Quaternary evolution of the drainage systems in the Southern and Central North Sea Basin deduced from high resolution reflection seismic and sedimentological data

Abstract

The North Sea shelf is one of the classic regions for studying Quaternary depositional and erosional sedimentary structures formed by discharging melt-water from retreating ice sheets and fluvial processes during periglacial shelf exposure. This provides an avenue to unravel the complex interplay between shelf morphology, changes in hydrodynamic regime, sediment influx and climate-driven eustatic changes. The ongoing political energy transition has prompted an intense geo-survey and sampling of the North Sea floor largely by industrial organizations, various Federal agencies such as the German Federal Maritime and Hydrographic Agency (BSH), German Federal Institute for Geosciences and Natural Resources (BGR) as well as some scientific institutions. The wealth of acquired data in the course of these various commercial, governmental and scientific expeditions has led to an improved understanding of the Quaternary geology of the North Sea sub-surface. Using a combination of tidally corrected acoustic profiles, shallower and deeper sediment cores and cone penetration test (CPT) data acquired from the German North Sea sector, this doctoral thesis aimed at contributing to the ongoing understanding of the morphology, the development of the drainage systems and the palaeo-landscape of the North Sea shelf since the Last Glacial Maximum. It also aimed at refining the stratigraphy and the geotechnical properties of the highly competent shallow sand units in the southern North Sea. The study unraveled the evolution, morphology and valley infill successions of the Elbe Palaeovalley (EPV). The EPV, located in the southeastern North Sea, occur as a shallow geomorphological trough spanning a length of about 210 km and width of about 40 km. The valley base is about 65 m below the present day sea-level. During the Marine Isotope Stage 2 sea level lowstand, this SE-NW oriented palaeo-drainage evolved as a braided fluvial system in a periglacial environment. Seismo-stratigraphic interpretation of the EPV infill successions revealed five major units. During deglacial sea-level rise, the EPV evolved into an estuary with tributaries, intertidal and subtidal flats. Towards the western flank of the EPV, the Palaeo-Ems, one of the known major tributaries that fed the southern head of the EPV was also identified and its overall course was seismo-stratigraphically described. The Palaeo-Ems river course was mapped as a buried, low gradient and meandering channel branching into two major pathways as it approaches a newly discovered delta flat at the western flank of the EPV. In its downstream direction, the Palaeo-Ems formed a unified depositional system with the early phase of the EPV. Based on available data, this study also shed light on the Palaeo Ems/EPV morpho-stratigraphic relationship for the first time. Ongoing sea-level rise since the early Holocene overwhelmed the adaptation capabilities of the joint system leading to the drowning of the whole drainage system. By focusing on the less well understood, regionally dominant sand units which were deposited after the retreat of the last glaciers, this study also refined the stratigraphic units and geotechnical parameters of the uppermost 50 m below the sea floor within the German North Sea sector. Two sandy units, the Aeolian Member and the Upper Fluvial Member, were identified as dominant deposits in the late- to post-Saalian geology within the study area. In addition, a detail seismic analysis revealed the occurrence of a Saalian Buried Valley Member believed to comprise fluvial deposits. Based on the integration of seismic facies analysis with core and CPT data, a detailed geotechnical parameter set for each identified stratigraphic unit within the study area was developed and correlated with those of the neighboring North Sea sectors. The findings from this study complements and details Coughlan et al. (2018) geotechnical and stratigraphic framework of the study area as well as the stratigraphic framework recently developed by the BSH (2021). These deductions offer a new insight about the soil competency of the North Sea sub-surface which is key for various offshore commercial and economic activities within the region including but not limited to wind farm developments. The findings from this study are thus crucial contributions in understanding the dewatering system and subsequently, the reconstruction of the palaeo-landscape development in the German North Sea sector since the Last Glacial Maximum. Lastly, the study contributes to an improved geotechnical understanding of the stratigraphy of the North Sea.