The geomorphology of the Axial Channel, southern North Sea: a complex glacio-fluvial and marine story

The Axial Channel is a prominent geomorphological feature seen on the present-day bathymetry of the southern North Sea. The 150 km long depression extends from the Norfolk Banks in the north to the Dover Strait in the south. It is believed to be a remnant of a large and complex drainage system that existed during the late Pleistocene, when ice sheets occupied parts of the North Sea region during three major glaciations: the Elsterian/Anglian (MIS12), Saalian/Wolstonian (MIS10-6) and Weichselian/Devensian (MIS5d-2) glaciations. The existence of these ice sheets was accompanied by a large fall in global sea level, causing the southern North Sea region to emerge and become isolated from the Atlantic. As a northern drainage route was blocked by coalescing ice sheets during their maximum expansion, glacial meltwater but also river water from the major West-European rivers (e.g. Scheldt, Meuse-Rhine, Elbe) followed a southern drainage route towards the Dover Strait. Understanding the evolution of the present-day Axial Channel is crucial to understanding the paleogeographic changes that affected the region over the course of multiple glacial-interglacial cycles.

A first step in understanding this evolution was performed by analysing the present-day bathymetry of this region and mapping the preserved geomorphological features. Available offshore bathymetry data were compiled in the region from 53° to 51° latitude North. This included the EMODnet Digital Bathymetry (DTM) map, at 20 m resolution, supplemented by high-resolution (up to 1 m resolution) bathymetry blocks from the UK Admirality Seabed Mapping Service (UK Hydrographic Office data ©Crown copyright and database right), covering most of the eastern part of the study area. Furthermore, in the framework of the WALDO project, seismic reflection data, including multi-channel sparker and high-resolution parametric sub-bottom profiler (TOPAS) data, have been gathered in the Axial Channel region.

Our bathymetric mapping revealed numerous geomorphological features on the plateau in the western part of the Axial Channel region. Our preliminary interpretation suggests a glacial origin for some features, such as the observed elongated deeps and north-south oriented scours. Furthermore, multiple palaeovalley systems, including a major west-east system, on this same plateau are witnesses of dry, not fully marine inundated periods in the southern North Sea. Our mapping further revealed multiple incisional phases shaping the present-day Axial Channel, including a distinct western escarpment, i.e. the eastern edge of the plateau. The seismic-reflection data were gathered to further investigate the incisional and infilling stages. It revealed additional incisional phases that could not be observed from bathymetric data alone. Furthermore, the reflection data allowed several infilling stages to be discerned in the northern part of the Axial Channel region, while no infilling sediments could be identified towards the south. This study illustrates the importance of combining bathymetric and seismic data to understand the evolution of large geomorphological features such as the Axial Channel.