PLEISTOCENE SEISMICITY ASSOCIATED WITH THE IMPOUNDMENT OF PROGLACIAL LAKE TIGHT, PIKE COUNTY, OHIO

Proglacial Lake Tight (PLT) formed when a pre-Illinoian glacial advance dammed the ancient Teays River system, ultimately impounding a massive 25,740 km² proglacial lake, with depths locally exceeding 75 meters, throughout the Ohio-West Virginia-Kentucky tri-state region. The relatively rapid impoundment of such an enormous mass of water would necessitate an isostatic readjustment of the lithosphere and presumably trigger reservoir-induced seismicity; however, evidence for this has been difficult to identify. The US Department of Energy’s Portsmouth Gaseous Diffusion Plant (PORTS) near Piketon in south-central Ohio is located approximately 30 km south of the location of the ice dam that formed PLT, and occupies an abandoned paleovalley of the Portsmouth River, a major tributary of the Teays River. As such, a complete, well preserved section of the Pleistocene Teays Formation is present in some locations – both the earlier fluvial component (Gallia Member; clayey sand and gravel of the Teays River and its tributaries) and the later lacustrine component (Minford Member; sedimentation within PLT). Decades of site studies at PORTS have allowed for a detailed characterization of the Teays Formation that has not been possible elsewhere. Here, the Minford Member is divided into a lower, transitional fluvio-lacustrine unit (silt, fine sand, and some clay), and an upper, wholly lacustrine unit (typically silty clay and clay). In only 25 borings out of more than 2,100 borings site-wide, isolated occurrences of “reworked” Gallia sediments (RWG) have been identified within or on top of the lower Minford; RWG is never found to penetrate the upper Minford. We interpret RWG as eruptive seismic features (clastic dikes and sand volcanoes), generated during an earthquake that occurred directly prior to PLT being impounded to its maximum extent. This interpretation is further supported by the intermittent occurrence at PORTS of the Gallia Member that has been compacted, dewatered, and subsequently cemented with bog iron. Because the Matuyama-Brunhes geomagnetic polarity reversal (770.2 ± 7.3 ka; Suganuma et al., 2015) has been documented in the Minford Member in West Virginia (Bonnett et al., 1991), and PLT existed for perhaps 10,000 years (Hoyer, 1976), the timing of this earthquake is suggested between 787 ka and 753 ka.