DEPOSITION AND PRESERVATION OF SPICULES WITHIN BIOSILICEOUS GLASS RAMPS

Recent investigations of regional-scale shelf deposits of biosiliceous chert, deemed "glass ramps", has raised questions regarding the relationship between the fossilized sponge spicules and the coastal environments along which they were found. This study tests models for spicule deposition along tropical Pangea coasts (eastern Panthalassa) during the earliest Jurassic. Models for local spiculite accumulation recorded in west central Nevada (New York Canyon area) will be contrasted with models of regional glass ramp growth recorded in Peru (Yauli Dome area of the central Peruvian Andes). New higher-detail analyses of microfacies from Peruvian samples will include: a) imaging of 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 degree of bioclast disturbance or transport. Preliminary 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 will evaluate the regional heterogeneity of on-shore vs. offshore spiculite accumulation and interpreted primary sponge habitats. In addition to comparing earliest Jurassic sites (records from Peru and Nevada) the microfacies analysis will be used to predict depositional models for regional spiculite accumulation of the Permian Spiculite Belt in western North America (records from Utah). Clarifying the relationship between the spicule remains and the cherts within which they are found will develop a better understanding of why, when, and how these biosiliceous glass ramps were formed.