Ground‐penetrating radar and sedimentological analysis of Holocene floodplains: Insight from the Tuross valley,New South Wales

Ground‐penetrating radar (GPR) has been used on an array of floodplain types on the lower Tuross River, in southeastern New South Wales, as part of an investigation into controls on channel‐floodplain relationships. Ground‐penetrating radar transects from two floodplains are presented, along with sedimentological detail from trenches dug along the profiles at key locations. Sedimentological investigations showed that 100 MHz antenna gave an approximation of overall bedding trends in the upper 3 m when automatic gain control processing was used. Spreading and exponential compensation processing provided insight into textural changes associated with increased silt content distal of the levee crest. One trench showed that thinning beds were responsible for onlapping reflectors. Signal attenuation at ～4 m depth below the raised floodplain surface resulted from a >50 cm‐thick bed of sandy clay. The close integration of GPR and sedimentological data produced an excellent dataset, that enabled form‐process associations and floodplain evolution to be established for these sandy floodplains. However, accurate subsurface assessment and interpretation must stem from carefully combined GPR and sedimentological datasets.     

A study of Holocene floodplain particle size characteristics with special reference to palaeochannel infills from the upper Severn basin,Wales, UK

Multiple sedimentary units from floodplain reaches at Welshpool on the upper River Severn and at the confluence of the Afon Tanat and Afon Vyrnwy (mid‐Wales, UK) were examined to ascertain if they have distinctive particle size characteristics. Changes in particle size characteristics and their possible relationship to known human and climatic impacts are also discussed. Ellipse plots of particle size characteristics from the River Severn floodplain at Welshpool show that coarse‐grained outwash deposits can be clearly discriminated from channel margin or palaeochannel sediments. In contrast, at the Afon Tanat–Vyrnwy study reach, this discrimination is not seen so clearly. The relationships between age and particle size characteristics from the most sampled sedimentary environment, palaeochannel infills, were also examined. The data from the River Severn floodplain at Welshpool show that palaeochannel sediments reveal a gradual but clear increase in particle size from the mid‐ to late Holocene towards the present day. Sediments deposited in the period 90–160 years BP are markedly coarser. It is suggested that these changes may be related to the combined effect of land‐use changes, metal mining impacts and changes in flood frequency and magnitude that occurred at this time within the upper Severn basin. In contrast, the particle size characteristics of post Late Devensian/Early Holocene units from Tanat–Vyrnwy palaeochannels were random with no discernible age–size patterns. It is suggested that the non‐systematic grain size distribution may be due to the steeper valley gradients of the Tanat–Vyrnwy system (and by inference higher stream powers) and its relatively narrow valley form enabling more effective coupling between coarser outwash deposits found on and at the edges of hillslopes and the valley floor. Although the two study reaches have undergone comparable environmental change during the Holocene and lie in the piedmont zone of their catchments, palaeochannel units of the same age possess distinctly different characteristics. Intrinsic reach‐scale geomorphic factors would appear to preclude the uniform application of particle size characteristics to determine alluvial response to environmental change. Consequently, care needs to be applied to the use of such data for environmental discrimination because the phenomenon of equifinality means that a specific set of sediment characteristics is not necessarily exclusive to specific fluvial environments in either space or time. Copyright © 2001 John Wiley & Sons, Ltd.     

Integrated sedimentology and ichnology of Late Jurassic fluvial point‐bars – facies architecture and colonization styles (Lourinhã Formation,Lusitanian Basin,western Portugal)

