Sediment distribution and depositional processes along the fluvial to marine transition zone of the Mekong River delta,Vietnam

The area of coastal rivers with a combination of fluvial, tidal and wave processes is defined as the fluvial to marine transition zone and can extend up to several hundreds of kilometres upstream of the river mouth. The aim of this study is to improve the understanding of sediment distribution and depositional processes along the fluvial to marine transition zone using a comprehensive dataset of channel bed sediment samples collected from the Mekong River delta. Six sediment types were identified and were interpreted to reflect the combined action of fluvial and marine processes. Based on sediment‐type associations, the Mekong fluvial to marine transition zone could be subdivided into an upstream tract and a downstream tract; the boundary between these two tracts is identified 80 to 100 km upstream of the river mouth. The upstream tract is characterized by gravelly sand and sand and occasional heterolithic rhythmites, suggesting bed‐load supply and deposition mainly controlled by fluvial processes with subordinate tidal influence. The downstream tract is characterized by heterolithic rhythmites with subordinate sand and mud, suggesting suspended‐load supply and deposition mainly controlled by tidal processes with subordinate fluvial influence. Sediment distributions during wet and dry seasons suggest significant seasonal changes in sediment dynamic and depositional processes along the fluvial to marine transition zone. The upstream tract shows strong fluvial depositional processes with subordinate tidal influence during the wet season and no deposition with weak fluvial and tidal processes during the dry season. The downstream tract shows strong coexisting fluvial and tidal depositional processes during the wet season and strong tidal depositional processes with negligible fluvial influence during the dry season. Turbidity maxima are present along the downstream tract of the fluvial to marine transition zone during both wet and dry seasons and are driven by a combination of fluvial, tidal and wave processes.     

Late Quaternary mass movement events in Lake El′gygytgyn,North‐eastern Siberia

Lake El′gygytgyn is situated in a 3·6 Myr old impact crater in North‐eastern Siberia. Its sedimentary record probably represents the most complete archive of Pliocene and Quaternary climate change in the terrestrial Arctic. In order to investigate the influence of gravitational sediment transport on the pelagic sediment record in the lake centre, two sediment cores were recovered from the lower western lake slope. The cores penetrate a sub‐recent mass movement deposit that was identified by 3·5 kHz echo sounding. In the proximal part of this deposit, deformed sediments reflect an initial debris flow characterized by limited sediment mixture. Above and in front of the debrite, a wide massive densite indicates a second stage with a liquefied dense flow. The mass movement event led to basal erosion of ca 1 m thick unconsolidated sediments along parts of its flow path. The event produced a suspension cloud, whose deposition led to the formation of a turbidite. The occurrence of the turbidite throughout the lake and the limited erosion at its base mainly suggest deposition by ‘pelagic rain’ following Stokes’ Law. Very similar radiocarbon dates obtained in the sediments directly beneath and above the turbidite in the central lake confirm this interpretation. When applying the depositional model for the Late Quaternary sediment record of Lake El′gygytgyn, the recovered turbidites allow reconstruction of the frequency and temporal distribution of large mass movement events at the lake slopes. In total, 28 turbidites and related deposits were identified in two, 12·9 and 16·6 m long, sediment cores from the central lake area covering approximately 300 kyr.     

Defining the shelf edge and the three‐dimensional shelf edge to slope facies variability in shelf‐edge deltas

