PHOSPHORITES AS UBER SHELL BEDS: THE RELATIONSHIP BETWEEN PHOSPHATE CONTENT AND SEDIMENT MATURITY IN CINCINNATIAN (ORDOVICIAN, KATIAN) LIMESTONES

Based on the idea that oceanic chemistry is the primary driver of phosphogenesis, widespread phosphate deposits have been interpreted as evidence of unusual marine chemistry or circulationpatterns. This study tests an alternative hypothesis, that slow sedimentation rates, coupled with episodic high-energy events under otherwise normal marine conditions led to reworking and phosphogenesis in the Cincinnati area Late Ordovician.

If this is the case, then phosphate content should increase with proxies for limestone maturity. These proxies include: 1) Dunham categories (mudstone, wackestone, packstone, grainstone) based on the relative abundance of shells, mud, and spar. 2) taphonomic condition of bioclasts (breakage, abrasion, bioerosion, and discoloration), and 3) the relative abundance of different bioclast types (aragonitic molluscs, calcitic brachiopods, bryozoans, and echinoderms).

Diagenetic phosphates in Cincinnatian limestones are microsteinkerns of molluscs, bryozoans, and other fossils. The concentration of these steinkerns varies among limestone units, which accommodates petrographic tests for these predictions of the reworking hypothesis.

To test the hyposthesis, we made a stratigraphic collection of vertical thin sections from the upper Fairview and lower Grant Lake formations at an old quarry face at Rice and Gage Streets, Cincinnati. For each of ~60 microfacies in each depositional unit, a 2 X 2 cm square was selected for study. Preliminary analysis is based on ranking squares; the amount of phosphate was estimated visually and (4 categories from <1% to >5%). Each square was also assigned a Dunham classification (expanded to 6 categories using mud content of intergranular space) and a breakage rank (4 categories from pristine to comminuted). Preliminary matrices (Chi2 p< 0.00001) appear to show a strong positive relationship between phosphate content and both Dunham maturity and fragmentation. Work is in progress to quantify phosphate content by particle counting, and to quantify fragmentation and relative bioclast abundances using point-counts.

The positive relationship between phosphate content and both textural and fragmentation proxies for maturity is better explained by a model based on sedimentary processes than by a model dependent on water-mass properties.