Late Ordovician-Early Silurian conodont biostratigraphy and thermal maturity, Hudson Bay Basin

Abstract

Abstract Hudson Bay Basin is one of the largest Palaeozoic sedimentary basins in North America. It was explored for hydrocarbon resources, at a reconnaissance scale, more than 30 years ago, although the exploration potential remains largely untested. The lower part of the basin succession comprises approximately 600–1040 m of Upper Ordovician (Bad Cache Rapids and Churchill River groups and Red Head Rapids Formation) and Lower Silurian (Severn River, Ekwan River, Attawapiskat and Kenogami River formations) strata. These formations mainly comprise carbonate rocks consisting of alternating fossiliferous limestone, evaporitic and reefal dolostone, and minor shale. A study of 4500 conodonts from 390 conodont-bearing samples from continuous cores and well cuttings from six exploration wells in the Hudson Bay Lowlands and offshore area has revealed 50 species representing 28 genera. The conodont studies have significantly improved our understanding of the Early Paleozoic geology by providing: 1. clear definition of conodont zones and their stratigraphic ranges. Seven zones are established for the Upper Ordovician–Lower Silurian interval, namely the Belodina confluens, Amorphognathus ordovicicus, Rhipidognathus symmetricus, Ozarkodina elibata, Kockelella? trifurcata and Distomodus staurognathoides interval zones, as well as the Pterospathodus celloni–P. eopennatus Assemblage Zone. 2. precise biostratigraphic control for the different formations. Upper Ordovician formations are dated as late Caradocian–late Ashgillian with the Maysvillian or part of Maysvillian and Gamachian missing: Lower Silurian formations are dated as early Rhuddanian–middle Telychian, Llandovery. 3. recognition of sea-level events based on the stratigraphic distribution and known ecologic partitioning of key conodont species in wells representing a shallow to deep gradient. 4. definition of the position of the Ordovician–Silurian boundary, typically associated with the global hiatus created by the terminal Ordovician glaciation. Most of the conodonts from the six wells studied have a Colour Alteration Index (CAI) value of 1, indicating little alteration of organic matter and that the strata have not reached burial temperatures greater than 80°C. However, slightly higher CAI values are recorded from the deepest part of the wells in the offshore area of the basin, indicating burial temperatures just within the oil window. The Late Ordovician Boas River oil shale, which is inferred to have significant potential as a source rock and occurs at the surface along the northern part of the basin, does not appear to be present in the wells.