history of the major rivers of southern Britain during the Tertiary, The

Journal of the Geological Society, Nov 2003 by Gibbard, P L, Lewin, J

Early Eocene

The Eocene opens with renewed significant marine transgression from the cast resulting in deposition of Thames Group sediments, the thickest and most extensive of which is the London Clay Formation (Fig. 3), deposited during the Early Eocene Yprcsian Stage (Daley 1999; Anderton 2000). During this period, submergence was most extensive towards the west and linked the London and Hampshire-Dieppe basins (Fig. 5) (Davis & Elliott 1957; King 1981). At least five transgression-regression cycles are represented, reflecting periods of sea-level rise followed by shallowing and coastline progradation (Flint 1988; Anderton 2000). Thin, marginal sand and silt-dominated sediments are rare in the London Basin but are found in the Flampshire-Dieppe Basin in Dorset and western Hampshire, where they were subjected to pedogenesis.

During deposition of the London Clay, the regional climate was very warm, the surrounding land to the north, west and south supporting dense tropical to subtropical and warm-temperate forests, with mangrove swamps on the coasts (Reid & Chandler 1933; Anderten 2000; Collinson & Cleal 2001b). Inland an 'upland flora' may have flourished, but temperate elements were also well represented in the shallow marine sediments (Daley 1972). The overall character of the flora shows affinities with the present-day Malay Pensinsula, characterized by high levels of precipitation and environmental stability (Daley 1972).

The nature of the land surface during London Clay times is poorly known but appears to have continued to be subdued with wide plains and no mountains in Wales (e.g. Wells & Kirkaldy 1966; Daley 1972). The major erosion phases seen earlier had ceased and instead had given way to overall slow degradation of exposed surfaces subjected to intense chemical weathering. This is seen in the vast volumes of clay, which may have been produced by extended earlier weathering and supplied to the basinal areas by the rivers from the north and west. Only limited evidence of actual channel systems is known at present. However, continual influx of river water has been invoked to explain the low-diversity foraminiferal assemblage in the Hampshire-Dieppe Basin by Murray & Wright (1974). In addition, incised fluvial channels, formed during local sea-lcvcl lowstands and then infilled by estuarine deposits during subsequent marine transgression, are recorded from the Portsmouth and Whitecliff members across the basin (Flint 1988). Davis & Elliot (1957) recognized that the basal pebble beds of the Hampshire basin region were derived from the SW, presumably by rivers (Fig. 5). Those workers envisaged several smaller streams flowing northwards.

Late Early-early Mid-Eocene

London Clay deposition was brought to an end by a rapid expansion of fluvial delta complexes towards the east in the late Ypresian-Lutetian (early Mid-Eocene) (Fig. 6). In the London Basin this transition is signalled by the sand-silt alternations of the claygatc Beds and the subsequent accumulation of the overlying Virginia Water-Bagshot Formation sands (Fig. 3). The latter are marine in the east, but to the west represent a vast delta complex (Wells & Kirkaldy 1966; Anderton 2000). The sediments are similar to the Reading deposits, consisting of crossbedded sands, with thin clay or silt partings and thin pebble beds, becoming considerably coarser towards their western limit. This transition from the London Clay to the overlying sands may reflect either depression of the basin, or uplift of the hinterland further west and NW, which encouraged river incision and potentially increased discharges of coarser terrigenous sediments. No later Paleogene fluvial or estuarine sediments are found in the London Basin.