Subaqueous Outwash Research Articles

Depositional Sequences of the Itararé Group in the Region of Mafra (SC) and their Regional Correlation

The Itararé Group, Permocarboniferous of the Paraná Basin, has its deposition associated with the Late Paleozoic Ice Age (LPIA), which encompassed multiple glacial advance-retreat cycles. Recognizing the nature of the processes that form these rocks is essential to understand the diversity of glacial and non-glacial depositional systems that were active during the LPIA. Previous authors have carried out sedimentological and stratigraphic studies in outcrops of the Itararé Group in the Santa Catarina and Paraná states, however, the integration between these areas and the delimitation of glacial cycles is still little known. Seeking to increase the knowledge on the stratigraphy of the Itararé Group, this work aims to investigate its facies in five shallow boreholes in the Mafra region, northern Santa Catarina state. In this region, 300 meters of cores sampled almost the entire stratigraphic succession of the Itararé Group. We further integrate our results with published data from Alfredo Wagner, Vidal Ramos, Presidente Getúlio, Doutor Pedrinho and São João do Triunfo in order to build a regional stratigraphic framework. A total of 33 sedimentary facies were recognized and organized in five genetic associations, corresponding to subaqueous outwash fans, rain-out and mud settling, thin-bedded turbidites, mass- transport deposits, and thick-bedded turbidites. The regional stratigraphic correlation allowed the recognition of 5 glacial cycles, corresponding to depositional sequences of deglaciation. They show signs of glacial influence that diminish toward the top, where deltaic deposits developed as climatic conditions improved. Diamictites are present in all areas and distributed preferentially in the lower and middle sequences. They consist of mass- transport deposits with origin attributed to gravitational instability due to sediment accumulation in melting and glacial retreat phases.

Morphology and sedimentology of the Rooskagh Esker, Co. Roscommon

Morphological and sedimentological investigations of the Rooskagh Esker, Co. Roscommon, indicate that there is a link between esker morphology and depositional environment. Sharp-crested ridges are underlain by glaciofluvial sediments laid down in subglacial tunnels and unroofed channels. Flat-topped segments are deltas, while fan-shaped areas are underlain by subaqueous outwash fans. Variation in morphology indicates that the esker is formed from repeated tunnel-to-ice-margin sections and is time-transgressive, indicating that the ice-marginal zone was characterised by stagnation zone retreat northward. The ice margin terminated in a proglacial lake.

The paths and timing of late Paleozoic ice revisited: New stratigraphic and paleo-ice flow interpretations from a glacial succession in the upper Itararé Group (Paraná Basin, Brazil)

This paper examines a glacial diamictite-bearing succession from the upper Itararé Group (Taciba Formation) in eastern Paraná Basin, Brazil. The object of study provides the opportunity to investigate in detail the late stages of glacial sedimentation during the Late Paleozoic Ice Age (LPIA) in this sector of SW Gondwana, with implications for glacial cyclicity and regional paleo-ice flow reconstructions. Sedimentology, geological mapping and palynological analysis allowed the recognition of four facies associations, comprising subaqueous outwash, mass-transport, tide-influenced delta-front, and tide-influenced delta plain deposits. The succession records at least two episodes of ice-margin advance and retreat into a marine-influenced environment and can be placed in the earliest Permian based on the palynomorph assemblage. Cross stratification in outwash facies and deltaic deposits indicate sediment transport to the SW, the same revealed by deformational structures in mass-transport diamictites derived from downslope resedimentation of glaciomarine sediments during deglaciation. A glacial source to NE is therefore indicated and is in agreement with paleo-ice flow directions obtained from previously studied localities to the north. This north-derived early Permian glaciation contrasts with glacial sources to the SE recognized in the lower and middle intervals of the Itararé Group. The scenario suggests a geometric reconfiguration of the Paraná Basin during the LPIA and the migration of ice centers with time during the Late Paleozoic Ice Age in SW Gondwana.

