Stratigraphic and structural framework of the Potsdam Group in eastern Ontario, western Quebec and northern New York State

Abstract

The presence of a major unconformity within the Potsdam Group is an important key to unlocking the geological history of this succession throughout its area of distribution in the Ottawa Embayment and Quebec Basin. The beds below the unconformity (Abbey Dawn and Covey Hill/Ausable formations) largely of continental origin up to 600 metres thick, is a complex assortment of red and grey quartzite cobble, quartz pebble and quartzarenite deposits of assumed Neoproterozoic to Early Cambrian age. The Covey Hill/Ausable formations have been further divided into four units of wide distribution beginning with the Jericho member of marginal marine origin, confined to the Quebec Basin bordering the Oka-Beauharnois Arch; succeeded and overlapped to the west by Hannawa Falls, Chippewa Bay and Edwardsville members of aeolian, fluvial and possible lacustrine origin. During and immediately following deposition of these members the beds were locally faulted, folded and subjected to a long period of subaerial erosion. This process was likely triggered by a plate tectonic event in progress during Neoproterozoic to early Early Cambrian time, when the continents were in a state of rifting and break-up, followed by the opening of the Iapetus Ocean. Above the unconformity are the white and grey Nepean and equivalent Cairnside and Keeseville formations up to 110 metres thick composed of quartzarenite and minor quartz pebble conglomerate of Late Cambrian to Early Ordovician age. These formations were deposited on the floor and margins of a seaway that entered the Ottawa Embayment from the east. The highland areas (arches) on all four sides of the embayment provided restricted marine depositional conditions that contributed to the precipitation of minor evaporates, a proliferation of stromatolites and a limited normal marine biota contained within the Nepean sandstone and its stratigraphic equivalents. Post-Potsdam deformation resulted in the rejuvenation of faults and local folding of Potsdam and younger Paleozoic strata throughout the Ottawa Embayment and Quebec Basin, the timing of which was likely associated with Taconian and Acadian orogenesis, and by the much later rifting of the North American continent that occurred with the break-up and separation of the continents in the Jurassic and Cretaceous.