Abstract

The Pridolian (Upper Silurian) Neigette breccia near Rimouski in the Québec Appalachians is made up of large reef limestone clasts that are identical to some of those in the Upper Silurian West Point Formation in the southeastern part of the Gaspé Belt. This occurrence indicates that the Upper Silurian West Point reef tract may have existed outside the Gaspé Peninsula in the Matapédia Valley area and that it developed along the northern edge of the Gaspé Belt. A close association of the breccia and the Neigette Fault, together with other features of the breccia, suggests that the Neigette breccia was deposited at the foot of a Pridolian normal fault that was active during Salinic extensional tectonism. Two main limestone clast types occur: those of the Anse à la Barbe Member and the Colline Daniel Member, which occur in the Upper Silurian West Point Formation in the southeastern part of the Gaspé Belt. Cement stratigraphy, based on stained thin sections and cathodoluminescence petrography, and carbon and oxygen stable isotope geochemistry, were used to decipher the diagenetic and porosity history of both limestone types.

The Anse à la Barbe-type limestone clasts encountered the same diagenetic evolution and had the same porosity history as the Anse à la Barbe limestone in the southeastern part of the Gaspé Belt. Diagenesis was marked by a freshwater dissolution event that created significant porosity, but early meteoric-influenced cement occluded all pore space rapidly in the depositional to shallow burial environment. This freshwater event was likely related to subaerial exposure during a late Ludlovian eustatic lowstand, as in southeastern Gaspé.

The Colline Daniel-type limestone diagenesis was marked by multiphase early subsea fracturing and subsequent cement and sediment infilling (neptunian dykes), likely related to synsedimentary Salinic tectonism. Brecciation may have accompanied the late stages of subsea fracturing. Except for neptunian dyke occurrence and brecciation that produced the Neigette breccia, the diagenetic and porosity history of the Colline Daniel-type limestone clasts is the same for the Colline Daniel limestone in the southeastern part of the Gaspé Belt. It is essentially a history of progressive burial. Temperature calculation indicates that the latest cement precipitated at a maximum burial depth of about 1500 m. Limestone porosity was occluded during deposition of the Upper Gaspé Limestones and Salinic block faulting and tilting. Fluid migration appears to have been toward the reef bodies. This might have formed potential subsurface reservoirs, if the hydrocarbons had been present in the transported fluids.

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