basin, the Sandstone Tongue is absent and the Fossil Butte Member overlies the Road Hollow Member.
Oriel and Tracey (1970) originally placed the Fossil Butte–Angelo Member contact at the base of the mudstone tongue of the Wasatch Formation (Fig. 2). This approach, however, only applies in the northernmost part of the basin where the mudstone tongue occurs, precluding application elsewhere. The mudstone tongue does not occur in the type section of the Fossil Butte Member at Fossil Butte, where Oriel and Tracey (1970) placed the contact several meters below the mudstone-tongue equivalent.
The proposed Fossil Butte Member–Angelo Member contact occurs 3–4-m above the k-spar tuff bed at the top of another ochre colored tuff bed (Figs. 2 and 3). The ochre tuff bed forms an ochre-colored slope just below the contact at Fossil Butte and in the Clear Creek area south of Fossil Butte. The contact is marked by an abrupt change in slope from steep outcrops or cliffs of the Fossil Butte Member to gradual slopes of the Angelo Member. Outside of this area, the ostracodal dolostone is a more reliable indicator of the proximity of the contact (Figs. 3 and 5). The ostracodal dolostone is approximately 30 cm below the k-spar tuff and occurs as 20-40-cm wide, 10–20-cm thick slabs along the outcrop.
This proposed contact is mappable and recognizable at most locations throughout the basin, occurs above the k-spar tuff bed, and is an isochronous horizon.
The transition from the Fossil Butte Member to the Angelo Member marks a major lithotype change from calcimicrite containing fossil fish to dolomicrite with calcite pseudomorphs after sodium carbonate, saline minerals, and the general disappearance of fossil fish in the lake center facies (Table 1).
Road Hollow Member
Buchheim (1994a, b) divided the Fossil Butte Member into “lower” and “middle” subunits. The proposed revisions for the Green River Formation in Fossil Basin include elevating the previous “lower unit” designation for the lower part of the Fossil Butte Member (Buchheim, 1994a, b) to formal member status as the Road Hollow Member. The Road Hollow Member is named after the canyon in which the Road Hollow type section is located (N41º40.231′, W110º49.441′, WGS 84, Figs. 4 and 5A–C type section).
The Road Hollow Member represents the earliest stages of lacustrine deposition in Fossil Basin (Buchheim et al., 2002). The lake first formed in the southern part of Fossil Basin and expanded northward as the lake grew (Fig. 6; Buchheim et al., 2002).
At the type section (Fig. 5A), the lower boundary of the Road Hollow Member is marked by the first occurrence of greenish-gray, calcareous, tuffaceous claystone with thin, interbedded KPLM at the base of the outcrop. At other locations this basal bed of the Road Hollow occurs above the uppermost sandstone of the Main Body of the Wasatch Formation (Figs. 3 and 6). The upper boundary of the Road Hollow Member is marked by the first occurrence of KRLM (i.e., lower oil shale of the Fossil Butte Member) 1–2 m below the lower sandwich bed of the Fossil Butte Member (see the Fossil Butte Member description above; Fig. 5B). In the southern part of the basin, the lower part of Sandstone Tongue of the Wasatch Formation interfingers with the upper part of the Road Hollow Member.
The Road Hollow Member is characterized by siliciclastic units composed of sandstone, siltstone, mudstone, and claystone, and carbonate units composed of KRLM, KPLM, bioturbated micrite (BM), KRLD, KPLD, ostracodal or gastropodal limestone, and tufa. The laminated micrites contain couplets consisting of 0.1–1.0-mm thick alternating organic and carbonate laminations. Volcanic tuff, bioturbated micrite, and dolomicrite units occur in the Road Hollow Member but are less common than in the Fossil Butte and Angelo Members.
The Road Hollow Member represents a shallow, fluvial–lacustrine depositional system. It consists of numerous fluvial to deltaic siliciclastic units interbedded with various lacustrine carbonate units. The profundal lacustrine carbonates (KRLM and KPLM) grade laterally into shallow lacustrine limestone and fluvial to deltaic sandstone and siltstone. A typical lithofacies succession in the Road Hollow Member consists of sandstone, siltstone, mudstone, and micrite (KPLM and KRLM) or limestone (Biaggi, 1989). This represents a deepening upward, transgressive sequence in the Road Hollow Member.
