EARLY JURASSIC SPICULITES: REGIONAL MODELS FOR THE GLASS RAMP DEPOSITS OF PERU

After the Triassic/Jurassic mass extinction, several coastal settings were overwhelmed with siliceous sponge deposits, also called spiculites, which can form “glass ramps”. Here we examine stratigraphy and microfacies from the Central Peruvian Andes, representing tropical deposition in a failed rift basin along the west coast of Pangea (eastern Panthalassa), to determine the fit of a few different regional-scale glass ramp models. Four previously-published stratigraphic successions of the Aramachay Formation across the La Oroya area are considered. Microfacies analyses include: a) imaging micro-pans using a petrographic Zeiss microscope with corresponding Zeiss Zen software; b) categorization of terrigenous grains and bioclasts by provenance, size, maturity, mineralogy, and preservation; c) construction of paragenetic sequences; d) interpretation of bioclast disturbance or transport intensity. Results support previous interpretations of a mid-ramp accumulation pattern for spicule deposition, with the majority of spicules being found between fair- and storm-weather wave bases. Further results evaluate the lateral heterogeneity of spiculite and sponge-ground accumulation, in contrast to biocalcifier, siliciclastic, and tuffaceous sediments. The project illuminates the frequently-overlooked paleoecological value of chert deposits, and clarifies the context of sponge expansion in the wake of the extinction.