Stratigraphic architecture and sedimentology of a Late Pleistocene subaqueous moraine complex,southwest Ireland

Glacigenic sediments exposed in coastal cliffs cut through undulatory terrain fronting the Last Glacial Maximum laterofrontal moraine at Waterville on the Iveragh Peninsula, southwest Ireland, comprise three lithofacies. Lithofacies 1 and 2 consist of interdigitated, offlapping and superimposed ice‐proximal subaqueous outwash and stacked sequences of cohesionless and cohesive subaqueous debris flows, winnowed lag gravels and coarse‐grained suspension deposits. These are indicative of sedimentation in and around small grounding line fans that prograded from an oscillating glacier margin into a proglacial, interlobate lake. Lithofacies 3 comprises braided river deposits that have undergone significant syn‐sedimentary soft‐sediment deformation. Deposition was likely related to proglacial outwash activity and records the reduction of accommodation space for subaqueous sedimentation, either through the lowering of proglacial water levels or due to basin infilling. The stratigraphic architecture and sedimentology of the moraine at Waterville highlight the role of ice‐marginal depositional processes in the construction of morphostratigraphically significant ‘end moraine’ complexes in Great Britain and Ireland. Traditional ‘tills’ in these moraines are often crudely stratified diamictons and gravelly clinoforms deposited in ice‐proximal subaqueous and subaerial fans. Copyright © 2011 John Wiley & Sons, Ltd.     

Glaciomarine facies within subglacial tunnel valleys: the sedimentary record of glacioisostatic downwarping in the Irish Sea Basin

Coastal exposures of Late Pleistocene sediments deposited after 19 000 yr BP near Dublin, Ireland, provide a window into the infill of a subglacially-cut tunnel valley. Exposures close to the steeply dipping bedrock wall of the valley show boulder gravels within multi-storey U-shaped channels cut and filled by subglacial meltwaters driven by a high hydrostatic head. Gravels are truncated by poorly sorted ice-proximal glaciomarine sediments that record the pumping of large volumes of subglacial debris along the tunnel valley to a tidewater ice sheet margin. The sedimentary succession is dominated by sediment gravity flow facies comprising interbedded diamict and massive, poorly sorted gravel facies interpreted as subaqueous debris flow deposits. Gravel beds show local inverse and normal coarse-tail graded facies recording the restricted development of turbulent flow. Sediment gravity flow deposits fill broad (<2 km) shallow (10 m) and overlapping channels. Penetrative deformation structures (e.g. dykes) are common at the base of channels. The same subglacially-eroded topography and glaciomarine infill stratigraphy can be identified on high resolution seismic profiles across nearly 600 km² of the western Irish Sea. Tunnel valleys are argued to have been exposed to glaciomarine processes by the rapid retreat of a calving tidewater ice sheet margin in response to marine flooding caused by glacio-isostatic downwarping below the last British Ice Sheet. The facies associations described in this paper comprise an event stratigraphy that may be found on other glaciated continental shelves.     

Late-Pleistocene tidewater glaciers and glaciomarine sequences from north County Mayo,Republic of Ireland

Late-Pleistocene deposits in north County Mayo were deposited in three main glacigenic environments. 1. Drumlins and basal tills were formed when ice moved from the Irish lowlands and local mountain catchments into Donegal Bay. 2. Gilbert-type deltas accumulated up to 80m I.O.D. on the lowlands and subaqueous moraines formed across minor valleys when marine-based ice grounded inland. 3. A thick drape of fossiliferous glaciomarine mud along the coastal fringe was deposited from meltwater plumes and by ice-rafting immediately outside of these ice limits. The muds contain an Elphidium clavatum-dominated, low-diversity microfauna which is characteristic of cold-water conditions adjacent to glacier termini. Valves of Macoma calcarea from the mud have been ¹⁴C dated at 16940 ± 120 and 17300 ± 100 BP. The high-level delta complex was deposited from tidewater glaciers in a peripheral depression adjacent to the drumlin ice limits of north County Mayo. Although the field evidence cannot be used to determine former sea level history with any accuracy, it poses general problems for sea level history and isostatic effects of the last major ice sheet in the west of Ireland. Raised glaciomarine sequences commonly occur in close association with drumlin ice limits elsewhere in Ireland and represent marine transgressions prior to glacial unloading. It is suggested that the magnitudes and patterns of crustal depression are greater and geometrically more complex at the margins of ice sheets in Ireland than hitherto realised.     

