Glacigenic Sediments Research Articles

Coastal geomorphology and evolution of Tierra del Fuego (Southern Argentina)

The northeastern Atlantic coast and the Beagle Channel are significant geomorphological areas of Tierra del Fuego (Southern Argentina). The northeastern Atlantic coast is located at the extra Andean lowlands (South- American Plate Domain). This coast line undergoes a macrotidal regime and is exposed to high energy waves and intense westerly winds. Extensive and wide beaches and littoral forms are composed of gravel and coarse sand. This zone was a free-ice area since 1,8 Ma B.P. Glacigenic deposits were re-worked by litoral processes that formed gravel beaches during sea level highstands of the. During the Holocene (i.e. approximately 5,000 years B.P.) gravel barriers plugged the inner estuaries of the palaeoembayments These barriers suggest a relative sea level fall of 0.214 m each 1,000 years, but a portion of this gradient could be due to wave dynamics since greater set-up of the storm waves enters the embayments. The growth of the northern gravel beach ridge plains and spits at the seaward flank of the embayments took place under limited sediment supply. The elongation of these littoral forms was triggered by erosion and sediment recycling at the seaward side (cannibalism), resulting in a significant landward retreat. Southward the gravel beach ridge plains underwent a regressive trend during the Holocene. They do not reveal either erosion, or sediment recycling, or significant landward retreat. The Beagle Channel connects the Pacific and Atlantic oceans. It is a 300 m depth basin separated from the Atlantic Ocean by a 30 m depth shallow sill. The Beagle Channel is located at the active seismotectonic setting of the Fuegian Andes (Scotia Plate Domain). It is a 5 km wide tectonic valley that was completely covered by ice during the Last Glaciation. After this period, glaciofluvial and glaciolacustrine environments developed in the basin. The Beagle valley was rapidly flooded by the sea immediately after the Younger Dryas, 11,000 year B.P. It undergoes a micro tidal range regime and shows a rugged, rocky shoreline where pocket gravel beaches develop in the embayments. Holocene raised beaches can be recognized in many places along the channel and their elevations vary considerably, reaching maximum elevations of 10 m above the present counterpart at ages of 6,000 years B.P. The estimated average tectonic uplift for this period is 1.5 to 2.0 mm/year.

Seismic morphology and distribution of inferred glaciomarine deposits along the East Antarctic continental margin, 20°E–60°E

Seismic-reflection profiles from the continental rise and abyssal plain along a nearly 2000-km long stretch of the East Antarctic margin have been studied, including the Cosmonaut Sea basin and adjacent parts of the Riiser-Larsen Sea to the west and the Cooperation Sea to the east. The recognition of glaciomarine deposits is based on their correlative basal reflector and acoustic characteristics. The lateral distribution of seismostratigraphic sediment thickness, internal architecture of reflectors and seafloor morphology reveal a number of deep-water depocentres with large paleo-channels. These main depocentres correspond to the coastal outlets of present-day ice streams and hence indicate longer-term stabilities in the ice-stream positions during the Neogene advance and retreat phases of the Antarctic Ice Sheet. Scattered paleo-channels and subordinate depocentres in the Cosmonaut Sea suggest that some of the ice streams might occasionally change their position or that also other ice streams were episodically involved during earlier glacial maxima. Several paleo-channels are > 30 km wide and > 500 m deep, the largest one being approximately 40 km wide and 1000 m deep. These channels show evidence of multiple incisions and have prominent, multi-storey levees. The giant conduits are considered to be formed through several extensive phases of mass wasting from the shelf edge and upper slope to the continental rise and adjoining abyssal plain. Downslope transport by sediment gravity flows predominates with a subordinate role of along-slope sediment drift by circumpolar contour current. The repetitive advances and retreats of the ice sheet and recharging of ice streams with terrigenous detritus allowed enormous amounts of glacigenic sediment to be transferred from the Antarctic interior to the shelf margin during glacial phases. It was probably a high rate of localized sediment supply by ice streams and a wide range of sediment grain sizes that promoted the long series of large and highly efficient turbidity currents. The study demonstrates that a seismostratigraphic mapping of deep-water depocentres and recognition of paleo-channels along a glaciated continental margin may provide important information on the long-term ice-sheet dynamics, particularly on the pattern of ice drainage represented by coastal ice streams.

