Glacial Material Research Articles

Factors Controlling the Retreat of Drumlin Coastal Cliffs in a Low Energy Marine Environment—Strangford Lough, Northern Ireland

The erosion of low cliffs of glacial material has been investigated around the shoreline of an enclosed sea basin (lough) with a very low wave energy environment. Erosion pins used at 16 sites monitored cliff retreat rates at monthly or 6-monthly intervals over a 3-year period. The total erosion during the 3-year study period ranged from 164.4 mm a−1 to 8.2 mm a−1. Cliff height, cliff angle, and the length and bearing of maximum fetch were found to be the most important parameters controlling erosion. Variations in cliff materials did not emerge as a significant factor controlling erosion rates. The shoreline on the eastern side of the lough suffers more rapid erosion than that on the western side because of its greater exposure to storms from the southwest quadrant. Current rates of erosion are sufficient on the eastern shore for some drumlins to have been completely eradicated or reduced to boulder shoals by erosion in the time that sea levels have been at present elevations.

Transport and fate of nitrate within soil units of glacial origin

Questions concerning the influence of soil type and crop cover on the fate and transport of nitrate (NO 3 − ) were examined. During a growing season, soils derived from glacial material underlying either corn or soybeans were sampled for levels of NO 3 − within the pore water. Measured levels of NO 3 − ranged from below detection limit to 14.9 g NO 3 − per kilogram of soil (g/kg). In fields with the same crop cover, the silty-clayey soil exhibited a greater decrease in NO 3 − levels with depth than the sandier soil. Crop uptake of NO 3 − occurs within the root zone; however, the type of crop cover did not have a direct impact on the fate or transport during the growing season. The soils underlying soybeans had an increase in NO 3 − levels following harvest, suggesting that the decomposition of the soybean roots contributed to the net gain of NO 3 − in the shallow soil. For all of the soil types, conditions below 100 cm are conducive for microbial denitrification, with both a high water saturation level (>60%) and moderate organic carbon content (1–2%). At depths below 100 cm, temporal differences in NO 3 − levels of over a magnitude, up to a 95% reduction, were recorded in the soil units as the growing season progressed. Physical properties that control the transport of NO 3 − or denitrification have a larger influence on NO 3 − levels than crop type.

Soils and Geomorphology in the Oregon Cascades: A Comparative Study of Illinoian-Aged and Wisconsin-Aged Moraines

Catena approaches to soil chronosequence studies in the Oregon Cascades are lacking in the literature. Here we compare pedogenesis with slope position on two different aged moraines. The younger Suttle Lake moraine (11 to 20 ka B.P.) contains a sequence of pyroclastic teprhas overlying glacial till and subsequently a series of buried soils. The older Jack Creek moraine (120 to 200 ka B.P.) is predominantly uniform in its parent material, glacial till. Both moraines show evidence of upslope erosion and down-slope deposition, being more active on the younger Suttle Lake moraine. Major differences in soil properties between the two moraines are associated with texture. Pyroclastic parent material is much coarser, with textures of sand and gravel, while older glacial till parent material typically displays a texture of loamy sand. Percent organic matter is higher in the finer glacial till. Harden's Profile Development Index values (i.e., PDI) were used to compare B-horizons of similar glacial till parent material. The PDI is of limited use in this study due to volcanism and glaciations producing alternating layers of parent material. However, B-horizons of similar parent material did obtain higher values on the older Jack Creek moraine. The index was also useful in identifying a much older buried B-horizon at the toeslope of the Suttle Lake moraine, possibly comparable to that of pre-Bull Lake-aged deposits in the Rocky Mountains. The use of soils as relative age indicators is cautioned for future work in the Oregon Cascades due to the complexity associated with an active volcanic landscape.

