Progressive soft sediment deformation within a subglacial shear zone—a hybrid mosaic–pervasive deformation model for Middle Pleistocene glaciotectonised sediments from eastern England

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Abstract

This paper presents a hybrid ‘pervasive’ and ‘mosaic’ model for the progressive deformation of a stratified diamicton complex in eastern England within an active subglacial shear zone. Sedimentary evidence from undeformed low-strain zones towards the base of the sediment pile indicates that the tectonised sediments were originally deposited as a series of subaqueous flows in a glaciolacustrine basin. These sediments have subsequently been deformed during a progressive subglacial shearing event. This event is divided into three stages: ductile folding and sediment remobilisation (D1), brittle faulting in the form of Reidel shears (D2) and hydrofracturing and sediment remobilisation (D3). The dominant control on the pattern and style of deformation appears to relate to the rate of thrust-induced till accretion, and the aquifer properties and pore water content and/or pressure of the deforming sediments.

Introduction

The introduction of a ‘soft deforming-bed’ model to explain the movement and transport of sediment beneath glaciers and ice sheets has revolutionised the fields of glaciology and glacial geology over the past 30 years (Boulton, 1986; Boulton and Hindmarsh, 1987). Considerable research has focussed upon the examination of evidence for the roles played by subglacial deforming beds in controlling the dynamics of the major Pleistocene ice sheets of North America and northern Europe (Boulton and Jones, 1979; Licciardi et al., 1998; Piotrowski et al., 2001). This has led to the assertion that the dynamics of these large continental-scale ice sheets may in-part be driven, and in-turn be drivers of past climate change via a series of complex feedback mechanisms that operate between terrestrial, oceanic and atmospheric systems (Broecker et al., 1992; Clark, 1994; Hughes, 1996; Clark et al., 1999). It has also been suggested that deformable beds exist beneath several contemporary ice masses including the West Antarctic Ice Sheet (Alley et al., 1987; Evans et al., 2006a, Evans et al., 2006b), and the melting of these ice sheets could exert a major influence on future climate and sea-level changes (Hughes, 1996; Bentley, 1997). An understanding of the nature and processes operating within subglacial deforming beds is therefore of critical importance.

Recent research on modern ice sheets has shown that rates of basal ice flow, even where ice is overriding deforming subglacial beds that typically encourages faster ice flow, can vary both temporally and spatially (Alley, 1989). Such basal-flow behaviour is referred to as ‘stick–slip basal ice flow’, where subglacial variations in pore water content and/or pressure, subglacial shear stress, drag and ice velocity at the ice–bed interface (IBI) create zones of basal ‘stick’ (or ‘stable’ beds) and ‘slip’ (or ‘deforming’ beds) (Fischer and Clarke, 1997; Hooke et al., 1997; Fischer et al., 1999). Evidence for ‘stick–slip basal ice flow’ has been recognised within the geological record, suggesting that the subglacial bed is in reality a ‘mosaic’ composed of zones of ‘stable’ and ‘deforming’ beds that change configuration in time and space (Piotrowski and Kraus, 1997; Piotrowski et al., 2004; Evans et al., 2006a, Evans et al., 2006b). Geological evidence from Pleistocene tills support theories proposed for the mechanics of ‘stick–slip basal flow’ inferred from contemporary ice sheets in that water-induced decoupling at the IBI (Hoffman and Piotrowski, 2001; Knight, 2002; Iverson et al., 2003) and zones of different permeability within the subglacial bed (Piotrowski et al., 2004; Evans et al., 2006b) are the primary drivers. This contrasts with the views of van der Meer et al. (2003) and Menzies et al. (2006), who argue that subglacial deformation is pervasive and overprints till facies accreted under different basal-flow regimes (e.g. lodgement or melt-out) such that all subglacial tills are deformation tills (‘tectomict’ of Menzies et al., 2006). In this paper, we contribute to this argument by examining the structural evolution of a Middle Pleistocene subglacial shear zone in northern East Anglia, UK. The variation in intensity of deformation recorded by these sediments reveals a complex pattern of relatively ‘higher-strain’ and ‘lower-strain’ domains, and these relate to changes in inter-granular pore water content driven by lithology and subglacial-till accretion.

Section snippets

Location and geological context

The location of the study site for this investigation is Bacton Green (National Grid Reference TG 338345), situated on the east coast of England (Fig. 1). Sections at Bacton Green consist of low coastal cliffs rising to a maximum observed height of 16 m above the level of the foreshore.

The region lies adjacent to the southern North Sea Basin and the pattern and chronology of lowland glaciation is currently the subject of considerable debate (Hamblin et al., 2000; Banham et al., 2001).

Methodology

The depositional setting and subsequent deformation history of the BGTM has been investigated using a range of macro- and micro-scale techniques. Sections were logged and described on the basis of their macro-scale features with particular emphasis placed on recording the type of bedding, sediment texture, structure (both sedimentary and tectonic), sediment colour (Munsell) and bed geometry. In addition, a total of eight samples of unconsolidated sand and diamicton material were collected from

Description and interpretation of the stratified diamicton complex

Evidence of glaciotectonic deformation within the BGTM varies in occurrence and nature spatially (Fig. 2). The basal 7 m of the till forms a highly heterogeneous ‘lower facies’ containing areas of minimal glaciotectonic disturbance, in which the original depositional character of the sediments can still be recognised, separated by a network of more highly deformed zones in which the primary sedimentary structures have been overprinted. By contrast, the upper 7 m of the till form an ‘upper mélange

Primary mode of sedimentation

The primary sedimentary features, where preserved within the ‘lower facies’, demonstrate that the BGTM forms part of a gradational continuum with the underlying glaciodeltaic sands of the MSM. This upwards transition from the MSM to the BGTM is thought to record a gradual change in depositional environments from a fine-grained outwash delta to a subaqueous debris fan associated with an ice advance into the glaciolacustrine basin. Sedimentary evidence indicates that the BGTM was deposited as a

Conclusions

The study of the stratified diamicton complex at Bacton Green contributes to an understanding of the mechanisms of subglacial deformation and its evolution in time and space.

  • Sedimentary features, where preserved, indicate that the stratified diamicton complex (BGTM) was deposited by processes of mass flow, rapid rain-out, and turbidity current activity within a standing body of water. The relationship of the ‘till’ to underlying sands, records the switch from deposition on a distal

Acknowledgements

The authors wish to thank Brian Moorlock and Helen Burke for their comments on an earlier version of the manuscript. Adrian Palmer is thanked for his assistance with the manufacturing of the thin sections. Jim Rose and anonymous referee are thanked for their constructive comments on the manuscript. Funding for this research was provided in two phases: (1) RHUL/BGS (NERC) studentship received by JRL between 2000 and 2003; (2) BGS (NERC) funding via the Quaternary Palaeoenvironments and

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