SPATIAL AND TEMPORAL VARIABILITY IN SHELL BEDS FROM DISTINCT NUTRIENT REGIMES IN THE PERMIAN PHOSPHORIA ROCK COMPLEX

The Permian Phosphoria Rock Complex (PRC) of ID and WY is an economically important P resource that accumulated in a shallow sea. The PRC hosts fossil accumulations in all facies; these shell beds are a valuable tool for the interpretation of ancient environments. The PRC records a shift from a nutrient-rich, sediment-starved, low-O2 environment (Meade Peak Mbr) to a later oligotrophic, normoxic carbonate shelf (Franson Mbr). Examining the faunal response to these shifts in environment at both the outcrop and petrographic scales will serve to capture this crucial interval before the end of the Paleozoic.

We assess two widespread shell beds—the phosphatic ‘Fish Scale Marker Bed’ (FSMB, base of Meade Peak Mbr) and Franson Mbr carbonates—at multiple localities across southern vs. northern areas as a vehicle to interpret changes in paleoenvironments. These localities parallel the north-south-trending paleo-coastline. We utilize trends in fossil size, taxa, mineralogy, and associated sediments within these shell beds to better reconstruct spatiotemporal shifts within an upwelling system in the ancient Phosphoria Sea.

The transition from FSMB to Franson yielded an increase in diversity of preserved fauna, and supported greater carbonate mineralization. A seafloor dominated by phosphatic inarticulate brachiopods and fish shifted to environments dominated by calcitic articulate brachiopods, crinoids, and bryozoans.

The FSMB is condensed and relatively thin (5 to 10 cm thick in south; 95 to 160 cm thick in north), with cm-scale layers at all localities. FSMB brachiopod and fish fragments are consistently ~2 mm diameter at all localities, and rarely up to 10 mm in the north.

The Franson Mbr ranges from 0 to 35 m thick in the south, with 4 to 60-cm-thick shell beds; the north has greater lithologic variability and silicification of carbonate fossils, while bed thicknesses remain comparable. In the south, Franson productid brachiopods are 5 to 70 mm and crinoid ossicles are 1 to 11 mm in diameter, with less common ramose bryozoans 5 to 80 mm long; brachiopod sizes are comparable in the north, while crinoids and bryozoans are smaller.

These PRC shell beds reflect increasing faunal diversity over this interval along a coastline whose northern margins experienced greater and coarser clastic sediment input, whereas the southern areas were subject to more intense and protracted nutrient input.