QUANTITATIVE ANALYSIS AND PALEOECOLOGY OF MIDDLE TO LATE TRIASSIC CARBONATE SHELF, FAVRET AND AUGUSTA MOUNTAIN FORMATION, CENTRAL NEVADA

The Middle and Late Triassic records the return of diverse marine communities after the severe effects of the end-Permian mass extinction. This diversification leads to the Mesozoic/modern adaptive radiation resulting in substantial changes in marine communities in comparison to their Paleozoic predecessors. We used microfacies analysis, thin-section point counts, and hand sample identification to track changes of abundance and diversity of fossil grains and general paleoecological trends through the Favret and Augusta Mountain Formations preserved in South Canyon, Central Nevada. This 800-meter section spans the Anisian through the Carnian (i.e., Middle to Late Triassic). In total, we examined over 500 hand samples and 120 thin-sections. In addition, we tracked carbon isotopic values of marine carbonate through time. Our results indicate that in the Late Anisian we observed an ammonoid dominated to pelagic, flat-clam, epifaunal benthic community within a muddy, quiet, open marine environment. Through time, epifaunal bivalves dominate within a slope environment, between the basin and platform indicated by a sedimentary breccia and soft-sediment deformation. An increase in infaunalization and shell-thickness followed. The first record of carbonate-platform construction appears in the early-Ladinian rocks. By the Late Ladinian, the presence of colonial scleractinian coral and algal-grain supported limestone suggests the slope environment gave way to a higher energy patch reef platform edge environment. Within this platform edge environment, we also observe epifaunal brachiopod communities. An abrupt change takes place within the early-Carnian; here, we observe a return to an open-marine basinal environment that contains abundant ammonoids and halobiid-bivalves. By late-Carnian times, presences of a small reef mound and layers packed with oncolites and large solenoporoid algae indicate that the carbonate depositional environment returned to a platform carbonate environment.