HIATAL SURFACES WITHIN THE PHOSPHORITE, ORGANIC-RICH, AND CHERT FACIES OF THE PERMIAN PHOSPHORIA FORMATION, SOUTHEASTERN IDAHO

The Phosphoria Sea occupied a restricted basin along the western edge of North America during the middle Permian, and is famous for its economically important phosphate deposits, thought to reflect a major upwelling system. The Phosphoria Formation comprises a distinctive suite of lithologies including phosphorite, organic-rich mudstone, dolostone, and thick intervals of chert. The absence of common subtidal siliciclastic facies suggests a sediment-starved system in which hiatal surfaces and condensed intervals should be common. New fieldwork using both natural and mine exposures in southeastern Idaho reveals that significant hiatal surfaces occur throughout both the Meade Peak and Rex Chert Members, which constitute the lower depositional sequence of the Phosphoria Fm Each surface is typically overlain by deeper water organic-rich mudstones or shales.

The basal sequence boundary of the Phosphoria Fm has a calcrete developed on top of the subjacent Grandeur Member carbonates of the Park City Fm, which is directly overlain by a condensed, bioclast-rich phosphorite bed. This sequence of the Phosphoria Fm is composed of numerous sub-meter- to meter-scale cycles of shallowing-up facies, demonstrating small-scale (fourth or fifth order) relative sea level fluctuations. The Meade Peak Member includes multiple small-scale cycles topped by well-developed, often bored, phosphatic nodules, peloids, and bioclastic material of brachiopods and fish. Shells and molds of bivalves are more frequently found within the beds in the top of shallowing-up cycles. Many of the flooding surfaces that mark the bases of cycles are notable for being either Thalassinoides-burrowed firmgrounds, or bored nodular hardgrounds with Trypanites ichnofacies. Similar types of surfaces occur in both the phosphorite- and dolomudstone-dominated Meade Peak Member and in the Rex Chert Member, which is primarily composed of spicular chert. The evidence of the repeated occurrence of these well-developed flooding surfaces in the Phosphoria Fm supports a sediment-starved depositional environment with oxygen levels sufficient for significant bioturbation.