Basal Debris Research Articles

Syn- and post-eruptive erosion, gully formation, and morphological evolution of a tephra ring in tropical climate erupted in 1913 in West Ambrym, Vanuatu

Syn- and post-eruptive erosion of volcanic cones plays an important role in mass redistribution of tephra over short periods. Descriptions of the early stages of erosion of tephra from monogenetic volcanic cones are rare, particularly those with a well-constrained timing of events. In spite of this lack of data, cone morphologies and erosion features are commonly used for long-term erosion-rate calculations and relative age determinations in volcanic fields. This paper offers new observations which suggest differing constraints on the timing of erosion of a tephra ring may be operating than those conventionally cited. In 1913 a tephra ring was formed as part of an eruption in west Ambrym Island, Vanuatu and is now exposed along a continuous 2.5 km long coastal section. The ring surrounds an oval shaped depression filled by water. It is composed of a succession of a phreatomagmatic fall and base-surge beds, interbedded with thin scoriaceous lapilli units. Toward the outer edges of the ring, base-surge beds are gradually replaced in the succession by fine ash-dominated debris flows and hyperconcentrated flow deposits. The inter-fingering of phreatomagmatic deposits with syn-volcanic reworked volcaniclastic sediments indicates that an ongoing remobilisation of freshly deposited tephra was already occurring during the eruption. Gullies cut into the un-weathered tephra are up to 4 m deep and commonly have c. 1 m of debris flow deposit fill in their bases. There is no indication of weathering, vegetation fragments or soil development between the gully bases and the basal debris flow fills. Gully walls are steep and superficial fans of collapsed sediment are common. Most gullies are heavily vegetated although some active (ephemeral) channels occur. These observations suggest that the majority of the erosion of such tephra rings in tropical climates takes place directly during eruption and possibly for only a period of days to weeks afterward. After establishment of the gully network, tephra remobilisation is concentrated only within them. Therefore the shape of the erosion-modified volcanic landform is predominantly developed shortly after the eruption ceases. This observation indicates that gully erosion morphology may not necessarily relate to age of such a landform. Different intensities of erosion during eruption (related to water supply or rainfall) are probably the major influence on gully spacing, modal depth and form. Longer-term post-eruption processes that could be indicators of relative age may include internal gully deepening (below basal debris flow fill sediments) and possibly widening and side-slope lowering due to undercutting and side-collapse.

Reply to discussion by P.J. Barnett on "Generation of heavy-mineral glacial indicator dispersal trains from a diabase sill, Nipigon Region, Northwestern Ontario"

Larson and Mooers 1753 Barnett (2006) poses a number of questions regarding some of the field observations and assumptions we used in our interpretation of a heavy-mineral dispersal train derived from an isolated eastern outlier of diabase of the Nipigon sills in northwestern Ontario (Larson and Mooers 2005b). He also brings attention to some details of the regional glacial geologic setting we did not address in our analysis and interpretation of the dispersal train, and highlights new field data (Barnett and Dyer 2005) relevant to interpretation of past glacial processes acting in the broader Nipigon region. We thank Barnett for his comments, and feel they serve in part to highlight the variety of problems regarding glacial erosion and entrainment and transport to which we believe our conceptual model of glacial indicator dispersal (Larson and Mooers 2004) is able to contribute. Barnett (2006) brings attention to the fact that tills exposed at surface in the Beardmore–Geraldton region contain a wide range (< 1 up to 23%) in carbonate content (Thorleifson and Kristjansson 1993). High carbonate contents in these tills are a clear signal of long-distance transport of carbonate-bearing debris from the Hudson Bay Lowlands. The occurrence of high-carbonate tills in patches of thick, streamlined drift has been interpreted by Hicock (1988) and Hicock and others (1989) to mean that all surface tills in this region were deposited at the base of a fastflowing ice stream. However, the ice stream model does not explain the widely disparate values in till carbonate occurring over distances of only a few kilometres; we believe a different interpretation explains the same data (Larson and Mooers 2005a). Tills sampled in our limited study area contained virtually none of the Paleozoic carbonate and Proterozoic greywacke clasts characteristic of high-carbonate tills found in the region. We believe this observation is inconsistent with an origin of the till-forming material by longdistance transport of debris from the Hudson Bay Lowlands by an ice stream, and rather that it reflects local erosion and entrainment and short-distance transport. However, the origin of the tills we sampled in our study area is not relevant to the interpretations we presented. Determining whether the till in our study area was generated by erosion and entrainment into the englacial basal debris layer of the ice sheet, by erosion and entrainment in a subglacial deforming debris layer, or by another process was beyond the scope of our study. We deliberately made no explicit assertion regarding the processes by which diabase was eroded and entrained, and transported. We did, however, attempt to document the rates at which the erosion and entrainment process worked. The only explicit assumptions we incorporated into our interpretations were of the debris mass per unit bed area (m) and of the velocity at which the basal debris layer was transported (u). These values were used to estimate the erosivity (E) recorded in the till sheet, and indirectly the absolute erosion and entrainment rate (e) of diabase into the basal debris layer. Examination of the equations presented in Larson and Mooers (2004), and the modified equations presented in Larson and Mooers (2005b), shows that increasing the estimated m results in increased estimates of the values of E and e. Increasing u likewise results in increased estimates of e. However, estimates of the erosion length scale (λ) and the relative rates of erosion of the diabase and greenstone are unaffected by varying these quantities. We feel that the m and u estimates that we incorporated into our interpretation of the heavy-mineral dispersal data represent reasonable values for an ice sheet, and that the resulting e estimates are likewise reasonable. It is important to note that the absolute values of erosion and entrainment we have estimated for diabase and greenstone lithologies lie within the range documented for modern glacial systems (cf. Hallet et al. 1996). Barnett (2006) questions our conclusion that the diabase sill in our study area was significantly more resistant to glacial erosion and entrainment than the Archean greenstones into which it was intruded. Notwithstanding the higher susceptibility of gabbro–diabase to physical and chemical

