A Comparison of The Orientation and Spacing of Joints in Limestone and Dolostone -- Cobourg Formation, Manitoulin Island Area, Ontario

ABSTRACT A single bed of carbonate rocks (Ordovician), approximately 20 cm thick, is exposed on a shallow-dipping dome and in adjacent troughs in an exposure on Goat Island. Ten dolostone ridges (megaripples), about 2 cm high, are enclosed by limestone and occupy the cap of the dome. The flanks and adjoining troughs are mainly dolostone. The orientations of joints in limestone, dolostone ridges and dolostone are similar; the average joint spacing is 105 cm, 38 cm, and 33 cm respectively, indicating joints are approximately three times more closely spaced in dolostone than in limestone. For a given rock type, joints in strongly developed sets (class intervals of high frequency in a rose diagram) are no more closely spaced than those in weakly developed sets. Joints occur in clusters, however, that are closely spaced in strong sets and widely spaced in weak sets. Dolomitization preceded joint formation. An original limestone bed probably consisted of calcilutite, calcarenite and calcarenite megaripples. Mg-bearing solutions travelled through the more permeable calcarenites and dolomitized them. This may have been achieved by meteoric waters while the Cobourg Formation was exposed to erosion or by solutions travelling along the Cobourg-Collingwood unconformity. The joint pattern is not simply the result of the upward propagation of joints in the Precambrian basement. The joints likely formed in response to the sinking of the Michigan Basin.