Meandering River Deposits Research Articles

Reconstructing sediment distribution in meandering river deposits through a simplified numerical modelling approach, with applications to the Holocene deposits of the Venetian Plain (Italy)

Abstract Many present-day alluvial floodplains display traces of abandoned meandering channel belts developed during the past millennia (i.e. mid to late Holocene). Deposits of these ancient rivers represent preferential pathways for groundwater flows and related environmental issues, such as contaminant propagation or saltwater intrusion in coastal areas. However, since formative bankfull flows in such old and abandoned routes are hard to estimate, fossil meanders have not been commonly addressed by morphodynamic numerical models, and most of them have been investigated following classical sedimentological approaches based mainly on punctual data derived from sedimentary cores. This study aims at investigating the sediment distribution within different fossil bends on the southern Venetian Plain (NE Italy), and relating such distribution to numerically modelled bed shear stresses used herein as a proxy of sediment sorting patterns. For this purpose, formative flows in the studied palaeomeanders are first inferred from measured sediment grain size and estimates of bend widths. Then, shear stress distributions are computed along the studied palaeobends using a 2D linearized model. Model results are finally compared with conductivity distributions gauged directly in the field through electromagnetic induction investigations in the frequency domain. Our results suggest significant correlations between shear stress distributions and sediment sorting estimated from conductivity data. We deem that the integration between sedimentological reconstructions and state-of-the-art numerical modelling can provide a solid contribution to predicting the spatial distribution of sediment properties within ancient meandering channel belts, with relevant implications for the understanding of shallow aquifer dynamics and soil management.

Application of seismic sedimentology in characterization of fluvial-deltaic reservoirs in Xihu sag, East China Sea shelf basin

The fluvial-deltaic reservoirs of the Oligocene Huagang Formation in the Xihu sag of the East China Sea shelf basin reflect rapid lateral change in sedimentary facies and poor morphology of conventional slice attributes, which bring difficulties to the reservoir prediction for subsequent exploration and development of lithologic reservoirs. The traditional seismic sedimentology technology is optimized by applying the characteristic technologies such as frequency-boosting interpretation, inversion-conventional −90° phase shift joint construction of seismic lithologic bodies, nonlinear slices, paleogeomorphology restoration, and multi-attribute fusion, to obtain typical slice attributes, which are conducive to geological form description and sedimentary interpretation. The Huagang Formation developed three types of sedimentary bodies: braided river, meandering river and shallow water delta, and the vertical sedimentary evolution was controlled by the mid-term base-level cycle and paleogeomorphology. In the early–middle stage of the mid-term base-level ascending cycle, braided channel deposits were dominant, and vertical superimposed sand bodies were developed. In the late stage of the ascending half-cycle and the early stage of the descending half-cycle, meandering river deposits were dominant, and isolated sand bodies were developed. In the middle–late stage of the descending half-cycle, shallow-water delta deposits were dominant, and migratory medium–thick sand bodies were developed. Restricted paleogeomorphology controlled the sand body distribution, while non-restricted paleogeomorphology had little effect on the sand body distribution. Based on reservoir characterization, the fault sealing type and reservoir updip pinch-out type structural lithological traps are proposed as the main directions for future exploration and development in the Xihu sag.

Updated repository for "Reconstructing sediment distributions in meandering river deposits through a simplified numerical modelling approach with applications to the Holocene deposits of the Venetian Plain (Italy)"

Updated repository for "Reconstructing sediment distributions in meandering river deposits through a simplified numerical modelling approach with applications to the Holocene deposits of the Venetian Plain (Italy)"

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Repository 2 for "Reconstructing sediment distributions in meandering river deposits through a simplified numerical modelling approach with applications to the Holocene deposits of the Venetian Plain (Italy)"

Repository for "Reconstructing sediment distributions in meandering river deposits through a simplified numerical modelling approach with applications to the Holocene deposits of the Venetian Plain (Italy)"

Repository for "Reconstructing sediment distributions in meandering river deposits through a simplified numerical modelling approach with applications to the Holocene deposits of the Venetian Plain (Italy)"

Sedimentology and basin-fill history of the Cenozoic succession of the Sylhet Trough, Bengal Basin, Bangladesh

