Stratigraphy of the Middle to Upper Ordovician foreland-basin succession, Ottawa Embayment: emphasis on the Turinian-Chatfieldian succession

Abstract

New contributions presented define the character of the depositional system and stratigraphic architecture of Middle to Upper Ordovician foreland succession of the Ottawa Embayment, central-east Canada. The Ordovician foreland succession is the local expression of the cratonic Tippecanoe I Megasequence that extends across southern Laurentia, developed within a retro-arc basin. This study investigates craton-interior intrabasinal response of sedimentation to tectonic, climatic, depositional and eustatic controls, and compares the stratigraphic fabric with equivalent strata along strike to the south in the much larger Appalachian Basin. The foreland stratigraphy is divided into eight (I-VIII) transgressive-regressive depositional sequence comprise platform succession and one (IX) regressive sequence forms basin succession. This define a tectonic cycle of net platform deepening over ~15 my, and subsequent basin-fill spanning ~3 my. This study reaffirms placement of the Sauk-Tippecanoe I (S-T) Megasequence boundary within the upper Beekmantown Group. Examination of the upper Turinian-Chatfieldian (Upper Ordovician) succession in the embayment reveals three stages of carbonate-platform development of which the intermediate stage (Watertown Formation, L'Orignal Formation, coeval Rockland Formation and lower Hull Formation) illustrates more intrabasinal (and structural) control on sedimentation compared to bounding stages that document regional continuity with extrabasinal stratigraphy. The upper Turinian-Chatfieldian interval was also examined using δ13C chemostratigraphy to provide higher resolution of stratigraphic correlation. Four positive excursions (E1 to E4) are mapped through the Ottawa Embayment into two outliers to the northwest along the Ottawa-Bonnechere graben. Chatfieldian Excursion E3 is the local expression of the regional (if not global) Guttenberg δ13C excursion (GICE). Improved resolution of the Guttenberg δ13C excursion in the embayment demonstrates amalgamated (E3 and E4) excursions across disconformities. Lateral variation in the magnitude of δ13C excursions reflect local modulation of regional (or global) events in response to changes in circulation, productivity, and accumulation rates. In comparison with laterally equivalent sedimentary basins in southern Laurentia, the Middle to Upper Ordovician foreland succession of the Ottawa Embayment illustrates an elevated sensitivity to syndepositional structural control. Tectonism played an important role on base level relative to an otherwise net eustatic rise through the Middle and Late Ordovician.