Glacigenic Succession Research Articles

The role of an ice-sheet, glacioisostatic movements and climate in the transformation of Middle Pleistocene depositional systems: a case study from the Reda site, northern Poland

ABSTRACT This study provides a new stratigraphic and palaeogeographic scheme for the time span between Marine Isotope Stages (MISs) 8 and 6. Based on detailed sedimentological and palynological studies, the roles of the Scandinavian Ice Sheet (SIS) advances and changing climate in the evolution of sedimentary environments is considered. At the Reda site, northern Poland, vertically stacked, 25 m thick glacigenic and fluvial successions were distinguished and divided into four depositional complexes. Two glacigenic units (R1 and R 2), representing two SIS advances in study area, are separated by two fluvial units (R2 and R3). The first SIS advance recorded at the Reda site took place at the end of MIS 8. Proglacial fluvial systems transported material along the ice margin towards the NW (R1 complex). Migration of the forebulge during deglaciation led to incision and opened the outflow towards the SW. During the younger part of MIS 7, the older fluvial unit (R2 complex) was deposited in a sand-bed braided fluvial system in boreal climate conditions. Local erosion and change in the sedimentation style into a meandering one under periglacial conditions took place (R3 complex). The progressive cooling of the climate towards the end of MIS 6 resulted in another SIS transgression and deposition of coarse-grained sediments (R4 complex) in the ice-marginal zone during deglaciation. SIS played a key role in the formation of R1 and R4 complexes. Glacial processes managed the reshaping the landscape through the erosion of older sediments and in the formation of accommodation space. Glacioisostatic movement controlled accommodation space, accumulation-erosion ratio, size of catchment and morphology of the valley.

Repeated ice streaming on the northwest Greenland continental shelf since the onset of the Middle Pleistocene Transition

Abstract. Ice streams provide a fundamental control on ice sheet discharge and depositional patterns along glaciated margins. This paper investigates ancient ice streams by presenting the first 3D seismic geomorphological analysis of a major glacigenic succession offshore Greenland. In Melville Bugt, northwest Greenland, six sets of landforms (five buried and one on the seafloor) have been interpreted as mega-scale glacial lineations (MSGLs) that provide evidence for extensive ice streams on outer palaeo-shelves. A gradual change in mean MSGL orientation and associated depocentres through time suggests that the palaeo-ice flow and sediment transport pathways migrated in response to the evolving submarine topography through each glacial–interglacial cycle. The stratigraphy and available chronology show that the MSGLs are confined to separate stratigraphic units and were most likely formed after the onset of the Middle Pleistocene Transition at ∼1.3 Ma. The MSGL record in Melville Bugt suggests that since ∼1.3 Ma, ice streams have regularly advanced across the continental shelf during glacial stages. High-resolution buried 3D landform records such as these have not been previously observed anywhere on the Greenland continental shelf margin and provide a crucial benchmark for testing how accurately numerical models are able to recreate past configurations of the Greenland Ice Sheet.

Upper Paleozoic glacigenic deposits of Gondwana: Stratigraphy and paleoenvironmental significance of a tillite succession in Northern Victoria Land (Antarctica)