Shelf‐edge deltas play a critical role in shelf‐margin accretion and deepwater sediment delivery, yet much remains to be understood about the detailed linkage between shelf edge and slope sedimentation. The shelf edge separates the flat‐lying shelf from steeper slope regions, and is observable in seismic data and continuous outcrops; however, it is commonly obscured in non‐continuous outcrops. Defining this zone is essential because it segregates areas dominated by shelf currents from those governed by gravity‐driven processes. Understanding this linkage is paramount for predicting and characterizing associated deepwater reservoirs. In the Tanqua Karoo Basin, the Permian Kookfontein Formation shelf‐slope clinothems are well‐exposed for 21 km along depositional strike and dip. Two independent methods identified the shelf‐edge position, indicating that it is defined by: (i) a transition from predominantly shelf‐current to gravitational deposits; (ii) an increase in soft‐sediment deformation; (iii) a significant gradient increase; and (iv) clinothem thickening. A quantitative approach was used to assess the impact of process‐regime variability along the shelf edge on downslope sedimentation. Facies proportions were quantified from sedimentary logs and photographic panels, and integrated with mapped key surfaces to construct a stratigraphic grid. Spatial variability in facies proportions highlights two types of shelf‐edge depositional zones within the same shelf‐edge delta. Where deposition occurred in fluvial‐dominated zones, the slope is sand rich, channelized with channels widening downslope, and rich in collapse features. Where deltaic deposits indicate considerable tidal reworking, the deposits are thin and pinch‐out close to the shelf edge, and the slope is sand poor and lacks channelization. Amplification of tidal energy, and decrease in fluvial drive on the shelf, coincides with a decrease in mouth bar and shelf‐edge collapse, and a lack of channelization on the slope. This analysis suggests that process‐regime variability along the shelf edge exercised significant control on shelf‐edge progradation, slope channelization and deepwater sediment delivery.     

鄂尔多斯盆地西南缘上三叠统延长组物源与沉积体系特征

以鄂尔多斯盆地西南缘延长组为例,通过对野外露头沉积特征、骨架颗粒、重矿物组合等分析,讨论了研究区物源与沉积体系特征。研究表明:早期沉积物源主要来自盆地西南方向,南部也有贡献。中、晚期沉积物源除来自西南方向和南部物源外,还受到来自盆地北东方向次要物源的影响。延长组长8之上与之下物源性质发生明显变化。延长组长8砂岩的岩屑以变质岩为主,少量沉积岩和火成岩;而延长组长6砂岩岩屑则以沉积岩为主,少量变质岩和火成岩,这表明长6沉积期盆地周缘、特别是盆地西缘抬升加剧,地层的剥蚀更强烈,致使大量较年轻的地层被剥蚀并加入源区母岩中。鄂尔多斯盆地延长期湖盆为北西—南东向展布,西南区主要发育辫状河和辫状河三角洲沉积体系,东北区主要发育曲流河三角洲沉积体系,在深湖区发育浊积扇沉积体系。     

Evaluating along‐strike variation using thin‐bedded facies analysis,Upper Cretaceous Ferron Notom Delta,Utah

Thin‐bedded delta‐front and prodelta facies of the Upper Cretaceous Ferron Notom Delta Complex near Hanksville in southern Utah, USA, show significant along‐strike facies variability. Primary initiation processes that form these thin beds include surge‐type turbidity currents, hyperpycnal flows and storm surges. The relative proportion of sedimentary structures generated by each of these depositional processes/events has been calculated from a series of measured sedimentological sections within a single parasequence (PS6–1) which is exposed continuously along depositional strike. For each measured section, sedimentological data including grain size, lithology, bedding thickness, sedimentary structures and ichnological suites have been documented. Parasequence 6–1 shows a strong along‐strike variation with a wave‐dominated environment in the north, passing abruptly into a fluvial‐dominated area, then to an environment with varying degrees of fluvial and wave influence southward, and back to a wave‐dominated environment further to the south‐east. The lateral facies variations integrated with palaeocurrent data indicate that parasequence 6–1 is deposited as a storm‐dominated symmetrical delta with a large river‐dominated bayhead system linked to an updip fluvial feeder valley. This article indicates that it is practical to quantify the relative importance of depositional processes and determine the along‐strike variation within an ancient delta system using thin‐bedded facies analysis. The wide range of vertical stratification and grading sequences present in these event beds also allows construction of conceptual models of deposition from turbidity currents (i.e. surge‐type turbidity currents and hyperpycnal flows) and storm surges, and shows that there are significant interactions and linkages of these often paired processes.     

