Subglacial Bed Conditions Research Articles

Elsterian ice‐sheet retreat in the southern North Sea: antecedent controls on large‐scale glaciotectonics and subglacial bed conditions

High‐resolution shallow sub‐bottom seismic survey data obtained for the Dudgeon windfarm site located 50 km offshore of the north Norfolk coast in the southern North Sea have revealed that the chalk bedrock in this area is overlain by sediments deposited during the Elsterian and Weichselian glacial periods. A buried N–S‐trending subglacial drainage channel (tunnel valley) filled with Swarte Bank Formation (MIS 12) sediments indicates that the maximum extent of the Elsterian ice‐sheet margin occurred further to the south. Detailed cross‐sections constructed from the seismic data reveal the presence of buried thrust‐block moraine system composed of deformed (folded and thrusted) Swarte Bank Formation sediments, lying beneath a younger sequence of sediments dominated by the glacigenic Bolders Bank Formation (MIS 2). The geometry of the folds and sense of offset on the thrusts, coupled with the morphology of the ridge‐like landforms within this buried moraine system, are consistent with deformation having occurred in response to ice‐push from the N/NE. The chalk bedrock that underlies the glaciotectonized sequence is thought to have provided an antecedent control on the location and preservation of this moraine system. A three‐phase model is proposed to explain the evolution of this thrust‐moraine complex that formed at the oscillating ice margin during the overall active retreat of the Elsterian ice sheet.

Microstructures, subglacial till deposition, and shear band development revealing up-section changes in shear–A study from Weissbach, Austria

Tills from an exposure in Wildschönau Valley, northern Austria were examined using microsedimentological techniques. The tills exhibit a range of microstructures indicative of soft sediment deformation within temperate subglacial bed conditions. The tills can be subdivided at the macroscale into a lower grey and upper red till both of which exhibit some sedimentological variations; however, at the micro-level the tills appear essentially identical. The microstructures in the tills are illustrative of structures developed during deformation both during and following their emplacement. Of note are the microshears within these tills that are demonstrative of changes in applied stress. Both low (<25°) and high angle (>25°) microshears were mapped and their fabric data analyzed. The microshears show a change in stress levels ascending through successive till units. The changes in stress are demonstrative of spatially and temporally changing rheological conditions undergone by the subglacial tills during deformation, ongoing deposition/ emplacement and stress localization. These findings indicate that microstructures reveal local deformation conditions in tills and a more detailed micro-history of paleo-stress.

Subglacial emplacement of tills and meltwater deposits at the base of overdeepened bedrock troughs

AbstractThe sedimentary infills of subglacially eroded bedrock troughs in the Alps are underexplored archives for the timing, extent and character of Pleistocene glaciations but may contain excellent records of the Quaternary landscape evolution over several glacial–interglacial cycles. The onset of sedimentation in these bedrock troughs is often reflected by diamicts and gravels directly overlying bedrock in the deepest basin segments. Subglacial or proglacial depositional environments have been proposed for these coarse‐grained basal units but their characteristics and origin remain controversial. This article presents results from drill cores that recovered a coarse‐grained basal unit in a major buried bedrock‐trough system in the Lower Glatt Valley, northern Switzerland. The excellent core recovery allowed a detailed study combining macroscopic, microscopic and geochemical methods and gives unprecedented insights into the transition from erosion to deposition in overdeepened bedrock troughs. These results show that the basal infill comprises diamicts, interpreted as subglacial tills, separated by thin sorted interbeds, originating from subglacial cavity deposition. The stacking of these units is interpreted to represent repeated switching between a coupled and decoupled ice–bed‐interface indicating an ever‐transforming mosaic of subglacial bed conditions. Decoupling in response to high basal water pressures is probably promoted by the confined subglacial hydraulic conditions resulting from the bedrock acting as aquitards, the narrow reverse sloping outlet and a large catchment area. While stratigraphic and lithological evidence suggests that erosion and the onset of basal sedimentation occurred during the same glaciation, different scenarios for the relative timing of infilling in relation to formation and glaciation of the bedrock trough are discussed. Overlying deltaic and glaciolacustrine sediments suggest deposition during subsequent deglaciation of the bedrock trough. The basal sediment characteristics are in agreement with previous reports in hydrogeological and seismic exploration and suggest the occurrence of similar basal successions in other subglacially overdeepened basins in the Alps and elsewhere.

