Mid‐Pleistocene thin‐skinned glaciotectonic thrusting of the Aberdeen Ground Formation, Central Graben region, central North Sea

Abstract

This paper presents the results of a high-resolution 2D seismic survey of Mid-Pleistocene glaciogenic sediments in the Central Graben region of the central North Sea. The survey has revealed that these sediments, which were originally interpreted as possessing a simple layercake stratigraphy, have in fact undergone major glaciotectonic thrusting and folding associated with the repeated southerly advance of an ice sheet across the region during the Mid-Pleistocene. The total observed length of the thrust-stacked section is approximately 5 to 6 km, comprising a series of discrete thrust slices, which range in length from < 300 m to > 700 m. The basal detachment of the thrust complex occurs at a depth of c. 220 m below sea bed within the upper Aberdeen Ground Formation. The ice-proximal limit of thrusting is marked by a large depression (>100 m deep, >1600 m wide) infilled by a sequence comprising at its base acoustically chaotic deposits, overlain by a well-bedded unit, passing upwards into tectonically deformed sediments. A thin-skinned glacitectonic model involving proglacial to ice-marginal glacitectonic thrusting followed by post-tectonic deposition is proposed. Initial ice advance led to the over-pressurising of groundwater within a laterally extensive sand sheet in the upper Aberdeen Ground Formation, promoting the formation of a major décollement surface at the base of the developing thrust stack. Detachment and transport of the thrusted sediments led to the contemporaneous formation of a "glacitectonic hollow" beneath the ice margin. Dewatering of the basal décollement, possibly due to water escaping from the thrusts which propagated upwards through the Aberdeen Ground, led to the cessation of forward propagation of the leading-edge of the thrust stack accompanied by extensive shortening and deformation at its ice-proximal end. Subsequent retreat of the ice exposed the tectonic hollow which became infilled with glacial outwash. This glacifluvial sequence was itself deformed during later readvance of the ice from the north. The proposed model provides clear evidence that Mid-Pleistocene ice sheet dynamics within the Central Graben of the North Sea was far more complex than previously thought.

How understanding past landscapes might inform present‐day site investigations: a case study from Dogger Bank, southern central North Sea

“…The recognition of a buried glacial landscape within the DBF (Figure 1(b) and (c)), coupled with the detailed analysis of seismic profiles in three key areas (Figures 2-5), have revealed that the Dogger Bank is, at least in part, composed of a complex moraine system comprising variably deformed (folded and thrusted) sediments. Comparable large-scale glaciotectonic complexes comprising folded and thrusted glaciogenic sediments have been described elsewhere within the North Sea Basin and adjacent areas (e.g., Andersen et al 2005;Phillips et al 2008;Burke et al 2009;Bakker and van der Meer 2003;Vaughan-Hirsch and Phillips 2017;Lee et al 2013Lee et al , 2017Pedersen and Boldreel 2017). The complexity of the structural and sedimentary architecture of the DBF within the foundation zone of the proposed offshore wind farm has profound implications for the siting and design of the turbine foundations.…”

Geomorphological Mapping and Spatial Analyses of an Upper Weichselian Glacitectonic Complex Based on LiDAR Data, Jasmund Peninsula (NE Rügen), Germany

“…They are characterized by an asymmetric mean longitudinal profile with a distal slope that is exceeding the proximal. However, composite ridges can be interpreted as surface expressions of fault-related folds, which show asymmetric profiles in relation to push direction, similar to push moraines [3,4,24]. An analysis of a glacitectonic environment, like Jasmund, needs to deal with this complication.…”

“…Pushing and thrusting of the advancing ice front, regardless of whether push moraines or composite ridges are formed, would very likely result in asymmetric cross profiles, usually with steeper distal and flatter proximal slopes (e.g., [4,13,32]). Thus, the flow-parallel topographic profile seems to be rather a weak tool to distinguish between the two landforms.…”

“…Glacitectonic complexes are small-scale fold and thrust belts in terms of geometry and formation processes [1][2][3][4]. However, they originate from ice-marginal or proglacial deformation instead of classic orogenesis [3,4]. The deformation can extend to a depth of a few tens of meters up to 200 m [5,6].…”

The Quaternary geology of the North Sea basin

“…The glacitectonic theme is continued in the paper by Vaughan-Hirsch and Phillips (2017). This paper presents the results of a high-resolution 2D seismic survey of mid-Pleistocene glaciogenic sediments in the Central Graben region of the central North Sea (10 on Fig.…”