A polyphase glacitectonic model for ice-marginal retreat and terminal moraine development: the Middle Pleistocene British Ice Sheet, northern Norfolk, UK

Abstract

Recent investigations from modern environments demonstrate that many terminal moraines do not simply record a single glacial maximum, but instead reveal a complex pattern of ice-marginal behaviour including polyphase retreat. Within this study, we examine the geomorphology, geology and internal structure of a terminal moraine complex -the 'Cromer Ridge' in north Norfolk to reconstruct patterns of ice-marginal behaviour. Previously, this landform was interpreted as the limit of a southern extension of the British Ice Sheet during a Middle Pleistocene glaciation. Evidence presented here reveals a more complicated pattern of ice-marginal behaviour with the 'Cromer Ridge' reinterpreted as a 'complex' comprising several ridge elements. We propose that the maximum ice extent lay further to the south, with the size and morphology of the largest ridge element (the 'Cromer Ridge' as previously defined) a facet of thrust-stacking at an ice-marginal stillstand. We recognise multiple oscillations of the ice-front recorded against a twelve-stage model for the decay of the southern margins of a fast-flowing lobe of North Sea ice. Changes in ice-marginal dynamics are identified by the superimposition and lateral and vertical evolution of glacitectonic styles. Differences between these various states, and switches between 'shallow' and 'deep' thinskinned glacitectonics, are strongly influenced by sub-marginal and proglacial water availability. Examination of the evidence for the morphostratigraphic proposals for the glacitectonic assemblage, within the context of the above interpretation, suggests that many of the 'glacigenic landforms' are erosional and a MIS 12 age of formation is favored although several anomalies remain to be explained. IntroductionTerminal moraines are an important landform within the glacial landsystem and are widely recognised within both contemporary (Boulton et al., 1996(Boulton et al., , 1999Bennett, 2001; Evans and Twigg, 2002; Evans, 2005;Benediktsson et al., 2010) and relict glacial environments (van der Meer, 1987; Lee, J.R., Phillips, E., Booth, S.J., Rose, J., Jordan, H.M., Pawley, S.M., Warren, M., Lawley, R.S. 2013. A polyphase glacitectonic model for ice-marginal retreat and terminal moraine development: the Middle Pleistocene British Ice Sheet, northern Norfolk, UK. Proceedings of the Geologists' Association, 124, 753-777. PRE-PROOF ACCEPTEF MANUSCRIPT.2 Hart, 1990;Van der Wateren, 1995;Harris et al., 1997;Phillips et al., 2002;Wilson, 2002;Thomas and Chiverrell, 2007;Johnson et al., 2013). Previously, they have been considered as providing a visible record of a maximum extent of glacier advance, although many now consider them to provide a more dynamic record of glacier retreat (e.g. Thomas, 1984;Thomas et al., 2004;Thomas and Chiverrell, 2007;Lüthgens and Böse, 2011; Clark et al., 2012).The formation of terminal moraines typically involves the complex interplay between ice-marginal to sub-marginal sedimentation and glacitectonics; the latter including proglacial and sub-margina...

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