channel arrangements and depositional styles in the sao lourenco fluvial megafan brazilian pantanal wetland abstract the brazilian pantanal is an extensive lowland tropical basin characterized by the presence of fluvial megafans and seasonally inundated savanna floodplain wetlands with an area of about 16 000 km 2 the sao lourenco is the second largest megafan in the pantanal three distinct fluvial channel styles that formed at different times during the late quaternary are found here a geomorphological and sedimentary assessment of these depositional patterns provides valuable insight on the environmental context of their evolution new optically stimulated luminescence data indicate that the upper five meters of sediment in the sao lourenco megafan has been accumulating since the late pleistocene ancient fan lobes located in upper and intermediate fan settings consist of medium and coarse grained fluvial sands and exhibit well preserved distributary braided paleochannels on their surfaces as the megafan evolved through time pleistocene lobes were incised by a prominent valley filled with holocene aged meander belt deposits which consist of silts interbedded with very fine sands and clays currently the incised valley is a zone of sediment bypass modern deposition occurs along the distal toe of the megafan system where lobes characterized by distributary channel levee ridges are widespread these features formed by progradation of avulsion belts into a broad swampy floodbasin which caused the lower portion of the meander belt to be abandoned the significant differences observed in intra fan morphology appear to be linked to the variability in effective precipitation fan lobes deposited with braided distributary channels occurred under relatively dry conditions in the late pleistocene by contrast aggradational meander belt deposits and lobes with distributary channel levee ridges formed during fluctuating precipitation conditions of the holocene when the pantanal emerged from deglacial aridity modern lobes form under heavy seasonal flooding and deposition occurs in response to very rapid and common avulsion events these results have implications for interpreting the complexity of megafan facies in similar continental basins

hydraulic and sedimentary processes causing anastomosing morphology of the upper columbia river british columbia canada abstract the upper columbia river british columbia canada shows typical anastomosing morphology multiple interconnected channels that enclose floodbasins and lateral channel stability we analysed field data on hydraulic and sedimentary processes and show that the anastomosing morphology of the upper columbia river is caused by sediment bedload transport inefficiency in combination with very limited potential for lateral bank erosion because of very low specific stream power 2 3 w m 2 and cohesive silty banks in a diagram of channel type in relation to flow energy and median grain size of the bed material data points for the straight upper columbia river channels cluster separately from the data points for braided and meandering channels measurements and calculations indicate that bedload transport in the anastomosing reach of the upper columbia river decreases downstream because of lateral channel stability no lateral storage capacity for bedload is created therefore the surplus of bedload leads to channel bed aggradation which outpaces levee accretion and causes avulsions because of loss of channel flow capacity this avulsion mechanism applies only to the main channel of the system which transports 87 of the water and 90 of the sediment in the cross valley transect studied because of very low sediment transport capacity the morphological evolution of most secondary channels is slow measurements and calculations indicate that much more bedload is sequestered in the relatively steep upper anastomosing reach of the upper columbia river than in the relatively gentle lower anastomosing reach with anastomosing morphology and related processes e g crevassing being best developed in the upper reach this confirms the notion of upstream rather than downstream control of upper columbia river anastomosis

hydrocarbon generation migration alteration entrapment and mixing in the central and northern north sea abstract there is little doubt that sediments of upper jurassic to lowermost lower cretaceous age particularly those of the kimmeridge clay formation and its equivalents are the source of the vast bulk of the oil in the central and northern north sea this is true both of oil in conventional mesozoic plays and in tertiary clastic reservoirs the timing of oil generation and migration ranges from the late cretaceous through to the present the oil productivity of the kimmeridge clay formation in the central graben moray firth and south viking graben areas of the north sea is estimated to be in excess of 250 billion barrels of which about 25 is accounted for in accumulations discovered to date there is strong areal differentiation between oil trapped in mesozoic reservoirs as opposed to upper cretaceous chalk and tertiary reservoirs this paper is particularly concerned with hydrocarbons in tertiary reservoirs which have a wide range of compositions and appear to have complex accumulation and degradation histories migration to chalk and younger tertiary reservoirs occurs often through thick shale sequences the evidence for the timing and mechanism of this process is considered and a preferred model for the vertical migration and subsequent lateral migration mixing and degradation is proposed