Methyl aminolaevulinate photodynamic therapy in practice: Treatment protocol

For solar keratoses and superficial basal cell carcinoma, loose crusts and debris are removed with a small curette or blade, and the lesion surface is roughened by gentle scraping. Damage to the surrounding normal skin should be avoided. For nodular BCC, the epidermal keratin layer (which can impede absorption) is removed with a scalpel blade or curette, and the exposed tumour material gently removed. Local anaesthesia is rarely used, but if required it should be used without adrenaline. Methyl aminolaevulinate (MAL) PDT treatment relies on the production of reactive oxygen species; adrenaline will cause vasoconstriction, leading to less tissue oxygenation. If bleeding occurs during lesion preparation, light pressure should be applied. Treatment can be continued once bleeding stops.

Depositional facies, environments and sequence stratigraphic interpretation of the Middle Triassic–Lower Cretaceous (pre-Late Albian) succession in Arif El-Naga anticline, northeast Sinai, Egypt

The Middle Triassic–Lower Cretaceous (pre-Late Albian) succession of Arif El-Naga anticline comprises various distinctive facies and environments that are connected with eustatic relative sea-level changes, local/regional tectonism, variable sediment influx and base-level changes. It displays six unconformity-bounded depositional sequences. The Triassic deposits are divided into a lower clastic facies (early Middle Triassic sequence) and an upper carbonate unit (late Middle- and latest Middle/early Late Triassic sequences). The early Middle Triassic sequence consists of sandstone with shale/mudstone interbeds that formed under variable regimes, ranging from braided fluvial, lower shoreface to beach foreshore. The marine part of this sequence marks retrogradational and progradational parasequences of transgressive- and highstand systems tract deposits respectively. Deposition has taken place under warm semi-arid climate and a steady supply of clastics. The late Middle- and latest Middle/early Late Triassic sequences are carbonate facies developed on an extensive shallow marine shelf under dry-warm climate. The late Middle Triassic sequence includes retrogradational shallow subtidal oyster rudstone and progradational lower intertidal lime-mudstone parasequences that define the transgressive- and highstand systems tracts respectively. It terminates with upper intertidal oncolitic packstone with bored upper surface. The next latest Middle/early Late Triassic sequence is marked by lime-mudstone, packstone/grainstone and algal stromatolitic bindstone with minor shale/mudstone. These lower intertidal/shallow subtidal deposits of a transgressive-systems tract are followed upward by progradational highstand lower intertidal lime-mudstone deposits. The overlying Jurassic deposits encompass two different sequences. The Lower Jurassic sequence is made up of intercalating lower intertidal lime-mudstone and wave-dominated beach foreshore sandstone which formed during a short period of rising sea-level with a relative increase in clastic supply. The Middle-Upper Jurassic sequence is represented by cycles of cross-bedded sandstone topped with thin mudstone that accumulated by northerly flowing braided-streams accompanying regional uplift of the Arabo–Nubian shield. It is succeeded by another regressive fluvial sequence of Early Cretaceous age due to a major eustatic sea-level fall. The Lower Cretaceous sequence is dominated by sandy braided-river deposits with minor overbank fines and basal debris flow conglomerate.

