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.     

Submarine debris flow deposits from the Upper Carboniferous Dwyka Tillite Formation in the Kalahari Basin, South Africa

ABSTRACT Four types of sediment gravity flow deposits occur interbedded with rhythmite shale, siltsone, mudstone and minor diamictite in a 230-m thick Carboniferous glacial sequence. Shear and plug zones are present in the cohesive debris flow deposits which have a diamictic texture. The high-density turbidity flow deposits which consist of coarse to medium-grained clastics, are characterized by both normal and reverse grading. The medium to fine-grained low-density turbidity flow deposits show normal grading and consist of Bouma units A, B, D and E. Deposition occurred by gravity flow, suspension settling and minor basal melt-out during ice retreat along the southern margin of the Kalahari Basin. Immediately basinwards of the ice grounding line a proximal diamictite facies consisting predominantly of cohesive and high-density turbidity flow deposits, and minor melt-out tills formed. A distal sedimentary facies of graded sandstone and siltstone units deposited by low-density turbidity flows and suspension settling of muds with ice-rafted debris is located basinwards.     

Glacial advance and retreat sequences in a Permo-Carboniferous section, central Transantarctic Mountains

Episodes of glacial advance and retreat can be recognized through analysis of vertical facies sequences in the Permo-Carboniferous Pagoda Formation of the Beardmore Glacier area, Antarctica. The formation includes a remarkably complete record of continental sedimentation near the terminus of a temperate glacier. Facies sequence is pre-eminent for inferring glacial advance and retreat. Other important criteria are abundance and geometry of sandstone interbedded with diamictite, diamictite character and nature of bed contacts. Using these characteristics advance and retreat sequences 5–60 m thick are recognized. A sharp contact, with a striated surface and erosional relief, overlain by structureless diamictite (lodgement till) is typical of grounded ice advance. Grounded ice retreat is characterized by structureless diamictite (lodgement till), overlain by crudely stratified diamictite (melt-out till) and then by diamictite interbedded with sandstone and conglomerate (flow till and glacio-fluvial or glacio-lacustrine deposits). Gradational contacts between shale overlain by diamictite and diamictite overlain by shale characterize advance and retreat, respectively, in subaqueous settings. Pauses in sediment accumulation, minor(?) fluctuations of the ice margin, and/or changes in subglacial dynamics are indicated by specific features within diamictite units such as probable frost-wedge casts, single layer boulder beds, sharp sedimentary contacts and changes in diamictite character. These minor(?) events are superimposed upon the main advance-retreat cycles. Study of both the overall facies sequence and of individual diagnostic structures, albeit in an incomplete stratigraphic record, permits a distinction between major and minor advance-retreat events. As many as six major advance-retreat cycles exist in some Pagoda sections, but the number of cycles present varies in different sections.     

Plio-Pleistocene sedimentation in Ferrar Fiord, Antarctica

A 166 m thick Plio-Pleistocene sequence of glacial sediments has been cored in Ferrar Fiord in the southwestern corner of the Ross Sea, Antarctica. The core has the following lithofacies: massive diamictite (33% of the core; interpreted as lodgement or waterlain till), weakly stratified diamictite (25%; waterlain till or proximal glaciomarine sediment), well-stratified diamictite (8%; proximal glaciomarine or glaciolacustrine sediment), sandstone (25%; sand of aeolian or supraglacial origin), mudstone(7%; derived from subglacial debris and transported offshore in suspension), and minor amounts of rhythmite and tuff. The range of facies in this polar setting differs from those normally found in subpolar and temperate glacier fiord settings in the high proportion of aeolian-derived sand and the low proportion of mudstone facies. The core can be divided into two sequences based on composition and texture. The sequence from 162 to 100 mbsf (metres below the sea floor) comprises alternations of diamictite dominated by basement lithologies and thin marine mudstone beds. It is Pliocene in age (4.9–2.0 Ma) and records several advances and retreats of ice through the Transantarctic Mountains and across the drill site from the west. The sequence from 100 mbsf to the sea floor, of Pleistocene age, consists of alternations of diamictite, interpreted as lodgement and waterlain till, and sandstone of aeolian origin deposited in a glaciolacustrine setting, similar to ice-covered lakes in the Dry Valleys today. These sediments have a high volcanic component, and hence are thought to have been derived by the grounding and advance of the Ross Ice Shelf from the east past volcanic Ross Island. This change in source is attributed to the rising Transantarctic Mountains increasingly containing East Antarctic ice. The Pleistocene sequence above 100 mbsf clearly represents polar glacial sedimentation, with alternations of till and glaciolacustrine sand. Mudstones from the Pliocene sequence beneath include palynomorphs, indicating times when the landscape was at least partially vegetated, but contain no evidence of meltwater influence.     

