Recurring lacustrine depositional successions in the Wilkins Peak Member, Green River: The basin-center evaporite perspective Formation

Elizabeth Klonowski; Tim Lowenstein; Alan Carroll; M. Elliot Smith; Matteo Paperini; Jeffrey Pietras

doi:10.31711/ugap.v50i.116

Elizabeth M. Klonowski Department of Geological Sciences and Environmental Studies, Binghamton University
Tim K. Lowenstein Department of Geological Sciences and Environmental Studies, Binghamton University
Alan R. Carroll Department of Geoscience, University of Wisconsin-Madison
M. Elliot Smith School of Earth Science and Environmental Sustainability, Northern Arizona University
Matteo Paperini Mine Engineering Dept., Solvay Chemicals, Inc.
Jeffrey T. Pietras Department of Geological Sciences and Environmental Studies, Binghamton University

DOI: https://doi.org/10.31711/ugap.v50i.116

Abstract

Mineralogy, petrographic textures, and sedimentary structures from the world’s largest trona deposit, the Wilkins Peak Member (WPM) of the early Eocene Green River Formation (GRF), Bridger subbasin, Wyoming, provide key data about depositional conditions and paleoenvironments. The 250 m-long WPM interval in the Solvay S-34-1 drill core analyzed in this study contains a detailed record of sedimentation in the Bridger subbasin at the deepest area of a hydrologically-closed basin during peak Cenozoic atmospheric CO2 concentrations. Large accumulations of trona (Na3(HCO3)(CO3)·2H2O), shortite (Na2Ca2(CO3)3), northupite (Na3Mg (CO3)2Cl), and halite (NaCl; now replaced by trona), occur in the lower half of the WPM. Modern saline lake environments such as Lake Magadi, Kenya, and the Dead Sea, Israel-Jordan, are useful analogues for interpreting paleolake conditions associated with evaporite deposition in the Solvay S-34-1 core. Solvay saline lake deposits are organized into meter-scale shallowing-upward successions, beginning with (1) oil shale overlain by (2) trona, in places interbedded with oil shale, followed by (3) peloidal dolomite grainstone and/or silty dolomitic mudstone, and (4) massive mudstone with disruption features or desiccation cracks, and/or siliciclastic sandstone with ripple cross-stratification. Based on observations of modern hypersaline lake environments, WPM evaporite deposition at the basin depocenter is interpreted to be controlled by inflow water composition and volume, evaporative concentration, and seasonally-driven lake temperature fluctuations, resulting in recurrent patterns in evaporite mineralogies and textures.