Lithostratigraphic revision of the Upper Cambrian Cairnside Formation, upper Potsdam Group, southwestern Quebec

The contact between the Covey Hill and overlying Cairnside formations (Potsdam Group) is exposed in the Ducharme quarry near the village of Covey Hill. It is the first time that the contact is identified from a surface section. The upper part of Covey Hill observed below this contact is characterized by a thickto medium-bedded, lightand medium-grey, coarse-grained, pebbly subarkose lithofacies. The Cairnside Formation consists of a lower unit of thickly bedded to medium-bedded, coarse-grained, burrowed, white quartz arenite and an upper unit characterized by beds similar to those of the lower unit with thinly bedded to lenticular, medium-grey, coarse-grained dolomitic sandstone. The upper contact of the Cairnside Formation is recognized from a core, and is placed at the top of the highest clean quartz arenite above which such lithofacies does not exist or is extremely subordinate relative to the dolomitic sandstone and sandy dolostone interbeds of the overlying Theresa Formation. 1 Contribution to the Appalachian Foreland and St. Lawrence Platform Architectures in Quebec, New Brunswick and Newfoundland NATMAP Project 1 Resume : Le contact entre les formations de Covey Hill et de Cairnside (Groupe de Potsdam) est present dans la carriere Ducharme, a proximite du village de Covey Hill. La description que nous presentons de ce contact est la premiere qui soit faite a partir d’une coupe affleurante. La partie superieure de la Formation de Covey Hill est caracterisee par un lithofacies de subarkose grossiere a caillouteuse de couleur grise, disposee en lits de grande a moyenne epaisseur. La Formation de Cairnside consiste en une unite inferieure de quartzarenite grossiere de couleur blanche, bioremaniee, a lits de grande a moyenne epaisseur et en une unite superieure similaire a la precedente, mais qui renferme des lits minces a lenticulaires de gres dolomitique grossier de couleur grise. Dans les coupes en forage, le contact superieur de la Formation de Cairnside est place au sommet du lit de quartzarenite le plus haut, au-dela duquel ce lithofacies n’existe plus ou est excessivement rare par rapport au gres dolomitique a interstrates de dolomie greseuse de la Formation de Theresa sus-jacente.