Fluvial response to rapid climate change during the Devensian (Weichselian) Lateglacial in the River Great Ouse, southern England, UK

Few Lateglacial (15,200-11,500 Cal BP) fluvial deposits are known from southern England, UK. This paper outlines a sedimentological investigation of a Lateglacial site near the village of Roxton in the middle reach of the River Great Ouse. Below the Holocene argillaceous alluvium, several large gravelly lithofacies units representing long-term fluvial processes are recognised based on their lower bounding surfaces and sedimentary features. In line with palaeontological evidence and radiocarbon dating, detailed facies analysis indicates that the river changed from a braided to a wandering/meandering regime in response to the warming-induced reduction in nival peak discharge and increase in vegetation cover consisting of birch woodland and grassland during the Lateglacial Interstadial (15,200-12,700 Cal BP). This was followed by a change back to a braided river in response to climatic deterioration during the Younger Dryas Chronozone (12,700-11,500 Cal BP). The interstadial floodplain consisted of both overbank fines and channel lateral or point-bar sands and gravels. The two suites of contemporaneous sediments in the geological succession may be easily mis-interpreted as independent climato-stratigraphical units due to their cross-cutting relationship and marked difference in facies assemblage. Lateral channel movement in such a system also implied that the floodplain underwent constant erosion, resulting in underdeveloped overbank sequences, insufficient to withstand post-depositional removal by erosion. This probably accounts for the rare preservation of fine-textured overbank deposits of the Lateglacial Interstadial in southern England.