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.     

Depositional architecture and evolution of progradationally stacked lobe complexes in the Eocene Central Basin of Spitsbergen

The down‐dip portion of submarine fans comprises terminal lobes that consist of various gravity flow deposits, including turbidites and debrites. Within lobe complexes, lobe deposition commonly takes place in topographic lows created between previous lobes, resulting in an architecture characterized by compensational stacking. However, in some deep water turbidite systems, compensational stacking is less prominent and progradation dominates over aggradation and lateral stacking. Combined outcrop and subsurface data from the Eocene Central Basin of Spitsbergen provide a rare example of submarine fans that comprise progradationally stacked lobes and lobe complexes. Evidence for progradation includes basinward offset stacking of successive lobe complexes, a vertical change from distal to proximal lobe environments as recorded by an upward increase in bed amalgamation, and coarsening and thickening upward trends within the lobes. Slope clinoforms occur immediately above the lobe complexes, suggesting that a shelf‐slope system prograded across the basin in concert with deposition of the lobe complexes. Erosive channels are present in proximal axial lobe settings, whereas shallow channels, scours and terminal lobes dominate further basinward. Terminal lobes are classified as amalgamated, non‐amalgamated or thin‐bedded, consistent with turbidite deposition in lobe axis, off‐axis and fringe settings, respectively. Co‐genetic turbidite–debrite beds, interpreted as being deposited from hybrid sediment gravity flows which consisted of both turbulent and laminar flow phases, occur frequently in lobe off‐axis to fringe settings, and are rare and poorly developed in channels and axial lobe environments. This indicates bypass of the laminar flow phase in proximal settings, and deposition in relative distal unconfined settings. Palaeocurrent data indicate sediment dispersal mainly towards the east, and is consistent with slope and lobe complex progradation perpendicular to the NNW–SSE trending basin margin.     

Evidence of iceberg-ploughing in a subaqueous ice-contact fan, glacial Lake Rinteln, NW Germany

The Coppenbrügge subaqueous ice‐contact fan complex of early Saalian age is about 10 km long and up to 10 km wide and is composed of offset‐stacked fan clinothems that are transgressive‐regressive sequences formed during an overall lake level rise. The individual fan bodies consist of coarse gravel in the ice‐proximal part, passing distally into sandy facies and showing large‐scale foreset bedding. The iceberg scour recognized in an open‐pit outcrop is up to 1.5m deep, up to 2m wide and cut in undisturbed mid‐fan deposits. The scour‐fill can be traced laterally for about 15m and consists of sheared sand and, in the frontal zone, of downbent overlying strata surrounded by a zone of deformed sediments. The deformed sediment produced by the iceberg keel's shearing of the trough walls is a sand mass containing angular soft‐sediment clasts that show internal folds and fractures. The basal surface of the deformed sediment is a nearly horizontal shear plane, steepening up laterally as a discrete thrust and showing a flat‐ramp‐flat geometry. The scour was formed by the iceberg keel's ploughing the substrate and pushing the sediment sideways and frontally, forming a ridge of deformed sediments at the trough end. This ridge was concurrently eroded by an accompanying meltwater underflow which apparently developed a horseshoe system of scouring vortices around the grounded iceberg. The current's scour was filled with massive, non‐stratified sand deposited rapidly from turbulent suspension. The iceberg eventually broke up and its keel part was buried. As these ice fragments gradually melted, the space was closed by normal faulting and downbending of overlying strata. The collapsing scour‐fill became partly liquified, and the resulting water‐escape structures cut the normal faults and the overlying deposits. Though produced chiefly by tangential shear strain, iceberg‐ploughing features are readily distinguishable from other glaciotectonic deformations. They can serve as a diagnostic criterion for glaciolacustrine or glaciomarine environments and the distinguishing of ice‐contact subaqueous fans from ice‐contact deltas in the stratigraphic record.     

