PALEOBIOLOGICAL IMPLICATIONS OF SKELETAL CONCENTRATIONS IN ORGANIC-RICH RECORDS: TAPHONOMY OF SHELL- AND FISH-RICH INTERVALS OF THE PERMIAN PHOSPHORIA FORMATION

Skeletal concentrations within the relatively sediment starved and eutrophic phosphorite, organic-rich mudstone, and spicular chert facies of the Permian Phosphoria Formation in Idaho vary somewhat predictably with sequence stratigraphic context, but are nonetheless distinct from well-characterized patterns in fully oxygenated meso- and oligotrophic marine records. Based on field and thin section observations, Phosphoria shell beds can be categorized into five types that imply different levels of time-averaging, differentiated by taphonomic complexity, lateral traceability, and sequence stratigraphic context. The first two types are only continuous at outcrop scale and reflect background levels of time-averaging between major stratigraphic surfaces. (1) Lenses or stringers of loosely to densely packed shells with minor damage, reflecting event concentrations. (2) Dispersed, unbroken but rarely articulated shells in organic-rich mudstone without extensive winnowing or hardground development.

Despite strong regional facies variation, several shell-rich beds persist across most of the basin. These shell beds are associated with significant stratigraphic surfaces, and thin sections reveal multiple phases of phosphatization, calcitization, and silicification. (3) Densely packed, current-oriented siliceous sponge spicules with subsidiary phosphatic grains and fish debris: these mantle parasequence-scale flooding surfaces, are only locally traceable, and reflect a moderate degree of time-averaging. (4) Loosely packed, damaged, phosphatic bioclasts with phosphatic nodules, often with evidence of hardground development. These hiatal beds occur at major flooding surfaces, are traceable for tens of kilometers, and are highly time-averaged. (5) Densely packed, highly damaged, dominantly phosphatic bioclasts associated with pervasive bioturbation and skeletal bioerosion. This hiatal bed mantles the basal sequence boundary, is traceable throughout nearly the entire basin, and reflects extensive winnowing and time-averaging. These lithologic characteristics indicate that even in settings of low sedimentation rate and nutrient enrichment, stratigraphic hiatuses regulate the abundance, preservation, and likely time-averaging of the shelly fossil record.