Burial diagenesis model for the Macumber Formation on Cape Breton Island - Implications for the tectonic evolution of the Windsor Group

Abstract

The Macumber Formation, a finely laminated limestone consisting of two thin units, represents a key stratigraphic marker at the base of the Visean Windsor Group. On Cape Breton Island the PbZn occurrences, and its upper boundary is in contact with rocks ranging in age from Vsean (early Carboniferous) to Westphalian (middle Carboniferous), the origin of the stratigraphic omissions being debtable. This first inorganic diagenesis study identifies nine post-depositional processes, including the stabilization of marine components, and evaporite and anhedral calcite precipitation. The δ18OPDB (-13.0 to 2.0‰), 87Sr/86Sr (0.7076 to 0.7079) and [Sr] (50 to 10000 ppm) of samples, including upper and basal whole-rock units and anhedral calcite cements, show two trends enveloping the entire field of data. The trends suggest that an evaporite-derived, non-radiogenic fluid and a clastic-derived radiogenic fluid mixed and interacted with the limestone sediments, in a progressively deeper burial environment. The evaporite-derived fluid mostly imprinted the top unit of the formation. The field investigations, microscope observations and geochemical results do not indicate a meteoric overorint as would be expected if an unconformity existed along the top of the formation. The burial history Documented here and the presence of fibrous calcite indicating bedding parallel shearing, support a detachment model to explain the stratigraphic omissions within the Windsor Group.