Résumés
Abstract
The Macumber Formation, a finely laminated limestone consisting of two thin units, represents a key strati-graphic marker at the base of the Visean Windsor Group. On Cape Breton Island the formation hosts numerous Pb-Zn occurrences, and its upper boundary is in contact with rocks ranging in age from Visean (early Carboniferous) to Westphalian (middle Carboniferous), the origin of the stratigraphic omissions being debatable.
This first inorganic diagenesis study identifies nine post-depositionat processes, including the stabilization of marine components, and evaporite and anhedral calcite precipitation. The B18Op0g (-13.0 to 2.0%o), 87Sr/86Sr (0.7076 to 0.7079) and [Sr] (SO 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 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 calcite indicating bedding parallel shearing, support a detachment model to explain the stratigraphic omissions within the Windsor Group.
RESUME
La Formation de Macumber, composed de calcaires fins en deux minces unites, reprisente un marqueur stratigraphique crucial a la base du Groupe de Windsor. A Tile du Cap Breton, la formation encaisse plusieurs indices Pb-Zn et constitue la limite inftricure d'une s6rie d'omissions stratigraphiquesa l'intlrieur du Groupe de Windsor, I'origine-desquelles est fortement ddbattue.
Cette premiere investigation de la diagenese inorganique place neuf processus en une succession post-depositionnelle incluant la stabilisation des Elements marins, et la precipitation d'evaporites et de ciments calcitiques anedriques. Les 818Opdb (-13.0 to 2.0%o), 87Sr/86Sr (0.7076 to 0.7079) et [Sr] (50 to 10000 ppm) de microechantillons des unites infirieures et suplrieures, et de ciments anedriques montrent deux tendences envcloppant Pcnsemble des rlsultats. Ces tendences suggerent qu'un fluide non-radiog<est;Snique, eVaporitiquc et un fluide radioglnique derive de roches clastiques, auraicnt affecte les sediments calcaires au cours de leur enfouissement progressif plus profond. Le fluide eVaporitique aurait principalement affecte 1'unite supirieurc de la formation. Les donn£es de terrain, les observation microscopiqucs et les rdsultats g6ochimiques n'indiqucnt pas I'empreinte mitiorique attendue si une discordance existait au sommet de la formation. L'histoirc diagdnltique ici documented, ainsi que I'existence de calcite fibreuse developp^c lc long dc plans de cisaitlement paralleles au litage supportent Ic modele de detachement pour expliquer les omissions stratigraphiques du Groupe dc Windsor.
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