Grounding-line Fan Research Articles

Lithofacies and sequence stratigraphic analysis of the glaciomarine lower Wynyard Formation (Pennsylvanian–early Permian, Tasmanian Basin)

Glaciation during the Late Paleozoic Ice Age climatic interval (~370–260 Ma) was likely dynamic; consisting of numerous ice centers that grew and shrank asynchronously through time. Improvements in understanding of the spatial and temporal depositional complexity of glaciomarine sedimentary systems have shown that in order to characterize the conditions of Late Paleozoic glaciation, glaciogenic depositional systems and their stratigraphy must fundamentally be understood at the local level. To that end, this study reexamines the physical sedimentology and sequence stratigraphy of the type-section of the Permo-Carboniferous, glaciomarine Wynyard Formation (Wynyard Tillite) of the Tasmanian Basin by describing a 415 m thick interval of the formation, beginning at its basal erosional unconformity with the Proterozoic Burnie (Oonah) Formation.The glaciogenic nature of the Wynyard Formation stratigraphy is indicated by the characteristics of clasts throughout the succession (striated, faceted, angular, and variable lithologies and grain sizes). Three glacial depositional environments were interpreted: a grounding zone wedge deposited in an ice-contact setting, glacier-proximal portion of a grounding line fan and/or morainal bank, and cyclopelites deposited in a glacier-intermediate to glacier-distal setting. Mass transport and turbidite deposits are common throughout all facies associations, as is soft sediment deformation likely caused by slumping. Several boulder pavements occur throughout the succession, indicating periodic glacier grounding. Together, these facies associations indicate that this Wynyard Fm succession is composed of glaciogenic sediments that were deposited in sub-aqueous, marine, predominantly proglacial environments. The sequence stratigraphic analysis indicates that this entire succession of the Wynyard Fm was likely deposited during the single retreat phase of the “Wynyard Glacier”, with one notable readvance over the area.The interpretations made by this work enhance the understanding of glaciation of the Tasmanian Basin during the LPIA through facies analyses and a sequence stratigraphic approach, which allowed for detailed subdivision of lithologies that enabled inferences regarding the type, timing, and extent of the “Wynyard glacier”. This Wynyard Fm succession was likely deposited during a single glacier retreat with some minor readvances that may have occurred on the order of decades to years. Additionally, the glacier's grounding line was likely never more than a few kilometers upglacier (south) of this location during deposition.These findings are significant, because the massive diamictites that comprise this succession had previously been considered homogenous and therefore resisted detailed interpretations. Constraining the often-complex depositional histories of glaciogenic strata in similar fashions across the Tasmanian Basin allows us to better understand how these glaciogenic deposits fit into the global climate system of the Late Paleozoic Ice Age.

Bedforms and associated facies of an Early Permian grounding-line fan: Exploring flow regime transitions of glacigenic density flows during the Late Paleozoic Ice Age

Grounding-line fans and their associated supercritical-flow bedforms are still poorly recognized in the upper Paleozoic glacial record. In order to fill this gap and to explore the products of the transition between upper and lower flow regimes, we examined a gravel-rich succession composed of conglomerates, sandstones, and diamictites deposited during the Late Paleozoic Ice Age (LPIA) in southeastern Paraná Basin, Brazil. To investigate this succession, we described the main bedforms and associated facies in laterally persistent outcrops and measured several vertical logs that were subsequently merged into a composite log. From base to top, the succession is composed of a muddy glacitectonized substrate, and a subglacial to ice-marginal facies association including a basal, boulder-rich, subglacial lag of local extent, and submarine deposits belonging to a grounding-line fan. In the grounding-line fan association, we identified bedforms related to concentrated flows that represent the complete spectrum of flow regime transition from supercritical to subcritical in a waning wall jet model. The proximal zone was dominated by highly turbulent supercritical flows, resulting in highly scoured conglomerates and deposition of unstable and stable antidunes. A transitional zone under transcritical flow conditions resulted in deposits represented by humpback dunes. In the distal zone, the flow behaved as subcritical, resulting in the deposition of dunes and climbing ripples. Debris flows were also active. These cohesive flows underwent dilution by water entrainment and erosion of the head, leading to the generation of turbulent portions in the head of the debris flows. This bipartite behavior of the flows resulted in thin sandstones capping diamictites, which compose the fabric of the glacigenic debrites observed in the study area. The described facies and bedforms point to the existence of a grounded ice margin at the final stages of the LPIA in the SE portion of the Paraná Basin.

