RIDING A PALE HORSE INTO THE GREAT DYING: THE EVAPORITES OF THE WHITEHORSE GROUP IN NORTHWEST TEXAS AND THE END-PERMIAN MASS EXTINCTION

A temporally calibrated terrestrial sequence of sedimentary strata encompassing the Permian-Triassic Boundary (PTB) is situated within the Whitehorse Group of northwest Texas, USA, in the Palo Duro Basin. The lower Whitehorse Group, defined as the portion of the Whitehorse Group that is stratigraphically positioned beneath the PTB, is predominantly composed of alternating red siltstones and gypsum-rich evaporite beds. Here we present detailed stratigraphic, petrographic, and mineralogical data for the bedded evaporites from the lower Whitehorse Group. The evaporite beds are dominated by gypsum with a small percentage of syn-sedimentary dolomite crystals, which are confirmed to be autogenic through petrographic analysis. Additionally, the bedded evaporites contain small quantities of aeolian quartz and hematite, and some beds contain selenite nodules, but their size and abundance appear to be random. Stratigraphic columns, encompassing ~100 meters of section, show variable thicknesses of the evaporite and the red siltstone beds, with the thickest evaporite beds near the PTB. The lowest bed in the measured stratigraphy is massive and crystalline with dispersed dolomite crystals. All other identified beds are composed of gypsum and dolomite grains (< 0.5 mm) and lack cements. Evaporite beds are pervasive until approximately 3 meters below the PTB, with the last evaporite bed exhibiting unique textural morphology. This bed is ~2.8 meters thick, extends laterally for many kilometers, and displays microbial growth structures, suggesting microbial communities were omnipresent across this region. Three distinct layers of microbial growth structures (thrombolites, domal, and wave-form stromatolites) are present. The stratigraphic order of these structures is consistent through this bed, suggesting important regional changes may have influenced their morphologies. These microbial structures become abundant leading into the End Permian Mass Extinction and are absent following the PTB. Future work will be to collect geochemical data (87Sr/86Sr, δ13C, δ18O, and δ34S) to refine our understanding of the local to regional, low latitude climatic and environmental conditions during the Guadalupian and Lopingian leading to the PTB.