CENTIMETER-SCALE CHARACTERIZATION OF SMALL SCALE CYCLES IN THE KOPE FORMATION (UPPER ORDOVICIAN), CINCINNATI, OHIO REGION: IMPLICATIONS FOR STRATIGRAPHIC RESOLUTION AND CORRELATION

The Kope Formation is an Upper Ordovician (Cincinnatian Series) mudrock-carbonate unit deposited in a storm-dominated, mixed siliciclastic-carbonate ramp setting. Lack of a regional fine-scale stratigraphy has obscured evidence of Kope depositional processes; previous stratigraphic analyses have characterized the Kope as a mosaic of localized microfacies. However, recent detailed studies demonstrate that these rocks are commonly correlatable at meter- and even decimeter-scale over large areas. The unit contains more than 40 meter-scale mudrock-siltstone successions capped by prominent thin skeletal packstone/ grainstone bed bundles; these parasequences stack into 5-10 meter intervals (submembers) showing general thinning upward of mudstones.

Preliminary centimeter-scale analysis of the 5-6 m Alexandria submember (middle Kope Fm.) probes the limits of high resolution event stratigraphy and provides insights into depositional processes. Each of the six component meter-scale shale-limestone parasequences displays unique attributes that permit unambiguous identification. In addition to characteristic spacing, each of the 5-20 cm capping limestone bundles displays stratinomic, faunal, and taphonomic attributes (e.g., re d-weathering dalmanellid brachiopods in bed 29; reworked concretions in bed 30a). Moreover, intervening mudstones also contain numerous thin (mm to cm), highly distinctive marker beds; these include a) single depositional events (e.g. graded siltstones; obrution beds with distinctive trilobite/crinoid faunas), b) time-averaged lag horizons (e.g. fragmentary graptolite beds; trilobite hash beds), and c) diagenetic beds (e.g. concretion horizons). Most of these horizons (~20) have been found at all Alexandria submember exposures over an area of several hundred square kilometers in southwest Ohio and northern Kentucky.

These results have several implications. They (1) indicate the efficacy of very high resolution correlation at regional scale, (2) imply widespread, spatially uniform, but temporally fluctuating environmental conditions and episodic sediment accumulation of regional scale, and (3) suggest that very high resolution correlation may be a necessary tool for paleoenvironmental interpretation of other siliciclastic-carbonate ramp units.