Abstract

The Late Ordovician Trenton Group of New York State and equivalent strata in south-central Canada peripheral to the Québec Embayment form a regional carbonate platform succession within the Taconian foreland basin. The upper and lower group boundaries record geodynamically controlled changes in regional subsidence, whereas a higher-order allocyclic stratigraphy traced south along the foreland interior of the eastern United States may identify shorter-term eustatic controls. A tectonomagmatic event extending across the foreland basin is recorded a few million years prior to the regional collapse of the foreland platform in the Québec Embayment, within the lower C. spiniferus (graptolite) Zone. Platform-margin retreat (∼50 km) in southern Québec is contemporaneous with a shale-rich limestone succession that thins (∼30–10 m) westward over ∼300 km into the Ottawa Embayment, eastern Ontario, west of the Appalachian structural front. This stratigraphic record coincides with catastrophic volcanism within the once distant volcanic arc terranes as recorded by a bentonite traced geophysically from the platform margin to the Michigan Basin. Core from the Ottawa Embayment demonstrates western-directed transport of lithic-bearing ash. The lithic material, representing reworked deep-water sedimentary and/or volcaniclastic sediment, was entrained by eruption of calc-alkaline felsic to basaltic andesitic, and trachyandesitic magmatic sources. The above geological history documents a short period of diminishing influence of subsidence moving into the platform interior—an event that enhanced, briefly, cross-platform oceanographic circulation as recorded by elevated shale abundance and change in macrofaunal assemblages. Short-lived differential subsidence of the continental crust in this region, in response to distal tectonism, may have been predicated by inherited weakness of a shallowly buried intracratonic (>400km) Precambrian fault system, manifest today by the Ottawa-Bonnechère Graben. The resulting thin (<10- to 30-m) stratigraphic record of the tectonomagmatic event is of similar thickness to high-order allostratigraphic patterns within the Taconic foreland interior of the eastern United States. Because there is a heterogeneous distribution of Precambrian fault systems underlying the Taconian foreland interior of eastern Laurentia, differential structural response of the foreland interior can be anticipated. The apparent regional allostratigraphy may contain subtle facies differences along the strike of the foreland basin that reflect this tectonic component.

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