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

Abstract

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.