EARLY TRIASSIC MICROBIAL SPHEROIDS IN THE VIRGIN LIMESTONE MEMBER OF THE MOENKOPI FORMATION, NEVADA, USA

摘要

Lower Triassic carbonate sedimentary rocks exhibit fabrics and facies indicative of reduced bioturbation and reduced abundance of skeletal animals and algae relative to their Permian counterparts. Widespread microbial mounds are one widely cited example. Micritic spheroids of probable microbial origin occur at a few horizons in the Spathian (uppermost Lower Triassic) Virgin Limestone Member of the Moenkopi Formation, exposed in southern Nevada. These spheroids (3–12 mm) consist of an irregular framework of micrite clots up to 100 µm in diameter and micrite-walled filaments 10 µm wide and <500 µm long. Based upon their microstructure, we suggest that these microbial spheroids formed through the rapid microbial precipitation of calcium carbonate from seawater. As such, they probably represent unattached analogues of Early Triassic microbial mounds. The microbial spheroids formed and lithified rapidly in shallow-water, high-energy settings. Their presence, in concert with other carbonate microbialites from the Moenkopi, is consistent with the observation that microbially mediated precipitation was an important carbonate depositional mode in the aftermath of the end-Permian mass extinction.