FLUVIAL SANDSTONE DEPOSITION IN AN UNDERFILLED LAKE BASIN: WILKINS PEAK MEMBER OF THE EOCENE GREEN RIVER FORMATION, WYOMING

Alan R. Carroll¹, Jeffrey T. Pietras^1,2, Brooke A. Swanson¹, M. Elliot Smith¹, and Cynthia A. Stiles^1
1 Dept. of Geology and Geophysics, University of Wisconsin, 1215 W. Dayton St., Madison WI 73706
² Present address: BP Exploration Alaska Inc., 900 E. Benson Blvd., Anchorage AK 99508
³ Department of Soil Science, University of Wisconsin, 1525 Observatory Drive, Madison WI 53706

The Eocene Green River Formation in Wyoming encompasses a wide variety of lacustrine and associated alluvial strata that were deposited in an intermontane basin bounded by the Sevier orogenic plateau and several basement-cored foreland uplifts. Peak rates of tectonically-controlled potential accommodation in the Greater Green River basin were attained during deposition of the evaporative Wilkins Peak member, based on evidence for the concurrent uplift of basin sills and on average accumulation rates determined from ⁴⁰Ar/³⁹Ar geochronology (Pietras et al., 2003a; Smith et al., 2003). The Wilkins Peak Member consists of carbonate-rich mudstone, siliciclastic sandstone, and Na-rich evaporites that represent profundal lacustrine to palustrine, fluvial, and saltpan environments respectively. These facies record at least 126 episodes of lake expansion and contraction, with an average duration of less than 10 ky for each sequence (Pietras et al., 2003b). The actual number sequences preserved varies with position in the basin, due to the interplay between varying magnitudes of lake expansion and a south-dipping depositional gradient. As few as one third of the sequences found in the southern part of the basin are present in more northerly sections.

Fluvial sandstone occurs in nine sheet-like, composite bedsets up to 25 m thick that can be traced over much of the western Greater Green River basin. Within each of these, arkosic silt to medium-grained sandstone alternates with lacustrine/palustrine mudstone at scales of 1-5 m. Evaporite minerals are absent. Provenance and paleocurrent relationships suggest that the sandstone was derived primarily from Precambrian rocks exposed on the eastern and northeastern basin margin. At exposures near the center of the basin (White Mountain), sedimentary structures are dominated by climbing ripple cross-lamination. Dm to m scale scour and fill, trough cross beds, parting lineation, and mudstone intraclasts are also common. Body fossils are absent, but trace fossils commonly include vertical burrows and trackways of mammals and wading birds. Preserved insect nests record occasionally depressed groundwater levels (c.f., Hasiotis, 2003). Vertic inceptisols found underlying some sandstone beds also attest to relatively long periods of exposure, perhaps hundreds of years. Collectively these observations suggest distal deposition by ephemeral streams entering a shallow lake margin, or possibly overbank deposition in interchannel ponds.

Intervals containing fluvial sandstone generally thicken southward, where evidence for more continuous flow in larger channels is common. For example, the middle part of the “D Bed” (Culbertson, 1961) at Firehole Canyon includes lateral accretion surfaces with up to 2-3 m of relief, bundled in continuous arrays tens of meters wide. Climbing ripples dominate the upper D Bed, except for wave ripples and possible palimpsest dune forms that occur at the top of the uppermost sandstone bed.

Deposition of Wilkins Peak Member fluvial sandstone appears to reflect at least two different orders of climatically-driven cyclicity. Individual sandstone beds and bedsets were deposited during periods of increasing runoff into the basin, resulting in an upward transition from fluvial facies that overlie exposure surfaces into lacustrine mudstone. The time required for these events is uncertain, but may have corresponded to the high frequency lake expansions discussed above. Over longer timescales, the deposition of the nine larger composite sandstone-mudstone bedsets corresponded to periods of generally decreased climatic humidity. As average lake levels dropped, alluvial fans at the eastern and northeastern basin margin were eroded and this reworked detritus was transported southwestward across the exposed basin floor by high frequency floods. Eventually long term increases in runoff again expanded Lake Gosiute over the fluvial intervals. Further work is needed to test this model, and to help integrate sandstone stratigraphy with recent advances in basin-scale chronostratigraphy, tectonics, and weathering history.

References

Hasiotis-S. T., 2003, New interpretations of complex trace fossils: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 192, p. 259-320 

Pietras, J. T., Carroll, A. R., and Rhodes, M. K., 2003a, Lake basin response to tectonic drainage diversion: Eocene Green River Formation, Wyoming, in Gierlowski-Kordesch, E. H., and Buchheim, H. P., eds., Lake Basins as Archives of Continental Tectonics and Paleoclimate: Journal of Paleolimnology Special Issue, v. 30, p. 115-125.

Pietras, J.T., Carroll, A.R., Singer, B.S., and Smith, M.E., 2003b, 10,000 yr depositional cyclicity in the early Eocene: Stratigraphic and 40Ar/39Ar evidence from the lacustrine Green River Formation: Geology, v. 31, p. 593-596.

Smith, M. E., Singer, B., and Carroll, A. R., 2003, 40Ar/39Ar geochronology of the Eocene Green River Formation, Wyoming: Geological Society of America, v. 115, p. 549-565.