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Sedimentation in an Ancient Playa-Lake Complex: the Wilkins Peak Member of the Green River Formation of Wyoming

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Sedimentation in an Ancient Playa-Lake Complex: the Wilkins Peak Member of the Green River Formation of Wyoming Sedimentation in an Ancient Playa-Lake Complex: The Wilkins Peak Member of the Green River Formation of Wyoming LAWRENCE1 A^^HARDIE I department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218 ABSTRACT INTRODUCTION The Wilkins Peak Member of the Green River Formation of The Eocene Green River Formation is perhaps the best-known Wyoming has been examined in outcrop with the object of recon- and most studied sequence of lacustrine deposits in the world. In- structing its depositional environment. Based on their assemblages vestigators with a great diversity of backgrounds and aims have of sedimentary structures, seven rock units are described, six of concerned themselves with these complex rocks, which present which define depositional subenvironments. These units are (1) problems for many specialties such as paleontology, stratigraphy, flat-pebble conglomerate, (2) lime sandstone, (3) mudstone, (4) oil sedimentation, mineralogy, and inorganic and organic geochemis- shale, (5) trona-halite, (6) siliciclastic sandstone, and (7) volcanic try. The geologic and paleolimnologic aspects of the formation tuff. Their respective subenvironments are (1) rapid transgression have occupied Bradley (1929, 1964) for a lifetime. Recent impor- of a shallow lake, (2) lake shore oscillating over a mud flat (slow tant contributions to field relations and stratigraphy have been transgression), (3) playa mud flats, (4) shallow lake with occasional made by Pipiringos (1962), Roehler (1965), and Culbertson desiccation, (5) seasonally dry salt lake, (6) braided stream, and (7) (1971). Gazin (1965), MacGinitie (1969), and McGrew (1971) not specific. have discussed the paleontologic aspects and climate. The trona These subenvironment deposits are arranged in depositional cy- beds found in the Green River Formation constitute not only the cles. We have observed four types of cycles involving flat-pebble world's largest deposit of sodium carbonate (Deardorff and Man- conglomerate (A), oil shale (B), mudstone (C), lime sandstone (D), nion, 1971), but they also present fascinating mineralogical and and also trona. These cycles are I: A-B-C, II: D-B-C, III: D-C, and geochemical problems (Bradley and Eugster, 1969). The assem- IV: B-trona-C. Individual cycles have been correlated over dis- blages of unusual minerals associated with the saline facies were tances of up to 24 km. described by Milton and Eugster (1959), Fahey (1962), and Milton The Wilkins Peak Member is thought to have been deposited in a (1971); the authigenic minerals of the tuffs by Surdam and Parker playa-lake complex, which consisted of a shallow, central playa (1972); and the clay minerals by Tank (1972). Special attention has lake that was surrounded by vast, normally exposed mud flats been paid to the oil shales so characteristic of the Green River For- fringed by alluvial fans. Evaporative concentration of mation (Bradley, 1931, 1970, 1973). A large number of organic bicarbonate-rich inflow waters led to saturation with respect to compounds have been identified (for example, see Robinson, 1969) calcite, most of which must have been deposited as cement within in these rocks. Economic significance is based on the fact that the alluvial fans. Evaporation continued in the capillary zone of the oil shales of Wyoming, Utah, and Colorado represent the largest mud flats, precipitating calcite first, then magnesian calcite, and potential reserve of hydrocarbons in the world (Duncan and Swan- eventually protodolomite. The carbonates accumulated as a soft son, 1965). micritic mud at the fringes of the playa mud flats. In spite of these extensive multidisciplinary efforts, many aspects During periods of desiccation, the muds were subject to crack- of the Green River rocks have remained mysterious, foremost ing, and the mud-crack polygons contributed sand- and silt-size among them the origin of the oil shales and the dolomitic mud- dolomitic micrite intraclasts that were transported to the central stones that are associated with both the oil shales and the evapo- lake by the next storm. When the central lake was large, oil shale rites. Further insight into these problems may be gained by a con- accumulated in it, with the organic matter derived from a flocculent sideration of the sedimentary environments in which these rocks ooze consisting of bottom-dwelling blue-green algae and fungi. were deposited. The general aspects of these environments have During dry periods the lake shrank, and trona and halite precipi- been discussed by Bradley (1964) and Roehler (1965). Most rocks tated in the central