Ottawa Embayment Research Articles

Paleokarst in the Lower Ordovician Beekmantown Group, Ottawa Embayment: Structural control inboard of the Appalachian orogen

Two paleokarsts, different in age, character, and origin, occur in dolostones of the Lower Ordovician Beekmantown Group of eastern Ontario, well within the interior of the Laurentian paleoplatform. Surficial to shallow subsurface ( p CO 2 and H 2 S concentrations arising from mixing of continentally derived pore waters with brines derived from dissolution of Beekmantown evaporites. Compared to the region9s geologic history, a pre–late Paleozoic age for formation of the regional paleokarst and mineralization is likely. The two platform-interior paleokarsts demonstrate unexpected links between tectonism, changes in paleohydrological patterns, and porosity development well inboard of the Appalachian orogen.

Hog's Back Formation: a new (Middle Ordovician) stratigraphic unit, Ottawa Embayment, eastern Ontario, Canada

The Hog's Back Formation, exposed in Ottawa, Ontario, is a new stratigraphic unit that disconformably overlies the Rockcliffe Formation and underlies, with apparent conformity, the Pamelia Formation of the Ottawa Group. The Hog's Back Formation is 14.3 m thick at its type section (Prince of Wales Falls, Ottawa) and thickens eastward to 27 m in Russell County, about 40 km southeast of Ottawa. It consists of lower, dark green shales with thin lime-mudstone and sandstone interbeds, and upper thin to thick beds of fine crystalline greenish grey sandy and calcareous dolostones with thin to medium (10–30 cm) pinkish grey, bioclastic packstone and grainstone interbeds. The latter thicken eastward and correlate with the "St. Martin calcarenites" of the Beaconsfield Member, Laval Formation, in the Montréal area. Together, these beds correlate with the Valcour Formation, Chazy Group, in the Lake Champlain area of New York. Conodonts and macrofossils indicate a Chazyan (Llandeilian) age for the Hog's Back Formation. The lower sandstone–shale–carbonate assemblage of the formation indicates sedimentation in peritidal environments, and documents initial regional flooding of nearshore sandy facies of the Rockcliffe Formation. The overlying carbonate facies represent sabkha environments punctuated by storm deposition. The gross shallowing-upward succession of the formation was terminated by renewed flooding associated with another regional transgressive event heralding deposition of the Ottawa Group.

Maturation thermique des roches mères dans les bassins des basses-terres du Saint-Laurent et dans quelques buttes témoins au sud-est du Bouclier canadien

Caradocian source rocks belonging to the Utica Group occur around and overlay the southeastern part of the Canadian Shield from the Ottawa embayment, in the southwestern part of St. Lawrence Lowlands, to the Anticosti Island basin. The kerogen of the source rocks is mainly made up of amorphous organic matter (transmitted light), resembling a solid bitumen framework in reflected light, and contains a small portion of zooclasts (graptolites and chitinozoans). The kerogen is often too mature to have preserved the structure of the original constituents, except those of graptolites and chitinozoans. Otherwise, UV fluorescent colonial alga and tasmanites are observed. In the eastern half of the St. Lawrence Lowlands, stratigraphic and tectonic burial is the main control of source rock thermal maturation. From an area located 50 km west of Quebec City, the estimated vitrinite reflectance ( R o-Std) of the source rock increases in both northeastern (Montmorency Falls) and southwestern (St. Maurice River) directions, from R o-Std of 1.2 to 1.5 and 1.7%, respectively. The source rocks of the Ottawa embayment and Montreal area (western half of the basin) are overmature due to hydrothermal and igneous activities (2.3%< R o-Std<2.8%).. In Chicoutimi and Lac Saint-Jean outliers, the source rocks show reflectances between 0.9 and 1.2%, slightly less than reflectance values in the Quebec City area ( R o-Std between 1.2 and 1.5%). On Anticosti Island, the thermal maturation is related to burial only and increases downdip of the inclined platform, from the northeast toward the southwest ( R o-Std between 0.9 and 2.1%). These observations suggest that the St. Lawrence platform and the southeastern Canadian Shield were buried by at least 3.5 km of Paleozoic strata prior to erosion to Grenville basement. This implies that most potential hydrocarbon fields are presumably also lost by erosion.