Onset of the late Paleozoic glaciation in the Lhasa terrane, Southern Tibet

The Late Paleozoic Ice Age is characterized by diverse stratigraphic records with extensive spatial-temporal distribution in the Gondwana. The Lhasa terrane located in northeastern Gondwana during the late Paleozoic developed successive glacial sedimentary records, but the depositional evidence, as well as age constraint for the onset of glaciation, is unknown. This paper provides the first full description of the basal glacial deposits in the Xainza area of Lhasa terrane, southern Tibet. Conodonts present in the carbonate overlying the glacial bed (1 m) dominantly consist of Declinognathodus intermedius, indicating that the initiation of the Late Paleozoic Ice Age in the Lhasa terrane took place in Bashkirian. Three facies associations were recognized: (i) tidal flat association (subtidal channel, bar, intertidal mixed flat), (ii) subglacial association (megaclasts and melt-out diamictite), and (iii) lonestone-bearing heterolithic association (grounding-line fan and iceberg). These facies associations are vertically arranged into a complete transgressive-retrogressive glacial succession. Ice movement direction reconstructed from the imbricated megaclasts, the concordant folds and the lateral distribution of subglacial deposits and glacial marine deposits is consistently from NNE to SSW. These characteristics suggest that the Bashkirian glaciation in the Lhasa terrane is irrelevant to western Australia, implying that these two blocks were separated during the Late Carboniferous.

A lithofacies analysis of a South Polar glaciation in the Early Permian: Pagoda Formation, Shackleton Glacier region, Antarctica

ABSTRACTThe currently favored hypothesis for Late Paleozoic Ice Age glaciations is that multiple ice centers were distributed across Gondwana and that these ice centers grew and shank asynchronously. Recent work has suggested that the Transantarctic Basin has glaciogenic deposits and erosional features from two different ice centers, one centered on the Antarctic Craton and another located over Marie Byrd Land. To work towards an understanding of LPIA glaciation that can be tied to global trends, these successions must be understood on a local level before they can be correlated to basinal, regional, or global patterns. This study evaluates the sedimentology, stratigraphy, and flow directions of the glaciogenic, Asselian–Sakmarian (Early Permian) Pagoda Formation from four localities in the Shackleton Glacier region of the Transantarctic Basin to characterize Late Paleozoic Ice Age glaciation in a South Polar, basin-marginal setting. These analyses show that the massive, sandy, clast-poor diamictites of the Pagoda Fm were deposited in a basin-marginal subaqueous setting through a variety of glaciogenic and glacially influenced mechanisms in a depositional environment with depths below normal wave base. Current-transported sands and stratified diamictites that occur at the top of the Pagoda Fm were deposited as part of grounding-line fan systems. Up to at least 100 m of topographic relief on the erosional surface underlying the Pagoda Fm strongly influenced the thickness and transport directions in the Pagoda Fm. Uniform subglacial striae orientations across 100 m of paleotopographic relief suggest that the glacier was significantly thick to “overtop” the paleotopography in the Shackleton Glacier region. This pattern suggests that the glacier was likely not alpine, but rather an ice cap or ice sheet. The greater part of the Pagoda Fm in the Shackleton Glacier region was deposited during a single retreat phase. This retreat phase is represented by a single glacial depositional sequence that is characteristic of a glacier with a temperate or mild subpolar thermal regime and significant meltwater discharge. The position of the glacier margin likely experienced minor fluctuations (readvances) during this retreat. Though the sediment in the Shackleton Glacier region was deposited during a single glacier retreat phase, evidence from this study does not preclude earlier or later glacier advance–retreat cycles preserved elsewhere in the basin. Ice flow directions indicate that the glacier responsible for this sedimentation was likely flowing off of an upland on the side of the Transantarctic Basin closer to the Panthalassan–Gondwanide margin (Marie Byrd Land), which supports the hypothesis that two different ice centers contributed glaciogenic sediments to the Transantarctic Basin. Together, these observations and interpretations provide a detailed local description of Asselian–Sakmarian glaciation in a South Polar setting that can be used to understand larger-scale patterns of regional and global climate change during the Late Paleozoic Ice Age.