Constraints on the Location of the Late Quaternary Reelfoot and New Madrid North Faults in the Northern New Madrid Seismic Zone, Central United States

The New Madrid North and Reelfoot Faults are believed to have ruptured during large-magnitude earthquakes on 23 January and 7 February 1812, respectively, based on the regional distribution of liquefaction, contemporary New Madrid seismic zone microseismicity, and historical accounts of earthquake-related damage. Although the location of the Reelfoot Fault generally is well constrained between the Kentucky Bend of the Mississippi River in Kentucky and the southeastern margin of the Mississippi Valley in Tennessee, the northern extent of the fault remains enigmatic in southeastern Missouri. This is equally true of the New Madrid North Fault, which has a postulated location based primarily on contemporary microseismicity and rupture scenario models of the 1811-1812 earthquake sequence. In this paper, we synthesize existing unpublished and published data with recently acquired subsurface and geomorphic information to clarify the locations of the Reelfoot and New Madrid North Faults in southeastern Missouri. On the basis of this data synthesis, we interpret that the Reelfoot Fault trends northwest across the Kentucky Bend of the Mississippi River as a northwest-facing scarp coincident with Des Cyprie Slough near New Madrid, Missouri, and anomalous elevated topography on southern Sikeston Ridge northwest of New Madrid. Furthermore, Quaternary faulting and folding imaged from seismic-reflection profiles across Des Cyprie Slough coincide with bedrock structural lineaments, a northeast-trending band of contemporary microseismicity, and a distinct northwest-trending post-Tertiary change in alluvial thickness. We trace the Reelfoot Fault as much as several kilometers northwest of the Mississippi River, where it either dies out or steps north-northeast to merge with the New Madrid North Fault. The New Madrid North Fault appears to be expressed geomorphically as left-stepping, en echelon northeast-trending fractures preserved in Pleistocene glacial outwash material comprising Sikeston Ridge. The fractures coincide with Quaternary faults and folds, as well as deeper Cretaceous and Paleozoic faults and flexures, imaged in geophysical profiles. In summary, the surface locations of the Reelfoot and New Madrid North Faults directly west-northwest of New Madrid, Missouri are constrained by geomorphic, geologic, geophysical, and historical seismological data sets and reflect transfer of strain from the northeast-verging Reelfoot reverse fault to the northeast-striking, dextral New Madrid North Fault.

Englacial phase changes and intergranular flow above subglacial lakes

AbstractGlaciers are permeated by liquid veins that line the triple junctions between ice grains and act as conduits for material transport. The density difference between water and ice produces a hydraulic gradient that drives liquid flow down towards the glacier bed. We investigate how variations in this transport rate are manifest in the lower regions of a glacier that floats on a subglacial reservoir. When the temperature rises towards the glacier bed, the associated permeability increase leads to more rapid fluid transport; internal melting supplies the changing flow. A reduction in hydraulic gradient results from surface energy effects, and causes a decreasing transport rate near the base; hence, freezing occurs within the polycrystalline ice. For representative values of the heat flux, soluble impurity loading and grain-size, the downward flux peaks at approximately 1–100mma–1 several tens of centimeters above the glacier bed. Beneath this level, the effects of surface energy control the fluid transport and cause the flow to reverse directions, pulling liquid upwards into the polycrystalline ice so that the flow does not introduce glacial material into the underlying reservoir.

Deltaic Infill of a Deglaciated Arctic Fjord, East Greenland: Sedimentary Facies and Sequence Stratigraphy