The Sylhet Trough, a petroleum province of the Bengal Basin, accommodates a huge thickness of Eocene to Recent sedimentary successions. However, the basin-fill history of the trough is poorly understood; specifically, constrains on timing of deposition of the individual units are yet to be established. Therefore, we aimed at establishing sedimentation and basin-fill history of the Sylhet Trough based on detailed lithofacies analysis of the outcropping Cenozoic succession. We have divided the entire Cenozoic succession into three megasequences which can be further sub-divided into nine lithostratigraphic units based on bounding discontinuities, such as transgressive erosion surface, regressive erosion surface, transgressive surface, marine flooding surface, and incised valley floor. The oldest is the Megasequence 1, comprised of shallow marine shelfal deposits overlain by shallow marine to nearshore deposits. In the middle, the Megasequence 2 is representing tide-dominated marine to coastal (deltaic) depositional systems with evidence of cyclic marine regression and transgression. Repetitive occurrence of incised channel, tidal inlet, tidal ridge/shoal, tidal flat and other tidal deposits are separated by shelfal deposits. The top of the Megasequence 2 is marked by a pronounced erosion surface interpreted as an incised valley floor indicating the final phase of marine regression followed by the gradual establishment of the overlying continental-fluvial depositional systems (i.e., the Megasequence 3). This youngest megasequence is characterized by stacked braided river sand bars that pass up-sequence into meandering river deposits. Based on the Cenozoic eustatic sea level curve, we suggest that the upper boundaries of the Megasequence 1 and Megasequence 2 are approximately at 39.5 Ma and 5.0 Ma, respectively.

Trace-fossil suites and composite ichnofabrics from meandering fluvial systems: The Oligocene Lower Freshwater Molasse of Switzerland

The Oligocene Lower Freshwater Molasse of Switzerland is host to spectacular outcrops of fluvial deposits. In distal areas of the Swiss Molasse Basin, deposits of a north-eastward draining meandering system are exposed. Combined sedimentologic and ichnologic observations in the Chli Sunnhalde and Roggwil quarries allow an improved understanding of animal-substrate interactions in deposits of meandering fluvial systems. Strata include meander-belt sandstone, levee sandstone and mudstone, crevasse-channel and splay sandstone, floodplain mudstone and sandstone, and paleosols. While the meander belt and the more proximal crevasse-channel deposits are typically unbioturbated, the more distal deposits show a clear biogenic overprint, with distal crevasse-channel, crevasse-splay, and floodplain deposits being intensely bioturbated.Simple grazing and locomotion trails (included in the Cochlichnus anguineus-Helminthoidichnites tenuis suite) were formed subaqueously in ponded areas of the floodplain. Meniscate burrows (Taenidium isp. suite) and root trace fossils occur along the margin of these floodplain water-bodies and at the top of crevasse-splay deposits. Some of the floodplain, crevasse-splay, and crevasse-channel deposits are pervasively bioturbated by crayfish burrows (Camborygma litonomos suite). Crevasse-splay deposits locally host escape trace fossils.Where cross-cutting relationships are apparent, a colonization sequence can be established as follows: the Cochlichnus anguineus-Helminthoidichnites tenuis suite is cross-cut by the Taenidium isp. suite, which in turn is cross-cut by the Camborygma litonomos suite and root trace fossils. Composite ichnofabrics are common in these meandering river deposits, recording progressive lowering of the water table in overbank subenvironments. Frequency of depositional events was a first-order control on degree of bioturbation. Position of the water table and degree of substrate consolidation exerted considerable influence over the types of trace fossils produced. Distinct bioturbation phases may result in the formation of different trace-fossil suites - representative of disparate ichnofacies - preserved on a single bedding plane. This study further supports the notion that meandering fluvial systems are characterized by trace-fossil suites belonging to the Scoyenia, Camborygma and/or Mermia ichnofacies.