Late Palaeozoic glacial cycles and subcycles in western Gondwana: Correlation of surface and subsurface data of the Paraná Basin, Brazil

The Paraná Basin, the largest basin in South America, received glacially derived sediments during the Late Palaeozoic Ice Age (LPIA) of the Gondwana supercontinent. Despite the importance of this basin for understanding the continental development of the Gondwana glaciation, and the fact that ca. 95% of this basin is not exposed at the surface, few attempts have been made to connect the exposed glacial strata to the subsurface record. In this paper, exposures of glacial cycles in the Upper Itararé Group in Santa Catarina State, southern Brazil, are analyzed, locally correlated and then linked to the three major glacial cycles previously described from subsurface studies along the basin. Together study areas (Doutor Pedrinho and Vidal Ramos) record five, partially comparable shorter glacial subcycles (relative to the major glacial cycles). These series comprise coarse-grained subaqueous outwash deposits, turbidite sand sheets, marine shales, and diamictites, the latter mostly derived from delta slope failure and ensuing resedimentation. In addition to sedimentological and genetic stratigraphic description and analysis, preliminary age determination based on the palynological content is also presented. Besides, a regional correlation of the described succession to the subsurface record is proposed based on well logs and core information. All the palynomorph associations identified from the exposed successions, which represent the upper third part of the Itararé Group, are related to the Subzone Protohaploxypinus goraiensis, base of the Vittatina costabilis Zone. This zone and correlated ones along the Gondwana are considered Early Permian in age. However, a first isotopic age recently obtained for the upper Itararé Group and published elsewhere is considered within a regional stratigraphic framework once it leads to new insights in terms of the LPIA time span recorded in the Paraná Basin.

Ice-Keel Scour Marks In the Geological Record: Evidence From Carboniferous Soft-Sediment Striated Surfaces In the Parana Basin, Southern Brazil

Abstract Soft-sediment glacially striated surfaces are common structures in ancient glacial successions, such as the Gondwanic Paleozoic record, and document ice erosion on non lithified beds. In most cases, they have been interpreted as the product of subglacial erosion beneath advancing marine or terrestrial glaciers, although direct evidence for glacial sedimentation in associated strata is infrequent. In this paper we document soft-sediment striated surfaces of Carboniferous age occurring in cross-laminated sandstones of the lower Itarare Group, southern Parana Basin, Brazil, and interpret them as scour marks generated by keels of floating ice. Striated surfaces occur in closely spaced, multiple stratigraphic horizons in the lower half of a deglacial sequence, whose facies characteristics indicate sedimentation in subaqueous outwash and deltaic settings. Surfaces are laterally discontinuous, commonly bordered by marginal berms, and in some cases capped by mudstones with dropstones; they have orientations that deviate from the regional paleo-ice flow indicated by glacial grooves carved on the pre-glacial basement. The characteristics observed are consistent with modern and Pleistocene ice-keel scour marks. In addition, soft-sediment striated surfaces from other Gondwanic localities have similar attributes, suggesting that ice-keel scour marks may be more common in the geological record than previously considered and that vertically repeated striated surfaces do not necessarily indicate multiple ice-sheet advances. Therefore, paleo-ice flow reconstructions based on soft-sediment striated surfaces need to be revaluated considering that the movement of free-floating ice masses is controlled by phenomena other than glacier advance.

Timing of the East Antrim Coastal Readvance: phase relationships between lowland Irish and upland Scottish ice sheets during the Last Glacial Termination

A submillennial ice readvance from upland centres of ice dispersal in west central Scotland into northeastern Ireland post-dates the retreat of lowland Irish ice immediately after the Killard Point Stadial (max. 16.5 cal ka BP). The dimensions of this southerly and westerly ice sheet readvance on the margins of the North Channel are reconstructed from subglacial bedform patterns, subglacial tectonic deformation of Tertiary lignite and glacigenic sediment, limiting moraines/outwash and glacial stratigraphy. Morainic ridges at Rams Island and Sandy Bay on the eastern margin of the Lough Neagh basin which mark this ice limit are perpendicular to a well-defined field of subglacial bedforms across east County Antrim. At the ice readvance limits glacial lakes impounded in the Lagan Valley were partially infilled with subaqueous outwash known as the Malone Sands. This water body drained south along the Dundonald/Comber Gap spillway providing sediment which formed extensive, late-glacial marine terraces at the northern end of Strangford Lough around 15–15.5 cal ka BP. The East Antrim Coastal Readvance is part of a much more extensive readvance southwards along the North Channel and adjacent lowlands associated with ice sheet reorganisation and ice sheet growth in west central Scotland. It is now termed the North Channel Readvance and may be similar in age to the Wester Ross Readvance moraines in northwestern Scotland.