Relay ramps as pathways for turbidity currents: a study combining analogue sandbox experiments and numerical flow simulations

Unlike for subaerial settings, the impact of subaqueous relay ramps on sediment dispersal is still poorly understood. A combination of analogue laboratory experiments in a sandbox with numerical flow calculations is used to simulate relay ramp topographies on rifting continental margins and to analyse the resulting turbidity current pathways and their deposits. Various scenarios are investigated, including inflow perpendicular and oblique to the relay ramp axis as well as flow constrained by an incised channel on the ramp and by a landward‐directed tilt of the ramp. Without channelling, most sedimentation takes place on the basin floor because the bulk of the flow follows the steepest gradient down the fault and into the rift basin. With a channel along the relay ramp, significant flow occurs initially down the ramp axis, but channel spillover and basinward ramp tilting combine to redirect much of the sediment down the fault slope into the basin. When the relay ramp has a landward‐oriented tilt, most of the current flows down the ramp and deposits its sediment load there and at the foot of the ramp. However, also here a considerable amount of the flow is shed over the hanging wall fault and into the basin, forming a secondary depocentre, while ponding redistributes thin deposits over a wider area of the basin. The quantitative dependence of these results on the specific ramp geometries remains to be investigated further but may bear great importance for refined sedimentary models in subaqueous rifted settings as well as for hydrocarbon exploration therein.     

Pliocene subaqueous fans and Gilbert-type deltas in maar crater lakes, Hopi Buttes, Navajo Nation (Arizona), USA

Maar craters of the Mio-Pliocene Hopi Buttes volcanic field of Arizona formed within a broad playa system, and accumulated a variety of lacustrine sedimentary deposits. Many craters initially held isolated, groundwater-fed lakes. Ephemeral streams crossing the playa entered some of the lake-filled craters, and built coarse grained Gilbert-type deltas and subaqueous fans along the margins of these craters. The small, coarse grained fans and deltas have many features in common with much larger coarse grained deltaic and fan deltaic deposits. However, the local production of coarse grained volcanic sediment, low gradients in the local stream catchment, steep subaqueous relief and the small size of the receiving ‘basins’resulted in a unique combination of features. Cone-shaped subaqueous fans initially formed at the mouths of incised feeder streams. The fans are small accumulations of steeply dipping gravelly tephra that consist almost entirely of overlapping lobes constructed by density-modified grain flows. Gravelly Gilbert-type tephra deltas formed in brimfull craters fed by a freely migrating feeder stream. They are concave lakeward, mimicking the underlying crater wall topography. Complex deltaic geometries are defined by topset strata that steeply onlap tall foreset beds. They suggest that feeding stream floods caused rapid and comparatively large variations in lake level within the small crater lakes. Bed-specific carbonate alteration is common, and probably resulted from both influx of detrital carbonate across the playa and alteration of tephra beds by carbonate-saturated lakewaters during between flood periods of high net evaporation.     

Sedimentology and depositional model for glaciolacustrine deposits in an ice-dammed tributary valley, western Tasmania, Australia

Quaternary sedimentary successions are described from the Linda Valley, a small valley in western Tasmania that was dammed by ice during Early and Middle Pleistocene glaciations. Mapping and logging of exposures suggest that an orderly sequence of deposits formed during ice incursion, occupation and withdrawal from tributary valleys. Four principal sediment assemblages record different stages of ice occupation in the valley. As the glacier advanced, a proglacial, lacustrine sediment assemblage dominated by laminated silts and muds deposited from suspension accumulated in front of the glacier. A subglacial sediment assemblage consisting of deformed lacustrine deposits and lodgement till records the overriding of lake-bottom sediments as the glacier advanced up the valley into the proglacial lake. As the glacier withdrew from the valley, a supraglacial sediment assemblage of diamict, gravel, sand and silt facies formed on melting ice in the upper part of the valley. A lacustrine regression in the supraglacial assemblage is inferred on the basis of a change from deposits mainly resulting from suspension in a subaqueous setting to relatively thin and laterally discontinuous laminated sediments, occurrence of clastic dykes, and increasing complexity of the geometry of deposits that indicate deposition in a subaerial setting. A deltaic sediment assemblage deposited during the final stage of ice withdrawal from the valley consists of steeply dipping diamict and normally graded gravel facies formed on delta foresets by subaqueous sediment gravity flows. The sediment source for the delta, which prograded toward the retreating ice margin, was the supraglacial sediment assemblage previously deposited in the upper part of the valley. A depositional model developed from the study of the Linda Valley may be applicable to other alpine glaciated areas where glaciers flowed through or terminated in medium- to high-relief topography.     

