PORTABLE XRF CHEMOSTRATIGRAPHY OF A PALEO-GLACIAL FORELAND BASIN, THE NANAIMO LOWLANDS, VANCOUVER ISLAND

A relatively thick and extensive succession of late Pleistocene proglacial, glacial and interstadial sediments occurs in the Strait of Georgia in British Columbia (and Puget Sound in Washington). Strata include the Dashwood and Vashon tills, the Cowichan Head, Quadra sand, and Capilano sediments. All of these units form either regional aquifers, or aquitards, hence understanding the spatial heterogeneity and lithochemistry are of high interest due to increasing groundwater issues in this area. Numerous studies have reviewed the lithostratigraphy, depositional environments and age relationship of these units; however, to-date there has been no systematic geochemical characterization. To resolve this knowledge gap the Geological Survey of Canada in collaboration with the Regional District of Nanaimo, British Columbia, has completed a chemostratigraphic study of 2 boreholes (~130 m deep) approximately 20 km apart, using a portable X-ray fluorescence spectrometer and multivariate statistical methods.

Results from the 234 analyses of the <63 µm (silt + clay) size fraction identify 15 elements that document the stratigraphic chemical variability. In the Dashwood till changes in both single element concentrations and multi-element associations indicate local variability within these sediments. The overlying lower Cowichan Head Formation displays similar elemental concentration trends as the underlying Dashwood till and is attributed to sediments sourced from Vancouver Island. In contrast, the upper Cowichan Head Formation and overlying Quadra sand have a geochemical signature (e.g. a relative increase in Sr concentration) reflecting a change in provenance to the Coast Mountains. The overlying Vashon Drift and Capilano Sediments can be differentiated from each other and the underlying Quadra Sand, by variations in the relative enrichment/depletion of the elements Ba, Fe, and Mg. To our knowledge these results are the first systematic geochemical characterization of late Pleistocene succession in the Nanaimo Lowlands.