POSSIBLE SEISMITES IN THE PLEISTOCENE TEAYS FORMATION, PIKE COUNTY, OHIO

Approximately 2,000 logged wells on the reservation of the Portsmouth Gaseous Diffusion Plant (PORTS) in Pike County, Ohio, provide an unprecedented view of the subsurface stratigraphy of the Pleistocene of southern Ohio. Prior to Pleistocene glacial advance, southern Ohio was drained by the north-flowing Teays drainage system. Pre-Illinoian ice dammed the Teays River, impounding the massive proglacial Lake Tight in its valleys for more than 7,000 years. The PORTS site spans a paleovalley, incised into lower Mississippian strata, of the Portsmouth River, a tributary of the Teays River. This paleovalley is filled by the Pleistocene Teays Formation, consisting of a lower Gallia Member and an upper Minford Member. The Gallia Member consists of unconsolidated, poorly sorted, medium- to coarse sand and gravel, and is interpreted as channel fill of the Portsmouth River. The Minford Member is characterized as a silty clay with fine sand, transitioning upward into a laminated clay with silt, and is interpreted as lacustrine in origin. In 25 wells located throughout the PORTS site, drillers and geologists have logged isolated occurrences of “reworked Gallia” sediments within the Minford Member. These occurrences do not exceed 6 feet in thickness, and are typically less than 3 feet in thickness. The characteristics of these sediments and their environment (i.e., saturated coarse clastics beneath clays, in a lake-bottom setting), as well as their isolated occurrences, suggest possible interpretation of these features as seismites. Isostatic readjustment of the lithosphere related to ice advance, the impoundment of Lake Tight, or both, is offered as a possible trigger mechanism for these seismic events.