Evidence of across-shelf transport of fine-grained sediments: turbidite-filled shelf channels in the Campanian Aberdeen Member, Book Cliffs, Utah, USA

The shore‐normal transport of fine‐grained sediments by shelf turbidity currents has been the focus of intense debate over the last 20 years. Many have argued that turbidity currents are unlikely to be a major depositional agent on the shelf. However, sedimentological, architectural, stratigraphic and palaeogeographic data from the Campanian Aberdeen Member, Book Cliffs, eastern Utah suggests otherwise and clearly demonstrates that storm‐generated and river flood‐generated underflows can transport a significant volume of fine‐grained sediments across the shelf. These across‐shelf flowing turbidity currents cut large subaqueous channel complexes up to 7 m deep, tens of kilometres basinward of their time‐equivalent shoreface. The shelf channels were filled with organic‐rich siltstones, mudstones and very fine‐ to fine‐grained Bouma‐like sandstone beds, including wave‐modified turbidites, hyperpycnites and classical turbidites. Deposition was above storm wave base. Palaeocurrent data reveal an overwhelmingly dominant across‐shelf (east–south‐east), offshore‐directed transport trend. Tectonic activity and/or concomitant palaeogeographic reorganization of the basin may favour the generation of these turbidite‐rich shelf deposits by altering the relative balance of wave versus fluvial energy. Increased erosion and sediment supply rates, because of tectonic uplift of the hinterland, may have increased the probability of fluvial dominance along the coastline and, hence, the possibility of submarine channelization in front of the river mouths. Additionally, the coastline may have become more sheltered from direct wave energy, thus allowing the fluvial processes to dominate. Seasonal increases in rainfall and storm activity may also favour the generation of across‐shelf underflows. On wave‐dominated shorelines, isolated shelf channels and lobes are most likely to be found down‐dip of fluvial‐feeder systems in relatively high sediment supply settings. These features are also most likely to occur in systems tracts that straddle a sequence boundary, especially those which are tectonically generated, as these would enhance the potential for altering basin morphology and, hence, the balance of fluvial and wave energy. Isolated shelf channels are recognized in older and younger strata in the Book Cliffs region, implying that wave‐supported gravity flows were a recurrent phenomena in the Campanian of Utah. It is probable that isolated shelf bodies are preserved in other stratigraphic intervals in the Cretaceous Western Interior of North America, and other basins worldwide, and are currently being overlooked or misidentified. Shoreface‐to‐shelf facies models should be revised to incorporate turbidite‐rich shelf deposits in some shelf settings.     

Late Quaternary sequence stratigraphy of Lake Malawi (Nyasa), Africa

High resolution seismic data, multichannel seismic data and sediment cores were used to examine the Songwe Sequence, the uppermost of four depositional sequences identifiable on multichannel seismic data from Lake Malawi (Nyasa). The sequence has a maximum thickness of about 115 m in two areas of the lake, but is typically less than 70 m thick over most of the basin. The sequence is distributed along the entire length of the 560 km long lake, and is concentrated in three main depocentres. ¹⁴C age dates from sediment piston cores are extrapolated to provide an age estimate of about 78 000 yr bp for the oldest sediments within the Songwe Sequence. In the North and Central bathymetric basins of the lake, high resolution seismic data indicate a dynamic depositional environment, dominated by turbidity and mass flow deposits. Seismic data from the southern basin show acoustically transparent sediments with relatively low amplitude internal reflections, indicative of pelagic and hemipelagic sedimentation. In many areas the Songwe Sequence is underlain by a pronounced angular unconformity, suggestive of a significant, prolonged, low lake stage prior to deposition of the sequence. Seismic reflectors within the Songwe Sequence can be correlated to younger low lake stages identified from sediment core data. Major late Quaternary low lake level stages in Lake Malawi, interpreted from features identified in the seismic data and sediment core analyses, are tentatively interpreted at 6000 to 10 000 yr bp , 28 000 to >40 000 yr bp , and prior to 78 000 yr bp . Budget calculations indicate mean sediment concentrations from catchment runoff during the period of deposition of the Songwe Sequence to be about 190 mg 1⁻¹, comparable to estimates of modern rainy season discharges from the major river systems. Erosion rates within the drainage basin are estimated to be higher than the African average by a factor of three or more, probably due to the high relief within the Lake Malawi catchment.     