ABSTRACT The deltaic fill of a deglaciated side-entry fjord in western Jameson Land, East Greenland, shows the record of an interplay between regional and local controls, including climate and relative sea-level change, onshore and offshore topography, and sediment yield. A four-phase model is suggested for the delta development and fjord filling. At the deglaciation stage (phase 1), the newly flooded fjord floor primarily accumulated sediment from reworked glacial material. These deposits were relatively thin, and draped the uneven sea-floor topography. The subsequent onset of paraglacial delta sedimentation (phase 2) was characterized by a high meltwater runoff and extreme sediment yield, and corresponded to the highstand and gradual fall of relative sea level. A Gilbert-type delta with a short distributary plain formed at the head of the tributary fjord, which received a very high amount of fine-grained sediment. The stage of enhanced delta progradation (phase 3) was characterized by decreased sediment yield and runoff, and a rapid fall of relative sea level. Delta progradation was fast because of rapidly decreasing accommodation space, and a solitary river channel deeply incised the delta. The equilibrium stage (phase 4) spanned most of Holocene time and involved a lowered rate of relative sea-level fall. Seasonal runoff supplied limited amounts of sediment to the river mouth, and the delta front became increasingly reworked by wave action and longshore drift. The side-entry fjord, a small deglaciated valley invaded by the sea, was filled chiefly by highstand and regressive systems tracts, whereas relatively little sea-floor aggradation was associated with the transgressive systems tract. The stratigraphic pattern documented in this paper contrasts with the pattern usually reported from nonglaciated incised-valley systems, which are filled mainly by the transgressive systems tract, typically involving a retreating estuarine system. The pattern also differs from some large, gradually deglaciated fjord valleys containing thick, transgressive, glaciomarine deposits accumulated during glacier retreat. The study demonstrates the large variability of valley fills and illustrates the importance of topography, sediment supply, and climate as well as the style of deglaciation on the sediment filling of a fjord valley.

Geochemistry of glacial sediments in the area of the Bend massive sulfide deposit, north-central Wisconsin

Geochemical exploration in northern Wisconsin has been problematic because of thick glacial overburden and complex stratigraphic record of glacial history. To assess till geochemical exploration in an area of thick glacial cover and complex stratigraphy samples of glacial materials were collected from cores from five rotasonic boreholes near a known massive sulfide deposit, the Bend deposit in north-central Wisconsin. Diamond drilling in the Bend area has defined a long, thin zone of mineralization at least partly intersected at the bedrock surface beneath 30–40 m of unconsolidated glacial sediments. The bedrock surface has remnant regolith and saprolite resulting from pre-Pleistocene weathering. Massive sulfide and mineralized rock collected from diamond drill core from the deposit contain high (10s to 10,000s ppm) concentrations of Ag, As, Au, Bi, Cu, Hg, Se, Te, and Tl. Geochemical properties of the glacial stratigraphic units helped clarify the sequence and source areas of several glacial ice advances preserved in the section. At least two till sheets are recognized. Over the zone of mineralization, saprolite and preglacial alluvial and lacustrine samples are preserved on the bedrock surface in a paleoriver valley. The overlying till sheet is a gray, silty carbonate till with a source hundreds of kilometers to the northwest of the study area. This gray till is overlain by red, sandy till with a source to the north in Proterozoic rocks of the Lake Superior area. The complex glacial stratigraphy confounds down-ice geochemical till exploration. The presence of remnant saprolite, preglacial sediment, and far-traveled carbonate till minimized glacial erosion of mineralized material. As a result, little evidence of down-ice glacial dispersion of lithologic or mineralogic indicators of Bend massive sulfide mineralization was found in the samples from the rotasonic cores. This study points out the importance of determining glacial stratigraphy and history, and identifying favorable lithologies required for geochemical exploration. Drift prospecting in Wisconsin and other areas near the outer limits of the Pleistocene ice sheets may not be unsuccessful, in part, because of complex stratigraphic sequences of multiple glaciations where deposition dominates over erosion.

