EARLY JURASSIC SPICULITES: PARAGENETIC SEQUENCES FOR THE GLASS RAMP DEPOSITS OF PERU

First-order stratigraphic and microscopic analyses demonstrate that siliceous demosponges expanded to form several large regions of spiculitic “glass ramps” in the aftermath of the Triassic/Jurassic mass extinction. The long-term paleoecological context of this expansion, however, remains poorly defined. Here we analyze sponge spicule microfacies to determine the variety and distribution of settings for sponge growth and bioclast accumulation. We select previously-published stratigraphic sections from the Aramachay Formation of the Central Peruvian Andes, which represents deposition along the tropical western edge of Pangea. We analyze bioclasts, grains, minerals, and other features to construct paragenetic sequences for both the spiculites and contemporaneous sediments. Our work presents several distinct models for the growth, deposition, and preservation of sponge fossils. The roles of microbiota and of early diagenesis are particularly important for spiculite preservation in inner-ramp settings. Taken together, these analyses support both the broader regional interpretation of the glass ramp system, and form the preliminary analyses required to evaluate any geochemical signal recorded by these peculiar chert rocks.