In the southern part of Fossil Basin, a major siliciclastic sequence overlies the Road Hollow Member. Oriel and Tracey (1970) identified this sequence as the Sandstone Tongue of the Wasatch Formation. Petersen (1987) suggested that this major siliciclastic sequence was deposited by a lacustrine, Gilbert-type delta. The Gilbert-type delta in Fossil Basin is similar to the Gilbert-type delta described by Surdam and Stanley (1980) in the Green River Basin. Siliciclastic/carbonate ratios indicate increasing siliciclastic input in the southern part of Fossil Basin (Biaggi, 1989). Sandstone lithologies range from very fine to coarse grained, homogeneous to thinly and thickly bedded, and vary from light tan to gray. Sedimentary structures in the sandstone units include ripple and trough cross bedding, current lineation, and small- and large-scale planar cross bedding.
Several Road Hollow Member units are fossiliferous, containing gastropods, ostracods, fish, leaves, and other plant debris (Fig. 5A, B). KPLM units contain the most fossils, but some limestone units contain gastropods and ostracods. Ostracods are the most abundant fossil in the Road Hollow Member. Fish are less abundant than in the Fossil Butte Member and include Knightia and Diplomystus (herring), Priscacara (percoid), and Phareodus (osteoglossid).
The Road Hollow Member is best developed in the southern part of Fossil Basin in the vicinity of Chicken Creek and Road Hollow. The type section for the Road Hollow Member is exposed along the north side of Road Hollow in the southern part of Fossil Basin (Figs. 4 and 5). North of the Road Hollow type section, at Fossil Butte, the Road Hollow Member consists of bioturbated micrite interbedded with laminated micrite, capped by a 4-m thick sequence of interbedded mudstone and laminated micrite (Biaggi, 1989; Fig. 5A, B). Continuing northward from Fossil Butte, the Road Hollow Member thins and grades laterally into the Wasatch Formation (Fig. 6). South of the Road Hollow section, siliciclastic units become increasingly abundant and replace the laminated carbonates in the southern part of Fossil Basin (Fig. 6). Structureless, indurated limestone also replaces some of the laminated micrite units in stratigraphic sections toward the south. Laterally, the Road Hollow Member interfingers with the Main Body of the Wasatch Formation at the lake margins.
Biaggi (1989; Biaggi and Buchheim, 1999) initially described the detailed stratigraphy of the “lower unit” (now Road Hollow Member). He divided the “lower unit” into four beds, one of which was previously established by Oriel and Tracey (1970) as the Sandstone Tongue of the Wasatch Formation (Fig. 5A, B). Rather than include the Sandstone Tongue in the Road Hollow Member, we recommend that the Sandstone Tongue remain a formal unit in the Wasatch Formation. We propose that the three units of the Road Hollow Member below the Sandstone Tongue of the Wasatch Formation be recognized as formal stratigraphic beds (Figs. 2, 3, and 5A, B). The stratigraphy from bottom to top includes: (1) the lowermost “Lower Shale,” overlain by the (2) “Lower White Marker” unit, that in turn is overlain by the (3) “Upper Limestone.” This sequence is capped by a major deltaic sequence, the Sandstone Tongue of the Wasatch Formation (Oriel and Tracey, 1970). These lithofacies were used to correlate sections across the basin (Fig. 6).
The Lower Shale lithofacies is characterized by brown and greenish-gray rocks and consists of alternating mudstone, calcimicrite, and siliceous calcimicrite. Proximal to the basin center (Road Hollow section, RH on Fig. 1), higher organic content produced KRLM and organic rich, mudstone units. A distinctive “brown layer” occurs at Road Hollow (formerly Chicken Creek section, Biaggi, 1989) and Bear Divide (BD on Fig. 1) sections, in the lower part of the sequence. It is composed of alternating mudstone and KRLM and forms up to 12 cyclic stratal packages totaling several meters in thickness (Figure 5). Although organic-matter rich, siliciclastic content prevented the expected deposition of KRLM in this “brown layer.” In the southern part of the basin, this subunit grades laterally into more siliciclastic units, represented by interbedded sandstone, siltstone, and mudstone with a few thin limestones.
The Lower White Marker lithofacies is very apparent in outcrop. A distinctive white color dominates this unit due to weathering of oil shales and calcimicrite, many of which are chalky and form long benches. Predominance of KRLM interbedded with a few siliciclastic units suggests a deeper stage of the lake, and it corresponds to major transgressions that deposited thinner KRLM as far south as Sheep Creek and other lake margin sections at Angelo Ranch and Bear Divide. The KRLM units are richest at the Road Hollow section and appear almost black due to their high organic-matter content. They contain abundant fossils, including fish, ostracods, insects, and plant remains (Fig. 5).
