VARIATION IN MACROFOSSIL ABUNDANCE AND PRESERVATION UNDER UPWELLING CONDITIONS: A CASE STUDY IN THE PERMIAN PHOSPHORIA FORMATION

Marine systems that experience coastal upwelling are characterized by nutrient enrichment, increased biological productivity, greater rain of organic matter, and dysoxic to anoxic conditions. A distinctive facies pattern of organic-rich mudstone, phosphorite, and chert, often in conjunction with shallow marine carbonates, records a gradient from the core to margin of upwelling cells in both ancient and modern seas. Siliciclastic sediment starvation also characterizes areas proximal to upwelling cells, resulting in hiatal surfaces with distinct accumulations of skeletal remains. This study evaluates variation in skeletal macrofossil close-packing, shell bed type, and taphonomic condition as a function of hiatal duration and nutrient enrichment (proximity to upwelling cell), using the coeval Permian Phosphoria and Park City Formations exposed in southeastern Idaho. The shallow carbonate ramp of the Phosphoria Sea occupied tropical latitudes during the Middle to Late Permian, and records multiple intervals of organic-rich deposits and phosphorites, indicative of high primary productivity and fluctuating oxygen conditions. Macrofossil assemblages in the six Phosphoria Formation members and three Park City Formation members include articulate and inarticulate brachiopods, mollusks, bryozoans, palaeoniscoid fish, and sponge spicules. Both carbonate and phosphatic skeletal material exhibit several types of preserved mineralogies, including original apatite or carbonate, secondary phosphatization, silicification, secondary calcite, and molds.