Lake basin response to tectonic drainage diversion: Eocene Green River Formation, Wyoming

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1 Journal of Paleolimnology 30: , Kluwer Academic Publishers. Printed in the Netherlands. 115 Lake basin response to tectonic drainage diversion: Eocene Green River Formation, Wyoming Jeffrey T. Pietras*, Alan R. Carroll and Meredith K. Rhodes Department of Geology and Geophysics, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA; *Author for correspondence ( Received 10 August 2001; accepted 11 December 2002 Key words: Wilkins Peak, Lacustrine, Sequence boundary, Tectonic control, Evaporite Abstract A previously unidentified major sequence boundary within the Eocene Green River Formation separates fluctuating profundal facies of the Tipton Shale Member from evaporative facies of the Wilkins Peak Member. During deposition of the Tipton Shale Member, rivers entered the basin from the north, across the subdued Wind River Mountains, and deposited the southward prograding deltaic complex of the Farson Sandstone Member. Boulderrich alluvial fan deposits overlie the Farson Sandstone adjacent to the Continental Fault, and correlate basinward to hypersaline lacustrine deposits of the Wilkins Peak Member. The alluvial fan deposits record a period of reverse motion on the Continental Fault and uplift of the southeastern Wind River Range, which diverted drainage away from the greater Green River Basin. This decreased inflow caused Lake Gosiute to shrink, exposing its bed to desiccation and erosion, and contributed to hydrologically-closed conditions and periodic evaporite deposition thereafter. This study is one of the first to demonstrate a direct relationship between movement along a specific basin-bounding structure, and a change in the overall style of lacustrine sedimentation. The identification of similar relationships elsewhere may challenge conventional interpretations of climate as the dominant factor influencing the character of lake deposits, and provide an important, but previously unexploited, approach to interpreting continental deformation and regional drainage organization. Introduction Lacustrine strata of long-lived lakes fundamentally record impoundment of paleo-drainage systems due to tectonic subsidence or uplift, and therefore offer unique and potentially important records of continental deformation. However, these records are rarely so exploited, due largely to the fact that our understanding of the *This is the first in a series of four papers published in this issue collected from the 2000 GSA Technical Session Lake basins as archives of continental tectonics and paleoclimate in Reno, Nevada. This collection is dedicated to Dr. Kerry R. Kelts; Drs. Elizabeth Gierlowski-Kordesch and H. Paul Buchheim were the guest editors of this collection. controls on lacustrine stratigraphy remains very poorly developed relative to marine systems. In addition, climate is conventionally perceived to be the principal control on lacustrine sedimentation. For example, cyclic packages of Quaternary lacustrine strata resulting from rapid lake level fluctuations are commonly interpreted to result from climatic forcing (e.g., Benson, 1981; Owen et al., 1990; Oviatt, 1997). These observations have been used to infer that cyclic stratigraphy in more ancient deposits was also forced by orbitallydriven climatic perturbations (e.g., Olsen, 1986; Roehler, 1993; Benson, 1999), even in the numerous cases where independent chronological evidence supporting this interpretation does not exist. Furthermore, the basin-scale occurrence of evaporites is commonly attributed to longer-term changes in precipitation vs. evaporation and transpiration (e.g., Castlefield Press: JOPL 900, CP, typeset, disc, Pips no.:

2 116 Langbein, 1961; Roehler, 1993; Gómez-Fernández and Meléndez, 1994). However, there is little correlation between precipitation/evaporation and any measure of lake size (Carroll and Bohacs, 1999) or chemistry (Bohacs et al., 2000) for modern lakes. Recent studies suggest that relatively subtle tectonic activity along key drainage divides can exert a first order control on basin hydrology, and consequently on evaporite deposition (e.g., Kowalewska and Cohen, 1998; May et al., 1999; Sáez et al., 1999). Yet, none of these studies have documented the detailed relationship between specific tectonic events and basin stratigraphy. The greater Green River Basin (Figure 1) of southwestern Wyoming provides an excellent opportunity to examine these detailed relationships, due to its continuous outcrop exposures and to the abundance of previous studies of nonmarine sedimentary facies and regional tectonics. The Green River Formation (Figure 2) consists of a mix of sedimentary facies, recording deposition in freshwater to hypersaline phases of Eocene Lake Gosiute (Culbertson, 1969; Hanley, 1976; Surdam and Stanley, 1979; Surdam and Stanley, 1980; Smoot, 1983; Roehler, 1991; Bohacs, 1998; and references therein). Continuous exposures of lacustrine and associated alluvial strata west and north of the Rock Springs Uplift (Figure 1) permit the identification, and direct correlation, of stratal boundaries between lacustrine facies associations near the basin center and syntectonic alluvial strata deposited at the basin margin. In this paper, we describe a lacustrine sequence boundary that separates the Tipton Shale Member from the overlying Wilkins Peak Member, and the relationship of this surface to the structural evolution of a major basin-bounding uplift. This paper aims to directly test the influence of a specific Laramide structural event on lake type evolution. Figure 1. Location of measured sections RS Rock Springs, BG Breathing Gulch, BT Boar s Tusk, and WC Whitehorse Creek within the greater Green River Basin. Maximum extent of individual members of the Green River Formation from Bradley and Eugster, 1969; Roehler, 1992b. Base map modified from Mallory, 1972.

3 117 Figure 2. Cross section A-A showing the regional stratigraphy of the greater Green River Basin, and the unconformable relationship between the Tipton Shale and Wilkins Peak members (modified from Roehler, 1991). See Figure 1 for location. Lake-basin types from Carroll and Bohacs (1999) OF overfilled, BF balanced fill, UF underfilled. Geologic setting The greater Green River Basin (Figure 1) was part of the foreland that formed adjacent to the Sevier Thrust Belt during late Cretaceous time. Final Sevier thrusting occurred during the late Paleocene and early Eocene along the Hogsback Thrust at the western border of the basin (DeCelles, 1994). This foreland was subsequently subdivided by basement cored block uplifts of the Laramide Orogeny during the Paleogene (Bell, 1954; Anderman, 1955; Keefer, 1965; Love, 1970; Dorr et al., 1977; Gries, 1983; Steidtmann et al., 1983; Roehler, 1992b). The Laramide Wind River and Uinta mountains, bordering the basin to the north and south respectively, were uplifted along reverse faults shown to cross cut parts of the Eocene Green River Formation (Steidtmann et al., 1983; Roehler, 1993). The Green River Formation (Hayden, 1869) records four major phases of Eocene Lake Gosiute, represented by the Luman Tongue, Tipton Shale Member, Wilkins Peak Member, and the Laney Member (Figure 2). These lacustrine strata grade laterally into the alluvial Wasatch Formation. During deposition of the Luman Tongue, Lake Gosiute is interpreted to have been a freshwater lake. The Luman Tongue is separated from the rest the Green River Formation by the overlying Niland Tongue of the Wasatch Formation. The remaining three members directly overlie one another, and record the shift in deposition in freshwater lakes (Tipton Shale Member), to evaporative lakes (Wilkins Peak Member), and then back to freshwater conditions in the Laney Member (Hanley, 1976; Roehler, 1992a; Roehler, 1993). Tipton Shale Member: overfilled to balanced fill basin The Tipton Shale Member is divided into the Scheggs Bed and partly correlative Farson Sandstone Member, and the Rife Bed (Figure 2). The base of the Tipton Shale Member is marked by an erosional scour. Sandstone beds that overlie this surface near the basin center grade laterally into pebble conglomerate beds at the basin margin (Figure 3). Fissile organic-rich calcimicrite (oil shale) and fish fossils typify both the Scheggs and Rife beds (Culbertson, 1969). However,