Quaternary environmental changes in tropical Lake Towuti,Indonesia, inferred from end-member modelling of X-ray fluorescence core-scanning data

Continental and marine sediments are composed of a mixture from different sources and are influenced by a variety of environmental factors and transport processes prior to deposition. For analysis and interpretation, these sources and processes are often challenging to disentangle. We show that end-member modelling of X-ray fluorescence (XRF) core-scanning data helps to overcome these challenges by unmixing different environmental signals from high-resolution sediment geochemical records. We apply this approach to a 100 m long lacustrine succession from Lake Towuti, Indonesia, to separate the regional climate and tectonic history from local ecological and diagenetic processes. The resulting six end-members (EMs) are interpreted to represent changes in ecological (EM1), climatic (EMs 2–4), tectonic (EM 5) and geomorphic (EM6) processes determining changes in sediment composition. Because end-member analysis allows for the tracking of transient and overlapping processes, climatic changes can be followed throughout the 100 m-long succession, suggesting alternating wet and dry periods in Central Sulawesi over long (several 100 000 years) time scales. We show that end-member analysis on elemental data sets offers a detailed and objective means to disentangle depositional processes in sedimentary successions resulting from varying tectonic and environmental factors involved in sediment formation and deposition.     

Suspended sediment transport,sedimentation, and resuspension in Lake Houston,Texas: Implications for water quality

Lake Houston is a man-made reservoir located northeast of Houston, Texas. The purpose of this investigation was to document suspended sediment transport, sedimentation, and resuspension in the lake with a view towards estimating the influence of sedimentation on water quality. Sediment traps were placed in strategic locations in the lake to collect suspended sediments. Samples were analyzed for bulk density, grain size, organic carbon, and a number of trace elements. These data were analyzed along with meteorological data to examine those factors which regulate suspended sediment input and dispersal, and the role of suspended sediments in controlling water quality within the lake. Sediment input to the lake depends primarily on the intensity of rainfall in the watershed. Sediment movement within the lake is strongly influenced by wave activity, which resuspends sediments from shallow areas, and by wind-driven circulation. The increased residence time of suspended sediments due to resuspension allows greater decomposition of organic matter and the release of several trace elements from sediments to the water column. Virtually all samples from sediment traps suspended between 1 and 5 m above the lake bottom contain medium to coarse silt, and even some very fine sand-sized material. This implies that circulation in Lake Houston is periodically intense enough to transport this size material in suspension. During winter, northerly winds with sustained velocities of greater than 5 m/sec provide the most suitable condition for rapid (<1 d) transport of suspended sediment down the length of the lake. Fluctuations in current velocities and the subsequent suspension/deposition of particles may explain variations in the abundance of coliform bacteria in Lake Houston.     

Sheetflow fluvial processes in a rapidly subsiding basin, Altiplano plateau, Bolivia

