The Pleistocene sea-level and neotectonic history of the eastern Solent, southern England

Abstract

In the eastern extremity of the Isle of Wight, near Bembridge, marine interglacial deposits occur at a variety of different elevations. The highest of these, the Steyne Wood Clay, is an estuarine deposit that lies between 38 and 40 m o.d . and rests on Bembridge Marls (Lower Oligocene). The Steyne Wood Clay, which had previously been assigned to the post-temperate substage of a Middle Pleistocene interglacial, has now yielded a diverse coccolith assemblage dominated by Gephyrocapsa oceanica and G. caribbeanica . The absence of both Pseudoemiliania lacunosa , with a last occurrence datum at ca. 0.475 M a B P. and Emiliania huxleyi , with a first occurrence datum at ca. 0.275 Ma B P, suggests deposition during this time interval. The dating of the Steyne Wood Clay is further constrained by palaeomagnetic data, indicating normal geomagnetic polarity, and by amino acid ratios consistent with an early M iddle Pleistocene age. An extended and revised list of Foraminifera and Ostracoda is given, including the description of Leptocythere steynewoodensis sp.nov. The low-level interglacial deposits make up the Bembridge Raised Beach, here formally defined as consisting of high-energy beach gravels, intertidal sands and organic muds, which represent a single fining-upwards sequence. Pollen analysis of the organic muds indicates that these accumulated during the early and late-temperate substages of the Ipswichian interglacial (Ip IIb - III) . Therm oluminescence dates of ca . 115 Ka BP have been obtained from sand lenses within the Raised Beach itself, which also support correlation with the Ipswichian. The Bembridge Raised Beach occupies an altitudinal range of 5-18 M O.D . and thickens rapidly in a westerly direction where it abuts a cliff cut into the Bembridge Marls. Details are given of the composition, morphology and sedimentology of the gravels constituting the Beach, and similarities to recent cuspate foreland and split accumulations are highlighted. A similar origin is proposed for this feature. The upper surface of the Beach has been soliflucted and deposits of matrixsupported gravel, rich in clay, thicken downslope in an easterly direction. This solifluction has been disturbed by cryoturbation. Both the in situ and soliflucted beach are mantled by brickearth, a reworked aeolian silt, which reaches a maximum thickness of 10 m. A Late Devensian age for this unit has been established by thermoluminescence dates in the range 16.0+1.5 to 21.5 + 2 K a BP. Near Lane End, a sedge-peat rich in plant macrofossils and insects occurs between two gravel units. These appear to post-date the Raised Beach and are interpreted as of fluvial origin. The gravel capping the cliffs at Priory Bay, the richest source of Palaeolithic artefacts on the Isle of Wight, occurs between 29 and 33 M O.D. and is also thought to be a fluvial aggradation unrelated to the Raised Beach. The relationship of these marine deposits to those occurring on the adjacent mainland are considered. The Steyne Wood Clay is correlated with the Slindon Sands at Boxgrove, part of the Goodwood-Slindon Raised Beach, which occur at an identical elevation and have produced a similarly diverse coccolith assemblage. Additional palaeontological evidence from Boxgrove suggests that the interglacial deposits should be assigned to a temperate stage falling in the latter part of the ‘Cromerian Complex’. Correlation of the Steyne Wood Clay and Slindon Sands with oxygen isotope stage 9, 11 or 13 seems very probable. Reasons for the occurrence of marine deposits of this age at ca. 40 M O.D.. are considered and it is thought that neotectonic activity is at least partly responsible. Mean rates of uplift of between 5.3 and 15.5 mm ka -1 have been calculated from age estimates for stages 9, 11 and 13 derived from the deep-sea record. However, it is unlikely that the uplift was uniform in either rate or direction. The diverse coccolith assemblages preserved in the Steyne Wood Clay and in the Slindon Sands indicate a full open connection with the marine waters of the central English Channel, and suggests that a thermocline was then present in the Channel at a time when the Straits of Dover were probably closed. The interglacial channel deposits on the modern foreshore of Bracklesham Bay near Earnley have produced a limited coccolith assemblage. Because the altitudinal and palynological differences between these deposits and the Steyne Wood Clay are so great, they are thought to belong to different interglacial stages. The Bembridge Raised Beach is thought to equate with similar deposits on the northern shore of the Solent at Selsey, Stone and West Wittering, which has also now yielded pollen, reported here.