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 strati graphic marker at the base o f the Visean Windsor Group. On Cape Breton Island the formation hosts numerous PbZn occurrences, and its upper boundary is in contact with rocks ranging in age from Visean (early Carboniferous) to Westphalian (middle Carboniferous), the origin o f the stratigraphic omissions being debatable.This first inorganic diagenesis study identifies nine post-depositional processes, including the stabilization of marine components, and evaporite and anhedral calcite precipitation. The (-13.0 to 2.0%o), 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 o f the for mation. 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 o f 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.La Formation de Macumber, composee de calcaires fins en deux minces unites, represente un marqueur stratigraphique crucial k la base du Groupe de Windsor. A Pile du Cap Breton, la formation encaisse plusieurs indices Pb-Zn et constitue la limite inferieure d'une serie d'omissions stratigraphiques k l'interieur du Groupe de Windsor, l'origine desquelles est fortement d6battue.Cette premiere investigation de la diagenese inorganique place neuf processus en une succession post-ddpositionnelle incluant la stabilisation des elements marins, et la precipitation d 'evaporites et de ciments calcitiques anddriques. Les 5 18OpD3 (-13.0 to 2.0%o), 87Sr/86Sr (0.7076 to 0.7079) et [Sr] (50 to 10000 ppm) de micro£chantillons des unites inferieures et superieures, et de ciments anedriques montrent deux tendences enveloppant l'ensemble des rdsultats. Ces tendences suggerent qu'un fluide non-radiogenique, evaporitique et un fluide radiogenique derive de roches clastiques, auraient affecte les sediments calcaires au cours de leur enfouissement progressif plus profond. Le fluide evaporitique aurait principalement affecte I'unite superieure de la formation. Les donnees de terrain, les observation microscopiques et les resultats geochimiques n'indiquent pas l'empreinte m6teorique attendue si une discordance existait au sommet de la formation. L'histoire diag6netique ici documentee, ainsi que Pexistence de calcite fibreuse ddveloppde le long de plans de cisaillement paralieies au litage supportent le modeie de detachement pour expliquer les omissions stratigra...

Deformation of the Macumber Formation, Antigonish Basin, Nova Scotia: implications for the Ainslie Detachment

“…Instead, the subhedral shape of the calcite crystals suggests that they are pseudomorphs after gypsum and that the crystal-laminated interval exhibits a primary fabric that records little post-depositional deformation other than compaction. Similar fabrics enveloping pseudomorphs (c) after gypsum have been described from undeformed limestone of the Macumber Formation in Cape Breton Island by Savard et al (1996). However, deformation is evident towards the base of the Macumber Formation where a series of large inclined to recumbent folds are developed that resemble the slump structures at MacIsaacs Point.…”