Transient impacts of Little Ice Age glacier expansion on sedimentation processes at glacier-dammed Iceberg Lake, southcentral Alaska

Iceberg Lake, a glacier-dammed proglacial lake in southern Alaska, contains a 1,500+ year varve record complicated by a history of episodic lake-level changes associated with fluctuations in ice-dam thickness and position. To better understand the basinwide glaciolacustrine response to late Holocene climate variability, we collected five cores from two areas in the lake, including a previously unexamined deepwater area distal from inlet streams. Based on eight AMS 14C dates, and correlations among our cores and previously documented outcrops, we describe ~1,000 years of stratigraphy from each area. Deposition at both areas was dominated by fine-grained varves, but cores from the distal area uniquely contain coarser deposits, including rhythmites and graded sand beds, that we attribute to deposition of a subaqueous outwash fan-delta between ~1250 and 1650 AD. We attribute this event to thickening of the impounding glacier and consequent incursion of the glacier margin, and an associated lateral moraine, into the lake. This result suggests an early onset of the Little Ice Age (LIA) glacial advance in this region. Changes in basinwide circulation and sedimentation associated with this event probably caused minor thickening of varves used previously to reconstruct summer temperatures, reducing sensitivity of that record to early LIA cooling. The basinwide impact of this event illustrates the potentially significant spatial and temporal variability of lacustrine sedimentary processes in dynamic glacial landscapes.

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

AbstractGlacigenic sediments exposed in coastal cliffs cut through undulatory terrain fronting the Last Glacial Maximum laterofrontal moraine at Waterville on the Iveragh Peninsula, southwest Ireland, comprise three lithofacies. Lithofacies 1 and 2 consist of interdigitated, offlapping and superimposed ice‐proximal subaqueous outwash and stacked sequences of cohesionless and cohesive subaqueous debris flows, winnowed lag gravels and coarse‐grained suspension deposits. These are indicative of sedimentation in and around small grounding line fans that prograded from an oscillating glacier margin into a proglacial, interlobate lake. Lithofacies 3 comprises braided river deposits that have undergone significant syn‐sedimentary soft‐sediment deformation. Deposition was likely related to proglacial outwash activity and records the reduction of accommodation space for subaqueous sedimentation, either through the lowering of proglacial water levels or due to basin infilling. The stratigraphic architecture and sedimentology of the moraine at Waterville highlight the role of ice‐marginal depositional processes in the construction of morphostratigraphically significant ‘end moraine’ complexes in Great Britain and Ireland. Traditional ‘tills’ in these moraines are often crudely stratified diamictons and gravelly clinoforms deposited in ice‐proximal subaqueous and subaerial fans. Copyright © 2011 John Wiley & Sons, Ltd.

Middle and Late Pleistocene glacial lakes of lowland Britain and the southern North Sea Basin

Focusing on lowland Britain and the southern North Sea Basin, this article reviews the sedimentary and geomorphic evidence for the main glacial lakes inferred during the Middle and Late Pleistocene and evaluates their impacts on drainage-basin development. Glacial lakes are best known from glaciations during Marine Isotope Stages (MIS) 12, 6 and 2, although glacial lakes have also been inferred during MIS 10 and 4. Some lakes – for example, Bosworth, low-level Humber and the lakes of the eastern Fenland margin – are reconstructed from unequivocal sedimentary evidence, including rhythmites and subaqueous outwash, whereas others lakes – for example, Lapworth and Fenland – are inferred mostly from erosional features and remain to be substantiated. The largest known glacial lake developed in the southern North Sea Basin between an ice sheet to the north and a chalk bedrock ridge in the Strait of Dover area, first during the Anglian/Elsterian glaciation of MIS 12 and again during the late Wolstonian/late Saalian Drenthe glaciation of MIS 6. The palaeohydological impacts of lake drainage are thought to include cutting of the Strait of Dover as a result of catastrophic drainage from the North Sea Lake during MIS 12, incision of a number of gorges and river valleys in England, and diversion or even reversal of major rivers such as the Thames and the proto-Soar/Avon system. Recently, varve chronologies have been correlated with the Greenland ice-core record, although caution is needed to discriminate between varves and non-annual rhythmites. Future work on Pleistocene glacial lakes needs to test chronologies of lake development by luminescence dating of glaciolacustrine sediments deposited in non-ice-proximal locations – (1) fine-grained rainout deposits and (2) wave-rippled sands deposited in shallow water – and to model the impacts of glacial isostasy in order to reconstruct lake extents. All of this work should be based on the rigorous application of sedimentology to interpret sedimentary facies and depositional environments.

