A RECORD OF CHANGING ICE MARGINAL CONDITIONS PRESERVED WITHIN LATE QUATERNARY SEDIMENTS OF THE BOWMANVILLE BLUFFS, ONTARIO, CANADA

Quaternary age sediments are well exposed along the north shore of Lake Ontario in the Bowmanville Bluffs (approximately 55 km east of Toronto) and provide a record of changing conditions along the southern margin of the Laurentide Ice Sheet during the Wisconsin glaciation. Interbedded glacial, glaciolacustrine and deltaic sediments exposed along the bluffs have been correlated with similar deposits exposed at the Scarborough Bluffs which lie approximately 50km to the west. However, no detailed sedimentological analyses of the Bowmanville sections have been conducted in recent years and there is a need to re-assess the paleoenvironmental record they contain. Detailed field based sedimentological logging and analysis of over 4 km of sediment exposed along the Bowmanville Bluffs identifies a series of matrix-supported diamicts, each with different structural and textural characteristics. Fine-grained silty-clay diamicts are exposed close to the base of several sections. Overlying sand-rich diamicts are massive to crudely bedded. These two distinct diamict types are separated by a discontinuous succession of rippled and deformed fine-grained sands. Deformation structures include syndepositional faults and water escape structures that suggest high sedimentation rates. The uppermost diamict exposed at the Bowmanville Bluffs is massive, with a silt-rich matrix and contains horizontal clast concentrations that resemble boulder pavements. In some areas this diamict is separated from the underlying sand-rich diamict by a discontinuous horizon of deformed silty sands. The Bowmanville Bluffs succession is interpreted to record sedimentation in a glacially-influenced lake basin in which ice marginal fluctuations allowed the deposition of silty clay glaciolacustrine diamicts, deltaic sands and silt and sand-rich subglacial deformation tills. Sedimentological analysis of this succession will significantly enhance understanding of Late Quaternary paleoenvironmental conditions in southern Ontario.