Geochemistry and sedimentology of Upper Ordovician mudrocks and limestones, eastern Ontario: A new sedimentary framework for the distal Taconic foreland

In eastern Ontario, Upper Ordovician (Edenian-Richmondian) strata, now preserved in the Ottawa Embayment, represents the final stages of foreland basin fill related to Taconic orogenesis. The mostly mudrock succession buried a shallow-water carbonate ramp (Lindsay Formation). Passage from carbonates to mudrocks is recorded by the Eastview Member (eastern Ontario) and Collingwood Member (southern and central Ontario). In eastern Ontario and at Craigleith, central Ontario, inboard of the Michigan Basin to the west, this unit consists of an interbedded limestone and petroliferous claystone. The latter lithology indicates intermittent dysoxic conditions along the ramp. Geochemistry of the shale suggests that increased organic productivity may have caused dysoxic conditions that preserved elevated organic carbon (2--6%). Mineralogy and geochemical proxies indicate the first introduction of sediment derived from erosion of the Taconic island arc. Along the eastern margin of the Michigan Basin, the Collingwood Member consists of inter-laminated/thinly bedded calciturbidites and organic-rich shale. This regional difference in carbonate stratigraphy may define formation of a distally-steepened ramp margin along the Michigan Basin, possibly arising from more rapid collapse of the carbonate platform. Subsequent shales of the lower Billings Formation (eastern Ontario) and the Blue Mountain Formation (southwestern Ontario) were deposited in dysoxic to anoxic deep-water basins, and record peak influx of mafic-rich, Taconic-derived, sediment.; In eastern Ontario, a K-bentonite (Russell Bed), dated (Ar-Ar) at 445 Ma, separates these deep-basin shales (lower Billings Formation) from subsequent influx of siliciclastic turbidites (upper Billings Formation), the latter deposited in an oxygenated basin. Deep-water ventilation, coincidental graptolite extinction, volcanism, and appearance of turbidites, combined, identify major basin reorganization and paleoceanographic changes at this time linked to Taconic orogenesis. The K-bentonite contains rare, altered glass spherules. Compositionally, source magmatism was mildly alkaline and basic, with a continental crustal component, when compared to the older rhyolitic to rhyodacitic K-bentonites of eastern North America that predate carbonate platform collapse. This geochemical change may also signal evolution of the magmatic arc along eastern North America or a basin-restricted event marking local fracturing of the continental basement due to distal thrust loading. (Abstract shortened by UMI.)...