Journal of Sedimentary Research, 2006, v. 76, 1197–1214 Research Article DOI: 10.2110/jsr.2006.096

HIGH-RESOLUTION STRATIGRAPHY OF AN UNDERFILLED LAKE BASIN: WILKINS PEAK MEMBER, EOCENE GREEN RIVER FORMATION, WYOMING, U.S.A.

JEFFREY T. PIETRAS* AND ALAN R. CARROLL 1University of Wisconsin-Madison, Department of Geology and Geophysics, 1215 West Dayton Street, Madison, Wisconsin 53706, U.S.A. e-mail: [email protected]

ABSTRACT: Lakes tend to respond noticeably to minor changes in sediment and water balance driven by climatic, tectonic, or geomorphic processes. This unique behavior of lacustrine basins can provide a high-resolution record of geologic processes within the continental setting, far from the globally averaged record of marine strata. The Wilkins Peak Member of the Eocene Green River Formation, in Wyoming, USA, is dominated by aggradation of repetitive sedimentary facies successions recording distinct lacustrine expansions and contractions. These lacustrine ‘‘cycles’’ consist of up to four successive facies associations: littoral, profundal–sublittoral, palustrine, and salt pan. Because they comprise disparate facies that may never have been simultaneously deposited in the basin, Wilkins Peak Member cycles are non-Waltherian successions that do not readily equate to any established sequence stratigraphic unit. The completeness of the stratigraphic record in the Wilkins Peak Member varies continuously across the basin. At least 126 cycles are present in the ERDA White Mountain #1 core near the basin depocenter, whereas only about one third as many are recognizable 53 km north, nearer to the basin margin. Cycle boundaries terminate northward by gradual amalgamation into palustrine facies, reflecting the interplay between varying lake levels and a south-dipping deposition gradient. Evidence for complete desiccation and hiatuses is commonplace even near the basin center; a truly complete record may not be present anywhere. Based on recent 40Ar/39Ar geochronology of tuffs, the average cycle duration in the White Mountain #1 core is approximately 10,000 years. However, the true average duration is shorter due to the presence of lacunae, and the time required for deposition of the thinnest cycles may have been less than 1000 years. No external driving mechanism is presently known for Eocene cycles of such short duration, raising the possibility that they are instead autogenic cycles related to geomorphic instability of the surrounding landscape. Evaporite deposition corresponded closely in time and space to maximum differential subsidence, suggesting that tectonic influences on drainage patterns and basin accommodation exerted a primary control. Differential accommodation was most pronounced during deposition of the lower Wilkins Peak Member and became progressively less so after that time. Based on structural and stratigraphic observations, a period of increased tectonic uplift commenced concurrent with deposition of the lower Wilkins Peak Member.

INTRODUCTION environments may occur repetitively within relatively short vertical successions (e.g., Gilbert 1882; Glenn and Kelts 1991; Talbot and Allen Much of our knowledge of continental climate and of terrestrial faunal 1996). This is especially true in underfilled basins, where stratal stacking and floral evolution comes from records preserved in lacustrine and patterns record repeated extreme fluctuations in base level (Carroll and related alluvial strata (e.g., Wolf 1978; McCune 1987; Stiassny and Meyer Bohacs 1999). 1999). Furthermore, because most long-lived lacustrine basins are A second problem common to many lacustrine deposits is the absence fundamentally tectonic in origin (Kelts 1988), they potentially provide of cosmopolitan marine fauna, which hinders biostratigraphic correlation highly resolved archives of orogenic processes and related landscape of these strata to the geologic timescale. Milankovitch-period deposi- modification (e.g., Yuretich 1989; Lambiase 1990). However, the tional cyclicity has therefore often been proposed as an alternative means sensitivity of lakes to paleoenvironmental change can also complicate of establishing high-resolution chronostratigraphy, on the basis that their stratigraphy, because of the potential rapidity of these changes orbitally controlled changes in solar insolation directly govern sedimen- relative to the time required for sediment deposition (e.g., Owen et al. tation rates and depositional environments in lakes (e.g., Bradley 1929; 1990; Oviatt et al. 1992; Scholz and Finney 1994; Benson et al. 1995). As Van Houten 1962; Olsen 1986; Fischer and Roberts 1991; Roehler 1993; a result, sedimentary facies representing vastly different depositional Fischer et al. 2004). However, the premise of externally regulated cyclicity has seldom been tested against other independent measures of elapsed or * Present address: BP Sakhalin, Inc., 501 Westlake Park Boulevard, Houston, absolute geologic time (Steenbrink et al. 1999; Pietras et al. 2003a). Texas 77079, U.S.A. Extreme Holocene lake-level changes are known to have occurred over

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FIG. 1.— Generalized geologic map of the greater green River Basin and the surrounding highlands showing the location of measured sections and selected Eocene faults (Witkind and Grose 1972; Love 1970; Steidtmann et al. 1983; Bradley 1995). Maximum extent of the Tipton Shale, Wilkins Peak, and Laney members is from Roehler (1993). Extent of evaporites is from Wiig et al. (1995). sub-precessional time scales for reasons unrelated to orbital forcing (e.g., construct the first detailed (centimeter- to decimeter-scale) regional cross Malde 1960; Johnson et al. 1996; Bouchard et al. 1998; Colman 1998; section of the entire Wilkins Peak Member. This cross section extends Reheis et al. 2002). Additionally, many previous proposals of Milanko- from near the basin center in the south to the basin margin in the north vitch cyclicity in lake deposits have been based either on single measured (Fig. 1). The unique basin-scale perspective it provides allows reinterpre- sections or wells (e.g., Olsen 1986; Fischer and Roberts 1991; Roehler tation of the controls on lacustrine evaporite deposition, of the nature of 1993) or else on one-dimensional composites of overlapping but the repetitive facies associations that dominate Wilkins Peak Member geographically separated intervals (e.g., Steenbrink et al. 1999; Olsen stratigraphy, and of the temporal significance of these ‘‘cycles.’’ and Kent 1996). It is therefore largely unknown to what extent geographic variation in sedimentary facies or in cycle preservation might REGIONAL SETTING AND STRATIGRAPHY affect the outcome of such studies. The Wilkins Peak Member of the Green River Formation of The greater Green River Basin (gGRB) formed between the Sevier fold southwestern Wyoming, U.S.A., is ideally suited for addressing these and thrust belt to the west, and several Laramide-style Precambrian-cored problems, due to its excellent exposures and to recent advances in its uplifts on the north, east, and south (Fig. 1; Dickinson 1979; Lawton 40Ar/39Ar geochronology (Smith et al. 2003). The general tectonic and 1985; Dickinson et al. 1988; Yuretich 1989). Eocene deposition of the climatic setting of the Green River Formation has been well characterized lacustrine Green River Formation in Lake Gosiute roughly coincided (e.g., Love 1970; Wolf 1978; Gries 1983; Steidtmann et al. 1983; Roehler with late pulses of shortening in the both the Sevier belt and Laramide 1991; Roehler 1993; Markwick 1994; Wilf 2000). Furthermore, previous uplifts (Love 1970; Steidtmann et al. 1983; Dickinson et al. 1988; DeCelles studies have established a sound fundamental understanding of the 1994). The occurrence of palms, ferns, gingers, and crocodilians indicate depositional environments recorded by these lacustrine strata (Culbertson a subtropical, humid climate with muted, equable seasonality and 1961; Eugster and Hardie 1975; Smoot 1983; Roehler 1992). In this study predominantly frost-free winters (MacGinitie 1969; Markwick 1994). we take advantage of the nearly continuous exposures along White Leaf-margin and leaf-area analyses yield interpreted mean annual Mountain, a cuesta on the western flank of the Rocks Springs Uplift, to temperatures (MAT) of approximately 15u to 23uC and mean annual JSR WILKINS PEAK MEMBER CYCLICITY AND BASIN EVOLUTION 1199

FIG. 2.—North–south stratigraphic cross section of the Green River Formation. Note the unique southward thickening of the Wilkins Peak Member compared to the Tipton Shale and Laney members, and the crosscutting relations between basin bounding faults and the lower Wilkins Peak Member. Modified from Roehler (1989).

precipitation (MAP) of 75 to 110 cm/year during deposition of the Green LITHOFACIES ASSOCIATIONS River Formation (Wing and Greenwood 1993; Wilf et al. 1998; Wilf The ten lithofacies below are defined on the basis of lithology, texture, 2000). Eocene climate models for southwestern Wyoming estimate similar and sedimentary and biogenic structures, and are grouped into 5 MAT and MAP values (Matthews and Perlmutter 1994; Morrill et al. lithofacies associations. This classification is broadly consistent with 2001). previous descriptions and depositional-environment interpretations (e.g., The Wilkins Peak Member consists of carbonate-rich lacustrine and Eugster and Hardie 1975; Smoot 1983). However, significant advances associated facies that were deposited during an underfilled basin phase have been made in understanding depositional processes in modern saline (Carroll and Bohacs 1999). It is enclosed by balanced-fill lake deposits of lakes and related fluvial systems in the two decades that have passed since the Tipton Shale and Laney members and grades laterally into variegated red and green alluvial mudstone and sandstone of the Cathedral Bluffs the previous generation of studies (e.g., Smoot and Lowenstein 1991; Tongue of the Wasatch Formation (Fig. 2). On the basis of recent Platt and Wright 1992; Tooth 2000a). Likewise, models for lake basin 40Ar/39Ar geochronology, the Wilkins Peak Member spans approximately evolution have been refined (e.g., Lowenstein and Hardie 1985; Kelts 1 My (Smith et al. 2003). The Wilkins Peak Member is 275 m thick at the 1988; Carroll and Bohacs 1999), justifying a reexamination of the Wilkins type locality near Rock Springs, Wyoming (Fig. 1), and thickens Peak Member. southward towards the Uinta Mountains, where it reaches 411 m (Roehler 1993). Main lithologies include tan to olive calcareous and Littoral Association dolomitic mudstone, kerogen-rich micro-laminated micrite, calcareous sandstone, arkosic sandstone, and bedded evaporites, including trona and Intraclastic Conglomerate Lithofacies.—The intraclastic conglomerate halite (Bradley 1959; Culbertson 1969; Smoot 1983). Insect remains, plant lithofacies is composed of granule to pebble-size mudstone intraclasts in debris, and trace fossils are the only commonly observed fossils. a matrix of calcareous very fine to fine-grained sandstone (Fig. 3). White Mountain exposes a complete section of the Wilkins Peak Bedding is wavy, and individual beds are commonly less than 10 cm Member where beds can be traced for over 50 km. We described six thick. Conglomerate beds unconformably overlie beds of calcareous and outcrop sections from the north end of White Mountain, south to the dolomitic mudstone. Intraclasts are typically flat with rounded corners, town of Rock Springs. From north to south they are Boar’s Tusk (BT), although some are more spherical. The composition of intraclasts is Breathing Gulch (BG), Apache Lane (AL), Microwave Reflector (MR), similar to that of the underlying calcitic and dolomitic mudstone beds. Stagecoach Boulevard (SB), and Kanda (KA) (Fig. 1). An additional Plant debris is common. section was described from the ERDA White Mountain #1 core (WM). Intraclasts presumably were derived from the underlying mudstone The WM core was collected from the fringe of the evaporite depocenter beds, which were at least partially lithified prior to being eroded (cf. (Fig. 1). Together these seven sections extend over 53 km, providing an Freytet and Plaziat 1982). Previous authors suggested that many of the oblique-dip view of the northern part of the southward-thickening ‘‘poker chip’’-like intraclasts originally formed as the tops of desiccation Wilkins Peak Member. polygons on an exposed lake plain that were subsequently eroded during 1200 J.T. PIETRAS AND A.R. CARROLL JSR

