Morphology, stratigraphy and genesis of buried mid-Pleistocene tunnel-valleys in the southern North Sea Basin

Conflicting views of the hydrology and dynamics of ice sheets apply to tunnel-valleys in sediment beds. Field data have been attributed to opposed models of superimposed glaciofluvial erosion, either beneath receding ice margins or by 'catastrophic' subglacial floods, whereas theoretical models infer a coupled response to the drainage of water through a deformable substrate. The southern reaches of the Elsterian glaciation are associated with large buried tunnel-valleys, locally over 400 m in relief. Their basal morphology and fill stratigraphy are examined over a 100x 150 km area of the southern North Sea Basin, using seismic reflection grids (including a 3D-volume over a 39x22 km area) supplemented by downhole data, mainly collected for hydrocarbon exploration. The results support a diachronous formation by drainage beneath a receding ice margin coupled to its bed. Basal contours define elongate basins up to 500 m in relief (575 m bsl), shallow relative to their widths (0.5-6 km) and steep-sided (5-40°), containing nested axial sub-basins 10-170 m in relief and 1-45 km long. The basins converge to the south in a rectilinear arborescent plan form. Local divergent or anastomosing patterns, and angular offsets of basal segments up to 20 km long, result from erosional overlap by younger basal elements to the north. The basal morphology is superimposed on trends in the incised Plio-Pleistocene substrata, which have only locally influenced cross-sectional form (stratal benches) and orientation (across faults). However, the basins increase eastwards in size and spacing in parallel with the thickness of sandy Pleistocene sediments. Contoured reflecting surfaces within the fill reveal three seismic sequences, from bottom to top: I axially downlapping clinoform surfaces, 0.5-3.5° in gradient and 3-20 km in extent, which dominate the fill with thicknesses up to 400 m beneath a depositional upper boundary up to 120 m in axial relief; the northwards prograding sequence includes discordant surfaces of erosion which coincide with plan-form complexity, separating younger cut-and-fill elements to the north; II subhorizontal surfaces which onlap basins at the surface of I and the upper walls with thicknesses up to 150 m; Ill complex surfaces which fill incised basins up to 70 m in axial relief at the surface of II with thicknesses up to 100 m and extend between some tunnel-valleys. Downhole data show sand-dominated assemblages beneath a mud-dominated cap. The sediments contain microfossils and secondary components (lignite, shell fragments, glauconite) reworked from series as young as early Pleistocene. A marine foraminiferal zone occurs within the upper muds. The succession records glaciofluvial to lacustrine deposition prior to marine conditions of the Holsteinian interglacial. Correlation to the seismic stratigraphy shows that progradation of glaciofluvial sands (I) was followed by subhorizontal deposition of sands and then muds in lake basins (II) and finally complex deposition in and between marine basins (III). Evidence for basal erosion to the south versus glaciofluvial fill progradation to the north are reconciled in a model of the contemporaneous excavation and backfilling of basins beneath the northwards receding ice margin. The rectilinear basal morphology, including erosional overlap by younger elements to the north, is attributed to shifting ice margin orientations and local readvances during recession. Clinoform surfaces (I) represent glaciofluvial backsets, previously observed or inferred within eskerine ridges, here attributed to a distributed system of submarginal streams rising to feed overand backlapping subaqueous outwash fans along a migrating grounding line. Overlying surfaces (II) record pro-glacial outwash of sands and increasingly distal accumulation of muds in lake basins, which elongated with ice recession and persisted until the marine transgression (III). The tunnel-valleys formed by reworking of sedimentary materials, by systems of streams within larger basins, beneath the outer tens of kilometres of the ice sheet. They are genetically equivalent to eskers, which record the headward evolution of englacial surface-to-bed drainage within melting ice margins during deglaciation. Their inverted morphology, and the observed relation of size and spacing to substrate (aquifer) thickness, are consistent with the response of a deformable sediment bed to seasonally large volumes of surface recharge. The tunnel-valleys record the diachronous coupling of substrate and glacier hydrology beneath the receding, melting ice sheet margin.