Ice-contact Fans Research Articles

From glacial erosion to basin overfill: a 240 m-thick overdeepening–fill sequence in Bern, Switzerland

Abstract. We drilled a 210 m-thick succession of Quaternary sediments and extended it 30 m upsection with information that we collected from an adjacent outcrop. In the 240 m-thick succession we identified 12 different lithofacies, grouped them into five facies assemblages, and distinguished two major sedimentary sequences. A sharp contact at 103 m depth cuts off cross-beds in sequence A and separates them from the overlying horizontal beds in sequence B. Although the lowermost facies assemblage of each sequence includes a till deposited during a period of ice cover, the two tills differ from each other. In particular, the till at the base of sequence A is dominated by large clasts derived from the underlying Molasse bedrock, whereas the till at the base of sequence B has no such Molasse components. Furthermore, the till in sequence A bears evidence of glaciotectonic deformation. Both tills are overlain by thick assemblages of subaqueous, most likely glaciolacustrine and lacustrine facies elements. The cross-bedded and steeply inclined sand, gravel, and diamictic beds of sequence A are interpreted as deposits of density currents in a subaqueous ice-contact fan system within a proglacial lake. In contrast, the lacustrine sediments in sequence B are considered to record a less energetic environment where the material was most likely deposited in a prodelta setting that gradually developed into a delta plain. Towards the top, sequence B evolves into a fluvial system recorded in sequence C, when large sediment fluxes of a possibly advancing glacier resulted in a widespread cover of the region by a thick gravel unit. Feldspar luminescence dating on two samples from a sand layer at the top of sequence B provided uncorrected ages of 250.3 ± 80.2 and 251.3 ± 59.8 ka. The combination of these ages with lithostratigraphic correlations of sedimentary sequences encountered in neighboring scientific drillings suggests that sequence B was deposited between Marine Isotope Stage 8 (MIS 8; 300–243 ka) and MIS 7 (243–191 ka). This depositional age marks the end of one stage of overdeepening–fill in the perialpine Aare Valley near Bern.

GIS dataset: geomorphological record of terrestrial-terminating ice streams, southern sector of the Baltic Ice Stream Complex, last Scandinavian Ice Sheet, Poland

Abstract. Here we present a comprehensive dataset of glacial geomorphological features covering an area of 65 000 km2 in central west Poland, located along the southern sector of the last Scandinavian Ice Sheet, within the limits of the Baltic Ice Stream Complex. The GIS dataset is based on mapping from a 0.4 m high-resolution digital elevation model derived from airborne light detection and ranging data. Ten landform types have been mapped: mega-scale glacial lineations, drumlins, marginal features (moraine chains, abrupt margins, edges of ice-contact fans), ribbed moraines, tunnel valleys, eskers, geometrical ridge networks, and hill–hole pairs. The map comprises 5461 individual landforms or landform parts, which are available as vector layers in GeoPackage format at https://doi.org/10.5281/zenodo.4570570 (Szuman et al., 2021a). These features constitute a valuable data source for reconstructing and modelling the last Scandinavian Ice Sheet extent and dynamics from the Middle Weichselian Scandinavian Ice Sheet advance, 50–30 ka, through the Last Glacial Maximum, 25–21 ka, and Young Baltic advances, 18–15 ka. The presented data are particularly useful for modellers, geomorphologists, and glaciologists.

Glacier dynamics during a phase of Late Quaternary warming in Patagonia reconstructed from sediment-landform associations

The geomorphological record in glaciated landscapes can provide important information for the study of the response of glaciers to rapid climate change. This study presents a new reconstruction of the glacial history of the northern Monte San Lorenzo ice cap, southern South America, during a period of accelerated warming and deglaciation following the Antarctic Cold Reversal (14.5–12.8 ka). We present a detailed geomorphological map of the valleys to the north of Monte San Lorenzo. Sediment-landform assemblages identified include lateral and terminal moraine ridges, flutes, deltas, ice-contact fans, palaeoshorelines, kame terraces and outwash plains. We map 14 primary ice-limits, 7 of which are newly identified, and 7 of which are from previous studies, mapped here in greater detail. We devise landsystem models to formalise and document spatial and temporal changes in glacial processes and environments. Our new geomorphological mapping and landsystem reconstructions provide an important insight into the response of temperate Patagonian glaciers to rapidly-warming climate.