Modification of sediment characteristics during glacial transport in high‐alpine catchments: Mount Cook area, New Zealand

The Mount Cook area in the Southern Alps of New Zealand is heavily glacierized with numerous peaks over 3000 m a.s.l. feeding several large valley glaciers. The region is subject to rapid tectonic uplift and heavy precipitation (up to 15 m per year). This paper describes the clast roundness, clast shape and textural characteristics associated with five glaciers (Fox, Franz Josef, Hooker, Mueller and Tasman) in terms of inputs to the glacier system, transport by the glaciers and reworking following glacial deposition. Inputs include rockfall, alluvial fan and avalanche material delivered to the surface of valley glaciers. Basal debris, where observed at the terminus of two glaciers, consists mainly of incorporated fluvial material. Following deposition, reworking is mainly by subglacial and proglacial streams. The dominant facies are (i) boulder gravel with mainly angular clasts on the steep slopes above the glaciers, (ii) sandy boulder gravel, with mainly angular and subangular clasts, forming lateral and end moraines, and (iii) sandy boulder/cobble gravel with mainly subrounded clasts, and sand, which represent glacially transported sediment reworked by braided rivers. Diamicton is rare in the contemporary glacial environment. Since most sediment associated with glaciers in the Southern Alps lacks unambiguous indications of glacial transport, interpretation of similar sediments in the geological record should not necessarily exclude the involvement of glacial processes.

The origin of massive diamicton in Marian and Potter coves, King George Island, West Antarctica

Marine sediment cores were obtained from in front of the tidewater glaciers in Marian and Potter coves in the South Shetland Islands in the austral summer of 1998–1999. Sedimentological and geochemical data from these cores document an advance of ice tongue for the deposition of clast-supported, massive diamicton, interpreted as having been produced by ice rafting in front of glacier margin and/or releasing of clasts from basal debris zones in the sub-ice tongue setting. A C-14 chronology for a core indicates that glacial advance took place ca. 1450–1700 yrs B.P., coincident with warm, humid phase in the study area. During this period, the glacier margin was likely to advance and release diamicton clasts, inferred from a reduction in the total organic carbon content, and an increase in sand and clasts within the diamicton facies. The glacial advance probably caused enhanced ice-edge blooms near the core sites, resulting in increased abundance of sea-ice related diatoms i.e.,Fragilariopsis curta andFragilariopsis cylindrus in the diamicton. The warm and humid conditions between 1450–1700 yrs B.P. might allow the intrusion of warm circumpolar deep water within the fjords, bringing about increased abundance of warm water form, i.e.,Fragilariposis kerguelensis. On the other hands, this warming condition probably prohibited the intrusion of Weddell Ice shelf water from the fjord, as evidenced by lack of cold water form,Thalassiosira antarctica, in the diamicton. Clearly, the response of the outlet glacier system along the periphery of the South Shetland Islands Ice Sheet during the late Holocene warm, humid period (1450–1700 yrs B.P.) was expansion. Thus the process of clast-supported massive diamicton formation is likely to be applicable to a number of areas of the modern and Quaternary Antarctic Peninsula.

Holocene climate and fjord glaciations in Northeast Greenland: implications for IRD deposition in the North Atlantic

The present distribution in Greenland of iceberg-producing outlet glaciers in the south and floating glaciers with bottom melting in the north is most likely controlled by the large contrast in climate. The Holocene glacial geological record in East Greenland shows that the boundary between the two types of outlet glaciers has moved up and down the coast in response to climate change. The difference in outlet glacier extent and dominant ablation mechanism is determinate for where ice rafted debris (IRD) contained in the bottom layers of the glaciers will be released by bottom melting. The icebergs (ice islands), presently detached from the northern floating glaciers are during long periods retained by semi-permanent sea ice, and will only occasionally escape to the sea. When eventually released, the ice islands contain little—if any—debris, as the debris contained in the bottom layer has already been released by intensive sub-glacial melting during the long stay of the ice-islands in coastal waters. The Holocene glacial geological record from Northeast Greenland is compared to the record of ice rafted debris (IRD) from North Atlantic deep-sea sediment cores. The comparison shows that transport by icebergs in the form of basal debris is unlikely to be the dominant transport mechanism of IRD to deposition sites in the North Atlantic during the Holocene. The ice rafted debris is more likely to be carried at the surface of sea- (or glacier) ice. This supports the result of previous studies by other workers that changes of atmospheric and ocean-surface circulation and temperature are the likely causes of Holocene cycles in IRD concentration in North Atlantic deep-sea sediments.

