Abstract

Geochemical analysis of the Ordovocian–Silurian boundary interval on Anticosti Island, Quebec, has revealed low Ir abundances for the lower Becscie Formation (0.005–0.039 ppb), comparable to those reported for the upper Ellis Bay Formation (0.005–0.058 ppb). When normalized as CaCO3-free values and ratioed to Al, a small amount of excess Ir (over background Ir levels) was observed in and on the upper surface of an oncolite platform bed in Member 7 (Laframboise) of the Ellis Bay Formation. Except for this bed, the abundances of Ir and other elements are relatively uniform throughout the section, but small enrichments are apparent in a few clay layers. As in other areas previously studied (South China, northwestern Canada, Scotland), the Ir maxima on Anticosti Island occur near the base of the graptolite persculptus Zone, and are attributable to condensation and slower sedimentation rates associated with the hardgrounds found within and on the upper surface of the oncolite platform bed. Although we cannot preclude a large extraterrestrial impact as the source for the Ir abundance maxima near the end of the Ordovician, which appear to be stratigraphically coeval and geographically widespread in the world, all of them are found to be associated with sedimentary condensation that may have resulted from glacio-eustatic sea-level fluctuations at the end of the Ordovician. From our data, we note that certain elements (e.g., Th, La, Lu, Sc, Cr, Ir, Fe, Al, Ca) show distinct abundance patterns in limestones, calcareous shales, and clays, apparently a result of increasing clay mineral contents.

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