Re-advance of Scottish ice into the Solway Lowlands (Cumbria, UK) during the Main Late Devensian deglaciation

The complex glacial geomorphology and stratigraphy of the Solway Lowlands (Cumbria, UK) reflects dynamic ice flow during the Main Late Devensian Glaciation, with numerous ice-flow shifts, and re-advances now recognised. The final incursion of Scottish ice into the region (Scottish Re-advance) during a late stage of deglaciation has provoked widespread debate and even scepticism arising from ill-defined marginal limits, and a paucity of landforms, sediments and dates. In an attempt to resolve some of these issues this paper has applied both geomorphological and stratigraphic techniques to critically review evidence pertaining to the Scottish Re-advance. A major deltaic landform-sediment assemblage at Holme St. Cuthbert; a thin, patchy till sheet; and eskers at Thursby and Sowerby Wood verify that Scottish ice re-advanced into the Solway Lowlands at a late stage of deglaciation. The thin, patchy till sheet associated with the re-advance is indicative of a short-lived pulse, with ice thought to have flowed rapidly across water-saturated sediment and into proglacial lakes dammed up against higher ground to the east. The Holme St. Cuthbert delta, which evolved from a subaqueous outwash fan marks a major still-stand and development of a large ice-contact lake in the vicinity of Wigton. The extent of the glaciation is difficult to elucidate, with the till sheet disparately exposed as far east as Lannercost although identification of the Blackhall Wood Re-advance Till member probably limits its SE extent to just south of Carlisle. Coeval re-advance of Lake District ice is possible given that a final stage lobate movement of ice out of the Vale of Eden and into the Solway Lowlands is now recognised, however there is no direct stratigraphic correlation or dating control.

Significance of soft-sediment clasts in glacial outwash, Puget Sound, USA

Soft-sediment clasts composed of silt and clay are contained within glacial outwash sands in the Puget Sound, Washington State, USA. The outwash was deposited during ice retreat of the Cordilleran ice sheet around 17 cal kyr BP. The soft-sediment clasts have a distinctive and consistent morphology and disposition within the sand beds. The sedimentology, sedimentary structures and presence of soft-sediment clasts suggest sand was deposited as proglacial outwash with silts and clays deposited in meltwater pools. Following drying-out of the pools and subaerial cracking, lumps of silt and clay were excavated by meltwater and transported distally as soft-sediment clasts within high-density flows. The most likely final depositional setting is as a Salisbury-type ‘delta’ in which subaqueous outwash grades distally into deeper water. This interpretation shows the power of soft-sediment clasts to inform on past processes and palaeogeography for which there is often little evidence in the geologic record.

Glaciovolcanic landsystems and large-scale glaciotectonic deformation along the Brekknafjöll–Jarlhettur, Iceland

This paper documents the glaciovolcanic landsystem of the Brekknafjöll–Jarlhettur ridge in Central Iceland. Glaciolacustrine diamict is found beneath, and in association with, a complex assemblage of pillow lava, lava breccias and hyaloclastites. Three depositional environments are identified: glaciolacustrine fan, pillow lava dome, and hyaloclastite fan. These subaqueous environments occurred both simultaneously and at different times along the volcanic fissures which underlie the ridge and have given rise to a complex facies architecture. This facies architecture provides evidence that the ridge evolved in a time transgressive fashion during several episodes of volcanism, some of which may have been punctuated by periods of ice erosion. Associated with the ridge are large-diapiric folds in diamict and gravel which form by the loading and lateral displacement of saturated diamict beneath the developing volcanic pile. A depositional model is presented which emphasises the glaciolacustrine component and the time transgressive nature of the glaciovolcanic landsystem. Much of the eruption occurred in subglacial to englacial lakes or vaults, which were probably linked by water and sediment exchange. The initial subglacial vaults appear to have extended beyond the fissure limits and were infilled by glaciolacustrine diamicts, subaqueous outwash and the eruption of pillow lava. This was followed by the eruption of hyaloclastite sand and breccia forming an elongated fan.