Ribbed moraines in northern Manitoba, Canada: characteristics and preservation as part of a subglacial bed mosaic near the core regions of ice sheets

Ribbed moraines are enigmatic glacial landforms for which different models, with contrasting paleoglaciologic implications, have been proposed to explain their formation. Despite the great deal of attention this type of landform has received over the last several decades, ribbed moraine fields in northern Manitoba, Canada are among the largest in the world but have been seldom studied. Ribbed moraines in this part of the world overlie the low-relief Canadian Shield, are not constrained by topography, and are part of a spatial subglacial-landform assemblage associated with drumlinoid ridges within palimpsest and relict-type Glacial Terrain Zones. Field observations herein provide new insights into the characteristics of these transverse-to ice-flow ridges at landscape (mapping and spatial analysis) and landform (internal structure using high-resolution shear wave (S-wave) seismic reflection surveys, sedimentological characteristics, clast-fabric analyses) scales. Two main types of ribbed moraine are recognized: ‘pristine’, high amplitude straight-crested ridges and secondarily-modified subdued ‘drumlinized’ ridges. Ribbed moraine in northeast Manitoba consist of massive, matrix-supported till at surface, which is similar in matrix texture and composition to the regional till sheet, though pristine moraines show a higher concentration of boulders. A seismic profile reveals subparallel-to surface layered stratigraphy with only minor folding and no major unconformities (stacking or faulting).The demonstrated fragmentary nature of ribbed moraine fields, the inherited signature of the till within these fields, the secondary patchy drumlinization of the ridges, and the more abundant granitoid outcrops in areas of intense modification all suggest that subglacial ribbed moraine formed from pre-existing sediments and were later preserved and/or partially reworked through a regional mosaic of shifting subglacial bed conditions. We therefore make the case that pristine (unmodified) moraines were preserved beneath stable sticky spots. We support a link between ribbed moraine and widespread cold-based and/or dewatered subglacial conditions in inner-core regions of ice sheets, but for reasons of preservation, and not necessarily formation.

Subglacial bed conditions during Late Pleistocene glaciations and their impact on ice dynamics in the southern North Sea

Passchier, S., Laban, C., Mesdag, C.S. &amp; Rijsdijk, K.F. 2010: Subglacial bed conditions during Late Pleistocene glaciations and their impact on ice dynamics in the southern North Sea. Boreas, Vol. 39, pp. 633–647. 10.1111/j.1502‐3885.2009.00138.x. ISSN 0300‐9483.Changes in subglacial bed conditions through multiple glaciations and their effect on ice dynamics are addressed through an analysis of glacigenic sequences in the Upper Pleistocene stratigraphy of the southern North Sea basin. During Elsterian (MIS 12) ice growth, till deposition was subdued when ice became stagnant over a permeable substrate of fluvial sediments, and meltwater infiltrated into the bed. Headward erosion during glacial retreat produced a dense network of glacial valleys up to several hundreds of metres deep. A Saalian (MIS 6) glacial advance phase resulted in the deposition of a sheet of stiff sandy tills and terminal moraines. Meltwater was at least partially evacuated through the till layer, resulting in the development of a rigid bed. During the later part of the Saalian glaciation, ice‐stream inception can be related to the development of a glacial lake to the north and west of the study area. The presence of meltwater channels incised into the floors of glacial troughs is indicative of high subglacial water pressures, which may have played a role in the onset of ice streaming. We speculate that streaming ice flow in the later part of the Saalian glaciation caused the relatively early deglaciation, as recorded in the Amsterdam Terminal borehole. These results suggest that changing subglacial bed conditions through glacial cycles could have a strong impact on ice dynamics and require consideration in ice‐sheet reconstructions.

