APPROACHING A SEQUENCE STRATIGRAPHIC FRAMEWORK FOR THE EARLY PALEOZOIC OTTAWA EMBAYMENT: PATTERNS OF BASIN-FILL AND TECTONISM IN THE PLATFORM INTERIOR

The Late Cambrian to Late Ordovician succession within the Ottawa Embayment preserves eleven stratal packages that illustrate an evolutionary succession of basin fill - in ascending order: cyclic (onlap-offlap), shallowing upwards, and stacking of successive deeper water facies - which comprises a series of carbonate/siliciclastic platforms that extend through passive (PST) first, then foreland (FST) basin settings. Such changing patterns of rhythmic stratigraphy characterize the platform interior's response to changes in accommodation space, subsidence, and sediment supply with tectonic evolution of the Laurentian margin. In detail: PST1: a basal, poorly defined, succession of arkosic sandstones and conglomerates (Covey Hill Fm) overlain by clean marine to fluvial quartzose sandstones (Nepean Fm). PST2 and 3: Theresa Fm (LCamb-EOrd), disconformably bounded, and the Beauharnois Fm (E-MOrd) of the Beekmantown Gp preserve onlap-offlap successions, the latter formation having more offshore dolomitic facies. PST4: Carillon Fm (MOrd) reveals paleoseismic events (fractures). Its upper contact is Sloss's Sauk-Tippecanoe sequence boundary, but new data illustrate a possible broader tectonic basin transition. FST1 and 2 (MOrd): Rockcliffe Fm, then the Hog's Back Fm both define unconformity bounded platform successions that grade laterally into open shelf carbonates in the Montreal region. FST3: Pamelia Fm (lower Ottawa Gp; MOrd) contains a shallowing upward, shale-to-limestone-to-dolostone, succession that thins dramatically onto a likely structural block in the Montreal region. FST4-8: Remaining formations (M-LOrd) of the Ottawa Gp define punctuated increases in water depth, although each unit reveals little vertical change. Platform strata were buried by synorogenic siliciclastics (FST9-11: Billings, Carlsbad, Queenston formations) preserving a basin-fill succession from anoxic bathyal to well oxygenated peritidal conditions. This is a work in progress: ongoing studies will better define the succession of platforms, boundary types, and shifts in facies of depositional sequences.