Abstract

Magnetostratigraphic studies of the late Palaeocene and early to middle Eocene formations in the Hampshire and London Basins indicate that the geomagnetic field maintained a predominantly reverse polarity throughout the period of deposition of these sediments. Eight discrete normal polarity magnetozones have been identified within this dominantly reverse polarity interval. Three of these lie within the Bracklesham Group, three within the London Clay Formation, one in the Oldhaven 'Formation', and one within the lowermost part of the Thanet Formation. Assuming that the magnetozone boundaries represent absolute time planes (except where they correspond with stratigraphic hiatuses), they may be used to provide a time framework for chrono-stratigraphic correlation between sections, and for investigating the dynamics of early Palaeogene sedimentation patterns in this region. The results indicate that the majority of the marine transgressive and regressive events which form the basis of King's (1981) subdivisions of the London Clay occurred very rapidly, and may be regarded as being effectively 'instantaneous'. In marked contrast, the boundaries between the sedimentary cycles of the Bracklesham Group (represented by the Wittering, Earnley, Selsey and Huntingbridge Divisions) are significantly diachronous. In relation to the absolute time planes provided by the magnetozone boundaries it is clear that the Bracklesham Group transgressive events occurred systematically earlier in the east than in the west, as might be expected for westward-directed transgressions. Certain biostratigraphic zonal boundaries within the Bracklesham Group appear also to have been influenced by the changing environmental conditions which accompanied these transgressive-regressive cycles.

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