FIG. 3.—Intraclastic conglomerate lithofacies (polished slab). Note the abundance of mudstone intraclasts at the base of this bed. flooding (Eugster and Hardie 1975; Smoot 1983). We adhere to these (GPT) determined by Fisher Assay analysis, with TOC (total organic previous interpretations that the intraclastic conglomerate lithofacies carbon) ranging up to 19% (Bohacs 1998; Carroll and Bohacs 2001). represents wave reworking of exposed lake-plain material into relatively The occurrence of kerogen-rich strata reflects the combined affects of shallow water, through a combination of sheet floods and wave erosion. primary productivity, preservation, and lack of significant dilution by inorganic sediment (Bohacs 1998). Much of the organic material Calcareous Sandstone Lithofacies.—The calcareous sandstone lithofa- preserved in the Wilkins Peak Member was derived from algae and cies is composed of interbedded very fine- to fine-grained calcareous bacteria (Bradley 1962; Tissot and Vandenbroucke 1983; Bohacs 1998). sandstone and thin mudstone beds (Fig. 4A). Beds are typically 10 to The preservation of organic material and sub-millimeter-scale lamination 50 cm thick, have sharp bases, and crop out as distinct white beds that suggests inhibition of burrowing organisms. This commonly results from can be traced for tens of kilometers. Grain types include carbonate, low oxygen levels in lake bottom waters (e.g., Demaison and Moore 1980; quartz, feldspar, mica, plant debris, and ooids (Fig. 4B). Mudcrack casts Talbot and Allen 1996). High salinity levels, inferred for the Wilkins Peak (Fig. 4C) and granule to pebble-size mudstone intraclasts are common Member from the presence of evaporites, also tend to limit benthic along the base of beds. Internally, sandstone beds contain wavy cross- organisms. Biomarker analyses from organic-rich beds in the Wilkins lamination and mudcracks, and preserve symmetrical ripple marks and Peak Member yields results consistent with deposition in low-oxygen, rare bedding-plane trace fossils along bed tops. Interbedded mudstone hypersaline waters (Carroll and Bohacs 2001). layers are typically less than 1 cm thick and tend to drape the tops of sandstone beds. Massive Calcareous Mudstone Lithofacies.—This lithofacies is similar Wave action is interpreted to be the dominant mode of sediment to the laminated kerogen-rich calcareous mudstone lithofacies. However, transport for the calcareous sandstone lithofacies, and is recorded by the millimeter-scale laminae are not as well preserved, beds are light brown to ubiquitous occurrence of nonparallel wavy lamination and symmetrical olive in color, and oil yields based on Fischer Assay analyses are lower ripple marks (cf. Allen 1981). The common occurrence of mudcracks (typically less than 10 GPT). The reduction of organic-richness and loss suggests frequent exposure and desiccation. Mudstone intraclasts were of well-developed lamination in this lithofacies presumably reflects the derived from underlying beds, similar to those found in the intraclastic activity of benthic organisms. This lithofacies suggests that bottom waters conglomerate lithofacies. Interbedded mudstone drapes suggests periods were more oxygenated than during deposition of the kerogen-rich of slack water allowing finer-grained material to settle from suspension lithofacies, implying shallower water depths. (cf. Reineck and Wunderlich 1968). Summary of Profundal–Sublittoral Association.—These strata represent Summary of Littoral Association.—This association records shallow- periods of relatively high lake level and lateral lake expansion. The warm- water deposition during shoreline transgression over a previously exposed equable climate inferred for the gGRB (see above) may have contributed and partially cemented lake plain. These deposits are common to the to the persistence of density stratification in a meromictic lake with littoral zone, where wave action is the primary transport process. In this oxygen-depleted bottom waters (cf. Tucker and Wright 1990; Talbot and depositional environment relatively small falls in lake level can expose Allen 1996). In addition, a more laterally expansive lake would have broad areas, particularly when the depositional gradient is very low, as tended to restrict the delivery of inorganic sediment to lake-marginal was the case for the gGRB. These high-frequency fluctuations result in areas, further concentrating organic matter in areas nearer the lake the repetition of wave-transported facies associated with numerous center. Fischer Assay oil yields and percentage of total organic carbon desiccation horizons. generally correlate with evidence from sedimentary facies for relatively deep-water conditions (Carroll and Bohacs 2001). Profundal–Sublittoral Association Palustrine Association Laminated Kerogen-Rich Calcareous Mudstone Lithofacies.—Sub-mil- limeter-scale laminated calcareous and dolomitic mudstone appears as Wavy-bedded Calcareous Siltstone–Sandstone Lithofacies.—The wavy- black or dark brown beds along freshly exposed surfaces and weather to bedded calcareous siltstone and sandstone lithofacies is composed of light a white or slightly blue-white color. Beds range in thickness from 0.1 to gray calcareous siltstone and sandstone that occur as centimeter-thick 2 m and can be traced laterally for tens of kilometers. The preservation of laminae interbedded with calcareous and dolomitic mudstone. Ripple organic debris suitable for petroleum generation gives rise to the term ‘‘oil cross-lamination and ripple marks are common. Many laminae are only shale’’ that has been applied to this lithofacies. Oil yields from kerogen- one ripple height in thickness, but composite siltstone–sandstone– rich strata in the Wilkins Peak Member reach up to 40 gallons per ton mudstone beds can be up to 2 m thick. JSR WILKINS PEAK MEMBER CYCLICITY AND BASIN EVOLUTION 1201

debris are common. Calcareous nodules, trace fossils (Fig. 5A), and insect remains occur locally. Many beds are completely brecciated, and in some cases evaporite minerals filled the voids between clasts (Fig. 5B). The local occurrence of wavy bedding suggests movement of grains by wave action. However, primary laminae are obscured by the formation of desiccation cracks, burrowing, and the growth of plant roots in most beds. These processes lead to brecciation and development of massive bedding. This lithofacies thus signifies an alternation between subaqueous deposition of mudstone and siltstone with subaerial modification. Previous studies argue for a primary or very early diagenetic dolomitization (Eugster and Surdam 1973; Eugster and Hardie 1975; Surdam and Wolfbauer 1975; Smoot 1983; Mason and Surdam 1992; Norris et al. 1996; Pitman 1996). Deep Springs Lake in California (Jones 1965) and several saline lakes of western Victoria, Australia (De Deckker and Last 1988), offer potential modern analogues of primary dolomite precipitation. In these playa-lake settings calcite is precipitated on the outer fringe of a playa surrounding a central lake, increasing the Mg/Ca ratio of residual waters. Dolomite precipitation occurs from these Mg- rich and sulfate-depleted waters along the lake margin when salinity and alkalinity are elevated by evaporation (Tucker and Wright 1990). Calcite and dolomite clasts can then be transported into the lake by wind and water.

Summary of Palustrine Association.—The intimate association of lacustrine carbonate accumulation and subaerial pedogenic modification common to these units suggests deposition in the littoral to eulittoral zones (Tucker and Wright 1990). Together these zones form the palustrine environment (Freytet and Plaziat 1982; Freytet and Verrecchia 2002). In this environment pedogenic modification occurs on local highs or away from the shoreline, while carbonate precipitation occurs in lower areas that alternate between wet and dry conditions (e.g., Platt and Wright 1992). Associated lacustrine environments derive sediment from these areas during floods. Palustrine environments are most expansive in low-gradient basins, like the gGRB, because minor changes in water inflow can result in frequent wetting and drying of very broad areas.

