DISAPPEARANCE OF PHOSPHATIC MICROFOSSILS IN THE UPPER CINCINNATIAN (UPPER ORDOVICIAN, LATE KATIAN): WATER MASS CHANGES OR FACIES TRACKING?

Sedimentary phosphate in the Cincinnatian Ordovician is generally found as sub-millimeter microsteinkerns of mostly gastropods and bryozoans, or as stereomic molds of echinoderm grains. Petrographic micro-diagenesis studies of Cincinnatian limestones suggest that these microsteinkerns formed in the shallow burial environment under dysoxic conditions, and accumulated in the active sediment zone over prolonged periods of slow or no sediment accumulation and repeated reworking of shell buildups.

Such phosphatic steinkerns are found throughout geologic time but are not evenly distributed. The requisite conditions for their formation is not well-understood. To what extent do global oceanic chemistry, basin structure, and local sedimentary processes contribute to the formation of these particles? To complicate the question, the post-Cambrian distribution of these phosphatic fossils is poorly known, leaving much to speculation.

For example, previous studies have suggested that the Richmondian is low in phosphate and that the Kope of the underlying Maysvillian is high in phosphate. The tapering off of phosphogenisis is taken as evidence of major changes—either climatic or oceanic that marked a shift from cold to warm climate, or, from cold upwelling water to warm continental water. Our preliminary studies of the Cincinnatian C3 sequence (near the midpoint of the succession) compared proximal peritidal to distal offshore facies. Our results suggest a strong relationship between water depth and phosphate steinkern development, with no steinkerns in peritidal sediments, and abundant steinkerns in offshore facies. Given that the Cincinnatian is a progradational system, with the Kope lacking any peritidal deposits in the region, and with the uppermost strata dominated by peritidal deposits, it follows that the observed decrease in phosphatic steinkerns could result as a simple artifact of facies tracking.

To explore this possibility, we sampled limestones from different positions on the paleo ramp at various points within the succession. Preliminary results suggest that in fact phosphatic particles are quite abundant in the Richmondian, particularly in the interval representing the “Richmondian Invasion”, an interval of rapid faunal change.