Causes of sudden, short‐term changes in ice‐stream surface elevation

Recent satellite‐borne observations of Antarctica's ice streams show sudden, spatially confined surface‐elevation changes that are interpreted as caused by subglacial water movement. Using a numerical model of idealized ice‐stream flow coupled to various simple treatments of subglacial bed conditions, we demonstrate that ice‐stream flow dynamics significantly modulates the surface‐elevation expression of processes taking place at the ice‐stream bed. This modulation means that observed surface‐elevation changes do not directly translate to basal‐elevation changes, e.g. inflation or deflation of subglacial water pockets, of equal magnitude and shape. Thus, subglacial water volume change is not directly proportional to the area integral of surface‐elevation changes. Model results show that ambiguities in interpretation of surface elevation changes can be overcome with additional measurements, such as of surface velocity change, and through development of methodology designed to understand transfer of basal change to surface change.

Radar‐derived bed roughness characterization of Institute and Möller ice streams, West Antarctica, and comparison with Siple Coast ice streams

Subglacial bed conditions exert a significant control on ice stream behavior and evolution, and can be characterized by determining bed roughness from FFT analysis of radar‐imaged basal reflectors. Here we assess bed roughness across Institute and Möller ice streams, West Antarctica, and compare our findings with bed roughness determined across the Siple Coast ice streams. We find that variations in bed roughness are spatially organized, and attribute this to the varying efficacy of subglacial erosion and deposition, with rougher (inland, slow‐flowing) regions largely manifesting preglacial topography, and smoother (downstream, fast‐flowing) regions evincing significant postglacial modification to the subglacial landscape. The observed similarities between bed roughness characteristics of IIS/MIS and the Siple ice streams suggest that IIS and MIS are largely underlain by wet, poorly consolidated sediments, and may therefore be vulnerable to the types of dynamical instabilities experienced by the Siple ice streams.

Radiocarbon constraints on the age of the maximum advance of the British–Irish Ice Sheet in the Celtic Sea

Reconstructions of the British–Irish Ice Sheet (BIIS) during the Last Glacial Maximum (LGM) in the Celtic Sea and southern Ireland have been hampered by a paucity of well-dated stratigraphic records. As a result, the timing of the last advance of the largest outlet of the BIIS, the Irish Sea Ice Stream, to its maximum limit in the Celtic Sea has been variously proposed as being pre-last glaciation, Early Devensian and LGM. The Irish Sea Till was deposited by the Irish Sea Ice Stream during its last advance into the Celtic Sea. We present 26, stratigraphically well constrained, new AMS radiocarbon dates on glacially transported marine shells from the Irish Sea Till in southern Ireland, which constrain the maximum age of this advance. The youngest of these dates indicate that the BIIS advanced to its overall maximum limit in the Celtic Sea after 26,000–20,000 14C yr BP, thus during the last glaciation. The most extensive phase of BIIS growth therefore appears to have occurred during the LGM, at least along the Celtic Sea and Irish margins. These data further demonstrate that the uppermost inland glacial tills, from the area of supposed “older drift” in southern Ireland, a region previously regarded as having been unglaciated during the LGM also date from the last glaciation. Thus most of southern Ireland was ice covered at the LGM. Advance of the BIIS to its maximum southern limit in the Celtic Sea may have been a short-lived glaciodynamic response facilitated by subglacial bed conditions, rather than a steady-state response to climate forcing alone.