Effects of Basal Debris on Glacier Flow

Glacier movement is resisted partially by debris, either within glaciers or under glaciers in water-saturated layers. In experiments beneath a thick, sliding glacier, ice containing 2 to 11% debris exerted shear traction of 60 to 200 kilopascals on a smooth rock bed, comparable to the total shear traction beneath glaciers and contrary to the usual assumption that debris-bed friction is negligible. Imposed pore-water pressure that was 60 to 100% of the normal stress in a subglacial debris layer reduced shear traction on the debris sufficiently to halt its deformation and cause slip of ice over the debris. Slip resistance was thus less than debris shearing resistance.

Reconstructions of western Ross Sea palaeo‐ice‐stream grounding zones from high‐resolution acoustic stratigraphy

This study analyses acoustic profiles of two mid‐shelf troughs, the JOIDES Basin and Pennell Trough in the western Ross Sea, Antarctica. These troughs are subglacial erosion features formed by repeated advance of streaming ice onto the Ross Sea continental shelf. Sediment wedge formations, interpreted to have been deposited at the most seaward grounding zone during the Last Glacial Maximum (LGM), are observed within the mid‐shelf region of these troughs. By correlating high‐resolution acoustic profiles with core samples, we resolve the stratigraphy of these wedge formations to assess the relationships between bathymetry, subglacial sediment distribution and palaeo‐ice‐stream configuration. The grounding zone wedge geometries were controlled by the pre‐existing topography. The JOIDES and Pennell Troughs were only partially infilled during the LGM. Axial diamict progradation from the landward margin of the troughs indicates enhanced flow (debris supply) along the axis of ice flow. Differences in grounding zone geometries indicate a decrease in basal debris deposition and/or supply during ice recession. The lack of a recessional ice‐shelf facies indicates that either the ice shelf was absent during retreat, or that there was no melt‐out of basal debris.

A glacigenic interpretation of a neoarchaean (≈2.78 Ga) volcanogenic sedimentary sequence in the Nnywane formation, Sikwane, Southeast Botswana

Neoarchaean volcano-sedimentary rocks of the Nnywane formation exposed at Sikwane, Botswana, exhibit palaeo-depositional features that allow the sequence to be interpreted with a glacigenic and volcanic-influenced depositional model. The sedimentary deposits are divided into two facies, a lower Grey facies of tuffitic sandstones, associated with diamictite and conglomerate units and shales, and an upper Red sandstone facies. A volcanic member is inter-layered with shales in the Grey facies, representing penecontemporaneous volcanism. Basal diamictite in the Grey facies contains striated and faceted clasts that reflect an origin as basal debris and which are typical of subglacial facies. Evidence supporting glaciation includes diamictite masses and isolated stones contained within shales, which are interpreted to indicate ice-rafting processes. The Grey facies is associated with a locally preserved smooth-textured tuffitic mudstone that is interpreted to represent glacial rockflour. Ice was probably formed in elevated areas during extensional tectonics associated with the major magmatic event of c.2.78 Ga in Southeastern Botswana, which is linked to the Limpopo Orogeny. Episodic sedimentation is evident in the Grey facies perhaps indicating the influence of glacial melt-waters and intermittent sedimentation associated with volcanic activities. The depositional environment consisted of a volcanic-determined and glacially influenced lake system.