Late Paleozoic glacial deposits are widespread in Gondwana. In Antarctica the best outcrops occur in the Central Transantarctic Mountains, Ellsworth Mountains, Pensacola Mountains, and in Southern Victoria Land, whereas they are rare and very scattered in Northern Victoria Land, which represents a linkage sector with Tasmania and eastern Australia. The aim of the present research is to define the stratigraphic setting and the environmental interpretation of a poorly studied Lower Permian glacigenic succession located near Orr Glacier in the Lanterman Range in Northern Victoria Land. It has great importance because it represents the only significant glacigenic succession for this wide area, marking and defining time and modality of a glacial-postglacial transition during the early Permian, a critical time-interval in the paleoenvironmental evolution of southern Gondwana. This succession, named here as the Lanterman Formation, lies directly on to crystalline basement, and is overlain by a thick Permian fluvial succession (Takrouna Formation). The results provided by sedimentological facies analysis and palynological analysis, integrated with a limited petrographic characterization, show a correlation framework between two isolated successions, outlining their stratigraphic architecture and subdivided into a lower glacial portion and an upper postglacial portion. The former, Asselian in age, is dominated by diamictite, sandstone, mudstone with lonestones, and carbonate deposits, suggesting repeated advance and retreat of a terrestrial glacier, with facies associations indicating environmental changes from subglacial to glaciofluvial, to glaciolacustrine. The latter is dominated by lacustrine mudstone with decreasing lonestones upward, thin bedded sandstone, conglomerate and a thick fluvial sandstone and pebbly sandstone succession at the top, signalling the transition to a fluvial system through time. The overall dataset suggests that the late Paleozoic glacigenic deposits of Northern Victoria Land record the evolution from a setting characterized by a terrestrial small ice cap that underwent repeated advances and retreats due to climate cycles in a lacustrine setting, into a postglacial paleoenvironmental scenario with a wide fluvial system draining northwards from Victoria Land towards the Tasmanian Basin. The results contribute to increasing knowledge of the timing and modalities of the Late Paleozoic Ice Age in Gondwana, particularly for polar regions such as Antarctica.

Glacitectonism, subglacial and glacilacustrine processes during a Neoproterozoic panglaciation, north‐east Svalbard

AbstractPalaeoenvironmental reconstruction of Neoproterozoic successions has been the subject of long‐standing debate, particularly concerning the interpretation of diamictites. The Wilsonbreen Formation of north‐east Svalbard is a 130 to 180 m thick diamictite‐dominated glacigenic succession deposited during a late Cryogenian (Marinoan) glaciation. Previous research has highlighted a complex sedimentary architecture with evidence of subaqueous, subglacial and non‐glacial conditions. This study combines well‐established sedimentological techniques with the first sedimentological application of the anisotropy of magnetic susceptibility technique in Neoproterozoic glacial sediments, to investigate the origin and palaeoenvironmental significance of glacigenic sediments within the Wilsonbreen Formation. A range of lithofacies occurs within the succession, dominated by massive diamictites, sandstones and conglomerates. Some of these facies display evidence of primary deformation and can be grouped into a Deformed Facies Association; these are interpreted to have been formed through glacitectonic deformation in a subglacial environment. Fabric investigation reveals that this deformation was associated with glacier flow towards the north. In addition, an Undeformed Facies Association records deposition in ice‐proximal and ice‐distal subaqueous environments. Taken together with intervening non‐glacial facies, the glacigenic sediments record a series of advance–retreat cycles, with ice flow involving sliding and sediment shearing below wet‐based ice.

The Hirnantian glacial landsystem of the Sahara: a meltwater-dominated system

During the Hirnantian (443 Ma), ice sheets expanded towards the present-day north over North Africa and the Arabian Peninsula when the western part of the Gondwana supercontinent straddled the South Pole. A glacigenic succession is exposed around the flanks of several Saharan cratonic basins, including Murzuq and Al Kufrah basins in Libya. As a result of the present-day hyper-arid context of the outcrops, a suite of palaeo-glacial landforms has been identified and, in some cases, mapped. These landforms occur along glacial erosion surfaces, defined as unconformities derived through direct ice sculpting, meltwater erosion or a combination of these processes. Unconformities, mostly resulting from ice sculpting and including glacially striated pavements and mega-scale glacial lineations (MSGLs), are identified at a number of locations along these erosion surfaces, in association with drumlinized features. The majority of reported examples are of ‘soft sediment character’; that is, generated through the deformation of unconsolidated sediment. Features mainly associated with meltwater erosion include tunnel valleys which occur in swarm-like belts, defining palaeo-grounding-line positions as the ice fronts retreated. A ‘smeared’ series of palaeo-ice-stream footprints, resulting from multiple advance–retreat cycles, is characteristic of the landsystem. Deformation of sediments into a series of push moraines, expressed as kilometre-scale fold–thrust belts, allows the locations of the former ice front to be identified. The Hirnantian glacial record of North Africa is a classic example of a continent-scale archive of an ancient palaeo-ice sheet. Expeditions to the Sahara during the late 1960s, focusing on the Algerian record, revealed a full suite of glacial indicators including striated pavements, pingos, roches moutonnees and a variety of meltwater channels (Beuf et al. 1971). Subsequently, largely as a result of their significance as hydrocarbon reservoirs, more recent studies re-evaluated these outcrops from a sedimentological perspective. Much effort focused on unravelling the glacial stratigraphy in neighbouring …

