POSTGLACIAL AND PRE-LAST-GLACIAL WEATHERING OF TILL ON THE HIGH PLATEAUS OF CENTRAL GASPESIE, QUEBEC, CANADA

Geochemical data from mineral exploration surveys are used to assess p ostglacial and pre-last-glacial weathering on the high plateaus of cen tral Gaspesie by comparing the distribution of 30 elements in the B an d C horizons of a thin soil developed on glacial deposits. Under postg lacial weathering, Fe2O3, TiO2, P2O5, Cr. Li. U. and V are immobile re lative to Al2O3, whereas the other elements are mobile and have been d epleted to various degrees from the B horizon. Loss-on-ignition value is the only parameter showing a significant enrichment from the C to t he B horizons. The apparently high degree of geochemical weathering of the glacial deposits in respect to their source bedrock lithologies s uggests that the deposits also contain previously weathered material. The spatial distribution of the geochemical weathering index values ca lculated for the C horizon shows no clear relation to the underlying b edrock formations. The high values are located on topographic highs, w ith maxima over deeply weathered bedrock occurrences, and the low valu es are located in glacial valleys. The spatial distribution of this we athering index in the glacial deposits shows the geochemical signature of a pre-last-glacial weathered mantle.