Quantitative Characterization of Reservoir Architecture of Meandering-River Deposits in Sparse Well Blocks: A Case Study of Chunguang Oilfield, Junggar Basin

Quantitative Characterization of Reservoir Architecture of Meandering-River Deposits in Sparse Well Blocks: A Case Study of Chunguang Oilfield, Junggar Basin

Sedimentary Evidences for the Depositional and Environmental Changes Regarding to Regression during Mid to Late Miocene in the Neogene Adana Basin (Southern Turkey)

In this study we investigate sedimentary and stratigraphic characteristics of the Mid to Late Miocene deposits to better understanding of the Tertiary evolution of the Adana Basin (Southern Turkey). For this purpose, detailed mapping, sedimentary survey and interpretations were made considering the stratigraphic relations in the Middle-Late Miocene units in the region. The depositional evolution of the Adana Basin starts during the Early Miocene as a foreland basin within the Çukurova region. An extensive marine transgression during the Early Miocene commenced the filling of the basin. The pre-Miocene deposition area was a large platform with an adjacent depression area. This depositional environment widely controlled the Miocene sedimentation after the Aquitanian-Burdigalian transgression. The Miocene sequences in the Adana Basin were deposited due to the northward transgression. The northerly sourced deltaic and littoral-brackish Kaplankaya Formation and a slightly deeper reefal Karaisalı Formation were deposited in the areas close to the shore. Documented by shallow to deeper marine sedimentation, the marine inundation of the region occurred during Aquitanian-Burdigalian and continued into the Serravallian causing the former coastline to migrate north. Synchronously tectonic subsidence further south cause southward progressive deepening. Therefore, various facies types, ranging from deep marine turbidites to terrestrial, deposited in the basin. The northern margin of the basin was experienced normal faulting in consequence of progressive tectonic and subsidence events during this marine transgression in the Langhian-Serravallian. Mapped and interpreted within three facies types, Güvenç Formation include; i) deep marine sediments, ii) offshore sediments and iii) shallow marine sediments. Deep sea deposits were accumulated in the early stages of the Early-Middle Serravalian period in the deeper part of the basin. During the Mid-to Late Serravallian offshore sediments were deposited in the southern parts of the basin. Further south, due to the continued regression, the shallow marine sediments were deposited in the basin during the Serravalian. Related to the neotectonic period of Anatolia, the transgressive sequences deposited within the basin were tilted during Late Serravallian to Early Tortonian period. The marine regression towards the south was accompanied by tectonic re-organisation in the Early Tortonian. Consequently, regressive sequences of the Tortonian age, the Kuzgun member of Kuzgun Formation, which involve shallow marine (beach) sediments and meandering river deposits are resting on the tilted Güvenç Formation with low angle unconformity.

Application of seismic architecture interpretation in enhancing oil recovery in late development Stage——Taking meandering river reservoir in Dongying depression as an example

Taking typical meandering river deposits of Ng52+3 in the west 7th block of Gudong Oilfield as an example, the influence of point-bar-sandstone superposition structure on flooding law and remaining oil distribution is studied by means of seismic architecture interpretation under the guidance of point-bar geometry. Meanwhile, the influence of morphological parameters and lithology of lateral accretionary surfaces on remaining oil and development characteristics is discussed. The seismic waveform and amplitude of point-bar boundary sandstones are obvious when they overlap with each other. The connectivity of superimposed zones between boundary sandbodies of point-bars is poor. The remaining oil near the superimposed zones is enriched when base-of-channel sandstone that infills abandonment (PI) and the earliest sandstone at the beginning of migration of the point bars (PI) superimposed with each other. Remaining oil is relatively not enriched when PI sandstones of point-bars are superimposed in superimposed zone. Remaining oil is relatively enriched in the upper point-bar sandstone at the superimposed zone. The effective shielding area effective distance to obstruct fluid flow of mudstone section of lateral accretionary surfaces is 50–80 m along the lateral direction of lateral-accretion surfaces, and the remaining oil is not enriched in the same point bar within the range of 50–80 m around oil and water wells. The physical property section of lateral-accretion surfaces in the middle and lower part of the surfaces has a certain blocking effect on flow and can obviously reduce the recovery degree. Seismic architectural interpretation plays an important role in predicting remaining oil enriched areas and prolonging oil field life in order to improve oil recovery.

