Upper Ordovician stratigraphy of Southampton Island, Northwest Territories

Abstract

The middle to late Caradoc Bad Cache Rapids Formation of Southampton Island consists of three "members": a locally present thin siliciclastic unit, overlain by a highly fossiliferous wackestone, capped by a mudstone, all deposited in a broad, shallow shelf setting. This formation is, possibly unconformably, overlain by the early Ashgill Boas River Formation, comprising a lower carbonate mudstone and an upper oil shale, deposited in a local, shallow subtidal, restricted basin during a transgression. Early to middle Ashgill strata of the Churchill River Formation are made up of a lower unit with coral-bearing wackestone, representing a return to normal marine conditions, and an upper unit of noncoralliferous wackestones, indicating a regional shallowing. The latter is capped by the Sixteen Mile Brook oil shale. The middle to late Ashgill Port Nelson Formation contains evaporite casts and algal laminites, suggesting a sabkha environment. Mound-like structures in this formation may have resulted from solution collapse brought about by the removal of evaporites after lithification.