WEATHERING IN THE HIGH PLATEAUS REGIONS OF GASPESIE - SOME ASPECTS

Weathering in the high plateaux regions of Gaspesie: some aspects. Twenty-six deeply weathered bedrock occurrences, formally described as saprolites, were discovered underlying the unconsolidated sediments in the plateaux regions of Gaspesie. These occurrences, which are erosional remnants of an old weathering surface, are preferentially preserved on slopes oriented to NNW and SSE. The weathered bed rock is distinct both mineralogically and geochemically from the solid bed rock and from the overlying deposits. The age of weathering is probably Tertiary. The preservation of these occurrences on slopes oriented in upglacial and downglacial directions is a new evidence for the invasion of Gaspesie by the continental Ice Sheet. Three younger weathering surfaces were identified in the sediments overlying the weathered bed rock. They are the present-day surface, an interstadial surface and an interglacial surface. The relative age of these surfaces is based on a comparison of the morphological, mineralogical, geochemical and stratigraphic attributes of the corresponding underlying sediments. 1) The surficial colluvium which underlies the postglacial weathering surface has been partially leached of Mn, Co, Ni, Cu, Pb, Zn, As and U and enriched in Cr and Fe, as compared with the underlying sediments. 2) A geochemically unaltered sediment composed of frost-shattered particles situated in intertill position underlies the next weathering surface. These sediments are believed to have formed under periglacial climatic condition during an interstadial period. 3) Strongly oxidized colluvium occurring in subtill position underlies the third weathering surface. This colluvium is geochemically similar to the surficial colluvial deposits and is, therefore, supposedly interglacial in age.