Glaciodynamically upthrusted bands of englacially transported debris in the Pleistocene of central Poland

Two kinds of buried structures are described from Dzirżenin, north-east of Warsaw, where they occur within a glaciofluvial landform: (1) narrow till ridges, showing vertically oriented structures, excavated from stratified gravel and sands; and (2) a narrow vertical zone of massive gravelly/sandy material, involving vertically oriented lens-like layers composed of massive sand with pebbles, or of diamicton. The gravelly/sandy zone is also closely surrounded by stratified glaciofluvial sediments. In spite of their vertical position and internal deformation, the till ridges and gravelly/sandy zone show non-tectonic contacts with the surrounding, stratified, undisturbed sediments. The glaciofluvial sediments that occur immediately next to the structures under discussion are characterized by the occurrence of comparatively coarse material and interbeddings of diamicton, which wedge out away from these structures. The gravelly/sandy zone separates different kinds of water-laid deposits. The buried structures are interpreted as former debris-laden bands, thrust upwards within the frontal part of the ice sheet and then transformed into still-frozen debris ridges projecting over the already dead ice. Further melting of the decaying ice resulted in abundant glaciofluvial sedimentation, and the debris ridges also supplied material for the deposition of the neighbouring stratified deposits. One of the ridges separated different glaciofluvial environments. The glaciofluvial sediments completely buried the ice-cemented ridges, which were finally transformed by a melting-out process into the till ridges and the gravelly/sandy zone. The former are interpreted as having been transformed from upturned debris-laden bands with a high concentration of debris or from the bands composed of frozen-up sediment slabs. The gravelly/sandy zone is interpreted as having (most probably) been deposited from upturned bands characterized by a lesser concentration of debris.     

Jet-efflux deposits of a subaqueous ice-contact fan,glacial Lake Rinteln,northwestern Germany

The coarse-grained, ice-contact, Porta Subaqueous Fan/Delta Complex was deposited in glacial Lake Rinteln at the margin of the Saalian ice sheet that advanced south of the Weser Chains, NW Germany. The ice-proximal depositional system was up to 15 km long and 10 km wide. The present study deals with ice-proximal subaqueous fan deposits, which are interpreted as products of a subcritical plane-wall outflow jet that periodically passed into a supercritical jet with hydraulic jump. The proximal facies assemblage consists of the coarse, clast-supported gravelly deposits of a hyperconcentrated (high-density) effluent and of related cohesionless debris flows attributed to the conduit or immediate proximal jet outflow zone of flow establishment. The intermediate facies assemblage, attributed to the outflow jet proximal zone of flow transition, is dominated by normally graded and cross-stratified gravels with scour structures at their bases; these gravels were deposited by a high-density effluent capable of forming mouthbar-like features. These deposits pass downcurrent into an assemblage of planar parallel-stratified and planar and trough cross-stratified sands and pebbly sands (partially interpreted as antidunes), with abundant scour structures and intercalated layers of fine sand/silt and silty mud, attributed to the jet distal zone of flow transition. The distal facies assemblage consists of trough cross-stratified sands and pebbly sands, and is attributed to the outflow jet proximal zone of established flow. The sedimentary succession as a whole has wedge-shape geometry, with a gentle fan-shaped inclination of the bedding from the southeast to the southwest. Repeated vertical alternations of supercritical and subcritical deposits and muddy interlayers can be attributed to temporary fluctuations in the meltwater outflow, whereas the overall upward fining of the succession indicates a net decline of meltwater discharges.     