Evidence for a grounding line fan at the onset of a basal channel under the ice shelf of Support Force Glacier, Antarctica, revealed by reflection seismics

Abstract. Curvilinear channels on the surface of an ice shelf indicate the presence of large channels at the base. Modelling studies have shown that where these surface expressions intersect the grounding line, they coincide with the likely outflow of subglacial water. An understanding of the initiation and the ice–ocean evolution of the basal channels is required to understand the present behaviour and future dynamics of ice sheets and ice shelves. Here, we present focused active seismic and radar surveys of a basal channel, ∼950 m wide and ∼200 m high, and its upstream continuation beneath Support Force Glacier, which feeds into the Filchner Ice Shelf, West Antarctica. Immediately seaward from the grounding line, below the basal channel, the seismic profiles show an ∼6.75 km long, 3.2 km wide and 200 m thick sedimentary sequence with chaotic to weakly stratified reflections we interpret as a grounding line fan deposited by a subglacial drainage channel directly upstream of the basal channel. Further downstream the seabed has a different character; it consists of harder, stratified consolidated sediments, deposited under different glaciological circumstances, or possibly bedrock. In contrast to the standard perception of a rapid change in ice shelf thickness just downstream of the grounding line, we find a flat topography of the ice shelf base with an almost constant ice thickness gradient along-flow, indicating only little basal melting, but an initial widening of the basal channel, which we ascribe to melting along its flanks. Our findings provide a detailed view of a more complex interaction between the ocean and subglacial hydrology to form basal channels in ice shelves.

Constraining the timing, kinematics and cyclicity of Mississippian-Early Pennsylvanian glaciations in the Paraná Basin, Brazil

Ice-contact deposits emplaced by Late Paleozoic Ice Age (LPIA) glaciers are rarely exposed due to a low degree of preservation and their capping by thick glaciomarine deglaciation sequences. In this paper, we present new data on glacial cyclicity, relative age constraints and paleoice flow interpretations for an ice-contact succession of the lowermost Itararé Group in eastern Paraná Basin (Brazil). The 80 m-thick ice-contact complex rests over the Itararé Group basal unconformity and comprises four stacked informal stratigraphic units regarded as the depositional/deformational record of three ice lobes advance/retreat cycles. Glacial cycle 1 comprises a sheet of massive diamictite resting on striated pavements carved on Devonian sandstones and topped by grooved/fluted surfaces, which indicate subglacial emplacement from a northward flowing ice lobe. The glacial cycle 2 succession was deposited in a proglacial to ice-marginal marine setting and subsequently deformed due to minor ice margin fluctuations of a westward/southwestward flowing grounded ice lobe. Glacial cycle 3 is interpreted as a thick overridden push moraine composed of grounding-line fan deposits capped by a subglacial diamictite emplaced by grounded ice advancing northwestward. A middle Visean-early Serpukhovian palynomorph assemblage was recorded in cycle two deposits, representing the oldest late Paleozoic glacial rocks in the Paraná Basin. The ice-contact complex elucidates multiple waxing and waning phases of the Windhoek Ice Sheet onto the eastern margin of the Paraná Basin during the Mississippian-Early Pennsylvanian, demonstrating that ice sheets of the early stages of the LPIA were also present in the Brazilian-African domain of southwestern Gondwana.

Glacial to postglacial submarine landform assemblages in fiords of northeastern Baffin Island

Fiord morphology plays a fundamental role in glacier flow dynamics and on ice-margin stability. As most of the present-day margins of the Greenland Ice Sheet lays in fiords, there is a need for understanding short- and long-term glacial dynamics in fiord settings. We investigate ice-sheet retreat patterns in previously glaciated submarine terrain of northeastern Baffin Island fiords to provide analogues for modern and future ice-sheet response to climate change and sea-level rise. Geomorphological maps constructed from the interpretation of swath bathymetry imagery in fiords of northeastern Baffin Island reveal a wide range of glacial to postglacial landforms that allow the reconstruction of past ice-sheet retreat dynamics. Ice-flow landforms such as mega-scale glacial lineations, crag-and-tails, and meltwater channels reveal the direction and behaviour of late-Foxe ice flow through the fiords. The presence of undisturbed elongated landforms within the fiords suggests that ice streams have probably been active until the late stage of deglaciation. Landforms transverse to ice-flow direction include grounding-zone wedges, frontal moraines, grounding-line fans, recessional moraines and De Geer moraines. These landforms are interpreted as the result of former standstills of the ice margin during deglaciation. The occurrence of grounding zones in deep (>800 m) part of the fiords contrasts with studies suggesting instability and rapid retreat of outlet glaciers over deep fiord basins. Sediment-filled basins, often characterised by the presence of turbidity channels, gullies and mass movement scars occur in-between the moraines. Sediment-filled basins with a ponded architecture between sills illustrate that most of the sediment accumulation was ice-proximal during deglaciation and characterised by gravity-driven flows. The proposed landform-assemblage model for northeastern Baffin fiords includes landforms typical of different fiord landsystems.