A rapid and synchronous initiation of the wide spread Cryogenian glaciations

The Neoproterozoic glaciations represent a remarkable milestone in Earth evolution due to their major influence on atmosphere, biosphere, and hydrosphere. The early Cryogenian, ‘Sturtian’ glaciations, evidence for which has been recorded worldwide, may have lasted for about 50 million years, but the precise timing and synchroneity of these glaciations have been disputed. Jiangkou glacigenic strata in South China can be attributed to the Sturtian glacial episode as they share sedimentological, stratigraphic and lithological similarities with Sturtian strata in South Australia. As such, dating the onset of the Jiangkou glaciation would contribute to testing the synchroneity of the worldwide Sturtian glaciation. In this paper, we demonstrate by means of SIMS U–Pb zircon dating that the Jiangkou glaciation in South China was initiated at 715.9±2.8Ma. The new age is, within uncertainty, synchronous with dates obtained from the lower parts of glacigenic successions in Laurentia and Oman, thereby indicating thereby indicating a synchroneity and global extent to the Sturtian glaciation. The synchronous occurrence of the Sturtian glaciation in mid- to low-latitude regions implies rapid ice sheet advancement in response to ice-albedo feedback, as supported by numerical climatic modeling.

Thermo-mechanical facies representative of fast and slow flowing ice sheets: the Weichselian ice sheet, a central west Poland case study

Abstract This study deals with an issue of thermo-mechanical facies, reflecting specific thermal and mechanical properties of the subglacial environment. The main objective of this study was to develop a model of glacitectionic deformation and its sedimentary record beneath fast and slow flowing ice sheets, based on investigations conducted in Wielkopolska (west central Poland). Sedimentary structures, mainly at the contact between subglacial tills and glacifluvial sediments, were recognized to delineate typical facies associations in a Weichselian glacigenic succession. Each association was interpreted as a record of the different depositional environments related to different subglacial conditions. Those investigations suggest the substratum was composed of frozen and dry, and wet and mobile spots, and four thermo-mechanical facies were distinguished: A – is representative of slower ice flow, dry and cold subglacial conditions, where driving stresses and normal effective pressure were high; B – is also related to slow ice flow and occurrence of cold subglacial permafrost, but with little amount of unfrozen water (however, higher than in facies A), with similar physical characteristics of the ice sheet as facies A; thermo-mechanical facies C and D represent wet and warm ice sole, with low normal effective pressure and driving stresses, thus lowering sediments’ shear strength and enabling high ice-flow velocities. We suggest that these facies have specific and non-random location, thereby revealing the relationship between subglacial thermo-mechanical conditions and ice sheet dynamics. Slow moving, cold-based ice occurred along ice sheet margins and inter-stream areas, whereas fast-moving, warm-based, well-lubricated ice, was typical of the axial parts of ice streams.