Bedrock detection and talus thickness assessment in the European Alps using geophysical methods

Three different geophysical techniques (ground-penetrating radar (GPR), 2D-resistivity, seismic refraction) were applied to measure sediment thickness of alpine talus slopes. The eight study areas covered a range of different rock types, aspects and elevations and are mainly situated in the north-eastern part of the European Alps. A total of more than 30 scree slopes were investigated. The emphasis was on the application of GPR. The other two methods were used additionally in five areas to verify the GPR results and to check the concurrence of the results. With all the techniques applied, certain difficulties of bedrock detection arose. GPR surveys using several frequencies seem to be the most powerful tool for detecting the bedrock surface and internal structures in these high-alpine environments. However, there frequently is no radar wave reflection at the debris–bedrock interface due to the rather similar dielectrical properties. This problem was overcome by using the characteristic reflection patterns of the subsurface units for delimitating debris from glacigenic sediments and bedrock. Typical radar facies are presented. The other two methods applied were impeded by the broad overlap between resistivity and seismic velocity values, respectively, of the subsurface layers. The widespread presence of compacted and/or glacigenic debris layers between surficial scree and bedrock rendered the interpretation difficult. The three methods coincide well in straightforward settings (e.g. debris superimposing bedrock). Additional sediment units like till, wet or compacted debris lead to considerable ambiguities and deviations between the results. Thus, the combined application of two or three techniques is strongly advisable to verify and improve the interpretation. The calculated Holocene rockwall retreat rates lay between 200 and 850 mm/ka which is well within the framework of previous measurements. The weathering rates at north faces seem to be roughly two times higher than at south faces in the areas of investigation.

Sub-ice shelf deposition during the late Devensian glaciation in western Ireland

Glacigenic sediments preserved around the margins of Clew Bay, western Ireland, record deposition in a tidally-influenced, sub-ice shelf environment during the late Devensian glaciation (∼ 21–13 14C kyr). Thick, massive to normally-graded glacial diamictons (tills) are interbedded with laminated to massive silt and clay beds. These sediments interbed with each other on scales ranging from millimetres to metres in a generally shallowing-up sequence associated with a decrease in accommodation space. Isolated dropstones and a downward-going clastic dike are also observed. These structures and sediments reflect a waterlain environment with rhythmically-varying levels of water energy and sediment supply. Tidally-driven uplift and downdraw of a retreating ice shelf is a likely control on sedimentation patterns, and glacial dynamics, of this sensitive, Atlantic-facing ice margin.

Fire and ice: unravelling the climatic and volcanic history of James Ross Island, Antarctic Peninsula

Back in the mid‐nineteenth century British explorer James Clark Ross took his ships, HMS Terror and HMS Erebus, farther south than anyone else had been. He now lends his name to James Ross Island, a part‐volcanic edifice that rises out of the sea off the north‐east tip of the Antarctic Peninsula. The island records a geological history dating back to the Cretaceous, though its great peaks are volcanic. The most recent rocks of the island record a monumental struggle between fire and ice, the volcanoes, and the ice sheets that cover them. The glacigenic sediments that are interspersed with the volcanic rocks contain rich fossil assemblages which suggest that at times, the climate was warmer, with the ice retreating. Their study may help us to delimit the patterns of climate change in the Antarctic Peninsula region as Earth's global climate warms.

Single grain optical dating of glacigenic deposits

Determining the age of glacigenic sediments is difficult for many geochronological methods because of the lack of suitable materials for analysis. Luminescence dating can be applied to the mineral grains making up the glacigenic sediments. However a major source of uncertainty in previous studies has been whether the mineral grains were exposed to sufficient daylight prior to deposition for the luminescence signal to be reset. Measurements of the optically stimulated luminescence signal from single sand-sized quartz grains offers the potential for explicitly identifying if a sediment contains grains that were not exposed to sufficient daylight to reset their signal. Statistical analysis of the resulting data can then reject those grains to allow the age of the sample to be determined. This study is the first to apply single grain optical dating to glacigenic sediments, and demonstrates the issues involved by analysis of samples from Chile and Scotland. Ages from 2.4±0.5 to 17.3±1.5 ka are produced. Comparison of the results with independent age control suggests that the ages are reliable. The results also show that the extent of bleaching at deposition varies considerably from one sample to another. For the most incompletely bleached sample, luminescence measurements based on the average of many hundreds or thousands of grains would have overestimated the age of the sample by ∼60 ka, but the single grain method proposed here was able to reliably date it.

