Lacustrine Deposition in the Bridger Formation: Lake Cosiute Extended'
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Lacustrine Deposition In The Bridger Formation: lake Cosiute Extended' LEONARD R. BRA ND2 1. M anuscri pt received Septemb er 30. 2004; Accepted M arc h' , 2007 2. Department of Earth and Biological Sci ences, Lom a Lind a U niversity, Loma Linda, CA 92350; lbrand wllu.ed u ABSTRACT The Green River Formation was de posited in Lake Gosiute , until the lacustrine system shifted to the more fluvial environment of the Bridger Formation. In the so uthern part of Bridger A exposures, sedi ment s become increasingly lacustrine and interfinger with the main bod y of the Laney Shale member of the Green River Formation. High er in Bridger A several widespread limestone marker beds are separated by mud stones. Bridger B also co nsists of mu dstones alternating with limestones. A nu mber of these lime stones have no w been mapped . They extend across the entire existing Bridger B exposures, and rep re sent basin-wide shallow lakes. The se lakes were filled by volcaniclastic input from episodes of volcanism to the nort h . The lacustrine de posits of the Green River Formation consist largely of laminated ke rogen rich micrites (oil sha les) that grad e laterally into massive limes tones or siliciclastic mudstones, whereas the lacustrine deposits of the Bridger Formation are wides pread, massive limestones depos ited in shal low, but very large lakes. Thus the large-scale lake that formed the Green River Formation did not really disappear. It became a shallow lake that periodically was filled by an episode of volcan iclastic depositio n in a fluvial-lacustrine system, only to reappear whe n bas in subsidence exceeded volcaniclastic input. INT RO D UCTION 69 M ODEL 74 LIM ESTO N ES IN TH E BRIDGER FORMATION 70 SUM M ARy 76 BRID GER LA KES AND BASIN TYPE 73 REFE RENCES 76 TU RTLE TAPHONOMY 73 INTRODUCTION replaced the Wasatch , finally covering the GRF and filling the basin (Fig. 2). The lacustrine Green River Formation (G RF) and its The sedime nt in the Bridger Formation has long been relationship with the asso ciated fluvia l de posits of the recognized as prima rily volcaniclastic (Sinclair, 1906; Wasatch an d Bridger Forma tions has been studie d since Koenig, 1960; Bradley, 1964; Gustav, 1974; West, 1976). the late 1800's and ea rly 1900's (Roehler, 1973, 1992a). Most of the volcan ic material apparently is fro m the Eocene Lake Gosiute filled a large part of the Green River Absaroka volca nic field in NWWyoming (Bradley, 1964), basin in SW Wyoming (Fig. 1) , and in this lake the GRF but the tuffs differ in co mposition from the rest of the was dep osited , with its largely laminated kerogen-rich Bridger volcanics and seem to have come from the Challis micrites (o il shales) that grade laterally into massive lime volcanic field in Idaho (Evanoff and Rossetti, 1992). stones or siliciclastic mudstones. The primarily fluvial Lake Gos iute fluctuated conside rably in size during its Wasatch Formation was deposited below an d along the history, as reflected in vertical changes in the lateral extent flanks of the GRF, and then the fluvial Bridger Formation of the GRF (Roehler, 1992c)(Fig. 2). During the time of the 'tbe Mou nta in Geologist, Vol. 44, No. 2 (April 2( 07), p 69-78 69 The Rocky Mountain Association of Geologists Leonard R. Brand Figure 1. Map of the Green River Basin, 0 107 showing the limits of the depositional basin, and the extent of exposures of 25 50 ! ! I J Green River Formation, Bridger units A, Miles B, and CE. 420 Laney shale member of the GRF, the lake was at its largest & b) (Fig. 3). From study of the sediments and the taphon extent (Surdam and Stanley, 1979), but during that time omy of the abundant fossil turtles a depositional model for volcaniclastic deposition increased, and the formation of Bridger B has been proposed (Buchheim et al., 2000; oil shales came to an end. It has been claimed that the Brand et al., 2000). This paper will enlarge on this model basin was filled and Lake Gosiute disappeared by the end and show why it indicates that Lake Gosiute extended into of the Laney interval, replaced by fluvial deposition of the Bridger time. Bridger Formation (Surdam and WoUl)auer, 1975; Surdam and Stanley, 1979). Several lines of evidence to be pre sented here indicate that Lake Gosiute did not end, but just LIMESTONES IN THE BRIDGER FORMATION changed character. The Bridger Formation consists of largely tuffaceous Bridger A contains several prominent, widespread lime floodplain deposits, with associated channel sandstones, stones (Fig. 4), separated by mudstones (McGrew and Sul deltaic and lacustrine sandstone and siltstone, and lime livan, 1971). In the southern part of Bridger A (northwest stone units (Koenig, 1960; Bradley, 1964; Gustav, 1974; corner of Fig. 3) the sediments become increasingly