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Abstract: Unconformities, Paleokarst, and Tectonic Activity Associated with the Silurian Lockport Dolomite, Western Ohio

Department of Geology, Bowling Green State University, Bowling Green, OH, 43403

In Ohio, the Lockport Dolomite is overlain by the Greenfield Dolomite; the units do not have a lateral facies relationships as proposed by some workers. Keys points are: (1) the two units are separated by a regional disconformity to paraconformity, (2) relationships identical to (1) can be observed within the confines of a single quarry exposure, (3) topographic highs (1-20 m relief) are present locally along the disconformity, and (4) the disconformity shows truncation of beds below and onlap of beds above.

The unconformity is a karst-modified regional surface with local sinks, rubble breccias (proto-soils), mottled (green-gray-red) paleosols in paleotopographic lows, mudcracks, and subaerial debris flows. Subsurface karst, present as much as 30 m below the unconformity, includes open caves, rubble-filled caves, solution widened fractures, stratigraphically and fracture controlled cavities, and pulverite. Discordant masses (up to 5 m diameter) of matrix to clast supported intraformational breccias, are common at four quarries and rare elsewhere. Laminated dolomitic clay mudstones and clay shales at the base of some such masses suggests they are cross sections of cavity and cave fills.

It is postulated that the unconformity between the Lockport Dolomite and the Greenfield Dolomite is a composite surface, different parts of which were subaerially exposed at different times for varying periods of time, as a result of block faulting in western Ohio during the Silurian. Hard evidence that syndepositional tectonics affected the Lockport Dolomite is indicated by an intra-formational unconformity at two localities, and abrupt facies and thickness changes in this unit.

AAPG Search and Discovery Article #90930©1998 AAPG Eastern Section, Columbus, Ohio