Sedimentology, Stratigraphy and Micropaleontology of Pre- and Early-Champlain Sea Fine-Grained Facies from the Foster Sand Pit, Ottawa, Ontario

The Foster sand pit exposes pre- to late-Champlain Sea sediments containing 4 litho/biofacies. These are, from oldest to youngest: Faciès 1- cross-stratified sand and diamicton deposited as late-glacial, ice-proximal subaqueous outwash. Faciès 2- laminated silt and clay rhythmites with a sparse Candona cf. C. subtriangulata ostracode fauna deposited in a deep, proglacial lake preceding the Champlain Sea, Fades 3- massive to rarely laminated silty clay with dominant foraminifera Elphidium bartletti, Protelphidium orbiculare and Cassidulina crassa represents the first glaciomarine deposition in Champlain Sea water 30-100 m deep with a paleosalinity of 22-33 %o. Faciès 4- mixed bedding of sand and mud, with a microfossil assemblage dominated by the Elphidium/Protelphidium complex representing an intertidal offlap sequence. Water depths during deposition of Faciès 4 dropped to 10-30 m. and paleosalinity dropped to 18-28 %o.. This study recognizes a pre-Champlain Sea proglacial lake, assigns subaqueous outwash deposits to an earlier time than previously believed and documents changes in bottom water characteristics of the early Champlain Sea based on sedimentary and microfossil evidence.

Till Stratigraphy and Late Wisconsinan Deglaciation of Southern Maine: A Review

At least two glaciations are recorded by the till stratigraphy of southern Maine. A more deeply weathered lower till is tentatively correlated with the early Wisconsinan (or older) Nash Stream Till in New Hampshire and its inferred equivalents in southern New England and Québec. The Laurentide Ice Sheet flowed south-southeastward across southern Maine in late Wisconsinan time and deposited the upper till. By about 14,000 years ago the ice sheet started to recede from the Maine coast, and the high peaks of the Mahoosuc Range emerged as nunataks in western Maine. Marine transgression accompanied déglaciation of lowland areas of southern Maine, with deposition of end moraines, deltas, and subaqueous outwash along the active ice margin, while thick clay deposits of the Presumpscot Formation accumulated on the ocean floor. The ice margin retreated quickly, reaching the marine limit in central Maine by 13,000 yr BP. The Pineo Ridge moraine system in eastern Maine, formerly thought to represent a major readvance, is reinterpreted as a glacial stillstand near the marine limit. Deglaciation inland from the marine limit in eastern and southwestern Maine occurred by recession of an active ice margin in some areas, and elsewhere by stagnation and downwasting of ice that was separated from the active ice sheet. Southern Maine was ice-free by 12,000 yr BP. but marine submergence persisted until about 11,000 years ago in the southwestern coastal lowland.

A Basin Analysis of the Wabigoon Area of Lake Agassiz, a Quaternary Clay Basin in Northwerstern Ontario

Information from a wide range of sources is integrated in a basin analysis of the Wabigoon Basin, a Quaternary clay basin located on the Canadian Shield in northwestern Ontario. The basin sediments were deposited between 10.9 ka and 9.5 ka, along the margin of the Rainy Lobe of the Laurentide Ice Sheet, which formed the northern boundary of proglacial Lake Agassiz. The basin architecture is dominated by four major elements: end moraines, eskers, kames and a clay plain, all of which overlie irregular bedrock topography. End moraines, eskers and kames are composed mainly of a fining upward sequence of gravels and sands. The geometry of these sedimentary units, and their sedimentary structures indicates they were deposited mainly by high and low-density turbidity currents, on ice-marginal subaqueous outwash fans. Eskers contain a core of coarse gravel and sand deposited within subglacial meltwater conduits, overlain by subaqueous fan sediments deposited at the conduit mouth. Esker ridges were formed during conduit filling events and flanking deposits were formed when a conduit remained in use during ice-marginal retreat. Where conduits were shortlived, isolated subaqueous fans (kames) were formed. A depositional model is proposed which relates moraine formation to catastrophic releases of subglacial meltwater and sediment simultaneously along the entire margin of the Rainy Lobe. The clay plain forms a broad blanket of fine-grained, rhythmically-bedded sediment which obscures bedrock topography, and often buries esker and kame deposits. Seismic profiles and overburden drilling reveal deep (50-70 m) bedrock lows beneath the clay plain. These lows, oriented sub-parallel to the ice margin, acted as sediment traps, and were infilled by the deposits of underflows generated at the ice margin.

