Comparison of litho-, bio-, and chemostratigraphy in two cores from the northeastern margin of the Michigan Basin (Manitoulin Island) and from within the Ottawa Embayment (eastern Ontario) identifies interbasinal differences of Late Ordovician platform foundering linked to Taconic orogenesis. Graptolite biostratigraphy defines an east-to-west younging (late Edenian to early Maysvillian) of platform burial. A regional unconformity likely caps the platform succession. In both basins, an increased supply of mafic material appears during the final stages of platform collapse, with the accumulation of organic-rich (<8%), petroliferous shales (Collingwood Member -Michigan Basin; Eastview Member -Ottawa Embayment). Both units preserve evidence for deposition coincident with increased dysoxic to possible anoxic bottom-water conditions, but the Collingwood Member accumulated under a relatively stable paleoceanographic environment. Rhythmic interbedding with platform limestone in eastern Ontario, combined with evidence for fluctuating paleoproductivity, suggests the depositional environment of the Eastview Member was more sensitive to higher order controls affiliated with tectonic, oceanographic, and (or) sea level variation. Such interbasinal differences likely reflect a greater rate of subsidence in the Manitoulin region transforming platform sedimentation to a distal ramp facies. In eastern Ontario, a lesser rate of subsidence maintained a shallower water, but open margin, setting. Burial of the Upper Ordovician platform, as preserved in eastern Ontario, occurred during peak dysoxic conditions, with deposition of a hemipelagic facies (Billings Formation) that marks the peak supply of clay-size mafic-derived sediment. Bottom-water ventilation occurred only with appearance of abundant Taconic-derived distal turbidites. An equivalent hemipelagic facies appears to be absent from the Manitoulin region. However, equivalent resedimented deposits are represented by the Blue Mountain Formation.Résumé : Des comparaisons de la litho-, bio-et chimiostratigraphie dans deux carottes provenant de la bordure du bassin de Michigan (île Manitoulin) et de la baie d'Ottawa (est de l'Ontario) identifient les différences entre les bassins de l'effondrement de plate-forme à l'Ordovicien tardif relié à l'orogenèse taconique. La biostratigraphie de graptolites définit un rajeunissement de l'est vers l'ouest (Edenien tardif à Maysvillien précoce) de l'enfouissement de la plate-forme. Une discordance régionale chapeaute sans doute la succession de la plate-forme. Dans les deux bassins, l'approvisionnement en matériaux mafiques croît durant les stages finals de l'effondrement de la plate-forme avec l'accumulation de shales pétrolifères, riches en matière organique (<8 %) (membre Collingwood -bassin de Michigan; membre Eastview -baie d'Ottawa). Les deux unités ont conservé des évidences d'une déposition qui coïncide avec des conditions d'eau de fond de plus en plus dyxosiques à possiblement anoxiques; cependant, le membre Collingwood s'est accumulé...

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A 6 cm thick K-bentonite, herein defined as the Russell Bed, occurs in an Upper Ordovician deep-basin shale succession in eastern Ontario, Canada, forming part of the distal Taconic foreland in eastern North America. The bed lies within the pygmaeus graptolite Biozone, which is about 451 to 452 Ma in age. Although some bentonites are reported from this interval in eastern North America, we are reporting the first set of compositional data for a bentonite of this age. Gamma-log correlation identifies a potential minimum distribution area of < 2 × 10 5 km 2 for the K-bentonite, covering part of southern Quebec, New York State and eastern Ontario. The deposit coincides with the first influx of distal turbdites into this shale basin, associated with Taconic flysch, and simultaneous abrupt ventilation of the once anoxic deep-water basin, which had formed initially after foundering of the Upper Ordovician carbonate platform. Concurrent intrabasinal extinction of several graptolite species suggests that change in sedimentation, palaeoceanography and volcanism were linked to a regional external process. Compositionally, the bentonite is distinct from the older Ordovician platform deposits in eastern North America. The deposit contains abundant titaniferous phlogopite with 1.6 % BaO, fluoroapatite with 2.5 % F, and dynamically shaped glass spherules now altered to clay. The spherules and clay matrix constitute 45 % of the bed and, compositionally, define an illite (> 90 %)smectite (I/S) structure with about 7.5 % K 2 O %. Age-dating by Ar-Ar analysis of the phlogopite crystals yielded a younger than expected (440-445 Ma) age. This difference, along with evidence of localized chloritization of phlogopite, likely reflects known post-Ordovician hydrothermal activity within the basin. On the basis of several geochemical proxies, the magmatic source of the Russell K-bentonite falls within the trachyandesite field and was Ba-enriched. Comparison of geochemistry and mineralogy with older, Middle to Late Ordovician and younger Early Silurian K-bentonites within the Taconic orogen along eastern Laurentia and Baltica reveals that the potential source magma for the Russell Bed was more mafic, more alkaline, and less fractionated than sources typical of older (platform) bentonites. Instead, it is more similar to the younger Llandovery bentonites of Scandinavia and Scotland. It remains uncertain if it signals local or regional compositional change in volcanic source in the northern Appalachians.

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