泥石流堆积物中细颗粒含量与渗透系数关系试验研究

泥石流拦砂坝通常建造在沟床堆积物上。泥石流堆积物的渗透性是影响坝底扬压力的关键因素。目前关于土体渗透性的研究主要集中于无黏性粗粒土,对泥石流堆积物这种宽级配土体渗透性的研究比较缺乏。选择北川县泥石流沟床堆积物作为试验材料,通过渗透试验,确定了影响宽级配土渗透性的细颗粒上限粒径;在此基础上,通过试验研究了细颗粒含量与渗透系数的关系。结果表明,泥石流堆积物中粗颗粒仅起骨架作用,细颗粒才是决定其渗透性能的关键;显著影响宽级配土渗透性的细颗粒上限粒径为0.1 mm;细颗粒（<0.1 mm）含量与渗透系数呈负指数关系,并且当细颗粒含量超过20%以后,泥石流堆积土的渗透性趋于稳定。     

The surface texture of the Saharan Debris Flow deposit and some speculations on submarine debris flow processes

Deep towed 30 kHz sidescan sonar data from the Saharan Debris Flow deposit, west of the Canary Islands, show spectacular backscatter patterns which are interpreted in terms of flow banding, longitudinal shears, lateral ridges (levees) and transported blocks. Identification of these features is based on high resolution seismic profiles and on a comparison with similar structures seen in better known environments including other marine debris flows and slides, subaerial sediment failures (particularly rock fall avalanches), glaciers and lava flows. Flow banding in the debris flow, picked out by bands of differing backscatter intensity, is on a scale of tens to hundreds of metres. It is considered to result from flow streaming of clasts, with variation in clast size between bands. This primary fabric is cut by a series of distinct flow-parallel longitudinal shears. Broad, high backscatter longitudinal bands along the edge of and within the debris flow are interpreted as lateral ridges associated with multiple flow pulses; the high backscatter possibly reflects either a concentration of coarse grained material or chaotic sediments deposited from a turbulent flow. Coherent, low backscatter patches are interpreted as rafted blocks, although streamlined haloes of high backscatter material around some blocks indicates differential movement between block and flow, possibly during the waning stages of the flow. A non-turbulent debris flow model is preferred, in which a raft of more or less coherent material is carried along by a base undergoing laminar flow. Speculatively, the lack of turbulent mixing preserves original sedimentological heterogeneity from the debris flow source area, possibly in the form of clast size distributions. These heterogeneous sediments are drawn out into a flow-parallel banding which is imaged as the flow-parallel backscatter intensity banding. The upper raft of material responds to cross-flow velocity differences, and perhaps to variations in the timing of flow movement, primarily by longitudinal shearing. More complex deformation of the flow banding occurs at the flow margins and around obstacles in the flow, where lateral velocity shear would be expected to be highest.     

Re-evaluation of coquinoid sandstone depositional model, Upper Jurassic of central Wyoming and south-central Montana

The most extensive Jurassic marine transgression in North America reached its maximum limits during the Oxfordian Age. At this time, siliciclastic sediments were being brought into the North American seaway from an uplifted zone to the west. Within this setting, complexes of sand ridges and coquinoid sands layers were deposited. Coquinoid sandstones appear to fill erosional scours and were interpreted as channel fills. Re-evaluation of these features in the light of recently discovered attributes of modern shelf sediments and processes has produced a revised model of coquinoid sand deposition in this setting. Coquinoid sandstones which fill ‘channel-like’ scours in the Oxfordian (Upper Jurassic) rocks of central Wyoming and south-central Montana, appear to have formed through the migration of sand waves across the crests of inner shelf sand ridges during periods of storm and tidal flow. Erosion in the zone of flow reattachment in the troughs between sand waves resulted in the development of shell lags. Migration of these scour zones as the sand waves advanced resulted in the deposition of sheet-like coquinoid sandstone bodies. Sand waves crossing the ridge crest tended to migrate more slowly and to be overstepped by later sand waves. Sand wave troughs thus buried have channel-like geometries with apparent epsilon bedding.     

Processes of alluvial fan sedimentation,basal rozel conglomerate formation,la têcte des hougues,Jersey, Channel Islands

The Rozel Conglomerate Formation, a coarse alluvial fan deposit, overlies the Bouley Rhyolite Formation beneath the headland called La Tete des Hougues, on the north coast of Jersey, Channel Islands. Here the rhyolites form an exhumed palaeotopography which in places is smoothly, but irregularly, eroded. Locally however, there is preservation of a deep Lower Palaeozoic weathering profile. Breccia is preserved spalled from these weathered profiles as a fossil scree deposit, which has subsequently been hydraulically reworked in a palaeotopographic low. Overlying the breccia is a prograding succession of alluvial fan sediments. A 4m thick, upwards-coarsening sequence of silty sandstones and fine pebble conglomerates is succeeded by the incision of a 3m deep channel. This is overlain by a 28m thick, upwards-fining sequence of coarse debris flow and streamflood conglomerates. The lower unit is interpreted as a prograding fan sector of reworked fan material, whilst the upper unit is thought to reflect the avulsive initiation and gradual abandonment of a fan lobe sourced from a northern hinterland.     