timescales mechanisms and controls of incisional avulsions in floodplain wetlands insights from the tshwane river semiarid south africa abstract avulsion relocation of a river course to a new position typically is assumed to occur more frequently in rivers with faster sedimentation rates yet supporting field data are limited and the influence of sedimentation rate on avulsion style remains unclear using analysis of historical aerial photographs optically stimulated luminescence dating of fluvial sediments and field observations we document three avulsions that have occurred in the last 650 years along the lower reaches of the semiarid tshwane river in northern south africa study of the modern river and abandoned reaches reveals that a downstream decrease in discharge and stream power leads to reduced channel size and declining sediment transport capacity bank erosion drives an increase in channel sinuosity leading to a decline in local channel slope and to a further decrease in discharge and sediment transport local sedimentation rates 10 mm a 1 occur within and adjacent to the channel so over time levees and an alluvial ridge develop the resulting increase in cross floodplain gradient primes a reach for avulsion by promoting erosion of a new channel on the floodplain which enlarges and extends by knickpoint retreat during periods of overbank flow ultimately the new channel diverts the discharge and bedload sediment from the older topographically higher channel which is then abandoned our findings support the assumption that avulsion frequency and sedimentation rate are positively correlated and we demonstrate that incisional avulsions can occur in settings with relatively rapid net vertical aggradation the late holocene avulsions on the semiarid tshwane river have been driven by intrinsic autogenic processes during meander belt development but comparison with the avulsion chronology along a river in subhumid south africa highlights the need for additional investigations into the influence of hydroclimatic setting on the propensity for avulsion

facies and facies architecture of paleogene floodplain deposits willwood formation bighorn basin wyoming usa abstract paleogene deposits of the willwood formation were analyzed in two areas of the bighorn basin to provide a better understanding of the facies and facies arrangement of floodplain deposits and the various processes that influence floodplain construction and facies variability despite similar facies and facies organization floodplain deposits in the two areas differ in grain size hydromorphy and maturity of the paleosols and scale and organization of ribbon sandstones in the avulsion deposits these differences appear to be controlled by basin position of the study areas and differences in avulsion between the areas two kinds of cumulative floodplain paleosols red and purple paleosols formed on overbank deposits the red paleosols were better drained and formed on less clay rich parent material intense mottling and iron oxide nodules indicate that the purple paleosols which formed on more clay rich parent material were poorly drained the grain size differences reflect basin position in particular distance from a local sediment source avulsion deposits are volumetrically important comprising about half of the floodplain deposits in both areas the avulsion deposits resemble the stage iii splays described by smith et al 1989 smith n d cross ia dufficy j p clough s r 1989 anatomy of an avulsion sedimentology 36 1 24 in terms of characteristics of their ribbon sandstones and the presence of thin sheet sandstones or siltstones paleosols that formed on the fine grained facies show morphologic differences interpreted as developmental differences and attributed to differing rates of avulsion in the two study areas this study shows that avulsion belt paleosols provide a key to understanding avulsion rate in the stratigraphic record other differences between the willwood avulsion deposits in the two areas and between these ancient deposits and the modern avulsion deposits described by smith et al may reflect 1 problems in clearly recognizing ancient avulsion deposits in the field and 2 sampling bias resulting from the areally restricted view provided by the stratigraphic record of an areally extensive modern depositional feature at the same time because exposures are three dimensional the willwood strata reveal aspects of the facies and facies arrangement of avulsion belts not readily observed in the modern ones

