ORIGINS OF CINCINNATIAN CALCAREOUS SILTS IN THE TYPE AREA: CONTRIBUTIONS BY ENDOSKELETAL ORGANISMS BORING AND MINING IN BRYOZOA

Sanctum laurentiensis Erickson and Bouchard, 2003, is a crudely excavated domichnium found within Cincinnatian Series upright and branching bryozoan zoaria (Erickson and Bouchard, 2003). We hypothesized that burrowing behavior by S. laurentiensis should have produced significant volumes of silt-sized carbonate grains having remnant bryozoan skeletal structure. Petrographic inspection of thin sections of 2 samples of calcareous siltstone (of calcitic composition), 1 bioclastic packstone, 1 calcareous siltstone of dolomitic composition, and 1 grainstone from within the Cincinnatian Series has revealed sand and silt-sized carbonate grains which exhibit what we interpret to be remnant bryozoan skeletal structure. These grains are inferred to have been removed (mined) from the endozones of the aforementioned zoaria by S. laurentiensis or another of several boring organisms. Insoluble residues and visual compositional estimates were produced to determine the percent composition of CaCO₃, mined bryozoan material and other calcareous elements in each sample.

Bryozoan skeletal material composed between 0.0 and 4.5 percent of the calcareous siltstones analyzed and 8.4 to 10 percent of the packstone and grainstone samples. This disparity in composition may be explained by future SEM studies of carbonate silt. Within the Kope and Fairview Formations of the Cinncinatian Series, nearly 30 percent of deepest (offshore) facies are represented by calcareous siltstones, whereas shallowest (nearshore) facies are generally represented by grainstone and packstone lags (Holland et al., 1997). Bryozoan-derived, silt-sized sediment was identified in both nearshore and offshore facies lithologies. Cincinnatian erect trepostome colonies thrived in nearshore, intertidal shelf settings. As such, the majority of bryozoan-derived sediment was likely produced in a similar setting. We believe that wave energy, especially in storms, winnowed skeletally derived carbonate silt grains from intertidal and shallow subtidal bryozoan thickets and transported it basinward where it produced the unusual 1.5 to 3 cm calcareous siltstones of the Kope and distal Fairview Formations. Conservative, first approximation estimates suggest that domicile production by S.laurentiensis and similar oranisms, may have been responsible for 8 to 10 percent of that silt production.