The Sauk-Tippecanoe Megasequence Boundary in an Interior Structural Corridor (Ottawa Embayment) of the Great American Carbonate Bank

Abstract

The Ottawa embayment forms an approximately 200-km (125-mi) structural indentation along the strike of the Precambrian Grenville province in central Canada. It follows the trace of a Neoproterozoic fault system that was reactivated episodically during the Phanerozoic. The Potsdam and Beekmantown Groups comprise the local Sauk megasequence. The former consists of a Neoproterozoic to Cambrian disconformable transition from immature feldspathic to very mature quartz arenites, culminating in restricted marine depositional conditions. The Beekmantown Group overlies the older succession with disconformity, and its lower and middle formations record a net onlap of increasingly deeper paleoenvironmental conditions; from peritidal (Theresa Formation) to shelfal (Beauharnois Formation) sandstone and dolostone facies, within a developing narrow seaway. The youngest formation, the Carillon Formation, consists of poorly fossiliferous seismite-bearing dolomudstone and abundant shale that abruptly onlap locally karstified and faulted Beauharnois strata. The Carillon Formation is early Middle Ordovician in age, post-Rangerian, but not younger than the Histiodella holodentata conodont Biozone. The formation contains abundant evidence of synsedimentary tectonisn that was coincident with far-field (1000 km [620 mi]) events along the St. Lawrence promontory, where uplift and deformation produced a prominent Sauk-Tippecanoe megasequence disconformity. Notable differences exist in the Middle Ordovician sea level record within the embayment compared with that along the Laurentian shelf margin, with deposition and syndepositional tectonism spanning formation of the outer-shelf Sauk-Tippecanoe megasequence boundary. A stratigraphic interval, the Carillon Formation, defines the Sauk-Tippecanoe transition within this interior structural corridor of the great American carbonate bank.