The Hudson Bay Basin underlies most of Hudson Bay, with exposed portions on the Northern and Central Hudson Bay Lowland to the southwest, and on Southampton, Coats and Mansel Islands to the north. The thickest section is toward the centre of Hudson Bay where between 6,000 and 10,000 feet of Phanerozoic strata have been variously interpreted by geophysical surveys. The Hudson Bay Basin is separated from the smaller James Bay Basin in the Southern Hudson Bay Lowland by the Patricia Arch. The Bell Arch is the bounding feature on the north. A possible sedimentary basin, the Foxe Basin, may exist north of this arch.
The composite stratigraphic succession in the Northern and Central Hudson Bay Lowland consists, in ascending order, of the Ordovician Bad Cache Rapids Group, with Portage Chute and Surprise Creek Formations; the Churchill River Group, with Caution Creek and Chasm Creek Formations; and Red Head Rapids Formation; and the Silurian Port Nelson, Severn River, Ekwan River and Attawapiskat Formations. The high Silurian Kenogami River Formation, the Devonian Sextant, Abitibi River, Williams Island and Long Rapids Formations, and the Cretaceous(?) Mattagami Formation, are known only in James Bay Basin, but may have their counterparts in the Hudson Bay Basin.
The Ordovician and Silurian are predominantly carbonate, and possibly 2,000 feet thick in the Central Lowland. The latter system contains porous reefs. The Devonian is of mixed lithology, mainly limestone and shale, and outcrops only in the Southern Lowland or James Bay Basin. Regional studies, however, indicate it extends along coastal areas of the Central Lowland and is on the mid-bay shoal in Hudson Bay. Cretaceous(?), known only from the Southern Lowland as a continental coal-bearing sequence, may be present in the Northern Lowland. Geophysical data for the Devonian and possibly younger systems indicate a minimum onshore thickness of 1,600 feet.
On Southampton, Coats and Mansel Islands, Ordovician and Silurian carbonates lithologically similar to their counterparts in the Lowland have been recognized. Ordovician has been found only on Southampton. It approaches 500 feet in thickness and contains an oil shale interval, 50 feet thick, in the uppermost part. Silurian is most widespread, apparently occupying all of Mansel and much of Coats and Southampton Islands. Thickness is uncertain but surface data suggest 1,000 feet or more.