lacus Buchheim et al., 2000). The Sage Creek Limestone, separat trine and interfinger with the Laney Member of the GRF ing Bridger Band C, was mapped across the basin by (McGrew and Sullivan, 1971). Some limestone beds were Bradley (964), but the other limestones were believed to traced from Bridger A into the GEF in this region (Wolf be local in extent (West, 1976; Roehler, 1992b). More bauer and Surdam, 1974). Bridger B also contains a num recently a number of sedimentary units, primarily lime ber of limestone units separated by mudstones (Fig. 4). stones, have been mapped, and it has become clear that Most of these limestones are continuous across all of the most limestones and some other units are continuous and existing Bridger B exposures (as determined by walking basin-wide (Brand, 1997; Evanoff et al., 1998; Murphey, out the limestone exposures during mapping) (Fig. 3), 2001; Murphey et al. in press a & b; Brand et al. in press a rather than interfingering with other sediments as would The Rocky Mountain Association of Geologists 70 LACUST1IINE DEPOSfT70N IN THE BRlDGh1? FORMA710N: LAKE G051rJ1E EX77XNDED Figure 2. Cross section through Rock Springs the Eocene Green River Forma tion and associated sediments in ~rir1(rpr Formation the Green River Basin (after E RoehlerI992a). The upper half D of the diagram shows the rela ?? tionship between Green River, ? Wasatch, Washakie, and Bridger Formations as proposed in this ? I? paper. Relationships between Bridger and Washakie Forma ?? tions are uncertain, but Roehler (1992b) suggested that some limestones can be correlated between these two formations. A Stratigraphy for Bridger A from McGrew and SuiIivan (1971), and for Bridger C - E from Mur phey (2001). 400 ~ 1i$ 200 ~ o Wasatch Formation be expected for limestones generated in local lakes. In a bursts, separated by sufficient time for a thin limestone to few cases, however, the unit is a resistant limestone over form in the lake. The upper limestone in each set tends to part of the basin, and continues across the rest of the basin be the most prominent one. However, where the Golden as a continuous limey mudstone that can be dearly traced bench limestone thins to the SE, the second limestone into the limestone unit. The Church Butte tuff in the south below it becomes more prominent and is the primary ern part of the basin is underlain by a thick, resistant lime bench-forming unit in that area. stone, which thins toward the north, and in the northern There are several widespread limestones in the lower half of Bridger 13 exposures it is a limey mudstone directly and middle Bridger C (Fig. 4), but limestones are much below the Church Butte tuff. The same phenomenon less common above middle Bridger C. It also appears that applies to the Black Mountain turtle layer. The Golden there are more localized limestones in Bridger C and D bench limestone changes in the opposite direction. It is a than in unit 13 (Murphey, 2001). With the exception of the prominent limestone through most of the basin, but in the Sage Creek Limestone, exposures of limestones in Bridger SE part of Bridger 13 exposures it thins and becomes less C-E are largely limited to the southern part of the Bridger distinct. In the SW part of the basin, in T15N and south, it Basin, so it cannot be determined whether some of these disappears. These facies changes are exceptions to the limestones once extended as widely over the basin as the general character of Bridger 13, in which facies change Bridger 13 limestones. stratigraphically, as described below, but each facies is lat The Bridger Formation limestones almost never contain erally uniform and continuous across the basin. oil shale (laminated micrite), but are massive limestones Some limestones (BMtl, Gbl, SCL) occur in sets of two (as determined by analysis of polished sections), usually a or three limestone units, separated by a few meters of few em to a few m thick. Also some Bridger limestones mudstone, Total thickness of these limestone/mudstone contain abundant ostracods and/or gastropods of the sets varies from 1.5 to 9 m. This seems to indicate that genus Goniobasis throughout their lateral extent. These are some volcanic episodes began with one or two preliminary both indicators of shallow water. Gastropods of the genus 71 The Rocky Mountain Association of Geologists Leonard R. Brand Figure 3. Map of several of the HIGHWAYS AND DIRT ROADS limestones in Bridger B. The Church RIVERS OR CREEKS Butte tuff and the Black Mountain -- SCI SAGE CREEK LIMESTONE turtle layer are each underlain by a TbBu UPPER BRIDGER B limestone. -- BMtI BLACK MOUNTAIN TIJRTLE LAYER TbBm MIDDLE BRIDGER B CB CHURCH BUTTE TUFF TbBI LOWER BRIDGER B -- LI LYMAN LIMESTONE WYOMING ~ TbBl f- NOT MAPPED RlI4W Rll3W R uz W RIIIW R 1I0W RI09W R 108 W Biornpbalaria are also abundant in most Bridger lime than the Bridger massive limestones.