SUBSURFACE STRATIGRAPHY OF THE UPPER ORDOVICIAN KOPE FORMATION IN SOUTHWEST OHIO: IMPLICATIONS FOR DEPOSITIONAL RATE AND MODE ON A SILICICLASTIC-CARBONATE RAMP

The Upper Ordovician (Cincinnatian Series) Kope Formation was deposited on a storm-dominated siliciclastic-carbonate ramp bordering an intracratonic basin, and consists of mudrocks with cyclically alternating grainstones and packstones. Most earlier models explained Kope mudrocks as the result of slow background sedimentation and limestones as products of rapid storm deposition. More recent studies have taken the opposite view: mudrock packages may represent rapid “dumps” of clastics while limestones are the “time-rich” products of long-term storm reworking, amalgamation and/or sediment starvation. These models cannot be tested fully until their predictions for stratigraphic trends can be checked across the entire ramp and basin. However, distal Kope facies, representing at least half the depositional setting, are present only in the subsurface north of Cincinnati and their stratigraphic relationships have been obscure.

To address this, a subsurface Kope stratigraphy has been constructed in southwest Ohio using wireline logs and cores, along with a small number of outcrops near Cincinnati. This framework reveals a number of relevant features. The mudrock packages show subtle facies changes but, for the most part, do not change significantly in thickness across the study area. Cores have revealed obrution deposits within the mudrocks that match stratigraphically and faunally with deposits found in proximal Kope exposures. Both of these findings suggest the mudrocks are chiefly the product of rapid sedimentation events that affected most of this area simultaneously (e.g. storm-blanketing). The major grainstone and packstone shell beds are also very widespread but show considerable variation in thickness, and examination of cores shows that many of these beds are amalgamated throughout the region. This implies a complex history for the limestones with multiple episodes of deposition, reworking, erosion, and bed stacking before final burial. These conclusions are in general agreement with predictions of the more recent depositional models for distal Kope stratigraphic trends, and suggest that limestones may have developed during periods of basin-wide siliciclastic starvation.