Integrated sedimentological and ichnological case studies of ancient meandering river systems have, for the most part, focused on the deposits of the fluvial–tidal transition zone; much less emphasis has been placed on the purely fluvial realm above the landward limit of tidal effects. This problem needs to be addressed so that in future the defining sedimentological and ichnological criteria assigned to ancient fluvial reaches are sufficiently well‐established to enable their separation from the down‐dip fluvial–tidal transition zone. Accordingly, a case study has been carried out on a well‐exposed meander belt deposit from the Late Jurassic Lourinhã Formation of the Lusitanian Basin, western Portugal (Praia Do Valmitão, Ribamar). Analysis indicates that the meander belt here comprised mixed‐load fluvial channels traversing a vegetated floodplain subject to a seasonal winter wet/summer dry palaeoclimate. This setting facilitated the development of both calcic palaeosols and shallow lakes on the adjacent floodplain. Critically, there is no evidence of the effects of tidal modulation on bedding structures, thereby establishing purely fluvial conditions. Heterolithic point‐bar deposits generated in this setting are bioturbated extensively by a trace fossil assemblage dominated by the meniscate trace Taenidium barretti , with Skolithos linearis , Planolites beverleyensis and Cylindricum isp. also recognized. A number of factors suggest that the Taenidium barretti producer in this case was a subaquatic organism living in an active fluvial channel setting (i.e. not colonizing subaerially exposed channel‐margin/floodplain deposits). Accordingly, there are some implications for current ichnofabric/ichnofacies models in the continental realm. Firstly, Taenidium ‐dominated ichnofabrics need not necessarily be confined to colonization beneath subaerially exposed surfaces; they might also be produced within submerged substrates. Secondly, there is scope to extend the range of the Scoyenia ichnofacies to include active fluvial channels and not simply those channels that were inactive or abandoned.     

The paleohydrological context of the Iron Age floodplain sites of the Mun River Valley,Northeast Thailand1

Many Iron Age sites on the flood plain of the Mun River in northeast Thailand are encircled by channels commonly known as “moats.” Also, the sites are closely associated with complex paleochannels of the river. A comparison between the seemingly human‐constructed moats and paleochannels provided an opportunity to assess the relationship between prehistoric human settlement and paleohydrological conditions. In this study, the results of physical, sedimentological, and geochemical analyses are used to characterize sediments deposited within the channels around the Iron Age site of Ban Non Wat and within a paleochannel at nearby Ban Non Ngiu. This allowed us to test the results of previous research that has suggested significant changes in the floodplain hydrology and the geoarchaeologically important conclusion that Iron Age human activity was associated with one particular paleohydrological phase. Our analyses broadly confirm the results of previous stratigraphic studies, but add detail regarding sedimentation processes. The evidence indicates that there are significant sedimentological differences within the complex of archaeological channel features, differences that provide critical evidence for the formation and sedimentation processes of the channels. More importantly, comparison between the archaeological features and the natural channel fills highlights the relationships between the archaeological sites and landscape. Drawing also on previously published chronological, geomorphological, and stratigraphical data, it is possible to place the sites into a floodplain hydrological regime that may have been unique to the Iron Age. Specifically, the moats may have been constructed in response to enhanced water availability on the floodplain. The sites, therefore, may reflect a human response to increased availability of water beyond the main river channels. This water supply, however, appears to have been short‐lived (centuries at most), and with its loss, the human adaptation to this enhanced natural resource became unviable. © 2008 Wiley Periodicals, Inc.     

Biosedimentary record of postglacial coastal dynamics: high‐resolution sequence stratigraphy from the northern Tuscan coast (Italy)

Integrating analysis of the benthic palaeoecological record with multivariate ordination techniques represents a powerful synergy able to provide an improved characterization of coastal depositional facies in a sequence stratigraphical perspective. Through quantitative analysis of benthic foraminifer, ostracod and mollusc associations from the postglacial succession of Core M3 (Arno coastal plain, Tuscany, Italy), and application of detrended correspondence analysis (DCA) to the mollusc sub‐data set, we offer a refined picture of stratigraphical variations in faunal content from a paralic depositional setting, and reconstruct the palaeoenvironmental gradients that account for such variations. Despite distinct ecological behaviours, and taphonomic and sedimentological constraints, a strong ecological control on meio‐ and macrofaunal biofacies and taxa turnover is documented across the study succession. Amongst all possible mechanisms that may play a role in ‘shaping’ fossil distribution, the ecological signal driven by salinity represents the most prominent factor controlling the composition of fossil associations in the cored succession. Molluscs can even provide outstanding quantitative estimates of palaeosalinity along the sampled core. When plotted stratigraphically, the three fossil sub‐data sets show consistent patterns of vertical evolution that enable prompt identification of the key surfaces for sequence stratigraphical interpretation in otherwise lithologically indistinguishable deposits. The concomitant maximum richness of species with strong marine affinity, paralleled by the highest DCA salinity estimates, allows recognition of the maximum flooding zone, dated to ～7.7 cal. ka BP, within a homogeneous succession of outer lagoon clays. These clays are sandwiched between early transgressive, swamp to inner lagoon deposits and overlying prograding coastal?alluvial plain facies.     