Geochemical weathering at the bed of Haut Glacier d'Arolla, Switzerland—a new model

AbstractWaters were sampled from 17 boreholes at Haut Glacier d'Arolla during the 1993 and 1994 ablation seasons. Three types of concentrated subglacial water were identified, based on the relative proportions of Ca2+, HCO3− and SO42− to Si. Type A waters are the most solute rich and have the lowest relative proportion of Si. They are believed to form in hydrologically inefficient areas of a distributed drainage system. Most solute is obtained from coupled sulphide oxidation and carbonate dissolution (SO–CD). It is possible that there is a subglacial source of O2, perhaps from gas bubbles released during regelation, because the high SO42− levels found (up to 1200 µeq/L) are greater than could be achieved if sulphides are oxidized by oxygen in saturated water at 0 °C (c.414 µeq/L). A more likely alternative is that sulphide is oxidized by Fe3+ in anoxic environments. If this is the case, exchange reactions involving FeIII and FeII from silicates are possible. These have the potential to generate relatively high concentrations of HCO3− with respect to SO42−. Formation of secondary weathering products, such as clays, may explain the low Si concentrations of Type A waters. Type B waters were the most frequently sampled subglacial water. They are believed to be representative of waters flowing in more efficient parts of a distributed drainage system. Residence time and reaction kinetics help determine the solute composition of these waters. The initial water–rock reactions are carbonate and silicate hydrolysis, and there is exchange of divalent cations from solution for monovalent cations held on surface exchange sites. Hydrolysis is followed by SO–CD. The SO42− concentrations usually are &lt;414 µeq/L, although some range up to 580 µeq/L, which suggests that elements of the distributed drainage system may become anoxic. Type C waters were the most dilute, yet they were very turbid. Their chemical composition is characterized by low SO42− : HCO3− ratios and high pH. Type C waters were usually artefacts of the borehole chemical weathering environment. True Type C waters are believed to flow through sulphide‐poor basal debris, particularly in the channel marginal zone. The composition of bulk runoff was most similar to diluted Type B waters at high discharge, and was similar to a mixture of Type B and C waters at lower discharge. These observations suggest that some supraglacial meltwaters input to the bed are stored temporarily in the channel marginal zone during rising discharge and are released during declining flow.Little of the subglacial chemical weathering we infer is associated with the sequestration of atmospheric CO2. The progression of reactions is from carbonate and silicate hydrolysis, through sulphide oxidation by first oxygen and then FeIII, which drives further carbonate and silicate weathering. A crude estimate of the ratio of carbonate to silicate weathering following hydrolysis is 4 : 1. We speculate that microbial oxidation of organic carbon also may occur. Both sulphide oxidation and microbial oxidation of organic carbon are likely to drive the bed towards suboxic conditions. Hence, we believe that subglacial chemical weathering does not sequester significant quantities of atmospheric CO2 and that one of the key controls on the rate and magnitude of solute acquisition is microbial activity, which catalyses the reduction of FeIII and the oxidation of FeS2. Copyright © 2002 John Wiley &amp; Sons, Ltd.

ABSTRACT: Deepwater Channelized Lobe Complexes : Architecture and Resevoir Potential. Some Insights From One Shallow 3D Seismic Example

Deepwater lobe elements are genetically related to channelized elements. They range the span from simple to complex morphologies and architectures. Simple lobes occur within shallow 3D seismic sections as 1 or 2 high amplitude reflections with a lenticular geometry. Depending on their relative position to parent-channel, they can be defined as crevassesplay, avulsion or terminal lobes. Simple lobe types form excellent reservoirs in both continuity and connectivity with generally good reservoir properties. Unfortunately with limited thickness and spatial extension, they barely pass economic thresholds in deepwater field developments unless associated to channelized reservoirs. Given high sediment influx, simple lobes may evolve to form compound lobes : channelized lobe complexes. These compound lobes are larger in size and more heterogeneous in both lithology and architecture. They consist of an association of different Architectural Elements (A.E.) : (1) basal debris flow, (2) extensive sand sheets dissected and/or overlain by (3) distributary channels with or without (4) levees, the whole capped by (5) hemipelagites. The distributary channels radiate from one single point (avulsion, crevasse or channel-mouth). They are narrow in proximal areas, may develop relatively thick levees and are mostly filled with relatively coarse-grained material. Downdip, distributary channels are wider with lower levees, if present, and are barely visible on seismic sections. Combination of various type of seismic attributes, mainly amplitude and coherency, mapping applied to one shallow example allow the identification and characterization of the A.E. and a better understanding of the architecture of these compound lobes.