Salt-Pan Association

Displacive Evaporite Lithofacies.—The displacive evaporite lithofacies is composed of zones of evaporite crystals, commonly shortite Na2Ca2(CO3)3 and trona Na3(HCO3)(CO3)?2(H2O), found within beds of the previously described lithofacies. Shortite occurs as randomly oriented euhedral grains ranging from microcrystalline to 1 mm to 3 cm across, or as discrete layers (Fig. 6A). Trona typically occurs as centimeter-scale radial blades (Fig. 6B) and microcrystalline aggregates. FIG. 4.—Calcareous sandstone lithofacies. A) Resistant layer of the calcareous sandstone interbedded with deeply eroded mudstone lamina (outcrop). B) These evaporite crystals crosscut and distort primary lamination in the Photomicrograph of a calcareous sandstone bed; q, quartz; f, feldspar; o, ooid; surrounding host lithology, and lithofacies boundaries. c, calcareous matrix. C) Mudcrack casts on the base of a calcareous sandstone bed Because these evaporite minerals both truncate and distort primary (hand sample). lamination they are interpreted to have formed in the unlithified lake- bottom sediment from supersaturated brines (cf. Lowenstein and Hardie Silt and sand grains of this lithofacies were transported by wave action 1985). Brines that form displacive evaporites are derived either directly as ripple trains, and mudstone interbeds were deposited by settling during from lake water or from interstitial waters (Smoot and Lowenstein 1991). slack-water periods (cf. Reineck and Wunderlich 1968). Isolated ripple trains suggest a sand-starved setting where coarse-grained material is Bedded Evaporite Lithofacies.—Evaporite minerals also form discrete available only during floods (Allen 1981). This lithofacies thus represents beds throughout the Wilkins Peak Member. Culbertson (1971) reported deposition in relatively shallow water, where silt- to sand-size sediments 42 evaporite beds, 25 of which are thicker than 1 m. These thicker beds were transported by wave action during intermittent floods. range in area from 427 to 1,870 km2 (Bradley and Eugster 1969). Their maximum extent is shown in Figure 1. The most common minerals Brecciated Carbonate Mudstone–Siltstone Lithofacies.—This lithofacies include trona and halite, but numerous other saline minerals are commonly occurs as recessive interbeds, 0.1 to 3 m thick, of olive and described in varied quantities (Bradley and Eugster 1969). Mudstone light brown calcareous and dolomitic and olive to gray calcareous interbeds containing wavy laminae are common within evaporite beds siltstone. Beds are internally massive, but wavy laminae are preserved (Fig. 6C). Evaporites are present as pseudomorphs in outcroppings locally. Mudcracks (Fig. 5A), angular mudstone intraclasts, and plant (Fig. 6D). 1202 J.T. PIETRAS AND A.R. CARROLL JSR

Bradley and Eugster (1969) described both bladed and granular textures of trona with inclusions of organic material, dolomicrite, silt, and pyrite that give beds a brownish color. They interpreted these beds, which are intercalated with profundal mudstone, to represent primary trona deposited subaqueously on the floor of Lake Gosiute. Similar trona deposits have been described from modern lakes Magadi and Bogoria (Eugster 1970; Renaut and Tiercelin 1994) and from the Neogene Baypazari Basin in Turkey (Helvaci 1998).

Summary of Salt-Pan Association.—This association represents the development of a salt pan (Lowenstein and Hardie 1985) during extended periods when evaporation exceeded influx of water to Lake Gosiute. The common interbedding of evaporite and mudstone layers suggests intermittent periods of flooding, freshening, and sediment advection from the surrounding lake plain, followed by evaporative concentration. Similar mudstone interbeds have been described associated with trona deposits of Searles Lake, California (Fairchild et al. 1998).

Fluvial Association

Siliciclastic Sandstone Lithofacies.—The siliciclastic sandstone lithofa- cies is distinct from other sandstone facies in the Wilkins Peak Member because of its brown color and almost ubiquitous occurrence of climbing ripples (Fig. 7A). Typical grain types include quartz, feldspar, and mica that range from silt to fine-grained sand. Individual beds range up to 2 m in thickness; however, composite sandstone–siltstone–mudstone bedsets are up to 25 m thick and can be traced across much of the basin (Culbertson 1961). Many beds consist of an amalgamation of several lenticular sandstone bodies stacked vertically and shingled laterally. In addition to climbing ripples, common sedimentary structures include decimeter to meter-scale scour and fill (Fig. 7B), trough cross-bedding, and parting lineation. Both calcareous and noncalcareous mudstone intraclasts are common in bed bases. Vertical burrows are common within beds, and more exotic trace fossils such as bird trackways (Fig. 7C), insect nests (Bohacs et al. 2002), and mammal footprints occur locally. Lenticular sandstone bodies in this lithofacies are interpreted to be sand-filled fluvial channels. These channels scoured into previously deposited mudstone and sandstone beds, creating laterally extensive amalgamated sandstone units. Climbing ripples signify rapid deposition from sediment-laden unidirectional flows (Allen 1970, 1971). Climbing ripples have been described in fluvial systems associated the waning stages of floods (Sneh 1983) and from the deposits of density flows in a standing body of water (Jopling and Walker 1968; Stanley 1974). Insect nests associated with some sandstone beds would require that that these intervals were above the water table for significant periods of time (Bohacs et al. 2002). For this reason we favor a fluvial origin for climbing ripples in this lithofacies.

Green to Brown Mudstone and Siltstone Lithofacies.—The brown mudstone and siltstone lithofacies is distinct from other mudstone facies because of its color and its typically noncalcareous nature. Sheet-like arkosic sandstone interbeds 1 to 20 cm thick, plant debris, carbonaceous material, and calcareous concretions are common. Beds are locally mottled red and green and weather to a blocky texture. Horizontal and

R

Fig. 5.—Brecciated calcareous mudstone–siltstone lithofacies (WM core). A) Burrows and mudcracks. B) Brecciated mudstone with shortite crystals between some clasts. JSR WILKINS PEAK MEMBER CYCLICITY AND BASIN EVOLUTION 1203

FIG. 6.— Displacive evaporite and bedded evaporite lithofacies. A) Photomicrograph of evaporite crystals (shortite) in a matrix of calcareous mudstone. Note how some laminae are both deflected and truncated by the growth of evaporite crystals. B) Bladed trona (WM core). C) Bedded trona overlain by calcareous mudstone (sample collected from the FMC trona mine). Some displacive evaporite (DE) crystals can be seen in the mudstone layer. D) Outcrop photo of bladed trona pseudomorphs. vertical surfaces marked by color change, differential cementation, and/or Summary of Fluvial Association.—This association corresponds to the modern fractures occur locally (Fig. 7D). nine sandstone–siltstone–mudstone marker beds (A through I) described This lithofacies is interpreted to represent floodplain deposits. Fine- by Culbertson (1961). These marker beds are up to 25 m thick in the grained material was deposited from suspension in abandoned channels, Wilkins Peak Member and can be correlated across the basin (Fig. 2). in an overbank setting, or in small lakes formed during flood intervals These beds are thought to represent the deposits of braided streams and (Miall 1996). Thin sandstone interbeds presumably represent sediment sheetfloods, and fine-grained overbank deposits (Smoot 1983). During that was transported as bedforms during occasional floods such as flood intervals shallow lakes may have formed, depositing thin units of crevasse splays (Miall 1992). Calcareous concretions and associated red siltstone and mudstone strata towards the basin center and in local lows, and green mottling suggest incipient pedogenesis, common to floodplain signifying temporary reestablishment of lacustrine environments. How- environments (Fenwick 1985; Retallack 1988). The vertical features ever, long-term discharge remained low, resulting in desiccation, returnof observed locally (Fig. 7D) also signify pedogenic alteration. They are a floodplain environment, and subsequent pedogenic alteration. These interpreted as pseudoanticlines that form by repeated swelling and interpretations may point toward a ‘‘dryland river’’ model for the fluvial shrinking of clays caused during seasonal wetting and drying in a soil association (cf. Picard and High 1973; Tooth 2000a, 2000b). However, profile (Yaalon and Kalmar 1978; Paik and Lee 1998). a full evaluation of this hypothesis is beyond the scope of the present 1204 J.T. PIETRAS AND A.R. CARROLL JSR

FIG. 7.—Siliciclastic sandstone and brown mudstone–siltstone lithofacies. A) Outcrop example of climbing ripples. B) Meter-scale scour and fill feature in a sandstone outcrop approximately 25 m in height. C) Casts of a bird trackway on a bedding plane of sandstone. D) Outcrop showing both vertical features (v) and horizontal features (h) in brown mudstone–siltstone interpreted as soil features. The ledge at the top of the photo (ss) is a sandstone bed. study. The fluvial association is much better exposed to the south of the littoral and profundal–sublittoral facies associations, into thicker inter- study area, offering an excellent opportunity for future work. vals of palustrine facies (Fig. 8). Overall thickness is highly variable, ranging between 0.14 and 5.70 m in this study. Cycles are bounded by WILKINS PEAK CYCLES widely traceable surfaces that commonly are marked by desiccation cracks or centimeter-scale scour. These surfaces are usually overlain by The littoral, profundal–sublittoral, palustrine, and salt-pan associa- intraclastic conglomerate and calcareous sandstone of the littoral tions occur in repetitive facies successions that represent periods of lake association. However, in some cycles profundal lithofacies directly overlie expansion and contraction (Fig. 8; Eugster and Hardie 1975; Smoot the basal surface. In all cases the cycle boundary is abrupt and reflects an 1983). These repetitive successions are referred to as ‘‘cycles.’’ We do this advancement of the shoreline and an increase in water depth. The in order to be consistent with many past studies of carbonate-rich marine profundal–sublittoral association represents the maximum lake expansion strata and both carbonate and clastic lacustrine deposits (e.g., Van during the deposition of an individual cycle. The upper contact of the Houten 1962), but we do not intend our use of this term to imply any profundal–sublittoral association is gradational with overlying mudstone specific periodicity or driving mechanism. and siltstone beds of the palustrine association, suggesting gradual Cycles in the Wilkins Peak Member are generally asymmetric in terms shoreline retreat and decreasing water depths. Most of the cycle thickness of facies thickness, grading upward from relatively thin intervals of records shoreline regression, in as much as palustrine facies typically JSR WILKINS PEAK MEMBER CYCLICITY AND BASIN EVOLUTION 1205