The Late Devensian (<22,000 BP) Irish Sea Basin: The sedimentary record of a collapsed ice sheet margin

The Late Devensian (<20 ka BP) glacial geology of the Irish Sea Basin (4000 km²) is an event stratigraphy recording the entry of marine waters into a glacio-isostatically-depressed basin, and the rapid retreat of the Irish Sea Glacier as a tidewater ice margin. Marine limits occur up to 140 m O.D. Across much of the central basin, the ice margin was uncoupled from its bed exposing a subglacially-scoured topography to glaciomarine processes. The Irish Sea Glacier was a major drainage conduit of the last British Ice Sheet; calving of the marine ice margin resulted in fast flow (surging) of ice streams recorded by drumlin fields around the northern basin margin and tunnel valleys. Rapid evacuation of the basin may have stranded large areas of dead ice in peripheral zones (e.g. Cheshire/Shropshire Lowlands) and initiated the collapse of the ice sheet.Thick wedges of ice-contact glaciomarine sediments were deposited during ice retreat as morainal bank complexes by successive tidewater ice margins stabilized at pinning points around the Irish Sea coast. Where morainal banks occur on the seaward side of drumlin swarms there is a clear sequential relationship between rapid ice loss from calving ice margins, the development of fast flowing ice streams, drumlinization and the pumping of subglacial sediment to tidewater. Raised delta complexes are locally associated with marine limits along the high relief coastal margins of Wales, east central Ireland, and the Lake District. Associated valley infill complexes record downslope resedimentation of heterogenous sediments into the marine environment during ice retreat. Co-eval offshore deposits are represented by well-stratified glaciomarine complexes that infill a subglacially-scoured topography that shows networks of tunnel valleys. Glaciomarine mud drapes occur well to the south of the maximum limit of grounded ice in the basin (e.g. North Devon, Scilly Islands, Southern Ireland). The age of these distal sediments, previously mapped as pre-Devensian tills, is constrained by amino acid ratios.Basin rebound following deglaciation was rapid, with over 100 m recovery in 3 ka, and was followed by a low marine still stand. Peat, accumulating in offshore areas now as much as 55 m below sea level has been drowned by the postglacial eustatic rise in sea level.The glacio-sedimentary model identified in this paper, involving rapid ice retreat and related sedimentation triggered by rising relative sea level, suggests that isotatic downwarping is an important mechanism for deglaciating continental shelves.     

Late Pleistocene morainal bank facies at Greystones, eastern Ireland: an example of sedimentation during ice marginal re-equilibration in an isostatically depressed basin

A late Pleistocene morainal bank is sited in a depocentre to the lee of a major rock ridge, near Greystones, in the western Irish Sea Basin. During deglaciation the ridge provided a pinning point during tidewater wastage northwards. Sedimentation patterns and palaeocurrent data show morainal bank growth by discharge from a single basal efflux located to the east or south-east of the ridge during ice marginal re-equilibration. The four lithofacies associations which are recognized from the western part of the formerly more extensive apron are related largely to variable jet and plume sedimentation. At the base of the 1.6 km long exposure, Lithofacies association 1 (massive mud, muddy diamict and laminated mud) was deposited from turbid plumes, variable ice rafting and traction current activity. Lenticular units of gravels within this mud bank record high energy pulses and sediment fluxes from the efflux jet. Lithofacies association 2 (sands, laminated muds and muddy diamict) is discontinuous and occurs within basins along a marked erosion surface cut in Lithofacies association 1. It is associated with a decrease in jet strength, traction currents and suspension sedimentation. Lithofacies association 3 is a tabular body of interbedded diamicts and gravels which is present along the entire section. It documents the decay phase of re-equilibration as the ice margin disintegrated catastrophically and released large volumes of heterogeneous sediment which was resedimented by quasicontinuous mass flow. Lithofacies association 4 consists of stratified and massive gravels within distributary channels cut into underlying facies and represents the last phase of meltwater activity. Sediment geometries, particularly sedimentary contrasts representing erosion surfaces at a variety of scales and abrupt textural contrasts are attributed to jet switching. Lithofacies association 1 (60%) and Lithofacies association 3 (30%) are the dominant facies. In favourable topographic settings this stratigraphic couplet is a signature for re-equilibrated ice margins in isostatically depressed basins dominated by tidewater fronts, rapid ice flux and high relative sea level. Morainal banks document rapid environmental change and in the Irish Sea Basin they form part of a deglacial event stratigraphy related to unstable tidewater margins and high relative sea level. Deglaciation was therefore controlled primarily by high relative sea level rather than climatic forcing. Facies variations should therefore not be used for stratigraphic correlations in place of direct stratigraphy. This type of situation may be more common than hitherto realized in Late Pleistocene, mid-latitude shelves where most of the preserved stratigraphy is characterized by complex, interbedded sequences formed when isostatic depression exceeded sea-level fall.     

