The Celtic Sea basins include the North Celtic Sea basin, Fastnet basin, St. George's Channel basin, Cardigan Bay basin, South Celtic Sea basin, and Bristol Channel basin. They form a series of linked, failed-rift basins.

The tectonic and sedimentary Mesozoic history of the area can be closely related to the staged opening of the North Atlantic. The Celtic Sea basins show gross similarities to other North Atlantic basins in orientation and timing of development.

Three discrete phases of extension and fault-controlled subsidence can be recognized, each followed by a period of thermally controlled passive subsidence:

Table

Reactivation of northeast-southwest Caledonian and east-west Variscan structural features is proposed as the major control on basin development. The deep pre-Mesozoic structures presumably interacted with the extension vectors associated with the opening of the North Atlantic to produce the complicated tectonic patterns seen within individual Celtic Sea basins.