Macro‐scale bed roughness of the siple coast ice streams in West Antarctica

AbstractSubglacial bed conditions are key to understanding ice stream behaviour and evolution, with bed roughness reflecting substrate composition and flow resistance. Here we present an analysis of bed roughness in the Siple Coast region of West Antarctica from airborne radio‐echo sounding data. The ice streams are associated generally with low bed roughness values, which decrease downstream. The bed of the slow‐flowing Ice Stream C (∼10 000 km2) is also characterized by being smooth at all scales (wavelengths ranging from 5 km to in excess of 40 km). Furthermore, the bed is smooth either side of Ice Stream C. This suggests the location of the ice stream is controlled by internal ice sheet dynamics rather than by bed morphology. If the ice stream were encouraged to migrate laterally, when active, there would be little resistance offered by the subglacial morphology. Other inter‐ice stream regions are rough, however, indicating a subglacial topographic influence on ice stream position. Bed roughness increases up‐flow of ice streams, which, unless the bed is modified, may limit the inland migration of these systems. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Evidence, from microstructures, of deformable bed conditions within drumlins, Chimney Bluffs, New York State

Discussion on the process of drumlin formation has been ongoing for over a century. At present, two dominant schools of thought prevail, viz. formation by (a) subglacial hydraulic mechanisms, and (b) due to subglacial sediment deformation. The latter mechanism depends on the existence of a subglacial deformable bed. Evidence from part of the large central New York drumlin field reveals evidence of syn-depositional deformation during the emplacement of diamictons that constitute the major portion of drumlins exposed along shore bluffs in Chimney Bluffs State Park. These diamictons cannot be lodgement till facies members but appear to comprise facies indicative of melange-style deposition under subglacial deformable bed conditions. The evidence obtained from microstructures and plasma fabric within these diamictons supports the existence of a deformable bed during diamicton emplacement. It is likely that formation of the drumlins occurred under these same subglacial bed conditions.

Patterns of glacial erosion and deposition in Prydz Bay and the past behaviour of the Lambert Glacier

Styles of glacial sedimentation and erosion in Prydz Bay respond to glacial and interglacial cycles and fall into three zones; an inner zone of net erosion, a middle zone of subglacially eroded and deformed transitional glacial marine deposits and an outer zone of subglacial till deposition and shelf progradation. The inner zone is the region of maximum basal shear stress and inner-shelf deep formation by enhanced erosion in areas where tributary glaciers converge with the extended Lambert Glacier. The middle zone is underlain by sediments deposited near the ice-grounding zone during retreat, both as blanket like deposits and as grounding-line moraines. This material is then deformed into elongate subglacial bedforms (megaflures or drumlins), a process that probably involves some erosion. Deforming subglacial bed conditions extend to the shelf edge within a valley crossing the shelf on the western side of Prydz Bay. The outer zone is a zone of net deposition of compact subglacial till and prograding continental slope deposits formed during full glacial conditions and glacial marine sediment formed during ice retreat. The inferred build up of ice on the Ingrid Christensen Coast may have been responsible for the development of the western ice stream that flowed in Prydz Channel. The geometry of seismic sequences in Prydz Channel suggests that this ice stream and its associated trough mouth fan developed after a major episode of shelf and slope erosion during the Pliocene.

Tunnel-valley formation in northwest Germany—geology, mechanisms of formation and subglacial bed conditions for the Bornhöved tunnel valley