The late Paleozoic El Imperial Formation,western Argentina: Glacial to post-glacial transition and stratigraphic correlations with arc-related basins in southwestern Gondwana

The El Imperial Formation of the San Rafael Basin records a succession of depositional environments during the latest Mississippian to earliest Permian that span before, during, and after the glaciation of west central Argentina. At the base of the formation, a restricted marine environment is recorded in mudstone containing marl and rippled and deformed sandstone beds. This unit, or sequence 1, is incised by a deltaic facies association composed of cross-bedded sandstone and conglomerate that form at least 5 stacked Gilbert deltas. The deltaic facies association grades upward into the glacially-influenced facies association, made up of stratified diamictite, mudstone with dropstones, and massive deformed sandstone, indicating deposition by wet-based tidewater glaciers that calved icebergs into the basin, with contributions from mass movement processes. The glacially-influenced facies association is overlain by mudstone and horizontally laminated and cross-bedded sandstone of the post-glacial open marine facies association, recording post-glacial transgression followed by relative sea level fall. The deltaic, glacially-influenced, and post-glacial open marine facies associations comprise sequence 2. Sequence 2 is incised by conglomerate of the upper fluvial member, or sequence 3.The strata of the El Imperial Formation are correlated to those of the other arc-related basins of western Argentina: Río Blanco, Calingasta–Uspallata, and Tepuel. A Bashkirian transgression and fluvial incision in the El Imperial Formation correlate with events in the Río Blanco and Calingasta–Uspallata Basins to the north, whereas glaciation continues to the south in the Tepuel Basin through the Early Permian. The deviating stratigraphic record of the Tepuel Basin may be the result of its higher latitudinal position during the Pennsylvanian–Early Permian and higher altitude due to either tectonic convergence of the Patagonian microplate or convergence along the Panthalassan margin of southwestern Gondwana.     

Bolla Bollana boulder beds: A Neoproterozoic trough mouth fan in South Australia?

The Bolla Bollana Formation is an exceptionally thick (ca 1500 m), rift‐related sedimentary succession cropping out in the northern Flinders Ranges, South Australia, which was deposited during the Sturtian (mid Cryogenian) glaciation. Lithofacies analysis reveals three distinct facies associations which chart changing depositional styles on an ice‐sourced subaqueous fan system. The diamictite facies association is dominant, and comprises both massive and stratified varieties with a range of clast compositions and textures, arranged into thick beds (1 to 20 m), representing stacked, ice‐proximal glaciogenic debris‐flow deposits. A channel belt facies association, most commonly consisting of normally graded conglomerates and sandstones, displays scour and fill structure of ca 10 m width and 1 to 3 m depth: these strata are interpreted as channelized turbidites. Rare mud‐filled channels in this facies association bear glacially striated lonestones. Finally, a sheet heterolithics facies association contains a range of conglomerates through sandstones to silty shales arranged into clear, normally graded cycles from the lamina to bed scale. These record a variety of non‐channelized turbidites, probably occupying distal and/or inter‐channel locations on the subaqueous fan. Coarsening and thickening‐up cycles, capped by dolomicrites or mudstones, are indicative of lobe build out and abandonment, potentially as a result of ice lobe advance and stagnation. Dropstones, recognized by downwarped and punctured laminae beneath pebbles to boulders in shale, or in delicate climbing ripple cross‐laminated siltstones, are clearly indicative of ice rafting. The co‐occurrence of ice‐rafted debris and striated lonestones strongly supports a glaciogenic sediment source for the diamictites. Comparison to Pleistocene analogues enables an interpretation as a trough mouth fan, most probably deposited leeward of a palaeo‐ice stream. Beyond emphasizing the highly dynamic nature of Sturtian ice sheets, these interpretations testify to the oldest trough mouth fan recorded to date.     