Submarine groundwater discharge site in the First Salpausselkä ice-marginal formation, south Finland

Abstract. Submarine groundwater discharge (SGD) has been implicated as a significant source of nutrients and potentially harmful substances to the coastal sea. Although the number of reported SGD sites has increased recently, their stratigraphical architecture and aquifer geometry are rarely investigated in detail. This study analyses a multifaceted dataset of offshore seismic sub-bottom profiles, multibeam and side-scan sonar images of the seafloor, radon measurements of seawater and groundwater, and onshore ground-penetrating radar and refraction seismic profiles in order to establish the detailed stratigraphical architecture of a high-latitude SGD site, which is connected to the Late-Pleistocene First Salpausselkä ice-marginal formation on the Hanko Peninsula in Finland. The studied location is characterized by a sandy beach, a sandy shore platform that extends 100–250 m seaward sloping gently to ca. 4 m water depth, and a steep slope to ca. 17 m water depth within ca. 50 m distance. The onshore radar and offshore seismic profiles are correlated based on unconformities, following the allostratigraphical approach. The aquifer is hosted in the distal sand-dominated part of a subaqueous ice-contact fan. It is interpreted that coarse sand interbeds and lenses in the distal fan deposits, and, potentially, sandy couplet layers in the overlying glaciolacustrine rhythmite, provide conduits for localized groundwater flow. The SGD takes place predominantly through pockmarks on the seafloor, which are documented on the shore platform slope by multibeam and side-scan sonar images. Elevated radon-222 activity concentrations measured 1 m above seafloor confirm SGD from two pockmarks in fine sand sediments, whereas there was no discharge from a third pockmark that was covered with a thin organic-rich mud layer. The thorough understanding of the local stratigraphy and the geometry and composition of the aquifer that have been acquired in this study are crucial for successful hydrogeological modelling and flux studies at the SGD site.

Asynchronous glacier dynamics during the Antarctic Cold Reversal in central Patagonia

We present 14 new 10Be cosmogenic nuclide exposure ages quantifying asynchronous readvances during the Antarctic Cold Reversal from glaciers in the Baker Valley region of central Patagonia. We constrain glacier and ice-dammed palaeolake dynamics using a landsystems approach, concentrating on outlet glaciers from the eastern Northern Patagonian Icefield (NPI) and Monte San Lorenzo (MSL). Soler Glacier (NPI) produced lateral moraines above Lago Bertrand from 15.1 ± 0.7 to 14.0 ± 0.6 ka, when it dammed the drainage of Lago General Carrera/Buenos Aires through Río Baker at a bedrock pinning point. At this time, Soler Glacier terminated into the 400 m “Deseado” level of the ice-dammed palaeolake. Later, Calluqueo Glacier (MSL) deposited subaerial and subaqueous moraines in the Salto Valley near Cochrane at 13.0 ± 0.6 ka. These moraines were deposited in an ice-dammed palaeolake unified through the Baker Valley (Lago Chalenko; 350 m asl). The Salto Valley glaciolacustrine landsystem includes subaqueous morainal banks, ice-scoured bedrock, glacial diamicton plastered onto valley sides, perched delta terraces, kame terraces, ice-contact fans, palaeoshorelines and subaerial push and lateral moraines. Boulders from the subaqueous Salto Moraine became exposed at 12.1 ± 0.6 years, indicating palaeolake drainage. These data show an asynchronous advance of outlet glaciers from the Northern Patagonian Icefield and Monte San Lorenzo during the Antarctic Cold Reversal. These advances occurred during a period of regional climatic cooling, but differential moraine extent and timing of advance was controlled by topography and calving processes.

