The varieties of limestone in the Middle Ordovician Cobourg Formation at Colborne, Ontario occur as follows: sparse biomicrite, 49%; fossiliferous micrite, 26%; biosparite, 23%; micrite, 2%; and packed biomicrite, trace.Some individual beds display a sequence of sedimentary structures composed of graded bedding at the base, followed by a zone of lamination, and structureless micrite at the top. Most disarticulated brachiopod and ostracod shells are convex upward in biosparite and convex downward in biomicrite and fossiliferous micrite. The percentage of brachiopod shells decreases towards the top of the quarry and changes laterally. Mechanical orientation patterns of elongate fossils show two dominant trends, NE–SW and NW–SE. Factor analysis suggests that the association of brachiopod shells and pelmatozoan fragments was controlled by a common environmental influence, that the bryozoa and ostracod remains were transported together with reworked quartz grains, whereas the distribution of trilobite fragments was controlled by another condition.The carbonate sedimentation occurred in a marine environment of generally low turbulence. Stronger currents were introduced sporadically to form graded and non-graded biosparites.