Sedimentation on an early Proterozoic continental margin under glacial influence: the Gowganda Formation (Huronian), Elliot Lake area, Ontario, Canada

Eight continuous cores up to 150 m long and spaced an average of 200 m apart yield a detailed local insight into the composition and architecture of an ancient continental margin sequence, the Gowganda Formation (early Proterozoic: Huronian) near Elliot Lake, Ontario. Nearby outcrops of similar facies provide important supplementary data on sedimentary structures. Continental glaciers provided an abundant supply of coarse debris but, apart from rafting of debris by floating ice, played little or no part in Gowganda sedimentation. The basal 50 m of the Gowganda Formation in the drill-hole area represents a continental slope depositional system. It consists mainly of gravelly and sandy sediment gravity flow deposits, interbedded with minor rain-out units of diamictite, and argillite containing dropstones. Ten types of sediment gravity flow deposit are distinguished. An overlying submarine-channel depositional system, 10–50m thick, consists of hemipelagic argillites containing dropstones and showing deformation structures. These are interbedded with well-sorted channel-fill sandstones. Submarine point bars 4·5 m thick (identified in nearby outcrops) demonstrate a meandering channel geometry. This channel-fill sequence probably formed during a period of high sea-level and reduced sediment supply, but the relationship to ice advance-retreat cycles is unclear. The subsurface sequence is completed by a blanket of massive rain-out diamictites up to 55 m thick, and a younger slope sequence of sediment gravity flow diamictites and sandstones. The stratigraphy is quite different in outcrop section 10 km to the west of the drill-holes, suggesting the presence of major lateral facies changes and/or internal erosion surfaces within the Gowganda Formation. This complexity of stratigraphy and depositional processes is probably a feature of many ancient glacial units, and points to the advisability of not making climatic or tectonic interpretations from a few generalized or composite sections.     

Depositional features of a late Weichselian outwash fan; central East Jylland,Denmark

Four major sedimentary facies are present in coarse-grained, ice-marginal deposits from central East Jylland, Denmark. Facies A and B are matrix-supported gravels deposited by subaerial sediment gravity flows as mudflows (facies A) and debris flows (facies B). Facies C consists of clast-supported, water-laid gravels and facies D are cross-bedded sand and granules. The facies can be grouped into three facies associations related to the supraglacial and proglacial environments: (1) the flow-till association is made up of alternating beds of remobilized glacial mixton (facies A) and well-sorted cross-bedded sand (facies D); (2) the outwash apron association resembles the sediments of alluvial fans in containing coarse-grained debris-flow deposits (facies B), water-laid gravel deposited by sheet floods (facies C) and cross-bedded sand and granules (facies D) from braided distributaries; (3) the valley sandur association comprises water-laid gravel (facies C) interpreted as sheet bars and longitudinal bars interbedded with cross-bedded sand and granules (facies D) deposited in channels between bars in a braided environment.The general coarsening-upward trend of the sedimentary sequences caused by the transition of bars and channel-dominated facies to debris-flow-dominated facies indicate an increasing proximality of the outwash deposits, picturing the advance and still stand of a large continental lowland ice-sheet. The depositional properties suggest that sedimentation was caused by melting along a relatively steep, active glacier margin as a first step towards the final vanishing of the Late Weichselian icesheet (the East Jylland ice) covering eastern Denmark.     