Early Silurian glaciation along the western margin of Gondwana (Peru, Bolivia and northern Argentina): Palaeogeographic and geodynamic setting

The record of the Ordovician–Silurian transition within the southern Peru–Bolivia Palaeozoic basin (extending across Peru, Bolivia and northern Argentina) is characterized by a diamictite-bearing unit which overlies several different Ordovician units, and underlies mid Silurian shales. We review the lithostratigraphy, sedimentology and biostratigraphy of this unit, revising stratigraphic relationships and several ideas and interpretations previously proposed. Analysis of facies and lithofacies associations allow for a reinterpretation of sedimentary processes and environments involved during its deposition. Turbidite and shale interbeds within the diamictites indicate a deep marine environment, with interbedded mud flows, debris flows, slumps and large displaced slabs providing evidence for sediment instability and resedimentation. Chitinozoan biostratigraphy indicates a Llandovery age, in contrast with previous proposals of a Hirnantian age, and suggests the need for more detailed sedimentologic and biostratigraphic studies, and a reassessment of this diamictite unit in relation to adjacent regions in South America. The resedimented character of the deposit explains some of the Ordovician fauna previously described, which must be considered as recycled from underlying units. In situ fauna needs to be reassessed, as it may be endemic, diachronic and/or indicative of migrations within Gondwana. Glacially-faceted and striated clasts, as well as large granitoid boulders within the resedimented materials, provide evidence for glaciation of the source area, and are interpreted as recycled from former glacigenic deposits. The evidence found in the Central Andes indicates a glaciomarine origin for the diamictites and corroborates glaciation of a western source area prior to the late Llandovery (Telychian). The precise age of glaciation in this part of Gondwana cannot be confirmed do to a lack of true tillites. Tectonic deformation and the resulting relief are respectively identified as the origin for sediment instability and for local glaciation along the active margin of western Gondwana during the Late Ordovician and Early Silurian. The new data imply that the Ordovician–Silurian boundary was not preserved in some areas of the basin, and that an erosional hiatus (disconformity with variable regional significance) is present between different Ordovician units and the diamictite unit. Late Ordovician (?late Caradoc–?early Ashgill) euxinic black shales locally present in the central Altiplano and Eastern Cordillera of Bolivia may represent the youngest Ordovician unit in the region, and more research should be focused on them. Paleogeographic reconstruction of the Early Silurian Peru–Bolivia Basin depicts the extension of the diamictite unit southwards into northern Argentina and western Paraguay, connecting with the Paraná basin through the Asunción Arch. To the north it continued into northern Peru and Ecuador, connecting with the Silurian record in Colombia and Venezuela, and to the northeast into Brazil (Solimões and Amazonas basins). The sedimentary record of glaciation in the Peru–Bolivia basin was due to the development of local ice fields to the west of the basin, related with tectonism and resulting reliefs along the active margin of Gondwana, and probably did not coincide with the development of the North African late Ashgillian ice cap.

Landscape response to deglaciation in a high relief, monsoon-influenced alpine environment, Langtang Himal, Nepal

Significant glacial fluctuations and rapid paraglacial reworking of glacigenic sediments characterize the Middle and Late Holocene of the Langtang Khola Valley, Central Nepal Himalaya. Geomorphic mapping and beryllium-10 cosmogenic radionuclide (CRN) dating of moraines and paraglacial fans were undertaken to test the existing paraglacial fan, terrace and moraine chronologies. The new dating compares favorably with prior studies that utilized radiocarbon, adding additional support to the assumption that fan and terrace formation are strongly linked to deglaciation. Fan and terrace denudation rates are so rapid in this region, averaging ∼33 mm/yr, that no depositional landforms older than 5 ka are preserved within 250 m of the valley floor. In this region, high rates of denudation during the Late Quaternary are driven by a combination of rapid tectonic uplift, numerous glacial fluctuations and intense weathering driven by an active monsoon climate. Extensive reworking of glacigenic sediments in Langtang during the latter half of the Holocene is consistent with studies completed in other areas of the Himalaya that are strongly influenced by the monsoon.

