PALEOECOLOGY OF GLASS RAMPS: NON-ACTUALISTIC SHALLOW-MARINE SPICULITES FROM THE PERMIAN OF NORTHWESTERN UTAH

Glass ramps are shallow-water biosiliceous deposits established on multi-million year timescales with no modern analogue. Cherts in Utah represent the southernmost expression of the most extensive and enduring glass ramp in Earth’s history, the great Permian spiculite belt. We present detailed analysis of sedimentological and paleontological conditions, including field observations and microfacies analysis, to determine the paleogeographical extent of spiculite formation and constrain environmental and biological controls on benthic sedimentation along inner- to middle-ramp zones in Middle Permian successions. Results confirm primary deposition of biogenic cherts composed of siliceous sponge spicules accumulated in high-energy, storm-influenced shelf environments. Localized patches of partially- to fully-silicified bioclastic carbonates occur sporadically in glass ramps, and are characterized by fine-grained bioclastic wackestones to packstones, in-situ brachiopod shell beds, and phosphatized bioclastic packstones and grainstones. Spiculite and carbonate ramp facies typically exhibit primary phosphatic and micritic textures, fabrics, and grains, indicating a microbial contribution to sediment formation, as well as secondary diagenetic features.

Permian deposits in Utah mark an intermediate sedimentation regime at the edge of the northern spiculite belt and contemporaneous tropical carbonates to the south. Sediment-starved, shallow-water facies dominated by bedded spiculitic cherts and contemporary phosphorites, with subordinate, isolated patches of low-diversity biogenic carbonate deposits indicate a spectrum of biosiliceous-carbonate depositional environments. Temperate, shallow-water spiculite-carbonate assemblages of glass ramps are poorly understood because they do not occur in modern oceans, but we speculate that these sedimentary regimes were more prevalent in ancient systems. This analysis establishes models for non-actualistic spiculite sedimentation and associated glass ramp facies and proposes ecological models for widespread sponge colonization and localized carbonates at the southern limit of the great Permian spiculite belt.