EARLY MIDDLE PLEISTOCENE FLUVIAL SEDIMENTS AT NORTON SUBCOURSE, NORFOLK; IMPLICATIONS FOR DRAINAGE BASIN EVOLUTION IN EASTERN ENGLAND

The early Middle Pleistocene drainage pattern of eastern England comprised a number of major eastwards-flowing rivers, including the ancestral River Thames and the Bytham River. The former was diverted and the latter destroyed by glaciation during OIS 12. These rivers supplied distinctive suites of gravels into the southern North Sea basin, forming shallow marine deposits, with equivalent on-shore deposits forming a series of fluvial terraces, which can be mapped on lithological and altitudinal grounds. At Norton Subcourse in Norfolk temperate climate deposits are exposed. These overlie marine gravels and are, in turn overlain by fluvial deposits of the Bytham River and glacial outwash deposits associated with Middle Pleistocene glaciation. The sediments at Norton Subcourse have yielded a range of biological data including large and small mammal remains, insects, molluscs and ostracods and also a well preserved pollen signal, from fine-grained sediments laid down in a large river channel. The ostracod assemblage indicates freshwater conditions. The biological data indicate that the deposits span the first half of a warm climate cycle, with summer temperatures at least as warm as those in the region today. The vegetation signal includes mixed deciduous taxa, typical of the early temperate sub-stage of an interglacial. The mammalian fauna includes hippopotamus, an extinct equid, hyaena and several rodent species, including the extinct water vole Mimomys, which is of biostratigraphical significance. On biostratigraphical grounds the interglacial probably correlates with the early part of the ‘Cromerian Complex’ (or possibly older), though attribution to a specific oxygen isotope stage is not yet possible. The sequence provides some temporal constraint on drainage basin evolution and changes in sea level in the region and also enables refinement of the early Middle Pleistocene stratigraphy of the region.