Late Wisconsinan grounding zones of the Laurentide Ice Sheet margin off the Québec North Shore (NW Gulf of St Lawrence)

Abstract Swath bathymetry data and seismic profiles collected in the NW Gulf of St Lawrence reveal a series of wedge-shaped depositional systems interpreted as grounding zone wedges (GZWs). Some segments of the GZWs change locally to form frontal moraines, or morainal banks, and subaqueous ice-contact fans, reflecting changes in either the nature of the ice margin or the rate of sediment input. These grounding zones (GZ) of the ice margin extend laterally along three isobaths at depths of 180 (GZ1), 120 (GZ2) and 80 (GZ3) m (±20 m) along the Québec North Shore shelf, the 120 m-deep GZ2 system being traceable over a distance of >300 km. Associated GZWs can occur in three geometries along a same isobath system: curvilinear, lobate and shelf-break. GZ systems were built during three distinct stages of stabilization of the marine-based southeastern margin of the Laurentide Ice Sheet following its rapid retreat across the deeper waters of the Laurentian Channel in the Gulf of St Lawrence after 14.8 cal ka BP. The occurrence of GZ along distinct isobaths indicates that bathymetry exerted a strong control on ice stabilization during deglaciation by reducing the relative water depth at the ice margin and thereby the buoyancy and rate of iceberg calving. However, fluctuations and re-advances of the ice margin are also recorded by the overprinting of a portion of the GZ2 system by the younger GZ3 system, potentially suggesting an additional response to climate-driven forcing.

3D architecture of cyclic-step and antidune deposits in glacigenic subaqueous fan and delta settings: Integrating outcrop and ground-penetrating radar data

Bedforms related to supercritical flows are increasingly recognised as important constituents of many depositional environments, but outcrop studies are commonly hampered by long bedform wavelengths and complex three-dimensional geometries. We combined outcrop-based facies analysis with ground-penetrating radar (GPR) surveys to analyse the 3D facies architecture of subaqueous ice-contact fan and glacifluvial delta deposits. The studied sedimentary systems were deposited at the margins of the Middle Pleistocene Scandinavian ice sheets in Northern Germany.Glacifluvial Gilbert-type deltas are characterised by steeply dipping foreset beds, comprising cyclic-step deposits, which alternate with antidune deposits. Deposits of cyclic steps consist of lenticular scours infilled by backset cross-stratified pebbly sand and gravel. The GPR sections show that the scour fills form trains along the delta foresets, which can locally be traced for up to 15m. Perpendicular and oblique to palaeoflow direction, these deposits appear as troughs with concentric or low-angle cross-stratified infills. Downflow transitions from scour fills into sheet-like low-angle cross-stratified or sinusoidally stratified pebbly sand, deposited by antidunes, are common. Cyclic steps and antidunes were deposited by sustained and surge-type supercritical density flows, which were related to hyperpycnal flows, triggered by major meltwater discharge or slope-failure events.Subaqueous ice-contact fan deposits include deposits of progradational scour fills, isolated hydraulic jumps, antidunes and (humpback) dunes. The gravel-rich fan succession consists of vertical stacks of laterally amalgamated pseudo-sheets, indicating deposition by pulses of waning supercritical flows under high aggradation rates. The GPR sections reveal the large-scale architecture of the sand-rich fan succession, which is characterised by lobe elements with basal erosional surfaces associated with scours filled with backsets related to hydraulic jumps, passing upwards and downflow into deposits of antidunes and (humpback) dunes. The recurrent facies architecture of the lobe elements and their prograding and retrograding stacking pattern are interpreted as related to autogenic flow morphodynamics.