FIG. 8.— Diagram of a typical lacustrine expansion–contraction cycle in the Wilkins Peak Member highlighting common sedimentary structures and the vertical distribution of lithofacies associations. The photo to the right is one complete cycle in the ERDA White Mountain #1 core. comprise two-thirds or more of the facies present. The profundal– and northward-terminating cycles indicate that accommodation was sublittoral association is absent in some cycles, in which case the greater towards the south (Fig. 11). The number of cycles decreases palustrine association directly overlies the littoral association. This systematically northward, from 126 in the WM core to 46 at BG and 34 at amalgamation most commonly occurs towards the basin margin BT. In contrast to the progressive basal onlap depicted by Roehler (1993), (Fig. 9). Conversely, several cycles in our most basinal section (WM) detailed correlations indicate that cycle boundaries gradually become are capped by bedded evaporites and/or zones of displacive evaporite unrecognizable northward, due to amalgamation within homogeneous mineralization, recording formation of a salt pan (Fig. 9). palustrine facies (Fig. 10; Pietras et al. 2003a). The maximum limit of any particular lake expansion can be roughly constrained between two specified measured sections, but its precise position is difficult or Lateral Correlation of Cycles impossible to identify. Roehler (1993) documented a maximum of 77 cycles from the ERDA Figure 12 displays changes through time in differential basin sub- White Mountain #1 core (WM; Fig. 1) based on Fischer Assay analysis sidence, based on a comparison of cycle thickness near the basin margin of retort oil yields. Peaks in oil yield were used as a proxy for lake to the thickness of the same interval near the basin center. Net sediment highstands. This approach, however, did not account for all episodes of accumulation was essentially uniform across the study area during lake expansion and contraction. Fischer Assay oil yields of the WM core deposition of the Tipton Shale Member, indicating that the basin floor were measured from homogenized samples that averaged 30 cm to over was relatively flat. In the Wilkins Peak Member, three major episodes of 1 m of core, whereas lacustrine cycles as thin as 14 cm can be recognized rapidly increased differential accommodation are evident, expressed by visually. Furthermore, some of the cycles are not sufficiently kerogen-rich intensified southward thickening of cycles and increased number of to be identified using Fischer Assay alone (Fig. 10). On the basis of cycles. Following each episode, a gradual shift back to more uniform detailed lithofacies descriptions, 126 cycles were identifiable in the WM sediment accumulation is recorded in the overlying cycles. The lowest core. such interval begins at the base the Wilkins Peak Member and extends Wilkins Peak Member cycles can be correlated from near the northern roughly to the position of the Grey Tuff (Fig. 11). The next interval is basin margin towards the basin center, primarily using exposures on the roughly constrained between the Grey and Main tuffs, and the final unit western flank of the Rock Springs Uplift (Fig. 11). Correlations are aided extends from the Main Tuff to the contact with the Laney Member. These by the presence of several distinctive volcanic air-fall tuffs and by the nine three intervals are similar to, but not identical to, the lower, middle, and composite bedsets of the fluvial association. Overall thickness patterns upper parts of the Wilkins Peak Member described by Roehler (1992). 1206 J.T. PIETRAS AND A.R. CARROLL JSR

FIG. 9.—A) Model for the development of lacustrine expansion–contraction cycles in the Wilkins Peak Member. CB 5 cycle boundary, MES 5 maximum expansion surface. B) Wheeler diagram showing how time is represented in Wilkins Peak Member expansion–contraction cycles.

Differential sediment accumulation in these three intervals decreased DISCUSSION systematically upward, returning to a relatively uniform configuration Sequence Stratigraphic Interpretation of Lacustrine Cycles during deposition of the Laney Member (Fig. 12). The distribution of evaporite beds also reflects this pattern of The stratigraphy of the Wilkins Peak Member is dominated by differential accommodation and indicates that the highest rates of repetition of lake expansion–contraction cycles, revealing the extreme accommodation occurred to the southwest of WM (Fig. 13). Three susceptibility of this system to high-frequency fluctuations in basin trends appear from this data: (1) the number of evaporite beds and their hydrology (Carroll and Bohacs 1999; Bohacs et al. 2000). A dilemma areal extent decreases upwards, (2) average evaporite bed thickness exists regarding the proper sequence stratigraphic interpretation of these decreases upwards, and (3) the locus of deposition migrated northward cycles: they could potentially be classified as either parasequences or as following deposition of the lower Wilkins Peak Member. Collectively, sequences, on the basis of the original definitions of those terms these observations suggest that evaporite deposition generally coincided (Mitchum et al. 1977; Van Wagoner et al. 1988). In thickness, Wilkins with periods of increased basin-floor slope and smaller lake surface areas, Peak Member cycles are closer to typical shallow marine and fluvial whereas more widespread and uniform lacustrine environments occurred parasequences than to sequences. The bulk of each cycle is represented by when the basin floor was relatively flat. regressive palustrine facies, which is also consistent with a shallowing- JSR WILKINS PEAK MEMBER CYCLICITY AND BASIN EVOLUTION 1207

FIG. 10.—Detailed correlation of measured sections highlighting termination of cycle boundaries into lake-margin facies. Shale oil yield cycles from Roehler, 1993. GPT, gallon per ton. upward parasequence. Bohacs (1998) noted that ‘‘lacustrine flooding distinctly divided lowstand from transgressive deposits. Only the surfaces’’ might reasonably be substituted for the ‘‘marine flooding surrounding bedrock surfaces provide any significant change in basin- surfaces’’ specified in the original parasequence definition (Van Wagoner floor slope, but presently there is no direct evidence for how high 1988). However, the relative magnitude of lake-level change associated shorelines climbed on these basin walls. with each cycle is extreme compared to the relative sea-level changes In contrast to typical marine carbonate sequences, the transgressive inferred from marine parasequences. Many Wilkins Peak Member cycles increments of Wilkins Peak Member cycles are quite condensed. Most of begin and end with complete or nearly complete desiccation, indicating the sediment deposited during transgression appears to have been that they record the entire base-level range of the lake. The absolute depth siliciclastic detritus and reworked carbonate lithologies rather than new, of Lake Gosiute during highstands is not known, but from comparison autochthonous production of carbonate. The enhanced biologic pro- with Holocene lakes and the preservation of laminated mudstone it ductivity that allows marine carbonate facies to ‘‘keep up’’ with rising sea probably reached tens or perhaps even hundreds of meters (e.g., level (cf. Kerans and Tinker 1997) does not appear to have played DeMaison and Moore 1980; Olsen 1990; Oviatt et al 1992; Scholz and a significant role in Wilkins Peak Member deposition. Progradation of Finney 1994). carbonate-rich palustrine deposits may have occurred during lake Taking the alternative view, Wilkins Peak Member cycles appear to highstands, but it is impossible to map such fine detail in this meet the basic definition of a sequence as a ‘‘relatively conformable homogeneous facies. succession of genetically related strata bounded at its top and base by Wilkins Peak Member cycles may not correspond to any established unconformities and their correlative conformities’’ (Mitchum et al. 1977). sequence stratigraphic unit because they do not represent true ‘‘Walther- The pivotal question is whether the cycle boundaries are merely short- ian’’ facies successions. Walther’s law of facies succession states ‘‘… that lived scour surfaces developed during transgression, or instead they only those facies and facies areas can be superimposed primarily which represent longer periods of basin-floor exposure and desiccation. This can be observed beside each other at the present time’’ (Middleton 1973). question is not easily answered, but because well-developed paleosols are Sequences and parasequences are both defined as successions of relatively rare it is possible that exposure times were relatively short. Furthermore, conformable, genetically related strata, which are generally taken to the cycles lack clearly defined lowstand facies. There are two possible imply that they adhere to Walther’s law. However, individual Wilkins reasons for this. First, the flux of weathering products (both physical and Peak Member cycles contain facies that may never have laterally chemical) into the basin was directly tied to the inflow of water, and coexisted within the gGRB. For example, evaporite beds lie directly over therefore would have decreased as lake level dropped. Much of the basin oil shale beds in some locations, but it is far from clear that both facies may therefore have experienced greatly decreased depositional rates were ever deposited simultaneously. Similar observations have been during dry periods. Second, the floor of Lake Gosiute had very low relief reported from Holocene underfilled lacustrine basins such as the Qaidam and lacked any significant geomorphic shelf–slope break that could have Basin in western China (Yang et al. 1995) and Death Valley, California 1208 ..PERSADAR CARROLL A.R. AND PIETRAS J.T. JSR FIG. 11.— High-resolution stratigraphic cross section of the Wilkins Peakmber Me at White Mountain. See Figure 1 for location of measured sections.ters A–ILet correspond to the nine units of the fluvial association (Culbertson 1961). CBT, Cathedral Bluffs Tongue. Detailedersions v of all measured sections appear in Pietras (2003). JSR WILKINS PEAK MEMBER CYCLICITY AND BASIN EVOLUTION 1209

