EARLY TRIASSIC (SMITHIAN) MICROBIAL BUILD-UPS, LOWER THAYNES FORMATION, SOUTHWEST UTAH

The aftermath of the end-Permian mass extinction was a period of delayed biologic recovery and unusual oceanic conditions. Early Triassic microbial build-ups, formed by trapping, binding, and precipitation of carbonate minerals by microbes on the seafloor, occur worldwide in the absence of reef-building metazoans. This study focuses on the oldest Triassic microbial build-ups in the western U.S., which occur in the late Smithian (~250 Ma) Timpoweap Member of the Lower Thaynes Formation of southwestern Utah. Microbial build-up facies and morphologies were documented in order to interpret their initiation, growth, and demise as a response to environmental controls. The importance of evaluating these post-extinction microbial build-ups is to better understand marine ecosystem response(s) to catastrophic reorganization of oceanic conditions.

Excellent outcrops along canyon walls of the Virgin River in southwestern Utah enable a unique opportunity to document the build-ups in 3-dimensions. The 0.5-2.5 m-thick build-ups occur along the base of a ~30 m-thick, northward-thickening shallow-water limestone tongue that represents the first Triassic marine transgression along the Utah shelf. They are overlain by shallow regressive mixed carbonate-siliciclastic facies. This transgressive-regressive marine sequence is overlain by northward-thinning progradational continental deposits of the Moenkopi Formation. Three stratigraphic sections, ~500 m apart, were measured on a bed-by-bed basis in sequence stratigraphic context to evaluate the build-up morphology, lateral variability, distribution, and facies associations. The build-ups sit conformably above a chert pebble conglomerate, and are conformably overlain by marine calcareous siltstones and sandstones. The microbial interval rapidly thins landward from 4 m to 1 m and interfingers landward with calcareous siltstones. The build-up cores are composed of fenestral lime mudstone, and high-angle build-up flanks are composed of oncoid-bivalve-gastropod wackestones. They have domal geometries, are commonly laterally-linked, and decrease in thickness from 2.5 m to 0.5 m over ~100 m landward distances. Future studies include evaluating the extent of build-ups in coeval offshore facies in northern Utah and Idaho.