REGIONAL STRATIGRAPHIC FRAMEWORK OF SURFICIAL SEDIMENTS AND BEDROCK BENEATH LAKE-ONTARIO

Approximately 2550 km of single-channel high-resolution seismic reflec tion profiles have been interpreted and calibrated with lithological a nd geochronological information from four representative piston cores and one grab sample to provide a regional stratigraphic framework for the subbottom deposits of Lake ontario. Five units overlying Paleozoic bedrock were identified and mapped. These are classified as informal units and represent, from oldest to youngest: (A) subglacial till (?) deposited by the Port Huron ice at the end of the Wisconsin glaciation ; (B) an ice-marginal (?) unit confined to the western part of the lak e that was probably deposited during retreat of the Port Huron ice sho rtly after 13 ka; (C) a regionally extensive unit of laminated glaciol acustrine clay that accumulated until about 11 ka; (D) a weakly lamina ted to more massive lake clay deposited during a period of reduced wat er supply and rising water levels after the drawdown of the high-level glacial lakes (Iroquois and successors); and (E) modern lake clay les s than 10 m thick that began accumulating around 6-8 ka with the subse quent return of upper Great Lakes drainage through the Ontario basin. Seismic reflections also define the configuration of the bedrock surfa ce and pre-glacial stream valleys incised in the bedrock surface. Seve ral anomalous bottom and subbottom features in the surficial sediments are mapped, such as discontinuous and offset reflections, furrows, ga s pockets, and areas of large subbottom relief. None of these features appear to be spatially correlative with the diffuse seismicity that c haracterizes the lake area or with deeper structures such as Paleozoic bedrock faults or crustal-penetrating faults in the Precambrian basem ent.