Sea level controls on the textural characteristics and depositional architecture of the Hueneme and associated submarine fan systems, Santa Monica Basin, California

Hueneme and Dume submarine fans in Santa Monica Basin consist of sandy channel and muddy levee facies on the upper fan, lenticular sand sheets on the middle fan, and thinly bedded turbidite and hemipelagic facies elsewhere. Fifteen widely correlatable key seismic reflections in high-resolution airgun and deep-towed boomer profiles subdivide the fan and basin deposits into time-slices that show different thickness and seismic-facies distributions, inferred to result from changes in Quaternary sea level and sediment supply. At times of low sea level, highly efficient turbidity currents generated by hyperpycnal flows or sediment failures at river deltas carry sand well out onto the middle-fan area. Thick, muddy flows formed rapidly prograding high levees mainly on the western (right-hand) side of three valleys that fed Hueneme fan at different times; the most recently active of the lowstand fan valleys, Hueneme fan valley, now heads in Hueneme Canyon. At times of high sea level, fans receive sand from submarine canyons that intercept littoral-drift cells and mixed sediment from earthquake-triggered slumps. Turbidity currents are confined to ‘underfit’ talweg channels in fan valleys and to steep, small, basin-margin fans like Dume fan. Mud is effectively separated from sand at high sea level and moves basinward across the shelf in plumes and in storm-generated lutite flows, contributing to a basin-floor blanket that is locally thicker than contemporary fan deposits and that onlaps older fans at the basin margin. The infilling of Santa Monica Basin has involved both fan and basin-floor aggradation accompanied by landward and basinward facies shifts. Progradation was restricted to the downslope growth of high muddy levees and the periodic basinward advance of the toe of the steeper and sandier Dume fan. Although the region is tectonically active, major sedimentation changes can be related to eustatic sea-level changes. The primary controls on facies shifts and fan growth appear to be an interplay of texture of source sediment, the efficiency with which turbidity currents transport sand, and the effects of delta distributary switching, all of which reflect sea-level changes.     

Depositional model for lacustrine nearshore subaqueous fans in a rift basin: The Eocene Shahejie Formation,Dongying Sag,Bohai Bay Basin,China

The origin of the fourth member of the Eocene Shahejie Formation in the northern steep slopes of the Minfeng Sub‐sag, Dongying Sag, China, was investigated by integrating core studies and flume tank depositional simulations. A non‐channelized depositional model is proposed in this paper for nearshore subaqueous fans in steep fault‐controlled slopes of lacustrine rift basins. The deposits of nearshore subaqueous fans along the base of steep border‐fault slopes of rift basins are typically composed of deep‐water coarse‐grained sediment gravity‐flow deposits directly sourced from adjacent footwalls. Sedimentation processes of nearshore subaqueous fans respond to tectonic activities of boundary faults and to seasonal rainfall. During tectonically active stages, subaqueous debris flows triggered by episodic movements of border‐faults dominate the sedimentation. During tectonically quiescent stages, hyperpycnal flows generated by seasonal rainfall‐generated floods, normal discharges of mountain‐derived rivers and deep‐lacustrine suspension sedimentation are commonly present. The results of a series of flume tank depositional simulations show that the sediments deposited by subaqueous debris flows are wedge‐shaped and non‐channelized, whereas the sediments deposited by hyperpycnal flows generated by sporadic floods from seasonal rainfall are characterized by non‐channelized, coarse‐grained lobate depositional bodies which switch laterally because of compensation sedimentation of hyperpycanal flows. The hyperpycnal‐flow‐deposited non‐channelized lobate depositional bodies can be divided into a main body and lateral edges. The main body can be further subdivided into a proximal part, middle part and frontal part. Normal mountain‐derived river‐discharge‐deposited sediments are characterized by thin‐bedded, fine‐grained sandstones and siltstones with a limited distribution range. Normal mountain‐derived river‐discharge‐deposited sediments and deep‐lacustrine mudstones are commonly eroded in the area close to boundary faults. A nearshore subaqueous fan can be divided into three segments: inner fan, middle fan and outer fan. The inner fan is composed of debrites and the proximal part of the main body. The middle fan consists of the middle part of the main body and lateral edges, normal mountain‐derived river‐discharge‐deposited fine‐grained sediments and deep‐lacustrine mudstones. The outer fan comprises the frontal part of the main body, lateral edges, and deep‐lacustrine mudstones. Based on the non‐channelized depositional model for nearshore subaqueous fans, criteria for stratigraphic subdivision and correlation are discussed and applied.     

