Les erratiques de dolomie à Rivière-Blanche, côte sud de l'estuaire maritime du Saint-Laurent : un indicateur de transport glaciaire et glaciel

Abstract

At Rivière-Blanche, a locality on the south shore of the lower St. Lawrence estuary, the clayey and rocky shore zone is largely covered by stones of various sizes and lithologies, from local and far-distant sources. A survey exceeding 31 000 boulders indicates that 31% are Precambrian clasts (granite, gneiss, anorthosite, etc.) from the Canadian Shield located on the north shore of the St. Lawrence, 40 km from Rivière-Blanche, and 69% are Appalachian lithologies, mainly (45%) sandstone and graywacke. There are also 1.5% of dolostone erratics, a lithology not widespread in the Canadian Shield nor in the Appalachians. Of the 1242 clasts of the various varieties of dolostone erratics observed, the grey coral (Cladopora) dolostone erratics are restricted to the Rivière-Blanche area, whereas many other varieties are common to both shores of the St. Lawrence estuary. The most likely far-distance source is the Proterozoic Mistassini sedimentary basin, while a few varieties such as the coral dolostone erratics are from the Appalachian Siluro-Devonian formations, of which the nearest outcrops are located in the northwestern area of Lake Matapedia, 25–30 km south of Rivière-Blanche. Whatever their source, the dolostone erratics were first transported by a late Wisconsinian regional ice stream before being released by icebergs in the Goldthwait Sea. The coral dolostone (Clapodora) erratics provide additional evidence for a northward ice flow between Lake Matapedia and Rivière-Blanche during an early phase of deglaciation of the St. Lawrence Valley after the formation of an Appalachian ice divide located approximately at the latitude of Lac-au-Saumon.