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Evolution of a Lower Paleozoic Continental Margin Carbonate Platform, Northern Canadian Appalachians

Noel P. James, Christopher R. Barnes, Robert K. Stevens, Ian Knight

The western margin of the northern Appalachian orogene and adjacent craton is composed of a lower Paleozoic, low-latitude carbonate platform which originally lay along the northern margin of Iapetus Ocean. Parts of the platform interior are now exposed in Quebec, much lies beneath the Gulf of St. Lawrence, and the outer shelf and deep-water deposits crop out in western Newfoundland. The shelf break and upper slope are nowhere exposed, but their nature has been determined from numerous clasts in sediment gravity flows redeposited on the lower slope and now stacked in allochthonous thrust complexes.

Four separate phases recording platform growth and demise can be differentiated, reflecting the interplay between tectonics, eustasy and the evolving lower Paleozoic biota. Phase 1, Preplatform Shelf, reflects initial siliciclastic deposition on rifted crystalline basement followed by a short phase of carbonate sedimentation dominated by archeocyathan buildups and ooid shoals that was terminated by offlap of thick quartz-arenites. Phase 2, Narrow, High-Energy Platform, is characterized by mixed siliciclastic-carbonate peritidal sedimentation throughout, manifest as three grand cycles. Contemporaneous deep-water sediments comprise basal welded conglomerates overlain by quartzose carbonate turbidites. Phase 3, Wide Low-Energy Platform, is an onlap package of muddy fossiliferous subtidal and peritidal carbonates arranged in the form of two unconformity-bounded megacycles. The adjacent deep-water slope was a narrow belt of debris flows and wide carbonate-shale apron, deposited in the lower part of an oxygen minimum zone. Phase 4, Foundered Platform, documents the initial uplift, faulting, subsidence, and fragmentation of the platform in a sequence of peritidal to subtidal to deep-water carbonates, reflecting the initial stages of the Taconic orogeny. The whole sequence is buried by synorogenic flysch and overlain by thrust sheets containing phases 1, 2, and 3 as well as oceanic lithosphere.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.