insolation (e.g., Bradley 1929; Fischer and Roberts 1991; Roehler 1993). The cycles we describe in the Wilkins Peak Member have specifically been attributed to 19–23 kyr precessional periodicities on the basis of Fischer Assay analysis of organic matter content (Roehler 1993; Fischer et al. 2004). Two direct observations challenge such interpretations. First, Fischer Assay data from the Wilkins Peak Member are not of high enough resolution to be used as a faithful proxy for lake level (see above). Secondly, the preservation of a variable number of cycles along a basin transect question the ability of any one section to represent a complete stratigraphic record. In fact hiatuses appear to be plentiful even in the most basinal sections (Bohacs 1998), suggesting that a truly complete stratigraphic record may not exist anywhere. Recent high-resolution 40Ar/39Ar age determinations from four tuffs (Smith et al. 2003) in the Wilkins Peak Member provide an opportunity to directly constrain average apparent cycle duration at different locations by counting the number of cycles preserved between dated tuffs (Fig. 11). Based on the number of cycles present between each tuff pair, the average apparent cycle duration decreases by approximately a factor of three from near the basin margin to near the center (Fig. 14; Table 1). All three intervals yield an apparent duration of approximately 10,000 years for cycles in the WM core. It should be noted however that these durations are maximum values, because they do not take into account hiatuses. These calculations also do not reflect the fact that cycle thicknesses in the subject interval vary by almost two orders of magnitude. For example, cycles in the middle Wilkins Peak Member range from 21 cm to 5.7 m (Fig. 15). Assuming that thinner cycles represent shorter increments of time, the shortest true cycle durations are therefore much less than 10,000 years, and in some cases may represent lacustrine episodes that lasted less than 1000 years. Durations of 10,000 years or less are too short to correspond to any known precessional period, even taking into account uncertainties in radioisotopic dating (Smith et al. 2003; Pietras et al. 2003a). Our study does not exclude the possibility that these short-period cycles combine to form longer cycles with precessional or other orbital periods. However, it is clear that alternative mechanisms are required for the genesis of sub- precessional cycles. Half-precessional frequencies observed in Upper Cretaceous marine deposits from Brazil have been attributed to the existence of an equatorial continent that was sensitive to both extremes of the precessional cycle (e.g., Short et al. 1991; Park et al. 1993). However, this mechanism is less plausible for an Eocene site in North America or for cycles with durations nearer 1000 than 10,000 years. Ice-related mechanisms (such as Heinrich events) seem unlikely, given deposition of the Green River Formation during the warmest period of the Cenozoic (Smith et al. 2003) in a presumably ice-free world. However, Pekar et al. (2005) recently proposed the existence of early and middle Eocene FIG. 12.— Graphical correlation of timelines in the Wilkins Peak Member glacioeustatic sea-level changes, suggesting that this assumption may between WM and BG. Inset figures show sediment accumulation across the basin require further examination. Other non-climatic mechanisms that have for each of the three portions of the Wilkins Peak Member. The nine sandstone– been implicated in causing expansion or contraction of lakes are siltstone–mudstone beds of Culbertson (1961) are labeled A–I. All nine beds occur at WM, but only bed D is present at BG. WP, Wilkins Peak Member. numerous, including tectonic modification of drainage divides (e.g., Sa´ez et al. 1999; Pietras et al. 2003b), diversion of rivers by volcanic flows (e.g., (Lowenstein et al. 1998). In these basins perennial lake deposits lie Bouchard et al. 1998), or vertical movements along basin outlets (e.g., directly beneath modern salt pans. Wilkins Peak cycles probably are best Malde 1960; Colman 1998; Reheis et al. 2002). These processes can occur considered as event beds that reflect comprehensive paleoenvironmental repetitively or as unique events, but they generally seem less than ideal for change across the basin rather than representing gradual migration of generating apparently ‘‘rhythmic’’ facies successions. Perhaps the most coexisting environments. As such they are analogous to some marine promising (and least explored) mechanism is geomorphic autogenesis, evaporite deposits, for which a ‘‘non-Watherian’’ origin has also been caused by migration of drainage divides or upstream propagation of argued (e.g., Kendall 1988). fluvial knickpoints (Hasbargen and Paola 2000). The temporal stability of alluvial landscapes over periods of 103–104 years is largely unknown but could potentially exert a first-order control on downstream basins Cycle Duration vs. Orbital Periodicity by modulating the flux of both water and sediment to downstream Previous authors have suggested that lacustrine expansion–contraction lakes. cycles in the Green River Formation were governed by changes in Interestingly, the variable updip-downdip preservation of cycles in the climatic humidity, in turn driven by orbitally controlled changes in solar Wilkins Peak Member acted as a natural frequency filter. Because fewer 1210 J.T. PIETRAS AND A.R. CARROLL JSR

FIG. 13.—Vertical, lateral, and areal extent of 25 potential economic trona beds (Culbertson 1971) in the lower, middle, and upper Wilkins Peak Member. Darker colors on the map represent stacked trona beds. Trona data from Wiig et al. (1995). Extent of Wilkins Peak Member is from Roehler (1992). WR, Wind River Mountains; RS Uplift, Rock Springs Uplift. cycles are preserved in updip locations (nearer the basin margin), their identify which cycles correspond to precession solely on the basis of their apparent frequency, based on cycle counts, is lower than cycles near the expression at one location. basin center. Assuming that Wilkins Peak cycles actually record a range of different frequencies, it is likely that shorter ‘‘true’’ frequencies are Evaporite Deposition in Laramide Basins progressively excluded from the record in sections nearer to the basin margins. At some geographic positions within the basin, the duration of Previous studies have implied that deposition of bedded evaporite in the shortest preserved cycles may in fact correspond to the range of the Wilkins Peak Member was caused by a period of markedly dry known precessional periods. However, the facies expression of Wilkins climate (e.g., Bradley and Eugster 1969; Roehler 1993), but presently Peak cycles is similar throughout much of the study area, regardless of there are no independent data that directly support this conclusion. their thickness or true duration. It would therefore be impossible to Paleofloral collections have been reported from the greater Green River JSR WILKINS PEAK MEMBER CYCLICITY AND BASIN EVOLUTION 1211

FIG. 14.—Estimate of apparent cyclicity determined for the lower, middle, and upper Wilkins Peak Member plotted against the position in the basin. Error bars reflect 2s uncertainty in 40Ar/39Ar age determinations (Smith et al. 2003), and decrease towards the basin center because the uncertainty is distributed between FIG. 15.—Histogram of cycle thicknesses for the middle Wilkins Peak Member. more cycles.

Basin (MacGinitie 1969; Wolfe 1978; Wilf 2000), but these lie and evaporite distributions (Fig. 13) reveal a marked increase in stratigraphically below and above the principal evaporite-bearing differential accommodation during deposition of the lowermost Wilkins intervals. On the basis of leaf-margin and leaf-area analysis of leaves Peak Member, consistent with tectonic loading of the southern basin collected from the Niland Tongue (below the Wilkins Peak Member), margin by renewed Uinta Mountain uplift (Hansen 1986; Roehler 1993; Wilf (2000) interpreted a mean annual temperature of 23.0 6 3.7uC and Bradley 1995). The detailed record presented in this study shows that mean annual precipitation of 100 cm/yr (standard error + 60.5, differential accommodation was greatest during deposition of the oldest 242.2 cm/y). For a second locality that spans the uppermost Wilkins Wilkins Peak Member strata and gradually decreased through time Peak to lowermost Laney members (Little Mountain), he interpreted (Fig. 12). Structural observations along the Uinta Mountain front mean average values of 19.6 6 2.1uC and 76.9 cm/yr (standard error reinforce this interpretation. For example, Roehler (1993) estimated + 33.2, 223.2 cm/yr). He further observed that changes in floral roughly 150 m of offset of the Sparks Ranch thrust fault (Fig. 1) during composition between these two sites were consistent with a climate that early Wilkins Peak Member deposition, on the basis of cross-cutting had become cooler and drier, with more seasonal rainfall. However, Wilf relations with the laterally equivalent lower Cathedral Bluffs Tongue. also pointed out that the Little Mountain flora was collected near the Farther west, along the center of the Uinta Mountains, crosscutting southern margin of the gGRB from an interval that records generally relationships demonstrate that the Henry’s Fork thrust fault (Fig. 1) was rising average lake levels and that this collection therefore may not be also active during the late early to early middle Eocene (Bradley 1995). A representative of the conditions that prevailed during periods of evaporite boulder conglomerate related to this fault has been correlated to the deposition. Fossil leaves have not been reported from the more evaporite- Tipton Shale Member and the overlying Cathedral Bluffs Tongue rich lower to middle Wilkins Peak Member, and therefore inferences of (Hansen 1986). On the northern basin margin, McGee (1983), Steidtmann a relatively dry paleoclimate are based on the occurrence of evaporite et al. (1983), and Pietras et al. (2003b) documented that movement on the beds themselves rather than on floral evidence. Continental Fault occurred simultaneously with the onset of Wilkins In contrast to its somewhat ambiguous paleoclimatic setting, the spatial Peak Member deposition. This reverse fault movement blocked drainages and temporal relationship of the Wilkins Peak Member to changing basin that had previously provided sediment and water to Lake Gosiute tectonics is relatively clear. Thickness patterns (Fig. 12; Roehler 1992) (Pietras et al. 2003b). Deposition of alluvial-fan conglomerate at the

TABLE 1.—Number of cycles present and average cycle duration for portions of the lower, middle, and upper Wilkins Peak Member constrained by dated tuffs (Smith et al. 2003).

