EARLY IMPOUNDMENT HISTORY OF PLEISTOCENE LAKE TIGHT: NEW INSIGHTS FROM SUBSURFACE STUDY OF THE TEAYS FORMATION, PIKE COUNTY, OHIO

Pleistocene Lake Tight (PLT) remains a poorly understood part of the Quaternary history of the Ohio Valley, despite being recognized for more than a century. Both the depth and areal extent of PLT were not well constrained until recently (see Bailey et al., 2014; Erjavec, 2014). The duration of impoundment of PLT is estimated between 6,500-7,000 and 11,000-25,000 years based on varve couplets in lacustrine sediments of the Teays Formation in West Virginia and Ohio, respectively, while the age of PLT is constrained only by the presence of the Matuyama-Brunhes magnetic polarity reversal in these same sediments. Furthermore, the initial impoundment phase of PLT has been completely unexplored. The extensive drilling program at the Portsmouth Gaseous Diffusion Plant (PORTS) in Pike County, Ohio, provides more than 2,000 logged borings which penetrate the Teays Formation in an abandoned paleovalley of the Portsmouth River, a major tributary of the Pleistocene Teays River. Here, as elsewhere, the Teays Formation is divided into a lower Gallia Member, consisting of coarse clastic fluvial sediments of the pre-PLT drainage; and an upper Minford Member. The Minford Member has long been interpreted regionally as a slackwater lacustrine deposit; however, at PORTS the Minford Member can be further divided into an upper, clearly lacustrine unit (colloquially “Minford Clay”), and a lower complex unit (“Minford Silt”) which indicates alternating phases of fluvial deposition (point bar and overbank deposits) and lacustrine deposition (varves). The Minford Silt is locally penetrated by occurrences of “reworked” Gallia sediments, which do not penetrate the Minford Clay, and which we interpret as seismites. The transitional nature of the Minford Silt is reconcilable with an episodic filling model in which smaller, temporary proglacial lakes periodically formed and drained prior to the ice dam reaching its final location on the Teays River near present Chillicothe, Ohio, and impounding PLT to its maximum extent. The number and duration of these temporary impoundments is not yet resolved, and it is unclear whether the lower PLT shoreline reported by Bailey et al. (2014) at an elevation of 825 feet corresponds to these early impoundments or to a later phase of PLT’s existence.