从端点走向连续:河流沉积模式研究进展述评

从河道类型的划分、河床演变与河型转换、河道沉积与河流砂体的建筑结构要素、河漫滩沉积、季节性河流与分支河流体系、河流沉积相模式、河流沉积学研究技术与方法等方面对国内外河流沉积模式的研究进展进行了综述,认为近十年来河流沉积学的理论和方法都发生了重要的变化。地貌学家、沉积学家和工程师认识到河道形态是连续可变的,而不是只有4~40多个端点类型。河床的演变受河床比降、流量变幅、河岸沉积物粒度构成、气候、植被以及构造沉降速率等多方面的影响。垂向剖面分析法难以对古河流类型做出正确的判断,运用建筑结构要素分析法重建河道内大型底形的地貌形态是河型判别和河流相模式重建的正确方法。河漫滩是河流沉积事件记录最为齐全的部位,对河漫滩、天然堤和泛滥平原沉积层序的研究能够揭示更多古河流沉积过程以及古环境、古气候和古生物方面的信息。对季节性河流、受季风强烈影响地区的河流、以及不同气候带河流所发育的独特沉积构造和建筑结构要素的研究不断增加。分支河流体系的概念得到越来越多的应用,但也得到不少质疑。我国学者应当注重对现代河流地貌形态和沉积过程的观察,把河床演变学的定量方法与沉积学的观点、理论和资料相结合,利用露头、三维地震资料和探地雷达技术建立河流砂体内部建筑结构信息数据库,加强对古河流河漫滩和泛滥平原的沉积过程、特征及其控制因素的研究,加强对不同构造和气候条件下河流沉积的差异性研究,不断发展河流沉积学研究技术,加强河流沉积学实验室建设和研究队伍建设,加强国际交流与合作,使我国河流沉积学为国家经济社会发展提供更加有力和有效的支撑,为推动国际河流沉积学发展做出中国人自己的贡献。     

Early Mississippian sandy siltstones preserve rare vertebrate fossils in seasonal flooding episodes

Flood‐generated sandy siltstones are under‐recognised deposits that preserve key vertebrate (actinopterygians, rhizodonts, and rarer lungfish, chondrichthyans and tetrapods), invertebrate and plant fossils. Recorded for the first time from the lower Mississippian Ballagan Formation of Scotland, more than 140 beds occur throughout a 490 m thick core succession characterised by fluvial sandstones, palaeosols, siltstones, dolostone ‘cementstones’ and gypsum from a coastal–alluvial plain setting. Sandy siltstones are described as a unique taphofacies of the Ballagan Formation (Scotland, UK); they are matrix‐supported siltstones with millimetre‐sized siltstone and very fine sandstone lithic clasts. Common bioclasts include plants and megaspores, fish, ostracods, eurypterids and bivalves. Fossils have a high degree of articulation compared with those found in other fossil‐bearing deposits, such as conglomerate lags at the base of fluvial channel sandstones. Bed thickness and distribution varies throughout the formation, with no stratigraphic trend. The matrix sediment and clasts are sourced from the reworking of floodplain sediments including desiccated surfaces and palaeosols. Secondary pedogenic modification affects 30% of the sandy siltstone beds and most (71%) overlie palaeosols or desiccation cracks. Sandy siltstones are interpreted as cohesive debris flow deposits that originated by the overbank flooding of rivers and due to localised floodplain sediment transport at times of high rainfall; their association with palaeosols and desiccation cracks indicates seasonally wet to dry cycles throughout the Tournaisian. Tetrapod and fish fossils derived from floodplain lakes and land surfaces are concentrated by local erosion and reworking, and are preserved by deposition into temporary lakes on the floodplain; their distribution indicates a local origin, with sediment transported across the floodplain in seasonal rainfall episodes. These deposits are significant new sites that can be explored for the preservation of rare non‐marine fossil material and provide unique insights into the evolution of early terrestrial ecosystems.     