interpreting avulsion process from ancient alluvial sequences guadalope matarranya system northern spain and wasatch formation western colorado alluvial deposits of the guadalope matarranya system oligocene ebro basin spain and the wasatch formation eocene western colorado provide time integrated records of the process of river channel avulsion these sequences consist of isolated channel belt sandstones incised into and abruptly overlain by flood plain siltstones indicating deposition by avulsive river systems the geometry and distribution of channel incisions suggest that avulsion was not controlled by tectonics climate or base level changes but formed by autocyclic processes measurements from 221 channel fills in the guadalope matarranya system and 38 from the wasatch formation allow us to statistically characterize channel geometries we infer to be associated with establishment and abandonment of individual river avulsions paleoflow depths in both systems average 1 4 to 1 6 m aggradation height superelevation of channel margin levees are on average 0 6 and 1 1 times paleoflow depth in the guadalope matarranya and wasatch systems respectively these results are consistent with values from recently avulsed modern rivers and suggest that 1 flow depth is the appropriate parameter against which to scale the critical superelevation necessary for channel avulsion and 2 the increase in potential energy due to channel perching drives the lateral instability that is needed for avulsion to be successful numerous stacked channel fills indicate repeated reoccupation of the same site by avulsing channels these reoccupation channels indicate that inherited flood plain topography here abandoned channel forms was an important control on the arrival site of newly avulsed channels comparison of our results to others suggests two end member types of avulsion can take place incisional avulsion seen here is characterized by an early incision phase followed by infilling by migrating bar forms aggradational avulsion begins with aggradation followed in time by stream integration into a single downcutting channel we suggest that the type of avulsion is strongly influenced by whether or not the adjacent flood plain is well or poorly drained in both cases subsequent aggradation and channel perching increase the chances that some triggering event will lead to avulsion

channel enlargement by avulsion induced sediment starvation in the saskatchewan river can localized deposition resulting from river avulsion produce downstream sediment starvation effects similar to those in channels downstream from dams we explore this question by examining a 130 km reach of the saskatchewan river canada downstream of an active 500 km 2 avulsion belt initiated in the 1870s progradation history and measurements of bedload and channel bed grain size indicate that little coarse sediment has escaped the avulsion belt since its inception comparisons of 17 channel cross sections surveyed in 1911 1954 1956 and 2011 2012 show that bankfull areas enlarged by deepening and widening in the upper two thirds of the study reach since 1911 between 1911 and 1954 1956 average channel area increased by 35 in the upper 45 km and as much as 44 locally from 1954 1956 to 2011 2012 additional enlargement occurred in the upper 35 km averaging 52 since 1911 these increases occurred despite a 24 decrease in mean annual discharge since 1913 these changes are interpreted as effects of sediment sequestration in the upstream avulsion belt net erosion by sediment deficient outflows from areas of avulsive deposition may be an underappreciated process in evolving floodplains

the potential application of stable isotopes for distinguishing indigenous versus migrated hydrocarbons in a mature shale sandstone sequence abstract bitumens isolated from kokelsumer heide no 1 corehole ruhr area f r g are indicative of the problem of resolving contributions of indigenous versus migrated fluids within a depth interval of approximately 50 m thermally mature shales containing types ii and iii organic matter and a coal seam 1 2 r m are all encountered in addition to interbedded sands that are present in the upper portion of this interval presented here is an approach based on the unique isotopic compositions of indigenous bitumens kerogens and migrating fluids which appears to enable the evaluation of the proportion of extracts which migrated from a source rock stable carbon isotope values of kerogens and of saturate and aromatic fractions of residual bitumens isolated from kokelsumer heide no 1 and reckelsumer heide no 2 corehole samples are used to estimate the relative contributions of the various sources to the bitumens preliminary results indicate that from 10 shale sequences to 100 sandstone sequences of the extractable materials i e bitumens could be characterized as a migrating fluid also stable carbon and nitrogen isotope values for kerogens and coal samples from the kokelsumer heide corehole can be used to verify previous geochemical and microscopic evidence for the existence of distinct kerogen types i and ii in this corehole

