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Coeval Tectonism and Epeiric Transgression on the Early Paleozoic Laurentian Platform Recorded by Strata of the Potsdam Group in the Northwestern Ottawa Embayment


In the Ottawa Embayment (OE) the Potsdam Group records Mid Cambrian to Early Ordovician terrestrial to marine siliciclastic deposition preceding westward platform-wide marine transgression. Many post-Ordovician faults dissect the modern basin, but it is unclear the role, if any, faulting had during Potsdam sedimentation and early transgression. Recent studies of the Potsdam in the northwestern OE, however, show distinct lateral changes in its isopach and constituent lithofacies across two regional normal faults. Based on these differences the ancient northwestern OE is subdivided into two sub-basins, a northern sub-basin (NsB) east of the N-S Gloucester Fault (GF), a southern sub-basin (SsB) south of the E-W Rideau Fault (RF) with an uplifted area ∼3500km2 in between. The Potsdam, irrespective of location in the northwestern OE, can be divided into three sequences (S1, S2, and S3) bounded by sequence boundaries and correlated across the northwestern OE. In the NsB the succession is thickest and each of the three sequences consists of fluvial (±eolian) strata overlain by marginal to fully marine strata, recording repeated lowstands and marine transgressions. In the SsB the three sequences are comparatively thin and consist of variably preserved eolian and fluvial strata (S1, S2) overlain by transgressive marine strata (S3). Fluvial strata in the NsB and SsB are mineralogically immature, especially near faults, regardless of stratigraphic position. Along the intervening uplifted area S1 is only locally preserved in the south and S2 forms a thin package in the north; however, S3 forms a widespread sheet of marginal marine and eolian facies across this area coeval with fully marine deposits in the NsB and SsB. Penecontemporaneous deformed beds in S3 strata west of the GF are interpreted to be seismities. The Potsdam-Beekmantown contact is a conformable lithological contact in the SsB and distal NsB but is an erosional discontinuity over the intervening uplifted area. The stratigraphy of the Potsdam Group in the northwestern OE indicates that faults were active on the Laurentian Platform before and during Potsdam deposition. Syn-depositional movement is indicated by structural deformation and erosion of the lowest sequence, seismities near a regional fault, mineralogically immature (first-cycle) fluvial deposits near faults and the persistence of fault-generated topography through the sedimentation of three sequences and into the final platform transgression.