Unit Duration Upper WP 260 6190 kyr Middle WP 430 6150 kyr Lower WP 310 6190 kyr Section Num. Cycles Ave. Duration (kyrs) Num. Cycles Ave. Duration (kyrs) Num. Cycles Ave. Duration (kyrs) WM 27 9.6 6 7.0 45 9.6 6 3.3 41 7.6 6 4.6 KA nm nm 32 13.4 6 4.7 nm nm SB nm nm 29 14.8 6 5.2 25 12.4 6 7.6 MR nm nm 22-28 18.2 6 8.2 19 16.3 6 10.0 AL nm nm 19-25 20.9 6 9.7 15 20.7 6 12.7 BG 16 16.3 6 11.9 16 26.9 6 9.4 12 25.8 6 15.8 BT 10 26 6 19.0 13 33.1 6 11.5 9 34.4 6 21.1 See Figure 11 for the stratigraphic position of dated tuffs. (nm – not measured). 1212 J.T. PIETRAS AND A.R. CARROLL JSR eastern basin margin in response to uplift of the Granite Mountains may basis of structural and stratigraphic observations, a period of also have coincided with Wilkins Peak Member deposition, but increased tectonic uplift commenced concurrent with the lower correlations between the conglomerate and the Green River Formation Wilkins Peak Member. We infer that tectonic modification of are presently not well-defined (e.g., Love 1970). regional drainage and basin subsidence patterns exerted a primary On the basis of the above structural histories, we infer that the long- long-term control on evaporite deposition. term shift from balanced-fill conditions during deposition of the Rife Bed of the Tipton Shale Member to underfilled conditions of the Wilkins Peak Member was principally a response to tectonic isolation of the basin. ACKNOWLEDGMENTS 40Ar/39Ar age determinations (Smith et al. 2003) on tuffs in the Green We thank Meredith Rhodes-Carson, Mike Smith, Kevin Bohacs, Reuben River Formation date this transition to between 51.25 6 0.31 and Johnson, Brooke Swanson, Brad Singer, and Jack Neal for stimulating 50.70 6 0.14 Ma (2s uncertainties). Differential subsidence of the discussions on the outcrop. Mike Smith provided insightful comments on greater Green River Basin mostly ended by 49.96 6 0.08 Ma (Fig. 11). a prior version of the manuscript. John Dyni, Cari Johnson, and Richard Wilkins Peak Member cycles deposited after that time (upper Wilkins Yuretich provided helpful reviews. This study was funded by donations from Peak Member) are relatively uniform in thickness across the basin, and Conoco, Texaco, the American Association of Petroleum Geologists Grants- evaporite and fluvial sandstone beds become rare. From the timing of in-aid program, the J. David Love Fellowship, the Department of Geology and Geophysics at the University of Wisconsin-Madison, by a grant from the return to balanced-fill conditions in the basin (i.e., the Laney Member), it Donors of The Petroleum Research Fund, administered by the American appears that approximately 1 My or less was required to restore the Chemical Society, and by National Science Foundation grants EAR-9972851 surrounding landscape to a state similar to that which existed before the (to Singer) and ATM-0081852 (to Singer and Carroll). onset of enhanced tectonism. REFERENCES CONCLUSIONS ALLEN, J.R.L., 1970, A quantitative model of climbing ripples and their cross-laminated deposits: Sedimentology, v. 14, p. 5–26. 1. The Wilkins Peak Member is dominated by repetitive, asymmetric ALLEN, J.R.L., 1971, Instantaneous sediment deposition rates deduced from climbing- facies successions that represent expansion and contraction of Lake ripple cross-lamination: Geological Society of London, Journal, v. 127, p. 553–561. Gosiute. The lower part of each cycle is relatively thin and records ALLEN, P.A., 1981, Wave-generated structures in the Devonian lacustrine sediments of lacustrine transgression that often culminated with deposition of south-east Shetland and ancient wave conditions: Sedimentology, v. 28, p. 369–379. BENSON, L., KASHGARIAN, M., AND RUBIN, M., 1995, Carbonate deposition, Pyramid laminated oil shale. The upper regressive part of each cycle is Lake subbasin, Nevada: 2, Lake levels and polar jet stream positions reconstructed thicker and most commonly consists of palustrine facies. Evaporite from radiocarbon ages and elevations of carbonates (tufas) deposited in the Lahontan facies deposited in a salt pan commonly cap cycles near the basin Basin: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 117, p. 1–30. BOHACS, K.M., 1998, Contrasting expressions of depositional sequences in mudrocks depocenter. Wilkins Peak Member cycles do not readily conform to from marine to nonmarine environs, in Schieber, J., Zimmerle, W., and Sethi, P., eds., any existing sequence stratigraphic nomenclature, and instead Shales and Mudstones I: Stuttgart, Schweizerbart’sche Verlagsbuchhandlung, p. should be considered as aggradational, non-Waltherian event beds 33–78. that record rapid and comprehensive paleoenvironmental change. BOHACS, K.M., CARROLL, A.R., NEDE, J.E., AND MANKIROWICZ, P.J., 2000, Lake-basin type, source potential, and hydrocarbon character: an integrated sequence-strati- 2. At least 126 Wilkins Peak Member cycles are present in the WM graphic-geochemical framework, in Gierlowski-Kordesch, E.H., and Kelts, K.R., eds., core near the basin center, in contrast to 77 cycles previously Lake Basins through Space and Time: American Association of Petroleum Geologists, identified using Fischer Assay data alone. The number of cycles Studies in Geology, no. 46, p. 3–33. BOHACS, K.M., HASIOTIS, S.T., AND DEMKO, T.M., 2002, The birds and the bees: insect preserved within a specific chronostratigraphic window decreases and vertebrate ichnofossil evidence for fluctuating groundwater table and lake level, systematically northward (toward the basin margin), reflecting an Green River and Wasatch formations, Eocene, Wyoming (abstract): Geological interplay between varying magnitudes of lake expansion and Society of America, Annual Meeting, Abstracts with Programs, v. 34, p. 556. a south-dipping depositional gradient. This interplay resulted in BOUCHARD, D.P., KAUFMAN, D.S., HOCHBERG, A., AND QUADE, J., 1998, Quaternary history of the Thatcher Basin, Idaho, reconstructed form the87Sr/86Sr and amino acid creation of a natural stratigraphic ‘‘frequency filter’’ and implies composition of lacustrine fossils: implications for the diversion of the Bear River into that the completeness of the resultant stratigraphic record varies the Bonneville Basin: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 141, p. continuously across the basin. Evidence for complete desiccation 95–114. BRADLEY, M.D., 1995, Timing of the Laramide rise of the Uinta Mountains, Utah and and hiatuses is commonplace even near the basin center, suggesting Colorado: Wyoming Geological Society, Field Conference Guidebook, Casper, that a complete or nearly complete chronostratigraphic record may Wyoming, p. 31–44. not exist anywhere. This study indicates the need for extreme BRADLEY, W.H., 1929, The varves and climate of the Green River Epoch: U.S. caution when interpreting periodic signals from other lake deposits; Geological Survey, Professional Paper 158-E, 110 p. BRADLEY, W.H., 1959, Revision of stratigraphic nomenclature of Green River especially when based on one-dimensional analysis of cycles or Formation of Wyoming: American Association of Petroleum Geologists, Bulletin, where independent chronostratigraphic control is absent. v. 43, p. 1072–1075. 3. The average apparent duration of cycles near the basin center is BRADLEY, W.H., 1962, Chloroplast in spirogyra from the Green River Formation of Wyoming: American Journal of Science, v. 260, p. 455–459. approximately 10,000 years, judging from dated tuff horizons BRADLEY, W.H., AND EUGSTER, H.P., 1969, Geochemistry and paleolimnology of the (Smith et al. 2003). The true duration of these cycles is shorter, trona deposits and associated authigenic minerals of the Green River Formation of due to the ubiquity of hiatuses. Because of the wide range in cycle Wyoming: U.S. Geological Society, Professional Paper 496B, 71 p. thickness (0.14–5.70 m), the shortest cycles most likely represent CARROLL, A.R., AND BOHACS, K.M., 1999, Stratigraphic classification of ancient lakes: Balancing tectonic and climatic controls: Geology, v. 27, p. 99–102. substantially less time than the average, possibly less than CARROLL, A.R., AND BOHACS, K.M., 2001, Lake-type controls on petroleum source rock 1,000 years. No external driving mechanism is presently known potential in nonmarine basins: American Association of Petroleum Geologists, for Eocene cycles of such short duration. It is therefore proposed Bulletin, v. 85, p. 1033–1053. COLMAN, S.M., 1998, Water-level changes in Lake Baikal, Siberia: Tectonism versus that some or all of the Wilkins Peak Member may be autogenic, climate: Geology, v. 26, p. 531–534. reflecting geomorphic instability in the drainage basin that CULBERTSON, W.C., 1961, Stratigraphy of the Wilkins Peak Member of the Green River surrounded Lake Gosiute. Formation, Firehole Basin Quadrangle, Wyoming: U.S. Geological Society, Pro- 4. Evaporite deposition in the Wilkins Peak Member corresponds fessional Paper 424D, p. 170–173. CULBERTSON, W.C., 1969, Oil shale in the Green River Formation, Green River Basin, closely to periods of maximum differential basin subsidence and is Wyoming, in Barlow, J.A. Jr., ed., Wyoming Geological Association Guidebook, 21st concentrated in the greater Green River Basin depocenter. On the Annual Field Conference, Casper, Wyoming, p. 191–195. JSR WILKINS PEAK MEMBER CYCLICITY AND BASIN EVOLUTION 1213

