GEOGRAPHIC AND STRATIGRAPHIC DISTRIBUTION OF SEISMITES—OPENING A NEW WINDOW TO OHIO’S SEISMIC HISTORY

Ohio generally is considered an aseismic state with simple geology, lacking major folding, faulting, and tectonic activity. However, past and present studies of bedrock outcrops, with an emphasis on seismites, reveal that the state has experienced significant earthquakes in geologic history. The convoluted beds between undeformed layers (seismites) are found in geologic formations across Ohio and are physical evidence of high-magnitude earthquakes caused by major tectonic activities during deposition of the layers. For the present study, outcrops of the reported soft-sediment deformation features and other geologic formations with deformation potential were visited for seismite verification and data collection for further analysis.

Over the past 30 years, small-scale seismites have been documented in Ordovician- and Silurian-age marine shale and carbonate layers exposed in southwest and western Ohio. The large-scale seismites in the Devonian-age Berea Sandstone and sandstone layers of the Bedford Formation, which are extensively exposed from northeast Ohio through central and south-central Ohio, have been reported as convoluted soft-sediment deformations since the 1950s. The current study examines seismites in Mississippian-age rock units exposed in southern, central, and northern Ohio, namely, the Cowbell Member of the Borden Formation, the Black Hand Member of the Cuyahoga Formation, the Fairfield and Berne Members of the Logan Formation, as well as undifferentiated units in the Logan Formation.

Rock units in Ohio that include seismite horizons range from Ordovician through Mississippian in age and were deposited in marine, nearshore, and deltaic environments. The size and distribution of the seismites in these sedimentary environments indicate their distance from regions of tectonic activity and active fault sources during the Acadian and Alleghenian Orogenies. The abundance of seismites in different geologic units reveals that Ohio experienced numerous significant seismic events during its Paleozoic history. Seismic activity from the end of the Paleozoic to the Pleistocene is unclear because of the absence of post-Paleozoic geologic formations.