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.     

An allostratigraphic approach to a non-marine basin: the north-western part of Cretaceous Kyongsang Basin, SE Korea

The north-western part of the Cretaceous Kyongsang Basin, south-east Korea, comprises alluvial deposits of conglomerate, gravelly sandstone, sandstone and mudstone which can be grouped into four allomembers bounded by stratigraphic discontinuities. The discontinuities trend NW–SE and are marked by distinct facies transitions, abrupt emplacement of conglomerate and thin but persistent mudstone beds. Sedimentary facies and architectural analyses reveal that each allomember formed a depositional system of fluvial channel networks draining toward the south-east with alluvial fans on the northern margin. Each allomember can be characterized by distinctive architecture of channel-fills, clast composition of conglomerate and sandstone/mudstone ratio. Successive units show an eastward shift in the locus of deposition, suggesting basinward relocations of alluvial systems. Such variations with time and space are interpreted to reflect changes in accommodation space and sediment supply during basin evolution, probably caused by fault movements. This study shows that detailed mapping, combined with architectural analysis, and the establishment of alluvial allostratigraphy can help assess changes in alluvial systems and structural development of the basin.     

The Maesan fan delta, Miocene Pohang Basin, SE Korea: architecture and depositional processes of a high-gradient fan-delta-fed slope system

The Maesan fan-delta-fed slope system in the Miocene Pohang Basin occurs between two Gilbert-type fan deltas. Detailed analysis of sedimentary facies and bed geometry reveals that the sequence is represented by 13 sedimentary facies. These facies can be organized into three facies associations, representing distinct depositional environments: alluvial fan (facies association I), steep-faced slope (facies association II), and basin plain (facies association III). Subaerial debris flows and dense, inertia-dominated currents were transformed into subaqueous sediment gravity flows in steep-faced slope environments. Further downslope, these flows were channelized and formed lobate conglomerate and sandstone bodies at the terminal edge of the channels (or chutes). Interchannel and interlobe areas were dominated by homogeneous mudstone and muddy sandstone, deposited by suspension settling of fine-grained materials. Part of the steep-faced slope deposits experienced large-scale slides and slumps. The chutes/channels, lobes and splays on the steep-faced slope of the Maesan system are similar to those in modern subaqueous coarse-grained fan-delta systems.     

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.     

Anatomy, geometry and sequence stratigraphy of basin floor to slope turbidite systems, Tanqua Karoo, South Africa

The Tanqua area of the Karoo basin, South Africa, contains five Permian deep-water turbidite fan systems, almost completely exposed over some 640 km². Reconstruction of the basin-fill and fan distributions indicates a progradational trend in the 450 m+ thick succession, from distal basin floor (fan 1) through basin-floor subenvironments (fans 2, 3 and 4) to a slope setting (fan 5). Fans are up to 65 m thick with gradational to sharp bases and tops. Facies associations include basin plain claystone and distal turbidite siltstone/claystone and a range of fine-grained sandstone associations, including low- and high-density turbidite current deposits and proportionally minor debris/slurry flows. Architectural elements include sheets of amalgamated and layered styles and channels of five types. Each fan is interpreted as a low-frequency lowstand systems tract with the shaly interfan intervals representing transgressive and highstand systems tracts. All fans show complex internal facies distributions but exhibit a high-frequency internal stratigraphy based on fan-wide zones of relative sediment starvation. These zones are interpreted as transgressive and highstand systems tracts of higher order sequences. Sandy packages between these fine-grained intervals are interpreted as high-frequency lowstand systems tracts and exhibit dominantly progradational stacking patterns, resulting in subtle downdip clinoform geometries. Bases of fans and intrafan packages are interpreted as low- and high-frequency sequence boundaries respectively. Facies juxtapositions across these sequence boundaries are variable and may be gradational, sharp or erosive. In all cases, criteria for a basinward shift of facies are met, but there is no standard 'motif' for sequence boundaries in this system. High-frequency sequences represent the dominant mechanism of active fan growth in the Tanqua deep-water system.     

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.     

