CORRELATION OF EUSTATIC SEA-LEVEL CHANGE TO LOWER SILURIAN BEDROCK DEPOSITION IN SOUTHWEST OHIO

The Llandoverian and Wenlockian Series of the Silurian witnessed changes in sea-level that greatly influenced carbonate deposition on the Cincinnati Platform. There were at least seven sea-level fluctuations in the order of 50 meters during the 30-million year span of the Silurian, with five occurring during the Early Silurian. Of these, there are four eustatic transgressions and regressions that can be correlated worldwide during the Llandoverian. It is believed that these one to three million year sea-level fluctuations are of third-order sequences, and in turn, are superimposed on a larger second-order rise and fall of sea levels that represents the entire Silurian period. Contributing fluctuations in eustatic sea-level may be interpreted from upward changes in the depositional facies and biofacies of the exposed bedrock as studied in two quarries in Miami County, Ohio.

The initial Silurian transgressive phases are limited and the distribution of sediments indicates deposition in valleys and erosional lows of the post-Ordovician surface topography. A former study by Ross indicated that additional Llandoverian sea-level highstands advanced progressively higher onto the craton as shown by changes in fossil assemblages with increasing water depth and by the areal extent of the deposits of each regression. The final transgressive sequence late in the Alexandrian is widespread, and its shelly fauna and reef structures are well preserved in the Brassfield Formation. In the early part of the Niagaran, a lowstand and erosional interval are evident, followed by a major transgression and highstand that continues through most of this series. The latest transgression of the Llandoverian was the most extensive. Evidence of this transgression and following regression are recorded in the Osgood Shale. The fourth transgression of the Early Silurian, and subsequent slow shallowing are responsible for the alternating carbonate and clastic deposition. The Laurel Limestone, Massie Shale, Euphemia Dolomite, Springfield Dolomite, and the Cedarville Dolomite formations are all indicative of this shallowing event.