Advances in Arabian stratigraphy: Origin and stratigraphic architecture of glaciogenic sediments in Permian-Carboniferous lower Unayzah sandstones, eastern central Saudi Arabia

ABSTRACT In the subsurface of eastern central Saudi Arabia, deposition of lower Unayzah sediments commenced in late Carboniferous (late Moscovian = late Westphalian-Stephanian) times during the earliest stages of the late Paleozoic Gondwanan glaciation. The Unayzah C member is the lowermost unit, and comprises quartzose sandstones laid down on an extensive glacio-fluvial outwash braidplain upon the pre-Unayzah unconformity (so-called “Hercynian unconformity”). They were overridden during (probably several) glacial re-advances, when the sediments were thrust over each other during the construction of push moraines ahead of the advancing ice sheets. The upper surface of the Unayzah C member represents the final sub-glacial contact of this longlived series of glacial advances and retreats, and demonstrably represents a significant post-Unayzah C unconformity. Subsequent (early Permian) deposits represent the final glacial retreat phase. They include the Unayzah B member and a new, un-named middle Unayzah member that underlies the Unayzah A member. The Unayzah B member is dominated by glaciogenic sediments, including highly deformed material attributed to seasonal push moraines (and hence representing minor glacial re-advances); ice-proximal, subaqueous outwash fans (high-energy lacustrine turbidites with associated dropstones, and massive diamictites); and ice-distal, glaciolacustrine deposits including laminites, stratified diamictites, ripple cross-laminated sandstones and mudrocks with associated dropstones. In contrast, the un-named middle Unayzah member is characterized by widespread, fine-grained red-beds, representing low-lying alluvial floodbasin deposits. Embedded within these occur aeolian and fluvial sandstones. A significant drainage event is thus implied between the Unayzah B member and the un-named middle Unayzah member. It is marked by a sharp contact between the two members that represents a regionally correlatable disconformity. The end of lower Unayzah deposition is represented in places by paleosol development, indicating a further disconformable contact, separating the un-named middle Unayzah member from the overlying Unayzah A member. This study significantly extends the record of the advance and retreat of the Gondwanan ice sheets in southern Saudi Arabia, and provides sufficient confidence to propose a degree of stratigraphic correlation with coeval deposits in Oman.

Onshore and offshore seismic and lithostratigraphic analysis of a deeply incised Quaternary buried valley system in the Northern Netherlands

High-resolution seismic data (onshore and offshore), geophysical borehole data as well as detailed lithofacies from airlift boreholes were acquired in northern Netherlands on and around the island of Ameland. Marine and land seismic data combined with information from land boreholes have been explored with the objective of providing a sedimentary model. Qualitative seismic facies analysis of the valley fill commonly shows a thin unit with high amplitude reflectors at the base. Thick units of variable seismic facies (transparent to high amplitude) occur higher up in the sequence. Onlap is common at mid–upper levels within the sandy valley fill (with clay in mm layering), and a transparent seismic facies, corresponding to firm clays, is common at the top. Almost all lithological unit boundaries recognised within core parameters correspond with seismic unconformities within error margins. Subunits contain multiple cyclical trends in gamma ray and grain size. Cyclical trends show lower order fluctuations in gamma radiation on a scale of less than 1 m. Gamma-ray pattern variability between units, e.g. in general coarsening-up or fining-up units, suggests migration of subaqueous outwash fans or ice margin fluctuations. Seismic results could support a headward excavation and backfilling process suggested by Praeg [Morphology, stratigraphy and genesis of buried Elsterian tunnel valleys in the southern North Sea basin [PhD thesis]: University of Edinburgh, 207 pp.; Journal of Applied Geophysics, (this volume)] as being responsible for the formation of buried valleys. On a lithological scale, a more complicated, detailed and cyclical pattern arises. Catastrophic processes are considered unlikely as being responsible for the infill sequence because of the observed small-scale facies variability and because of the presence of diamicton layers. Diamicton layers at the base of basal unconformities as well as higher in sequence could suggest subglacial deformation by grounded ice before and during the valley-fill process.

