| Summary: | Tunnel-valleys are morpho-stratigraphical entities within glaciated sedimentary lowlands, which record large-scale channelised drainage beneath ice sheets. The southern North Sea Basin contains large examples, locally over 400 m in relief, cut and filled during the maximum extent of the mid-Pleistocene Elsterian glaciation. These are examined over a 100 x 150 km area of the UK/Dutch sector using seismic reflection and downhole data, collected mainly in the course of hydrocarbon exploration. The reflection dataset comprises over 12,000 line-km of profiles (≤1 km grids), and a 3D-seismic volume over a 39 x 22 km area. New information on basal morphology and fill stratigraphy is used to support a model of time-transgressive formation by en- and sub-glacial drainage beneath the ice margin during its deglacial recession. Evidence for basal erosion to the south, erosional overlap to the north, and glaciofluvial fill progradation to the north are reconciled in a model of contemporaneous headward excavation and backfilling or elongate basins during ice margin recession. This resulted in an axially diachronous valley base and fill sequence, younger to the north. Clinoform surfaces (sequence I) are identified as sub-marginal <I>backsets</I>, not previously observed at such a scale. They are explicable as subaqueous outwash from a distributed system of subglacial streams feeding grounding line fans, within the outer 5-20 km of ice-filled basins. Overlying surfaces (sequence II) record proglacial outwash of sands, and increasingly distal accumulation of muds, in lake basins which elongated with ice recession and persisted until the interglacial marine transgression. The physical characteristics of the tunnel-valleys indicate reworking of Cenozoic sedimentary materials (most of which remain within them) by drainage beneath the outer 50 km of the former ice sheet margin during its recession. Their size and spacing in relation to substrate thickness, and their arborescent geometry, are consistent with non-catastrophic models of tunnel development as a form of stable subglacial drainage over and through deformable aquifers. Sub-marginal drainage is dominated by surface supply, which exceeds basal melting by two orders of magnitude or more during deglaciation. Tunnel-valleys are therefore argued to form in response to both substrate characteristics and englacial hydrology during deglaciation, in a marginal zone which migrates with the ice sheet as it melts and passes through its own margin.
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