Abstract

The Middle to lower Upper Ordovician (Chazyan) platform succession of eastern North America extends landward of the ancient Quebec Embayment along the Laurentian margin, part of a foreland basin peripheral to an arc-collisional plate boundary. The platform grades from seaward reefal carbonate (Chazy Group, Champlain Valley) into platform- and foreland-interior tide- and wave-dominated siliciclastics (Rockcliffe Formation) and restricted peritidal muddy carbonate (Hog's Back Formation). The latter two formations occur in the sedimentary Ottawa Embayment, which overlies the trace of a Neoproterozoic rift. Prominent tectonostratigraphic surfaces bound and subdivide the Chazyan platform-interior succession, and are characterized by abrupt marine transgressions, local synsedimentary faults, and seismogenic beds. The depositional history includes: (1) accumulation of shallow-marine siliciclastics (Rockcliffe Fm) across a differentially faulted paleoplatform; (2) abrupt regional retrogradation (∼ 100 km) of this siliciclastic system, replaced by an intraplatform shale basin (lower Hog's Back Formation); (3) basin fill and development of a low-energy muddy carbonate platform interior (upper Hog's Back Formation) coincident with regional fall in sea level and change in subsidence along the Laurentian margin; and (4) abrupt high-frequency base-level changes preceding regional onlap of Mohawkian (Turinian) shale contemporary with local synsedimentary deformation and rapid subsidence along the distal arc-collisional plate boundary. Structurally controlled sedimentation patterns in the platform (and foreland) interior document episodic tectonism coincident with relatively sustained deposition along the outer platform (the Chazy Group). This difference is interpreted to be a response to foreland-interior flexuring, modified by local reactivation of the Precambrian rift, that was coincident with preferential subsidence at seaward sites more proximal to structural loading along the arc-collision plate boundary.