Jet-efflux deposits of a subaqueous ice-contact fan,glacial Lake Rinteln,northwestern Germany

The coarse-grained, ice-contact, Porta Subaqueous Fan/Delta Complex was deposited in glacial Lake Rinteln at the margin of the Saalian ice sheet that advanced south of the Weser Chains, NW Germany. The ice-proximal depositional system was up to 15 km long and 10 km wide. The present study deals with ice-proximal subaqueous fan deposits, which are interpreted as products of a subcritical plane-wall outflow jet that periodically passed into a supercritical jet with hydraulic jump. The proximal facies assemblage consists of the coarse, clast-supported gravelly deposits of a hyperconcentrated (high-density) effluent and of related cohesionless debris flows attributed to the conduit or immediate proximal jet outflow zone of flow establishment. The intermediate facies assemblage, attributed to the outflow jet proximal zone of flow transition, is dominated by normally graded and cross-stratified gravels with scour structures at their bases; these gravels were deposited by a high-density effluent capable of forming mouthbar-like features. These deposits pass downcurrent into an assemblage of planar parallel-stratified and planar and trough cross-stratified sands and pebbly sands (partially interpreted as antidunes), with abundant scour structures and intercalated layers of fine sand/silt and silty mud, attributed to the jet distal zone of flow transition. The distal facies assemblage consists of trough cross-stratified sands and pebbly sands, and is attributed to the outflow jet proximal zone of established flow. The sedimentary succession as a whole has wedge-shape geometry, with a gentle fan-shaped inclination of the bedding from the southeast to the southwest. Repeated vertical alternations of supercritical and subcritical deposits and muddy interlayers can be attributed to temporary fluctuations in the meltwater outflow, whereas the overall upward fining of the succession indicates a net decline of meltwater discharges.     

Middle to Late Weichselian Norwegian Channel Ice Stream deposits and morphology on Jæren, southwestern Norway and the eastern North Sea area

Glacial lineations on a bank area and a coastal lowland, both bordering the Norwegian Channel, are studied with regard to morphology and distribution by means of side-scan sonar data, detailed digital maps and fieldwork. Their genesis and age are further elucidated through stratigraphic and sedimentologic information from excavations in one typical coast-parallel drumlin. Four excavated sections revealed four lithologic units: Prodeltaic glaciomarine sand, glaciofluvial gravel, glaciomarine diamicton and deformation till. After Middle Weichselian delta progradation, glaciomarine diamicton was deposited and later subglacially reworked by a northwards flowing glacier. The two upper diamictons form the main volume of the ridge, which is interpreted as a drumlin, and imply a reinterpretation of the Jæren part of the so-called Lista moraine. Preconsolidation of glaciomarine diamicton suggests a maximum ice thickness of 500 m during drumlin formation, indicating an ice surface slope of 1 m/km. The occurrence of sediments that provided low basal shear stresses, and the orientation of drumlins and megaflutes indicating ice confluence both point to high glacier flow velocities and suggest that an ice stream, rather than a slower moving part of the ice sheet, occupied the Norwegian Channel during the Late Weichselian maximum. Deformation till overlying, more or less, undeformed glaciomarine diamicton suggests that high glacier velocities during periods of low driving stresses were possible due to a subglacial deformable layer.     

Depositional evidence for marginal oscillations of the Irish Sea ice stream in southeast Ireland during the last glaciation

Late Devensian/Midlandian glacial deposits on the southeast Irish coast contain a record of sedimentation at the margins of the Irish Sea ice stream (ISIS). Exposures through the Screen Hills reveal a stratigraphy that documents the initial onshore flow of the ISIS ('Irish Sea Till') followed by ice stream recession and readvances that constructed glacitectonic ridges. Ice-contact fans (Screen Member) were deposited in association with subglacial deformation tills and supraglacial/subaqueous mass flow diamicts. In SE Ireland, the ISIS moved onshore over proglacial lake sediments which were intensely folded, thrust and cannibalized producing a glacitectonite over which laminated and massive diamictons were deposited as glacitectonic slices. Ice marginal recession and oscillations are documented by: (a) ice-proximal, subaqueous diamict-rich facies; (b) isolated ice-contact glacilacustrine deltas; (c) syn-depositional glacitectonic disturbance of glacilacustrine sediments and overthrusting of ice-contact outwash; (d) offshore moraine ridges; and (e) changing ice flow directions and facies transitions. Diagnostic criteria for the identification of dynamic, possibly surging, ice-stream margins onshore include thrust-block moraines, tectonized pitted outwash and stacked sequences of glacitectonites, deformation tills and intervening stratified deposits. In addition, the widespread occurrence of hydrofracture fills in sediments overridden and locally reworked by the ISIS indicate that groundwater pressures were considerably elevated during glacier advance. The glacigenic sediments and landforms located around the terrestrial margins of the ISIS are explained as the products of onshore glacier flow that cannibalized and tectonically stacked pre-existing marine and glacilacustrine sediments. Localized tectonic thickening of subglacially deformed materials at the former margins of glaciers results in zones of net erosion immediately up-ice of submarginal zones of net accretion of subglacial till. The more stable the ice-stream margin the thicker and more complex the submarginal sedimentary stack.     

