WIDESPREAD LOWER TRIASSIC (SMITHIAN) MICROBIAL MOUND COMPLEX ACROSS SOUTHERN UTAH: FACIES ARCHITECTURE AND IMPLICATIONS FOR POST-P-T PALEOCEANOGRAPHIC CONDITIONS

Lower Triassic marine deposits commonly include microbial limestones and inorganic carbonate precipitates, many of which directly overlie the Permian-Triassic (P-T) boundary. Their occurrence with low diversity metazoans, widespread anoxic deposits, and widely fluctuating δ¹³C and δ¹⁸0 isotope trends reflect continuing deleterious oceanic conditions associated with the end-Permian extinction (EPE). This study describes and interprets microbial mounds that developed across southern Utah ~<2 My after the EPE (Smithian) in a <15 m thick limestone unit in the lower Thaynes Formation. The mounds (<0.5 m synoptic relief; ~6 m of total aggradational relief) are dome-shaped and laterally linked (over <10 m distances). Mound facies include peloid-oncoid packstone/grainstone and microbially-laminated peloidal lime mudstone/wackestone; they contain minimal fossils and siliciclastics, abundant cement-filled fenestrae and stromatactis, and minor <2 cm-thick thrombolite crusts, calcite fans, and syndepositional pyrite.

The lower Thaynes microbial limestones lie between siliciclastic-rich facies, and likely formed rapidly during transgression and maximum flooding of the Utah shelf. The mounds nucleated on transgressive peloidal grainstones and silty peloidal lime mudstones in moderate energy subtidal environments. Low turbidity waters are suggested by the scarcity of fine siliciclastics within microbial facies. During maximum sea-level rise, the mounds attained their greatest aggradational height. Decreased growth rates during sea-level highstand/fall are interpreted based on an upward decrease of synoptic relief; buildup termination by accommodation loss and increased siliciclastic influx during sea-level fall/lowstand is interpreted from overlying nearshore quartz sand- and silt-rich facies. These relationships cannot explain the absence of microbialites in the overlying >6 Smithian–Spathian depositional sequences (<1 My), or global microbialite development during the Early Triassic. Previous work indicates extreme sea surface temperatures (SST) and widespread anoxia during the Smithian; the large areal extent of microbialites and inorganic precipitates in Utah suggests increased carbonate saturation states and reduced metazoan competition related to high SST and anoxia.