On the origin of the Itararé Group basal nonconformity and its implications for the Late Paleozoic glaciation in the Paraná Basin, Brazil

Outcrop studies of the Itararé Group in the Alfredo Wagner area, along the eastern margin of the Paraná Basin in Brazil, reveal a partially exhumed, NNW-SSE oriented glacial paleovalley. The Carboniferous Rio do Sul Formation represents the basal interval of the glacially influenced Itararé Group, and rests directly on Precambrian basement rocks. This nonconformity is thought to represent an areal scouring surface, as indicated by the series of glacially-carved linear depressions that were incised into the glaciated bedrock during glacial advances. These large-scale erosional landforms differ in their extent by orders of magnitude, and include wide, deep troughs as well as narrower, shallower gouges. Glacial troughs and gouges usually display smaller-scale, subglacial erosion landforms, such as polished and grooved striations on their walls and floors. These indicate the NNW motion of a warm-based, grounded glacier. In addition, large-scale erosional features represent local topographic lows within the major glacial paleovalley. These local lows are large enough to contain part of the succeeding deglacial deposits, which thin toward and onlap the wall of these depressions. We grouped these deglacial deposits into six genetic facies associations (FA-1 to -6) within three fining-upward glacial cycles. FA-1 is composed of black shale, and represents an initial marine flooding event. It can be correlated with the regionally widespread Lontras Shale. FA-2 is composed of sandstones and mudstones associated with flood-derived hyperpycnal flows, and occasionally with surge-type turbidites. FA-3 is represented by plastered or injected, matrix-supported conglomerates emplaced as lodgment tills. FA-4 comprises matrix- and clast-supported conglomerates, sandstones, and mudstones, attributed to ice-rafting and concentrated flows in grounding-line fan facies. This unit also contains ice-keel dragging features, carved into soft substrate. FA-5 is composed of dropstone-bearing black shale, related to a second major marine flooding event within the paleovalley. FA-6 comprises sandstones associated with channelized, surge-type turbidites, forming the second turbidite system in the succession. As these deposits are confined to a paleovalley, we infer that sedimentation was influenced by topographically controlled ice streams, instead of non-confined, widespread ice lobes as previously suggested. Reconstructing South America and southern Africa in their positions during the LPIA suggests that the glacial valleys then present in the Namibia area may have extended further, into the eastern margin of the Paraná Basin. In this scenario, confined outlet glaciers would have flowed out from the Namibian Windhoek ice cap, and terminated along the eastern margin of the Paraná Basin.

Facies and depositional architecture according to a jet efflux model of a late Paleozoic tidewater grounding-line system from the Itararé Group (Paraná Basin), southern Brazil

During the Late Paleozoic, the Gondwana supercontinent was affected by multiple glacial and deglacial episodes known as “The Late Paleozoic Ice Age” (LPIA). In Brazil, the evidence of this episode is recorded mainly by widespread glacial deposits preserved in the Paraná Basin that contain the most extensive record of glaciation (Itararé Group) in Gondwana. The Pennsylvanian to early Permian glaciogenic deposits of the Itararé Group (Paraná Basin) are widely known and cover an extensive area in southern Brazil. In the Doutor Pedrinho area (Santa Catarina state, southern Brazil), three glacial cycles of glacier advance and retreat were described. The focus of this article is to detail the base of the second glacial episodes or Sequence II. The entire sequence records a deglacial system tract that is represented by a proximal glacial grounding-line system covered by marine mudstones and shales associated with a rapid flooding of the proglacial area. This study deals with the ice proximal grounding-line systems herein interpreted according to lab model named plane-wall jet with jump. Detailed facies analysis allowed the identification of several facies ranging from boulder-rich conglomerates to fine-grained sandstones. No fine-grained deposits such as siltstone or shale were recorded. According to this model, the deposits are a product of a supercritical plane-wall outflow jet that changes to a subcritical jet downflow from a hydraulic jump. The hydraulic jump forms an important energy boundary that is indicated by an abrupt change in grain size and cut-and-fill structures that occur at the middle-fan. The sedimentary facies and facies associations show a downflow trend that can be subdivided into three distinct stages of flow development: (1) a zone of flow establishment (ZFE), (2) a zone of transition (ZFT), and (3) an established zone (ZEF). The proximal discharge is characterized by hyperconcentrated-to-concentrated flow due to the high energy and sediment-laden nature of the flows. At the transitional zone, a hydraulic jump produces a rapid shift of conglomeratic to sandy facies with associated scour features. Towards the distal zones, the jet detaches to originate a vertical turbulent jet characterized by more diluted flows. Discussion of fan facies and architecture within a framework of jet-efflux dynamics provides an improved understanding of grounding-line fans systems that produce coarse-grained strata commonly enclosed by fine-grained rocks. The results have clear implication in terms of prediction of facies tract and geometry of oil and gas reservoirs deposited under similar conditions. And also can be useful to identifying the position of a glacial terminus through time.

