Geoffrey Boulton

Glacial geology and glaciology of the last mid-latitude ice sheets

Satellite imagery and data from ground surveys are used to reconstruct the integrated pattern of the principal longitudinal and transverse features produced on a continent-wide scale by the last ice sheets in Europe and North America. From modern analogues, it is argued that most longitudinal features reflect flow in the outer zone of the ice sheet, and that most major transverse features reflect relatively stable ice-sheet margins. These principles are tested and, using them alone, detailed patterns for the decay of the last ice sheets in North America, Europe and the British Isles are produced, and periods during which they attained near steady-states identified. These patterns can be calibrated by dated sequences to yield deglaciation isochrons. Application of glaciological models to these geological reconstructions generates detailed prediction of net ablation for the period of ice-sheet decay and, by using evidence of last glaciation stratigraphy, models of the dynamic behaviour of the ice sheets throughout the last glacial period are constructed. These enable volumetric changes, oceanic isotopic changes and erratic dispersal pathways to be reconstructed. Erratic dispersal patterns give a good indication of the long-term distribution of centres of ice sheet mass. Discrepancies between predicted and empirical oceanic isotopic records indicate ways in which the conventional continental timescale of glacial change must be altered to fit the better-dated deep ocean record. In addition discrepancies between predicted and empirical erratic dispersal patterns suggest that conventional views of ice-sheet behaviour based on high latitude models may be inappropriate to the dynamically more active mid-latitude ice sheets based in large part on deformable sediment beds.

Palaeoglaciology of an ice sheet through a glacial cycle:: the European ice sheet through the Weichselian

Abstract Satellite images provide unique means of identifying large-scale flow-generated lineations produced by former ice sheets. They can be interpreted to reconstruct the major elements which make up the integrated, large-scale structure of ice sheets: ice divides; ice streams; interstream ridges; ice shelves; calving bays. The evolving palaeoglaciological structure of the European ice sheet during its decay from the Last Glacial Maximum (LGM) is reconstructed by reference to these components and in the context of a new map showing isochrons of retreat. During the retreat phase in particular the time-dependent dynamic evolution of the ice sheet and the pattern of ice stream development are reconstructed. Crossing lineations are widespread. The older ones are suggested to have formed during molten bed phases of ice sheet growth and preserved by frozen bed conditions during the glacial maximum, particularly in areas which lay, during deglaciation, beneath ice divides and inter-ice stream ridges, both areas of slow flow and possibly frozen bed conditions. Four phases of growth (A1 to A4) and five phases of decay (R1 to R5) are used to describe the major climatically and dynamically determined stages in the evolution of the ice sheet through the last glacial cycle. The growth and decay patterns are quite different and associated with major shifts in the ice divide, reflecting growth from the Fennoscandian mountains and decay away from marine influenced margins. These patterns were determined by the locations of nucleation areas; spatial patterns of climate; and calving at marine margins.The prevalence of streaming within the retreating ice sheet suggests that the mean elevation of the ice sheet was lower than predicted from glaciological models which do not include streaming, and that this might reconcile glaciological models and earth rheology models which infer paleao-ice sheet thickness by inverting sea level data.

Processes of Glacier Erosion on Different Substrata

Most theories of glacier movement and subglacial erosion have assumed that glaciers rest on rigid bedrock surfaces. Whilst this is probably correct for much of the bed area of most modern glaciers, deformable sediments do occur beneath them and formed a substantial area of the beds of large ice sheets during glacial periods. Observations and theories are presented and reviewed about the processes of glacier erosion of rock and unlithified sediment beds both when they are frozen and unfrozen. Erosional bedrock landforms, such as roches moutonnees , indicate two principal subglacial erosional processes, plucking and abrasion. Where supraglacially derived debris is unimportant, plucking provides the tools which abrade the bed, and must be a quantitatively more important process than abrasion, though more localized. Where plucking is suppressed, erosion rates must be slow. Subglacial measurements of abrasion rates beneath a temperate glacier are used to test an earlier abrasional theory (Boulton, [ C 1974]). The form of the predicted abrasion-rate curve for changing ice velocity and pressure is verified. This theory successfully simulates two-dimensional erosional bedforms. Subglacial observations demonstrate how flow basal ice around the flanks of bedrock obstacles causes streaming of debris to occur. It is suggested that this streaming process is primarily responsible for the longitudinally lineated form of large-scale surfaces typical of glacially eroded bedrock. Plucking and abrasion also occur beneath cold ice, though at slow rates, and are probably restricted to places where the ice thickness is small. Where the glacier bed is composed of unlithified sediment, subglacial measurements show that deformation can produce very large discharges of subglacial material, which makes this a potential agent of very rapid subglacial landform production. The heterogeneity of subglacial sediment leads to spatially variable rates of deformation, and it is suggested that relatively stronger parts of the sediment body may form the nuclei for drumlin and mega-flute formation. Whereas unlithified unfrozen sediment deforms beneath the glacier rather than being incorporated within it, ice-cemented subglacial sediments can behave like bedrock, because of their relative rigidity, and are readily plucked and incorporated englacially. They may also deform beneath the glacier.

