BURIAL DIAGENESIS MODEL FOR THE MACUMBER FORMATION ON CAPE-BRETON ISLAND - IMPLICATIONS FOR THE TECTONIC EVOLUTION OF THE WINDSOR GROUP

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 formation hosts numero us Pb-Zn occurrences, and its upper boundary is in contact with rocks ranging in age from Visean (early Carboniferous) to Westphalian (middl e Carboniferous), the origin of the stratigraphic omissions being deba table. This first inorganic diagenesis study identifies nine post-depo sitional processes, including the stabilization of marine components, and evaporite and anhedral calcite precipitation. The delta(18)O(PDB) (-13.0 to 2.0 parts per thousand), Sr-87/Sr-86 (0.7076 to 0.7079) and [Sr] (50 to 10000 ppm) of samples, including upper and basal whole-roc k units and anhedral calcite cements, show two trends enveloping the e ntire field of data. The trends suggest that an evaporite-derived, non -radiogenic fluid and a clastic-derived radiogenic fluid mixed and int eracted with the limestone sediments, in a progressively deeper burial environment. The evaporite-derived fluid mostly imprinted the top uni t of the formation. The field investigations, microscope observations and geochemical results do not indicate a meteoric overprint as would be expected if an unconformity existed along the top of the formation. The burial history documented here, and the presence of fibrous calci te indicating bedding parallel shearing, support a detachment model to explain the stratigraphic omissions within the Windsor Group.