CARBONATES AND ASSOCIATED SEDIMENTARY-ROCKS OF THE UPPER VISEAN TO NAMURIAN MABOU GROUP, CAPE-BRETON ISLAND, NOVA-SCOTIA - EVIDENCE FOR LACUSTRINE DEPOSITION

The Upper Visean to Namurian Mabou Group of Cape Breton Island conform ably overlies highest marine carbonate strata of the Windsor Group and unconformably (? also conformably) underlies the mainly fluviatile Cu mberland Group. Regionally, the Mabou Group, dominated by fine grained elastic rocks, comprises a lower grey facies assemblage of proposed l acustrine origin, and an upper red facies of fluviatile origin. The gr ey facies is represented by the Hastings Formation in western Cape Bre ton Island, by the Cape Dauphin Formation in the Sydney Basin, and by the lower parts of the MacKeigan Lake Formation in the Loch Lomond Bas in of southeastern Cape Breton Island. The red facies comprises the Po mquet Formation in western Cape Breton Island, the Point Edward format ion in the Sydney Basin, and the upper part of the MacKeigan Lake Form ation. The ubiquitous presence of thin, laterally discontinuous limest one beds in the lower grey facies of the Mabou Group is a useful guide to stratigraphic position. These carbonate beds include wackestones a nd grainstones containing intraclasts and ooids as well as ostracods a nd serpulids. Stromatolites are the most common carbonate rock type wi thin the grey facies of the Mabou Group. Faunal elements characteristi c of normal marine environments are conspicuously absent. The carbonat e rocks and associated siliciclastic sediments indicate that depositio n of the Mabou Group grey facies occurred within a shallow subaqueous environment undergoing intermittent periods of sub-aerial exposure, a setting consistent with lacustrine conditions, traditionally postulate d for lower Mabou Group sedimentation. Initially elevated salinities r eflecting the last stages of marine evaporite sedimentation gave way p rogressively to brackish and then fresh-water conditions as the Late V isean arid climate moderated during earliest Namurian time.