Pleistocene lake outburst floods and fan formation along the eastern Sierra Nevada, California: implications for the interpretation of intermontane lacustrine records

Variations in the rock flour fraction in intermontane lacustrine sediments have the potential to provide more complete records of glacier fluctuations than moraine sequences, which are subject to erosional censoring. Construction of glacial chronologies from such records relies on the assumption that rock flour concentration is a simple function of glacier extent. However, other factors may influence the delivery of glacigenic sediments to intermontane lakes, including paraglacial adjustment of slope and fluvial systems to deglaciation, variations in precipitation and snowmelt, and lake outburst floods. We have investigated the processes and chronology of sediment transport on the Tuttle and Lone Pine alluvial fans in the eastern Sierra Nevada, California, USA, to elucidate the links between former glacier systems located upstream and the long sedimentary record from Owens Lake located downstream. Aggradation of both fans reflects sedimentation by three contrasting process regimes: (1) high magnitude, catastrophic floods, (2) fluvial or glacifluvial river systems, and (3) debris flows and other slope processes. Flood deposits are represented by multiple boulder beds exposed in section, and extensive networks of large palaeochannels and boulder deposits on both fan surfaces. Palaeohydrological analysis implies peak discharges in the order of 10 3–10 4 m 3 s −1, most probably as the result of catastrophic drainage of ice-, moraine-, and landslide-dammed lakes. Cosmogenic radionuclide surface exposure dating shows that at least three flood events are represented on each fan, at 9–13, 16–18 and 32–44 ka (Tuttle Fan); and at ∼23–32, ∼80–86 ka, and a poorly constrained older event (Lone Pine Fan). Gravels and sands exposed in both fans represent fluvial and/or glacifluvial sediment transport from the Sierra Nevada into Owens Valley, and show that river systems incised and reworked older sediment stored in the fans. We argue that millennial-scale peaks in rock flour abundance in the Owens Lake core reflect (1) fluctuations in primary subglacial erosion in the catchments in response to glacier advance–retreat cycles; (2) short-lived pulses of sediment delivered directly by catastrophic flood events; and (3) sediment released from storage in alluvial fans by fluvial and glacifluvial incision and reworking. As a result of this complexity the coarse sediment peaks in lake deposits may not simply reflect periods of increased glaciation, but likely also reflect changes in sediment storage and flux controlled by paraglacial processes. Current dating evidence is inadequate to allow precise correlation of individual flood or incision events with the Owens Lake rock flour record, although given the widespread occurrence of flood deposits in fans along the eastern margins of the Sierra Nevada, it is clear that fan deposition and incision played a very important role in modulating the delivery of glacigenic sediment to Owens Lake.

Subglacial and proglacial glacitectonic deformation in the Neoproterozoic Port Askaig Formation, Scotland

The recognition and interpretation of glacigenic sediments in the ancient rock record has important implications for understanding processes and patterns of climate change. Diamictite facies from the Neoproterozoic Port Askaig Formation, Scotland, have been variously interpreted as evidence for grounded glacier ice, mass-movements on a glacially influenced submarine slope, and subaqueous mass movements in a non-glacial environment. These contrasting interpretations imply widely differing degrees of ice cover, and have contrasting implications for Neoproterozoic (“Snowball”) glaciations. In this paper, we focus on the Great Breccia and the Disrupted Beds, two controversial units in the Port Askaig Formation. We present evidence that they were formed by the proglacial and subglacial deformation, respectively, of pre-existing shallow marine facies during an advance of grounded ice. Quaternary examples provide remarkably exact analogues for the Great Breccia and Disrupted Beds, illustrating the potential value of applying litho- and tectono-facies models to ancient sedimentary successions.