Late Pleistocene glaciolacustrine sedimentation and paleogeography of southeastern Michigan,USA

The geomorphic, stratigraphic and sedimentological characteristics of glaciolacustrine sediments in the metropolitan Detroit, Michigan area were studied to determine environments of deposition and make paleogeographic reconstructions. Nine lithofacies were identified and paleoenvironments interpreted based on their morphostratigraphic relationships with relict landforms. The sediments studied are found southeast of the Defiance and Birmingham moraines lying beneath a lowland characterized by a low morainal swell (Detroit moraine) and a series of lacustrine terraces that descend progressively in elevation southeastward. The glaciolacustrine sediments were deposited approximately 14.3–12.4 kA BP during the Port Bruce and Port Huron glacial phases of late Wisconsinan time, and are related to proglacial paleolakes Maumee, Arkona, Whittlesey, Warren, Wayne, Grassmere, Lundy and Rouge. The glaciolacustrine section is typically 2–4 m thick and consists of a basal unit of wavy-bedded clayey diamicton overlain by a surficial deposit of stratified and cross-stratified sand and gravel. The basal unit is comprised of subaqueous debris flow deposits that accumulated as subaqueous moraine in paleolake Maumee along the retreating front of the Huron lobe. The surficial deposits of sand and gravel were formed by traction, resulting from lacustrine wave activity and fluvial processes, in lakebed plain, beach ridge and deltaic depositional settings. Much of the lake-margin sand and gravel was derived from clayey diamicton by lacustrine wave action and winnowing, and that associated with paleolakes of the Port Huron phase is largely reworked Port Bruce sediment. Paleogeographic reconstructions show that the Defiance, Birmingham and Detroit moraines, Defiance and Rochester channels, and the Rochester delta, were deposited penecontemporaneously as paleolake Maumee expanded northward across the map area. A unique type of wavy bedform is characteristic of clayey diamicton deposited by subaqueous mass flow in the study area that is useful for differentiating sediment: 1) deposited by mass flow in subaqueous vs. subaerial settings, and 2) deposited by subaqueous mass flow vs. basal till. These bedforms are a useful tool for identifying subglacial meltwater deposits, and facilitate the mapping and correlation of glacial sediments based on till sheets. The map area provides a continental record of ice sheet dynamics along the southern margin of the Laurentide ice sheet during Heinrich event H-1. The record reveals rapid glacial retreat (～ 0.8 km/yr) contemporaneous with the discharge of a large volume of meltwater. Evidence in the study area for subglacial meltwater is problematic, but indications that periglacial conditions persisted in the map area until ～ 12.7 kA BP, and extended for 200 km or more south of the ice front suggest that a frozen substrate may have contributed to instability of the LIS.     

Sedimentary facies analysis and depositional model of gravity-flow deposits of the Yanchang Formation,southwestern Ordos Basin,NW China