Eiríksjökull plateau icefield landsystem, Iceland

ABSTRACTA 1:9500 scale map of the Eiríksjökull plateau icefield and its post-Little Ice Age (LIA) foreland geomorphology and surficial geology is presented as a modern exemplar of an asymmetrically developed mountain glacier typical of the style of glacierization that dominated during mid-latitude Quaternary cold stages. Features regarded as diagnostic for this setting include (a) ice-cored hummocky terrain, indicative of controlled moraine construction in polythermal snouts, and localized breach lobe development, incremental stagnation and rock glacierization, all indicative of a debris-charged glacier snout; (b) fluted till and moraines, indicative of temperate basal ice conditions up-ice from polythermal glacier margins and (c) glacifluvial and debris flow deposits, occurring as steep fans emanating from glacier snouts at the plateau edge and in ice-contact fans or ramps fed directly by debris-covered glacier margins at the LIA. Although this plateau icefield landsystem is similar to those previously reported from Iceland, a remarkable debris-covered snout/ice-cored moraine complex on the foreland of Klofajökull is a more extreme example of the depositional zone that characterizes the valleys surrounding the more sediment-starved plateau ice dispersal centres.

Architecture of Late Ordovician glacial valleys in the Tassili N'Ajjer area (Algeria)

The architecture of three Late Ordovician glacial valleys was studied in detail in the Tassili N'Ajjer (SE Algeria) outcrops. The valleys are oriented south–north, 2 to 5km wide, and up to 250m deep. The valley-fills revealed a very complex sedimentary architecture with significant lateral facies changes. Several glacial cycles induced the formation of Glacial Erosion Surfaces (GES) at the base and within the glacial valleys.The first type of GES shows a sharp and steep-angled contact without striations or associated syn-sedimentary deformation, suggesting that subglacial meltwater was the dominant erosive agent. A second type associated with the deformation of pre-glacial and syn-glacial sediment, suggests that ice was in contact with the valley floor. Four facies associations are proposed: FA1: subglacial tillite; FA2: Sub-to pro-glacial ice contact fans; FA3: Proglacial sub-aqueous gravity flows; and FA4: outwash fans.The stratigraphic architecture of three of the main valleys reveals a complex polyphase infill. At least two main cycles of ice-sheet advance and retreat can be interpreted from the sedimentary succession of each valley. Minor glacial cycles by ice oscillations also occur locally. GES morphology and the facies sequence suggest that the Iherir valleys were initiated by meltwater erosion in subglacial channels, whereas the Dider and Ouarsissen valleys were part of a large ice stream pathway. Above the valley-fill and the interfluves, a sand-rich unit of stacked lobes and channels is interpreted as submarine outwash fans deposited during final ice retreat.A glacial sequence found between two GES comprises fluvio-glacial or ice-contact fan deposits, fluvio-glacial eskers and tills. These sediments were deposited subglacially or at the glacier front during the ice maximum phase and/or the early ice retreat phase. During the ice retreat, interbedded subaqueous gravity flow deposits and diamictites filled the glacially cut topography as the sea invaded the valleys. Maximum ice retreat was associated with high water fluxes and sediment discharge, causing a sand-dominated outwash fan to prograde out over the valleys and interfluves. This outwash fan was supplied mainly by flood activity at the ice front, involving high-density sustained flows. The dominant facies consists of giant aggrading climbing dunes filling channels or constructing sandy lobes downstream.

