MICROFACIES OF PANTHALASSAN BENTHIC COMMUNITIES BEFORE AND AFTER THE END-TRIASSIC EXTINCTION FROM THE PERUVIAN PUCARÁ GROUP

Metazoan-rich carbonate depositional systems collapsed following the End-Triassic mass extinction. In the aftermath of this catastrophe for carbonate reefs, there is an observed expansion of siliceous sponges in Hettangian and Sinemurian deposits from Nevada, U.S.A., Austria, and Peru. The Pucará Group of Peru is of interest as the basal Chambará formation preserves a metazoan carbonate ramp system while the overlying Aramachay contains a siliceous sponge ramp dating from the Early Jurassic. The ecological relationship between these regional communities of bio-calcifiers and bio-silicifiers, however, remains unclear.

Here we examine previously-published stratigraphic sections of the Pucará Group to define microfacies that span across the Triassic-Jurassic boundary, thereby including paleo-communities from before and after the End-Triassic extinction. We analyze both carbonate and siliceous bioclasts alongside trace fossil evidence, terrigenous grains, matrix composition, and early diagenetic fabrics to define microenvironments and elucidate their spatial, temporal, and taxonomic associations. We combine these analyses with preliminary strontium and carbon isotopic data derived from coeval carbonate material, screened by cathodoluminescence techniques, to provide greater constraint on the location of the Triassic-Jurassic boundary within the studied sections. It is the goal of this research to better characterize the dynamics of the siliceous sponge expansion within the context of the Pucará Group, as well as the responses of marine ecosystems following an environmental catastrophe.