Sedimentary environments and depositional characteristics of the Middle to Upper Eocene whale‐bearing succession in the Fayum Depression,Egypt

The discovery of whale fossils from Eocene strata in the Fayum Depression has provoked interest in the life and lifestyle of early whales. Excellent outcrop exposure also affords the dataset to develop sedimentological and stratigraphic models within the Eocene strata. Previous work generally asserts that the thick, sand‐rich deposits of the Fayum Depression represent shoreface and barrier island successions with fine‐grained lagoonal and fluvial associations capping progradational successions. However, a complete absence of wave‐generated sedimentary structures, a preponderance of thoroughly bioturbated strata and increasingly proximal sedimentary successions upwards are contrary to accepted models of the local sedimentological and stratigraphic development. This study considers data collected from two Middle to Upper Eocene successions exposed in outcrop in the Wadi El‐Hitan and Qasr El‐Sagha areas of the Fayum Depression to determine the depositional affinities of Fayum strata. Based on sedimentological and ichnological data, five facies associations (Facies Association 1 to Facies Association 5) are identified. The biological and sedimentological characteristics of the reported facies associations indicate that the whale‐bearing sandstones (Facies Association 1) record distal positions in a large, open, quiescent marine bay that is abruptly succeeded by a bay‐margin environment (Facies Association 2). Upwards, marginal‐marine lagoonal and shallow‐bay parasequences (Facies Association 3) are overlain by thick deltaic distributary channel deposits (Facies Association 4). The capping unit (Facies Association 5) represents a transgressive estuarine depositional environment. The general stratigraphic evolution resulted from a regional, tectonically controlled second‐order cycle, associated with northward regression of the Tethys. Subordinate cycles (i.e. third‐order and fourth‐order cycles) are evidenced by several Glossifungites‐ichnofacies demarcated discontinuities, which were emplaced at the base of flooding surfaces. The proposed depositional models recognize the importance of identifying and linking ichnological data with physical–sedimentological observations. As such – with the exception of wave‐generated ravinement surfaces – earlier assertions of wave‐dominated sedimentation can be discarded. Moreover, this study provides important data for the recognition of (rarely reported) completely bioturbated sand‐dominated offshore to nearshore sediments (Facies Association 1) and affords excellent characterization of bioturbated inclined heterolithic stratification of deltaic deposits. Another outcome of the study is the recognition that the whales of the Fayum Depression are restricted to the highstand systems tracts, and lived under conditions of low depositional energy, low to moderate sedimentation rates, and (not surprisingly) in fully marine waters characterized by a high biomass.     

A reconstruction of the palaeohydrological conditions of a flood‐plain: a multi‐proxy study from the Grabia River valley mire,central Poland

This study investigated the Grabia River valley mire in central Poland to reconstruct its palaeoenvironmental conditions from the Younger Dryas to the present. We analysed sedimentological, biological and geochemical data from the palaeo‐oxbow lake and valley mire to identify the principal hydrological trends, especially episodes of high water level. During the Lateglacial and Holocene, the Grabia River had a meandering channel, and its hydraulic parameters and the channel dimensions changed in response to climatic oscillations and vegetation development. We identified phases of high flood intensity and high groundwater level that correlate with regional and supraregional climatic events. The frequency and timing of palaeohydroclimatic oscillations show strong similarities to records from other sites in Poland and the rest of Europe. We show that various analytical methods, namely, pollen, plant macrofossils, Cladocera, Chironomidae, sedimentological, geochemical and radiocarbon data, can be effective tools for reconstructing past hydroclimatic changes in palaeo‐oxbow lakes and investigating the effects of past climate changes on river environments. The high sensitivity of the biota, especially Cladocera, to changes in water level permits the reconstruction of palaeoecological changes, especially flood episodes that occurred in the river valley. In particular, the increase in the proportion of sediment‐associated Cladocera and pelagic taxa was closely correlated with floods. Through comparisons with the palaeobiological data, geochemical data allowed the identification of humid phases within the fen associated with a rising groundwater table, direct fluvial activity (floods) and alluvial deposition. We also discuss the limitations of palaeohydrological reconstructions based on these proxies, especially on fossil aquatic invertebrates.     