Ice-stream landform assemblage in Kveithola, western Barents Sea margin

Distinct assemblages of landforms are produced by retreating ice streams (e.g. Stokes & Clark 1999; Wellner et al. 2006). The composition of landforms is governed by ice-stream retreat dynamics, implying that ice-stream dynamics can be reconstructed from these geomorphic patterns. This method has been used to decipher the retreat dynamics of an ice stream that occupied the Kveithola Trough on the western Barents Sea margin, presumably during the Late Weichselian deglaciation (Rebesco et al. 2011; Ruther et al. 2012; Bjarnadottir et al. 2013). Kveithola Trough contains a diverse assemblage of landforms related to the presence and withdrawal of an ice stream, as well as to postglacial processes (Fig. 1a, b), described in detail by Bjarnadottir et al. (2013). Here, the acoustic-stratigraphic units defined by Bjarnadottir et al. (2013) are used, with a focus on seafloor morphology relating to the ice-stream landform assemblage. Fig. 1. Multibeam swath bathymetry from Kveithola Trough, western Barents Sea. ( a ) Swath-bathymetric image showing trough geomorphology. White polygons indicate locations of detail images (c–f). White lines indicate profiles in Figures 2 and 3. VE×8. ( b ) Location of study area (red box; map from IBCAO v. 3.0). ( c ) Oblique view of ice-stream onset zone, showing drumlins and channels. VE×20. ( d ) Oblique image showing multiple generations of grounding-line fans downstream of a GZW. VE×20. ( e ) Oblique view of MSGLs on/beneath GZWs. VE×35. ( f ) Oblique view of diverging MSGLs, shelf edge and gullies on upper slope. VE×16. Images (c–f) are viewed from the black stippled lines. Acquisition system Kongsberg Simrad EM300. Frequency 30 kHz. Grid-cell size 10 m. Kveithola is on the western margin of the Barents Sea, 40 km NW of Bear Island (Fig. 1b). The trough is c. 90–120 km long, 10–15 km wide (slightly wider at the seaward end) and 200–700 …

Geomorphic relations among glacial Lake Algonquin and the Munising and Grand Marais moraines in eastern Upper Michigan, USA

Data obtained from ground-penetrating radar (GPR), optically stimulated luminescence (OSL) dating, and geostatistical reconstructions of Lake Algonquin water planes indicate that high-standing outwash aprons flanking the Munising moraine's southern margin in eastern Upper Michigan terminate as large, coalescing Gilbert-type ice-contact deltas graded to the Main level of Lake Algonquin. Nearly 30km of GPR imagery provides clear evidence of topset and foreset beds diagnostic of deltaic deposition. Landform assemblages suggest that the deltas likely evolved from subaqueous grounding line fans formed along a retreating ice margin characterized by widespread stagnation. In time, these deltas aggraded to the Main Lake Algonquin level as the ice margin stabilized. An OSL date of 12.5±1.1ka on a nearby beach ridge associated with the Main Algonquin shoreline is consistent with this interpretation. These new data, coupled with the presence of nested fans at the downstream end of meltwater channels incised into the delta, indicate that Lake Algonquin was falling as delta deposition waned, eventually reaching a level concordant with the Lower Orillia level before final abandonment. Large, regionally extensive heads of outwash marking the crest of the Munising moraine are graded to the upper margins of these deltas and, by inference, must also correlate with Main Lake Algonquin and the associated Two Rivers deglaciation ca. 13–12.5ka. The deltas, in turn, are conspicuously crosscut by outwash aprons that are graded to levels much lower than any recognized upper group Lake Algonquin shoreline. Because these aforementioned heads of outwash, part of Drexler and others' Grand Marais moraine, correlate with the Marquette moraine farther west, they likely date to the Marquette advance ca. 11.5ka. Thus, landforms related to Two Rivers and to Marquette glaciations appear to comingle across the study area. In the absence of extensive exposures and datable materials, our work highlights the fact that crosscutting relationships and distinct differences in base level can provide an important first step for untangling the complicated landform relationships in this part of Michigan.