Ice-marginal environment and ecosystem prior to initial advance of the late Palaeozoic ice sheet in the Mount Butters area of the central Transantarctic Mountains, Antarctica

Basal rocks of the Upper Carboniferous to Lower Permian Pagoda Formation at Mount Butters provide an unusual view of periglacial conditions in the central Transantarctic Mountains region prior to the initial advance of the Gondwanide ice sheet. These rocks were deposited on a high relief unconformity that developed on granite. Deposition within relief on the unconformity, possibly in the lee of a granite buttress, protected the rocks from erosion during subsequent overriding by the ice sheet. The succession reflects deposition in a glacial‐fed to ice‐contact lake that contained a freshwater crustacean fauna. Centimetre‐ to decimetre‐scale basal layers include breccia and coarse‐grained sandstone. The occurrence of breccia resting on weathered granite suggests sedimentation as scree and as mass flow deposits. Overlying decimetre‐to metre‐scale stratified diamictites interbedded with metre‐scale, coarsening‐upward successions of siltstone to cross‐laminated sandstone suggest lacustrine deposition by suspension settling, rain out of ice‐rafted debris, and deltaic progradation. Thin zones with abundant conchostracans and/or with prolific trace fossils, in addition to less common remains of other crustaceans, attest to the presence of a low diversity benthic fauna. Conchostracans are concentrated in a series of thin beds that reflect moderately lengthy, perhaps seasonal, periods of free‐flowing water. Patchy vertical and lateral distribution of intense bioturbation and profuse trace fossils probably reflect repeated colonization events during times of favourable environmental conditions. Massive diamictite overlies the basal rocks and indicates that the ice‐marginal lake was subsequently overridden by the late Palaeozoic ice sheet. Occurrences of lodgement till, glacitectonite and deformation till suggest deposition from temperate or warm‐based ice, whereas underlying lacustrine and deltaic deposits, along with a crustacean and trace fossil fauna, suggest temperate periglacial conditions. Previous studies have stressed that upper Palaeozoic glacigenic deposits in Antarctica, and in Gondwanaland, record deglaciation events. In contrast, rocks at Mt. Butters provide an unusual glimpse into an ice‐margin lake and its fauna just prior to ice sheet advance.     

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.     

Glaciation and deglaciation of the Libyan Desert: The Late Ordovician record

Detailed outcrop studies at the flanks of Al Kufrah Basin, Libya, reveal the nature of glacially-related sedimentation and post-depositional deformation styles produced in association with the Late Ordovician glaciation, during which ice sheets expanded northward over North Africa to deposit the Mamuniyat Formation. At the SE basin flank (Jabal Azbah), the Mamuniyat Formation is sand-dominated, and incises interfingering braidplain and shallow marine deposits of the Hawaz Formation. The glacially-related sediments include intercalations of mud-chip bearing tabular sandstones and intraformational conglomerates, which are interpreted as turbidite and debrite facies respectively. These record aggradation of an extensive sediment wedge in front of a stable former ice margin. An increase in mudstone content northward is accompanied by the occurrence of more evolved turbidites. A widespread surface, bearing streamlined NW–SE striking ridges and grooves, punctuates this succession. The structures on the surface are interpreted to have formed during a regional north-westward ice advance. Above, siltstones bearing Arthrophycus burrows, and Orthocone-bearing sandstones beneath tidal bars testify to glaciomarine conditions for deposition of the underflow deposits beneath. By contrast, the northern basin margin (Jabal az-Zalmah) is appreciably different in recording shallower water/paralic sedimentation styles and major glaciotectonic deformation features, although facies analysis also reveals northward deepening. Here, a siltstone wedging from 8 to 50 m toward the north was deposited (lower delta plain), succeeded by climbing ripple cross-laminated sandstones up to 60 m in thickness (distal through proximal delta mouth bar deposits) with occasional diamictite interbeds. These rocks are deformed by thrusts and > 50 m amplitude fault-propagation folds, the deformation locally sealed by a diamictite then overlain by conglomeratic lag during ultimate deglaciation. Integrating observations from both basin margins, a model of fluvial-dominated delta systems feeding a pulsed debrite and turbidite fan system in a shallow proglacial shelf is proposed.     

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.     

