Lake Basin Response to Tectonic Drainage Diversion: Eocene Green River Formation, Wyoming
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Journal of Paleolimnology 30: 115–125, 2003. 115 © 2003 Kluwer Academic Publishers. Printed in the Netherlands. 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 (e-mail: [email protected]) 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 fluctuat- ing 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. Boulder- rich 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 simi- lar relationships elsewhere may challenge conventional interpretations of climate as the dominant factor influenc- ing the character of lake deposits, and provide an important, but previously unexploited, approach to interpreting continental deformation and regional drainage organization. Introduction controls on lacustrine stratigraphy remains very poorly developed relative to marine systems. In addition, cli- Lacustrine strata of long-lived lakes fundamentally mate is conventionally perceived to be the principal record impoundment of paleo-drainage systems due to control on lacustrine sedimentation. For example, cy- tectonic subsidence or uplift, and therefore offer unique clic packages of Quaternary lacustrine strata resulting and potentially important records of continental defor- from rapid lake level fluctuations are commonly inter- mation. However, these records are rarely so exploited, preted to result from climatic forcing (e.g., Benson, due largely to the fact that our understanding of the 1981; Owen et al., 1990; Oviatt, 1997). These obser- vations have been used to infer that cyclic stratigraphy in more ancient deposits was also forced by orbitally- driven climatic perturbations (e.g., Olsen, 1986; Roehler, 1993; Benson, 1999), even in the numerous cases where *This is the first in a series of four papers published in this issue independent chronological evidence supporting this collected from the 2000 GSA Technical Session ‘Lake basins as interpretation does not exist. archives of continental tectonics and paleoclimate’ in Reno, Nevada. Furthermore, the basin-scale occurrence of evap- This collection is dedicated to Dr. Kerry R. Kelts; Drs. Elizabeth Gierlowski-Kordesch and H. Paul Buchheim were the guest editors orites is commonly attributed to longer-term changes of this collection. in precipitation vs. evaporation and transpiration (e.g., Castlefield Press: JOPL 900, CP, typeset, disc, Pips no.: 5119286 116 Langbein, 1961; Roehler, 1993; Gómez-Fernández and 2) consists of a mix of sedimentary facies, recording Meléndez, 1994). However, there is little correlation deposition in freshwater to hypersaline phases of Eocene between precipitation/evaporation and any measure of Lake Gosiute (Culbertson, 1969; Hanley, 1976; Sur- lake size (Carroll and Bohacs, 1999) or chemistry dam and Stanley, 1979; Surdam and Stanley, 1980; (Bohacs et al., 2000) for modern lakes. Recent studies Smoot, 1983; Roehler, 1991; Bohacs, 1998; and refer- suggest that relatively subtle tectonic activity along key ences therein). Continuous exposures of lacustrine and drainage divides can exert a first order control on ba- associated alluvial strata west and north of the Rock sin hydrology, and consequently on evaporite deposi- Springs Uplift (Figure 1) permit the identification, and tion (e.g., Kowalewska and Cohen, 1998; May et al., direct correlation, of stratal boundaries between lac- 1999; Sáez et al., 1999). Yet, none of these studies have ustrine facies associations near the basin center and documented the detailed relationship between specific syntectonic alluvial strata deposited at the basin mar- tectonic events and basin stratigraphy. gin. In this paper, we describe a lacustrine sequence The greater Green River Basin (Figure 1) of south- boundary that separates the Tipton Shale Member from western Wyoming provides an excellent opportunity to the overlying Wilkins Peak Member, and the relation- examine these detailed relationships, due to its continu- ship of this surface to the structural evolution of a major ous outcrop exposures and to the abundance of pre- basin-bounding uplift. This paper aims to directly test vious studies of nonmarine sedimentary facies and the influence of a specific Laramide structural event on regional tectonics. The Green River Formation (Figure 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. 117 Figure 2. Cross section A-A’ showing the regional stratigraphy of the greater Green River Basin, and the unconformable relationship be- tween 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 Lake Gosiute is interpreted to have been a freshwater lake. The Luman Tongue is separated from the rest the The greater Green River Basin (Figure 1) was part of Green River Formation by the overlying Niland Tongue the foreland that formed adjacent to the Sevier Thrust of the Wasatch Formation. The remaining three mem- Belt during late Cretaceous time. Final Sevier thrusting bers directly overlie one another, and record the shift occurred during the late Paleocene and early Eocene in deposition in freshwater lakes (Tipton Shale Mem- along the Hogsback Thrust at the western border of ber), to evaporative lakes (Wilkins Peak Member), and the basin (DeCelles, 1994). This foreland was subse- then back to freshwater conditions in the Laney Mem- quently subdivided by basement cored block uplifts ber (Hanley, 1976; Roehler, 1992a; Roehler, 1993). 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, Tipton Shale Member: overfilled to balanced fill 1992b). The Laramide Wind River and Uinta moun- basin tains, bordering the basin to the north and south respec- tively, were uplifted along reverse faults shown to The Tipton Shale Member is divided into the Scheggs cross cut parts of the Eocene Green River Formation Bed and partly correlative Farson Sandstone Member, (Steidtmann et al., 1983; Roehler, 1993). and the Rife Bed (Figure 2). The base of the Tipton The Green River Formation (Hayden, 1869) records Shale Member is marked by an erosional scour. Sand- four major phases of Eocene Lake Gosiute, represented stone beds that overlie this surface near the basin center by the Luman Tongue, Tipton Shale Member, Wilkins grade laterally into pebble conglomerate beds at the Peak Member, and the Laney Member (Figure 2). These basin margin (Figure 3). Fissile organic-rich calci- lacustrine strata grade laterally into the alluvial Wasatch micrite (oil shale) and fish fossils typify both the Formation. During deposition of the Luman Tongue, Scheggs and Rife beds (Culbertson, 1969). However, 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 Creek, this interval consists of planar-laminated, fine- association of Hanley (1976) is abundant only in the grained sandstone and muddy-sandstone beds inter- Scheggs Bed, suggesting that conditions in the lake preted as topsets of a delta plain facies association. became more saline during deposition of the Rife Bed. The progradational geometry of the Farson deltas, The Farson Sandstone Member (Roehler, 1991) is in- and the presence of a freshwater fauna suggest that terpreted as deltaic and lake-margin deposits that the basin was overfilled during the deposition of the generally prograded into Lake Gosiute from the north Scheggs Bed. In contrast, the trend to more saline con- during deposition of the Scheggs Bed. It is mapped ditions in the overlying Rife Bed suggests an intermit- across the entire southern margin of the Wind River tent closed