Tertiary Rivers: Palaeogeographical setting
At the beginning of the Palaeogene Britain stood at latitude 40°N, 12°S of its position today. The Earth’s climate, which had been extremely warm during the Cretaceous ‘hot-house’, continued to be both warm and without the extreme fluctuations of the later Quaternary. An overall, long-term trend of cooling climates through the Tertiary is first seen in changing floral assemblages from late in the early Eocene culminating in the warm temperate climate of the late Neogene.
Immediately before the Palaeogene, the British region was submerged beneath the shallow Late Cretaceous Chalk sea. Some areas probably remained emergent, as in Wales, Scotland and Fennoscandia, but were of low relief. Emergence became more general by the Maastrichtian (Late Cretaceous) and by the Danian (Early Palaeocene) was widespread. This change was accompanied by a replacement of the longstanding deposition of marine carbonates by clastic sediments. This is attributed to increased erosion of both the emergent and submergent regions consequent on the widespread uplift associated with the opening of the North Atlantic region in late Palaeocene times, mentioned above. Compared to this Late Cretaceous / Early Tertiary uplift and erosion, later Oligocene and Miocene events appear to have been relatively mild, regionally restricted and related to basin compression and inversion.
From the beginning of the Tertiary, Britain was surrounded by depositional basins. To the west the Atlantic Ocean- Greenland Sea basin began to open, with its periodic connections to the Channel, Paris and Western Approaches basins in the south. The long-established North Sea basin, already referred to, lay to the east. Finally, the extension of the northern North Sea basin to the east was the west-east orientated Northwest European basin that extended into modern Poland. This basin also formed a major depositional feature throughout the Tertiary and early Pleistocene.
The Tertiary deposits of lowland Britain, particularly those of the Palaeogene in the main depositional Hampshire-Dieppe and London basins, characteristically record alternating transgression and regression sequences that have been attributed to global eustatic sea-level cycles (Fig. 2). Pulsed tectonism has also been invoked. Although initially restricted to the eastern end of the London Basin, later transgressions became more extensive, some reaching the Hampshire-Dieppe Basin. Over 20 such transgressions have so far been identified. The older Eocene strata (London Clay, Bracklesham Group, Barton Group) represent major transgressive periods, involving several individual transgressive events. By contrast, few are found in the non-marine Solent Group of the Hampshire region. Overall the sea-level through the Cenozoic parallels that of global temperature showing a long-term downward trend that culminates in the glacioeustatic lowstands that typify the Quaternary (Fig.2). The sea-level record represented in the British sequences reflects the interplay of the long-term trend with substantial eustatically-driven oscillations, themselves modified by local tectonic activity. Attempts to relate the local sea-level record to global curves are hindered by the need to untangle these interacting drives.
Fig.2 Eustatic sea-level curve (1) and ocean bottom water temperature (2) during the Tertiary (after Haq et al. 1987; Savin 1977).
The Tertiary sequences in southern Britain are summarised in figure 3 below.
Fig.3 Summary of the main Tertiary sequences in lowland Britain, showing the major lithostratigraphical units.