鄂尔多斯西部地区中—晚奥陶世克里摩里期—乌拉力克期岩相古地理

中奥陶世克里摩里期,鄂尔多斯西部地区为镶边陆架的碳酸盐岩台地,自东向西依次发育开阔台地、台地边缘浅滩、台缘斜坡—斜坡脚、广海陆棚和深水海槽相带,从浅水区经由碎屑流搬运来的块状钙质角砾岩在台缘斜坡—斜坡脚相带集中堆积,形成厚度不等的透镜体夹于正常深水灰泥石灰岩和泥岩中。乌拉力克期发生较大规模构造运动,盆地东部整体抬升,西部边缘发生裂陷,沉积范围以同生正断层为界,随着海平面的上升沉积环境演变为相对闭塞的深水斜坡—盆地,沉积一套富含笔石的泥页岩地层,并不时有陆源克里摩里组垮塌的石灰岩沉积物被带入盆地,形成数量不等的多套角砾岩夹层。     

Sediment record of short-lived ice-contact lakes, Burroughs Glacier, Alaska

Sediments deposited in two small ice-contact lakes with low rates of sediment input have been studied in subaerial exposures. Sediment characteristics are a function of the water source (glacial meltwater versus non-meltwater), proximity to the glacier margin and lake shore, amount of supraglacial debris, and lake duration. Calving Lake expanded (and later partially drained) as a calving ice margin retreated. Nearshore deltas contain 1 × 10⁵ m³ stratified sand and gravel deposited at rates up to 1 m/yr during a 9-yr interval. Deltaic sediment contains types A and B ripple-drift cross-lamination, draped lamination, and scour surfaces caused by variations in water-flow velocity and the amount of sediment settling from suspension. Most water inflow came from non-subglacial meltwater sources and was sediment-poor, so overflow and interflow sedimentation processes dominated the offshore environment. Offshore sediment generally contains massive silt or silt interbedded with fine-grained sand deposited at rates of 1.3-1.5 cm/yr. Iceberg gravity craters observed on the lake plain were formed when icebergs impacted the lake floor during calving events. In Bruce Hills Lake, proximity to glacier ice and the presence of supraglacial sediment formed coarsening-upward successions when debris fell directly from an ice ledge onto silty lacustrine sediment.     

An origin for laminated glacimarine sediments through sea-ice build-up and suppressed iceberg rafting

Laminated glacimarine sediments are observed in visual core logs and x-radiographs from Scoresby Sund and Nansen Fjord, east Greenland. They are mostly underlain and overlain by massive or stratified glacimarine diamicton (Dmm or Dms), which is a product of iceberg delivery of heterogeneous debris and, in Scoresby Sund, reworking by deep-drafted iceberg keels. The laminated sediments are AMS radiocarbon dated to two cold periods since the last, Late Weichselian deglaciation: the Younger Dryas stadial (Milne Land Stadial in east Greenland) and the Little Ice Age. During cold climatic events, multiyear shorefast sea ice ('sikussak') formed in these fjords and trapped the icebergs. Fine-grained, laminated muds (Fl) were deposited in Scoresby Sund when the flux of icebergs was suppressed, but turbid meltwater continued to provide some sediment flux to the fjord systems, varying through time to produce laminations. In Nansen Fjord, thinner and often massive mud layers (Fm) resulted from shorter intervals of sea-ice cover with no ice rafting. Stratified diamicton layers (Dms), which alternate with mud deposition to produce a laminated unit, probably represent intervening times of more open conditions with iceberg rafting. In Scoresby Sund, foraminifera are either absent from the laminated unit or begin to appear towards the end of its deposition. The absence of both benthic and planktonic foraminifera also suggests that multiyear sea ice was covering the core sites. There is no evidence of macrofaunal activity, and bioturbation is absent from the laminated sediments. Satellite data show that multiyear shorefast sea ice is present in several areas of the high Arctic today, and this traps icebergs calved from interior ice-cap drainage basins. Thus, the process of laminated glacimarine sediment formation is likely to be applicable to a number of areas of the modern and Quaternary Arctic.     

Sedimentary facies and environmental history of the Late-glacial glaciomarine Fossvogur sediments in Reykjavík, Iceland