Superfícies estriadas em arenitos do Grupo Itararé produzidas por gelo flutuante, sudeste do estado do Paraná

Permocarboniferous glacial pavements has already been found in several places in southern Brazil. In the southeast of Parana State there are many occurrences, as the well-known striated surface produced over Devonian basement rocks in the locality of Witmarsun. However, most of the pavements are in fact soft-sediment striated surfaces preserved in the lower part of the glacial Itarare Group itself. At the base of a deglacial sequence exposed in the valley of the Salto River near the Palmeira town, several soft-sand striated surfaces are distributed in four different stratigraphic levels inside cross-stratified sandstones interpreted as deposits of subaqueous outwash fans. Scour marks indicate paleoglacial movement towards the azimuth 341o, coherent with the advance of glaciers postulated in regional paleogeographic reconstructions for Permocarboniferous of south Brazil. The surfaces are discontinuous, sometimes slightly concave and associated with linear grooves, and the preservation was favored by thin laminae of fines. These characteristics and the fact that they are not covered by coarse facies like diamictites, allow us to interpret them as soft-sand striated surfaces produced by floating ice. The highly consistent orientation of striations and grooves suggests that they were formed near the grounding zone of an ice shelf in a marine environment, but the surfaces could be ice keel scour marks produced by free-floating ice masses.

Facies architecture within a regional glaciolacustrine basin: Copper River, Alaska

This paper defines the principal architectural elements present within the Pleistocene, glaciolacustrine basin-fill of the Copper River Basin in Alaska. The Copper River drains an intermontane basin via a single deeply incised trench through the Chugach Mountains to the Gulf of Alaska. This trench was blocked by ice during the last glacial cycle and a large ice-dammed lake, referred to as Lake Atna, filled much of the Copper Basin. Facies analysis within the basin floor allows a series of associations to be defined consistent with the basinward transport of sediment deposited along calving ice margins and at the basin edge. Basinward transport involves a continuum of gravity driven processes, including slumping, cohesive debris flow, hyperconcentrated/concentrated density flows, and turbidity currents. This basinward transport results in the deposition of a series of subaqueous fans, of which two main types are recognised. (1) Large, stratified, basin floor fans, which extend over at least 5 km and are exposed in the basin centre. These fans are composed of multiple lobes, incised by large mega-channels, giving fan architectures that are dominated by horizontal strata and large, cross-cutting channel-fills. Individual lobes and channel-fills consist of combinations of: diamict derived from iceberg rainout and the ice-marginal release of subglacial sediment; multiple units of fining upward gravels which grade vertically into parallel laminated and rippled fine sands and silts, deposited by a range of density flows and currents derived from the subaqueous discharge of meltwater; and rhythmites grading vertically into diamicts deposited from a range of sediment-density flows re-mobilising sediment deposited by either iceberg rainout or the ice-marginal release of sediment. (2) Small, complex, proximal fans, which extend over less than 2 km, and are exposed in the southern part of the basin. These fans are composed of coalescing and prograding lobes of diamict and gravel deposited both directly by subaqueous meltwater and from sediment-density flows. These lobes are cross-cut by a range of sand and gravel-filled troughs and channels cut by subaqueous outwash, and either overlie or are overlain by horizontal sheets of gravel and diamict deposited from a range of sediment-density flows. The fans are, therefore, characterised by a complex, and laterally variable facies, architecture. Water depth, proglacial topography, stability of meltwater portals and sediment supply may all be important in determining the type of subaqueous fan present at any one location. We suggest that the Copper River basin-fill is dominated by packages of sediment containing multiple subaqueous fans with individual fans separated by units of diamict. Each sediment package is in turn separated from the next by a palaeo-landsurface shaped by interstadial/interglacial fluvial processes and by volcanic debris flows.

Sedimentology of a glaciofluvial landsystem, Lough Ree area, Central Ireland: implications for ice margin characteristics during Devensian deglaciation

Eskers in a glaciofluvial landsystem in the Lough Ree area, Central Ireland, exhibit two orientations. The most southerly esker in the area, the Athlone Esker, forms part of a dendritic esker system with paleocurrent directions indicating eastward drainage, while the Rooskagh Esker and associated eskers and kames immediately to the north were formed during southward drainage. Sediments indicate that sharp-crested, steep-sided sections of both ridges were formed within subglacial tunnels, while fan-shaped and flat-topped areas were formed as subaqueous outwash fans and deltas in standing water immediately in front of an ice-margin. The deposition of ice-marginal deposits indicating southward paleoflows against the side of the eastward flowing Athlone Esker indicates a 90° shift in the direction of ice surface slope, and a similar shift in the likely orientation of the ice margin. The absence of any evidence of intermediate ice- or water-flow directions indicates that the shift reflects a recession of ice, followed by a readvance from the north.