Description and correlation of Eocene rocks in stratigraphic reference sections for the Green River and Washakie basins, Southwest Wyoming

Abstract

Stratigraphic reference sections of the Wasatch, Green River, and Bridger (Washakie) Formations were measured on outcrops in the Green River and Washakie basins adjacent to the Rock Springs uplift in southwest Wyoming. The Washakie basin reference section is 7,939 feet thick and consists of 708 beds that were measured, described, and sampled to evaluate the origin, composition, and paleontology of the rocks. The reference section in the Green River basin is 6,587 feet thick and consists of 624 beds that were measured and described but were not sampled; this section was measured to correlate rock units with those in the Washakie basin. Columnar sections that have been prepared combine information on the Stratigraphic nomenclature, age, depositional environments, lithologies, and fossils of each bed in the reference sections. Eocene strata in the Green River and Washakie basins have been correlated biostratigraphically, chronostratigraphically, and lithostratigraphically. The time boundaries of the lower, middle, and upper Eocene rocks in the reference sections are located partly from biostratigraphic investigations and partly from chronostratigraphic investigations. The time boundaries agree with North American land mammal ages, which were used in the report because of their historical precedence. Major Stratigraphic units and key marker beds correlated between the reference sections appeared remarkably similar in thickness and lithology, which suggests that most depositional events were contemporaneous in both basins. One major difference between the sections is that the upper 875 feet of upper Eocene rocks in the Washakie basin is missing by nondeposition or by post-Eocene erosion in the Green River basin. Rocks sampled in the Washakie basin reference section were examined petrographically and were analyzed using heavy mineral separations, X-ray techniques, and assays. The mineralogy suggests that source rocks in the lower part of the Eocene were mostly of plutonic origin and that source rocks in the upper part of the Eocene were mostly of volcanic origin. Economically significant beds of oil shale and zeolite were identified by the analyses. Fossil pollen Manuscript approved for publication November 30, 1990. collected from 22 beds was used to identify plants that characterize five successive climates present in the Washakie basin during the Eocene Epoch. Dl D2 WASATCH, GREEN RIVER, AND BRIDGER (WASHAKIE) FORMATIONS BOUNDARY OF GREATER GREEN RIVER BASIN Lonetree McKinnon 7(530 50 MILES FIGURE 1. General locations (shaded) of Eocene reference sections, Green River and Washakie basins. For detailed locations, see index maps on plates 1 and 2. EOCENE ROCKS IN GREEN RIVER AND WASHAKIE BASINS, SOUTHWEST WYOMING D3 ACCESSIBILITY OF THE REFERENCE SECTIONS The reference section in the Green River basin is accessible by Wyoming Highway 530 south of Green River, Wyo., and Wyoming Highway 373 southwest of Rock Springs, Wyo. The reference section in the Washakie basin is accessible by Wyoming Highway 430 southeast of Rock ...

Synoptic reconstruction of a major ancient lake system: Eocene Green River Formation, western United States

“…13). Integrations of the collective solute loads for all of the upstream and downstream basins during such expansions may explain, for example, the dolomitic stromatolite horizons in the otherwise alluvial Cathedral Bluffs Member of the Wasatch Formation in the Washakie subbasin (Roehler, 1992b), and shortterm salinity increases interpreted by Keighley et al (2003) in the otherwise overfi lled transitional interval in the Uinta Basin.…”

“…Although coarse sediment was deposited in these basins, their lack of lacustrine strata implies that an unknown fraction of that sediment was transported through them. Molluscan faunas and other fl uvial-lacustrine indicators in the upper LaClede bed of the Laney Member indicate that the Greater Green River Basin likely overfl owed into the Piceance Creek Basin (Roehler, 1992b). Lake Uinta expanded greatly in extent (the Mahogany zone of the upper Parachute Creek Member and upper member), becoming one vast saline lake that covered both the Uinta and Piceance Creek Basins and overlapped the Douglas Creek arch (Fig.…”

“…Alternatively, the association of leaf fossils with laminated facies may indicate that preservation of fl ora was limited to relatively short (<100 k.y. ), climatically induced phases of lake expansion (Pietras et al, 2003b), whereas evaporite deposition occurred only during intervening dry periods (Burnside and Culbertson, 1979;Roehler, 1992b;Dyni, 1996). Further research is needed to help resolve these issues.…”

Capture of high-altitude precipitation by a low-altitude Eocene lake, western U.S

“…Dolomite and mudcracks increase near the tops of cycles, sometimes in association with evaporite minerals or mineral casts. There are 10-11 cycles below an ~10 m dolomitic siltstone bed named the "buff marker" by Roehler (1992;Rhodes et al, 2007). Two to three additional stromatolite-bearing cycles are present above the buff marker.…”

Reply

“…A stratigraphic comparison of the radioisotopic ages from the Laney Member and Bridger Formation reinforces the conclusion that the age of the Church Butte tuff may be overestimated. West of the Rock Springs Uplift, the Analcite and Church Butte tuffs are separated by Ͼ400 m of continuous strata (Roehler, 1992a), yet the ages of these tuffs determined by and Murphey et al (1999) are indistinguishable at the 95% confidence level (Fig. 1).…”