The glacial sedimentology and geomorphological evolution of an outwash head/moraine-dammed lake, South Island, New Zealand

Extensive exposures through the glacial landforms around southern Lake Pukaki, New Zealand, comprise seven lithofacies (LFs 1–7). LFs 1–3 are grouped together as LFA 1 (Pukaki Member) and record pulsed subaqueous grounding line fan progradation, cohesionless debris flows, underflow activity and rhythmite deposition by suspension settling, iceberg rafting of dropstones, and pulsed traction current activity. Localized disturbance of these deposits by glacitectonic deformation and multi-generational hydrofracture fills records minor readvances by the glacier snout and the emplacement of a glacitectonite (LF 4) derived from cannibalization of glacilacustrine sediments. LFs 4–6 are grouped together as LFA 2 (Twizel Member) and record direct glacigenic deposition of glacitectonite (LF 4), subglacial traction till (LF 5) and supraglacially dumped boulder rubble (LF 6). Stratigraphic relationships between LFA 1 with LFA 2 record the oscillatory behaviour of the former Tasman Glacier snout when it formed a calving margin in a proglacial and locally supraglacial lake dammed by a glacitectonically disturbed outwash head and lateral moraine ridges. This is entirely consistent with the landform–sediment record of its coeval terrestrial margins, where flutings and push moraines are diagnostic of active temperate glacier recession from a glacially overridden outwash head, the latter being recorded by the vertically stacked sub-horizontally bedded and coarse-grained gravels of LF 7 (Waitaki Member). Previous proposals that late Pleistocene lake damming was initiated by an ice-cored moraine arc appear unfounded, because the glacilacustrine deposits only lie above the altitude of the outwash head/lateral moraine arc in locations where they have been glacitectonically compressed. Alternatively, it is proposed that the overdeepened subglacial topography was produced by the construction of an outwash head, leading to a glacilacustrine sediment sink which operates at times when the expanded Tasman Glacier actively retreats from the outwash head apex. The changing landsystem imprint related to the shrinkage of the Tasman Glacier records spatio-temporal landsystem change, involving evolution from a coupled landsystem to a moraine-dammed to an uncoupled landsystem.

Processes and products of grounding-line fans from the Permian Pagoda Formation, Antarctica: Insight into glacigenic conditions in polar Gondwana

Glacimarine grounding-line fans are subaqueous depositional features that form due to the expulsion of sediment-laden subglacial waters at the grounding-lines of temperate tidewater glaciers. Few studies of ancient grounding-line fans exist. This study describes and interprets several ancient glacimarine grounding-line fans and their physical characteristics using the Lower Permian Pagoda Formation along Tillite Glacier in the central Transantarctic Mountains, Antarctica as a deep-time example. Wedge-shaped sandstone bodies are used to identify major depositional processes and products that occurred during fan formation. The depositing processes on these fans operated in tandem; as tractive freshwater discharge from beneath the glacier(s) was progressively replaced down the fans, in the direction of flow, by buoyant upwelling and the development of overflow plumes. Tractive flow deposits are characterized by trough and climbing cross-stratification, while deposits resulting from settling from meltwater plumes consist of horizontal laminations and abundant soft-sediment deformational structures (load, flame, and diapiric structures). Secondary features, such as thrust and normal faults, resulted from advance and retreat dynamics of the ice front; whereas, mass flow deposits (convolute bedding, recumbent and S-shaped folds, slumps and slides) resulted from resedimentation. Because glacimarine grounding-line fans require specific depositional conditions to develop, their identification is useful in constraining environmental conditions in the rock record. For the Pagoda Formation, the occurrence of grounding-line fan deposits is significant, as these strata were originally interpreted as deposits of a large terrestrial ice sheet centered over the location of the present Transantarctic Mountains that extended outward and covered much of Gondwana for up to 90millionyears. However, this study demonstrates that these rocks were deposited in a glacimarine setting. Results, combined with other recent studies, suggest that multiple small temperate ice sheets occurred in South Polar Antarctica during the late Paleozoic ice age rather than a single massive ice sheet.