The anatomy of a deep water mud-mound complex to the southwest of the Dinantian platform in Derbyshire, UK

ABSTRACT An elongate Waulsortian mud-mound complex developed at Dovedale on a ramp to the southwest of a developing carbonate platform in Derbyshire during Chadian (early Viséan) times. The complex occupied an area of approximately 6 km² and grew to a maximum relief of 80 m with longitudinal and transverse valleys developed near the southern margin. Five mound associated facies have been identified: mound core, mound flank (fine), mound flank (coarse), intermound (fine) and intermound (coarse). The mound core facies is a massive skeletal wackestone with comminuted sponge debris, foraminifera, ostracodes and crinoid debris set in a matrix of clotted micrite. The mound flank sediments display moderately inclined bedding surfaces. While the mound flank (fine) contains sponge debris, the mound flank (coarse) is dominated by articulated crinoid columnals, and includes algal-encrusted micritized intraclasts and coarse peloids. The well-bedded intermound (fine) facies is bituminous and micritic while the intermound (coarse) facies is composed of skeletal-peloidal-intraclast grainstones which locally contain calcified algae. Although the fauna is diverse, the density of colonization by metazoans was low and the supply of macrofossil debris modest. The clotted micrite texture is interpreted as the product of micro-organisms which precipitated and trapped fine-grained sediment. The mud-mound complex is dominated by the bathymetric assemblages B and C proposed by Lees, Hallet & Hibo which on their model of the Belgian Waulsortian, indicate depths of between 220 and 280 m. Intercalation of assemblages B/C and C/D on the northern margin of the complex is interpreted as the result of local storm disturbance. A deep water drift is postulated to explain the NW-SE alignment of the complex which probably fitted the ‘export model’ of Bosence, Rowlands & Quine. Beneath the sediment surface, phreatic flow eroded unlithified sediments and developed interconnected cavities which were filled by cement and sediment relatively eariy. Mound instability triggered the opening of fissures which filled with crinoid debris, peloids, indurated lithoclasts and micrite.     

Transformation,partitioning and flow–deposit interactions during the run‐out of megaflows

Four megabeds (I to IV) were recognized throughout the Cerro Bola inlier, a glacially influenced depositional area of the Carboniferous Paganzo Basin, south‐western La Rioja Province, Argentina. Such anomalous thick beds are associated with the collapse of an unstable basin margin after periods of large meltwater discharge and sediment accumulation. Failure of these previously deposited sediments triggered mass flows and associated turbidity currents into the basin. Megabed I is up to 188 m thick and was deposited during a transgressive stage by re‐sedimentation of ice‐rafted debris. Also part of the transgressive stage, Megabeds II, III and IV are up to 9 m thick and are associated with a dropstone‐free period of flooding. Megabeds were subdivided into three divisions (1 to 3) that represent a spectrum of flow properties and rheologies, indicative of a wide range of grain support mechanisms. These divisions are proposed as an idealized deposit that may or may not be completely present; the Cerro Bola megabeds thus display bipartite or tripartite organization, each division inferred to reflect a rheologically distinct phase of flow. Division 1 is a basal layer that consists of clast‐supported and matrix‐supported, pebble conglomerate, rarely followed by weak normally graded to ungraded, very coarse‐ to coarse‐grained sandstone. This lower interval is interpreted to be the deposit of a concentrated density flow and is absent in bipartite megabeds. Division 2 is represented by a mud‐rich sandstone matrix with dispersed granule to pebble‐size crystalline and mudstone clasts. It also includes fragments of sandstone up to boulder size, as well as rafts of cohesive muddy material and wood fragments. Division 2 is interpreted to be a result of debris‐flow deposition. A debrite‐related topography, resulting from the freezing of high yield strength material, captures and partially confines the succeeding upper division 3, which fills the topographic lows and pinches out against topographic highs. Division 3 is rich in mudstone chips and consists of very coarse‐grained, dirty sandstones grading upward to siltstones and mudstones. It is interpreted to be a deposit of a co‐genetic turbidity current. Spectral gamma ray and petrographic analyses indicate that both debrite and co‐genetic turbidite have high depositional mud content and are of similar composition. One of the megabeds is correlated with an initial slump‐derived debris flow, which suggests that the mass flow becomes partitioned both at the top, generating a co‐genetic turbidity current and, at the base, segregating into a concentrated density flow that seems to behave as a gravelly traction carpet.     