Preservation of meandering river channels in uniformly aggrading channel belts

Channel belt deposits from meandering river systems commonly display an internal architecture of stacked depositional features with scoured basal contacts due to channel and bedform migration across a range of scales. Recognition and correct interpretation of these bounding surfaces is essential to reconstruction of palaeochannel dimensions and to flow modelling for hydrocarbon exploration. It is therefore crucial to understand the suite of processes that form and transfer these surfaces into the fluvial sedimentary record. Here, the numerical model ‘NAYS2D’ is used to simulate a highly sinuous meandering river with synthetic stratigraphic architectures that can be compared directly to the sedimentary record. Model results highlight the importance of spatial and temporal variations in channel depth and migration rate to the generation of channel and bar deposits. Addition of net uniform bed aggradation (due to excess sediment input) allows quantification of the preservation of meander morphology for a wide range of depositional conditions. The authors find that the effect of vertical variation in scouring due to channel migration is generally orders of magnitude larger than the effect of bed aggradation, which explains the limited impact bed aggradation has on preservation of meander morphology. Moreover, lateral differences in stratigraphy within the meander belt are much larger than the stratigraphic imprint of bed aggradation. Repeatedly produced alternations of point bar growth followed by cut‐off result in a vertical trend in channel and scour feature stacking. Importantly, this vertical stacking trend differs laterally within the meander belt. In the centre of the meander belt, the high reworking intensity results in many bounding surfaces and disturbed deposits. Closer to the margins, reworking is infrequent and thick deposits with a limited number of bounding surfaces are preserved. These marginal areas therefore have the highest preservation potential for complete channel deposits and are thus best suited for palaeochannel reconstruction.     

Floodplain sedimentology of the Squamish River, British Columbia: relevance of element analysis

Element analysis of modern-day floodplains provides a framework for characterizing associations amongst depositional forms, the processes responsible for them and their local depositional environment. From interpretation of the spatial association of elements, mechanisms of floodplain evolution can be analysed. The Squamish River, in southwestern British Columbia, is a high-energy, gravel-based river, which exhibits a distinct downstream gradation in channel planform type. The floodplain sedimentology of this river is evaluated using an element approach. Five elements, defined on the basis of their morphological outline, position within sediment sequences and sedimentological character, describe the floodplain sedimentology: (i) top-stratum, (ii) chute channel; (iii) ridge; (iv) bar platform; (v) basal channel gravels. The sedimentological composition of each element is described. Each of these units relates directly to morphostratigraphic units which make up contemporary bars of the Squamish River. Associations among facies defined at the bedform scale, morphostratigraphic units on bar surfaces and elemental floodplain features are described and explained. The vertical stacking arrangement of elements is analysed in trenches (dug perpendicular to the main channel) and in bank exposures. Two elemental sedimentology models are proposed. In the first model, bar platform sands are discontinuous above basal channel gravels. Chute channel, ridge and proximal topstratum elements form thick sequences above. The second model is characterized by sequences in which distal top-stratum deposits are observed. In these instances, bar platform sands are better preserved beneath the distal top-stratum element, with proximal top-stratum elements above. The applicability of these models is determined primarily by position on the floodplain. Chute channel reworking of floodplain sediments and replacement by top-stratum elements is the dominant process marginal to contemporary bars. Sites in which channel avulsion has resulted in preservation of distal top-stratum deposits in the midsequence of the present-day channel banks determine the occurrence of the second model. Although channel planform style changes down-valley in the study reach from braided to meandering, these two models apply in each reach. It is concluded that processes operative at the element scale, rather than the channel planform scale, determine floodplain sedimentology.     

Geoarchaeological simulation of meandering river deposits and settlement distributions: A three‐dimensional approach

Fluvial processes have the potential to obscure, expose, or even destroy portions of the archaeological record. Floodplain aggradation can bury and hide archaeological features, whereas actively migrating channels can erode them. The archaeological record preserved in the subsurface of a fluvial system is potentially fragmented and is three‐dimensionally complex, especially when the system has been subjected to successive phases of alluviation and entrenchment. A simulation model is presented to gain insight into the threedimensional subsurface distribution, visibility, and preservation potential of the archaeological record in a meander‐floodplain system as a function of geomorphic history. Simulation results indicate that fluvial cut‐fill cycles can strongly influence the density of archaeological material in the subsurface. Thus, interpretation of floodplain habitation based solely upon features visible in the shallow subsurface (through traditional techniques such as aerial photography and geophysical prospection) can be misleading. In the examples, the loss of archaeological record by channel migration ranges between 45% and 90% over 12,000 years for channel belt‐dominated systems, decreasing to 10 to 30% for rivers where the floodplain width is a multiple of channel belt width. The modeling presented can be used to test excavation strategies in relation to hypothesized scenarios of stratigraphic evolution for archaeological sites. © 2006 Wiley Periodicals, Inc.     

