^PSRecords of Punctuated Tectonism in Platform-Interior Graben Systems (Ontario, Canada) Far-Flung from Contemporaneous Taconic Orogenesis in the Northern Appalachians*

Sajal Sharma¹, George R. Dix¹, Mario Coniglio², Aicha Achab³, and John F. V. Riva³

Search and Discovery Article #30038 (2005)

Posted September 27, 2005

*Poster presentation at AAPG Annual Convention, Calgary, Alberta, June 19-22, 2005

Click to view poster in PDF format.

¹Carleton University, Ottawa, ON ([email protected])

²University of Waterloo

³I.N.R.S.-E.T.E., Universitè de Quèbec, Quèbec, QC

Abstract

Ordovician basins along the craton-interior of the Ottawa-Bonnechère and Temiskaming graben systems of eastern and northern Ontario, respectively, form an interconnected structural corridor (up to 600 km in length) extending northwest of the Taconic Orogen. Both graben systems preserve cryptic evidence of Late Ordovician tectonic events far-flung from the present structural limits of the orogen, events that help define the changing regional influence and patterns of foreland basin development. In both graben systems, collapse of a regional carbonate platform and formation of local deep-water shale basins occurred by ~452 Ma. In the Ottawa-Bonnechère graben, in eastern Ontario, a K-bentonite punctuates deposition of organic-rich shales within the pygmaeus graptolite Biozone (~451 to 452 Ma). It immediately precedes the onset of distal turbidity flows; and, unlike older platform-hosted K-bentonites in eastern North America, its geochemistry identifies greater incorporation of continental crust within the magmatic source. Whether the bentonite is either regional in extent or is a more local basin-restricted eruptive is uncertain, but its composition and timing suggests it was linked to fundamental changes in Taconian basin architecture. Within the Temiskaming graben, a seafloor hydrothermal event is recorded within slightly younger shales (manitoulinensis graptolite Biozone; ~449-451 Ma) by a microfossil-rich (conodont, chitinozoan) phosphatic sandy-shale, which contains rare (<5%) fluorite and common (~60 %) microcrystalline ferroan dolomite. Hydrothermal fluids briefly stimulated productivity, and likely arose along basement faults controlled by Taconian crustal stress. Hydrothermal activity may help explain some dolomitization of Ordovician and Silurian strata in this basin.