fluvio lacustrine sedimentary processes and landforms on the distal paraguay fluvial megafan brazil abstract tropical fluvio lacustrine depositional processes along the toe of the paraguay fluvial megafan were investigated with a focus on lake uberaba lu the largest lake in the pantanal wetlands a limnogeological analysis of lu was conducted using remote sensing imagery lake water chemistry sedimentological and radiochemical data from lake floor samples and short cores echo sounding revealed that lu is deepest z max 3 5 m near its southern shoreline and outlet the lake is hydrologically open throughout the year and is characterized by a dilute ca 2 na hco 3 chemistry key environmental controls on sedimentation in lu include inflowing transverse and axial rivers and water level changes which influence the composition mixing accumulation and the organization of facies structureless brown sandy silts with relatively low organic carbon toc mean 1 7 wt and low to moderate biogenic silica bisi mean 1 9 wt comprise most offshore lake floor sediments whereas northern lake margin sediments consist of peaty silts and sandy silts whose origins are linked to inflowing paraguay megafan distributary channels sediment core stratigraphy revealed that extant lacustrine deposits are underlain by oxidized clayey silts and sands suggesting recent subaerial exposure of the basin floor sedimentary datasets conclusively suggest that lu is a shallow overfilled lake basin that is strongly influenced as a depositional system by its persistently open hydrology shoreline channel density and absence of margin coincident topography this study provides new insights on lacustrine landform development in distal megafan settings which is important for improving our understanding of fluvial landscapes

sedimentology geochemistry and reservoir potential of the organic rich qusaiba shale tabuk basin nw saudi arabia abstract the qusaiba shale formation of the qalibah group is the most prolific source rock for the paleozoic petroleum system in saudi arabia and is recently considered as a potential unconventional shale gas reservoir detailed sedimentological petrographical and geochemical analyses were undertaken on an 31 m thick outcrop of this organic rich shale exposed in the northwest of tayma city nw of saudi arabia to provide a better understanding of the vertical and lateral lithofacies variations and the effects of such variations on reservoir properties x ray diffraction analysis reveals that clay minerals average 50 and quartz average 28 dominate the bulk mineralogical composition while k feldspar plagioclase and pyrite occur as common to minor components in the analyzed samples an integration of the sedimentological description petrography and mineralogical composition allows the subdivision of the investigated outcrop section into six lithofacies these are laminated organic rich shale graptolitic concretion massive organic rich shale organic lean shale siltstone shale interbedded and cross stratified siltstone sandstone lithofacies toc content in the laminated and massive organic rich shale lithofacies ranges between 0 5 and 6 1 wt average 2 52 wt then it decreases upward 0 04 0 35 wt average 0 18 wt in the overlying organic lean shale and siltstone lithofacies trace elements cu mo as ni co u and v show positive excursions in the lowermost 20 m of the organic rich dark grey shale interval and display abrupt depletions at the contact with overlying organic lean shale the enrichment factors of these trace elements are characterized by similar distribution patterns as their measured concentrations like the elements these factors also exhibit a significant depletion at the contact between the organic rich shale and the overlying organic lean shale out of all the redox indices v v cr v v ni th u and mo s toc were found to best distinguish between the organic rich and the overlying organic lean intervals the laminated organic rich shale lithofacies indicate deposition of hemipelagic sediments by a relatively low energy quiet anoxic bottom water most likely in a sulfidic conditions that were associated with a strongly restricted bottom water mass circulation at the time of its deposition this restricted hydrographic condition was likely enhanced in the study area due to the paleotopography that was created by the deglaciation of the early silurian time the toc richness common occurrence of pyrite and high enrichment of redox elements mo u and v in the organic rich shale intervals support the existence of anoxic bottom water at the time of deposition the abrupt decrease of the toc and the redox sensitive elements and their associated indices from the underlying organic rich lithofacies to the overlying organic lean shale lithofacies is likely due to a gradual increase in oxygen level this improved bottom water oxygenation condition might have led to a reduction in organic productivity and limited organic matter preservation in the organic lean shale lithofacies the graptolitic concretion lithofacies found in the sequence refers to an in situ syngenetic to early diagenetic origin formed close to the sediment water layer when there was little or no deposition or at periods characterized by low sedimentation rates the siltstone fine sandstone lithofacies in the topmost part of the studied section is interpreted to indicate the dominance of high energy and highly oxygenated shallow bottom water conditions during deposition probably in a middle to inner shelf depositional settings the kerogen in qs is a mixture of type iii gas prone and type iv inert kerogen tmax values with an average of 430 c and vro values average 0 58 calculated from the tmax assigned the studied section as immature qualitative evaluation of porosity allows the identification of three different types of porosity including inorganic porosity organic porosity and natural fracture porosity although the ductile clay minerals are the most abundant minerals in the lower part of the studied qs average 56 the brittle minerals quartz feldspar and pyrite form an average of 42 and exhibit a uniform distribution of brittleness in the organic rich interval