Sediment distribution around glacially abraded bedrock landforms (whalebacks) at lago tranquilo, chile

Whalebacks are convex landforms created by the smoothing of bedrock by glacial processes. Their formation is attributed to glacial abrasion either by bodies of subglacial sediment sliding over bedrock or by individual clasts contained within ice. This paper reports field measurements of sediment depth around two whaleback landforms in order to investigate the relationship between glacigenic deposits and whaleback formation. The study site, at Lago Tranquilo in Chilean Patagonia, is situated within the Last Glacial Maximum (LGM) ice limits. The two whalebacks are separated by intervening depressions in which sediment depths are generally 0.2 to 0.3 m. Two facies occur on and around the whalebacks. These facies are: (1) angular gravel found only on the surface of the whalebacks, interpreted as bedrock fracturing in response to unloading of the rock following pressure release after ice recession, and (2) sandy boulder‐gravel in the sediment‐filled depressions between the two whalebacks, interpreted as an ice‐marginal deposit, with a mixture of sediment types including basal glacial and glaciofluvial sediment. Since the whalebacks have heavily abraded and striated surfaces but are surrounded by only a patchy and discontinuous layer of sediment, the implication is that surface abrasion of the whalebacks was achieved primarily by clasts entrained in basal ice, not by subglacial till sliding.

Cenozoic alongslope processes and sedimentation on the NW European Atlantic margin

Based on studies of sediment accumulations deposited from-and erode by-alongslope flowing ocean currents on the European continental margin from Porcupine (Ireland) to Lofoten (Norway), the evolution of the Cenozoic paleocirculation was reconstructed as part of the STRATAGEM project. There is evidence of ocean current-controlled erosion and deposition in the Rockall Trough, in the Faeroe-Shetland Channel and on the Vøring Plateau since the late Eocene, although the circulation pattern remains ambiguous. The late Palaeogene flow in the Rockall Trough was almost probably driven by southerly-derived Tethyan Outflow Water. The extent and strength of any northerly-derived flow is uncertain. From the early Neogene (early-mid-Miocene), there was a massive regional expansion of contourite drift development both in the North Atlantic and in the Norwegian-Greenland Sea. This was most probably related to the development of the Faroe Conduit, the opening of the Fram Strait and the general subsidence of the Greenland-Scotland Ridge. These may have combined to cause a considerable acceleration in the exchange and overflow of deep waters between the Arctic and Atlantic Oceans. An early late Neogene (late early Pliocene) regional erosional event has been ascribed to a vigorous pulse of bottom-current activity, most probably the result of a global reorganisation of ocean currents associated with the closure of the Central American Seaway. During the late Neogene, contourites and sediment drifts developed in deep-water basins, between units of glacigenic sediments as well as infill of several paleo-slide scars. These sediments were derived from areas of bottom-current erosion as well as from the development of Plio-Pleistocene prograding sediment wedges, incorporating the extensive sediment supply derived from shelf-wide ice sheets. Presently a profound winnowing prevails along the shelf and upper slope due to the inflowing currents of Atlantic water. Depocentres of sediments derived from the winnowing are located (locally) in lower slope embayments and in slide scars.

A provisional Quaternary and Neogene lithostratigraphical framework for Great Britain

AbstractThis paper presents an overview of a provisional lithostratigraphical framework for the Quaternary and Neogene deposits of Great Britain (England, Scotland and Wales) (onshore). The objective is to provide a workable framework to aid future Quaternary mapping and research, and a stratigraphical scheme capable of use in a wide variety of applications. Using the full hierarchy, a supergroup, group and subgroup lithostratigraphy, based upon the primary mapping unit, the formation, is proposed. It is recommended that some classes of lithogenetically-defined deposits which, at present cannot be accorded formational status, should be assigned informally to one of the proposed groups or subgroups. The framework distinguishes one superficial deposits supergroup within which seven groups are defined: i) Crag Group, marine deposits (Late Pliocene to Early Pleistocene); ii), Dunwich Group, fluvial deposits (pre-Anglian/pre-Elsterian); iii) Residual Deposits Group; iv) British Coastal Deposits Group, coastal and marine deposits (Anglian to Holocene); v) Britannia Catchments Group, fluvial, organic and mass movement deposits (Anglian to Holocene) within broadly defined catchment areas related to Late Devensian to present-day physiography; vi) Albion Glacigenic Group, glacigenic deposits (pre-Devensian/pre-Weichselian), and vii) Caledonia Glacigenic Group, glacigenic deposits (Devensian/Weichselian). North of the Devensian (Weichselian) ice-sheet limit, a series of glacigenic subgroups are defined geographically for the two glacigenic groups on the basis of mappable formations of till. The subgroups include associated formations of glaciofluvial and glaciolacustrine deposits. Consequently some of the glacigenic water-lain units may extend beyond the Devensian limit. Catchment subgroups of the Britannia Catchments Group are proposed for formations and lithogenetic units defined within broad present-day physiographic regions by major river drainage systems that have developed since Middle Pleistocene time. Lithostratigraphical description and correlation of formations will aid the refinement of the proposed framework and enable the development of lithostratigraphical maps and three-dimensional models. As well as offering a unified framework for onshore Quaternary and Neogene deposits the proposed supergroup, group and subgroup structure may prove useful for a wide range of regional applications (e.g. hydrological, hydrogeological, engineering).