1246 Co-operation and competence in cancer care

A systematic compilation and characterisation of many reports of neotectonic crustal deformation in Norway (both on local and regional scales) has identified two neotectonic faults in northern Norway. The Stuoragurra Fault is a large reverse fault in Finnmark County. The Nordmannvikdalen fault is a much smaller normal fault in Troms County. The Stuoragurra postglacial fault can be followed, in several discontinuous sections, for 80 km, in a NE–SW direction. The fault has up to 10 m of displacement. During 1998, two trenches were made across the fault. The hanging wall was seen to be thrust upwards over the footwall, with 7 m vertical displacement evident from displaced glacial contacts. The fault did not penetrate the overlying glacial materials, but rather folded them, forming a blind thrust. Large liquefaction and other deformation structures were found in the glaciofluvial sediments in both trenches. Veins of angular and subangular pebbles from the local bedrock penetrate more than 10 m laterally from the thrust plane and into the sediments in the footwall. It is thought that these veins were injected during the fault activity. The major deformation of the sediments has a décollement plane that continues laterally in the E/B horizon contact of the modern soil on top of the footwall. This may indicate that an initial pedogenesis had taken place before the fault activity occurred, however no macro plant fossils to support this were found in the possible buried soil. Deformational structures seen in the trench can be explained as a result of one major fault event. The Nordmannvikdalen postglacial fault is a NW–SE trending normal fault, dipping to the NE. The fault offsets till on the NW slope of Nordmannvikdalen. The escarpment varies in height from 0.5 to 1.5 m, with a trench often present between the hanging wall and the footwall. The fault locally splits into two subparallel branches, however this is probably only in the glacial overburden. Ground penetrating radar (GPR) profiles show two prominent bedrock reflectors, dipping approximately 40° to the NE. An upward extrapolation of the northernmost reflector through the overburden coincides with the position of the surface scarp. The bedrock is well exposed along a streamed parallel to the GPR profiles. Bedding and foliation cannot be responsible for the reflectors visible in the GPR profiles. Gravitational forces have been ruled out as the origin of the fault. Although both faults probably formed shortly after Weichselian deglaciation, there is evidence that isostatic uplift was not the only force acting. Quaternary tilting of Fennoscandia due to ridge push, or mantle convection, may also play a role in neotectonic activity in Norway.

The origin and significance of debris‐charged ridges at the surface of storglaciären, northern sweden

Storglaciären is a 3.2 km long polythermal valley glacier in northern Sweden. Since 1994 a number of small (1–2 m high) transverse debris‐charged ridges have emerged at the ice surface in the terminal zone of the glacier. This paper presents the results of a combined structural glaciological, isotopic, sedimentological and ground‐penetrating radar (GPR) study of the terminal area of the glacier with the aim of understanding the evolution of these debris‐charged ridges, features which are typical of many polythermal glaciers. The ridges originate from steeply dipping (50–70°) curvilinear fractures on the glacier surface. Here, the fractures contain bands of sediment‐rich ice between 0.2 and 0.4 m thick composed of sandy gravel and diamicton, interpreted as glaciofluvial and basal glacial material, respectively. Structural mapping of the glacier from aerial photography demonstrates that the curvilinear fractures cannot be traced up‐glacier into pre‐existing structures visible at the glacier surface such as crevasses or crevasse traces. These curvilinear fractures are therefore interpreted as new features formed near the glacier snout. Ice adjacent to these fractures shows complex folding, partly defined by variations in ice facies, and partly by disseminated sediment. The isotopic composition (δ18O) of both coarse‐clear and coarse‐bubbly glacier ice facies is similar to the isotopic composition of the interstitial ice in debris layers that forms the debris‐charged ridges, implying that none of these facies have undergone any significant isotopic fractionation by the incomplete freezing of available water. The GPR survey shows strong internal reflections within the ice beneath the debris‐charged ridges, interpreted as debris layers within the glacier. Overall, the morphology and distribution of the fractures indicate an origin by compressional glaciotectonics near the snout, either at the thermal boundary, where active temperate glacier ice is being thrust over cold stagnant ice near the snout, or as a result of large‐scale recumbent folding in the glacier. Further work is required to elucidate the precise role of each of these mechanisms in elevating the basal glacial and glaciofluvial material to the ice surface.

Remediation of the Wells G & H Superfund Site, Woburn, Massachusetts.

