Eocene sedimentation and tectonics in the Hampshire Basin

Abstract

Sedimentary evidence in the Eocene (Cuisian-Lutetian) of the Hampshire Basin indicates important intra-Eocene movement on the Isle of Wight and Purbeck Monoclines, and on the Ridgeway Fault. Evidence for syn-depositional movement includes Jurassic and Cretaceous chert and flint pebbles in the Eocene, the distribution of which suggests derivation from both E and W. At Whitecliff Bay, two units of cross-bedded sand have northward-directed palaeocurrents and may have been deposited as localized fans of debris reworked from earlier Tertiary sediments on the upwarped side of the monocline. Three units of laminated muds are intercalated with marine sediments in the eastern part of the basin and suggest periods of low salinity, tideless conditions. These episodes are attributed to the periodic isolation of the Hampshire Basin from the fully marine Dieppe Basin to the SE. This may have been the result of intermittent movement on a tectonic barrier to the SE of Selsey. The onset of uplift and subsequent exposure of the Chalk along the southern margin of the basin proceeded unevenly from ?early Cuisian to late Lutetian times. Movement on the Portsdown Anticline probably occurred over a similar period. The Isle of Wight and Purbeck Monoclines are the superficial expressions of faults in the Palaeozoic basement, and not the result of lateral ‘Alpine’ pressures. A minor series of syn-depositional folds trend NW-SE across the basin and may reflect a component of sinistral strike-slip on basement faults.