During the deposition of the Chang-7 (Ch-7) and Chang-6 (Ch-6) units in the Upper Triassic, gravity flows were developed widely in a deep lake in the southwestern Ordos Basin, China. Based on cores, outcrops, well-logs and well-testing data, this paper documents the sedimentary characteristics of the gravity-flow deposits and constructs a depositional model. Gravity-flow deposits in the study area comprise seven lithofacies types, which are categorised into four groups: slides and slumps, debris-flow-dominated lithofacies, turbidity-current-dominated lithofacies, and deep-water mudstone-dominated lithofacies. The seven lithofacies form two sedimentary entities: sub-lacustrine fan and the slump olistolith, made up of three and two lithofacies associations, respectively. Lithofacies association 1 is a channel–levee complex with fining-/thinning-upward sequences whose main part is characterised by sandy debris flow-dominated, thick-bedded massive sandstones. Lithofacies association 2 represents distributary channelised lobes of sub-lacustrine fans, which can be further subdivided into distributary channel, channel lateral margin and inter-channel. Lithofacies association 3 is marked by non-channelised lobes of sub-lacustrine fans, including sheet-like turbidites and deep-lake mudstones. Lithofacies association 4 is represented by proximal lobes of slump olistolith, consisting of slides and slumps. Lithofacies association 5 is marked by distal lobes of slump olistolith, comprising tongue-shaped debris flow lobes and turbidite lobes. It is characterised by sandy debris flow, muddy debris flow-dominated sandstone and sandstone with classic Bouma sequences. Several factors caused the generation of gravity flows in the Ordos Basin, including sediment supply, terrain slope and external triggers, such as volcanisms, earthquakes and seasonal floods. The sediment supply of sub-lacustrine fan was most likely from seasonal floods with a high net-to-gross and incised channels. Triggered by volcanisms and earthquakes, the slump olistolith is deposited by the slumping and secondary transport of unconsolidated sediments in the delta front or prodelta with a low net-to-gross and no incised channels.     

Deglaciation sequences in the Permo-Carboniferous Karoo and Kalahari basins of southern Africa: a tool in the analysis of cyclic glaciomarine basin fills

The Late Westphalian to Artinskian glaciomarine deposits of the Karoo and Kalahari basins of southern Africa consist of massive and stratified diamictite, mudrock with ice-rafted material, sandstone, silty rhythmite, shale and subordinate conglomerate forming a cyclic succession recognizable across both basins. A complete cycle comprises a resistant basal unit of apparently massive diamictite overlain by softer, bedded stratified diamictite, sandstone and mudrock with a total thickness of as much as 350 m. Four major cycles are observed each separated by bounding surfaces. Lateral facies changes are present in some cycles. The massive diamictites formed as aprons and fans in front of the ice-grounding line, whereas the stratified diamictites represent more distal debris-flow fans. The sandstones originated in different environments as turbidite sands, small subaqueous outwash channel sands and delta front sands. The rhythmites and mudrock represent blanket deposits derived from turbid meltwater plumes. Cycles represent deglaciation sequences which formed during ice retreat phases caused by eustatic changes in the Karoo and Kalahari basins. Evidence for shorter-term fluctuation of the ice margin is present within the major advance-retreat cycles. Hardly any sediment was deposited during lowstand ice sheet expansion, whereas a deglaciation sequence was laid down during a sea-level rise and ice margin retreat with the volume of meltwater and sediment input depending on temporary stillstands of the ice margin during the retreat phase. The duration of the cycles is between 9 and 11 Ma suggesting major global tectono-eustatic events. Smaller cycles probably linked to orbital forcing were superimposed on the longer-term events. A sequence stratigraphic approach using the stacking of deglaciation sequences with the ice margin advance phases forming bounding surfaces, can be a tool in the framework analysis of ancient glaciomarine basin fills.     

A critical review and re-investigation of the Pleistocene deposits between Cranfield Point and Kilkeel,Northern Ireland: Implications for regional sea-level models and glacial reconstructions of the northern Irish Sea basin

The coastline of County Down includes sites that are pivotal to understanding the history of the last glaciation of the northern Irish Sea Basin in relation to relative sea level and regional glacial readvances. The cliff sections display evidence that has been used to underpin controversial models of glaciomarine sedimentation in isostatically-depressed basins followed by emergent marine and littoral environments. They also provide crucial evidence claimed to constrain millennial-scale ice sheet oscillations associated with uniquely large and rapid sea-level fluctuations. This paper reviews previous work and reports new findings that generally supports the ‘terrestrial’ model of glaciation, involving subglacial accretion and deformation of sediment beneath grounded ice. Deep troughs were incised into the till sheet during a post Late Glacial Maximum draw-down of ice into the Irish Sea Basin. Ice retreat was accompanied by glaciomarine accretion of mud in the troughs during a period of high relative sea level. The trough-fills were over-ridden, compacted, deformed and truncated during a glacial re-advance that is correlated with the Clogher Head Readvance. Grounding-line retreat accompanied by rapid subaqueous ice-proximal sedimentation preserved a widespread subglacial stone pavement. Raised beach gravels cap the sequence. The evidence supports an uninterrupted fall in relative sea level from c. 30?m that is consistent with sea level curves predicted by current glacio-isostatic adjustment modelling. Critical evidence previously cited in support of subaerial dissection of the troughs, and hence rapid fall and rise in relative sea level prior to the deposition of the glaciomarine muds, is not justified.     