Remediation of ground water and soil contamination at the Wells G & H Superfund Site, Woburn, Massachusetts, uses technologies that reflect differences in hydrogeologic settings, concentrations of volatile organic compounds (VOCs), and costs of treatment. The poorly permeable glacial materials that overlie fractured bedrock at the W.R. Grace property necessitate use of closely spaced recovery wells. Contaminated ground water is treated with hydrogen peroxide and ultraviolet (UV) oxidation. At UniFirst, a deep well completed in fractured bedrock removes contaminated ground water, which is treated by hydrogen peroxide, UV oxidation, and granular activated carbon (GAC). The remediation system at Wildwood integrates air sparging, soil-vapor extraction, and ground water pumping. Air stripping and GAC are used to treat contaminated water; GAC is used to treat contaminated air. New England Plastics (NEP) uses air sparging and soil-vapor extraction to remove VOCs from the unsaturated zone and shallow ground water. Contaminated air and water are treated using separate GAC systems. After nine years of operation at W.R. Grace and UniFirst, 30 and 786 kg, respectively, of VOCs have been removed. In three years of operation, 866 kg of VOCs have been removed at Wildwood. In 15 months of operation, 36 kg of VOCs were removed at NEP. Characterization work continues at the Olympia Nominee Trust, Whitney Barrel, Murphy Waste Oil, and Aberjona Auto Parts properties. Risk assessments are being finalized that address heavy metals in the floodplain sediments along the Aberjona River that are mobilized from the Industri-Plex Superfund Site located a few miles upstream.

DEPOSITION AND LEACHING OF SULPHATE AND BASE CATIONS IN A MIXED BOREAL FOREST IN EASTERN FINLAND

Monthly fluxes of sulphate (SO4 2-) and base cations(Ca2+, Mg2+, K+) were studied from 1993 to 1996 as precipitation passed through forest vegetation and surfacesoil layers in an area receiving low and declining levels of atmospheric sulphate pollution. The canopy was dominated by mature Norway spruce (Picea abies Karsten) and the soilwas a podzol developed on glacial till material. The mean annual bulk deposition of SO4 2- collected in the open was 136 molc ha-1 and that of Ca2+, Mg2+ and K+ was 44, 11 and 25 molc ha-1, respectively. The annual total throughfall deposition of SO4 2- was 318 molc ha-1 and that of Ca2+, Mg2+ and K+ was 151, 64 and 181 molcha-1, respectively. Sulphate was the dominant anion accompanying the base cations leached from the canopy. More than half (58%) of the annual total throughfall deposition ofSO4 2- was retained by the O-horizon and only 15% leached from below the B-horizon. The annual leaching of Ca2+, Mg2+ and K+ from below the B-horizon was14, 25 and 9% of the annual total throughfall deposition, respectively. The transport of base cations through the soil was predominantely countered by SO4 2- anions.

The Lake Erie Agricultural Systems for Environmental Quality project: an introduction.

In the last part of the twentieth century, recognition became widespread of the important effect of agricultural runoff on the health of aquatic ecosystems in the Lake Erie basin and elsewhere. Because of the efforts to remediate Lake Erie, the "dead lake" among the Laurentian Great Lakes, a number of research and demonstration projects were undertaken in the Lake Erie basin to evaluate and foster adoption of conservation tillage and other farming techniques that would reduce runoff while maintaining productivity. In addition, intensive water quality studies of long duration were begun on major tributaries to Lake Erie during this time. The Lake Erie Agricultural Systems for Environmental Quality (LEASEQ) project examined governmental programs, changes in agriculture, and changes in water and soil quality during the period 1975-1995, and sought to evaluate the linkages among these factors. The study area is characterized by extensive agricultural land use of soils developed from glacial materials deposited on Paleozoic sedimentary bedrock, mostly limestone. Tile drainage is extensive, particularly in slow-draining clay-rich lacustrine soils in the lower reaches of the watersheds. This paper introduces the study area, its geology, geography, soils, and agricultural history. In addition, we provide an overview of the LEASEQ concept and introduce the 11 other papers in this series, which provide a detailed exposition of the results of our studies.

Late Pleistocene Detrital Sediment Yield of the Jura Glacier, France

Measures of present-day glacial erosion vary widely with the technique employed. This paper quantifies the glacial material trapped in a proglacial lake during the Würm glacial period. The Combe d'Ain site was occupied by a meltwater lake where all the detrital material entering it from the Jura glacier accumulated. Sediment yield is computed from three factors: (1) the size of the sediment source area, (2) the length of time the system operated, and (3) the volume of sediment trapped. The sediment budget of the lake system suggests a detrital sediment yield of 4400±1700 metric tons per square kilometer and per calendar year. This represents a denudation rate of 1.6±0.6 mm per year, illustrating that mechanical erosion by the Jura glacier is more intensive than other processes of erosion.

