Abstract The Lower–Middle Ordovician Beekmantown Group of the St. Lawrence Lowlands (eastern Ontario and southwestern Quebec) consists of basal inner-shelf dolomitic sandstones (Theresa Formation) and inner- to middle-shelf carbonates (Beauharnois and Carillon Formations). The stratigraphic relationship of the three formations has been considered in the past as a normal vertical succession. Integration and reinterpretation of the existing lithostratigraphic and biostratigraphic data support a new chronostratigraphic relationship, particularly between the Theresa and Beauharnois Formations, as well as correlations with the outer-shelf deposits preserved in the Philipsburg tectonic slice (PTS). Variations in the lithostratigraphic pattern are pronounced along a cross sectional depositional profile from inner shelf to outer shelf. The data demonstrate that the Beekmantown Group is thickest in the region south of Montreal and gradually becomes thinner toward eastern Ontario (Ottawa embayment) and the region north of Montreal (Laurentian highs). The lower boundary of the Theresa Formation is diachronous, becoming younger toward the craton. In the region south of Montreal, Theresa Formation sandstones are temporally equivalent to the lower part of the middle-shelf carbonates assigned to the Ogdensburg Member of the Beauharnois Formation. The Theresa Formation sandstones pinched out toward the outer shelf and replaced by Ogdensburg-like facies. The latter apparently merges with the outer-shelf carbonates of the Wallace Creek and Morgan Corner Formations. The outer-shelf strata of the Hastings Creek and Naylor Ledge Formations correlate with the upper part of the Ogdensburg Member. The Naylor Ledge Formation is truncated by the Sauk-Tippecanoe unconformity and overlain by the lower Whiterockian (?Rangenian) Luke Hill Formation. The onset of the Sauk-Tippecanoe unconformity is older in the outer-shelf rocks of the PTS and becomes younger toward the inner shelf, where it separates the middle Whiterockian Carillon Formation from the Chazyan strata of the St. Lawrence Lowlands.

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 synde-positional 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.