Abstract

In eastern Ontario, a Middle Ordovician disconformity that separates predominantly sandstone of the Rockcliffe Formation from underlying dolomite of the Carillon Formation (Beekmantown Group) has, historically, been considered equivalent to the Sauk-Tippecanoe sequence boundary. The character of the formation contact varies considerably, and includes (1) rare, serrated micropinnacles (centimetres in height); (2) smoothed, irregular to undulose erosional bedforms along shallow (<30 cm) incised channels at least ten's of metres in width; (3) a planar boundary with no obvious evidence of erosion; (4) compaction effects across the boundary, as well as within uppermost Carillon and lowermost Rockcliffe strata; and (5) interbedding of Carillon- and Rockcliffe-like lithologies. The latter two observations suggest that, locally, the disconformity represents a relatively minor gap in time. Local, irregular paleotopography is interpreted as some combination of paleokarst, mechanical erosion, and differential sediment loading. The Carillon-to-Rockcliffe transition, from peritidal carbonates to marine or estuarine sandstones, is punctuated by local, anomalous structural and stratigraphic features: ptygmmatically folded fractures, local disconformities, and deposition, then differential erosion of high-energy fluvial-dominated sediments. Differential erosion, fracturing, and paleokarst are local, anomalous attributes of the pre-Carillon paleosurface as well. Combined, these events bracket Carillon deposition, and are interpreted to reflect local platform-interior tectonism coincident with onset of distal Taconic orogenesis. Platform-interior expression of distant, developing changes in plate-boundary dynamics may be linked to incipient basement reactivation along an underlying, shallowly buried Proterozoic fault system, later manifest as the Ottawa-Bonnechere Graben.

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