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

The Tertiary sequences in southern Britain are summarized in Figure 3.

Paleocene

Much of what is now Britain became emergent during the early Paleogene (Daley 1999; Anderton 2000); the maximum uplift occurred in the NW, causing a regional tilt towards the east and SE (Murray 1992). Indeed, the form of the modern landmass was already identifiable (Lovcll 1977), 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 SW 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 Paleocene arenites preserved in the adjacent basins (Dalcy 1999). However, much of England was of low relief, with southeastward-aligned drainage (Murray 1992), encouraged by general subsidence of the SE (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 Paleocene, southern central and SE Britain were lowlying and readily inundated by marine transgressions from the east (Murray 1992), which resulted in the Paleogene deposits being laid down across the region. This thick, mainly clastic sedimentary sequence was almost certainly accompanied by marked subsidence for millions of years (Pomcrol 1973).

The earliest Tertiary deposit in Britain is the marine Thanct Sand Formation (Thanetian), a series of slightly glauconitic sands that rest directly on 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, which 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 (Wells & Kirkaldy 1966; Anderton 2000).

At least one major river, sometimes referred to as the 'Eocene Mississippi' or 'Amazon' (e.g. Wooldridgc & Gill 1925; Wooldridge & Ewing 1935; Wells & Kirkaldy 1966) 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 (e.g. Crane & Golding 1991). Such breccias could have originated from bank undercutting during channel migration (Wells & Kirkaldy 1966). Exotic clast assemblages, including quartz and lyditcs, the latter derived from rocks of Early Cretaceous and Late jurassic age of the NW (Wooldridge & Gill 1925; Wooldridge & Ewing 1935), are also found. They are associated with mottled clays representing weathered, low-energy floodplain accretionary sediments and locally with lignite units up to l m thick (Hester 1965). Smaller tributaries may also have contributed to the delta complex from the north, west and even possibly the SW. These predominantly fining-upward type sequences clearly indicate the occurrence in Reading times (late Thanetian, c. 55 Ma ago) of at least one substantial, probably actively meandering proto-Thames river from the NW (Hester 1965), possibly rejuvenated by contemporary earth movements. This was transporting pebbly sand to clay-sized material and forming a major delta complex (Fig. 4).