Organic Matter Type, Maturity, Depositional Environmental Characteristics, and Liquid Hydrocarbon Potential of Late Carboniferous Kozlu Bituminous Coal and Coaly Shale Beds (Zonguldak-Amasra Basin, NW Anatolia, Turkey): An Application of Biomarker Geochemistry

The Upper Carboniferous (Westphalian A) Kozlu Formation consisting chiefly of sandstone, thick coal layers, shale, and conglomerate levels crops out in the Zonguldak-Amasra Basin, NW Anatolia. The Kozlu Formation contains a total of 20 mineable coal layers with the thickness varying from 0.5 to 6 m. The lower level of formation is represented by lacustrine deposits, whereas the upper part is made up of thick flood plain sediments and meandering river deposits bearing laterally continuous coal levels. In this study, coal samples taken from coal layers within the Kozlu Formation at the Kozlu Underground Coal Mining site were evaluated using the data obtained from pyrolysis/total organic matter (TOC), gas chromatography, and gas chromatography–mass spectrometry. The average total organic matter (TOC) value of Kozlu coals is 40.28%. The coals are characterized by relatively high hydrogen index (HI) value (average 262 mg HC/gTOC) and very low oxygen index value (2 mg CO2/gTOC). Pyrolysis data indicate that coals contain dominantly type II and less amount of type III kerogen, and Tmax values range from 454 to 468 °C. In gas chromatographs, the recorded distribution consists predominantly of low-carbon-numbered n-alkanes and subordinately of high-carbon-numbered n-alkanes and the terrigenous/aquatic ratio value is very low (0.05–0.09). Pristane abundance is greater than that of phytane, and Pr/Ph ratios are in the range of 1.11–1.60. The sterane abundances in the Kozlu coals are in the following order: C29 > C28 > C27. Coals have high C19 and C20 tricyclic terpane concentrations and high (C19 + C20)/C30 ratio, high C30* (diahopane) and C29Ts concentrations, and high C30*/C29Ts ratio and low C31R/C30 and C29/C30 hopane ratios. The dibenzothiophene-to-phenanthrene ratio of Kozlu coals is found to be very low (0.04–0.14). Based on the pyrolysis and biomarker data, the Kozlu coals are interpreted as being deposited in a suboxic–oxic continental environment in which there is effective input of clay and dominantly terrestrial (with significant lipid-rich components) and bacterial organic matter. High Tmax values, CPI values close to 1, low moretane/hopane (0.23–0.12), equilibrated 22S/(22S + 22R) homopane (for C32), 20S/(20S + 20R) C29 sterane (0.52–0.54) and TA(I)/TA(I + II) steroid ratios, high ββ/(ββ + αα) sterane (0.51–0.55), C30*/C29Ts, C30*/(C30* + C30H), MPI-3(α/β) (1.24–1.41), and MDR and MA(I)/MA(I + II) steroid are indicative of mature–late mature organic matter. Ro values between 0.9 and 1.25% determined from Tmax (454–468 °C) values indicate “high-volatile bituminous B–medium-volatile bituminous” rank for Kozlu coals. Kozlu coals having HI values (up to 331 mg HC/gTOC) extremely higher than those of classical coals indicates that these coals have significant oil and gas generation potential, and high S1 (average 6.04 mg HC/g rock) and S2 (110.08 mg HC/g rock) values imply that they generate notably high amount of liquid hydrocarbons and still have generation potential.

Meandering river deposits in sediment cores, the Middle Jurassic Alma Field, Southern Danish Central Graben

Fluvial deposits are amongst the most important terrestrial hydrocarbon reservoirs, but the complex nature of these deposits is challenging in subsurface reservoir characterisation. This study is the first detailed facies analysis of the meandering river deposits of the Middle Jurassic Alma Field situated in the southern Danish North Sea. The fluvial sandstones and their associated deposits are described and interpreted based on studies from two core sites (Alma-1X and Alma-2X). The facies analysis of the cores demonstrates the presence of three meandering river facies associations: Channel deposits, channel margin deposits and floodplain deposits. The channel deposits comprise channel thalweg and point bar sediments, the channel margin deposits include crevasse channel and crevasse splay sediments, while the floodplain deposits comprise overbank and backswamp sediments. The point bar deposits are composed of fine- to medium-grained sandstones but can contain intervals of finer grained sediments, particularly in their upper parts where they can grade into muddy sandstones or true heterolithic deposits. Preserved sand body thicknesses (channel thalweg and point bar deposits) in both Alma cores have a mean value of 2.6 m and a maximum value of 4.35 m (Alma-1X) and 6.55 m (Alma-2X). Using maximum values of channel deposit thicknesses, and assuming the preservation conditions are met, the width of the largest ancient channel belt in Alma-1X would be between 90 m and 200 m or around 900 m, depending on whether the fluvial system is mud-rich or sand-rich. The same method applied to Alma-2X gives a width of the largest channel belt between 130 m and 330 m or around 1300 m. Fluvial sediments of the Middle Jurassic Scalby Formation (north-east England) were deposited in a sandy meandering river with sedimentary characteristics corresponding to those observed in the Alma cores. Outcrop analogue investigations of this formation were carried out to examine the architecture of the fluvial facies in a two-dimensional section with emphasis on channel thalweg and point bar deposits. Combined evidence from core analysis and outcrop analogue studies suggests that the fluvial deposits in the Alma Field represent a mixed-load meandering river system with sandy point bars. The meandering river system developed on a coastal plain with overbank fines and organic-rich backswamp deposits. The mud-rich or heterolithic deposits in the upper part of the point bar facies intervals are noteworthy and could indicate markedly fluctuating discharge in a mixed-load river.

