The limestone of the Middle Ordovician Cobourg Limestone at Colborne, Ontario, occurs, in decreasing order of occurrence, as sparse biomicrite, fossiliferous micrite, biosparite, micrite, and packed biomicrite.

Individual beds display a sequence composed of graded bedding at the base, overlain by a zone of lamination, and structureless micrite. This resembles the sequence in terrigenous turbidites of the Alps and other places. Most disarticulated brachiopod and ostracod shells are convex upward in biosparite and convex downward in biomicrite and fossiliferous micrite. The percentage of brachiopod shells increases toward the bottom of the quarry; a variation in the percentage across the quarry has been found. The percentages of echinoderm, bryozoan, and ostracod fragments have a high positive correlation with the quantity of quartz grains, whereas the percentages of brachiopod shells and trilobite fragments do not. Orientation patterns of elongate fossils show two dominant trends, northeast-s uthwest and northwest-southeast.

Sedimentation of the Cobourg Limestone was characterized by weak currents. Strong currents were introduced sporadically to form graded and non-graded biosparites, which are suggested tentatively as products of turbidity currents.

All relations between geological entities were calculated by computer.

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