PALEOWEATHERED GRANITOID SURFACES OF SOUTHERN NOVA SCOTIA: ENVIRONMENTAL IMPLICATIONS OF SAPROLITES

In addition to insignificant post-glacial weathering effects, the results of three distinct weathering events are being recognized, studied and contrasted within the granitoids of the South Mountain Batholith of southwestern Nova Scotia: 1) Pre-Carboniferous, 2) Pre-Triassic, and 3) Pre-Pleistocene in age. 1) Weathering of granitoids prior to deposition of Devono-Carboniferous sediments produced a well-developed saprolite, an in situ weathering profile, in which feldspars and micas are altered to clay minerals, and only weak preservation of granitic texture is evident. Subsequent burial and relithification have masked the original weathering mineralogy in this horizon. 2) Beneath the Triassic clastic sedimentary rocks there is a thick horizon of weathered granitoid that has not been relithified. This argillaceous saprolite exhibits an intensifying-upward weathering zonation typical of weathered horizons, with increased proportions of clay minerals at the upper levels. 3) In contrast, a younger arenaceous or sandy saprolite of pre-Pleistocene age can be found beneath glacial till at a number of locations throughout the batholith. This younger weathered horizon exhibits mechanical weathering, and incipient chemical weathering. These paleoweathering horizons are significant in that they attest to relatively warmer climates at several times in the geologic past. Their pattern of preservation implies some of them were much thicker and more widely distributed before partial erosion. Paleoweathered regolith was a probable sediment source for industrial-quality quartz sand in the Carboniferous sequence, the economic silica sands and clay deposits of Cretaceous age, and a sandy till of Pleistocene age. The remnants preserved in situ today are significant groundwater aquifers, and their geochemistry, mineralogy and texture enhance mobility and migration of toxic elements such as uranium, radon and arsenic. They are also mechanically weak, therefore their presence should be considered when selecting construction and waste disposal sites.