The third subunit is the Upper Limestone lithofacies. It is characterized by its golden brown color. A thin unit of KRLM occurs at the base of this unit and grades upward into alternating limestone, siltstone, and mudstone. Abundant gastropods and ostracods, and in some localities pelecypods, occur in the limestone.
Thick sandstone units cap ridges in the southern half of Fossil Basin and form the Sandstone Tongue lithofacies. This subunit is the Sandstone Tongue of the Wasatch Formation and was studied in detail by Petersen (1987). He described it as a Gilbert-type delta, produced by influxes of siliciclastic material into the southern end of Fossil Lake (Fig. 6).
The Road Hollow Member was known only as a relatively thin unit of the Fossil Butte Member below the Sandstone Tongue of the Wasatch Formation when Oriel and Tracey (1970) first described the Fossil Butte Member of the Green River Formation. The Road Hollow Member is less than 5 m thick at the Fossil Butte type section. However, Oriel and Tracey (1970) reported that oil-well drilling encountered oil shales in the vicinity of Clear Creek, suggesting that a thicker, better-developed lacustrine sequence might be present. Exploration of Fossil Basin since that time has revealed that a significant lacustrine sequence does exist that has since been well documented (Biaggi, 1989; Buchheim, 1994a, b) as the “lower unit” of the Green River Formation in Fossil Basin. The unit is significant because it is widespread, mappable, the thickest unit of the Green River Formation in Fossil Basin, and represents a distinctive depositional and climatic phase of Fossil Lake. In addition, the Road Hollow Member is followed by deposition of the Sandstone Tongue of the Wasatch Formation, a distinctive sandstone deposited by a prograding delta. Sandstone Tongue deposition was followed by Fossil Lake's most extensive and significant transgression. The Sandstone Tongue is easily mapped and provides a reliable stratigraphic marker in Fossil Basin.
In addition to proposing the new Road Hollow Member, we also propose minor, but specific revisions to the contact boundaries of the Fossil Butte and Angelo Members. The proposal of the Road Hollow Member divides the Fossil Butte Member into two members. The proposed uppermost boundary of the Fossil Butte Member is approximately 9 m below Oriel and Tracey's (1970) original boundary at the Fossil Butte type section. We propose this revision because the boundary proposed by Oriel and Tracey (1970) at the type section does not match their Mudstone Tongue boundary at other sections in the basin. The miscorrelation inadvertently set up two different upper boundaries for the Fossil Butte Member in Fossil Basin. The proposed member-boundary change is significant because the newly revised Fossil Butte Member is only 8 m thick at the type section. In addition, the revised upper boundary represents a significant lithological change from laminated calcimicrite containing fossil fish to laminated dolomicrite without fossil fish. The boundary also represents a change from a relatively freshwater lake to a hypersaline–alkaline lake. Calcite pseudomorphs after saline minerals are abundant above this boundary, often disrupting the once-laminated sediment into a highly chemically disrupted sediment, referred to as chemoturbated dolomicrite (Loewen, 1999, p. 12). The proposed boundary is readily correlated throughout most of the basin and forms gentle, weathered slopes compared to the bold, cliff-forming outcrops of the Fossil Butte Member. In addition, a dark ochre tuff bed occurs at the contact and forms an ochre-colored slope below a distinctive “white marker bed” in the region from Elk Mountain north to Fossil Butte. Approximately 4 m below the contact is the “k-spar tuff”, an 8–25-cm thick tuff bed that allows precise stratigraphic correlation from Little Muddy Creek north to Schuster Basin. South of Little Muddy Creek, the Green River Formation becomes increasingly dominated by fine-grained siliciclastics and begins to lose its white, cliff-forming nature. In these areas, the contact is located 3–4 m above the ostracodal dolostone (approximately 30 cm below the k-spar tuff), a reliable stratigraphic marker at these marginal locations (Figs. 3 and 5).