Late Quaternary multiple incised valley systems: An unusually well‐preserved stratigraphic record of two interglacial valley‐fill successions from the Arno Plain (northern Tuscany,Italy)

Un‐fragmented stratigraphic records of late Quaternary multiple incised valley systems are rarely preserved in the subsurface of alluvial‐delta plains due to older valley reoccupation. The identification of a well‐preserved incised valley fill succession beneath the southern interfluve of the Last Glacial Maximum Arno palaeovalley (northern Italy) represents an exceptional opportunity to examine in detail evolutionary trends of a Mediterranean system over multiple glacial–interglacial cycles. Through sedimentological and quantitative meiofauna (benthic foraminifera and ostracods) analyses of two reference cores (80 m and 100 m long) and stratigraphic correlations, a mid‐Pleistocene palaeovalley, 5 km wide and 50 m deep, was reconstructed. Whereas valley filling is chronologically constrained to the penultimate interglacial (Marine Isotope Stage 7) by four electron spin resonance ages on bivalve shells (Cerastoderma glaucum), its incision is tentatively correlated with the Marine Isotope Stage 8 sea‐level fall. Above basal fluvial‐channel gravels, the incised valley fill is formed by a mud‐prone succession, up to 44 m thick, formed by a lower floodplain unit and an upper unit with brackish meiofauna that reflects the development of a wave‐dominated estuary. Subtle meiofauna changes towards less confined conditions record two marine flooding episodes, chronologically linked to the internal Marine Isotope Stage 7 climate‐eustatic variability. After the maximum transgressive phase, recorded by coastal sands, the interfluves were flooded around 200 ka (latest Marine Isotope Stage 7). The subsequent shift in river incision patterns, possibly driven by neotectonic activity, prevented valley reoccupation guiding the northward formation of the Last Glacial Maximum palaeovalley. The applied multivariate approach allowed the sedimentological characterization of the Marine Isotope Stage 7 and Marine Isotope Stage 1 palaeovalley fills, including shape, size and facies architecture, which revealed a consistent river‐coastal system response over two non‐consecutive glacial–interglacial cycles (Marine Isotope Stages 8 to 7 and Marine Isotope Stages 2 to 1). The recurring stacking pattern of facies documents a predominant control exerted on stratigraphy by Milankovitch and sub‐Milankovitch glacio‐eustatic oscillations across the late Quaternary period.     

Fluvial styles,palaeohydrology and modern analogues of an exhumed,Cretaceous fluvial system: Cerro Barcino Formation,Cañadón Asfalto Basin,Argentina

Superb surface exposures of a fluvial palaeochannel-belt 2–5 km wide and 13 km long occur in Cretaceous sedimentary rocks of the Cerro Barcino Formation (Las Plumas Member), Chubut Group, Cañadón Asfalto Basin, Argentina. The exceptional plan-view exposures of fluvial sandbodies allowed identification of a large number of WNW–ESE sinuous ridges that represent high-sinuosity, braided and low-sinuosity fluvial palaeochannels, crevasse and chute palaeochannels, and crevasse-splay deposits. Predominance of low-sinuosity palaeochannels and low values of the width/thickness ratio of most them suggest high stability of their margins, probably controlled by the cohesiveness of tuffaceous floodplain deposits. Morphological and sedimentological observations provide evidence of abrupt lithofacies changes in palaeochannel fills, fine-grained intercalations in multi-storey palaeochannels and occurrence of different fluvial styles, implying temporal/spatial variations in palaeodischarge and/or slope conditions. Comparison of the near-identical plan view morphology of the Cretaceous palaeochannel belt with the nearby Chubut River, plus the comparison of palaeohydrological data of the exhumed palaeochannels with hydrological data from this modern analogue, gives evidence of a very similar fluvial behaviour. This provides an integrative tool to be employed in the study of other ancient fluvial successions, including the reconstruction of palaeofluvial hydrological parameters in planetary geology.     