Coarse-grained submarine fan and slope apron deposits in a Cretaceous back-arc basin, Antarctica

The Cretaceous of west James Ross Island, Antarctica represents the proximal fill of a late Mesozoic back-arc basin that was probably initiated by oblique extension during the early development of the Weddell Sea. The succession records sedimentation in two contrasting depositional systems: a laterally persistent slope apron flanking the faulted basin margin interrupted both spatially and temporally by coarse-grained submarine fans. Slope apron deposits are dominated by thinly interbedded turbiditic sandstones and mudstones (mudstone association), interspersed with non-channelized chaotic boulder beds, intraformational slump sheets and isolated exotic blocks representing a spectrum of mass-flow processes from debris flow to submarine gliding. Localized sand-rich sequences (sandstone-breccia association) represent sandy debris lobes at the mouths of active slope chutes. The submarine fan sediments (conglomerate association) are typified by coarse conglomerates and pebbly sandstones, interpreted as the deposits of high-density turbidity currents and non-cohesive debris flows. Three assemblages are recognized and are suggested to represent components of the inner channelled zone of coarse-grained submarine fans, from major fan channels through ephemeral, marginal channels or terraces to levee or interchannel environments. The occurrence of both slope apron and submarine fan depositional systems during the Early and Mid-Cretaceous is attributed to localized input of coarse arc-derived sediment along a tectonically active basin margin. Periods of extensive fan development were probably linked to regional tectonic uplift and rejuvenation of the arc source region; cyclicity within individual fan sequences is attributed to migration or switching of fan channels or canyons. Slope apron sedimentation was controlled largely by intrabasinal tectonics. Local unconformities and packets of amalgamated slide sheets and debris flow deposits probably reflect episodic movement on basin margin faults. Differential subsidence across the basin margin anchored the basin slope for at least 20 Myr and precluded basinward progradation of shallow marine environments.     

Sedimentary Response of Different Fan Types to the Paleogene–Neogene Basin Transformation in the Kuqa Depression, Tarim Basin, Xinjiang Province

A group of alluvial fans formed in the early Paleogene represent marginal sedimentary facies at the foot of the South Tianshan Mountain, Kuqa Depression, Tarim Basin, Xinjiang province. Two types of fans occurred in the middle–late Paleogene Kumugeliemu and Suweiyi formations: one alluvial, and the other fan delta deposited in a lacustrine setting. Within the early Neogene Jidike Formation, coastal subaqueous fans developed, probably in a deeper water lacustrine setting. The three types of fans are stacked vertically in outcrop with the sequence in ascending order: bottom alluvial, middle fan-delta, and top subaqueous. The subaqueous is a typical coarse-fan deposit occurring in the glutinite member of the Jidike Formation in some wells. Laterally, from the foreland to the lacustrine settings, the distribution pattern of sedimentary facies represents the same three fan types sequentially. The spatial distribution of these fans was controlled by the Paleogene–Neogene Basin transformation, and evolution with different types of fans developed in the Kuqa Depression in response. In the Paleogene, the Kuqa Depression was a rift basin where an alluvial fan was deposited in the foreland setting, which, by early Neogene, became a foreland basin when the lake level changed. With any rise in lake level, fan-deltas migrated from lacustrine to foreland settings, whereas when the lake level fell, fan migration was reversed. In the early Neogene, with increasing slope and rising lake level, fans progressed and covered the previous fan-delta and lacustrine mudstone. Eventually, subaqueous fans developed, forming the present spatial configuration of these three fan types.     

Ice-margin retreat and grounding-zone dynamics during initial deglaciation of the Storfjordrenna Ice Stream,western Barents Sea

Processes occurring at the grounding zone of marine terminating ice streams are crucial to marginal stability, influencing ice discharge over the grounding-line, and thereby regulating ice-sheet mass balance. We present new marine geophysical data sets over a ~30×40 km area from a former ice-stream grounding zone in Storfjordrenna, a large cross-shelf trough in the western Barents Sea, south of Svalbard. Mapped ice-marginal landforms on the outer shelf include a large accumulation of grounding-zone deposits and a diverse population of iceberg ploughmarks. Published minimum ages of deglaciation in this region indicate that the deposits relate to the deglaciation of the Late Weichselian Storfjordrenna Ice Stream, a major outlet of the Barents Sea–Svalbard Ice Sheet. Sea-floor geomorphology records initial ice-stream retreat from the continental shelf break, and subsequent stabilization of the ice margin in outer-Storfjordrenna. Clustering of distinct iceberg ploughmark sets suggests locally diverse controls on iceberg calving, producing multi-keeled, tabular icebergs at the southern sector of the former ice margin, and deep-drafted, single-keeled icebergs in the northern sector. Retreat of the palaeo-ice stream from the continental shelf break was characterized by ice-margin break-up via large calving events, evidenced by intensive iceberg scouring on the outer shelf. The retreating ice margin stabilized in outer-Storfjordrenna, where the southern tip of Spitsbergen and underlying bedrock ridges provide lateral and basal pinning points. Ice-proximal fans on the western flank of the grounding-zone deposits document subglacial meltwater conduit and meltwater plume activity at the ice margin during deglaciation. Along the length of the former ice margin, key environmental parameters probably impacted ice-margin stability and grounding-zone deposition, and should be taken into consideration when reconstructing recent changes or predicting future changes to the margins of modern ice streams.     