Errata: Neogene fjordal sedimentation on the western margin of the Lambert Graben, east Antarctica

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

Coupling between hillslopes and channels in upland fluvial systems: implications for landscape sensitivity, illustrated from the Howgill Fells, northwest England

The sensitivity of upland fluvial systems depends on the magnitude and frequency of sediment and flood producing events, modified by the internal coupling characteristics of the system. This paper assesses the role of hillslope/channel coupling for the sensitivity of upland geomorphic systems, using evidence from a 30-year monitoring programme of geomorphic change in the Carlingill valley, Howgill Fells, northwest England. In the hillslope zone, there is little sediment supply to the stream system. Locally, slope failures occur in response to extreme events (six such events in 30 years within the ca. 6 km 2 Carlingill valley). In the footslope coupling zone, basally induced gullies are major sediment sources to the stream. Sediment-production events occur ca. 30 times per year, feeding sediment to basal debris cones. Stream floods which can entrain these sediments occur once every ca. 2–5 years. Stream channel morphology is adjusted to this regime. Downstream of gullies, channels are wide and braided; elsewhere they are narrow and single-thread. As the gullies develop, however, this coupling weakens and the eroding slopes eventually stabilise by revegetation. Over the 30-year monitoring period, there has been a progressive trend towards gully stabilisation, and an associated reduction in channel instability. Massive destabilisation may occur in response to rare extreme flood events. Such an event (return period >100 years) occurred in neighbouring Langdale and Bowderdale valleys in June 1982. That event destabilised the system, causing slope failures, fan deposition, and in some places, a switch in channel style to wide braided channels. Since 1982 there has been a progressive recovery by slope stabilisation and single-thread sinuous channels have become reestablished. A different style of extreme event occurred in Carlingill in October 1998, in response to the wettest week in the 30-year period. A slope failure fed debris flows 400 m downslope, almost coupling with the channel system. Future climatic change could render the system prone to destabilisation, through either extreme flood events or through major slope failures.

Glacial geomorphology of bonney drift, taylor valley, antarctica

Taylor Dome is a small dome located at the edge of the East Antarctic Ice Sheet (EAIS) just inland of the Transantarctic Mountains (TAM) in the McMurdo Sound region of Antarctica. Taylor Dome reaches 2400m elevation and is connected to inland Dome C by a broad ice divide that extends into the interior EAIS. Taylor Glacier originates on Taylor Dome and flows eastward for 90 km into Taylor Valley in the TAM, to terminate at about 57m elevation in the west lobe of Lake Bonney. Taylor Glacier was less extensive than now during the last glacial maximum and has since expanded so that it now occupies its most extensive Holocene position. Bonney drift in central and lower Taylor Valley represents the penultimate advance of Taylor Glacier. Geomorphological analysis of Bonney drift reveals extensive lateral moraines and drift remnants on valleywalls up to 300m elevation that document an expanded Taylor Glacier which flowed through the central into the lower portion of the valley. In addition, lateral moraines of valley‐wall alpine glaciers merge into the Bonney laterals, affording evidence for concurrent advance alpine glaciers. The most striking geomorphologic features of Bonney drift are hummocky moraines deposited on the floor of central Taylor Valley. Widespread lacustrine sediments, including in situ and reworked carbonate plates with the δ18 O signature of Taylor Glacier, are associated with these hummocky moraines. Similar hummocky moraines are now being formed at the Taylor Glacier snout as it advances into the western lobe of Lake Bonney. Glaciological modeling shows that Taylor Glacier is frozen at its base except near its snout, both now and during the time of the last global glaciation. The observed basal debris in the easternmost 1.5–2.0km of the glacier is the sediment source for the frontal hummocky thrust moraines. This basal sediment is frozen onto the glacier base in a lacustrine environment, rather than resulting from wet‐based conditions. The implication is that the nearly identical Bonney hummocky moraines in central Taylor Valley likewise result from downvalley expansion of a frozen‐based Taylor Glacier into its proglacial lakes. This model for the formation of hummocky moraine forms the basis for interpreting the numerous uranium/thorium dates of lacustrine carbonates associated with Bonney drift.