根据泥石流沉积物计算泥石流容重的方法研究

泥石流的容重是泥石流的最重要的参数之一,用简单易行的方法获得泥石流的容重因泥石流灾害的评估和泥石流的防治的需要显得日益重要。分析了泥石流的组成颗粒中3个分别代表粗颗粒,细颗粒和粘粒颗粒的颗粒粒径,即2 mm、0.05 mm和0.005 mm,及它们的百分含量与泥石流容重的关系。在对泥石流的组成颗粒的研究中发现泥石流中的粗颗粒,细颗粒和粘粒颗粒的百分含量与泥石流容重都有一定的关系,但因地区的差异这种关系不是唯一的。粘性泥石流中的粘粒含量与物源区的粘粒含量相当,稀性泥石流的粘粒含量远大于物源区的粘粒含量。用粗颗粒和细颗粒百分含量计算的泥石流的容重与泥石流观测样和沉积样对比有较好的一致性。根据泥石流沉积物计算的泥石流容重应服从根据沉积物的特征判断的泥石流类型和框定的泥石流容重范围。     

Application of electromagnetic techniques in survey of contaminated groundwater at an abandoned mine complex in southwestern Indiana,U.S.A.

In part of a large abandoned mining complex, electromagnetic geophysical surveys were used along with data derived from cores and monitoring wells to infer sources of contamination and subsurface hydrologic connections between acidic refuse deposits and adjacent undisturbed geologic materials.Electrical resistivity increases sharply along the boundary of an elevated deposit of pyritic coarse refuse, which is highly contaminated and electrically conductive, indicating poor subsurface hydrologic connections with surrounding deposits of fine refuse and undisturbed glacial material. Groundwater chemistry, as reflected in values of specific conductance, also differs markedly across the deposit's boundary, indicating that a widespread contaminant plume has not developed around the coarse refuse in more than 40 yr since the deposit was created. Most acidic drainage from the coarse refuse is by surface runoff and is concentrated around stream channels.Although most of the contaminated groundwater within the study area is concentrated within the surficial refuse deposits, transects of apparent resistivity and phase angle indicate the existence of an anomalous conductive layer at depth (>4 m) in thick alluvial sediments along the northern boundary of the mining complex. Based on knowledge of local geology, the anomaly is interpreted to represent a subsurface connection between the alluvium and a flooded abandoned underground mine.     

The utilization of seismic interpretation to identify and describe the shallow subsurface structures in the southeastern Mediterranean Sea