Grounding‐line dynamics during the last deglaciation of Kveithola, W Barents Sea, as revealed by seabed geomorphology and shallow seismic stratigraphy

A marine geophysical study reveals a complex deglaciation pattern in the Kveithola trough, W Barents Sea. The data set includes multibeam swath bathymetry and sub‐bottom sediment profiler (chirp) data acquired for the whole extent of a palaeo, marine‐terminating ice stream, along with high‐resolution single‐channel seismic data from chosen profiles. The multibeam data show a geomorphic landform assemblage characteristic of ice streams. The results of a combination of seismic and chirp unit stratigraphy reveal that the seabed geomorphology is governed by a deeper‐lying reflector. The reflector dominates surface expressions of several subglacial and ice‐marginal units, each connected to a separate episode of ice‐margin stillstand/advance. Analysis of the combined data set has resulted in a conceptual model of the ice‐stream retreat. The model depicts complex deglaciation of a small, confined ice‐stream system through episodic retreat. It describes the formation of several generations of grounding‐zone systems, characterized by high meltwater discharges and the deposition of fine‐grained grounding‐line fans. The inferred style of grounding‐zone deposition in Kveithola deviates from that of other accounts, and is suggested to be intermediate in the previously described continuum between morainal banks and grounding‐line wedges. The results of this paper have implications for grounding‐zone theory and should be of interest to modellers of grounding‐line dynamics and ice‐stream retreat.

Proglacial deposition and deformation in the Upper Carboniferous to Lower Permian Wynyard Formation, Tasmania: A process analysis

The Wynyard Formation of Tasmania, Australia, provides the youngest evidence of grounded ice during the Late Carboniferous–Early Permian in the Tasmania Basin of southeastern Gondwana during the late Paleozoic ice age. Within the Wynyard Formation, four facies associations are recognized: 1) massive diamictite, 2) stratified diamictite, 3) conglomerate and sandstone, and 4) deformed mudstone and fine sandstone. A detailed facies analysis was performed in order to interpret the depositional processes and environments. The massive diamictite facies association contains massive diamictite and sandstone and was deposited primarily by iceberg rain-out in a subaqueous morainal bank setting. The stratified diamictite facies association is composed of stratified diamictite, sandstone, and conglomerate, and was deposited by debris flows and iceberg rain-out, and deformed by glacial pushing. The conglomerate and sandstone facies association is made up of cross-stratified, channelized conglomerate and pebbly sandstone that were deposited by glacial outwash on grounding-line fans. The deformed mudstone and fine sandstone facies association is composed of pervasively deformed pebbly mudstone and fine-grained sandstone that exhibit volcano, flame, and dyke structures. This facies association was deposited by a combination of suspension settling, iceberg rain-out, and soft sediment deformation, in a quiet distal proglacial setting. The facies analysis supports the interpretation that the Wynyard Formation was deposited by a wet-based, tidewater glacier or glaciers. The depositing glacier(s) occupied a ~ 40 km-wide trough in northwest Tasmania. Based on the distribution of the Wynyard Formation and the presence of glacial deposits in other eroded troughs in Tasmania, glaciation occurred in the region within broadly eroded valleys. The Wynyard Formation and overlying mudstone of the Inglis Formation provide a glacial–postglacial signature similar to those in South Africa, Antarctica, and eastern Australia during the latest Carboniferous–Early Permian, therefore the climate warmed in southern Gondwana following this glacial interval. The continued glaciation in eastern Australia was a result of regional topography and oceanographic processes.

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

AbstractGlacigenic 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.

Glacimarine sedimentation processes at Kronebreen and Kongsvegen, Svalbard

AbstractTidewater glaciers deposit sediment at their terminus, thereby reducing the relative water depth. Reduced water depth can lead to increased glacier stability through decreased rates of iceberg calving, glacier thinning and submarine melting. Here we investigate sedimentation processes at the termini of Kronebreen and Kongsvegen, Svalbard. We mapped the fjord floor bathymetry in August 2009 and calculate sedimentation rates based on our bathymetry and that from a similar study in 2005. A grounding-line fan is developing near the current position of the subglacial stream. An older, abandoned grounding-line fan that likely formed between ∼1987 and 2001 is degrading near the middle of the ice front. Our findings indicate that sediment gravity flows reduce the height of the sediment mound forming at the glacier terminus. The future impact of glacimarine sedimentation processes on glacier stability will depend on the net balance between the observed gravity flows and sediment deposition.

Stratigraphic signature of the late Palaeozoic Ice Age in the Parmeener Supergroup of Tasmania, SE Australia, and inter-regional comparisons

