Tertiary rivers: Palaeocene
Fig. 4 Palaeogeography of the Late Palaeocene Thanetian Stage.
Much of what is now Britain became emergent during the early Palaeogene the maximum uplift occurring in the north-west, causing a regional tilt towards the east and south-east. Indeed the form of the modern landmass was already identifiable, in contrast to conditions during the Late Cretaceous submergence. Where it existed, the extensive Chalk cover was being actively dissected by both mechanical and chemical subaerial denudational processes to produce a residuum of flints. Where the substrate beneath the Chalk was exposed it was subjected to active erosion and older rocks were also being removed and transported. These included the crystalline basement rocks of the massif areas of the South-West Peninsula, Scotland, Wales and Brittany which were subjected to block-faulting. The nature of the landscape of these massifs is unknown but it seems likely that they reached significant altitudes or were undergoing continual uplift to judge from the thick sequences of Palaeocene arenites preserved in the adjacent basins. However, much of England was of low relief, with south-eastwards-aligned drainage, encouraged by general subsidence of the south-east (Fig. 4). The derivation of thick weathering mantle material suggests that long exposure rather than rapid uplift was required to develop the considerable volumes of chemically-weathered materials produced during the Tertiary. In the Late Palaeocene, southern central and south-eastern Britain were low-lying and readily inundated by marine transgressions from the east, which resulted in the Palaeogene deposits being laid down across the region. This thick mainly clastic sedimentary sequence was almost certainly accompanied by marked subsidence for millions of years.
The earliest Tertiary deposit in Britain is the marine Thanet Sand Formation (Thanetian), a series of slightly glauconitic sands that rest directly with the eroded transgressive surface on Chalk, restricted to the eastern part of the London Basin but with equivalents in Belgium and the Paris Basin (Fig. 3). This formation is overlain by the Lambeth Group that includes the Woolwich Formation, a series of laguno-marine sediments that pass westwards into the Reading Formation. The latter are a series of clays and associated sediments of fluvial origin that accumulated in a deltaic complex.
At least one major river, sometimes referred to as the ‘Eocene Mississippi’ or ‘Amazon’ but more realistically a proto-Thames system, entered the London Basin in the area of the modern Chilterns from Hertfordshire, Middlesex, Buckinghamshire and Berkshire. Here cross-stratified gravels and sands, accompanied by silty clay lenticular channel fills associated with inwashed plant material and frequent clay-clast breccias, occur. Such breccias could have originated from bank undercutting during channel migration. Exotic clast assemblages, including quartz and lydites, the latter derived from Lower Cretaceous and Upper Jurassic rocks of the north-west are also found. They are associated with mottled clays representing weathered, low-energy floodplain accretionary sediments and locally with lignite units up to 1 m thick. Smaller tributaries may also have contributed to the delta complex from the north, west and even possibly the south-west. These predominantly fining-upward type sequences clearly indicate the occurrence in Reading times ( late Thanetian = c.55 my ago) of at least one substantial, probably actively-meandering proto-Thames river from the NW, possibly rejuvenated by contemporary earth movements. This was transporting pebbly sand to clay-sized material and forming a major delta complex (Fig. 4).
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 clays, deposited by rivers from the west and south-west. This is, in principle, the first evidence of a proto-Solent River system. Heavy-mineral analysis of the sands clearly indicates a possible Armorican contribution at this time but according to Daley 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 predominantly comprises mottled multi-coloured clays; the colouration is thought to result from subaerial weathering. Whilst these sediments may in places be of lagoonal origin, it is generally more likely that they represent floodplain 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. 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 to those of the modern south-eastern USA. Further west at Studland Bay, the Reading Formation comprises fluvial channel-fill current-bedded granular sands containing fragmentary plant material.
It therefore appears that the Thanetian 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 south-east. 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 multi-coloured mottled clays, pedologically-modified and associated lignitic units, and sedimentation typically found in vertically –accreted floodplain sequences, are associated with fining-upward channel fills and point-bar accumulations. The clay-clast breccias typify meandering river sediments in tropical region. Here rivers have cohesive banks, densely-vegetated floodplains, and predominantly transport fines but also coarser sediment during floods. Both the proto-rivers Solent 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.