ABSTRACT This three-day field trip focuses on the stratigraphy and the structural characteristics of the late- and post-Taconian sedimentary basins of the southern Québec Appalachians, with a particular emphasis on N-to-S and W-to-E structural and lithological variations. In order to discuss various aspects of the regional structural evolution of these basins, we will visit a series of key outcrops following three sections, the Beauce/Thetford-Mines sections, the Sherbrooke section, and the Coaticook section. RÉSUMÉ Cette excursion de trois jours se concentre sur la stratigraphie et les caractéristiques structurales des bassins sédimentaires tardi- et post-Taconien des Appalaches du sud du Québec, en mettant l’accent sur les variations structurales et lithologiques du nord au sud et d’ouest en est. Afin de discuter des divers aspects de l’évolution structurale régionale de ces bassins sédimentaires, nous visiterons une série d’affleure ments clés en suivant trois sections, soient les sections de Beauce/Thetford-Mines, de Sherbrooke, et de Coaticook.

The recession of the late Wisconsinan ice sheet from central Maine was characterized by several modes of deglaciation. Below the limit of marine submergence in the Augusta-Skowhegan area, the retreat of the ice margin is documented by end moraines. A DeGeer-type esker system and accompanying marine deltas were deposited in a succession of narrow, topographically controlled zones of stagnant ice that bordered the active ice margin. Immediately inland from the marine limit, glaciofluvial deposits were graded to the sea. Examples occur in the Pleasant, Piscataquis, and upper Kennebec River Valleys. The large volumes of these deposits and the presence of morphosequences suggest that they were derived from active ice standing a short distance inland from the sea. However, in certain areas, as between Dexter and Guilford, melting of large stagnant ice blocks produced little or no waterlaid sediment. Moraines are also present above the marine limit although their significance in determining ice-marginal positions is not well understood. They are particularly characteristic of basins underlain by phaneritic plutonic rocks. Glaciofluvial and lacustrine deposits in northwestern Maine and the deposits of small cirque glaciers on Mt. Katahdin are among the youngest glacial deposits within the study area.

The deglaciation of the study area started around 13,500-13,000 years B.P. and was distinguished by the existence of an Appalachian ice mass that produced glacial erosion marks, three morainic segments, numerous eskers, and various accumulations of ice-contact stratified drift. Field data allow for the reconstruction of an active phase of this Appalachian ice, which was quickly followed by a stagnating phase. The separation of the northward-flowing Appalachian ice from the main southeastward-flowing Laurentide Ice Sheet was produced by an upstream progression of a calving bay in the St. Lawrence River Valley. This progression stopped in the valley bottleneck near Québec City, where the zone of separation shifted inland onto the present south shore of the St. Lawrence River. From this point the separation of the Appalachian ice took place on land and was not controlled by the calving bay. Near this zone of separation, the Laurentide ice produced the Highland Front Moraine. It was not until the retreat of the Laurentide ice front from the Highland Front Moraine position that marine water could invade the St. Lawrence Lowlands. A tentative chronology of events is presented.