4 118 Figure 3. Correlation of measured sections across the Tipton Shale/Wilkins Peak contact showing the progradational geometry of the Farson Sandstone Member, and details along the sequence boundary. See caption on Figure 1 for localities. the freshwater Pisidiidae-Goniobasis-Valvata mollusk association of Hanley (1976) is abundant only in the Scheggs Bed, suggesting that conditions in the lake became more saline during deposition of the Rife Bed. The Farson Sandstone Member (Roehler, 1991) is interpreted as deltaic and lake-margin deposits that generally prograded into Lake Gosiute from the north during deposition of the Scheggs Bed. It is mapped across the entire southern margin of the Wind River Mountains, extending southward to the center of the greater Green River Basin (Figure 1). The stratal geometries of two sandstone intervals observed within the Farson Sandstone Member record progradation to the southwest (Figure 3). The lower interval consists of fine-to-medium-grained sandstone deposited as southwestward dipping deltaic foresets with approximately 15 m of relief, indicating water depths of at least this great recorded in the Whitehorse Creek section (Figure 4). Individual clinoforms can be correlated down dip to very-fine-grained sandstone beds with abundant gastropods, and up dip to thin pebble lags composed mainly of quartz, chert, schist, and gneiss (McGee, 1983; Pietras et al., 2000). Deposition of the upper interval extended farther southwest, as far south as the Breathing Gulch section (Figure 3). At Whitehorse Creek, this interval consists of planar-laminated, finegrained sandstone and muddy-sandstone beds interpreted as topsets of a delta plain facies association. The progradational geometry of the Farson deltas, and the presence of a freshwater fauna suggest that the basin was overfilled during the deposition of the Scheggs Bed. In contrast, the trend to more saline conditions in the overlying Rife Bed suggests an intermittent closed hydrology lending evidence for balanced fill conditions (cf. Carroll and Bohacs, 1999). Wilkins Peak Member: underfilled basin Lithologies in the Wilkins Peak Member (Bradley, 1959) include tan to olive marlstone, organic-rich calcimicrite, thin calcareous sandstone beds, arkosic sandstone beds, and bedded trona and halite (Bradley and Eugster, 1969; Culbertson, 1969; Smoot, 1983). During extended lake lowstands the basin center was the site of a playa where 25 evaporite beds (thicker than 1 m) were deposited ranging from 427 1,870 km 2 in extent (Bradley and Eugster, 1969). Fossils representing a freshwater fauna, such as fish or gastropods, are absent in the Wilkins Peak Member, supporting the interpretation of hypersaline

5 119 Figure 4. Farson Sandstone Member delta at Whitehorse Creek. Foresets dip to the left (southwest), and are 15 m thick on the far left of the photo. See location of this outcrop in Figure 7. conditions inferred from the presence of evaporite deposits. Insect remains are the only common macroscopic fossils observed. The presence of numerous desiccation surfaces and bedded evaporites, suggest that the Wilkins Peak Member was deposited in an underfilled basin (Carroll and Bohacs, 1999). The Wilkins Peak Member thins northward and interfingers with the Cathedral Bluffs Tongue of the Wasatch Formation (Figure 3). In the Whitehorse Creek section, the Cathedral Bluffs Tongue is composed of red and green silty-mudstone with a basal pebble layer (Figure 5A) and rare well-rounded granitic boulders (Figure 5B). These alluvial deposits coarsen northward rapidly into an angular boulder-rich conglomerate that is interpreted to be part of an alluvial fan complex (McGee, 1983). This conglomerate facies reaches approximately 60 m in thickness on Pacific Butte (Figure 6) before terminating along the Continental Fault. The lateral extent of coarse-grained material is restricted to Pacific Butte and a similar butte to the east where the conglomerate facies is approximately 120 m thick (Figure 6). The surface of both buttes dips southwestward at approximately 5. We interpret them as exhumed Eocene alluvial fans. Basal Wilkins Peak Member sequence boundary The Wilkins Peak Member and laterally equivalent Cathedral Bluffs Tongue unconformably overlie the Rife Bed of the Tipton Shale Member (Figure 3). This contact is abrupt across the study area, and marked by subaerial erosion. Decimeter-scale scour into profundal facies of the Rife Bed marks the contact in the Rock Springs section (Figure 5C). Here, a thin, very-finegrained sandstone bed of the Wilkins Peak Member infills the scour, and contains rip-ups of the underlying organic-rich calcimicritic Rife Bed. Mudcracks are present along this contact northward at both the Breathing Gulch and Boar s Tusk sections (Figure 5D). A 1 m thick stromatolite bed was deposited on this surface in the Breathing Gulch section during initial transgression of the Wilkins Peak Member. Concurrently, a thin very-fine-grained sandstone bed overlain by a 1 cm thick oolite bed was deposited in the Boar s Tusk section. Along the northern basin margin, at Whitehorse Creek, the Cathedral Bluffs Tongue overlies the Rife Bed. Here, the contact is marked by meter-scale incision, and is overlain by a pebble conglomerate that grades upward into silty-mudstone (Figure 5A). We interpret the contact between the Tipton Shale