Although facies models of braided, meandering and anastomosing rivers have provided the cornerstones of fluvial sedimentology for several decades, the depositional processes and external controls on sheetflow fluvial systems remain poorly understood. Sheetflow fluvial systems represent a volumetrically significant part of the non‐marine sedimentary record and documented here are the lithofacies, depositional processes and possible roles of rapid subsidence and arid climate in generating a sheetflow‐dominated fluvial system in the Cenozoic hinterland of the central Andes. A 6500 m thick succession comprising the Late Eocene–Oligocene Potoco Formation is exposed continuously for >100 km along the eastern limb of the Corque syncline in the high Altiplano plateau of Bolivia. Fluvial sandstone and mudstone units were deposited over an extensive region (>10 000 km²) with remarkably few incised channels or stacked‐channel complexes. The Potoco succession provides an exceptional example of rapid production of accommodation sustained over a prolonged period of time in a non‐marine setting (>0·45 mm year⁻¹ for 14 Myr). The lower ≈4000 m of the succession coarsens upward and consists of fine‐grained to medium‐grained sandstone, mudstone and gypsum deposits with palaeocurrent indicators demonstrating eastward transport. The upper 2500 m also coarsens upward, but contains mostly fine‐grained to medium‐grained sandstone that exhibits westward palaeoflow. Three facies associations were identified from the Potoco Formation and are interpreted to represent different depositional environments in a sheetflow‐dominated system. (i) Playa lake deposits confined to the lower 750 m are composed of interbedded gypsum, gypsiferous mudstone and sandstone. (ii) Floodplain deposits occur throughout the succession and include laterally extensive (>200 m) laminated to massive mudstone and horizontally stratified and ripple cross‐stratified sandstone. Pedogenic alteration and root casts are common. (iii) Poorly confined channel and unconfined sheet sandstone deposits include laterally continuous beds (50 to >200 m) that are defined primarily by horizontally stratified and ripple cross‐stratified sandstone encased in mudstone‐rich floodplain deposits. The ubiquitous thin‐sheet geometry and spatial distribution of individual facies within channel sandstone and floodplain deposits suggest that confined to unconfined, episodic (flash) flood events were the primary mode of deposition. The laterally extensive deposition and possible distributary nature of this sheetflow‐dominated system are attributed to fluvial fan conditions in an arid to semi‐arid, possibly seasonal, environment. High rates of sediment accumulation and tectonic subsidence during early Andean orogenesis may have favoured the development and long‐term maintenance of a sheetflow system rather than a braided, meandering or anastomosing fluvial style. It is suggested here that rapidly produced accommodation space and a relatively arid, seasonal climate are critical conditions promoting the generation of sheetflow‐dominated fluvial systems.     

Fluvial to tidal transition in proximal,mixed tide‐influenced and wave‐influenced deltaic deposits: Cretaceous lower Sego Sandstone,Utah, USA

Facies models for regressive, tide‐influenced deltaic systems are under‐represented in the literature compared with their fluvial‐dominated and wave‐dominated counterparts. Here, a facies model is presented of the mixed, tide‐influenced and wave‐influenced deltaic strata of the Sego Sandstone, which was deposited in the Western Interior Seaway of North America during the Late Cretaceous. Previous work on the Sego Sandstone has focused on the medial to distal parts of the outcrop belt where tides and waves interact. This study focuses on the proximal outcrop belt, in which fluvial and tidal processes interact. Five facies associations are recognized. Bioturbated mudstones (Facies Association 1) were deposited in an offshore environment and are gradationally overlain by hummocky cross‐stratified sandstones (Facies Association 2) deposited in a wave‐dominated lower shoreface environment. These facies associations are erosionally overlain by tide‐dominated cross‐bedded sandstones (Facies Association 4) interbedded with ripple cross‐laminated heterolithic sandstones (Facies Association 3) and channelized mudstones (Facies Association 5). Palaeocurrent directions derived from cross‐bedding indicate bidirectional currents which are flood‐dominated in the lower part of the studied interval and become increasingly ebb‐directed/fluvial‐directed upward. At the top of the succession, ebb‐dominated/fluvial‐dominated, high relief, narrow channel forms are present, which are interpreted as distributary channels. When distributary channels are abandoned they effectively become estuaries with landward sediment transport and fining trends. These estuaries have sandstones of Facies Association 4 at their mouth and fine landward through heterolithic sandstones of Facies Association 3 to channelized mudstones of Facies Association 5. Therefore, the complex distribution of relatively mud‐rich and sand‐rich deposits in the tide‐dominated part of the lower Sego Sandstone is attributed to the avulsion history of active fluvial distributaries, in response to a subtly expressed allogenic change in sediment supply and relative sea‐level controls and autocyclic delta lobe abandonment.     