Facies and Architecture of a Carboniferous Grounding-line System From the Guandacol Formation, Paganzo Basin, Northwestern Argentina

New outcrops of Middle Carboniferous glacigenic deposits found in the Guandacol Formation (western Paganzo Basin) are described in this paper. The study locality of Los Pozuelos Creek (northwestern Argentina) includes coarse-grained diamictites, rhythmites, laminated pebbly mudstones and shales that represent an expanded column of the Gondwanic glaciation in this region. Thirteen lithofacies recorded at the measured section have been grouped into three facies associations. Facies Association I is composed of coarse-grained massive and stratified diamictites (lithofacies Dmm, Dms, Dmg, Dcs), laminated siltstones with dropstones (Fld) and interstratified sandstones and mudstones (Fl, Sr). These rocks represent both tillites and resedimented diamictites closely associated to small water bodies where laminated siltstones with dropstones and stratified sandstones and mudstones were deposited. Facies Association II comprises couplets of matrix-supported thinly bedded diamictites (Dmld) and laminated mudstones with dropstones (Fld). This facies association results from the combination of three different processes, subaqueous cohesionless debris flows, coeval rainout of ice-rafted debris and settling of fine-grained particles from supension. Finally, Facies Association III is made up of laminated mudstones without dropstones, thin marl levels and scarce fine- to very fine-grained sandstones. This assemblage clearly suggests sedimentation in a deep marine environment below the wave base.The architecture of the glacigenic deposits has been investigated using photomosaic panels. The geometry of the depositional bodies and facies suggest that Los Pozuelos Creek outcrops exhibit a well preserved three-dimensional example of a grounding-line system. In particular, three different subenvironments of a morainal bank were interpreted: a bank-front, a bank-core and a bank-back. The bank-front assemblage is characterized by coarse-grained, mainly resedimented, diamictites grading laterally to prograding clinoforms composed of interbedded matrix-supported thinly bedded diamictite and mudstones. The bank-core assemblage is formed by a stacking of coarse-grained diamictites where at least five major erosional surfaces, bounding four multistory diamictite bodies, can be recognized. Finally, the bank-back assemblage corresponds to discontinuous intervals of striated lodgement till, and coarse-grained resedimented diamictites showing important post-depositional deformation. The retrogradational stacking of the morainal banks indicate an overall glacial retreat and a glacioeustatic sea-level rise. Erosional surfaces at the base of each morainal bank suggest intervening short term episodes of ice advance.The new data presented here confirm the existence of "true" tillites in western Paganzo Basin and suggest several (at least four) pulses of glacial advance and retreat during the Namurian glaciation in the region and permit a more refined interpretation of the glacial deposits in the Huaco area.     

Neogene fjordal sedimentation on the western margin of the Lambert Graben, East Antarctica

The Lambert Graben is occupied by the world’s largest fjord system, through which flows the Lambert Glacier, the Amery Ice Shelf and their tributaries. Along the western margin of the graben, in the northern Prince Charles Mountains, remnants of uplifted Miocene and Pliocene strata of the glacigenic fjordal Pagodroma Group total more than 800 m in thickness. These sediments provide evidence for a dynamic East Antarctic ice sheet during the Neogene Period. Each of the four Pagodroma Group formations defined from this region rests unconformably on either Proterozoic or Permo‐Triassic rocks. The unconformity surfaces represent parts of the walls and floors of Neogene fjords. For these surfaces to have been eroded, the ice must have been grounded out as far as the continental shelf in Prydz Bay. The Pagodroma Group was deposited by wet‐based glaciers discharging into a fjordal setting and includes lithofacies that are quite different from those produced by modern Antarctic ice masses. The principal lithofacies are massive diamicts and soulder gravels, deposited both close to a calving, grounded glacier terminus and from icebergs. The few stratified diamicts are the product of more distal iceberg sedimentation. An ice‐transported gravel lithofacies includes rockfall debris derived from palaeofjord walls and mixed with subglacially derived diamicts. Some lithofacies contain evidence of subaquatic slumping and gravity flowage. Volumetrically minor lithofacies include laminites, with some exposures exhibiting large ice‐rafted clasts. The laminites represent less proximal, mainly ice‐free fjordal sediments, resulting either from tidal‐current sorting of suspended sediment originating from subaquatic glaciofluvial discharge, or from turbidity currents derived from unstable subaquatically deposited glacigenic sediment. The Pagodroma Group provides a record of multiple glaciation by dynamic, sliding glaciers carrying large amounts of both basal and supraglacial debris. The closest modern analogues, in terms of the thermal and dynamic characteristics of the Neogene Lambert Glacier, appear to be the fast‐flowing tidewater glaciers of East Greenland. These glaciers originate from the interior ice sheet and discharge large volumes of icebergs; the resulting lithofacies are predominantly diamicts.     