The Road Hollow Member records the origins and initial history of Fossil Lake. The lake was restricted in size with its center located in the vicinity of Road Hollow. Deposition of the Road Hollow Member was controlled by continuous basin subsidence and rapid infill where sediment influx kept up with, or intermittently exceeded, basin accommodation. This formed a sequence of alternating fluvial and lacustrine beds. The lake remained relatively shallow and restricted in size, never achieving the depths and expansive size of the succeeding Fossil Butte Member lacustrine system. The Road Hollow lake system, however, deposited a 125-m thick sequence of fluvial-lacustrine rocks. The lake remained fresh during this period and represents an overfilled lake stage in the Carroll and Bohacs (1999) three-stage model of overfilled, balanced-fill, and underfilled lakes. Overfilled lake basins occur when accommodation rates are exceeded by available water and sediment supply over the depositional time span of the unit. Lake-level fluctuations are minimal because water inflow approximates outflow. These freshwater lakes are closely related to fluvial systems and mires.
The Road Hollow lake evolved and expanded to fill much of Fossil Basin during the subsequent balanced-fill lake phase (Buchheim and Eugster, 1998) of the Fossil Butte Member. Maturity was reached during Angelo Member deposition (Buchheim, 1994a) when the underfilled hypersaline stage of Fossil Lake culminated lacustrine deposition in Fossil Basin. An analogous succession of lake-basin phases was deposited in the Green River Basin, just kilometers to the east of Fossil Basin. These include the Luman Tongue (time equivalent to the Road Hollow Member), the Tipton Member (time equivalent to the Fossil Butte Member), and the Wilkins Peak Member (time equivalent to the Angelo Member). The Green River Basin lake phases and chronology are discussed in detail by Bohacs et al. (2000) and Smith et al. (2008) respectively. The relatively thick sequence of the Road Hollow Member is divided into three subunits or beds, to allow detailed vertical and lateral comparisons, and to provide a context for detailed stratigraphic studies, correlations, and mapping. The beds, in ascending order, are the Lower Shale Bed, the Lower White Marker Bed, and the Upper Limestone Bed. The Lower Shale Bed consists of alternating claystone (shales in part) and KPLM (fossiliferous and tuffaceous in part) but is predominantly siliciclastic. The lower section reflects cyclic deposition and many of the laminated beds contain abundant fossils (primarily fish and ostracods). The Lower White Marker Bed is noticeable in outcrop due to the weathering of its structureless micrite and kerogen-rich laminated micrite (oil shale) beds. Some of these beds contain significant amounts of dolomite.
The Lower Shale bed was deposited in a shallow, flood-plain lake in which carbonate deposition was diluted by relatively high influxes of siliciclastic material. The Lower White Marker Bed was formed in a deeper lake conducive to deposition of KRLM (oil shale) and represents a transgressive phase during Road Hollow Member time (Biaggi, 1989). The Upper Limestone Bed represents the continuing long-term transgression (with a minor regression) and eventual stabilization of Fossil Lake. This situation set up the stable conditions that allowed the progradation into Fossil Lake (Sandstone Tongue of the Wasatch Formation) of a large delta system that occupied the southern half of the basin (Petersen, 1987).
This paper proposes establishment of the Road Hollow Member of Green River Formation and provides revisions of boundaries for the Fossil Butte and Angelo Members of the Green River Formation. These revisions are needed in order to systematize the thick sequence of lacustrine rocks that dominate the Green River Formation in the southern half of Fossil Basin and to better understand the depositional systems responsible for their deposition. In addition, the Road Hollow Member is divided into three subunits or beds that allow detailed stratigraphic studies and correlations. The studies that have allowed these revisions represent over 25 years of detailed geologic investigations of Fossil Basin. These studies and the revisions detailed in this paper have allowed us to complete detailed mapping of the Green River Formation and associated rocks. Detailed 1:24,000-scale mapping of these members is complete and slated for publication as 12 quadrangle maps by the Wyoming State Geological Survey. Eventually, the maps will be incorporated into the state survey's GIS system and become available to the public as both digital and paper versions. In conclusion, the stratigraphic studies and geologic mapping have provided a foundation to successfully correlate and map lacustrine beds from Fossil Butte National Monument south to Interstate Highway 80 and into the Green River Basin (Fig. 1).
We are grateful for the logistical support provided by A. K. Aase and the staff at Fossil Butte National Monument, financial support from the U.S. National Park Service and Loma Linda University, field and laboratory efforts from graduate students Mark Loewen and Tom Amato, and diverse contributions from the numerous summer undergraduate students who helped us through the years.
- Received March 4, 2011.
- Revision received September 13, 2011.
- Accepted November 11, 2011.