Large‐scale architecture in non‐marine basins: the response to the interplay between accommodation space and sediment supply

Sedimentary successions of non‐marine basins can be considered in terms of accommodation space and sedimentary supply changes. Changes in accommodation space controlling the large‐scale architecture of non‐marine basins are different in areas with high and low sedimentary supplies. Uplift of intrabasinal monoclines and anticlines reduced the available accommodation space, resulting in changes in both the geometry of the depositional sequences and the large‐scale architecture of fluvial, mudflat and shallow carbonate lacustrine deposits. Main drainage fluvial systems record areas with a high sedimentary supply, while mudflats and shallow fluctuating lakes represent areas that received less sediment. Two end members in the large‐scale architecture of main drainage fluvial system in the Almazán Basin (Spain) are: (i) ribbon‐shaped channel fills with low interconnectivity which pass laterally into mudflats dominated by mudstones and evaporites and into palustrine and shallow carbonate lacustrine deposits (mainly in the A2 depositional sequence); and (ii) sheet‐like channel fills with high interconnectivity laterally correlated with stacked calcretes in the marginal mudflats (in the upper part of A3). Ribbon‐shaped channel fills formed in areas of high accommodation space and sheet‐like channel fills formed in areas of reduced accommodation space.     

Controls on sedimentation in distal margin palaeovalleys in the Early Tertiary Alpine foreland basin, south-eastern France

The influence of palaeodrainage characteristics, palaeogeography and tectonic setting are rarely considered as controls on stratigraphic organization in palaeovalley or incised valley systems. This study is an examination of the influence of source region vs. downstream base level controls on the sedimentary architecture of a set of bedrock-confined palaeovalleys developed along the distal margin of the Alpine foreland basin in south-eastern France. Three distinct facies associations are observed within the palaeovalley fills. Fluvial facies association A is mainly dominated by poorly sorted, highly disorganized, clast-to-matrix-supported cobble-to-boulder conglomerates that are interpreted as streamflood deposits. Facies association B comprises mainly yellow siltstones and is interpreted as recording deposition in an estuarine basin environment. Estuarine marine facies association C comprises interstratified estuarine siltstones and clean, well-sorted washover sandstones. The sedimentary characteristics of the valley fill successions are related to the proximity of depositional sites to sediment source areas. Palaeovalleys located proximal to structurally controlled basement palaeohighs are entirely dominated by coarse fluvial streamflood deposits. In contrast, distal palaeovalley segments, which are located several kilometres downstream, contain successions showing upward transition from coarse fluvial facies into estuarine central basin fines, and finally into estuarine-marginal marine facies. Facies distributions suggest that the fluvial deposits form wedge-shaped, downstream-thinning sediment bodies, whereas the estuarine deposits form an upstream-thinning wedge. The vertical stacking of fluvial to estuarine to marginal marine depositional environments records the fluvial aggradation and subsequent transgression of relatively small bedrock-confined river valleys, which drained a rugged, upland terrain. Facies geometries suggest that a fluvial sediment wedge initially prograded downvalley, in response to high bed load sediment yields. Subsequently, palaeovalleys became drowned during the passage of a marine transgression, with the establishment of estuarine conditions. Initial fluvial aggradation and subsequent marine flooding of the palaeovalleys is a consequence of the interaction of high local rates of sediment supply and relative sea-level rise driven by flexural subsidence of the basin.     