Glacitectonic deformation in the Chuos Formation of northern Namibia: implications for Neoproterozoic ice dynamics

The Chuos Formation is a diamictite-dominated succession of Cryogenian age, variously interpreted as the product of glaciomarine deposition, glacially related mass movement, or rift-related sediment remobilisation in a non-glacial environment. These interpretations have wide ranging implications for the extent of ice cover during the supposedly pan-global Neoproterozoic icehouse. In the Otavi Mountainland, northern Namibia, detailed analysis of soft-sediment deformation structures on the macro- and micro-scale support glacitectonic derivation in response to overriding ice from the south/south-east. Overall, the upward increase in strain intensity, predominance of ductile deformation features (e.g. asymmetric folds, rotational turbates and necking structures, clast boudinage, unistrial plasmic fabrics) and pervasive glacitectonic lamination support subglacial deformation under high and sustained porewater pressures. In contrast, soft-sediment structures indicative of mass movements, including flow noses, tile structures, and basal shear zones, are not present. The close association of subglacial deformation, abundant ice-rafted debris and ice-contact fan deposits indicate subaqueous deposition in an ice-proximal setting, subject to secondary subglacial deformation during oscillation of the ice margin. These structures thus reveal evidence of dynamic grounded ice sheets in the Neoproterozoic, demonstrating their key palaeoclimatic significance within ancient sedimentary successions.     

The internal sediment architecture of a drumlin,Port Byron,New York State,USA

An exposure within the central portion of a large drumlin at Port Byron, New York State, USA, part of the large New York drumlin field, reveals a sequence of steeply dipping cemented sands and gravels of proglacial, ice-contact deltaic origin overlain by a thin till veneer. The sands and gravels appear to have been deposited within the proximal proglacial environment during a late retreat phase of the Laurentide Ice Sheet sometime prior to being overridden by subsequent ice and drumlinized. During deposition of the ice-contact delta, escaping subglacial regelation-meltwater permeated the proximal deltaic sediment pile and calcium carbonate was released, in a series of pulses, to form pore-occluding calcite cement within the sand and gravel porespaces. The calcium carbonate precipitated into the sands and gravels due to a reduction in hydrostatic pressure and CO₂ outgassing of the meltwater as it exited from beneath the ice sheet. Once cemented, these deltaic sediments were considerably stronger and acted afterward as an obstacle around which the future ice advance streamed and, in turn, produced the characteristic drumlin shape. In overriding the ice-contact deltaic sediments, the ice sheet emplaced a thin layer of till which exhibits syndepositional deformation features indicative of being emplaced as a deforming bed layer beneath the advancing ice sheet. Micromorphological analysis of the overlying till shows that no interstitial or intraclastic calcite occurs within the till.     

Sedimentological and deformational criteria for discriminating subglaciofluvial deposits from subaqueous ice‐contact fan deposits: A Pleistocene example (Ireland)

A pit located near Ballyhorsey, 28 km south of Dublin (eastern Ireland), displays subglacially deposited glaciofluvial sediments passing upwards into proglacial subaqueous ice‐contact fan deposits. The coexistence of these two different depositional environments at the same location will help with differentiation between two very similar and easily confused glacial lithofacies. The lowermost sediments show aggrading subglacial deposits indicating a constrained accommodation space, mainly controlled by the position of an overlying ice roof during ice‐bed decoupling. These sediments are characterized by vertically stacked tills with large lenses of tabular to channelized sorted sediments. The sorted sediments consist of fine‐grained laminated facies, cross‐laminated sand and channelized gravels, and are interpreted as subglaciofluvial sediments deposited within a subglacial de‐coupled space. The subglaciofluvial sequence is characterized by glaciotectonic deformation structures within discrete beds, triggered by fluid overpressure and shear stress during episodes of ice/bed recoupling (clastic dykes and folds). The upper deposits correspond to the deposition of successive hyperpycnal flows in a proximal proglacial lake, forming a thick sedimentary wedge erosively overlying the subglacial deposits. Gravel facies and large‐scale trough bedding sand are observed within this proximal wedge, while normally graded sand beds with developed bedforms are observed further downflow. The building of the prograding ice‐contact subaqueous fan implies an unrestricted accommodation space and is associated with deformation structures related to gravity destabilization during fan spreading (normal faults). This study facilitates the recognition of subglacial/submarginal depositional environments formed, in part, during localized ice/bed coupling episodes in the sedimentary record. The sedimentary sequence exposed in Ballyhorsey permits characterization of the temporal framework of meltwater production during deglaciation, the impact on the subglacial drainage system and the consequences on the Irish Sea Ice Stream flow mechanisms.     