Late Devonian glacigenic and associated facies from the central Appalachian Basin, eastern United States

Late Devonian strata in the eastern United States are generally considered as having been deposited under warm tropical conditions. However, a stratigraphically restricted Late Devonian succession of diamictite-mudstone-sandstone within the Spechty Kopf and Rockwell Formations that extends for more than 400 km along depositional strike within the central Appalachian Basin may indicate otherwise. This lithologic association unconformably overlies the Catskill Formation, where a 3- to 5-m-thick interval of deformed strata occurs immediately below the diamictite strata. The diamictite facies consists of several subfacies that are interpreted to be subglacial, englacial, supraglacial meltout, and resedimented deposits. The mudstone facies that overlies the diamictite consists of subfacies of chaotically bedded, clast-poor mudstone, and laminated mudstone subfacies that represent subaqueous proximal debris flows and distal glaciolacustrine rhythmites or varvites, respectively. The pebbly sandstone facies is interpreted as proglacial braided outwash deposits that both preceded glacial advance and followed glacial retreat. Both the tectonic and depositional frameworks suggest that the facies were deposited in a terrestrial setting within the Appalachian foreland basin during a single glacial advance and retreat. Regionally, areas that were not covered by ice were subject to increased rainfall as indicated by wet-climate paleosols. River systems eroded deeper channels in response to sea-level drop during glacial advance. Marine facies to the west contain iceborne dropstone boulders preserved within contemporaneous units of the Cleveland Shale Member of the Ohio Shale. The stratigraphic interval correlative with sea-level drop, climate change, and glacigenic succession represents one of the Appalachian Basin's most prolific oil- and gas-producing intervals and is contemporaneous with a global episode of sea-level drop responsible for the deposition of the Hangenberg Shale/Sandstone of Europe. This interval records the Hangenberg biotic crisis near the Devonian-Carboniferous boundary.

Sedimentary environment of Ediacaran glacigenic diamictite in Guozigou of Xinjiang, China

The Talisayi Formation found in the Guozigou region on the southern side of Sayram Lake of Xinjiang, China is a Neoproterozoic glacigenic diamictite succession that has not been adequately researched. According to its isotopic geochronology and regional stratigraphic correlation, the Talisayi Formation is equivalent to the Ediacaran glacial sediments. The present paper provides the geochemical data of the Talisayi Formation that reveals the tectonic setting and sedimentary environment in this area during the Ediacaran period. The oxides of Si, K, Na and Al and their ratios of matrix materials from glacigenic diamictites and bulk rock of siltstone and argillaceous siltstones indicate that the sedimentary environment of this formation evolved from a tectonically active region to a passive continental margin. Values of CIA (carbonate-corrected Chemical Index of Alteration) ranging from 54 to 74 show a low-middle weathering action under a glacial environment. Total organic carbon (TOC), V/Cr, Ni/Co and U/Th ratios suggest that the sediments deposited in shallow water with an oxic and weak hydrodynamic condition. There is an abrupt rise in TOC, V/Cr, Ni/Co and U/Th ratios respectively in the overlying Lower Cambrian dolomitic limestones and in the bottom of Talisayi Formation, indicating a rapidl increase in biologic productivity and a variation in redox conditions, which was probably caused by seawater mixing of deep anoxic water and surface oxic water.

‘Cap carbonates’ and Neoproterozoic glacigenic successions from the Kimberley region, north‐west Australia

AbstractThe term ‘cap carbonate’ is commonly used to describe carbonate units associated with glacigenic deposits in Neoproterozoic successions. Attempts to use carbonate units as stratigraphic markers have been counfounded by inconsistent identification of ‘cap carbonates’ and a somewhat broad use of the term. Systematic sedimentological and geochemical analysis of carbonate rocks (mostly dolomite) associated with glacigenic deposits from the Neoproterozoic succession of the Kimberley region, north‐western Australia, shows that it is possible to characterize such units by their specific mineralogical, sedimentological, petrographic, geochemical and stratigraphic features. Hence, it is possible to differentiate true ‘cap carbonates’ from other carbonate units that are associated with glacigenic deposits. In the Kimberley successions two broad carbonate types are identified that reflect two stratigraphically distinct depositional realms. Carbonate rocks from the Egan Formation and Boonall Dolomite (the youngest carbonate units in the succession) are characterized by sedimentary components and features that are consistent with deposition on shallow platforms or shelves, analogous to Phanerozoic warm‐water carbonate platform deposits. In contrast, dolomite from the Walsh, Landrigan and Moonlight Valley Tillites preserves a suite of sedimentary and geochemical characteristics that are distinctly different from Phanerozoic‐like carbonate rocks; they are thin (ca 6 m), laterally persistent units of thinly laminated dolomicrite/dolomicrospar recording δ13C fluctuations from −1‰ to −5‰. These latter features are consistent with a ‘Marinoan‐style cap‐carbonate’ rock described from other Neoproterozoic successions. The similarity and broad distribution of these rocks in Australia, when considered within the context of genetic models suggesting a global oceanographic–atmospheric event, support their use as a lithostratigraphic marker horizon for the start of the Ediacaran Period at ca 635 Ma.

