Abstracts Category : Other

Integrated High-Resolution Chemostratigraphic and Cyclostratigraphic Analysis of the Paleotropical Carbonates Spanning the Ordovician-Silurian Boundary at the West End of Anticosti Island, Eastern Canada

by Alain Mauviel

The carbonate storm-dominated sedimentary succession superbly exposed on Anticosti Island in Eastern Canada represents one of the most complete and well-preserved paleotropical stratigraphic records spanning the Ordovician-Silurian (O-S) boundary. We sampled the nearly continuous coastal outcrop exposed at low tide along the west coast of Anticosti Island for high-resolution 13C and 18O chemostratigraphy. These new isotopic curves comprise more than 500 data points spaced at ~0.6 m intervals; for a total of 320 m of strata across the O-S boundary. The 13C curve displays two distinctive positive excursions in the Hirnantian Ellis Bay Formation; a small lower excursion (+2.5) and an upper larger excursion (+4.5). These two positive isotopic carbon excursions provide a distinctive chemostratigraphic signature for regional and global correlations with other Hirnantian sections. The continuing descending 13C trend, at least 30 m above the currently interpreted O-S boundary, suggests a readjustment of that boundary on the Anticosti succession. The 18O curve, similarly to the Quaternary 18O marine curve, is tightly coupled with multi-order cyclic facies changes. Our tightly coupled lithological and oxygen isotopic data suggest that the Anticosti succession was influenced by glacio-eustatic fluctuations during the end-Ordovician. Furthermore, the estimated duration of these multi-order cycles supports an astronomical forcing. A primary isotopic signal record is also supported by the lack of significant covariance between 13C and 18O, by the excellent microfabric preservation of both macro and microfossils in petrographic, cathodoluminescence, and SEM microscopy, and by little or no diagenetic resetting as suggested by the trace element geochemistry, which is unusual in the deep geological time.