HIATAL ASSOCIATION OF SKELETAL CONCENTRATIONS IN HIGH-PRODUCTIVITY RECORDS: PALEOECOLOGICAL IMPLICATIONS FOR THE PHOSPHORIA ROCK COMPLEX

High-productivity systems are known to host a diversity of bioelemental sediments, and high nutrient input has the potential to influence skeletal concentrations through multiple interacting ecological, taphonomic, and diagenetic effects. New field observations on the classic, high-productivity record of the Permian Phosphoria Rock Complex (PRC) in a sequence-stratigraphic context reveals a mosaic of aerobic to dysaerobic (rarely quasi-anaerobic) facies that accumulated largely above storm wave base and a multitude of complex discontinuity surfaces in its SE Idaho depocenter.

Skeletal concentrations range from single-event tempestites to physically and diagenetically complex hiatal accumulations, with the type of concentrations related largely to hiatal duration, which is inferred independently from discontinuity surfaces (bedding planes, bed-set boundaries, flooding surfaces of parasequences, and maximum flooding and starvation surfaces).

Each type of diastem or hiatus is characterized by a distinctive type of skeletal concentration, though variations often depend on the host lithology, which is tied to both paleo-bathymetry and paleo-nutrient regimes in these largely bioelemental phosphorites, cherts, and organic-rich shales. Ichnofauna and macrofaunal body fossils indicate variable paleo-oxygen conditions, but oxygenation does not seem to have dictated the abundance of skeletal material as much as hiatal duration does. Discontinuity surfaces are in fact the best way to prospect skeletal remains. The association with hiatuses suggests that skeletal concentrations have the potential to be highly time-averaged, i.e. beyond that expected for assemblages from intervening depositional increments, with the complexity of their taphonomic history varying with the duration of the hiatus. Relative durations of time-averaging vary among the types of shell beds, with rare census-type shell beds preserved, within-habitat time-averaging very common, and further environmental condensation in the most time-averaged shell beds.

Thus, the taphonomic condition of fossil material, and indeed its occurrencewithin the PRC, appears to be governed first by the duration of hiatus, and second by the interplay of ancient nutrient regimes on oxygenation.