6 120 Figure 5. (A) Mudstone of the Tipton Shale Member unconformably overlain by pebble conglomerate of the Cathedral Bluffs Tongue and Whitehorse Creek section. (B) Well-rounded granitic boulder at the base of the Cathedral Bluffs Tongue at Whitehorse Creek approximately 1 m in diameter. (C) Tipton Shale/Wilkins Peak sequence boundary at Rock Springs section marked by sandstone filled scour into oil shale. (D). Mudcrack casts at the base of the Wilkins Peak Member at Breathing Gulch section. Specimen is 50 cm across. Figure 6. Geologic map of the Whitehorse Creek section and surrounding features. Note the presence of two exhumed alluvial fans terminating along the trace of the Continental Fault. See Figure 1 for location. Modified from Zeller and Stevens (1969).

7 121 and Wilkins Peak members as a sequence boundary; based on the observation that it is an unconformable surface that separates two genetically unrelated packages of rock (Mitchum et al., 1977; Van Wagoner et al., 1988). This surface thus marks a fundamental shift in the style of sedimentation in Lake Gosiute from overfilled to underfilled deposits. Discussion Previous workers have inferred that evaporite deposition in the Wilkins Peak Member occurred due to a long-term shift to a hotter and drier climate (Bradley and Eugster, 1969; Roehler, 1993). Studies based on crocodilian distribution (Markwick, 1994), leaf-margin analysis (Wolfe, 1978; Wing and Greenwood, 1993; Wilf, 2000), and mean leaf area (Wilf et al., 1998; Wilf, 2000) have produced mean annual temperature (MAT) estimates of ºC, and mean annual precipitation (MAP) of cm/yr for the area during deposition of the Green River Formation. The paleotemperature estimates, associated with deposition in Eocene Lake Gosiute, are warmer than the conditions determined for the underlying Paleocene fluvial and alluvial deposits of the Fort Union and Wasatch formations (~12 19 ºC). This observation is consistent with global proxies of paleoclimate that suggest the early Eocene was the warmest period during the Cenozoic (Zachos et al., 2001). However, detailed evidence for climate change across the Tipton-Wilkins Peak boundary is equivocal at best. Only two floral assemblages constrain the boundary. The lower assemblage was collected from the Niland Tongue of the Wasatch Formation, located stratigraphically below the Tipton Shale Member (Figure 2; Wilf, 2000). The upper assemblage was collected at the contact between the Wilkins Peak and Laney members (MacGinitie, 1969; Wolfe, 1978; Wilf, 2000). In the most recent reinvestigation of the paleobotanical record, Wilf (2000) showed that the MAT increased by 4 ºC, and the MAP decreased by 25 cm/ yr between the Niland Tongue and upper Wilkins Peak assemblages. This data suggests that the fluvial/alluvial Niland Tongue was deposited during a wetter period than the lacustrine upper Wilkins Peak and lower Laney members. This is inconsistent with the notion that large lakes form during wetter times, and contract during climatically warmer-drier periods. Furthermore, Surdam and Stanley (1980) correlated lacustrine strata between the greater Green River Basin and the Piceance Creek and Uinta basins to the south, based on stratigraphic patterns and potassium-argon dates (Mauger, 1977) of interbedded volcanic tuffs. They showed that evaporite deposition occurred synchronously between the Piceance Creek Basin and the Wilkins Peak Member, however evaporite deposition in the Uinta Basin occurred later. Smith (2002) confirmed this correlation based on high precision 40 Ar/ 39 Ar geochronology of the same volcanic tuffs. It is likely that Eocene climate for these basins was similar based on their proximity, however the long-term lake type fluctuated independently. A modern analogue to this situation is represented by Utah Lake and Great Salt Lake. Separated by only 50 km, both are within the same climate regime, however Utah Lake is a freshwater lake that drains into the hypersaline Great Salt Lake. Combined, these observations suggest that factors other than regional climate controlled longterm lake type. Alternatively, the Tipton Shale-Wilkins Peak sequence boundary can be explained by tectonic reorganization of the Lake Gosiute drainage basin in response to renewed uplift of the southern Wind River Mountains along the Continental Fault (Figure 1). The most recent movement of the Continental Fault was extensional, resulting in a Neogene graben (Love, 1954; Zeller and Stephens, 1969; Steidtmann and Middleton, 1991). However, this fault has been interpreted as an Early Eocene reverse fault, up thrown to the north, based on the association with the nearby Wind River thrust system and the shift in sedimentation from the deltaic Farson Sandstone Member to the overlying alluvial fan deposits of the Cathedral Bluffs Tongue (McGee, 1983; Steidtmann et al., 1983; Steidtmann and Middleton, 1991). As seen on Figures 6 and 7, the surface trace of the Continental Fault at Whitehorse Creek is just 2.5 km north of Reds Cabin Monocline, which formed in response to thrusting along the buried Wind River Fault. Deformation of Reds Cabin Monocline postdates deposition of the Cathedral Bluffs Tongue. The abrupt increase in maximum grain size from pebbles to boulders, and a shift from rounded to angular clasts across the sequence boundary lends more support to the development of high relief near the Whitehorse Creek section. We propose that uplift of the Wind River Mountains along the Continental Fault contributed to the diversion of rivers that had flowed into the greater Green River Basin during deposition of the Tipton Shale and Farson Sandstone members. The result was a rapid decrease in lake area and the ensuing deposition of evaporites and associated underfilled lacustrine strata of the Wilkins Peak Member in the basin center, and alluvial deposition at the basin margin (Figure 8).