portions. are interpreted as lacustrine; those of the Bridger basin, for in- An understanding of the Wilkins Peak sediments can be achieved stance, are believed to have been deposited in Lake Gosiute, a lake only by considering as inseparable the hydrologic, sedimentary, thought to have existed throughout most of Green River time, ap- geochemical, and biologic processes responsible for their forma- proximately 4 m.y. tion. Key words: sedimentary petrology, Eocene, oil shale, deposi- Bradley and Eugster (1969) focused particularly on trona ac- tional environments, sedimentary cycles, hydrochemistry, alkaline cumulation and suggested the existence of a stratified lake, with the brines, carbonate deposition, trona. deeper parts of the basin occupied by a strong brine of sodium Geological Society of America Bulletin, v. 86, p. 319-334, 19 figs., March 1975, Doc. no. 50307. 319 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/3/319/3433491/i0016-7606-86-3-319.pdf by guest on 26 September 2021 320 EUGSTER AND HARDIE NOHTHHN MARGIN CENTER OF BASIN alents at the margins of the Green River basin belong to the Wasatch Formation, whose main body underlies the Green River Formation. The stratigraphic relations are shown schematically in Figure 1. The lateral extent of the Bridger basin is shown in Figure 2, which approximately delineates the hydrographic basin and the maximum extents of the Wilkins Peak and Laney stages. The basin is essentially free of structural complications, with all rocks dipping very gently (less than 2°), except for the immediate area of the Rock Springs uplift, at the south margin, and along the northwest mar- gin. Bradley (1964) considered the uplift to have formed an island in Lake Gosiute, although later geochemical and sedimentological evidence indicates that uplifting postdates Tipton and probably Wilkins Peak time as well (Wolfbauer, 1971, p. 6). Only a small percentage of the Wilkins Peak rocks are accessible in outcrop, the principal belt being shown in Figure 3. However, the outcrop belt, together with section A located in the Big Island mine, provides Figure 1. Stratigraphie relations of Green River Formation in Bridger coverage from the alluvial fans to near the center of the basin. basin (after McGrew, 1971). ROCK UNITS AS RECORDS OF carbonate—sodium chloride composition. Based on a preliminary DEPOSITIONAL SUBENVIRONMENTS examination of the sedimentary structures of the Wilkins Peak Member, Eugster and Sufdam (1973) challenged this model and Bradley (1931, 1964) recognized a number of rock types in the suggested that it be replaced by a playa-lake model. In their view, Wilkins Peak Member of the Bridger basin. The following oil shale and trona are the only true lacustrine deposits, whereas lithologies were defined: claystone, siltstone, mudstone, marlstone, the dolomitic mudstones and calcareous siltstones were deposited shale, papery shale, and oil shale. Additional terms used were on the broad playa flats fringing the central lake and thus were sub- limestone, limy sandstone, sandstone, and edgewise conglomerates. jected to frequent drying and wetting. An understanding of the physical conditions under which these rocks accumulated has implications beyond the origin of a particu- lar rock type. They involve questions of hydrology, climate, geochemical balance, brine evolution, mineral formation and transport, as well as biologic processes. What is most needed at this point is to clarify some of these questions by a detailed investiga- tion of the sedimentary and geochemical features that are diagnos- tic of a particular depositional process and environment. This paper presents such an analysis for the Wilkins Peak Member of the outcrop belt west of the Rock Springs uplift. GEOLOGIC SETTING The Green River Formation is middle-early to early-middle Eocene in age (Gazin, 1965) and consists of three principal mem- bers: Tipton Shale, Wilkins Peak, and Laney, with the following re- spective maximum thicknesses: 60, 370, and 520 m. Lateral equiv- N Figure 2. Inferred outline of hydrographic basin of Gosiute Lake. Approximate maximum extent of Lake Gosiute during Laney and Wilkins Peak stages. [Modified after Bradley and Eugster (1969).] For map showing Figure 3. Outcrop belt of Wilkins Peak Member and locations of mea- probable extent of oil shale, see Culbertson (1969). sured sections; see Fig. 15 (after Bradley, 1964). Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/3/319/3433491/i0016-7606-86-3-319.pdf by guest on 26 September 2021 SEDIMENTATION IN AN ANCIENT PLAYA-LAKE COMPLEX 321 Subsequent workers have adopted this terminology (for example, Stuart, 1965; Roehler, 1965; Culbertson, 1971), which is essen- tially a pétrographie field terminology. Stratigraphie sections are measured in terms of this field classification. In addition, in his type sections, Bradley (1964) noted the
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