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

Reading facies-type sediments also occur in the Hampshire-Dieppe Basin (Fig. 3). Here they comprise thin transgressive pebbly sands, resting on Chalk, overlain by sands and mottled elays (Hester 1965; Anderton 2000), deposited by rivers from the west and SW. This is, in principle, the first evidence of a proto-Solcnl River system. Heavy-mineral analysis of the sands clearly indicates a possible Armorican contribution at this time (Morton 1982) but according to Daley (1972) they are unlikely to be of fluvial origin because 'the latter was periodically separated from Britain'. Deposited during regression of the sea, the Reading Formation of the Isle of Wight, the central Channel and mainland localities such as Felpham (Bone 1986; Collinson & deal, 2001a) predominantly comprises mottled multicoloured clays; the coloration is thought to result from subacrial weathering (Buurman 1975, 1980; Daley 1999). Although these sediments may in places be of lagoonal origin (Ellison 1983), it is generally more likely that they represent fioodplain vertical accretion complexes that have been periodically affected by pedogenesis to produce hydromorphic gley soils of fluvial to fluviomarine origin. These soils formed under a warm climate with a distinct rainy season (Buurman 1975, 1980). Associated channel-fill sandy clay breccia and lignite are known from Sussex and the Central Channel. At the former a diverse flora, including trees in growth position, indicates a dense floodplain swamp forest of warm, seasonally humid climate, comparable with those of the modern southeastern USA (Collinson, in Bone 1986; Collinson & Clcal 200Ia). Further west at Studland Bay, the Reading Formation comprises fluvial channel-fill current-bedded granular sands containing fragmentary plant material (Daley 1999).

It therefore appears that the Thanctian was the first period when substantial evidence of significant fluvial activity is represented in both the Hampshire-Dieppe and London basins (Fig. 4). Palaeocurrents and provenance indicators suggest drainage alignment towards the SE. In both regions the rivers seem to have adopted flow channels dominated by sands with clay-plug type abandoned channel fills and notably clay-clast breccias in the London region. The multicoloured mottled clays, pcdologically modified and associated lignitic units, and sedimentation typically found in vertically accreted floodplain sequences, arc associated with fining-upward channel fills and point-bar accumulations. The clay-clast breccias typify meandering river sediments in tropical regions (Miall 1996). Here rivers have cohesive banks, densely vegetated floodplains, and predominantly transport fine material but also coarser sediment during floods. Both the proto-rivers Soient and Thames and others were already present at this time. Their preservation and disposition, in the uppermost parts of the sequence, indicate that they may have been extending their courses seaward to follow the regressing sea.

Eocene