| Résumé | (disponible en anglais seulement)
As part of the Geomapping for Energy and Minerals Program of Natural Resources Canada in northern Canada, multidisciplinary studies have led to a reassessment
of the hydrocarbon potential of the Hudson Platform. The depositional history of the Hudson Platform represents the filling of the large, interior, continental Hudson Bay Basin and of the adjacent, but smaller basins, known as the Moose River, Foxe
and Hudson Strait Basins.
Of the biostratigraphic studies hitherto carried out, the chitinozoan studies have led to a refining, by means of new and revised conodont data, of the previously established stratigraphic framework. This has enabled us
to harmonize the stratigraphic nomenclature of the Early Paleozoic succession.
The Middle-Upper Ordovician to Upper Devonian succession of the Hudson Platform has a maximum thickness of about 2500 m in the centre of the basin. Middle Ordovician
strata are only known in the Foxe Basin.
During the Paleozoic, the Hudson sedimentary succession was driven by major sea-level variations of shallow and confined, interior, continental sea that developed on Laurentia at equatorial latitudes under
arid conditions. Transgressive episodes are represented by shallow limestone and deeper dark shale settings, while regressive episodes are indicated by gaps in the sedimentary succession and by shallow dolomite, evaporite and reef settings.
As the
regressive depositional settings of the succession were not favorable to chitinozoans (a group of extinct, marine, organic-walled microorganisms), the sampling was restricted to the more favorable, argillaceous shaly limestone and dark shale
intervals representing the major, transgressive pulses of the interior, cratonic sea. Notwithstanding the discontinuous sampling, an Early Paleozoic chitinozoan succession of the Hudson Platform can be formulated from 136 productive core, cuttings
and hand rock samples. The samples are from two dark shale horizons of the Amadjuak Formation (Foxe Basin) and from coeval strata of the Hudson Bay Basin, now constituting the Manitoba and Ontario Lowlands and the Hudson Bay offshore.
By
integrating the chitinozoan information obtained from the Hudson Platform succession to those from Eastern Canada and the U.S. mid-Continent, it has been possible to document a total of four Late Ordovician, two Early Silurian and one Middle Devonian
chitinozoan assemblages.
The late Chatfieldian-early Edenian chitinozoan associations recovered from the Amadjuak Formation are primarily characterized by the constant and abundant occurrence of small to medium size, hairy, finger-shaped forms of
the genus Acanthochitina. These are succeeded by specimens of Hercochitina duplicitas.
The Edenian-early Maysvillian associations first encountered in the Bad Cache Rapids Group are recognizable by the constant occurrence of Belonechitina
seriespinosa, Spinachitina bulmani and forms of Belonechitina aff. micracantha, the latter being characterized by a well-developed, serrated aperture.
Late Maysvillian- early Richmondian associations are reported from the upper Bad Cache Rapids
Group (Portage Chute Formation) and the Boas River Shale. They are characterized by Belonechitina cf. schopfi americana, Cyathochitina hyalophrys, Acanthochitina agrestis, Eisenackitina ripae and blunt forms of Hercochitina similar to H.
normalis.
The mid- to late Richmondian associations extracted from the Churchill Group (Caution Creek and Chasm Creek formations) are characterized by the occurrence of Hercochitina normalis, Parachitina curvata and Tanuchitina laurentiana in
association with the A. agrestis, Cy. hyalophrys, Cy. vaurealensis, H. crickmayi and H. grandispina productive levels.
The Early Silurian chitinozoan assemblages documented from the Severn River and Ekwan formations are of Aeronian and Telychian
or late Telychian- early Wenlockian age based on the presence or absence of Belonechitina visbyensis in association with Conochitina edjelensis, C. proboscifera and Eisenackitina dolioliformis, which have a longer stratigraphic range (Aeronian to
early Wenlockian).
Finally, the youngest chitinozoan associations are of Middle Devonian age. They come from a few cuttings in the interval extending from the Stooping River to William Island formations. The assemblage is typical of the Eifelian
and Givetian chitinozoan associations of the American mid-Continent. It consists of Ancyrochitina cornigera, Eisenackitina aranea, E. inflata, Fungochitina callawayensis and F. spinosa.
The positive results obtained from this first regional
chitinozoan investigation of the Early Paleozoic Hudson sedimentary succession encourage one to apply a more refine sampling to improve this preliminary chitinozoan zonation and to calibrate it by integrating it with the abundant conodont data and
the sparse graptolite data. The resulting stratigraphic framework will be useful to future petroleum exploration programs of the cratonic sedimentary basins related to the Hudson Platform. From a more global perspective, the chitinozoan results
obtained so far from the Hudson Platform will contribute to a better understanding of the regional biodiversity changes of the margins and the interior, continental basins of the Laurentian paleocontinent during the Paleozoic. |