A detailed account of facies relationships in the Late-glacial Fossvogur beds is presented for the first time. A new interpretation in terms of sedimentary processes has been synthesized in a palaeoenvironmental reconstruction that incorporates the results of a recently completed, systematic ¹⁴C dating project for the Fossvogur beds. The present sedimentological analysis has revealed three marine fossiliferous facies and several diamictite facies. The two uppermost marine facies are separated by a horizon of local deformation and erosion which is ascribed to increased activity at a tidewater glacier margin in Fossvogur. The fact that marine fossiliferous sediments below and above this horizon have been confined to the Allerød chron indicates a temporary expansion of glaciers in the Reykjavík region towards the end of, but within, the Allersd. The relative sea level must have been at least 20 m higher than at present before the expansion, and it was probably even higher during and after the expansion. The total absence of facies indicating either lodgement or melt-out processes, and the abundance of diamictites interpreted as debris-flow deposits as well as frequent erratics in the marine mudrocks favour an interpretation based on a glaciomarine model for the Allerød deposition in Fossvogur. It is suggested that the relatively quiet, submarine conditions indicated by facies towards the top of the Fossvogur beds display continued transgression and an increased distance to the source of sediment supply during the Younger Dryas.     

Sedimentation at the margins of a late Pleistocene ice-lobe terminating in shallow marine environments, Dundalk Bay, eastern Ireland

Late Pleistocene morainic sequences around Dundalk Bay, eastern Ireland, were deposited in a variety of shallow, glaciomarine environments at the margins of a grounded ice lobe. The deposits are essentially ice-proximal delta-fan and -apron sequences and are divided into two lithofacies associations. Lithofacies association 1 occurs as a series of morainal banks formed at the southern margin of the ice lobe in a body of water open to influences from the Irish Sea. The morainal banks consist mainly of diamictic muds deposited from turbid plumes and by ice-rafting with minor occurrences of turbidites, cross-bedded gravels (subaqueous outwash) and massive boulder gravels (high-density debris flows). Lithofacies association 2 was deposited in a narrow arm of the sea at the north-eastern margin of the ice lobe. The deposits consist mainly of a series of coalescing, ice-proximal Gilbert-type fan deltas which are interbedded distally with tabular and lens-shaped subaqueous deposits. The latter are mainly ice-rafted diamictons, debris-flow deposits and subaqueous sands and gravels. Both lithofacies associations are draped by diamictons formed by a combination of rain-out, debris flow and traction-current activity. At a few localities the upper parts of the sequence have been sheared by minor oscillations of the ice sheet margin. These sequences form part of an extensive belt of glaciomarine deposits which border the drumlin swarms of east-central Ireland. Lithostratigraphic variability is partially related to the arrival of large volumes of debris at the ice lobe margin when the main lowland ice sheet surged during drumlin formation. Complex depositional continua of this type lack any major erosional breaks and should not be used either as climatic proxies or for stratigraphic correlations.     

The sedimentary record of drifting ice (early Wisconsin Sunnybrook deposit) in an ancestral ice-dammed Lake Ontario, Canada

Outcrops of pebbly mud (diamict) at Scarborough in Southern Ontario, Canada (the so-called Sunnybrook ‘Till’) are associated with the earliest incursion of the Laurentide Ice Sheet (LIS) into mid-continent North America some 45,000 years ago. The Sunnybrook is a blanket-like deposit containing deepwater ostracodes and occurs conformably within a thick (100 m) succession of deltaic and glaciolacustrine facies that record water depth changes in a large proglacial lake. Contextual evidence (associated facies, sedimentary structures, deposit geometry and landforms) indicates a low energy depositional setting in an ice-dammed ancestral Lake Ontario in which scouring by floating ice masses was an important process. U-shaped, iceberg-cut scours (with lateral berms) up to 7 m deep, occur on the upper surface of the Sunnybrook and are underlain by ‘sub-scour’ structures that extend several meters below the scour base. Ice-rafted concentrations of clasts (‘clast layers’), grooved surfaces formed by floating ice glissading over a muddy lake floor (‘soft sediment striations’) and melanges of sand and mud mixed by grounding ice keels (‘ice keel turbates’) are present and are all well known from modern cold environments. The wider significance of this depositional model is that the LIS margin lay east of Scarborough and did not overrun Southern Ontario. This finding is in agreement with recent data from the Erie Basin of Canada, Ohio, and Indiana where deposits formerly correlated with the Sunnybrook (and thus implying an extensive early Wisconsin ice sheet) are now regarded as Illinoian. A speculative hypothesis is proposed that relates deposition of the Sunnybrook and two younger deposits of similar sedimentology, to surge-like instabilities of the southern LIS margin.     