Late Ordovician ice-marginal processes and sea-level change from the north Gondwana platform: Evidence from the Valongo Anticline (northern Portugal)

In the Valongo Anticline, northern Portugal, the Upper Ordovician-age Sobrido Formation records glacioeustatic sea-level changes and glacially-influenced depositional environments on the north Gondwana platform associated with the Hirnantian glaciation. New stratigraphic data from the Sobrido Formation, taking into account lateral facies variations not exposed in the type section, allow for a better understanding of Hirnantian glacial sedimentary processes, sea-level changes and palaeogeography. The lower member of the Sobrido Formation is composed of quartzites alternating with siltstones and mudstones. The transition to the upper member is most commonly an erosional contact marked by a ferruginous horizon. The lowermost part of the upper member contains a thinly laminated and rhythmically-bedded mudstone that is capped by a thin (mm-scale) shelly bed and overlain by massive diamictites. Laterally, conglomerate beds that have erosion bases are rhythmically interbedded with slates and diamictites that sometimes contain outsized mudstone clasts. The uppermost part of the upper member is very variable in thickness laterally and contains massive to laminated diamictite beds intercalated with quartzites, conglomerates and slates. Massive diamictites commonly exhibit thin horizons with Fe-oxides and occasionally manganese, phosphate and oolitic chamosite. The lower member of the Sobrido Formation is interpreted as ice-proximal outwash that is transitional laterally to subaqueous ice-contact fans formed by debris and mass flows. Erosion followed by chemical weathering of the lower member and variably within the lowermost part of the upper member is interpreted as a response to sea-level fluctuations and development of spatially-variable anoxic conditions and groundwater migration within isolated marine basins. Facies within the upper member of the Sobrido Formation are interpreted as ice-distal (laminated diamictites) and ice-proximal deposits (massive diamictites, quartzites and conglomerates). Groove casts and hummocky cross-stratification within quartzite beds suggest deposition in a shallow shelf setting. The Upper Ordovician–Lower Silurian transition is locally marked by a ferruginous horizon. Black basal quartzites are interpreted to record development of an erosional unconformity during Silurian sea-level rise. In other areas a chloritoid layer indicates proximity to dry land. These palaeoenvironmental interpretations suggest dynamic changes in sedimentary and geochemical processes in response to sea level and related palaeogeographic changes during the Upper Ordovician and transitional to the Lower Silurian on the north Gondwana platform.

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

Abstract 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.

A glacial geomorphological map of the Monadhliath Mountains, Central Scottish Highlands

The Monadhliath Mountains in the Central Scottish Highlands are dominated by an extensive plateau area that has received little Quaternary research attention during the last century. Previous studies have focussed on the geomorphology and sedimentology of isolated valleys and therefore the 1:57,500 map presented here provides the first systematic assessment of the glacial geomorphology of the region as a whole, covering an area of approximately 840 km2. Results of mapping from remotely sensed imagery and in the field reveal a large number of moraines and ice-marginal meltwater channels both within outlet valleys and on the plateau, alongside a wide range of glacial, periglacial and fluvial features including glaciolacustrine landforms and sediments, lake overflow channels, ice-contact fans, blockfields, solifluction lobes, alluvial fans, debris cones, river terraces and rock slope failures. The spatial distribution of the majority of landforms indicates that the region was dominated by at least one local plateau icefield glaciation. The map now provides a framework for further analysis, which will greatly improve understanding of the extent and dynamics of former ice masses in the region.

Depositional architecture and sequence stratigraphic correlation of Upper Ordovician glaciogenic deposits, Illizi Basin, Algeria