Exhumed channel sandstone networks within fluvial fan deposits from the Oligo-Miocene Caspe Formation, South-east Ebro Basin (North-east Spain)

The Oligo‐Miocene Caspe Formation corresponds to the middle fluvial facies of the wider Guadalope‐Matarranya fluvial fan, located in the South‐east Ebro foreland basin (North‐east Spain). At the time of the Caspe Formation deposition, this sector of the Ebro basin underwent a very continuous, moderate sedimentation rate. Lithofacies comprise deposits from channellized and unchannellized flows. Channellized flow lithofacies form multi‐storey ribbon‐like sandstone bodies that crop out as extensive sandstone ridges belonging to exhumed channel networks. Width/thickness ratios of these channel‐fill bodies average close to six. Sinuosity is usually low (most common values around 1·1), although it can be high locally (up to 2). Thicknesses range from a few metres to 15 m. Unchannellized flow lithofacies form tabular bodies that can be ascribed to overbank deposits (levées, crevasse splays and fine‐grained floodplain deposits) and also to frontal lobes, although recognition of this last case requires exceptional outcrop conditions or geophysical subsurface studies. The unchannellized flow lithofacies proportion ranges from 75% to 97·8%. Methods applied to this study include detailed three‐dimensional architectural analysis in addition to sedimentological analysis. The architecture is characterized by an intricate network of highly interconnected ribbon‐like sandstone bodies. Such bodies are connected by three kinds of connections: convergences, divergences and cross‐cuttings. Although the Caspe Formation lithofacies and architecture resemble anastomosed channels (low topographic gradient, high preservation potential, moderate aggradation rate, high lateral stability of the channels, dominance of the ribbon‐like morphologies and high proportion of floodplain to channel‐fill sediments), an unambiguous interpretation of the channel networks as anastomosed or single threaded cannot be established. Instead, the observed architecture could be considered as the product of the complex evolution of a fluvial fan segment, where different network morphologies could develop. A facies model for aggrading ephemeral fluvial systems in tectonically active, endorheic basins is proposed.     

Palaeochannel controlled Cretaceous sandstone-type uranium deposit of Lostoin area,Mahadek basin,Meghalaya

Sandstone–type uranium deposit hosted by reduced feldspathic sandstone sequence of lower part of Cretaceous Mahadek Formation occurs at Lostoin area of Mahadek basin and is confined within a prominent palaeochannel of NNE-SSW trend with flow towards SSW. The channel-fill comprises of about 10 m to 55 m grey to dark grey coloured, reduced, coarse to medium grained feldspathic sandstone along with abundant carbonaceous matter (5 to 15% by visual estimation) and is overlain by 5 m to 80 m of oxidized sub-feldspathic sandstone. Correlatable medium grade (～ 0.1% U₃O₈) uranium mineralization is intercepted in the boreholes aligned along the palaeochannel. Uraniferous solutions generated from the fertile provenance of south Khasi batholith flow through the channel and precipitated uranium at suitable reduction traps. Gentle slope of 2° to 3° of the basement channel facilitated prolonged interaction of uraniferous solution with reductants causing ore grade concentration. It has been envisaged that the control of palaeochannel and palaeobasement topography of the ore body would continue towards the SSW beyond Wahblei river.     

Environmental responses to Lateglacial climatic fluctuations recorded in the sediments of pre‐Alpine Lake Mondsee (northeastern Alps)