Late Quaternary deposition and facies model for karstic Lake Estanya (North-eastern Spain)

Lake Estanya is a small (19 ha), freshwater to brackish, monomictic lake formed by the coalescence of two karstic sinkholes with maximum water depths of 12 and 20 m, located in the Pre‐Pyrenean Ranges (North‐eastern Spain). The lake is hydrologically closed and the water balance is controlled mostly by groundwater input and evaporation. Three main modern depositional sub‐environments can be recognized as: (i) a carbonate‐producing ‘littoral platform’; (ii) a steep ‘talus’ dominated by reworking of littoral sediments and mass‐wasting processes; and (iii) an ‘offshore, distal area’, seasonally affected by anoxia with fine‐grained, clastic sediment deposition. A seismic survey identified up to 15 m thick sedimentary infill comprising: (i) a ‘basal unit’, seismically transparent and restricted to the depocentres of both sub‐basins; (ii) an ‘intermediate unit’ characterized by continuous high‐amplitude reflections; and (iii) an ‘upper unit’ with strong parallel reflectors. Several mass‐wasting deposits occur in both sub‐basins. Five sediment cores were analysed using sedimentological, microscopic, geochemical and physical techniques. The chronological model for the sediment sequence is based on 17 accelerator mass spectrometry ¹⁴C dates. Five depositional environments were characterized by their respective sedimentary facies associations. The depositional history of Lake Estanya during the last ca 21 kyr comprises five stages: (i) a brackish, shallow, calcite‐producing lake during full glacial times (21 to 17·3 kyr bp ); (ii) a saline, permanent, relatively deep lake during the late glacial (17·3 to 11·6 kyr bp ); (iii) an ephemeral, saline lake and saline mudflat complex during the transition to the Holocene (11·6 to 9·4 kyr bp ); (iv) a saline lake with gypsum‐rich, laminated facies and abundant microbial mats punctuated by periods of more frequent flooding episodes and clastic‐dominated deposition during the Holocene (9·4 to 0·8 kyr bp ); and (v) a deep, freshwater to brackish lake with high clastic input during the last 800 years. Climate‐driven hydrological fluctuations are the main internal control in the evolution of the lake during the last 21 kyr, affecting water salinity, lake‐level changes and water stratification. However, external factors, such as karstic processes, clastic input and the occurrence of mass‐flows, are also significant. The facies model defined for Lake Estanya is an essential tool for deciphering the main factors influencing lake deposition and to evaluate the most suitable proxies for lake level, climate and environmental reconstructions, and it is applicable to modern karstic lakes and to ancient lacustrine formations.     

Discussion and Reply: Shaping the Australian crust over the last 300 million years: Insights from fission track thermotectonic imaging and denudation studies of key terranes

Ellis Fjord is a small, fjord‐like marine embayment in the Vestfold Hills, eastern Antarctica. Modern sediment input is dominated by a biogenic diatom rain, although aeolian, fluvial, ice‐rafted, slumped and tidal sediments also make a minor contribution. In areas where bioturbation is significant relict glaciogenic sediments are reworked into the fine‐grained diatomaceous sediments to produce poorly sorted fine sands and silts. Where the bottom waters are anoxic, sediments remain unbioturbated and have a high biogenic silica component. Three depositional and non‐depositional facies can be recognised in the fjord: an area of non‐deposition around the shoreline; a relict morainal facies in areas of low sedimentation and high bioturbation; and a basinal facies in the deeper areas of the fjord.     

Seismic‐stratigraphic record of a deglaciation sequence: from the marine Laflamme Gulf to Lake Saint‐Jean (late Quaternary,Québec,Canada)

The stratigraphy of the last deglaciation sequence is investigated in Lake Saint‐Jean (Québec Province, Canada) based on 300 km of echo‐sounder two dimensional seismic profiles. The sedimentary archive of this basin is documented from the Late Pleistocene Laurentidian ice‐front recession to the present‐day situation. Ten seismic units have been identified that reflect spatio‐temporal variations in depositional processes characterizing different periods of the Saint‐Jean basin evolution. During the postglacial marine flooding, a high deposition rate of mud settling, from proglacial glacimarine and then prodeltaic plumes in the Laflamme Gulf, produced an extensive, up to 50 m thick mud sheet draping the isostatically depressed marine basin floor. Subsequently, a closing of the water body due to glacio‐isostatic rebound occurred at 8.5 cal. ka BP, drastically modifying the hydrodynamics. Hyperpycnal flows appeared because fresh lake water replaced dense marine water. River sediments were transferred towards the deeper part of the lake into river‐related sediment drifts and confined lobes. The closing of the water body is also marked by the onset of a wind‐driven internal circulation associating coastal hydrodynamics and bottom currents with sedimentary features including shoreface deposits, sediment drifts and a prograding shelf‐type body. The fingerprints of a forced regression are well expressed by mouth‐bar systems and by the shoreface–shelf system, the latter unexpected in such a lacustrine setting. In both cases, a regressive surface of lacustrine erosion (RSLE) has been identified, separating sandy mouth‐bar from glaciomarine to prodeltaic muds, and sandy shoreface wedges from the heterolithic shelf‐type body, respectively. The Lake Saint‐Jean record is an example of a regressive succession driven by a glacio‐isostatic rebound and showing the transition from late‐glacial to post‐glacial depositional systems.     