The Middle Pleistocene glaciolacustrine environment of an ice‐dammed mountain valley,Sudeten Mountains,Poland

This article reports on an Early Saalian proglacial lake formed between the Scandinavian Ice Sheet and the front of the Sudeten Mountains, Poland. Sediments investigated at Mys?ów point to a transition from glacifluvial to glaciolacustrine environments. The bulk of the sediments was deposited in deep‐water Gilbert‐type deltas (A–E complexes). A delta plain (topset) gradually passes into a subaerial plateau and then a clastic shoreline and the subaquatic slope of a prograding delta (foreset). The glaciolacustrine lithofacies represent a number of lake‐basin environments, from marginal subaqueous slopes to distal parts of a subaqueous fan. Glaciolacustrine and glaciodeltaic deposits locally reach ?50–70 m in thickness. Analyses of A–E complexes indicate that the lake existed for more than 130 years and that its origin and evolution were closely connected with the ice front. This case study records lake sedimentation at an ice‐sheet margin with cohesionless gravity flows, turbidity currents, debris‐avalanching and, to a much lesser degree, parapelagic suspension fall‐out and ice‐raft dumping. In the initial stage, the lake extended more than 10 km to the south, and the deposition was relatively slow. In the second stage, recession of the ice sheet caused rapid growth of a delta. The third and ultimate stage coincided with the final glacial recession, with rapid deposition occurring only on the lake bottom. The model of the glaciolacustrine environment presented here may also be applicable to many other proglacial lakes in mountain areas.     

Time-transgressive deposits of repeated depositional sequences within interlobate glaciofluvial (esker) sediments in Köyliö, SW Finland

ABSTRACT Evidence of conspicuous repeated seasonal to annual deposition of glaciofluvial and glaciolacustrine sequences within a structurally complex interlobate esker segment in SW Finland is presented. The time‐transgressive, overlapping depositional sequences consist of deposits from two successive melt seasons, including three vertically stacked lithofacies associations: (1) massive to stratified coarse gravels = summer deposits; (2) trough and ripple cross‐stratified fine‐grained deposits = autumn to winter deposits; and (3) sandy stratified beds = spring deposits. The depositional environment of each lithofacies association involves a transition from subglacial or submarginal tunnel to a subaqueous re‐entrant environment, which then passes to a proglacial glaciolacustrine environment. The study also presents evidence of headward extension of subglacial tunnel deposits, related to the rapid shifting of a tunnel expansion point during the increasing spring discharge, which occupied the old tunnel exit: this mode of annual deposition has not been reported previously in esker studies. The good preservation of the rhythmic lithofacies associations is suggested as resulting from interlobate depositional conditions associated with rapidly decaying icestreams. Therefore, the depositional model may provide a key to recognizing time‐transgressive interlobate eskers that form an important geomorphological and sedimentological record of meltwater activity during the last deglaciation of the Fennoscandian and Laurentide ice sheets. The identification of time‐transgressive interlobate eskers and associated palaeo‐icestream behaviour is an essential step forward for more accurate models of ice sheet behaviour and palaeoclimatic reconstructions.     

Subglacial and subaqueous processes near a glacier grounding line: sedimentological evidence from a former ice-dammed lake, Achnasheen Scotland

Subglacial and subaqueous sediments deposited near the margin of a Late-glacial ice-dammed lake near Achnasheen, northern Scotland, are described and interpreted. The subglacial sediments consist of deformation tills and glacitectonites derived from pre-existing glaciolacustrine deposits, and the subaqueous sediments consist of ice-proximal outwash and sediment flow deposits, and distal turbidites. Sediment was delivered from the glacier to the lake by two main processes: (1) subglacial till deformation, which fed debris flows at the grounding line; and (2) meltwater transport, which fed sediment-gravity flows on prograding outwash fans. Beyond the ice-marginal environment, deposition was from turbidity currents, ice-rafting and settling of suspended sediments. The exposures support the conclusion that the presence of a subglacial deforming layer can exert an important influence on sedimentation at the grounding lines of calving glaciers.