PLEISTOCENE MIGRATION OF THE OHIO RIVER AND GREEN RIVER MOUTH NEAR OWENSBORO, KENTUCKY

Surficial geologic mapping, funded by the USGS STATEMAP program, has identified a series of bedrock and topographic features suggesting multiple abandoned paths of the Ohio River and lower Green River near Owensboro, Ky. This study combines new surficial geologic mapping, geomorphic mapping, geophysics, geoarchaeology, and published geochronology to outline a relative sequence of changes in the fluvial river system, with reasonable quantitative constraints on timing of those changes.   Bedrock topography was investigated using subsurface well data, seismic shear-wave refraction profiles, and seismic reflection profiles. The resulting interpreted bedrock surface suggests a previous course of the pre-Illinoian(?) Ohio River was farther south than the current surface course of the river, flowing south of the Bon Harbor Hills. Illinoian and Wisconsinan outwash filled the valley to a depth of more than 120 ft by 23,500 years ago. A major undocumented event, perhaps seismic movement along an adjacent fault system west of Owensboro, caused temporary redirection of Ohio River flow north of the Rockport Hills instead of around the south end of that upland. Recent studies have documented other possible seismically induced river adjustments downstream. During this time, the mouth of the Green River was likewise displaced, the lower stretch of the Green River flowing east from Curdsville through the location of downtown Owensboro to join the Ohio somewhere near Rockport. Subsequent flow of the Ohio River was rerouted to south of the Rockport Upland, and breached the col between the Rockport Upland and the Bon Harbor Hills. Multiple sloughs record the successive relocation of the mouth of the Green River westward toward its approximate present location north of Spottsville between about 12,000 and 6,000 years ago.   The location and extent of these paleovalleys and paleochannels have significance for geotechnical planning and seismic-hazard assessment of the Owensboro area, as the paleochannel sediments have lower shear-wave velocities than adjacent deposits and thus higher potential for ground-motion amplification and other detrimental seismic behaviors.