The shallow subsurface structures of the offshore Nile Delta particularly in the southeastern Mediterranean were dealt through the interpretation of 40 two-dimensional seismic reflection lines. The interpretations of seismic reflection data indicated that the principle sedimentary processes affecting the study area include three main structural groups according to their origin and development. The first group of structures comprises of gravity-driven structures, which include slides, slumps, turbidities, and debris flow. Slides are present in three different forms on seismic sections: slide sheets, slides with scar, and wedges of slide materials. Slumps have many geometrical shapes: lenses, spoon-shaped slumps, and slumped blocks bound by growth faults. Debris flows are present as transparent unit (due to the dispersion of seismic waves at debris boundaries), whereas turbidities appear on the seismic profiles, which are formed of closely spaced parallel thin reflectors analogous to their thin stratified bedded layers. The second group of structures is syn-depositional structures, which include growth faults, and tilted and rotated fault blocks. Growth faults are listric in shape and usually dip seaward; displacements along the fault plane increase with depth. Some of these faults are incipient, and some are complicated and intersected by secondary antithetic faults. Most of the growth faults soles out basin wards and in the evaporites layer. Fault blocks are formed due to the Messinian evaporite movement vertically and horizontally due to its mobility as a consequence to the pressure resulted from the overloading of Pliocene sediment. The third group of structures comprises evaporite flow structures such as diapiric structures and graben collapse structures. The surface of the Messinian evaporites was folded during its flow as a consequence to the lateral compression acted on the mobile strata of the Messinian evaporites to form diapiric triangular structures and creates a stress zone faulting and fractures system. These conditions led to the formation of collapse structures or graben collapse structures.     

Pseudo-ice-wedge casts of Connecticut,northeastern United States

Since 1965, ice-wedge casts have been reported in deposits of sand and gravel in Connecticut. These are wedge forms up to 1.1 m wide and many meters high. Most are single forms, not in polygonal array. They are found in adjoining states as well. Their distribution, dimensions, structure, and fabric and an assessment of the former physical environment preclude their origin as permafrost features. They appear to be tension fractures produced by the loading of coarse clastics on fine clastics near and below the water table where sediments creep toward a stream or depression. Locally movement started with kettle formation during deglaciation. However, some wedges cut horizontal layers of iron-coated sand and gravel and must be younger than those distinctly postglacial phenomena. Moreover, modern B horizons of the overlying soil have moved down into some wedges more than 2 m, indicating that fracturing is still active today. Complex fracture fillings in bedrock also have been attributed to a permafrost origin, but this too seems unlikely.     

Evidence for carbonate platform failure during rapid sea‐level rise; ca 14 000 year old bioclastic flow deposits in the Lesser Antilles

Bioclastic flow deposits offshore from the Soufrière Hills volcano on Montserrat in the Lesser Antilles were deposited by the largest volume sediment flows near this active volcano in the last 26 kyr. The volume of these deposits exceeds that of the largest historic volcanic dome collapse in the world, which occurred on Montserrat in 2003. These flows were most probably generated by a large submarine slope failure of the carbonate shelf comprising the south‐west flank of Antigua or the east flank of Redonda; adjacent islands that are not volcanically active. The bioclastic flow deposits are relatively coarse‐grained and either ungraded or poorly graded, and were deposited by non‐cohesive debris flow and high density turbidity currents. The bioclastic deposit often comprises multiple sub‐units that cannot be correlated between core sites; some located just 2 km apart. Multiple sub‐units in the bioclastic deposit result from either flow reflection, stacking of multiple debris flow lobes, and/or multi‐stage collapse of the initial landslide. This study provides unusually precise constraints on the age of this mass flow event that occurred at ca 14 ka. Few large submarine landslides have been well dated, but the slope failures that have been dated are commonly associated with periods of rapid sea‐level change.     

贺兰山苏峪口泥石流物源启动模型试验分析

文中通过模型试验,对试验现象中泥石流启动模式和堆积特征进行观测,研究了贺兰山苏峪口泥石流的形成受沟床坡度、土体含水率、粗颗粒含量三个因素影响的状况,初步探讨了贺兰山东麓泥石流在三个因素影响下的变化情况,试验结果表明：三个因素对泥石流影响由大到小分别是粗颗粒含量,沟床坡度、土体含水率;且粗颗粒含量越低、沟床坡度越大、含水率越高越容易发育泥石流。细颗粒含量较高时,泥石流的类型为沟道侵蚀型,粗颗粒含量较高时,泥石流类型为堵溃型。     

Terrestrial-marine transition in the Cambrian rocks of Kangaroo Island, South Australia

