A new geological framework for the Middle Ordovician Carillon Formation (uppermost Beekmantown Group, Ottawa Embayment): onset of Taconic foreland deposition and tectonism within the Laurentian platform interior

Abstract

The Carillon Formation underlies the Ottawa Embayment, a craton-interior extension of the regional Laurentian paleoshelf, defined today by the Ottawa–Bonnechere Graben. Facies within the embayment during the Middle Ordovician identify a mixed-sediment (carbonate, siliciclastic), tide-influenced, peritidal and schizohaline basin, host to a mosaic of dolomitic tidal mudflats, shallow lagoons or ponds, and channels. Skeletal metazoans were rare, but bioturbation increased with episodic influx of less saline water carrying sand and mud, possibly in response to regional climate change. Stratigraphic analysis reveals that the formation coincides with a third-order eustatic sea level cycle, but higher order stratigraphic patterns show no obvious eustatic link. Instead, a stepwise onlap geometry coincided with local faulting that produced widespread syndepositional sediment deformation. In particular, stages of faulting and post-Carillon erosion are mapped along the present-day trace of a northwest–southeast-oriented lineament, the Gloucester Fault, within the present graben. Other present-day northwest–southeast-oriented faults of the graben may also possess Middle Ordovician histories. Platform-interior tectonism and development of the Carillon Formation coincided with the onset of Taconic deformation along the outer paleoshelf (western Newfoundland). The formation defines the initial stage of foreland basin sedimentation in the embayment. Platform-interior faulting identifies a predisposed structural weakness associated with underlying Neoproterozoic structure, possibly enhanced by a regional structural connection (an ocean-to-onland transform fault system) with the distal (~1200 km), convergent, plate boundary.