Preliminary results from a drilling programme aimed at evaluating the oil shale potential of the Whitby Formation (Upper Ordovician) of Ontario showed that the Craigleith Member of this unit possessed the greatest promise. However, some preliminary lithostratigraphic observations contradicted those of earlier workers, prompting a detailed analysis of the lithostratigraphy, log response, and organic geochemistry of this unit of organic-rich interbedded shale and limestone, also known informally as the "Collingwood shales." Previously grouped with overlying non-calcareous shales, these strata are found to be in gradational contact with underlying limestones of the Lindsay Formation. This, together with a sharp decrease in carbonate and organic carbon contents at their top, leads to their redefinition as the Collingwood Member of the Lindsay Formation. Analysis of subsurface data shows that the unit now termed the "Collingwood Formation" in well records does not contain any of the Collingwood strata as originally defined, and that the potential oil shales of the newly defined Collingwood Member have a significantly restricted distribution. The non-calcareous blue-grey shales previously referred to the upper part of the Whitby Formation are redefined as the Blue Mountain Formation, which is dominated by the brown and grey slightly organic and calcareous Rouge River Member only in the area east of Toronto.

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