Hydro-stratigraphic correlation by portable X-ray fluorescence spectrometry based chemostratigraphy

Abstract

In glacial basins stratigraphic correlation is commonly based on lithostratigraphic methods. Correlation can be significantly improved through the use of geophysical and geochemical properties; however widespread subsurface geophysical data is often limited. For groundwater studies, the collection of sediment geochemistry data is often beyond the scope and budget of many programs and is generally not included as a part of routine data collection. Portable X-ray fluorescent (pXRF) spectrometry has proven to be a successful, cost effective tool to characterize the chemostratigraphy of glacially derived sediments and to improve the interpretation of downhole geophysics, micropaleontology results, and pore water geochemistry. Data collected from this method has now become a routine part of borehole studies within the Groundwater Program at the GSC. Analytical protocols have been developed to utilize portable X-ray fluorescence spectrometry (pXRF) to obtain precise and accurate data for a suit of up to 14 elements detected in the <63 microns grain size fraction (Ba, Ca, Cu, Fe, K, Mn, Ni, Rb, S, Sr, Ti, V, Zn, Zr). This protocol was developed through the analyses of over 10,000 samples obtained from multiple glacial basins across Canada, and verified against traditional laboratory methods (fusion, four acid, aqua regia digestions) using >500 samples. The introduction of chemostratigraphic techniques to samples collected from boreholes establishes chemical and related mineralogical variations within sediments and contributes to information collected by sediment description, grain size data, downhole geophysical and stratigraphic correlations. Geochemical data also provides an opportunity to establish a chemostratigraphic framework that complements other stratigraphic correlation techniques, for example lithostratigraphy and biostratigraphy. Results have demonstrated the ability of chemical analyses obtained from pXRF spectrometry to identify stratigraphic units, refine sedimentological interpretations, and correlate within glacial basins. The addition of geochemical analyses has refined paleogeographic interpretations; provenance studies, and provides information to support 3-D geological models with increased confidence in stratigraphic correlations. A pilot study in the Greater Toronto Area (GTA) of 10 borehole cores, sampled at approximately one metre interval (1057 sample analysis), has provided a proof of concept for planning of a series of chemostratigraphic transects across southern Ontario. Samples will be collected from OGS and GSC archival material for 18-20 boreholes for approximately 2000 samples. Additionally reanalysis of a suite of samples from the NATMAP (< 50 samples) orientation sample transect in the GTA will provide a link with surface geochemical sampling and the subsurface chemostratigraphic data.