Numerical reconstructions of the Eurasian Ice Sheet and climate during the Late Weichselian

Geological investigations undertaken through the Quaternary Environments of the Eurasian North programme established ice-sheet limits for the Eurasian Arctic at the Last Glacial Maximum (LGM), sedimentary records of palaeo-ice streams and uplift information relating to ice-sheet configuration and the pattern of deglaciation. Ice-sheet numerical modelling was used to reconstruct a history of the Eurasian Ice Sheet compatible with these geological datasets. The result was a quantitative assessment of the time-dependent behaviour of the ice sheet, its mass balance and climate, and predictions of glaciological products including sediments, icebergs and meltwater. At the LGM, ice cover was continuous from Scandinavia to the Arctic Ocean margin of the Barents Sea to the north, and the Kara Sea to the east. In the west, along the continental margin between the Norwegian Channel and Svalbard, the ice sheet was characterised by fast flowing ice streams occupying bathymetric troughs, which fed large volumes of sediment to the continental margin that were deposited as a series of trough mouth fans. Ice streams may also have been present in bathymetric troughs to the north between Svalbard and Franz Josef Land. Further east, however, the ice sheet was thinner. Across the Kara Sea, the ice thickness was predicted to be less than 300 m, while on Severnaya Zemlya the ice cover may have been thinner at the LGM than at present. It is likely that the Taymyr Peninsula was mainly free of ice at the LGM. In the south, the ice margin was located close to the shoreline of the Russian mainland. The climate associated with this ice sheet is maritime to the west and, in stark contrast, desert-like in the east. Atmospheric General Circulation Modelling has revealed that such a contrast is possible under relatively warm north Atlantic conditions because a circulation system develops across the Kara Sea, isolating it from the moisture-laden westerlies, which are diverted to the south. Ice-sheet decay began through enhanced iceberg calving in the deepest regions of the Barents Sea, which caused a significant ice embayment within the Bear Island Trough. By about 12,000 years ago, further iceberg calving reduced ice extent to the northern archipelagos and their surrounding shallow seas. Ice decay was complete by about 10,000 years ago.     

Sedimentary architecture and glacial hydrodynamic significance of the stratified Oak Ridges Moraine,southern Ontario,Canada

High-quality subsurface data provide new insights into the formation of Oak Ridges Moraine (ORM), an ~80 km³ sequence of stratified meltwater deposits resting >200 m above adjacent Lake Ontario. The ORM sedimentary succession comprises a three-part regional architecture: (i) ~north–south channel sand–gravel; (ii) channel-capping rhythmites; and (iii) east–west ridge sediments. The ORM depositional sequence overlies a regional unconformity with a cross-cutting channel network resulting from ~north–south meltwater floods that transitioned progressively (falling stage) from a ~NNE to ENE flow direction (parallels Lake Ontario depression). Seismic profiles delineate the channels and channel fill characteristics of bank-to-bank channel sedimentation of thick gradational gravel–sand–mud sequences. Channel-capping mud (~100–236 rhythmites) within multiple channels beneath the ORM landform mark a widespread interval of low-energy, seasonally controlled subglacial pond deposition. During this quiescent period ice-sheet thickness adjusted to flood-induced stretching/thinning and re-profiled slopes. New ice gradients led to east–west flow and deposition of the overlying third element, a sequence of high-energy confined esker–fan sediments along ORM ridge. Close, sequential timing (~329 varve years) of channel, basin and ridge-forming architectural elements supports naming this assemblage the ORM formation. Proposed ORM floods are analogous to Icelandic jökulhlaups based on the size, geometry and sedimentology. The observed rhythmite interval between flood events represents a short period (~236 years) of regional meltwater storage prior to east–west ORM flooding. The ORM channel and overlying esker-fan sediment ridge represent two closely timed meltwater drainage events rather than formation by coalescing ice streams. The scale and timing of the ORM flood events are linked to rapid sea-level rise, ~13.5 ka BP. This high-resolution ORM sedimentological record may provide insights into depositional and glaciogenic controls of other large, stratified moraines. The ORM data indicate deposition in response to hydrodynamic events (outbreak floods, re-profiled ice) rather than direct climate forcing.