TEPHROCHRONOLOGY OF ORDOVICIAN BENTONITES FROM THE MONTREAL AREA: ZIRCON AND APATITE CHEMISTRY AS A TOOL FOR STRATIGRAPHIC CORRELATIONS

Ordovician successions of the St. Lawrence Platform in southern Quebec (Canada) contain altered volcanic ash beds, known as K-bentonites, which are found throughout eastern North America. The chemical characteristics of the zircon and apatite contained in these bentonites can provide a fingerprint for individual ash layers, which can then be used to correlate the bentonites across and between sedimentary basins. In the Montreal area, the stratigraphy of the St. Lawrence Platform contains Cambrian sandstones of the Potsdam Group at the base, overlain by the Ordovician Beekmantown, Chazy, Black River, and Trenton groups, which are a series of alternating limestones, shales and dolomites that grade upward into rhythmites and turbidites in the Utica and Lorraine groups.

Field work performed in Montreal combined with drill core and spectral gamma data from the City of Montreal and private companies allowed us to identify 13 centimetric K-bentonite clay beds in the Trenton Group. 13 additional beds have been collected in Ontario, New-York and Vermont in supposedly equivalent formations. The zircons in these bentonites can be hydrothermally altered, but a chemical abrasion procedure prior to analysis allows us to remove altered domains leaving only fresh material for analysis. The volatile (Cl, F, OH), major, minor and trace element composition of the separated apatite and zircon crystals were measured using an electron microprobe and LA-ICP-MS.

Our new results allow us to establish chemical fingerprints of the different bentonite beds, and to gain insights into their formation. Cl-F ratio, combined with dissimilarity calculations and multidimensional scaling of apatite (using major, minor & trace elements) as a stratigraphic correlation tool makes it possible to precisely link sedimentary units across the Montreal area and understand their links with the Ontario and New England beds. Precise correlations can be proposed between samples that are several km apart. In addition, the chemistry of these minerals such as Sr/Y and LREE ratios in apatite and Ti in zircon allows us to determine that the bentonites have an arc origin. Also our data allow us to assess the impact Ordovician volcanic eruptions may have had on the climate at the time. Future work will include high precision CA-ID-TIMS U-Pb dating and Hf isotope analysis.