Investigation of the sedimentary record of pre‐Alpine Lake Mondsee (Upper Austria) focused on the environmental reaction to rapid Lateglacial climatic changes. Results of this study reveal complex proxy responses that are variable in time and influenced by the long‐term evolution of the lake and its catchment. A new field sampling approach facilitated continuous and precisely controlled parallel sampling at decadal to sub‐annual resolution for µ‐XRF element scanning, carbon geochemistry, stable isotope measurements on ostracods, pollen analyses and large‐scale thin sections for microfacies analysis. The Holocene chronology is established through microscopic varve counting and supported by accelerator mass spectrometry ¹⁴C dating of terrestrial plant macrofossils, whereas the Lateglacial age model is based on δ¹⁸O wiggle matching with the Greenland NGRIP record, using the GICC05 chronology. Microfacies analysis enables the detection of subtle sedimentological changes, proving that depositional processes even in rather large lake systems are highly sensitive to climate forcing. Comparing periods of major warming at the onset of the Lateglacial and Holocene and of major cooling at the onset of the Younger Dryas reveals differences in proxy responses, reflecting threshold effects and ecosystem inertia. Temperature increase, vegetation recovery, decrease of detrital flux and intensification of biochemical calcite precipitation at the onset of the Holocene took place with only decadal leads and lags over a ca. 100 a period, whereas the spread of woodlands and the reduction of detrital flux lagged the warming at the onset of the Lateglacial Interstadial by ca. 500–750 a. Cooling at the onset of the Younger Dryas is reflected by the simultaneous reaction of δ¹⁸O and vegetation, but sedimentological changes (reduction of endogenic calcite content, increase in detrital flux) were delayed by about 150–300 a. Three short‐term Lateglacial cold intervals, corresponding to Greenland isotope substages GI‐1d, GI‐1c2 and GI‐1b, also show complex proxy responses that vary in time. Copyright © 2011 John Wiley & Sons, Ltd.     

Alluvial records of medieval and prehistoric tin mining on Dartmoor,southwest England

The role of tin mining in the society of prehistoric Dartmoor and its impact on the local landscape have long been discussed despite equivocal evidence for prehistoric mine sites. A fluvial geomorphological approach, using floodplain stratigraphy, combined with sediment geochemistry and mineralogy, was employed to identify prehistoric tin mining at the catchment scale. Waste sediment, released during hydraulic mining of alluvial tin deposits, caused downstream floodplain aggradation of sands with a diagnostic signature of elevated Sn concentration within the silt fraction. At a palaeochannel site in the Erme Valley, sediment aggradation buried datable peat deposits. A period of aggradation postdating cal. A.D. 1288–1389 is consistent with the 13th century peak in tin production identified in the documentary record. An earlier phase of aggradation, however, occurred between the 4th and 7th centuries A.D., providing evidence of late Roman or early Post Roman tin mining activity on Dartmoor. © 2004 Wiley Periodicals, Inc.     

Role of permeability barriers in alluvial hydromorphic palaeosols: The Eocene Pondaung Formation,Myanmar

This study examines the lateral distribution of hydromorphy in the fine‐grained alluvial deposits of the Eocene Pondaung Formation, central Myanmar. Through detailed outcrop analysis and using a combined sedimentological and pedological approach, this study proposes a reconstruction of Pondaung overbank floodplain palaeoenvironments. The variations of hydromorphic features in the different overbank sub‐environments are then discussed and used to build a model of hydromorphic variability in alluvial deposits. Two main architectural associations with distinctive lithofacies and pedogenic features were identified, corresponding to different sub‐environments: heterolithic deposits and extensive mudstone successions. The heterolithic deposits display variegated fine‐grained lithofacies and contain poorly developed palaeosols with gley and vertic features, which are interpreted to reflect a seasonal wetlands landscape, developed in actively aggrading avulsion belts. Extensive mudstone successions with Vertisols that locally exhibit mukkara‐style pseudogley features are interpreted to represent a distal open‐forested environment. The palaeosols of both sub‐environments display dense local hydromorphic variations they are also characterized by a gradual shift from gley‐dominated to pseudogley‐dominated features with increasing distance from the avulsion belt. The clay‐dominated lithology of the floodplain parent material, which forms numerous subsurface permeability barriers, is shown to have acted as a fundamental control in limiting water‐table dynamics in coarse‐grained parts of the succession, thereby favouring hydromorphic variability. Palaeosol sequences of the Pondaung Formation contrast with the soil‐landscape associations described in other studies and provide an alternative model with which to account for the hydromorphic variability in poorly drained, alluvial soils. The model proposed as an outcome of this study demonstrates that hydromorphic variations can be dramatic in floodplains where permeability barriers are numerous. Further, the model stresses the importance of undertaking detailed lateral palaeosol analyses prior to making interpretations regarding hydromorphic variability.