Recent research in eastern Australia has established that rather than being a single, long-lived epoch, the late Palaeozoic Ice Age comprised a series of glacial intervals each 1–8 million years in duration, separated by non-glacial intervals of comparable duration. In order to test whether the glacial events recognized in New South Wales and Queensland have broader extent, we conducted a reappraisal of the Parmeener Supergroup of Tasmania, southeast Australia. A facies analysis of the Pennsylvanian to Permian section was carried out, allowing rationalization of the succession into four recurrent facies associations: a) glacigenic facies association, restricted to the basal Pennsylvanian/earliest Permian Wynyard Formation and correlatives, b) glacially/cold climate-influenced to open marine shelf facies association, which accounts for large parts of the Permian succession, c) deltaic facies association, which specifically describes the Lower Permian “Lower Freshwater Sequence” interval, and d) fluvial to estuarine facies association, which specifically addresses the Upper Permian Cygnet Coal Measures and correlatives. Indicators of sediment accumulation under glacial influence and cold climate are restricted to four discrete stratigraphic intervals, all of which indicate that glaciation was temperate in nature. The lowermost of these, incorporating the basal Wynyard Formation and its correlatives, and overlying Woody Island Formation, shows evidence of proximal glacial influence (subglacial, grounding-line fan and ?fjordal facies), and is likely a composite of one or more Pennsylvanian glacial event(s) and an earliest Permian (Asselian) glacial. The second, of late Sakmarian to early Artinskian age, comprises an initial more proximal ice-influenced section and an overlying more distal ice-influenced interval. The third (Kungurian to Roadian) and fourth (Capitanian) intervals are both distal glacimarine records. The four intervals are of comparable age to glacials P1–P4, respectively, recognized in New South Wales and Queensland (notwithstanding apparent discrepancies of < 2 million years in age), and display similar facies characteristics and vertical contrasts to those intervals. Accordingly, it is concluded that the late Palaeozoic stratigraphy of Tasmania preserves a glacial/cold climate record correlatable to that of mainland eastern Australia, lending support to the hypothesis that these events were widespread across this portion of Gondwana.

Modern glacimarine processes and potential future behaviour of Kronebreen and Kongsvegen polythermal tidewater glaciers, Kongsfjorden, Svalbard

Abstract Glacimarine dynamics and associated sedimentary processes are closely tied to glacial regime and reflect dominant climatic conditions. Quantitative measurements for subpolar glaciers, such as sediment yield, are limited especially near glacial termini where most sediment accumulates. Here we characterize the modern glacimarine environment, quantify sediment flux and yield, document landform genesis and hypothesize potential future behaviour of Kronebreen and Kongsvegen glaciers in inner Kongsfjorden, Svalbard. A minimum of 6.74×10 3 g m −2 d −1 (at least 300 mm a −1 ) of glacimarine sediment is building a grounding-line fan via submarine stream discharge from Kronebreen. Average daily sediment flux to the ice-contact basin is recorded to be 2.6×10 3 g m −2 d −1 or an average annual flux of 1.56×10 5 g m −2 a −1 . We measure an average annual ice-contact sediment yield of 1.20×10 4 tonnes km −2 a −1 associated with the rapid genesis of grounding-line landforms. With forecasted warming we expect meltwater volumes and sediment flux to increase. Grounding-line deposits may aggrade above water, tending to stabilize the terminus at least initially if the sediment is sufficient to counteract total terminus ablation. This would hold until either the glaciers next surge or climatic warming ablates the glaciers through surface melting.

Ice-marginal sedimentation associated with the Late Devensian Welsh Ice Cap and the Irish Sea Ice Stream: Tonfanau, West Wales

This paper provides the first comprehensive description and interpretation of Late Devensian glacigenic deposits exposed in a 2 km-long cliff section at Tonfanau, Mid-West Wales. The section offers crucial insight concerning glacier dynamics and sedimentation on the eastern margin of the Irish Sea Basin. Six lithofacies are identified: diamicton, sandy gravel, well-rounded gravel, well-sorted sand, muddy sand, and laminites. Two diamicton horizons are present: an upper brown clast-rich sandy diamicton, and a lower grey clast-poor muddy diamicton, the latter being only sporadically exposed by wave action. Sand and gravel occur in close association, as well as minor mud and gravel successions. The sedimentary succession at Tonfanau records the dynamics and deposition of both Welsh and Irish Sea Ice. The lowermost part of the exposed section records Welsh ice extending into Cardigan Bay, depositing a diamicton originating from the adjacent hinterland to the West. As the Irish Sea Ice Stream flowed south, it impinged upon the present coastline at Tonfanau, depositing sediment with a northern provenance on top of predominantly Welsh material, as indicated by clasts from the Irish Sea Basin, Anglesey and the Llŷn Peninsula. The depositional environment for the Irish Sea materials is interpreted as one where a fluctuating ice margin generated a grounding-line fan complex in an ice-contact lake that merged laterally with lake sediments derived from suspension and turbidity currents. Succeeding facies indicate over-riding by Irish Sea Ice. Finally, the upper few metres of the succession was subjected to cryoturbation under a periglacial climate.