Polish Pleistocene stratigraphy - A review of interglacial stratotypes

AbstractThe Pleistocene stratigraphy of Poland is based on palynological analyses of buried organic lake sediments and dating with radiocarbon or thermoluminescence (TL), as well as on the lithologic-petrographical analyses of tills and other glacigenic deposits to which TL-dating was also applied. Rarely were sediments of the stratotype interglacial profiles to which palaeomagnetic methods and TL-dating were applied, examined by mineralogical, chemical and isotopie б180 and б13С analyses, except the Augustovian, Mazovian and Eemian interglacials. The data was recorded in both exposures and boreholes. Overall, tills of 8 glaciations have been identified: Narevian (Menapian), Nidanian, Sanian 1 (Elsterian 1), Sanian 2 (Elsterian 2), Liviecian (Fuhne), Krznanian (Drenthe), Wartanian (Warthe) and Vistulian (Weichselian). Substantial palynological evidence with complete pollen sequences and absolute dating exists for organic sediments of five interglacials: Augustovian (Bavelian Complex or Cromerian I), Ferdynandovian (Voigtstedt), Mazovian (Holstein), Zbójnian (Dömnitz) and Eemian. Other interglacials i.e. Malopolanian (Cromer Complex - II or III Interglacial) and Lubavian (Schöningen) are not sufficiently supported palynologically and especially the stratigraphie position and age of the Lubavian Interglacial is debated.

Facies and Architecture of a Carboniferous Grounding-line System From the Guandacol Formation, Paganzo Basin, Northwestern Argentina

New outcrops of Middle Carboniferous glacigenic deposits found in the Guandacol Formation (western Paganzo Basin) are described in this paper. The study locality of Los Pozuelos Creek (northwestern Argentina) includes coarse-grained diamictites, rhythmites, laminated pebbly mudstones and shales that represent an expanded column of the Gondwanic glaciation in this region. Thirteen lithofacies recorded at the measured section have been grouped into three facies associations. Facies Association I is composed of coarse-grained massive and stratified diamictites (lithofacies Dmm, Dms, Dmg, Dcs), laminated siltstones with dropstones (Fld) and interstratified sandstones and mudstones (Fl, Sr). These rocks represent both tillites and resedimented diamictites closely associated to small water bodies where laminated siltstones with dropstones and stratified sandstones and mudstones were deposited. Facies Association II comprises couplets of matrix-supported thinly bedded diamictites (Dmld) and laminated mudstones with dropstones (Fld). This facies association results from the combination of three different processes, subaqueous cohesionless debris flows, coeval rainout of ice-rafted debris and settling of fine-grained particles from supension. Finally, Facies Association III is made up of laminated mudstones without dropstones, thin marl levels and scarce fine- to very fine-grained sandstones. This assemblage clearly suggests sedimentation in a deep marine environment below the wave base. The architecture of the glacigenic deposits has been investigated using photomosaic panels. The geometry of the depositional bodies and facies suggest that Los Pozuelos Creek outcrops exhibit a well preserved three-dimensional example of a grounding-line system. In particular, three different subenvironments of a morainal bank were interpreted: a bank-front, a bank-core and a bank-back. The bank-front assemblage is characterized by coarse-grained, mainly resedimented, diamictites grading laterally to prograding clinoforms composed of interbedded matrix-supported thinly bedded diamictite and mudstones. The bank-core assemblage is formed by a stacking of coarse-grained diamictites where at least five major erosional surfaces, bounding four multistory diamictite bodies, can be recognized. Finally, the bank-back assemblage corresponds to discontinuous intervals of striated lodgement till, and coarse-grained resedimented diamictites showing important post-depositional deformation. The retrogradational stacking of the morainal banks indicate an overall glacial retreat and a glacioeustatic sea-level rise. Erosional surfaces at the base of each morainal bank suggest intervening short term episodes of ice advance. The new data presented here confirm the existence of "true" tillites in western Paganzo Basin and suggest several (at least four) pulses of glacial advance and retreat during the Namurian glaciation in the region and permit a more refined interpretation of the glacial deposits in the Huaco area.