A.J. Maltman, B. Hubbard and M.J. Hambrey, eds. 2000. Deformation of glacial materials. London, Geological Society (Special Publication 176). 352 pp. ISBN 1-86239-72-X, hardback. List price: £79.00/$132.00; Geological Society of London member price: £35.00/$58.00; American Association of Petroleum Geologists member price: £48.00/$80.00.

A.J. Maltman, B. Hubbard and M.J. Hambrey, <i>eds.</i> 2000. Deformation of glacial materials. London, Geological Society (Special Publication 176). 352 pp. ISBN 1-86239-72-X, hardback. List price: <i>£</i>79.00/$132.00; Geological Society of London member price: <i>£</i>35.00/$58.00; American Association of Petroleum Geologists member price: <i>£</i>48.00/$80.00.

Paradox of downstream fining and weathering-rind formation in the lower Hoh River, Olympic Peninsula, Washington

Although downstream fining of clasts is typical in modern and ancient river systems, the Hoh River, a cobble-and-boulder-bed river in Washington state, contains surprisingly little downstream fining of the coarsest tail of the grain-size distribution along its lower 63 km. Mean rate of fining of the coarsest size fraction is only 0.24 mm/km, barely significant given uncertainties in measurement. In addition, these same clasts have weathering-rind thicknesses that change very little along the length of the river (they decrease by 0.01 mm/km). Because weathering rinds are well developed in this setting and both tumbler studies and field observations show that abrasion of weathering rinds greatly accelerates clast diminution, there seems to be a paradox between predicted and observed downstream fining rates. Field study shows that detritus is constantly being resupplied to the river by erosion of late glacial materials along the river9s cutbanks and tributaries. The clasts supplied range in size but include abundant coarse grains with very thick weathering rinds. The continuous resupply of grains strongly attenuates the rate of downstream fining, despite the fact that these weathered grains abrade relatively rapidly. Thus, the dominance of reworked glacial debris overwhelms any processes by which grains are reduced in size in the modern river. These results suggest that relatively infrequent glaciation can have a long-lived effect on river sedimentation.

Neotectonic faulting in northern Norway; the Stuoragurra and Nordmannvikdalen postglacial faults

A systematic compilation and characterisation of many reports of neotectonic crustal deformation in Norway (both on local and regional scales) has identified two neotectonic faults in northern Norway. The Stuoragurra Fault is a large reverse fault in Finnmark County. The Nordmannvikdalen fault is a much smaller normal fault in Troms County. The Stuoragurra postglacial fault can be followed, in several discontinuous sections, for 80 km, in a NE–SW direction. The fault has up to 10 m of displacement. During 1998, two trenches were made across the fault. The hanging wall was seen to be thrust upwards over the footwall, with 7 m vertical displacement evident from displaced glacial contacts. The fault did not penetrate the overlying glacial materials, but rather folded them, forming a blind thrust. Large liquefaction and other deformation structures were found in the glaciofluvial sediments in both trenches. Veins of angular and subangular pebbles from the local bedrock penetrate more than 10 m laterally from the thrust plane and into the sediments in the footwall. It is thought that these veins were injected during the fault activity. The major deformation of the sediments has a décollement plane that continues laterally in the E/B horizon contact of the modern soil on top of the footwall. This may indicate that an initial pedogenesis had taken place before the fault activity occurred, however no macro plant fossils to support this were found in the possible buried soil. Deformational structures seen in the trench can be explained as a result of one major fault event. The Nordmannvikdalen postglacial fault is a NW–SE trending normal fault, dipping to the NE. The fault offsets till on the NW slope of Nordmannvikdalen. The escarpment varies in height from 0.5 to 1.5 m, with a trench often present between the hanging wall and the footwall. The fault locally splits into two subparallel branches, however this is probably only in the glacial overburden. Ground penetrating radar (GPR) profiles show two prominent bedrock reflectors, dipping approximately 40° to the NE. An upward extrapolation of the northernmost reflector through the overburden coincides with the position of the surface scarp. The bedrock is well exposed along a streamed parallel to the GPR profiles. Bedding and foliation cannot be responsible for the reflectors visible in the GPR profiles. Gravitational forces have been ruled out as the origin of the fault. Although both faults probably formed shortly after Weichselian deglaciation, there is evidence that isostatic uplift was not the only force acting. Quaternary tilting of Fennoscandia due to ridge push, or mantle convection, may also play a role in neotectonic activity in Norway.