The upper part of the Lower Cambrian succession in northeast Kangaroo Island comprises three interbedded facies associations. The fine-grained association is composed of siltstone, mudstone and minor sandstone. It contains flat lamination and abundant ripple cross-lamination which shows bipolar palaeocurrents, and occurs in combinations of flaser bedding, lenticular bedding and wavy lamination. Although body fossils are relatively rare, trilobite traces and desiccation cracks are common, and the association is interpreted as a predominantly subtidal to intertidal deposit. The conglomerate facies association contains horizontally bedded cobble to boulder conglomerate, with subordinate trough cross-stratified coarse sandstone to granule/pebble conglomerate. Fabrics and structures in the coarse conglomerates are consistent with alluvial transport (stream and debris flow), but not beach deposition. The conglomerate association is attributed to tectonic uplift and erosion of a Precambrian-Lower Cambrian succession developed adjacent to the present north coast of Kangaroo Island. Southward progradation of an alluvial fan complex occurred across east-west oriented tidal flats on which limited wave activity reworked sand and fine gravel, but not coarser material. The sandstone facies association mainly comprises trough cross-stratified and plane-laminated sandstone, the latter with current lineation predominantly sub-parallel to the east-west shoreline. Trough cross-stratification is ascribed to onshore waves and longshore currents, and current lineation to predominantly shore-parallel tidal currents, augmented by longshore drift and storm surge. Tectonic movements gave rise to cycles of transgression and regression as tidal and alluvial processes dominated alternately.     

珠江口盆地白云凹陷晚渐新统-早中新统沉积特征及演化规律

以珠江口盆地白云凹陷钻井、测井、地震和古生物资料为基础,结合前人研究成果,系统的分析了白云凹陷晚渐新统到早中新统沉积相发育特征及凹陷沉积充填演化过程。研究结果表明,珠海组下部发育大型陆架边缘三角洲沉积,地震反射特征表明该三角洲发育三期;钻遇水下分支河道、支流间湾、河口坝及远砂坝微相,沉积物以富砂为特征,发育冲刷-充填构造、递变层理及较粗的水平潜穴等多种构造;珠海组上部为浅海相,沉积物以海相砂泥岩互层为主,此时期陆架坡折带位于白云凹陷南坡;珠江组沉积时期,海平面升降旋回频繁,陆架坡折带迁至凹陷北坡。随着古珠江流域扩大,沉积物输入量增加,在珠江组下部发育了深水扇,沉积物以细-粗砂岩为主,夹少量粉砂岩及深海泥岩,发育颗粒流、液化流、浊流及碎屑流等四种主要的重力流,钻遇内扇水道,中扇废弃水道及水道间漫溢沉积,外扇深海泥沉积;珠江组上部为半深海相,沉积物以深海泥岩为主。      

恒定渗流作用下泥石流起动过程冲刷试验分析

渗流是泥石流水动力条件主要来源之一,不同渗流流量具有不同的渗流力和冲刷力,从而可引起不同规模泥石流。通过开展室内水槽试验,利用测压管量测渗流过程中的孔隙水压力,并结合高清摄像技术从微观角度记录堆积土体内部细颗粒的运移、骨架颗粒的坍塌现象,以此分析研究土体渗透破坏、起动形成泥石流过程中的渗流和冲刷作用。在此基础上设定水槽坡度为7°,调节恒定渗流流量分别为120、170、265、320 ml/s,分析不同恒定渗流流量对固体堆积物失稳、泥石流起动过程中流态变化的影响。分析结果表明,在恒定渗流流量作用下,堆积土体内部细颗粒迁移、骨架颗粒坍塌造成土体颗粒重排列、孔隙水压力上升进而导致土体抗力降低是泥石流土体颗粒失稳、起动、冲刷的重要原因;随着渗流流量增加,流速迅速上升,土体内孔隙水压力逐步增大,骨架颗粒的失稳、移动主要受渗流及水流冲刷两方面共同作用,堆积土体颗粒的移动分别表现出缓慢小幅滑动后稳定、过渡型滑动和快速流滑现象。     

