PALEOECOLOGICAL AND PHYSICAL EVIDENCE OF A COMPLEX MOSAIC OF OXYGEN CONDITIONS IN THE PHOSPHORIA SEA, PERMIAN OF SOUTHEASTERN IDAHO

Fine-scale fieldwork and microfacies analysis is providing new insights into depositional and non-depositional environments for the Permian Phosphoria Formation, which is famous for commercial-grade phosphorites and long thought to reflect paleo-upwelling along the tropical northwestern margin of Pangaea. The Fm is generally considered to record a low-oxygen, quiescent, siliciclastic sediment-starved setting. However, physical sedimentary structures and trace and body fossils indicate that the phosphorite, siltstone, carbonate, and chert facies that comprise parasequences represent a spatial and temporal mosaic of aerobic to quasi-anaerobic conditions in inner to outer ramp settings, largely above storm wave base. For example, coarse-grained phosphorites that cap many parasequences throughout the Fm exhibit current ripples, erosional contacts, truncated and reworked grains, and graded bedding; siltstones exhibit low-angle current ripples; and multi-mm sponge spicules in cherts are commonly current-oriented.

Evidence of benthic metazoans exists throughout the Phosphoria. Large remnant lenses of normative shelf limestone (Franson Mbr) host dense, storm-generated shell beds of brachiopods and crinoids, indicating aerobic conditions, but macrofossils are most common regionally in hiatal beds associated with multiple ranks of flooding and regressive surfaces. Thalassinoides burrows occur throughout the Rex Chert Mbr, and macrofaunal encrusters and fish debris mantle flooding surfaces that cap densely packed sponge spicules, indicating aerobic phases. In the Meade Peak Mbr, paracycles with laminated black shales are still bounded by burrowed firmgrounds and include coarse phosphorites with orbiculoid brachiopods and conodonts, signaling upper dysaerobic intervals. Nodular phosphorites and carbonate concretions associated with hiatuses commonly have Trypanites borings and fish debris, pointing to lower to upper dysaerobic phases. The uppermost, thin-bedded siltstones of the Fm rarely yield body fossils, but host abundant Chondrites, signaling quasi-anaerobic bottom water. Trace and body fossils thus support a mosaic of aerobic to quasi-anaerobic conditions in the distal Phosphoria Sea, with much of the seabed experiencing moderately high-energy conditions.