CONSTRUCTIVE MICRITE ENVELOPES FROM THE LOWER TRIASSIC VIRGIN LIMESTONE: FURTHER EVIDENCE OF AN EARLY TRIASSIC MICROBIAL BLOOM IN WESTERN NORTH AMERICA

Lower Triassic sedimentary rocks from southwestern North America contain a variety of unusual carbonate facies and fabrics, including widespread microbialites. Documented microbial fabrics in shallow-water facies have included domal and stratiform stromatolites in carbonates as well as wrinkle structures in clastic facies. Coated grains are common in shallow water facies from the Virgin Limestone (Moenkopi Formation); petrographic analysis reveals that the grains are commonly coated with constructive micrite envelopes, in which micrite was precipitated onto the surface of allochems, as opposed to being the product of destructive micritization by endolithic microborers. Evidence for a constructive as opposed to a destructive origin for the micrite envelopes are as follows: 1) the micrite envelopes appear to be building out from the grain surface; 2) the micrite envelopes commonly exhibit internal structure (typically smooth to irregular laminations); micritization tends to destroy most if not all internal microfabric; 3) the contact between the micrite envelope and the allochem is smooth, as opposed to the irregular contacts and microborings associated with destructive micritization; 4) the micrite envelopes may be interlaminated with cements, including low-Mg calcite in some thin sections; 5) the micrite envelopes are typically of varying thickness, and often are thicker on one side of the coated grain than the other; and, 6) a variety of types of grains have micrite envelopes, including calcareous siltstone intraclasts that contain up to 30% quartz silt grains and exhibit silt-free micrite envelopes, which would not be expected if endolithic borers were attacking calcite phases and leaving behind the quartz. The micrite that comprises the envelopes is often dense or cloudy, suggesting a microbial origin, and is commonly pyrite-rich, suggesting that the coatings grew on an organic-rich substrate, such as a biofilm. Constructive micrite envelopes within the Virgin Limestone therefore provide further evidence of a widespread microbial bloom following the Permian-Triassic mass extinction that is likely due to the combination of a lessening of grazing pressures coupled with environmental conditions that enhanced the likelihood of carbonate precipitation.