Bed sediment sources and mixing in the glacierized upper Fraser River watershed, east-central British Columbia

摘要

The geochemical, mineralogical and lithological composition of modern stream bed material is examined in order to characterize sources and evaluate downstream mixing of sediments in the upper Fraser River drainage basin, British Columbia. The <63 μm fraction is emphasized for its relative mobility and ease of analysis using instrumental neutron activation. Overall, the composition of the stream sediments closely reflects bedrock distribution. Samples dominated by limestone and dolostone, calcite and dolomite, and related elements (Ca, Mg, Sr etc.) correspond to Lower and Middle Cambrian carbonate bedrock largely confined to the Moose River sub-basin. Clastic and non-quartzite metamorphic lithologies, primary and secondary aluminosilicate minerals and related elements (Al, Cs, Rb etc.) are largely derived from Miette Group bedrock and associated with the uppermost Fraser River sub-basin. Except in the case of the Moose River/Fraser River junction, the determination of proportional tributary contributions is complicated by variable or delayed mixing, localized floodplain or valley side sources, and limited contrast between source areas. At present the Moose River sub-basin contributes a greater proportion of the total and fine-grained sediment loads of the combined Fraser River than would be expected from drainage basin area alone. The imbalance is related to greater relief, precipitation and runoff in the Moose River sub-basin; however, the spatial association of carbonate-rich stream sediments, ice cover and carbonate bedrock exposure indicates that glaciers play a particularly important roll in generating fine-grained fluvial sediment. Since differences in glacier cover and glacier potential in the two major sub-basins are likely to be persistent, and since relative sediment yields from the sub-basins can be determined from sediment composition, a potential indicator of glacier variation and climate change during the Holocene is therein available.