The strangest 0.05% of the geological history

The historical geological perspective is the main cause that much more is known about young rocks than about older ones. Most is therefore, obviously, known about Quaternary rocks, but this does not imply that the Quaternary is understood much better than the older periods. On the contrary, facies distributions — particularly in glacigenic successions — seem often so much more chaotic than in successions from other periods that the Quaternary could be considered as the strangest 0.05% in the geological history. It is most unlikely, however, that the Quaternary differs so much from other periods. It must therefore be concluded that many of the apparent discrepancies between Quaternary and older rocks either are due to different approaches followed during investigations, or reflect a geologically still immature situation, because destructive agents have had insufficient time to remove material or to obscure specific characteristics. In spite of these probably fundamental differences between Quaternary and older sediments — that will disappear with time — it is likely that the application of Quaternary investigation methods to older rocks — and that of hard-rock research methods to Quaternary successions — may contribute substantially to a better understanding of both the remote geological past and the nearby geological history.

Late Neoproterozoic stromatolites in glacigenic successions of the Kimberley region, Western Australia: evidence for a younger Marinoan glaciation

Tungussia julia, a Neoproterozoic III stromatolite, occurs in the Egan Formation (Louisa Downs Group), the uppermost glacigenic unit in the Kimberley region of Western Australia. Identification of this stromatolite is significant because it allows biostratigraphic correlation of the Egan Formation with the Julie Formation of the Amadeus Basin (Supersequence 3 of the Centralian Superbasin) and the upper Wonoka Formation of the Adelaide Geosyncline. Both the Julie and Wonoka Formations are stratigraphically younger than the Marinoan glacial sediments of the Adelaide Geosyncline and Centralian Superbasin. This implies that the Egan Formation records a younger glaciation in the late Neoproterozoic in Australia.

Neoproterozoic glaciation in the Broken Hill area, New South Wales, Australia

The Neoproterozoic sedimentary rocks on the eastern margin of the Adelaide Geosyncline include two major glacigenic successions. The lower of these, the Yancowinna Sub-Group (as much as 5,000 m thick), can be divided into four stratigraphic units deposited in a northwest-trending graben. Dramatic thickness variations are attributed to contemporaneous normal fault movements. The thin basal granite-boulder diamictite/conglomerate of Unit 1 is considered to represent ice-proximal deposition in a temperate glacial setting. Unit 2 is a heterogeneous mudstone-dominated succession formed from fine material provided by meltwater streams during glacial recession when the basin was starved of coarse debris. Coarser grained facies were deposited by sediment gravity-flow mechanisms. The thick diamictites of Unit 3 are interpreted as rain-out deposits formed during a second glacial advance. Unit 4 records the terminal glacial recession. Paleo-current measurements suggest that the sediment source was the Curnamona Cratonic Complex to the northwest. These rocks are the most easterly preserved Neoproterozoic glacigenic deposits of the Adelaide Geosyncline and are therefore relevant to comparisons with regions such as western North America, which were possibly adjacent, prior to the breakup of a Neoproterozoic supercontinent.

