An epeiric glass ramp: Permian low-latitude neritic siliceous sponge colonization and its novel preservation (Phosphoria Rock Complex)

Abstract

Glass ramps are shallow-marine depositional settings in which siliceous sponge meadows dominate coastal environments. They are increasingly recognized throughout the Phanerozoic and represent a biosiliceous counterpart to neritic carbonate factories. Detailed reexamination of the Permian Tosi Chert in the Bighorn Basin indicates that it records a glass ramp that extended over at least 75,000 km2. Outcrops, cores, and wireline logs are used to discriminate previously unidentified shallow subtidal to peritidal facies in its landward extent. These facies indicate that sponge meadows ranged from variably oxygenated offshore settings through low-energy, well-oxygenated, and saline shallow subtidal settings, with spicules transported into supratidal environments affected by enterolithic evaporite growth. This range of subenvironments is largely unique among glass ramps. This is the result of the Tosi's accumulation in an epicontinental sea where waves impinged offshore but frictional attenuation caused low-energy nearshore environments. As a result, the Tosi shares similarity with epeiric sea carbonate deposition and is referred to herein as an epeiric glass ramp. The low palaeolatitude of the Tosi and hot and arid desert it bordered also contributed to its uniqueness as shallow waters were warmer and more saline than higher-latitude counterparts. As a result, a minor sea-level fall at the termination of biosiliceous deposition was associated with increased lagoonal circulation and refluxing brines that caused evaporite and dolomite precipitation within the upper Tosi. Preservational attributes of the Tosi also add to the range of unique traits that can be used to reconstruct neritic biosiliceous environments. These include three disparate colours of chert (black, grey, and purple) related to the host strata and diagenetic redox conditions, early chertification that preserved sedimentary structures within nodules, and nodule shape related to bioturbation intensity. The Tosi glass ramp thus expands the known extent and context of Permian glass-ramp deposition along the western Laurentian margin and illustrates key properties that will aid future glass ramp identification.