The Bornhöved tunnel valley consists of a relatively broad, elongated depression filled primarily with tills, and a narrow channel in the axial part filled with outwash, glaciolacustrine sediments and tills. The tunnel valley is at least 13 km long, 222 m deep (bottom at 191 m b.s.l.), and extends parallel to the axis of a peripheral sink between two Permian salt diapirs in the substratum. It is suggested that the initiation of the tunnel valley was controlled by a differential response of the bed materials to ice overriding on top of and between the salt structures. The Bornhöved tunnel valley is a polygenetic feature that resulted from different erosional processes throughout all three main glaciations in northern Germany. During the Elsterian Glaciation glaciotectonic squeezing occurred, to be later replaced by subglacial meltwater erosion. During the ice retreat and the Holsteinian Interglacial, the valley was filled with fine-grained glaciolacustrine and marine sediments, which were subsequently removed and redeposited further down-ice as a 200-m-thick glaciotectonic melange by the first Saalian advance. The second Saalian advance caused only minor changes and the tunnel valley was reactivated as a subglacial channel during the Weichselian Glaciation. It is suggested that at the time of the first Weichselian ice advance a large subglacial water reservoir developed in the area of the Baltic Sea basin and caused a rapid, surge-like ice movement. As the ice sheet advanced out of the Baltic Sea basin, drainage of the water reservoir was prevented by the ice toe overriding the permafrost on the Saalian highlands. During the ice retreat, frozen ground was left beyond the ice margin and subglacial meltwater catastrophically drained through the tunnel valley with a discharge roughly estimated at about 3.75 × 10 3 m 3/s. It seems possible that drainage of the subglacial water collector through the Bornhöved tunnel valley and other similar channels south of the Baltic Sea led to increased ice-sheet basal friction, and was one reason for successively smaller extents of the subsequent Weichselian advances. Evidence against pervasively deforming bed materials in the vicinity of the Bornhöved tunnel valley in the Weichselian Glaciation indicates that the valley served as an efficient drainage path for porewater from the subglacial sediments.

Unstable Behavior of the Laurentide Ice Sheet over Deforming Sediment and Its Implications for Climate Change

Geologic records of fluctuations of the Laurentide ice sheet margin following the most recent glacial maximum ( ca. 20,000 14C yr B.P.) identify fundamental differences in ice-sheet behavior depending on subglacial bed conditions. Rapid and irregular icemargin fluctuations occurred only over areas of deforming sediment, indicating nonclimatic forcing controlled by the inherent instability of coupled ice sheet-deforming sediment dynamics. In contrast, largely uninterrupted ice-margin retreat with no evidence of significant readvance occurred over rigid-bed areas, indicating stable behavior. Unstable ice-sheet behavior was most pronounced from 15,000 until 10,000 14C yr B.P., by which time most of the ice margin had retreated onto a rigid bed. Unstable ice-sheet behavior would have been an integral component in controlling variable fluxes of icebergs and meltwater, as well as meltwater routing, to the North Atlantic, thus affecting thermohaline circulation. The abrupt climate oscillations in the North Atlantic region that ended at 10,000 14C yr B.P. may thus have their origin in the inherently unstable behavior of the Laurentide ice sheet overriding deforming sediment.

Microstructures in diamictons—evidence of subglacial bed conditions

Examination of microstructures within diamictons from southern Ontario, Canada, and southern Bavaria, and Baden-Württemburg, Germany reveal evidence illustrative of certain subglacial bed conditions. The microstructures from Ontario were obtained from a complex glacial mélange unit that forms the internal structure of an isolated drumlin/megaflute on the north shore of Lake Erie. The evidence from Germany was taken from individual drumlins within drumlin fields in the Inn-Chiemsee, Iller, Rhein and Salzach glacier areas. The structures within the diamicton from Ontario reveal clear evidence of massive shear associated with high externally applied strain. There are kink-band arrays, high angle shear zones, crenulation and sedimentary “dyke” and water escapes structures. In the German diamictons there is a lack of evidence of high levels of bulk deformation. There is, however, a very high carbonate content. Some evidence points to the possibility that these diamictons may contain intraclasts of paleosols. Interpretation of both sets of diamictons suggests that the Ontario sediments were subject to conditions at the bed of an ice mass best typified as a deformable bed. The German sediments may be interpreted as: (1) being so saturated that any deformation must have been by inter-grain slip (pervasive deformation) and thus no structures are recorded; or (2) being already cemented by the carbonates and sufficiently rigid to cause strain to be taken up elsewhere; or (3) were frozen prior to transposition and were again too rigid to suffer deformation; or (4) suffered no deformation whatsoever. The analyses of these two sets of sediments have important implications for subglacial conditions before and during drumlin and other subglacial bedform development.