DIVERSITY OF STROMATOLITES IN THE LOWER TRIASSIC VIRGIN LIMESTONE (MOENKOPI FORMATION) NEAR BLUE DIAMOND, NV

Numerous studies have attempted to shed light on environmental conditions that enabled the formation of anachronistic facies in the aftermath of the Permian-Triassic mass extinction. Unusual carbonates, such as flat pebble conglomerates and large seafloor cements, and the proliferation of microbialites are indicative of the presence of alkaline and anoxic seawater, and such hostile environmental conditions would have hindered the post-extinction biotic recovery. The presence of well developed, laterally extensive domal stromatolites in the Spathian Virgin Limestone Member of the Moenkopi Formation near Blue Diamond, NV, supports the model of stressed environmental conditions along the shallow shelf. These stromatolites are stratigraphically lower than those reported at Lost Cabin Springs, and are the earliest observed microbialites in the Virgin Limestone.

Stromatolites occur in micritic limestone as laterally linked hemispheroids, isolated hemispheroids, and meter-scale lozenge-shaped masses. Laterally linked hemispheroids up to 20 cm in diameter have well defined wavy laminae; in several samples, centimeter-scale fingering is apparent. Isolated domes have a similar hemispherical shape and wavy laminae, but at a smaller scale, <10 cm. Lozenge-shaped masses appear to be aggregates of columnar domes; polished slabs of the stromatolite aggregates reveal closely packed decimeter-scale columns with constant diameter. In all three forms, the appearance of layering is enhanced by the concentration of oxidized iron minerals (formerly pyrite?) along laminae.

Surrounding units include hummocky cross-stratified lime mudstone, beds of large (>3 cm) oriented bivalve shells, and mudchip horizons, which are indicative of occasional storm events and deposition in a subtidal environment. These stromatolites, therefore, represent an additional example of subtidal microbialite growth following the Permian-Triassic mass extinction. Widespread microbialites are thought to represent the combination of a dampening of burrowing and grazing behaviors in the wake of the extinction, coupled with environmental conditions that likely enhanced the growth of microbial and abiotic calcium carbonate.