Three major sediment units are recognized in the surficial sediments of Lake Ontario on the basis of extensive sampling and echo sounding: (1) glacial till and bedrock; (2) glaciolacustrine clay; (3) postglacial mud. Sand and silt are minor units in the Ontario surficial sediments. The sediment distribution pattern is essentially simple with a natural superposition of sediment units reflecting the glacial and postglacial history of the lake. The inshore region of Lake Ontario is composed of glacial till and bedrock. The till is overstepped in deeper water by glaciolacustrine clays which are themselves overstepped by postglacial muds. Two northsouth sills, composed of glaciolacustrine clay, subdivide the main basin of Lake Ontario into three distinct basins of postglacial mud accumulation. The surficial sediments contain variable amounts of quartz, feldspar, clay minerals, organic carbon and calcite. Quartz and feldspar contents are greatest in the coarser inshore sediments while clay minerals and organic carbon are greatest in the finer offshore sediments. Carbonate is generally low throughout the lake. Illite is the dominant clay mineral with lesser amounts of chlorite and kaolinite. Mean rates of sedimentation ranging from 114 to 309 g/m 2 /yr are estimated for the basins of modern mud accumulation. The primary source of detrital material is believed to be the extensive glacial deposits of southern Ontario and northern New York State. An estimated 55 percent of this material is derived from the drainage basin with the remaining 45 percent being the result of shoreline and lake bottom erosional processes. The textural characteristics, defined by moment measures, of the modern sediments can be interpreted as resulting from the varying degrees of mixing between a sand and a clay size end member population; the former occurring in the nearshore zone and the latter in the offshore basin deposits. Both end member populations are leptokurtic and skewed due to the introduction of silt size material in the form of CaCO 3 probably as the product of lakewide precipitation. Mean groin size and standard deviation, predictably show trends from nearshore to offshore, with decreasing grain size, related to increasing water depth and hence to decreasing energy from a wave- and current-active nearshore region to an offshore region of quiet water with sedimentation from suspension. In addition, skewness and kurtosis show trends relative to mean grain size and standard deviation which can be explained in terms of increasing distance from shore. Variations in the compositional phases of the sediment in terms of quartz, clay and organic carbon, and also variations in the redox potential can in turn be related directly to textural characteristics of the sediment and consequently to the depositional environment.

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