Gravity-Driven Consolidation of Granular Slurries: Implications for Debris-Flow Deposition and Deposit Characteristics

ABSTRACT Fresh debris-flow deposits consolidate under their own weight. How quickly they consolidate (dissipate excess pore-fluid pressure and compact) affects their resistance to remobilization as well as their sedimentologic and stratigraphic characteristics. Here, analysis of small-volume (0.05 m3) noncohesive debris-flow slurries and larger (10 m3) experimental debris-flow deposits reveals the nature, rate, and magnitude of consolidation of typical debris-flow deposits. A simple, linear, one-dimensional model describing the diffusion of excess pore-fluid pressure satisfactorily approximates the overall timing and magnitude of consolidation of noncohesive debris-flow deposits. The model and measurements of pore-fluid pressure demonstrate that changes in fluid pressure and effective stress evolve upward from the base of a deposit, and show that hydraulic diffusivities of muddy slurries containing about 5 to 50 wt% mud are remarkably similar, about 10-6-10-7 m2/s. By comparison, sandy-gravel debris-flow deposits containing <2 wt% mud have higher hydraulic diffusivities, 10-4 m2/s. Pore-fluid seepage across a permeable basal boundary accelerates consolidation response time in the lower stratum compared to that over a no-flow boundary. However, changes in sediment fabric resulting from porosity changes alter hydraulic properties of basal debris and retard expected decay of fluid pressure immediately above the bed. This result suggests that fluid infiltration to the substrate does not contribute significantly toward debris-flow deposition. Low hydraulic diffusivities promote high and persistent pore-fluid pressure in debris flows, key factors enhancing mobilization. Elsewhere, pore-fluid pressures nearly sufficient to liquefy debris have been shown to persist through transit and deposition. Here, I show that significant dissipation of such fluid pressure is restricted to postdepositional consolidation. Therefore, neither uniform decay of excess pore-fluid pressure nor intrinsic viscoplastic yield strength explain debris-flow deposition. Instead, debris-flow deposition results from friction concentrated along flow margins where high pore-fluid pressures are absent. Sustained high pore-fluid pressure following deposition fosters deposit remobilization, which can mute or obliterate stratigraphic evidence for multiple events. A thick deposit of homogeneous, poorly sorted debris can result from mingling of soft deposits and recurrent surges rather than from a single flow wave if deposit consolidation time greatly exceeds typical sediment emplacement times.

Volcanic ash bands in the Frontier Mountain and Lichen Hills blue-ice fields, northern Victoria Land

Dust bands in the blue-ice of the Frontier Mountain meteorite trap (northern Victoria Land, Antarctica) were previously reported as upthrust basal debris. Four of them have now been sampled at Frontier Mountain and Lichen Hills. The absence of local rocks and sedimentary fragments, the ubiquitous abundant volcanic glass with no evidence for abrasion, the igneous minerals, the chemical compositions of glass and minerals and the bulk chemical compositions indicate that they are volcanic ash bands (tephra) and not glacial debris. Although hardly distinguishable in the field, the different volcanic ash bands are discriminated using mineralogical and chemical data, as well as particle size, abundance and vesicularity of glass. Chronological constraints, particle size and chemical compositions localize the source for the Frontier Mountain and Lichen Hills tephra within the recent activity of the Mount Melbourne Volcanic Province in northern Victoria Land; possible emission centres are the Pleiades (40 ± 50 ka to 3 ± 14 ka) and/or Mount Rittmann (3.97 Ma to present).

Late Pleistocene–Holocene retreat of the West Antarctic Ice-Sheet system in the Ross Sea: Part 2—Sedimentologic and stratigraphic signature

Sedimentologic, geotechnical, geochemical, and accelerator mass spectrometer (AMS) radiocarbon data from two marine geologic cruises in the Ross Sea have allowed us to constrain facies relationships and temporal changes in the West Antarctic Ice Sheet. The selection of core sites was facilitated by the use of multibeam bottom imagery and a good quality (CHIRP) subbottom reflection system. A complex but consistent succession of facies documents a number of environments from subglacial to open marine. Massive, mud-rich diamictons have low water contents, contain (in places) a calcareous microfossil assemblage, show minimal textural variation, and contain low and uniform total organic carbon values. This reflects a subglacial setting. This unit passes upward into a stratified, thin, granulated facies consisting of pelletized, sandy, muddy gravel that is loosely compacted and contains a variable water content and concentrated horizons of pebble-sized clasts. This facies reflects the lift-off zone or thin water film between the basal debris and sea floor. Overlying this unit are silty clays that contain a well-sorted, very fine-grained sand component. There are no coarse grains within this unit. This facies is, in part, laminated and reflects deposition beneath an ice shelf, near and away from the grounding-line zone. The ice-shelf facies passes upward into a siliceous mud and ooze unit that represents deposition in an open-marine setting. A sandy, volcaniclastic-rich subfacies marks the transition from ice shelf to open-marine environments found at the calving line. The 86 AMS radiocarbon dates on organic matter provide an accurate chronology for 19 cores. Ice-shelf conditions were established in the outer Drygalski Trough by 11 ± 0.25 ka and perhaps earlier. This transition took place in the JOIDES Basin by 10–8 ka. The calving front of the Ross Ice Shelf passed over the Drygalski Trough at 74°S by 9.5 ± 0.25 ka. The timing of deglaciation in the Ross Sea calls into question current models for the contribution of Antarctic glacial ice to Holocene sea-level rise and suggests that recession was relatively gradual and more closely aligned with Northern Hemisphere deglaciation and its associated eustatic pulse.

