SULFUR ISOTOPES AS A PROXY FOR SEDIMENTATION RATE IN A MIXED SILICICLASTIC-CARBONATE CYCLIC SUCCESSION: EXAMPLE FROM THE UPPER ORDOVICIAN (KATIAN) KOPE FORMATION, OHIO, INDIANA, KENTUCKY

The Upper Ordovician (Katian Stage) Kope Formation is an exemplar of mixed siliciclastic/carbonate cyclicity. Although long studied by many researchers, including Prof. Paul Potter, the sedimentological mechanisms recorded in the cycles of alternating sparsely fossiliferous siliciclastic mudstone and skeletal limestone are still debated. Most previous studies suggested storm wave winnowing to explain the shell beds as skeletal lags, whereas we suggest that that they record slow accumulation during a reduction in siliciclastic input perhaps associated with minor transgressions. Key to this debate is the trend in sedimentation rate, especially in the transition between mudstone to limestone. In this study, sulfur isotope analysis of disseminated pyrite is used as a proxy for sedimentation rate (SR) as the ∂³⁴S_pyrite is a function of organic matter burial rate during early diagenesis. Mudstone, siltstone and limestone beds from five meter-scale cycles were analyzed for ∂³⁴S. Results show a consistent pattern of increasing ∂³⁴S values (rising SR) through the mudstone interval with a high (~+18.00‰) at ~3/4 of the cycle followed by a sharp decrease in ∂³⁴S (rapid slowing of SR) in the last and first 10% of cycle mudstone, including the basal limestone (minimum ~-1.00‰). This interpretation of the sedimentation rate is bolstered by plotting Nb/Y vs. Zr/TiO₂ as a means of differentiating between sediment sources. The siliciclastic component in the limestone beds has an elemental fingerprint distinct from the mudstone intervals. The isotopic and elemental findings suggest that the mudstone portion of a cycle reflects increased rates of prograding siliciclastic sediment accumulation. The limestone portion of a cycle was formed when input from the primary source of siliciclastic sediment, presumably the Taconic Orogen, was significantly reduced. This starvation of marine borne sediments permitted relative concentration of muds with a distinctive trace elemental signature suggesting a distinct secondary provenance. We suggest that these represent windblown sediments from a distinct source area. This example draws attention to the utility of ∂³⁴S and Nb/Y vs. Zr/TiO₂ analyses as valuable tools to better understand sedimentological changes in mixed marine siliciclastic/carbonate deposition.