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