New evidence for a grounded Irish Sea glaciation of the Isles of Scilly, UK

New geomorphological and sedimentological data support previously published reconstructions of an Irish Sea Glacier advance as far south as the northern shores of the Isles of Scilly in the Celtic Sea (49°58′N). Offshore, boulder-covered bars with cores of diamicton are interpreted as moraines, which together with onshore moraine ridges and trimlines mark the maximal extent of the Irish Sea Glacier. At Bread and Cheese Cove, St. Martin's, a heterogeneous sediment unit, wedged between periglacial gelifluctates, is interpreted as a mixture of glacitectonized and cannibalized glacilacustrine and marine deposits as well as primary subglacial tills. Macrofabric signatures reflect a subglacial origin, but the whole unit has been subject to down-slope mass movement. Micromorphological analysis indicates highly variable response to deformation and glacitectonism, with most features relating to ductile behaviour and wet deformation. The characteristics suggest deposition during a short-lived advance of the Irish Sea Glacier over pre-existing marine and contemporaneous proglacial lacustrine sediments. Once blocks of stratified sediment and diamicton had been driven onshore, they were subject to paraglacial remobilization, resulting at most locations in complete disaggregation. The distribution of re-worked glacigenic sediments coincides with the glacial limits inferred from the geomorphological evidence.

Mixed diatom assemblages in glacigenic sediment from the central Ross Sea, Antarctica

We have investigated diatom assemblages and their distribution in piston and trigger cores from six sites in the central Ross Sea, including analysis of absolute abundance of diatom valves and diatom fragments, with the aim of characterising different glacigenic sediment facies and sediment packets. These data provide a new perspective on the current texturally based classification of diamicton, mud, and diatomaceous mud. All samples investigated include diatom taxa characteristic of the modern Ross Sea flora, thus all deposits evaluated are interpreted to be upper Quaternary in age. Reworked diatoms are ubiquitous, but they vary in terms of species content and concentration. Sediment units classified as diamicton are enriched in robust reworked valves of long-ranging taxa such as Paralia spp., Stephanopyxis spp., and Stellarima microtrias, but different diamicton packets can be distinguished based on variable concentrations of biostratigraphically constrained taxa including Denticulopsis spp., Thalassiosira spp., and Actinocyclus spp. Absolute abundance of diatoms and diatom fragments provide another useful criterion for distinguishing sedimentary processes. Together, these criteria permit interpretations of past glacial and glacial marine processes with greater certainty than allowed by textural analyses alone. We are able to distinguish between tills deposited beneath ice streams from those emplaced by slow-moving ice—information that is important in ice sheet reconstruction.

Actual paraglacial progradation of the coastal zone in the Kongsfjorden area, western Spitsbergen (Svalbard)

Abstract This research was carried out on the Brøgger Peninsula, northwest Spitsbergen, Svalbard (79°N 12°E). In the western part of Spitsbergen, cold-based valley glaciers have retreated more than 1 km from their Little Ice Age limits, and glacial meltwater has extensively reworked glacigenic sediments on exposed glacier forelands. In such areas, a paraglacial sediment transport regime has become predominant, with runoff as the dominant process. A combination of GIS, DEM, aerial photographic and field data was employed to estimate shoreline progradation at sandur outflows. Average shoreline progradation is estimated to amount to 3 m/annum over the last 30 years, a period of uninterrupted sediment provision from the glacial runoff system.

