The geology of Poole Harbour

The last 400 million years of the geological history of Poole Harbour and the region adjacent to it are reviewed. This is facilitated by the extensive exploration for oil which has been undertaken by BP in the development of the Wytch Farm oilfield, and the exploration wells made by English China Clay in search of economically viable deposits of kaolinite. The closure in the Carboniferous of the Rheic Ocean lead to the formation of east-west thrusts, now deep beneath southern England, which have determined the structural grain of the region. A series of en echelon thrusts run beneath east Dorset and the Isle of Wight. These thrusts have been reactivated extensionally during the opening of the Atlantic (Jurassic– Cretaceous), and compressionally during the early Alpine Orogeny in the Palaeogene, when collision between Africa and Europe commenced. Hydrocarbon accumulations were formed by burial of organic-rich Early Jurassic sediments on the southern, downthrown, side of a fault. This burial resulted in maturation of oil, which subsequently migrated up the fault plane to become trapped beneath Wytch Farm. Inversion in the Palaeogene, 40– 50 million years ago, created the Hampshire Basin, and caused the uplift of the Chalk ridges of Purbeck and the Isle of Wight. A Proto-Solent drainage pattern, with a catchment to the north and west, thus developed in the Palaeogene. This brought kaolinite from the granites of Devon into the region, and valuable ‘ball clays’ were deposited during the Eocene Period. The Holocene transgression, 10,000 years ago, caused the sea to break through the Chalk ridge joining the Isle of Wight and Dorset, and created the western Solent Channel.