ORDOVICIAN K-BENTONITES OF THE ST. LAWRENCE PLATFORM IN THE MONTRÉAL AREA, CANADA: LITHOLOGICAL, GEOPHYSICAL AND GEOCHEMICAL CHARACTERIZATION

During the Middle to Late Ordovician, the eastern continental margin of Laurentia was exposed to multiple explosive island-arc volcanic events associated to the ongoing Iapetus subduction. These volcanic ash deposits are known as K-bentonite beds within the St. Lawrence platform (SLP). They usually consist of volcanic glass altered into an assemblage of kaolinite and/or illite-smectite. Due to their wide dispersion, K-bentonite beds represent excellent stratigraphic markers. In North America, two such widespread horizons are the Millbrig horizon, a sequence of multiple ash beds, and the Deicke horizon, a single ash bed.

In the Montreal area of the SLP, we have identified two such beds in the Ordovician Chazy Group (Laval Formation), and seven beds in the Trenton Group (Tétreauville and Rosemont formations). Preliminary interpretation suggests that some of these bentonites relate to either the Deicke or the Millbrig horizons, or both. In the Chazy Group, a yellow-to-orange sandy layer is associated with one of the ash beds. On the field, these beds display a high gamma-ray activity, commonly attributed to their K content, but also to the presence of Th and U. Laboratory gamma-ray spectral analyses can quantify these radionuclides. The results indicate that K-bentonites of the Trenton Group are enriched in Th (mean value of 27.5 ± 0.7 ppm) when compared to the Chazy Group (mean value of 11.9 ± 0.3 ppm). XRD analyses indicate that kaolinite is the main clay mineral component (22 %), with a lesser amount of illite-smectite (11%) for both groups of K-bentonites. Primary crystals, such as microcline, albite, apatite and zircons, are present in some samples. ICP-MS analyses indicate a rhyodacite/dacite to trachyandesite composition. Principal component analysis of the geochemical data is successful to distinguish the Chazy- from the Trenton-hosted beds, as well as similar-looking lithologies such as shales beds and altered Cretaceous sills. On-going geochronological analyses will provide U-Pb zircon ages for both bentonites and will be use to compare our results with the age of the Millbrig horizon, dated at 453.36 ± 0.38 Ma (Oruche et al., 2018) in the Orignal quarry of eastern Ontario.