Morphology and spacing of river meander scrolls

Many of the world's alluvial rivers are characterised by single or multiple channels that are often sinuous and that migrate to produce a mosaicked floodplain landscape of truncated scroll (or point) bars. Surprisingly little is known about the morphology and geometry of scroll bars despite increasing interest from hydrocarbon geoscientists working with ancient large meandering river deposits. This paper uses remote sensing imagery, LiDAR data-sets of meandering scroll bar topography, and global coverage elevation data to quantify scroll bar geometry, anatomy, relief, and spacing. The analysis focuses on preserved scroll bars in the Mississippi River (USA) floodplain but also compares attributes to 19 rivers of different scale and depositional environments from around the world.Analysis of 10 large scroll bars (median area = 25 km2) on the Mississippi shows that the point bar deposits can be categorised into three different geomorphological units of increasing scale: individual ‘scrolls’, ‘depositional packages’, and ‘point bar complexes’. Scroll heights and curvatures are greatest near the modern channel and at the terminating boundaries of different depositional packages, confirming the importance of the formative main channel on subsequent scroll bar relief and shape. Fourier analysis shows a periodic variation in signal (scroll bar height) with an average period (spacing) of 167 m (range 150–190 m) for the Mississippi point bars. For other rivers, a strong relationship exists between the period of scroll bars and the adjacent primary channel width for a range of rivers from 55 to 2042 mis ~50% of the main channel width. The strength of this correlation over nearly two orders of magnitude of channel size indicates a scale independence of scroll bar spacing and suggests a strong link between channel migration and scroll bar construction with apparent regularities despite different flow regimes. This investigation of meandering river dynamics and floodplain patterns shows that it is possible to develop a suite of metrics that describe scroll bar morphology and geometry that can be valuable to geoscientists predicting the heterogeneity of subsurface meandering deposits.

O-257 蛇行河川堆積物の不均質性と透水性 : 上部更新統常総層を例として

O-257 蛇行河川堆積物の不均質性と透水性 : 上部更新統常総層を例として

Late Miocene fluvial distributary system in the northern Danube Basin (Pannonian Basin System): depositional processes, stratigraphic architecture and controlling factors of the Piešťany Member (Volkovce Formation)

Aim of the study was to investigate the detailed stratigraphic architecture of the alluvial Volkovce Formation, which was deposited in the northern Danube Basin during the interval between 10 and 6 Ma. Two type locality exposures were subject to facies analysis and interpretation of depositional processes and >500 boreholes served for correlation of spatial variability in the character of the formation. The study was focused on the northern Blatne and western Risňovce depressions, which are partly separated by the Považský Inovec Mountains horst. It was revealed, that a mostly gravelly fluvial distributary system of the Piesťany Member was deposited in the northern Blatne depression, partly confined by the exposed horst and by incised palaeotopography. The sequence is composed of amalgamated braided river channel bodies with no overbank strata, in response to high sediment supply and low accommodation. The palaeo-stream then crossed the horst in its southern part towards the Risňovce depression, where there was four times more accommodation. The submerged horst acted as a boundary, where the depositional processes changed to a meandering river and significant overbank deposits started to be preserved. Only small streams were present in the northern Risňovce depression, which led to very low sediment supply compared to the high accommodation there. The depositional system was affected mainly by the incised palaeotopography, by the location of sediment input together with spatial differences in accommodation rate caused by the activity of the Ripňany Fault on the southeastern side of the Považský Inovec Mts.