CULBERTSON, W.C., 1971, Stratigraphy of the trona deposits in the Green River LAMBIASE, J.J., 1990, A model for the tectonic control of lacustrine stratigraphic Formation, southwest Wyoming: University of Wyoming, Contributions to Geology, sequences in continental rift basins, in Katz, B.J., ed., Lacustrine Exploration: Case v. 10, p. 15–23. Studies and Modern Analogues: American Association of Petroleum Geologists, DECELLES, P.G., 1994, Late Cretaceous–Paleocene synorogenic sedimentation and Memoir 50, p. 265–276. kinematic history of the Sevier thrust belt, northeast Utah and southwest Wyoming: LAWTON, T.F., 1985, Style and timing of frontal structures, thrust belt, Central Utah: Geological Society of America, Bulletin, v. 106, p. 32–56. American Association of Petroleum Geologists, Bulletin, v. 69, p. 1145–1159. DE DECKKER, P., AND LAST, W.M., 1988, Modern dolomite deposition in continental, LEIGH, T.R., 1998, Wyoming trona: An overview of the geology: Wyoming State saline lakes, western Victoria, Australia: Geology, v. 16, p. 29–32. Geological Survey, Public Information Circular, 40, p. 1–13. DEMAISON, G., AND MOORE, G.T., 1980, Anoxic environments and oil source bed LOVE, J.D., 1970, Cenozoic geology of the Granite Mountains area, central Wyoming: genesis: American Association of Petroleum Geologists, Bulletin, v. 64, p. 1179–1209. U.S. Geological Survey, Professional Paper 495-C, 154 p. DICKINSON, W.R., 1979, Cenozoic plate tectonic setting of the Cordilleran region in the LOWENSTEIN, T.K., AND HARDIE, L.A., 1985, Criteria for the recognition of salt-pan United States, in Armentrout, J.M., Cole, M.R., and TerBest, H. Jr., eds., Cenozoic evaporites: Sedimentology, v. 32, p. 627–644. Paleogeography of the Western United States: SEPM, Pacific Section, Paleogeogra- LOWENSTEIN, T.K., LI, J., AND BROWN, C.B., 1998, Paleotemperatures from fluid phy Symposium III, p. 1–13. inclusions in halite: method verification and a 100,000 year paleotemperature record, DICKINSON,W.R.,KLUTE,M.A.,HAYES, M.J., JANECKE, S.U., LUNDIN,E.R., Death Valley, CA: Chemical Geology, v. 150, p. 223–245. MCKITTRICK, M.A., AND OLIVARES, M.D., 1988, Paleogeographic and paleotectonic MACGINITIE, H.D., 1969, The Eocene Green River flora of northwestern Colorado and setting of Laramide sedimentary basins in the central Rocky Mountain region: northeastern Utah: University of California, Publication in Geological Sciences, v. 83, Geological Society of America, Bulletin, v. 100, p. 1023–1039. 140 p. EUGSTER, H.P., 1970, Chemistry and origin of the brines of Lake Magadi, Kenya, in MALDE, H.E., 1960, Evidence in the Snake River plain, Idaho, of a catastrophic flood Morgan, B.A., ed., Fiftieth Anniversary Symposia, Mineralogy and Geochemistry of from Pleistocene Lake Bonneville: U.S. Geological Survey, Professional Paper 400B, Non-marine Evaporites: Mineralogical Society of America, Special Paper 3, p. p. 295–297. 215–235. MARKWICK, P.J., 1994, ‘‘Equability,’’ continentality, and Tertiary ‘‘climate’’: The EUGSTER, H.P., AND HARDIE, L.A., 1975, Sedimentation in an ancient playa-lake crocodilian perspective: Geology, v. 22, p. 613–616. complex: The Wilkins Peak Member of the Green River Formation of Wyoming: MASON, G.M., AND SURDAM, R.C., 1992, Carbonate mineral distribution and isotope Geological Society of America, Bulletin, v. 86, p. 319–334. fractionation: An approach to depositional environment interpretation, Green River EUGSTER, H.P., AND SURDAM, R.C., 1973, Depositional environment of the Green River Formation, Wyoming, USA: Chemical Geology, v. 101, p. 311–321. Formation of Wyoming: A preliminary report: Geological Society of America, MATTHEWS,M.D.,AND PERLMUTTER, M.A., 1994, Global cyclostratigraphy: an Bulletin, v. 84, p. 1115–1120. application to the Eocene Green River Basin, in DeBoer, P.L., and Smith, D.G., FAIRCHILD, J.L., LOVEJOY, M.E., AND MOULTON, G.F., JR., 1998, A new technology for eds., Orbital Forcing and Cyclic Sequences: International Association of Sedimentol- the soda ash deposits near Trona, California, in Dyni, J.R., and Jones, R.W., eds., ogists, Special Publication 19, p. 459–481. Proceedings of the First International Soda Ash Conference, Volume II: Laramie, MCCUNE, A.R., 1987, Lakes as laboratories of evolution: Endemic fishes and Wyoming, Wyoming State Geological Society, p. 143–151. environmental cyclicity: Palaios, v. 2, p. 446–454. FENWICK, I., 1985, Paleosols, problems of recognition and interpretation, in Boardman, MCGEE, L.C., 1983, Laramide sedimentation, folding and faulting southern Wind River J., ed., Soils and Quaternary Landscape Evolution: New York, Wiley Interscience, p. Range, Wyoming [Ph.D. dissertation]: Laramie, Wyoming, University of Wyoming, 3–21. 92 p. FISCHER, A.G., AND ROBERTS, L.T., 1991, Cyclicity in the Green River Formation MIALL, A.D., 1992, Alluvial Deposits, in Walker, R.G., and James, N.P., eds., Facies (lacustrine Eocene) of Wyoming: Journal of Sedimentary Petrology, v. 61, p. Models: Geological Association of Canada, p. 119–142. 1146–1154. MIALL, A.D., 1996, The Geology of Fluvial Deposits; Sedimentary Facies, Basin FISCHER, A.G., GRIPPO, A., AND TEERMAN, S., 2004, Eocene Green River Formation: Analysis, and Petroleum Geology: New York, Springer, 582 p. Orbital cycles versus argon–argon chronology (abstract): Geological Society of MIDDLETON, G.V., 1973, Johannes Walther’s Law of the Correlation of Facies: America, Abstracts with Programs, v. 36, no. 5, 99 p. Geological Society of America, Bulletin, v. 84, p. 979–987. FREYTET, P., AND PLAZIAT, J.C., 1982, Continental carbonate sedimentation and MITCHUM, R.M., JR., VAIL, P.R., AND THOMPSON, S, III, 1977, Seismic stratigraphy and pedogenesis—Late Cretaceous and Early Tertiary of South France, in Purser, B.H., global changes of sea level, Part 2: The depositional sequence as a basic unit for ed., Contributions to Sedimentology: Stuttgart, Schweizerbart’sche Verlag, v. 12, stratigraphic analysis, in Payton, C.E., ed., Seismic Stratigraphy-applications to 213 p. Hydrocarbon Exploration: American Association of Petroleum Geologists, Memoir FREYTET,P.,AND VERRECCHIA, E.P., 2002, Lacustrine and palustrine carbonate 26, p. 53–62. petrography: an overview: Journal of Paleolimnology, v. 21, p. 221–237. MORRILL, C., SMALL, E.E., AND SLOAN, L.C., 2001, Modeling orbital forcing of lake level GLENN, C.R., AND KELTS, K.R., 1991, Sedimentary rhythms in lake deposits, in Einsele, change: Lake Gosiute (Eocene), North America: Global and Planetary Change, v. 29, G., Ricken, W., and Seilacher, A., eds., Cycles and Events in Stratigraphy: Berlin, p. 57–76. Springer, p. 188–221. NORRIS, R.D., JONES, L.S., CORFIELD, R.M., AND CARTLIDGE, J.E., 1996, Skiing in the GILBERT, G.K., 1882, Contributions to the history of Lake Bonneville, in Powell, J.W., Eocene Uinta Mountains? Isotopic evidence in the Green River Formation for snow ed., Report of the Director of the United States Geological Survey: Washington, melt and large mountains: Geology, v. 24, p. 403–406. D.C., Government Printing Office, p. 167–200. OLSEN, P.E., 1986, A 40-million-year lake record of early Mesozoic orbital climatic GRIES, R., 1983, North–south compression of Rocky Mountain foreland structures, in forcing: Science, v. 234, p. 842–848. Lowell, J.D., ed., Rocky Mountain Foreland Basins and Uplifts: Rocky Mountain OLSEN, P.E., 1990, Tectonic, climatic, and biotic modulation of lacustrine ecosystems; Association of Geologists, Guidebook, p. 9–32. examples from Newark Supergroup of eastern North America, in Katz, B.J., ed., HANSEN, W.R., 1986, History of faulting in the eastern Uinta Mountains, Colorado and Lacustrine Exploration: Case Studies and Modern Analogues: American Association Utah, in Stone, D.S., and Johnson, K.S., eds., New Interpretations of Northwest of Petroleum Geologists, Memoir 50, p. 209–224. Colorado Geology: Rocky Mountain Association of Geologists, p. 5–17. OLSEN, P.E., AND KENT, D.V., 1996, Milankovitch climate forcing in the tropics of HASBARGEN, L.E., AND PAOLA, C., 2000, Landscape instability in an experimental Pangea during the Late Triassic: Palaeogeography, Palaeoclimatology, Palaeoecology, drainage basin: Geology, v. 28, p. 1067–1070. v. 122, p. 1–26. HELVACI, C., 1998, The Beypazari trona deposit, Ankara Province, Turkey, in Dyni, OVIATT, C.G., CURREY, D.R., AND SACK, D., 1992, Radiocarbon chronology of Lake J.R., and Jones, R.W., eds., Proceedings of the First International Soda Ash Bonneville, eastern Great Basin, USA: Palaeogeography, Palaeoclimatology, Palaeo- Conference Volume II: Laramie, Wyoming, Wyoming State Geological Society, p. cology, v. 99, p. 225–241. 67–103. OWEN, R.B., CROSSLEY, R., JOHNSON, T.C., TWEDDLE, D., KORNFIELD, L., DAVISON, S., JOPLING, A.V., AND WALKER, R.G., 1968, Morphology and origin of ripple-drift cross- AND ECCLES, D.H., 1990, Major low levels of Lake Malawi and their implications for lamination, with examples from the Pleistocene of Massachusetts: Journalof speciation rates in cichlid fishes: Royal Society [London], Proceedings, Series B, Sedimentary Petrology, v. 38, p. 971–984. Biological Sciences, v. 240, p. 519–553. JOHNSON, T.C., SCHOLZ, C.A., TALBOT, M.R., KELTS, K., RICKETTS, R.D., NGOBI, G., PAIK, I.S., AND LEE, Y.I., 1998, Desiccation cracks in vertic palaeosols of the Cretaceous BEUNING, K., SSEMMANDA, I., AND MCGILL, J.W., 1996, Late Pleistocene desiccation of Hasandong Formation, Korea: genesis and palaeoenvironmental implications: Lake Victoria and rapid evolution of cichlid fishes: Science, v. 273, p. 1091–1093. Sedimentary Geology, v. 119, p. 161–179. JONES, B.F., 1965, The hydrology and mineralogy of Deep Springs Lake, Inyo County, PARK, J., HONDT, S.L.D., KING, J.W., AND GIBSON, C., 1993, Late Cretaceous California: U.S. Geologic Survey, Professional Paper 502A, 56 p. precessional cycles in double time: a warm-Earth Milankovitch response: Science, KELTS, K., 1988, Environments of deposition of lacustrine petroleum source rocks: An v. 261, p. 1431–1434. introduction, in Fleets, A.J., Kelts, K., and Talbot, M.R., eds., Lacustrine Petroleum PEKAR, S.F., HUCKS, A., FULLER, M., AND LI, S., 2005, Glacioeustatic changes in the Source Rocks: Geological Society of London, Special Publication 40, p. 3–26. early and middle Eocene (51–42 Ma): hallow-water stratigraphy from ODP Leg 189 18 KENDALL, A.C., 1988, Aspects of evaporite basin stratigraphy, in Schreiber, B.C., ed., Site 1171 (South Tasman Rise) and deep-sea d O records: S Geological Society of Evaporites and Hydrocarbons: New York, Columbia University Press, p. 11–65. America, Bulletin, v. 117, p. 1081–1093. KERANS, C., AND TINKER, S.W., 1997, Sequence stratigraphy and characterization of PICARD, M.D., AND HIGH, L.R., JR., 1973, Sedimentary Structures of Ephemeral carbonate reservoirs: SEPM, Short Course Notes 40, 130 p. Streams: New York, Elsevier, 223 p. 1214 J.T. PIETRAS AND A.R. CARROLL JSR