About this title

The flow of glacier ice can produce structures that are striking and beautiful. Associated sediments,too, can develop spectacular deformation structures, and examples are remarkbly well preserved in Quaternary deposits. Although such features have long been recognized, they are now the subject of new attention from glaciologists and glacial geologists. This collection of papers addresses how the methods for unravelling deformation structures evolved in recent years by structural geologists can be used for glacial materials, and the opportunities offered to structural geologists by glacial materials for studying deformation in rocks.

Deformation of glacial materials: introduction and overview

Deformation of glacial materials: introduction and overview

Neoproterozoic glacial and gravitational sedimentation on a continental rifted margin: The Jequitaı́–Macaúbas sequence (Minas Gerais, Brazil)

This paper is a contribution to the knowledge of the sedimentation of Neoproterozoic sequences, known as the Jequitaı́ Formation and Macaúbas Group. These sequences are present along the transitional zone between the São Francisco Craton and the Brasiliano (≌600 Ma) Araçuaı́ fold belt in Minas Gerais, Brazil. A sedimentological study of these Neoproterozoic sequences enables us to distinguish between true continental and marine glacial facies and glacial material reworked by various subaqueous gravitational processes. The cratonic Jequitaı́ Formation consists of massive and stratified diamictites up to 100 m thick. This diamictite association is tentatively interpreted as glaciomarine in origin. It continues eastward, in the Araçuaı́ fold belt, as the metasedimentary Macaúbas Group, which is composed of metadiamictites, quartzites and schists from 5–12(?)km thick. The Macaúbas Group consists of resedimented glacial material deposited by subaqueous debris flows and turbidity currents. A depositional model is proposed for the Jequitaı́–Macaúbas glacial/gravitational sequence. From west to east, a glaciomarine sequence, possibly deposited from an ice-sheet and slightly reworked by gravitational processes, was reworked along the São Francisco cratonic border and generated a slope apron system made up of diamictites associated with turbidites and rhythmites.

Ice-sheet numerical modeling and marine geophysical measurements of glacier-derived sedimentation on the Eurasian Arctic continental margins

Long-range side-scan sonar images of the Barents Sea continental margin have been analyzed in conjunction with results from previous geophysical investigations to determine a qualitative model for sedimentation over the Bear Island and Storfjorden trough mouth fans. These data indicate that gravity-driven debris flows are major processes in the downslope transport of glacial material, delivered to the shelf break when ice sheets advanced across the continental shelf. During late Weichselian time, ∼4000 km 3 of sediments were deposited over the Bear Island fan (280 000 km 2 ) while ∼700 km 3 of sediments were deposited over the Storfjorden fan (40 000 km 2 ). A numerical ice-sheet model, including sediment deformation and transport beneath ice streams, reconstructs the glacial conditions required to transport large volumes of sediment to the late Weichselian Eurasian continental margin. Model results indicate that glaciation of the Eurasian High Arctic occurred after 28 ka, and that ice streams within bathymetric troughs were active by ca. 25 ka. The maximum ice-sheet thickness over the Barents Sea was about 1400 m; there was a secondary dome −1 (0.13 cm yr −1 averaged over the fan); the rate was 6 cm yr −1 (equivalent to 0.6 cm yr −1 over the fan) over the Storfjorden trough mouth. The modeled sediment volume at the continental margin of the Bear Island and Storfjorden troughs agrees well with the volumes of late Weichselian sediment inferred from seismic records from these large prograding submarine fans. Sensitivity experiments show that adjustments to model environmental inputs do not significantly affect the results.