Glacial geology of outer Bjørnøyrenna, southwestern Barents Sea

The Upper Cenozoic glacigenic succession in outer Bjørnøyrenna (Bear Island Trough), southwestern Barents Sea is investigated utilizing data from six shallow boreholes (< 143 m below sea bed) and seismic profiling. Overdeepened troughs in the lowermost part of a large Plio-Pleistocene wedge at the shelf margin are interpreted as glacial erosion forms. This suggests that the overlying succession, which basinward attains a thickness of several km, was deposited during a period with glaciations. Large scale possible sand ridges in the stratigraphic lower part of this succession are confined to two north-south trending zones parallel to the present shelf break. They probably formed in a continental shelf/coastal environment with exceptionally high sediment supply and fluctuating relative sea level related to glaciations. A core from a channel fill in the lower part of the succession consists of possibly glaciofluvial gravelly sand intercalated with till. Most of the other cores are dominated by Middle and Upper Pleistocene muddy, massive, partly fissile diamictons interpreted to be mainly tills. Undrained shear strength of these cores ranges from about 20 kN m −2 to more than 300 kN m −2, and both underconsolidated intervals compared to the present burial depth are found. An underconsolidated interval comprising Eemian interglacial, biourbated marine sediments overlain by laminated Weichselian glaciomarine sediments on the outher shelf, is covered by 50–100 m of muddy diamicton. This diamicton is suggested to be mainly till deposited by a late Weichselian ice sheet which was grounded to more than 500 m below present sea level. Earlier Weichselian ice expansions did not reach the outer shelf. Based on the stratigraphy in the shallow cores and seismic interpretation we suggest that there have been five major expansions of grounded ice to the shelf break since ca. 440 ka. The present study suggests that glacial erosion was a major process in the Late Cenozoic denudation of the Barents Sea, and in large areas perhaps as much as 200–250 m of preexisting strata could have been eroded by glaciers since about 440 ka.

Late Proterozoic stratigraphy and the Canada-Australia connection

New stratigraphic studies in southeastern Australia and northwestern Canada permit a detailed comparison of the Late Proterozoic successions in these two areas. The remarkable similarities support the recent suggestion that the two regions were juxtaposed prior to the opening of the Pacific Ocean. The proposed correlations include a detailed breakdown of Late Proterozoic glacigenic successions and comparison of the overlying latest Precambrian, which involves a series of shoaling-upward sequences. The hypothesis of contiguity could be further tested by searching in the Cordilleran region for Late Proterozoic meteorite-impact ejecta recently documented in southeastern Australia and by isotopic investigation of correlative shales and carbonate units.

Sturtian glacial deposition in the vicinity of the Yankaninna Anticline, north Flinders Basin, south Australia

The Late Proterozoic (Sturtian) glacigenic succession in the vicinity of the Yankaninna Anticline, in the central North Flinders Basin, varies in thickness from 60 to 1500 m. These dramatic thickness changes are attributed in part to a pre‐existing topography, but were probably also controlled by contemporaneous faulting. Despite variations in thickness, a common, probably palaeoclimatically‐controlled, four‐fold stratigraphy is present throughout the region. Palaeocurrents, mainly from ripple cross‐laminations in resedimented rocks, indicate a centrifugal pattern of sediment dispersal around the Yankaninna Anticline. Together with megabreccias containing fragments from the underlying Burra Group, this evidence suggests that the area now occupied by the Yankaninna Anticline may have been a submarine(?) high in the central part of the North Flinders Basin during the Sturtian glaciation. Although two distinct Sturtian glacigenic diamictite‐bearing units are present in the area, no evidence was found for an ...

