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  • 1
    Language: English
    In: Quaternary Science Reviews, 2008, Vol.27(19), pp.1848-1871
    Description: Detailed analysis and interpretation of the stratigraphy and structures developed within polydeformed sediments exposed in the coastal section between West Runton and Sheringham (eastern England) has allowed the characterisation of the glacitectonic signature associated with the advance of a major Mid-Pleistocene (Anglian) ice sheet. The sequence of pre-glacial deposits and glacial sediments laid down during earlier southerly directed ice advances (D1–D2) is deformed and disrupted by an ice advance from the west/southwest that marks a major change in ice flow dynamics in northern East Anglia. The simplest interpretation of this deformation event (D3) is in terms of a progressive proglacial to subglacial deformation model. Proglacial deformation occurred in advance of the ice margin and was dominated by thrusting. Positive topographic features which developed above the propagating thrusts controlled sediment dispersal patterns within outwash sandur, leading to the formation of small sub-basins between these morainic ridges. The accretion of the wedge-shaped, proglacial thrust moraines to the main push moraine formed at the margin of the advancing glacier may have temporarily stalled forward motion of the ice. Thrusting and large-scale folding associated with ice-marginal deformation led to the stacking of material excavated from further up-ice onto the stoss-side of these accreted thrust-related moraines that allowed the glacier to override the obstruction. Subglacial deformation was highly variable in its style and intensity ranging from heterogeneous folding and thrusting, through to more pervasive ductile shearing associated with the formation of a subglacial shear zone. The thickness and complexity of this shear zone increases up-ice, where it is characterised by a thick glacitectonic mélange. The pore water content of the deforming sediments controlled the pattern of deformation within the shear zone. However, an increase in the efficiency of the drainage system towards the front of the glacier would have led to a rapid thinning of this water-enhanced zone of ductile shear. Evidence from north Norfolk suggests that zones of preferential pore water flow within the glacier bed are a major controlling factor on the location of the subglacial shear zone. Consequently, the bulk of the forward motion of the glacier may not be accommodated by shearing within the sediments immediately below its base, but occur at a deeper level within the deforming bed.
    Keywords: Sciences (General) ; Geology
    ISSN: 0277-3791
    E-ISSN: 1873-457X
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  • 2
    Language: English
    In: Quaternary Science Reviews, 2008, Vol.27(13), pp.1350-1362
    Description: 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.
    Keywords: Sciences (General) ; Geology
    ISSN: 0277-3791
    E-ISSN: 1873-457X
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  • 3
    Language: English
    In: Quaternary Science Reviews, Feb 1, Vol.109, p.88(23)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.quascirev.2014.09.010 Byline: Jonathan R. Lee, Oliver J.W. Wakefield, Emrys Phillips, Leanne Hughes Abstract: Subglacial drainage systems exert a major control on basal-sliding rates and glacier dynamics. However, comparatively few studies have examined the sedimentary record of subglacial drainage. This is due to the paucity of modern analogues, the limited recognition and preservation of upper flow regime deposits within the geological record, and the difficulty of distinguishing subglacial meltwater deposits from other meltwater sediments (e.g. glacier outburst flood deposits). Within this study, the sedimentological and structural evolution of a subglacial to subaerial (ice-marginal/proglacial) drainage system is examined. Particular emphasis is placed upon the genetic development and preservation of upper flow regime bedforms and specifically recognising them within a subglacial meltwater context. Facies are attributed to subglacial meltwater activity and record sedimentation within a confined, but progressively enlargening, subglacial channel system produced under dune to upper flow regime conditions. Bedforms include rare large-scale sinusoidal bedding with syn-depositional deformation produced by current-induced traction and shearing within the channel margins. Subglacial sedimentation culminated with the abrupt change to a more ephemeral drainage regime indicating channel-abandonment or a seasonal drainage regime. Retreat of the ice margin, led to the establishment of subaerial drainage with phases of sheet-flow punctuated by channel incision and anastomosing channel development under diurnal, ablation-related, seasonal discharge. The presence of extensive hydrofracture networks demonstrate that proglacial groundwater-levels fluctuated markedly and this may have influenced later overriding of the site by an ice stream. Author Affiliation: (a) British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK (b) British Geological Survey, West Mains Road, Edinburgh, EH9 3LA, UK Article History: Received 30 April 2014; Revised 1 September 2014; Accepted 4 September 2014
    Keywords: Groundwater ; Bedforms ; Sedimentary Structures ; Sediments (Geology)
    ISSN: 0277-3791
    Source: Cengage Learning, Inc.