The interrelation of glaciotectonic and glaciodepositional processes within the glacial environment

Abstract In recent years it has been recognised that ice/sediment coupling occurred beneath the Quaternary ice sheets that advanced over the soft sediments of lowland areas. This paper looks in detail at the effects of this coupling on the sediments, which results in glaciotectonic deformation, and also discusses the interaction of deformation and deposition within the subglacial environment. Two types of glaciotectonic deformation are discussed that are produced by the active movement of ice: (1) proglacial tectonics at the margin, which include compressive fold styles such as listric thrusts and faults and open folding; (2) subglacial tectonics formed beneath the glacier, which include fold styles resulting from simple shear and represent a soft rock purely dynamic shear zone. Styles of deformation associated with stagnant ice are also investigated. We argue that glaciotectonic deformation is a common phenomenon and an integral part of geological record from continental ice sheets. It is suggested that zones of deformation within the sediment are related to similar zones of strain in the ice sheet, and complex deformation sequences are produced by the superimposition of these differing styles upon one another as the ice sheet advances and retreats. It is also argued that subglacial deposition and deformation are related and that on soft beds underformed till is rare, whilst deformed till is very common.

Theory of glacial erosion, transport and deposition as a consequence of subglacial sediment deformation

A theory of erosion, transport and deposition of unlithified sediments by glaciers is presented. It predicts the large-scale areal distribution of zones and rates of erosion and deposition in time and space through a complete glacial cycle, together with the resultant intensity of large-scale lincations (drumlins) which will be incised in the landscape. The theory also predicts the dispersal patterns of subglacial lithologies, together with the form of dispersal trains derived from distinctive sources and the vertical and horizontal distribution of lithologies within a till. It predicts major erosional discontinuities within tills and the formation of boulder pavements. It suggests that the dominant proportion of the lowland tills produced by Pleistocene mid-latitude ice sheets was generated by subglacial deformation and explains why they are predominantly fine-grained. The theory is based on an analysis of glacier-dynamic processes and therefore can be used to infer the dynamic behaviour of former ice sheets from the distribution of tills and their lithologic composition.

Modern Arctic glaciers as depositional models for former ice sheets

Sedimentary sequences currently forming at the margins of Spitsbergen glaciers are identical in thickness and detail to many Pleistocene and pre-Pleistocene glacigenic sequences. The transport of considerable volumes of englacial debris leads directly to the predominance of supraglacial till deposition, giving hummocky till surfaces and till plains. The association of supraglacial outwash with flow till produces tripartite till/outwash/till sequences, and multitill sequences, which are the result of a single glacier retreat phase. Complex tectonic structures, often with systematic regional trends are described, which are not the result of ice pushing but of downslope flow and collapse of supraglacial sequences above melting ice. New classifications are suggested for ice-contact stratified deposits and till, both of which depend upon position of deposition, supraglacial, englacial or subglacial It is suggested that existing models for the interpretation of ancient tills and the sequences in which they lie are often too simple and lead to erroneous stratigraphic and palaeogeographic conclusions. Till is too often interpreted solely as lodgement till, and it is suggested that many Pleistocene and earlier sequences, currently thought of as products of repeated glacier advance and readvance, may be perfectly normal products of a single retreat phase by a glacier with a thick englacial debris load. Ways of reconstructing the structural character of ancient ice margins are presented and it is also suggested that the thermal regimes of past ice sheets can be reconstructed from the nature of their deposits. The last Pleistocene ice sheet in Britain is thought to have been composed, at its maximum extent, of cold ice in the marginal zone and temperate ice in the internal zone.