Major element geochemistry and paleoclimatology of the Permo-Carboniferous glacigene Dwyka Formation and postglacial mudrocks in southern Africa

A Chemical Index of Alteration (CIA) is used to quantify the degree of weathering of fine-grained materials in the glacigenic Permo-Carboniferous Dwyka Formation and succeeding stratigraphic units of southern Africa. Diamictites in the Dwyka Formation have low CIA values compatible with their glacigenic origin. Interbedded mudrocks show a wide scatter of values, suggesting that at least some of them were deposited during periods when ice had retreated from the source areas. Samples from the Prince Albert shale, which immediately overlies the Dwyka Formation, show high CIA values, suggesting a rapid transition to a much more intensive weathering regime (warmer climatic conditions?). Samples from the succeeding Whitehill shale have a low average value, so that weathering was once more inhibited, perhaps by a temporary return to a colder climate. The highest stratigraphic units sampled, the “upper shales”, have an average CIA value comparable to that of “average” shales, and suggesting “normal” weathering conditions. Major element geochemical data can supplement sedimentological and paleontological information and provide a valuable contribution to the detailed paleoclimatic interpretation of such glacigenic successions.

Multiple Wisconsinan Glacigenic Sequences at Wedron, Illinois

Lithofacies and their relationships, sedimentological and glaciotectonic structures, pebble fabrics, and laboratory data were used to delineate seven facies associations in a classic upper Wisconsinan glacigenic succession near Wedron, Illinois. Each association is interpreted to represent a different depositional environment. Proglacial facies include massive and laminated silt (lacustrine), bedded sand (fluvial), and massive silt (eolian). Subglacial facies consist of homogeneous diamicton (lodgement till) and variable diamicton and sorted sediment deformed by folding and shearing (deformed till). An ice-marginal facies of heterogenous diamicton and sorted sediment (supraglacial redeposited sediment) and a paraglacial facies of laminated silt (lacustrine) also are present. Stratigraphic relationships among the facies allow the delineation of three glacigenic sequences from which three glacial events are inferred. Each sequence consists of proglacial fluvial and/or lacustrine sediment, subglacial till and fluvial deposits, and supraglacial and ice-marginal sediment-flow, fluvial, and lacustrine deposits. The complete nature of these glacigenic sequences suggests that the area was located primarily in the depositional zone of the Laurentide Ice Sheet during the late Wisconsinan. Erosional contacts and localized sorted sediment between three lithologically distinct till facies in the lowest sequence are interpreted to be subglacial in origin; the three facies were deposited during the same event, rather than separate events as previously inferred. Spatial and temporal variability among the subglacial processes of erosion, transportation, deformation, and lodgement is evident within that sequence.

Middle Pleistocene glacial and glaciomarine sedimentation in the west central North Sea

A sequence of Middle Pleistocene (approximately early ‘Cromerian Complex’) sediments has been subdivided into subglacial, proximal glaciomarine, distal glaciomarine and marine facies. The subglacial facies represents lodgement till deposited during the final stages of ice‐sheet advance. At the onset of ice‐sheet retreat, streams deposited their load into a shallow‐water; glaciomarine environment; gravelly sediments immediately in front of the ice‐grounding line and finer material, in suspension, to more distal areas. Ice‐rafting, slumping and traction currents were also active within the glaciomarine environment. The lithofacies characteristics of this sequence are consistent with deposition from a grounded tidewater ice‐sheet. The glacigenic succession is restricted to the Forth Approaches area, which implies that the ice‐sheet had a limited offshore extent.

The Late Devensian glaciation along the border of northeast Wales

AbstractThe glacigenic succession along the border of northeast Wales records the advance, coalescence and subsequent uncoupling of Welsh and Irish Sea ice‐sheets during the Late Devensian cold stage. During coalescence, extensive basal tills were deposited and considerable diversion of pre‐existing drainage was accomplished by subglacial meltwater. Analysis of borehole data and quarry exposure indicates that a complex assemblage of ice‐contact, proglacial fluviatile and lacustrine environments was developed during uncoupling and subsequent retreat. Neither of the previously proposed models for the formation of the so‐called Wrexham ‘delta‐terrace’ are adequate to explain the wide variety of sedimentary successions observed and the feature is a complex, diachronous structure that reflects rapidly changing depositionai conditions, including ice‐front alluvial fans, sandar and proglacial and ice‐contact lakes, formed at the margin of a retreating ice‐sheet.