Abstract Upper Ordovician glaciogenic deposits are profoundly important as hydrocarbon reservoirs across North Africa, such as within the Illizi Basin of SE Algeria. In this study we present a new sedimentological and sequence stratigraphic model for Upper Ordovician glaciogenic deposits based on the analysis of core descriptions and wireline logs from 25 wells in the Tiguentourine Field. Within the glaciogenic succession, two ice advance–retreat cycles can be defined, consisting of glaciomarine ice-contact fan deposits and tillites. Deposits of the marine ice-contact fan systems generally show a retrogradational stacking pattern from ice-proximal to ice-distal deposits. This pattern is attributed to the deposition in front of a retreating ice sheet. The proximal marine ice-contact fan deposits consist of massive or low-angle cross-bedded pebbly sandstone. They are interpreted as the deposits of turbulent, high-energy plane-wall jets, emerging from subglacial meltwater conduits. These jet-efflux deposits are up to 60 m thick and interbedded with deposits of cohesive and non-cohesive debris flows. The jet-efflux deposits are overlain by fine-grained, thick-bedded massive sandstone. These mid-fan deposits build up the bulk of the glaciomarine fans and are interpreted as deposits of underflows, generated at the point of flow-detachment, where marine meltwater jets become buoyant and large volumes of sediment fall-out from suspension. In the upper part of the fan succession massive sandstones pass upwards into mud-prone massive sandstones, interpreted as deposits of cohesive sandy debris flows. The most ice-distal deposits are muddy sandstones and mudstones deposited by waning low-density turbulent flows and suspension fall-out. The best reservoir properties within the glaciogenic succession are attributed to the proximal and medial deposits of the ice-contact fans such as coarse-grained jet-efflux deposits and sustained high-density turbulent flow deposits. However, the mud content within the massive sandstones is highly variable and influences the reservoir quality. Both glacial depositional sequences infill 60–175 m deep, elongated depressions, which are interpreted as subglacial tunnel valleys. These tunnel valleys acted as depocentres for the glaciomarine fan deposits. After final deglaciation and post-glacial transgression, organic-rich shale was preferentially deposited in underfilled tunnel valleys.

The formation of Alpine lateral moraines inferred from sedimentology and radar reflection patterns: a case study from Gornergletscher, Switzerland

Abstract Studies on the formation of alpine lateral moraines are rare; consequently, their internal structure and their modes of formation are relatively poorly understood. We present here sedimentological and ground-penetrating radar (GPR) data from a lateral moraine complex where an exposure allows radar facies to be compared to the field evidence. The moraine, which is slightly asymmetric with a slightly steeper distal slope, consists of alternating clast- and matrix-supported stratified diamicts and intercalated sorted sediment units which are all subparallel to the moraine surface. They are interpreted as subaerial debris flows and fluvial sediments, respectively. The GPR, using frequencies of 50, 100 and 200 MHz, allows tracing of very similar subparallel reflectors to c. 10 m into the subsurface. These are interpreted to represent the diamict units while the thickness of the sorted sediment units is below GPR resolution. Our data suggests that a two or three frequency approach supplemented by common-midpoint (CMP) measurements works well in glaciated high-mountain environments. Based on our sedimentological and GPR data we develop a conceptual framework for the formation of successive lateral moraines which involves ice-contact fan sedimentation, followed by collapse and incremental reworking of the proximal side during glacier retreat.

The stratigraphy, timing and climatic implications of glaciolacustrine deposits in the middle Rakaia Valley, South Island, New Zealand

Stratigraphic interpretations and infrared stimulated luminescence (IRSL) ages document the glacial history of the Middle Rakaia Gorge section of the Rakaia Valley, Canterbury, New Zealand, during the last two glacial cycles. Sheets of glaciolacustrine sediments several tens of metres in thickness can be traced at least 10 km upstream of the Rakaia Gorge. The stratigraphic package is capped by outwash gravels associated with the last glacial maximum (LGM) advances. The dominant inferred sedimentary environments in the sequence are 1) pro-glacial and paraglacial lake beds, 2) sub-aqueous ice-contact fans, 3) sub-aqueous mass flow deposits 4) supraglacial dump material and 5) outwash gravels. Syndepositional deformation, associated with glacitectonic deformation, is common. The stratigraphy records glacier margin oscillations, including six significant advances. These occurred in early MIS 6, mid-MIS 6, MIS 5b (c100–90 ka), MIS 5a/4 (c 80 ka), mid-MIS 3 (c 48 ka), and late MIS 3 (c 40ka). All the post-MIS 6 advances can be corroborated from other sites in New Zealand and the timings appear to coincide with both Southern Hemisphere insolation minima and maxima, suggesting variable combinations of climatic forcing in New Zealand glaciation.

