Pleistocene subglacial tunnel valleys in the central North Sea basin: 3‐D morphology and evolution

Abstract

Four phases of cross-cutting tunnel valleys imaged on 3-D seismic datasets are mapped within the Middle-Late Pleistocene succession of the central North Sea basin (Witch Ground area).In plan the tunnel valleys form complex anastomosing networks, with tributary valleys joining main valleys at high angles. The valleys have widths ranging from 250 to 2300 m, and base to shoulder relief varying between 30 and 155 m, with irregular long-axis profiles characteristic of erosion by water driven by glaciostatic pressures. The youngest phase of tunnel valleys are smaller and have a thinner infill than the older generations. The fill of the larger valleys comprises three seismic facies, the lowermost of which has high amplitudes and is discontinuous. The middle facies consists of wedge-shaped packages of low-angle dipping reflectors and is overlain by a facies characterised by sub-horizontal reflectors, which onlap the valley margins. The seismic character, and comparison with lithologies identified in other northwest European Pleistocene tunnel valleys both onshore and offshore, suggests that the lower two seismic facies are most likely sand and gravel-dominated, while the uppermost facies consists of glaciolacustrine and marine muds. The 3-D morphology of the valley margins combined with the geometry of the infill packages suggest that episodic discharge of subglacial meltwater was responsible for incising the valleys and depositing at least some of the infill. Proglacial glaciofluvial deposits are inferred to account for some of the fill overlying the subglacial deposits. Glaciolacustrine and marine muds filled remaining valley topography as the ice sheet retreated. The preserved valley margins are shown to be time-transgressive erosion surfaces that record changes in geometry of the tunnel valley system as it evolved through time, implying that valleys associated with each ice-sheet advance/retreat cycle were dynamic and probably long-lived. Within the constraints of the existing stratigraphy the oldest tunnel valleys in the Witch Ground area of the central North Sea are most likely to be Marine Isotope Stage (MIS) 12 (Elsterian, ca. 470 ka) in age and the youngest pre-MIS 5e (last interglacial, ca. 120 ka). If each tunnel valley phase was formed during the retreat of a major ice sheet then four glaciations with ice coverage of the central North Sea are recorded in the pre-Weichselian, Middle-Late Pleistocene stratigraphy.

The northern sector of the last British Ice Sheet: Maximum extent and demise

“…Here we describe the morphology and spatial distribution of sea-bed landforms should also be noted that whereas these channels retain bathymetric expression, due to only partial sediment infill, they form part of a more extensive system of north to northwest-trending channels in the central and northern North Sea Basin that have been subsequently buried by sediment (Stoker et al, 1985;Gatliff et al, 1994;Praeg, 2003;Fitch et al, 2005;Lonergan et al, 2006). Whether open or infilled, the bases of all these channels generally lie between 50 and 100 m below the surrounding sea bed.…”

“…Key indicators of glacial activity on the UK continental shelf include several generations of tunnel valleys in the North Sea (Cameron et al, 1987;Wingfield, 1989Wingfield, , 1990Ehlers and Wingfield, 1991;Huuse and Lykke-Andersen, 2000;Praeg, 2003;Lonergan et al, 2006), and moraines preserved to the NE and NW of Britain (e.g. Rokoengen et al, 1982;Stoker et al, 1985Stoker et al, , 1993Selby, 1989;Hall and Bent, 1990;Stoker and Holmes, 1991;Austin and Kroon, 1996).…”

“…Radiocarbon dates between 21.7 ka and 17.7 ka 14 C BP, obtained from lignitised wood sampled from glacimarine deposits adjacent to the eastern margin of the Wee Bankie moraine (Holmes, 1977), have traditionally formed the basis for interpreting these moraines as the maximum eastern limit of the last BIS, at around 18-22 ka BP (Sutherland, 1984;Boulton et al, 1985Boulton et al, , 1991Boulton et al, , 2002Cameron et al, 1987;Hall and Bent, 1990;Lambeck, 1991;Bowen et al, 2002;. However, such a limit fails to reconcile the occurrence of subglacial tunnel valleys largely formed during the Late Devensian (MIS 2), located east of these moraines (Wingfield, 1989;Ehlers and Wingfield, 1991;Lonergan et al, 2006).…”

Bedform signature of a West Antarctic palaeo-ice stream reveals a multi-temporal record of flow and substrate control

“…6B,C). We interpret these features as tunnel valleys formed beneath an ice sheet or icesheet margin during multiple subglacial meltwater discharge events, or modified by constant, steady-state meltwater erosion over successive glacial phases (Ó Cofaigh, 1996;Lowe and Anderson, 2003;Lonergan et al, 2006;). …”

Glaciogenic reservoirs and hydrocarbon systems: an introduction

“…Praeg (2003) has great promise for reservoir prediction (Kristensen et al 2008), it does not account for presently unfilled valleys. Although the origin of tunnel valleys as ice-marginal meltwater conduits is generally accepted (Ussing 1903;Ehlers et al 1984;O'Cofaigh 1996;Huuse & Lykke-Andersen 2000a;van der Vegt et al 2012), there are opposing views (Lonergan et al 2006;Stewart & Lonergan 2011), and it is still debated how many, how frequent, how long and how vigorous the meltwater flow events would have been to form specific tunnel valleys and tunnel valley families (Boulton & Hindmarsh 1987;Huuse & Lykke-Andersen 2000a;van der Vegt et al 2012). As for any debated geological feature, it is unlikely that a single genetic model will fit all occurrences, unless the model is sufficiently broad in terms of parameters such as flow modes and frequencies, role of glacial occupation, and presence or absence of foreland permafrost.…”