8 122 Figure 7. Oblique aerial photograph of the Whitehorse Creek section looking northwest towards the Wind River Mountains. Note the deltaic deposits of the Farson Sandstone Member overlain by an exhumed alluvial fan (Pacific Butte) of the Cathedral Bluffs Tongue that terminates along the trace of the Continental Fault. Sandstone provenance provides further evidence for tectonic drainage reorganization. McGee (1983) showed that sand grains in the Farson Sandstone Member contain a heavy mineral assemblage similar to that of a Precambrian iron formation exposed in the core of the Wind River Mountains (Figure 1). The streams supplying sand to the Farson Sandstone Member therefore must have crossed the present position of the Wind River drainage divide. Furthermore, McGee (1983) interpreted variations in sandstone provenance between Figure 8. Schematic illustration of Early Eocene uplift of the southern Wind River Mountains, and consequent river diversion and evolution from an overfilled to underfilled lacustrine basin.

9 123 beds within the Farson Sandstone Member to indicate subtle shifts in source area caused by tectonic activity in the southern Wind River Mountains. Conclusions Structural and stratigraphic field relations across the greater Green River Basin provide a well-constrained example of lacustrine evaporite deposition in response to a specific tectonic event. Because many lake basins are fundamentally associated with areas of active orogenic uplift, the identification of long-term changes in lacustrine facies associations offers an important (and under-utilized) means for helping to decipher uplift histories. In addition, this study documents a previously unrecognized lacustrine sequence boundary from one of the most intensely studied lacustrine basins in the world. The use of sequence stratigraphy is widely accepted for marine basins, where stratal surfaces are formed by relative changes in sea level. Relative sea level change encompasses eustatic sea level fluctuations, as well as regional and global tectonics. Recent studies (e.g., Tang et al., 1994; Currie, 1997) have applied these same concepts to nonmarine basins where sediment supply, regional tectonics, and local climate control sequence development. Furthermore, Carroll and Bohacs (1999) argued that changes in lakebasin type, presumably marked by sequence stratigraphic surfaces, results from the balance between tectonically controlled potential accommodation and the water+sediment fill rate commonly controlled by climate. However, episodic events such as faulting (described in this paper), catastrophic failure of basin sills (e.g., Malde, 1960; Stearns, 1962), or the diversion of rivers by volcanic flows (e.g., Bouchard et al., 1998) can alter the sediment + water fill rate. 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