PIETRAS, J.T., 2003, High-resolution sequence stratigraphy and strontium isotope STANLEY, K.O., 1974, Morphology and hydraulic significance of climbing ripples with geochemistry of the lacustrine Wilkins Peak Member, Eocene Green River superimposed micro-ripple-drift cross-lamination in lower Quaternary lake silts, Formation, Wyoming, U.S.A. [Ph.D. dissertation]: University of Wisconsin– Nebraska: Journal of Sedimentary Petrology, v. 44, p. 472–483. Madison, 372 p. STEENBRINK, J., VAN VUGT, N., HILGEN, F.J., WILJBRANS, J.R., AND MEULENKAMP, J.E., PIETRAS, J.T., CARROLL, A.R., SINGER,B.S.,AND SMITH, M.E., 2003a, 10 k.y. 1999, Sedimentary cycles and volcanic ash beds in the lower Pliocene lacustrine depositional cyclicity in the early Eocene: Stratigraphic and 40Ar/39 Ar evidence from succession of Ptolemais (northwest Greece): Discrepancy between 40Ar/39Ar and the lacustrine Green River Formation: Geology, v. 31, p. 593–596. astronomical ages: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 152, p. PIETRAS, J.T., CARROLL, A.R., AND RHODES, M.K., 2003b, Lake basin response to 283–303. tectonic drainage diversion, Eocene Green River Formation, Wyoming: Journal of STEIDTMANN, J.R., MCGEE, L.C., AND MIDDLETON, L.T., 1983, Laramide sedimentation, Paleolimnology, v. 30, p. 115–125. folding, and faulting in the southern Wind River Range, Wyoming, in Lowell, J.D., PITMAN, J.K., 1996, Origin of primary and diagenetic carbonates in the lacustrine Green ed., Rocky Mountain Foreland Basins and Uplifts: Rocky Mountain Association of River Formation (Eocene), Colorado and Utah: U.S. Geological Survey, Bulletin Geologists, Guidebook, p. 161–167. 2157, 17 p. STIASSNY, M.L.J., AND MEYER, A., 1999, Cichlids of the Rift Lakes: Scientific American, PLATT, N.H., AND WRIGHT, V.P., 1992, Palustrine carbonates and the Florida v. 280, p. 64–69. Everglades: towards an exposure index for the fresh-water environment: Journal of SURDAM,R.C.,AND STANLEY, K.O., 1979, Lacustrine sedimentation during the Sedimentary Petrology, v. 62, p. 1058–1071. culminating phase of Eocene Lake Gosiute, Wyoming (Green River Formation): REHEIS, M.C., STINE, S., AND SARNA-WOJCICKI, A.M., 2002, Drainage reversals in Mono Geological Society of America, Bulletin, v. 90, p. 93–110. Basin during the late Pliocene and Pleistocene: Geological Society of America, SURDAM, R.C., AND WOLFBAUER, C.A., 1975, Green River Formation, Wyoming: Bulletin, v. 114, p. 991–1006. A playa-lake complex: Geological Society of America, Bulletin, v. 86, p. 335–345. REINECK, H.E., AND WUNDERLICH, F., 1968, Classification and origin of flaser and TALBOT, M.R., AND ALLEN, P.A., 1996, Lakes, in Reading, H.G., ed., Sedimentary lenticular bedding: Sedimentology, v. 11, p. 99–104. Environments; Processes, Facies and Stratigraphy, Oxford, Blackwell Science, p. RENAUT, R.W., AND TIERCELIN, J.J., 1994, Lake Bogoria, Kenya Rift Valley—A 83–124. sedimentological overview, in Renaut, R.W., and Last, W.M., eds., Sedimentology TISSOT, B.P., AND VANDENBROUCKE, M., 1983, Geochemistry and pyrolysis of oil shales, and Geochemistry of Modern and Ancient Saline Lakes: SEPM, Special Publication in Miknis, F.P., and McKay, J.F., eds., Geochemistry and Chemistry of Oil Shales: 50, p. 101–123. Washington, DC, American Chemical Society, Symposium Series 230, p. 1–11. OOTH RETALLACK, G.J., 1988, Field recognition of paleosols, in Reinhardt, J., and Sigleo, T , S., 2000a, Process, form and change in dryland rivers: a review of recent W.R., eds., Paleosols and Weathering through Geologic Time: Geology Society of research: Earth-Science Reviews, v. 51, p. 67–107. America, Special Paper 216, p. 1–20. TOOTH, S., 2000b, Downstream changes in dryland river channels: the Northern Plains of arid central Australia: Geomorphology, v. 34, p. 33–54. ROEHLER, H.W., 1989, Correlation of surface sections of the intertongued Eocene Wasatch and Green River formations along the west flank of the Rock Springs Uplift TUCKER, M.E., AND WRIGHT, V.P., 1990, Carbonate Sedimentology: Cambridge, in Southwest Wyoming: U.S. Geological Survey, Miscellaneous Field Studies Map, Massachusetts, Blackwell Science, 482 p. MF-2104. VAN HOUTEN, F.B., 1962, Cyclic sedimentation and the origin of analcime-rich Upper Triassic Lockatong Formation, west-central New Jersey and adjacent Pennsylvania: ROEHLER, H.W., 1991, Revised Stratigraphic nomenclature for the Wasatch and Green River formations of Eocene age, Wyoming, Utah, and Colorado: U.S. Geological American Journal of Science, v. 260, p. 561–576. Survey, Professional Paper 1506B, 38 p. VAN WAGONER, J.C., POSAMENTIER, H.W., MITCHUM, R.M., JR., VAIL,P.R,SARG, J.F., LOUTIT, T.S., AND HARDENBOL, J., 1988, An overview of the fundamentals of sequence ROEHLER, H.W., 1992, Correlation, composition, areal distribution, and thickness of stratigraphy and key definitions, in Wilgus, C.K., Hastings, B.S., Ross, C.A., Eocene stratigraphic units, Greater Green River Basin, Wyoming, Utah, and Posamentier, H.W., Van Wagoner, J.C., and Kendall, C.G.St.C., eds., Sea-Level Colorado: U.S. Geological Survey, Professional Paper 1506E, 49 p. Changes: An Integrated Approach: SEPM, Special Publication 42, p. 39–45. ROEHLER, H.W., 1993, Eocene climates, depositional environments, and geography, WIIG, S.V., GRUNDY, W.D., AND DYNI, J.R., 1995, Trona resources in the Green River Greater Green River Basin, Wyoming, Utah, and Colorado: U.S. Geological Survey, Basin, Southwest Wyoming: U.S. Geological Survey, Open-File Report 95–476. Professional Paper 1506F, 74 p. WILF, P., 2000, Late Paleocene–early Eocene climate changes in southwestern Wyoming: ´ SAEZ, A., CABRERA, L., JENSEN, A., AND CHONG, G., 1999, Late Neogene lacustrine Paleobotanical analysis: Geological Society of America, Bulletin, v. 112, p. 292–307. record and Palaeogeography in the Quillagua–Llamara basin, Central Andean fore- WILF, P., WING, S.L., GREENWOOD, D.R., AND GREENWOOD, C.L., 1998, Using fossil arc (northern Chile): Palaeogeography, Palaeoclimatology, Palaeoecology, v. 151, p. leaves as paleoprecipitation indicators: An Eocene example: Geology, v. 26, p. 5–37. 203–206. SCHOLZ, C.A., AND FINNEY, B.P., 1994, Late Quaternary sequence stratigraphy of Lake WING, S.L., AND GREENWOOD, D.R., 1993, Fossils and fossil climate: the case for equable Malawi (Nyasa), Africa: Sedimentology, v. 41, p. 163–179. continental interiors in the Eocene: Royal Society [of London], Philosophical SHORT, D.A., MENGEL, J.G., CROWLEY, T.J., HYDE, W.T., AND NORTH, G.R., 1991, Transactions, v. 341, p. 243–252. Filtering of Milankovitch cycles by Earth’s geography: Quaternary Research, v. 35, p. WITKIND, I.J., AND GROSE, L.T., 1972, Figure 1: Areal geologic map of the Rocky 157–173. 40 39 Mountain region and environs, in Mallory, W.W., ed., Geologic Atlas of the Rocky SMITH, M.E., SINGER, B., AND CARROLL, A., 2003, Ar/ Ar geochronology of the Mountain Region: Rocky Mountain Association of Geologists, p. 34. Eocene Green River Formation, Wyoming: Geological Society of America, Bulletin, WOLF, J.A., 1978, A paleobotanical interpretation of Tertiary climates in the Northern v. 115, p. 549–565. 40 39 Hemisphere: American Scientist, v. 66, p. 694–703. SMITH, M.E., SINGER, B., AND CARROLL, A., 2004, Ar/ Ar temporal constraints on YAALON, D.H., AND KALMAR, D., 1978, Dynamics of cracking and swelling clay soils: Eocene uplift, subsidence, and paleohydrology in the Laramide Foreland, Western displacement of skeletal grains, optimum depth of slickensides, and rate of intra- U.S. (abstract): American Geophysical Union, Eos, Transactions, v. 85, Fall Meeting pedonic turbation: Earth Surface Processes, v. 3, p. 31–42. Supplement, Abstract T51D-07. YANG, W., SPENCER, R.J., KROUSE, H.R., LOWENSTEIN, T.K., AND CASAS, E., 1995, Stable SMOOT, J.P., 1983, Depositional subenvironments in an arid closed basin; Wilkins Peak isotopes of lake and fluid inclusion brines, Dabusun Lake, Qaidam Basin, western Member of the Green River Formation (Eocene), Wyoming, U.S.A.: Sedimentology, China: Hydrology and paleoclimatology in arid environments: Palaeogeography, v. 30, p. 801–827. Palaeoclimatology, Palaeoecology, v. 117, p. 279–290. SMOOT, J.P., AND LOWENSTEIN, T.K., 1991, Depositional environments of non-marine YURETICH, R., 1989, Paleocene lakes of the central Rocky Mountains, western United evaporites, in Melvin, J.L., ed., Evaporites, Petroleum and Mineral Resources: New States: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 70, p. 53–63. York, Elsevier, Developments in Sedimentology 50, p. 189–347. SNEH, A., 1983, Desert stream sequences in the Sinai Peninsula: Journal of Sedimentary Petrology, v. 53, p. 1271–1279. Received 3 October 2005; accepted 9 March 2006.