Facies-Related Variability of Trace Fossils in the Green River Formation, Wy: Clues to Paleoecological Dynamics in Underfilled and Balanced Fill Lake-Types

Scott, Jennifer J.¹; Smith, Michael E.²; Carroll, Alan R.³; Williams, Eric M.³; Buatois, Luis A.¹; Renaut, Robin W.^1
1 Geological Sciences, University of Saskatchewan, Saskatoon, SK, Canada.
² Geology, Sonoma State University, Rohnert Park, CA.
³ Geology and Geophysics, University of Wisconsin-Madison, Madison, WI.

Changing environmental conditions control the dynamic interactions between animals and sediments, and result in spatially and temporally variable trace fossil assemblages. In lake basins, the parameters that influence trace fossils respond to changes in the hydrologic balance of the lake. This study aims to characterize the trace fossil composition, distribution, and relationship with sedimentary facies influenced by lake-level rise and fall in balanced fill and underfilled lake-types of the Green River Formation, WY. Stratigraphic, sedimentologic, and ichnologic investigations were undertaken in lacustrine and alluvial facies associations at the basin margin and basin center of the Bridger Basin.

Several facies-related trace fossil associations were identified in the Tipton and Wilkins Peak Members and correlative Cathedral Bluffs Member of the Wasatch Formation. In deltaic, shoreline, and lake-margin strata deposited at both the basin margin and basin center, trace fossil suites differ between the lake-types. Different degrees of bioturbation and patterns of suite overprinting also show variability in substrate stability, salinity, and substrate saturation related to lake-level fluctuations. Balanced fill facies at the basin margin are dominated by post-desiccation backfilled burrows in several surfaces that were exposed during lake-level fall. These examples contrast with pre-desiccation Planolites-dominated suites in overfilled facies at the basin margin. The basin center Tipton Member comprises kerogen-rich micro-laminated micritic carbonates that lack bioturbation and reflect anoxic bottom water conditions.

Underfilled facies at the basin margin contain trace fossil suites dominated by post-desiccation Scoyenia and backfilled trace fossils, but include pre-desiccation suites where there was freshwater input. Basin center trace fossils in Wilkins Peak shoreline and evaporitic lake-plain deposits show lower diversity suites dominated by vertebrate tracks, simple horizontal trace fossils, and a lack of backfilled structures. Basin centre siliciclastic fluvial/floodplain deposits of the Wilkins Peak also differ in trace fossil suite composition, with ichnofossils in soft, wet substrates and a much lower degree of suite overprinting than at the basin margin. Trace fossils in high energy deposits are restricted to the tops of bedsets and reflect a drop in water levels, instantaneous sedimentation rate, and/or energy which led to more stable substrates.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009