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  • 4
    Language: English
    In: Quaternary Science Reviews, 2011, Vol.30(23), pp.3481-3500
    Description: Sand intraclasts are common within the Bacton Green Till Member, a glacitectonic mélange subjected to polyphase deformation during the Middle Pleistocene in North Norfolk, UK. The intraclasts range from a few tens of centimetres to 〉10 m in length and have sharp contacts with the surrounding till. Sand within the intraclasts is unconsolidated and contains well-preserved primary stratification. The wrapping of glacitectonic foliation around the intraclasts and the development of folds relating to mechanical instabilities indicate that the intraclasts acted as competent masses within a more easily deformable fine-grained till that accommodated the majority of the strain. Sharp contacts and distinctive heavy-mineral assemblages indicate little intermixing between the sand and till. Five hypotheses about the entrainment and evolution of the intraclasts are tested against sedimentological, structural and mineralogical observations. The most reasonable hypothesis attributes the intraclasts to glacitectonic deformation of “warm” permafrost. Initial ice advance caused large-scale thrusting of proglacial permafrost that led to the stacking of pre-glacial and ice-marginal sediments that were subsequently deformed sub-marginally to generate the intraclasts. Preservation of primary stratification within the intraclasts is attributed to deformation at temperatures slightly below the pressure-melting point, when pore ice cemented the intraclasts as rigid bodies. At the same time deformation was concentrated into the surrounding finer-grained till because of its significant liquid water content and ductile rheology. It is concluded that the intraclasts provide a criterion to identify past glacier–permafrost interactions and a potential means of differentiating between subglacial deformation under unfrozen and partially-frozen conditions. ► We examine the origin and significance of sand intraclasts within tills. ► Occur as competent masses within deformed tills in north Norfolk. ► Cohesion associated with the development of an ice cement. ► Rheological contrast within deformed tills consistent with sub-freezing temperatures. ► Intraclasts are potentially diagnostic of glacier–permafrost interactions.
    Keywords: Palaeoglaciology ; Glacial Sedimentology ; Glacitectonic Deformation ; Permafrost ; Sciences (General) ; Geology
    ISSN: 0277-3791
    E-ISSN: 1873-457X
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  • 5
    Language: English
    In: Quaternary Science Reviews, 01 February 2015, Vol.109, pp.88-110
    Description: Subglacial drainage systems exert a major control on basal-sliding rates and glacier dynamics. However, comparatively few studies have examined the sedimentary record of subglacial drainage. This is due to the paucity of modern analogues, the limited recognition and preservation of upper flow regime deposits within the geological record, and the difficulty of distinguishing subglacial meltwater deposits from other meltwater sediments (e.g. glacier outburst flood deposits). Within this study, the sedimentological and structural evolution of a subglacial to subaerial (ice-marginal/proglacial) drainage system is examined. Particular emphasis is placed upon the genetic development and preservation of upper flow regime bedforms and specifically recognising them within a subglacial meltwater context. Facies are attributed to subglacial meltwater activity and record sedimentation within a confined, but progressively enlargening, subglacial channel system produced under dune to upper flow regime conditions. Bedforms include rare large-scale sinusoidal bedding with syn-depositional deformation produced by current-induced traction and shearing within the channel margins. Subglacial sedimentation culminated with the abrupt change to a more ephemeral drainage regime indicating channel-abandonment or a seasonal drainage regime. Retreat of the ice margin, led to the establishment of subaerial drainage with phases of sheet-flow punctuated by channel incision and anastomosing channel development under diurnal, ablation-related, seasonal discharge. The presence of extensive hydrofracture networks demonstrate that proglacial groundwater-levels fluctuated markedly and this may have influenced later overriding of the site by an ice stream.
    Keywords: Subglacial Drainage ; Upper Plane Bed ; Helicoidal Flow ; Sinusoidal Bedding ; Hydrofracturing ; Groundwater ; Sciences (General) ; Geology
    ISSN: 0277-3791
    E-ISSN: 1873-457X
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  • 6
    Language: English
    In: Quaternary Science Reviews, 01 February 2018, Vol.181, pp.123-143
    Description: Published conceptual models argue that much of the forward motion of modern and ancient glaciers is accommodated by deformation of soft-sediments within the underlying bed. At a microscale this deformation results in the development of a range of ductile and brittle structures in water-saturated sediments as they accommodate the stresses being applied by the overriding glacier. Detailed micromorphological studies of subglacial traction tills reveal that these polydeformed sediments may also contain evidence of having undergone repeated phases of liquefaction followed by solid-state shear deformation. This spatially and temporally restricted liquefaction of subglacial traction tills lowers the shear strength of the sediment and promotes the formation of “transient mobile zones” within the bed, which accommodate the shear imposed by the overriding ice. This process of soft-bed sliding, alternating with bed deformation, facilitates glacier movement by way of ‘stick-slip’ events. The various controls on the slip events have previously been identified as: (i) the introduction of pressurised meltwater into the bed, a process limited by the porosity and permeability of the till; and (ii) pressurisation of porewater as a result of subglacial deformation; to which we include (iii) episodic liquefaction of water-saturated subglacial traction tills in response to glacier seismic activity (icequakes), which are increasingly being recognized as significant processes in modern glaciers and ice sheets. As liquefaction operates only in materials already at very low values of effective stress, its process-form signatures are likely indicative of glacier sub-marginal tills.
    Keywords: Glacier Bed Deformation ; Till Liquefaction ; Pressurised Meltwater ; Icequakes ; Sciences (General) ; Geology
    ISSN: 0277-3791
    E-ISSN: 1873-457X
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