The environment in and around ice-dammed lakes in the moderately high relief setting of the southern Canadian Cordillera

During decay of the Cordilleran Ice Sheet, ˜13 000–10000 cal. yr BP, numerous ice-dammed, ribbon-shaped lakes developed within the moderately deep valleys of the Interior Plateau of British Columbia. We describe the pattern and characteristics of lake sediments within the Thompson Valley, propose a palaeoenvironmental model for glacial lakes Thompson and Deadman and explore their implications for the palaeogeography of Cordilleran Ice Sheet decay. Seventeen glaciolacustrine lithofacies are identified within deltas, subaqueous fans and lake-bottom beds. Sediments accumulated at high rates and by a diversity of sediment dispersal and depositional processes: hyperpycnal and surge-type turbidity currents, grain flows and debris flows. Megascale subaqueous failures (tens of metres thick) were facilitated by high sedimentation rates. The palaeoenvironmental model highlights: (i) high rates of basin infilling; (ii) the dominant role of tributary rivers, rather than valley-occupying ice, in delivering water and sediment to lakes; and (iii) the role of melt cycles, jökulhlaups and hyperpycnal flows in sediment delivery. These conditions, in combination with a lack of organics and a fining upward sequence in lake sediments, suggest that glacial lakes Thompson and Deadman were coeval with dwindling plateau ice.     

Dimensions and architecture of late Pleistocene submarine lobes off the northern margin of East Corsica

Sandy lobe deposits on submarine fans are sensitive recorders of the types of sediment gravity flows supplied to a basin and are economically important as hydrocarbon reservoirs. This study investigates the causes of variability in 20 lobes in small late Pleistocene submarine fans off East Corsica. These lobes were imaged using ultra‐high resolution boomer seismic profiles (<1 m vertical resolution) and sediment type was ground truthed using piston cores published in previous studies. Repeated crossings of the same depositional bodies were used to measure spatial changes in their dimensions and architecture. Most lobes increase abruptly down‐slope to a peak thickness of 8 to 42 m, beyond which they show a progressive, typically more gradual, decrease in thickness until they thin to below seismic resolution or pass into draping facies of the basin plain. Lobe areas range from 3 to 70 km² and total lengths from 2 to 14 km, with the locus of maximum sediment accumulation from 3 to 28 km from the shelf‐break. Based on their location, dimensions, internal architecture and nature of the feeder channel, the lobes are divided into two end‐member types. The first are small depositional bodies located in proximal settings, clustered near the toe‐of‐slope and fed by slope gullies or erosive channels lacking or with poorly developed levées (referred to as ‘proximal isolated lobes’). The second are larger architecturally more complex depositional bodies deposited in more distal settings, outboard more stable and longer‐lived levéed fan valleys (referred to as ‘composite mid‐fan lobes’). Hybrid lobe types are also observed. At least three hierarchical levels of compensation stacking are recognized. Individual beds and bed‐sets stack to form lobe‐elements; lobe‐elements stack to form composite lobes; and composite lobes stack to form lobe complexes. Differences in the size, shape and architectural complexity of lobe deposits reflect several inter‐related factors including: (i) flow properties (volume, duration, grain‐size, concentration and velocity); (ii) the number and frequency of flows, and their degree of variation through time; (iii) gradient change and sea floor morphology at the mouth of the feeder conduit; (iv) lobe lifespan prior to avulsion or abandonment; and (v) feeder channel geometry and stability. In general, lobes outboard stable fan valleys that are connected to shelf‐incised canyons are wider, longer and thicker, accumulate in more basinal locations and are architecturally more complex.