Geoarchaeology in a meandering river: A study of the BK site (1.35 Ma), Upper Bed II, Olduvai Gorge (Tanzania)

BK site hosts 8 archaeological levels, preserved inside the channel of a meandering river. This river is not found in a floodplain, but encased in a wide karstified carbonate surface with no edaphic development. The river at BK has been recurrently used by hominins for carcass processing in the river channel, coinciding with a high concentration of vegetation and water resources along the channel banks. The spatial distribution of the different archaeological levels is the result of a complex sedimentary record. Such complexity in meandering river deposits is due to processes such as erosion, transport and sedimentation occurring simultaneously and coarse and very fine sediments being deposited at the same time throughout the same isochronal surface. BK site offers a unique opportunity, thanks to its abundance in archaeological remains and the quality of the outcrop, to geoarchaeologically depict and describe an archaeological site in a meandering channel. The geology of this fluvial environment is considered regarding a) fluvial architecture, b) facies distribution and c) fluvial dynamics. Furthermore, the abundance of archaeological remains and megafauna found inside the channel at BK contrasts with the absence of any remains outside the channel. We hypothesize that this area was preferred by hominins since it offers a high concentration of water resources and vegetation, as well as a greater protection against predators than an open plain. A detailed reconstruction of the paleolandscape will try to uncover the reason behind the huge contrast existing between the abundance of remains found inside and outside the channel, leading to the interpretation that an ecological or landscape related factor is conditioning the location and formation of the archaeological assemblage.

Quantitative Relation of the Point BarWidth and Meander Belt Width of Subsurface Reservoir

Due to large lateral oscillation amplitude, it tends to form complex meander belt in meandering river, which makes the internal sand body size and distribution characteristics of meandering river deposits in different region very complex. Subsurface reservoir architectural elements analysis based on the experience formula summarized from the outcrop and modern deposition research tends to have greater uncertainty. Microfacies sandbody size was measured through fine reservoir architecture research on mature area of target oilfield, it is concluded that meandering river microfacies sandbody scale is closely related to the size of the sedimentary system, the point bar width has the highest degree in related to meander belt width in the narrow banding patterns. Under different curvature, meandering river point bar length and width present a certain positive correlation, when the curvature is less than 1.7, the correlation is higher. The quantitative relation of different configuration unit of the target oilfield was established，it provided a quantitative basis for fine research and geological modeling on less well area of target oilfield and similar oilfield.

River response to climate and sea level changes during the Late Saalian/Early Eemian in northern Poland – a case study of meandering river deposits in the Chłapowo cliff section

Abstract Fluvial sediments in the Chłapowo cliff section were studied in order to reconstruct their palaeoflow conditions and stratigraphical position. Lithofacies, textural and palaeohydraulic analyses as well as luminescence dating were performed so as to achieve the aim of study. Sedimentary successions were identified as a record of point bar cycles. The fluvial environment probably functioned during the latest Saalian, shortly after the retreat of the Scandinavian Ice Sheet. Discharge outflow was directed to the northwest. The river used the older fluvioglacial valley and probably was directly connected to the Eem Sea. Good preservation and strong aggradation of point-bar cycles were related to a rapid relative base level rise. The meandering river sediments recognised showed responses to climate and sea level changes as illustrated by stratigraphical, morphological and sedimentological features of the strata described. The present study also revealed several insights into proper interpretation of meandering fluvial successions, in which the most important were: specific lithofacies assemblage of GSt (St, Sp) → Sl → SFrc → Fm (SFr) and related architectural elements: channel/sandy bedforms CH/SB → lateral accretion deposits LA → floodplain fines with crevasse splays FF (CS); upward-fining grain size and decreasing content of denser heavy minerals; estimated low-energy flow regime with a mean depth of 1.6–3.3 m, a Froude number of 0.2–0.4 and a sinuosity of 1.5.