Sediment deformation beneath glaciers and its coupling to the subglacial hydraulic system

Abstract The extent and style of shear deformation in sediments beneath modern glaciers and the geological evidence for such deformation in deposited sediments are reviewed. New evidence is presented from beneath a modern glacier of the spatial and temporal patterns of water pressure fluctuation and of time dependent patterns of deformation in sediments. It is concluded that in most experimental sites beneath soft-bedded modern glaciers, deformation is a significant or major contributor to glacier movement and the resultant discharge of till is large enough to make sediment deformation a major till forming process. Particular modes of deformation facilitate incorporation of underlying material into the till, whilst the capacity of a deforming till to absorb strain can protect the underlying strata from deformation, leading to the commonly found relationship where till overlies other strata with a sharp planar interface. It is argued that the almost ubiquitous occurrence of drumlins on the beds of former ice sheets is a reflection of the widespread occurrence of sediment deformation beneath them, with important implications for the coupling of ice sheet flow and bed properties. It is argued that the mechanical behaviour of the subglacial system is not simply determined by till properties but largely controlled by the subglacial water pressure regime determined by the nature of subglacial drainage. Results of field experiments show how the nature of the basal hydraulic system can play a vital role in controlling the coupling between the glacier and till deformation processes. They show that rapid glacier advances can produce undrained loading of sediments, that effective pressure may increase either upwards or downwards in a till according to the direction of drainage and that interstitial water pressures in subglacial sediments can show large and rapid variations, producing strong variations in the rate and distribution of strain and in the partitioning of basal movement between sliding and deformation.

Groundwater flow beneath ice sheets: Part II — Its impact on glacier tectonic structures and moraine formation

Abstract Meltwater flowing as groundwater from beneath the margin of an ice sheet determines the distribution of sub-surface heads and effective pressures. A subglacial groundwater flow model is used, together with an ice sheet loading model, to compute the magnitudes and directions of the principal effective stresses in the subsurface, from which the distribution of different types of sub-surface failure in the subglacial and proglacial zones are deduced. Zones of hydrofracturing, shear fracture and pervasive shear failure are distinguished. Beneath the ice sheet divide area, intact rocks of high tensile strength may fail. Hydrofracturing and liquefaction are two coupled processes which lead to the formation of upward-filled and downward-filled sediment dykes and till wedges. Quicksand conditions are developed where strong vertical seepage pressures occur, producing sediment diapirism. It is suggested that subglacial permeability magnitude may be the product of a self organising process. Certain types of moraine (extrusion moraines) are suggested to be a consequence of upward movement and surface extrusion of sediment driven by rising groundwater. It is suggested that groundwater over pressure associated with narrow proglacial permafrost plates are conductive to the formation of large push moraines, and that many large ancient and modern examples are produced in this setting.

THE ORIGIN OF GLACIALLY FLUTED SURFACES­ OBSERVATIONS AND THEORY

Studies of Auted surfaces beyond the margins of glaciers in Spitsbergen, Iceland, Norway and the Alps show that almost all emanate from rigid obstructions, commonly boulders in till. Field relations of Autes are described and it is shown that a relati onship exists between Aute height and the height of the initiating obst ruction. Subglacial observations indicate that Autes form when till is intruded into tunnels which tend to open up in the lee of obstacles. The pattern of strain implied by this process is shown to be reAected by micro- and macrofabrics in the till. The commonly found occurrence of an average spacing between Autes does not arise because of some rhythmic or periodic mechanism in the glacier, but is produced by the random seeding of boulders which themselves generate Autes. It is suggested that the term Aute be used as a genetic rather than a descriptive term, and be restricted to long parallel-sided ridges which reAect accurately the direction of ice movement and which form when deformable subglacial materials are intruded into tunnels which tend to open up on the lee sides of single

Sedimentary and sea level changes during glacial cycles and their control on glacimarine facies architecture

Abstract In part 1 a simple sedimentation model is presented in which subglacial icecontact, proglacial ice-contact, inner and outer proximal, and distal glacimarine zones are distinguished. In these, facies associations are controlled by: (a) distance from the glacier and the diffusion of glacially-derived water masses and their suspended sedimentary (including berg-transported) debris into nearby oceanic waters; (b) the form and depth of the sea bed, and its position in relation to the continental margin (which help determine water mass character and tidal, wave-driven and geostrophic currents and the stability of sea-bottom sediments). In part 2 a model of eustatic/isostatic sea level change in the vicinity of a generic ice sheet through a whole glacial cycle is developed. The sedimentation model from part 1 is linked to the sea level model to predict patterns of marine sedimentation near to an ice sheet through a glacial cycle, in inner shelf, outer shelf and continental slope environments. Comparisons are made between theoretical models and observed patterns of glacimarine facies architecture produced during the last glaciation of Britain, continental Europe and Spitsbergen. The phase relationship between local ice sheet advance and retreat and that of the Laurentide ice sheet, the principal determinant of global eustatic change, is a major determinant of large scale sedimentation patterns.