Ichnology of a Late Palaeozoic ice-marginal shallow marine succession: Talchir Formation, Satpura Gondwana basin, central India

The Late Palaeozoic, glaciomarine Talchir Formation in the Satpura Gondwana basin, central India hosts sparse to profuse trace fossils of various kinds, although sporadically. Among the three principal depositional systems that gave rise to the Talchir succession, deposits of the ice-contact fan delta system and the glacial outwash braidplain delta system lack trace fossils of any kind. On the other hand, within the deposits of the storm wave-dominated, shoreface–shelf system a variety of animal traces is present including simple locomotion traces and non-descript bioturbation. Three trace fossil assemblages could be recognized within these deposits. Assemblage-I is dominated by Cylindrichnus of the Skolithos ichnofacies that occurs in storm laid, medium to fine sandstones deposited in shoreface and transitional shoreface–inner shelf environments. In contrast, assemblage-II comprising dominantly horizontal, grazing-cum-deposit-feeding traces Cochlichnus, Helminthopsis, Phycosiphon, Planolites, Archaeonassa and Psammichnites occurs on decimeter-thick sandstone beds interlayered with mudstones deposited in lower shoreface to inner shelf environments. Trace fossil assemblage-III is known from a 6.9 m thick, outer shelf to lower shoreface, progradational succession showing a vertical transition from dark gray mudstone to fine sandstone. The middle part of this particular section shows Rhizocorallium and flat-type Zoophycos, whereas the upper part hosts profuse Cylindrichnus along with Rosselia and Teichichnus in lesser proportions . This assemblage is comparable to the marine ichnofacies succession combining elements of both Skolithos and Cruziana ichnofacies. The characteristic features of the Talchir trace fossil assemblages are: (i) low ichnodiversity, (ii) presence of forms typically found the in marine environment, (iii) co-occurrences of vertical and horizontal traces of Skolithos and Cruziana ichnofacies, (iv) presence of simple structures formed by trophic generalists, (v) abundance of certain forms, and (vi) presence of monospecific suites. These characters are akin to those of brackish water ichnofaunas. In the Talchir succession, the transition from an ice-contact fan delta system to a relatively ice-free open marine system indicates an enormous flux of fresh water into the sea reducing its overall salinity. Predominance of annelid and worm traces and the rarity of crustacean burrows in the Talchir strata also suggest a stressful environment due to the prevailing cold climate and a lower than normal marine salinity. It thus appears that mixing of fresh water during deglaciation might be a key controlling factor in shaping the characters of ichnofaunas in glacially influenced shallow marine environments.

An inter–ice-stream glaciated margin: Submarine landforms and a geomorphic model based on marine-geophysical data from Svalbard

Well-preserved submarine landforms from the continental shelf and fjords of northwest- ernmost Svalbard provide an example of ice-sheet deposition in an inter-ice-stream setting. EM1002 swath-bathymetric imagery covering 1280 km 2 was examined. At the shelf edge, a distinctive and continuous belt of hum- mocky topography represents the grounding zone of a slow-moving Late Weichselian ice sheet. Active ice on the outer shelf is inferred from subtle lineations orientated parallel to fl ow. Low-amplitude transverse moraines crosscut the lineations, suggesting ice retreat across the outer shelf with brief stillstands. On the middle and inner shelf, large moraine ridges indicate multiple stillstands during de- glaciation. There are arcuate moraine ridges at fjord mouths. Streamlined crag-and-tail landforms are preserved from when ac- tive full-glacial ice fl owed out of the fjords. Clusters of smaller transverse ridges indi- cate slow retreat of grounded ice through the fjords. Holocene sedimentation is by rainout from sediment-rich meltwater, producing smooth basin fi ll. Small slides from the fjord walls are common. Little Ice Age glacier readvance produced another set of terminal moraines and smaller retreat moraines in the innermost fjords. A schematic model of this inter-ice-stream glacial landform assem- blage summarizes the geomorphic record. It is compared with a model derived from sev- eral Svalbard cross-shelf troughs occupied by fast-fl owing Late Weichselian ice streams. In general, the seafl oor morphology of continen- tal margins affected by ice streams is domi- nated by streamlined, subglacially produced landforms oriented in the former ice-fl ow direction, interrupted by major grounding- zone wedges formed during temporary halts in ice retreat. By contrast, between ice streams, shelf and fjord morphology records submarine landforms of various dimensions oriented mainly transverse to ice fl ow, pro- duced at slowly retreating, grounded ice- sheet margins. There is little evidence for channeled subglacial water fl ow in the form of eskers and ice-contact fans on the Sval- bard margin, implying that basal water is drained or advected mainly within soft sub- glacial sediments.

