Thunder Bay, which is partly silled to the south by an east-northeast trending horst, contains three separate basins. Recent sediments, which cover the bay-floor, grade from a thin veneer of sand less than 3 centimeters thick, along the bay-shelf, to a sequence of clay-silts up to 14 meters thick, in the central parts of the basins. Tight folding of the Recent clay-silts in the deepest part of the central basin is probably caused by downslope slumping. Geochemical investigations of the bottom-sediments show that there appears to be a tendency towards negative Eh and low pH values in the deep-water areas. In the vicinity of the Kaministikwia Delta anomolously negative Eh and high pH measurements were recorded which were probably caused by industrial pollutants entering the bay from the Kaministikwia River. The Recent sediments unconformably overlie Pleistocene varved sediments of undetermined thickness. The Recent clay-silt section and the Pleistocene varved section each form a typical syndiagenetic sequence with a relatively thin, upper oxidized zone (initial stage), and a lower reduced zone (early burial stage).

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