LITHOLOGY OF BOULDERS AT MITIS-BAY, SOUTH SHORE OF THE LOWER ST.-LAWRENCE ESTUARY - AN EXAMPLE OF COMPOSITE GLACIAL AND GLACIEL TRANSPORT

Lithology of boulders at Mitis Bay, south shore of the Lower St. Lawre nce Estuary: an example of composite glacial and glaciel transport. Th e lithology of boulders forming intertidal pavements and barricades in the Mitis Bay, is composed of 40% Precambrian, 25.8% limestone, 18.3% shale (including slate, mudrock and clayrock), 11.5% limestone, 3.6% quartzite and 0.8% conglomerate. This average is based on 52 countings for a total of 29 932 clasts. In the Precambrian category, anorthosit e boulders, a lithology occurring only in the Laurentidian Shield, acc ount for 0.8%. In the share category, 1.6% of clasts are red slates. C orral limestone and dolostone boulders were also observed. The source of the Precambrian clasts is most likely the Laurentidian Shield on th e north shore of the Saint Lawrence estuary over 55 km away, while bou lders of sedimentary lithologies belong to the Cambro-ordovician forma tions in the coastal zone of the south shore and of the Silurian forma tions over 35 km inland to the SE of the Mitis Bay area. Excepted for boulders of proximal or local origin, most others have been transporte d over many tenth of kilometers and sometime over 100 km by Appalachia n and Laurentidian glacier ice. Because most boulders are found in mar ine clay deposits, final deposition is attributed to icebergs. This st udy clearly demonstrates the importance of icebergs as a sedimentary a gent during the first millenia of the Goldthwait Sea.