The sedimentary signature of ice-contact sedimentation and deformation at macro- and micro-scale: A case study from NW Scotland

Studies on the genesis of subaerial debris flows and associated deposits are relatively rare in the literature, especially in an ice-marginal context of moraine formation. The present contribution reports results from both the macro- and micro-scales of a subaerial depositional setting in order to contribute to closing this gap. At the macroscale, alternating loose, stratified, clast- and matrix-supported diamicts and finely laminated sand units indicate deposition of debris flows and fluvial units in a subaerial, ice-marginal setting that were stacked up to form a terrestrial ice-contact fan. Macroscale and micromorphological analyses show that this fan displays evidence of a three-phased formation: (a) overriding and glaciotectonisation of pre-existing sediments followed by retreat and burial of this core by (b) ice-contact fan deposition dominated by water-rich fluvial deposition with relatively little debris flow activity and (c) a switch to a gravitational sedimentation style with dominantly debris flow deposition and fewer and thinner fluvial units. Thin sections of both the diamict and laminated sand units show evidence of deposition of a mud and fine sand-rich slurry being expelled from the tops of advancing mass flows. Water-rich fine-grained slurries appear to have been progressively overridden and deformed in response to ductile shear occurring at the base of individual flows. Liquefaction and remobilisation of sand within laminated deposits occurred during such basal shear events, resulting in the injection of liquefied sediments into variably deformed laminated sands and clays. Deformation is more likely to have taken place through internal movement of the sediment due to changing porewater conditions and loading upon emplacement. Our approach confirms previous results that highlight the possibilities of increasing the accuracy of sedimentological investigations through combined sedimentological analyses at varying scales.

Transitional deposits between the end moraine and outwash plain in the Pomeranian glaciomarginal zone of NW Poland: a missing component of ice-contact sedimentary models

The study area near Chelm in Western Pomerania, NW Poland, is located between the end moraine and outwash plain of the glaciomarginal zone of the Pomeranian Phase (Weichselian glaciation) Four assemblages of deposits are exposed in the Chem pits: gravelly diamictons derived from debrisflows, sandy diamictons derived from hyperconcentrated flows, alternating sand and gravel deposited by sheetflows and sandy deposits filling shallow, braided-stream channels. The first facies association predominates in the proximal part of the transition zone and the last one in the distal part. This spectrum of facies associations, some resembling those of the end moraines and others those of the adjacent outwash plain, is considered to be unique, and is attributed to the formation of an array of coalescent ice-contact fans (‘transition’ fans). Architectural element analysis highlights the southwards change from debrisflow processes accompanied by sheetflows to hyperconcentrated flows and further to channelized streamflow. This transition zone in the present case is c. 1–km wide. It is suggested that analogous transitional zones, with similar or different widths, may have been developed in other glaciomarginal belts and remain to be recognized.

Upper Weichselian submarine fluvioglacial deposits of ice-contact fans in the southeastern Barents Sea

Upper Weichselian submarine fluvioglacial deposits of ice-contact fans in the southeastern Barents Sea