The prehistory and palaeogeography of the great Indian desert : Bridget Allchin, Andrew Goudie, and Karunakara Hegde. Academic Press, London/New York/San Francisco, 1978, 370 pp., 73 figs., 85 tables, 12 plates. Bibliography, Index, £25/-, U.S. $48.90

Seismic reflection profiles (3.5 kHz) were obtained along more than 3500 km of shiptrack in Lake Superior within the last 2 yr. The acoustic character of profiles is categorized as: (I) a single, strong reflector at the lake floor, (II) a thick, acoustically transparent layer overlying a strong reflector, and (III) relatively thick sediment with internal acoustic reflectors. These profiles, in conjunction with sediment cores from the area, reveal that varved glacial-lacustrine sediment settled out preferentially in a trough between Isle Royale and the north shore, and to a lesser extent in other topographic depressions; bottom currents generated by storm waves prevent clay accumulation on till or bedrock in the open lake wherever the bottom is shallower than 100 m; bottom currents prevent deposition or erode bottom sediment in certain deep-water (> 200 m) valleys; and lacustrine sediment is disturbed by creep or slumping off Grand Portage, Minnesota, and by other processes such as dewatering in many other areas. These factors complicate sedimentation in Lake Superior, and must be considered when investigating any aspect of the lake sediment.     

End moraine construction by incremental till deposition below the Laurentide Ice Sheet: Southern Ontario,Canada

Eyles, N., Eyles, C., Menzies, J. & Boyce, J. 2010: End moraine construction by incremental till deposition below the Laurentide Ice Sheet: Southern Ontario, Canada. Boreas, 10.1111/j.1502‐3885.2010.00171.x. ISSN 0300‐9483. Just after 13 300 ¹⁴C a BP in central Canada, the retreating Ontario lobe of the Laurentide Ice Sheet briefly re‐advanced westwards through the Lake Ontario basin to build a large end moraine. The Trafalgar Moraine (27 km long, 4 km wide) is composed of a distinctly red‐coloured silt‐rich till (Wildfield Till, up to 16.5 m thick) formed by the reworking of proglacial lake deposits and soft shale bedrock. The moraine has a pronounced ramp‐like longitudinal form passing upglacier into fluted till resting on exposed shale. Analysis of water well stratigraphic data, drilled sediment cores, downhole gamma‐ray logs and exposures in deep test pits shows that within the moraine the Wildfield Till is built of superposed beds up to 7 m in thickness. These are inferred to result from the repeated incremental deposition of fine‐grained debris being moved towards the ice margin as a deforming bed such as identified at modern glaciers. A total till volume of 0.81 km³ was produced in a very brief time‐span along a transport path probably no greater than 10 km in length. Subglacial mixing of pre‐existing sediment and soft shale was clearly a very effective process for generating and moving large volumes of till to the ice margin. Similar till‐dominated end moraines occur widely around the margins of the Great Lake basins, where the markedly lobate margin of the retreating Laurentide Ice Sheet re‐advanced repeatedly into proglacial lakes and over fine‐grained sediment. This suggests the wider applicability of the till transport and incremental depositional model presented here.