Heterolithic meandering-channel deposits from the Neoproterozoic of NW Scotland: Implications for palaeogeographic reconstructions of Precambrian sedimentary environments

Pre-Silurian fine-grained meandering river deposits are apparently rare in the rock record. Most modern-day river dynamics are influenced by vegetation through bank stabilization, fine-grained sediment production and retention, and runoff control, leading to the development of highly sinuous, single-channel systems. In contrast, pre-vegetation river dynamics are poorly understood, in part because there are no modern-day analogues for completely non-vegetated meandering river systems, particularly under humid climates. Some models attribute the paucity of fine-grained sediments described from studies of pre-vegetation fluvial deposits to lower rates of chemical weathering in the absence of land plants. The architecture of precambrian, fine-grained meandering stream deposits is here described for the first time.The Allt na Béiste Member at the base of the Applecross Formation of the Torridon Group (Neoproterozoic, NW Scotland) is characterized by heterolithic deposits, preserving a varied suite of fluvial forms including inclined heterolithic strata, lateral-accretion sets, up to 8m-thick muddy floodplain deposits with preserved crevasse-splays. Successions of laterally-accreting strata are interbedded with metre-scale channel-fill sandstones, in a succession up to 190m-thick related to the early stages of the Applecross Formation fluvial system, as large-scale rivers buried the Lewisian palaeovalleys over which fluvial and lacustrine sediments of the Diabaig Formation had been deposited. The lacustrine fine-grained sediments, coupled with relatively low gradients of the shallow lacustrine environments and denuded, highly weathered Lewisian basement, apparently provided considerable amounts of fine-grained sediment which added sufficient cohesion to this fluvial system to induce the adoption of laterally-migrating, meandering channel planforms at the expense of multi-thread braided channels.Our data suggest that, given appropriate conditions, pre-vegetation meandering channel planforms were indeed able to develop, without the buffering effects of land plants. The paucity of fine-grained sediments in the pre-vegetation rock record may be more a consequence of preservation-related issues than the actual paucity of such sediments. These results provide novel insights into the characteristics of Earth's landscape prior to the Silurian, and provide potential analogues for meandering channels interpreted from satellite imagery of Mars.

Terrace styles and timing of terrace formation in the Weser and Leine valleys, northern Germany: Response of a fluvial system to climate change and glaciation

In glaciated continental basins accommodation space is not only controlled by tectonics and sea-level but also by the position of ice-sheets, which may act as a regional base-level for fluvial systems. Although the Pleistocene terrace record of major river systems in northwestern Europe has been investigated by many authors, relatively little attention has been paid to base-level changes related to glacier advance–retreat cycles and how these regional changes in base-level interacted with river catchment processes. This study provides a synthesis of the stratigraphic architecture of Middle Pleistocene to Holocene fluvial terraces in the upper Weser and middle Leine valley in northern Germany and links it to glaciation, climate and base-level change. The depositional architecture of the fluvial terrace deposits has been reconstructed from outcrops and high-resolution shear wave seismic profiles. The chronology is based on luminescence ages, 230Th/U ages, 14C ages and Middle Palaeolithic archaeological assemblages.The drainage system of the study area developed during the Early Miocene. During the Pleistocene up to 170 m of fluvial incision took place. A major change in terrace style from strath terraces to cut-and-fill terraces occurred during the early Middle Pleistocene before Marine Isotope Stage MIS 12, which may correlate with climate deterioration and the onset of glaciation in northern central Europe. During this time a stable buffer zone was established within which channels avulsed and cut and filled freely without leaving these vertical confines. Climate was the dominant driver for river incision and aggradation, whereas the terrace style was controlled by base-level changes during ice-sheet growth and decay. A major effect of glacio-isostatic processes was the post-Elsterian re-direction of the River Weser and River Leine.The Middle Pleistocene fluvial terraces are vertically stacked, indicating a high aggradation to degradation ratio, corresponding with a regional base-level rise during glacier advance. At the beginning of the Late Pleistocene the terrace style changed from a vertical to a lateral stacking pattern, which is attributed to a decrease in accommodation space during glacier retreat. The formation of laterally attached terraces persisted into the Holocene.Major incision phases took place during MIS 5e, 5d, 5c, and probably early MIS 4, early MIS 3 and MIS 2 (Lateglacial). During MIS 5e and the Lateglacial the braided river systems changed into meandering rivers, indicated by preserved organic-rich flood-plain and point bar deposits. The Late Pleistocene braided river systems (MIS 5c to MIS 3) are characterized by a high sinuosity, which may be a direct effect of an increased downstream gradient after deglaciation when the channel lengthened and the river adjusted to the increased gradient by increasing sinuosity. These Middle Pleniglacial fluvial deposits are unconformably overlain by Lateglacial to Holocene meandering river deposits, which form laterally attached terraces, recording millennial-scale channel shifts. The lack of Late Pleniglacial deposits might be related to Late Weichselian forebulge formation.