Les erratiques de dolomie à Saint-Fabien-sur-Mer, côte sud de l'estuaire maritime du Saint-Laurent (Québec) : un traceur de transport glaciaire et glaciel exceptionnel / Dolostone erratics at Saint-Fabien-sur-Mer, south shore of the Lower St. Lawrence est

Abstract The lithology of boulders on the surface of Anse à Mercier, a clayey tidal flat at Saint-Fabien-sur-Mer (Québec), is composed of 57% erratics from the Laurentidian Shield and 43% from the Appalachians. Of the 20 counts made for a total of 9900 clasts, the minimum, maximum and median Shield's erratics are respectively 44%, 69.3% and 54%. During detailed surveys made between 1990 and 2002, 952 dolostone erratics, an uncommon lithology in the Shield and the Appalachians, were observed. There are various types of dolostone erratics including dolostone with stromatolites. About 30% of the erratics are striated and most are subrounded to rounded. Small to medium size clasts are more common: 7.5% weigh less than 1 kg, 72.5%, 1 to 20 kg, 15.2%, 20 to 100 kg and 4. 7% more than 100 kg. Dolostone erratics originated from the Mistassini Proterozoic sedimentary bassin in the Laurentidian Shield, approximately 400 km to the NE, and from the Silurian formations in the Appalachians to the SE, the first outcrops of carbonated formations located about 30 km from the present shoreline. Initially transported by the Wisconsinan glaciers, these dolostone erratics were ice rafted and released mainly by icebergs, during the postglacial Goldthwait Sea. The concentration of boulders on the surface of the Anse à Mercier tidal flat results from erosion of a marine clay deposit dated 12.5 to 10 ka and from shore ice activity during the Holocene. Although shore ice processes are still active, clasts concentrated into a large boulder pavement are presently relatively stable.