东非鲁武马盆地始新统超深水重力流砂岩储层特征及成因

鲁武马盆地古近系-新近系发育多套超深水、超大型、富含天然气藏的重力流沉积砂体。以始新统砂体为解剖对象,分析区内重力流砂岩储层特征及成因。结果表明砂体以巨厚层状产出于深海泥岩内部,并与周围泥岩截然接触,测井曲线表现出宏观均一性;岩心揭示此类巨厚砂体是由多期单砂体叠置而成,单砂体是由底部高密度颗粒流和顶部低密度浊流两部分组成,且经历过强底流改造。鲁武马河流三角洲强大物源供给决定了区内砂体分布面积和体积规模;深海滑塌、块体搬运等重力流沉积过程控制了沉积体粒序构造和内部结构;海底区域性强底流持续冲刷并携带走单砂体顶部细粒沉积物,残留了底部“干净”的中粗粒砂岩;多期沉积事件和频繁水道迁移决定了砂体纵、横向叠加展布,并最终形成了区内厚度巨大、岩性宏观均一且连通性极好的超大型深水重力流沉积砂岩储层。     

Architecture of an Oligocene fluvial ribbon sandstone in the Ebro Basin,North‐eastern Spain

Fluvial ribbon sandstone bodies are ubiquitous in the Ebro Basin in North‐eastern Spain; their internal organization and the mechanics of deposition are as yet insufficiently known. A quarrying operation in an Oligocene fluvial ribbon sandstone body in the southern Ebro Basin allowed for a three‐dimensional reconstruction of the sedimentary architecture of the deposit. The sandstone is largely a medium‐grained to coarse‐grained, moderately sorted lithic arenite. In cross‐section, the sandstone body is 7 m thick, occupies a 5 m deep incision and wedges out laterally, forming a ‘wing’ that intercalates with horizontal floodplain deposits in the overbank region. Three architectural units were distinguished. The lowest and highest units (Units A and C) mostly consist of medium‐grained to coarse‐grained sandstone with medium‐scale trough cross‐bedding and large‐scale inclined stratasets. Each of Units A and C comprises a fining‐up stratal sequence reflecting deposition during one flood event. The middle unit (Unit B) consists of thinly bedded, fine‐grained sandstone/mudstone couplets and represents a time period when the channel was occupied by low‐discharge flows. The adjoining ‘wing’ consists of fine‐grained sandstone beds, with mudstone interlayers, correlative to strata in Units A and C in the main body of the ribbon sandstone. In plan view, the ribbon sandstone comprises an upstream bend and a downstream straight reach. In the upstream bend, large‐scale inclined stratasets up to 3 m in thickness represent four bank‐attached lateral channel bars, two in each of Units A and C. The lateral bars migrated downflow and did not develop into point bars. In the straight downstream reach, a tabular cross‐set in Unit A represents a mid‐channel transverse bar. In Unit C, a very coarse‐grained, unstratified interval is interpreted as deposited in a riffle zone, and gives way downstream to a large mid‐channel bar. The relatively simple architecture of these bars suggests that they developed as unit bars. Channel margin‐derived slump blocks cover the upper bar. The youngest deposit is fine‐grained sandstone and mudstone that accumulated immediately before avulsion and channel abandonment. Deposition of the studied sandstone body reflects transport‐limited sediment discharges, possibly attaining transient hyperconcentrated conditions.     

Sedimentary structures of cold-climate coastal dunes, Eastern Hudson Bay, Canada

A study of sedimentary structures of cold-climate coastal dunes along Eastern Hudson Bay, Canada, showed that in areas of very sparsely vegetated surfaces, tractional sedimentary processes are dominant while in partly and well vegetated areas, grainfall deposition is the dominant sedimentary process. Alternating fine and coarse sand strata are characteristic of the vegetated foredune ridge. Coarse strata are interpreted to be mainly related to niveo-aeolian deposition. In poorly vegetated areas deformed strata associated with the melting of snow were preserved. A vaguely stratified strata interpreted as a dimpled surface as well as coarse lenses with irregular basal contact were found. Such distinctive sedimentary structures may represent good indicators of cold-climate conditions and could serve as criteria to identify aeolianites that may have been deposited in similar cold-climate environments in palaeoenvironmental reconstructions.