Seismic stratigraphy of the Plio-Pleistocene Ross Island flexural moat-fill: a prognosis for ANDRILL Program drilling beneath McMurdo-Ross Ice Shelf

Ross Island volcanic complex began forming with the emplacement of the basaltic shield volcanoes of Mt. Bird and Mt. Terror between ca. 4.6 and 1.3 Ma, though it has developed most significantly over the last 1 Ma during an eruptive phase resulting in the 3794-m-high composite vent of Mt. Erebus. Throughout this time, loading of the lithosphere at the southern end of the Terror Rift by the Ross Island volcanic pile has progressively depressed the crust, resulting in a subcircular flexural moat around the periphery of the island. Multichannel seismic reflection data collected from the McMurdo-Ross Ice Shelf (MRIS) reveal the stratigraphic architecture of the moat-fill on the southeastern side of Ross Island. The moat region has accommodated a well-stratified, regionally extensive sedimentary succession of at least 1.2 km below the seafloor in the deepest part of the depression. Three seismic stratigraphic units are identified that generally thicken and dip towards Ross Island and are bounded by angular (onlap) unconformities. We infer that the three units were deposited in accommodation space created during discrete phases of volcanic load-induced subsidence: 1. Unit III. Moderate to low-amplitude discontinuous reflectors are dislocated and tilted by normal faulting and interpreted to represent coarse-grained glacigenic and fine-grained marine sediments with likely intercalated volcanic ash. These strata may have started to accumulate during loading of the crust by Mt. Bird between ca. 4.6 and 3.8 Ma. 2. Unit II. Moderate to high-amplitude continuous reflectors that onlap Unit III and are interpreted to represent coarse-grained glacigenic and fine-grained marine sediments with likely intercalated volcanic ash. These strata are inferred to have accumulated in the crustal depression resulting from loading by Mt. Terror between ca. 1.8 and 1.3 Ma. 3. Unit I. Relatively continuous low-amplitude to seismically-opaque reflectors (Unit IB), onlap Unit II and grade upwards into moderate to high amplitude reflectors below the seafloor (Unit IA). Unit IB is interpreted to represent fine-grained pelagic marine sediments and volcanic ashes which may have accumulated during a phase of rapid subsidence at the initiation of Mt. Erebus loading between ca. 1.0 and 0.8 Ma. Unit IA is interpreted to represent progressive infilling of the flexural moat with more coarse-grained, probably glacigenic sediments during continuation of Mt. Erebus loading between ca. 0.8 and 0.2 Ma. These phases of load-induced subsidence may have periodically overdeepened the moat, making it likely that the sedimentary fill is relatively continuous and has largely escaped erosion by ice grounding during past glacial expansions of the West Antarctic Ice Sheet (WAIS) and the MRIS. The age relationships provided by radiometric dating of the various volcanic loads imply that the sediments here have the potential to provide a relatively continuous and high-resolution glacimarine record of the past behavior of the WAIS–MRIS and the climate history of the western Ross Sea region back to ∼5 Ma. This stratigraphic record is scheduled to be drilled by the ANDRILL Program in the austral summer of 2006. From the interpretation of seismic facies, we present a stratigraphic prognosis for the proposed drill site.

Map and GIS database of glacial landforms and features related to the last British Ice Sheet

A review of the academic literature and British Geological Survey mapping is employed to produce a ‘Glacial Map’, and accompanying geographic information system (GIS) database, of features related to the last (Devensian) British Ice Sheet. The map (1:625000) is included in a folder and GIS data are freely available by web download (http:www.shef.ac.ukgeographystaffclark_chrisbritice.html). Emphasis is on information that constrains the last ice sheet. The following are included: moraines, eskers, drumlins, meltwater channels, tunnel valleys, trimlines, limit of key glacigenic deposits, glaciolacustrine deposits, ice‐dammed lakes, erratic dispersal patterns, shelf‐edge fans and the Loch Lomond Readvance limit of the main ice cap. The GIS contains over 20 000 features split into thematic layers (as above). Individual features are attributed such that they can be traced back to their published sources. Given that the published sources of information that underpin this work were derived by piecemeal effort over 150 years, then our main caveat is of data consistency and reliability. It is hoped that this compilation will stimulate greater scrutiny of published data, assist in palaeoglaciological reconstructions and facilitate use of field evidence in numerical ice‐sheet modelling. It may also help direct field workers in their future investigations.