Debris entrainment and transfer in polythermal valley glaciers

AbstractModes of debris entrainment and subsequent transfer in seven “normal” and five surge-type glaciers in Svalbard (76–79° N) are outlined in the context of the structural evolution of a glacier as the ice deforms during flow. Three main modes of entrainment and transfer are inferred from structural and sedimentological observations: (i) The incorporation of angular rockfall material within the stratified sequence of snow/firn/superimposed ice. This debris takes an englacial path through the glacier, becoming folded. At the margins and at the boundaries of flow units the stratified ice including debris is strongly folded, so that near the snout the debris emerges at the surface on the hinges and limbs of the folds, producing medial moraines which merge towards the snout. The resulting lines of debris are transmitted to the proglacial area in the form of regular trains of angular debris. (ii) Incorporation of debris of both supraglacial and basal character within longitudinal foliation. This is particularly evident at the surface of the glacier at the margins or at flow unit boundaries. It can be sometimes demonstrated that foliation is a product of strong folding, since it usually has an axial planar relationship with folded stratification. Foliation-parallel debris thus represents a more advanced stage of deformation than in (i). Although the presence of basal debris is problematic, it is proposed that this material is tightly folded ice derived from the bed in the manner of disharmonie folding. The readily deformed subglacial sediment or bedrock surface represents the plane of décollement. (iii) Thrusting, whereby debris-rich basal ice (including regelation ice) and subglacial sediments are uplifted into an englacial position, sometimes emerging at the ice surface. This material is much more variable in character than that derived from rockfalls, and reflects the substrate lithologies; diamicton with striated clasts and sandy gravels are the most common facies represented. Thrusting is a dynamic process, and in polythermal glaciers is probably linked mainly to the transition from sliding to frozen bed conditions. It is not therefore a solely ice-marginal or proglacial process.

Debris entrainment and transfer in polythermal valley glaciers

AbstractModes of debris entrainment and subsequent transfer in seven “normal” and five surge-type glaciers in Svalbard (76–79° N) are outlined in the context of the structural evolution of a glacier as the ice deforms during flow. Three main modes of entrainment and transfer are inferred from structural and sedimentological observations: (i) The incorporation of angular rockfall material within the stratified sequence of snow/firn/superimposed ice. This debris takes an englacial path through the glacier, becoming folded. At the margins and at the boundaries of flow units the stratified ice including debris is strongly folded, so that near the snout the debris emerges at the surface on the hinges and limbs of the folds, producing medial moraines which merge towards the snout. The resulting lines of debris are transmitted to the proglacial area in the form of regular trains of angular debris. (ii) Incorporation of debris of both supraglacial and basal character within longitudinal foliation. This is particularly evident at the surface of the glacier at the margins or at flow unit boundaries. It can be sometimes demonstrated that foliation is a product of strong folding, since it usually has an axial planar relationship with folded stratification. Foliation-parallel debris thus represents a more advanced stage of deformation than in (i). Although the presence of basal debris is problematic, it is proposed that this material is tightly folded ice derived from the bed in the manner of disharmonie folding. The readily deformed subglacial sediment or bedrock surface represents the plane of décollement. (iii) Thrusting, whereby debris-rich basal ice (including regelation ice) and subglacial sediments are uplifted into an englacial position, sometimes emerging at the ice surface. This material is much more variable in character than that derived from rockfalls, and reflects the substrate lithologies; diamicton with striated clasts and sandy gravels are the most common facies represented. Thrusting is a dynamic process, and in polythermal glaciers is probably linked mainly to the transition from sliding to frozen bed conditions. It is not therefore a solely ice-marginal or proglacial process.