geochemical effects of primary migration of petroleum in kimmeridge source rocks from brae field area north sea ii molecular composition of alkylated naphthalenes phenanthrenes benzo and dibenzothiophenes abstract variations with depth in the quantitative distributions of di and tricyclic alkylaromatic hydrocarbons and sulfur heterocycles determined for a total of thirty two core samples from two thick shale intervals and thirteen thin interbedded shale layers of the upper jurassic age kimmeridge clay formation brae field area north sea reveal in part the effects of depletion with hydrocarbon expulsion trends of increasing depletion with approach to the nearest shale sandstone contacts as documented by a decrease in carbon normalized yields are more distinctive for the alkylnaphthalenes as compared to the alkylphenanthrenes and alkyldibenzothiophenes but less drastic than previously observed for long chain alkanes cf part i leythaeuser et al 1988 lowest yields consistently were encountered at the outermost 1 2 metres of the thick shale intervals and for the thin shale layers where the relative expulsion efficiencies were much higher for c15 to c30n alkanes 80 95 than for c2 and c3 alkylnaphthalenes 30 40 in contrast c2 and c3 alkylbenzothiophene yields were higher in these shale portions than further inside the thick shale which is interpreted to result from a post expulsion selective replenishment from the adjacent oil accumulation expulsion was not associated with any detectable fractionation among c1 and c3 alkylnaphthalenes or c1 and c2 alkylphenanthrenes as evidenced by a mass balance approach and concentration ratios of individual alkylaromatic isomers commonly used as maturity indicators based on a reconstruction of the quantitative distributions of aromatic hydrocarbons and sulfur aromatics expelled from thin shale layers from early mature through to mature stages of thermal evolution it was verified that the methylphenanthrene index mpi 1 and methyldibenzothiophene ratio mdr of the expelled oil reflects the average of the maturity levels during which the source rock has expelled petroleum

conditions for branching in depositional rivers it is often taken for granted that rivers organize transport into a single active channel in some net depositional environments however flow of water and sediment is distributed in several stable channels such branching rivers may be confined in valleys anabranching or anastomosed or unconfined on deltas distributaries and their existence confronts us with the very basic question of what governs the spatial organization of channel patterns in sedimentary landscapes current models for equilibrium channel morphology cannot predict the occurrence of branching rivers because they do not consider dynamical processes such as avulsion i e the rapid abandonment of a channel in favor of a new path at lower elevation the requisite conditions for avulsion have been the subject of ongoing debate here we resolve the conditions leading to channel avulsion and show that branching rivers occur when avulsion is the dominant mechanism of lateral channel motion a compilation of field and laboratory data demonstrates that avulsion frequency scales with the time required for sedimentation on channel beds to produce a deposit equal to one channel depth from the relative rates of bank erosion and channel sedimentation we derive a dimensionless mobility number that accurately predicts the conditions under which anabranching and distributary channels occur results may be directly applied to modeling landscape evolution over human and geologic time scales and for inverting formative environmental conditions from channel deposits on earth and other planetary surfaces

predicting delta avulsions implications for coastal wetland restoration river deltas create new wetlands through a continuous cycle of delta lobe extension avulsion and abandonment but the mechanics and timing of this cycle are poorly understood here we use physical experiments to quantitatively define one type of cycle for river dominated deltas the cycle begins as a distributary channel and its river mouth bar prograde basinward eventually the mouth bar reaches a critical size and stops prograding the stagnated mouth bar triggers a wave of bed aggradation that moves upstream and increases overbank flows and bed shear stresses on the levees an avulsion occurs as a time dependent failure of the levee where the largest average bed shear stress has been applied for the longest time r 2 0 93 these results provide a guide for predicting the growth of intradelta lobes which can be used to engineer the creation of new wetlands within the delta channel network and improve stratigraphic models of deltas

bifurcation dynamics and avulsion duration in meandering rivers by one dimensional and three dimensional models at river bifurcations water and sediment are divided over two branches the dynamics of the bifurcation determine the long term evolution centuries of the downstream branches potentially leading to avulsion but the dynamics are poorly understood the long term evolution can only be studied by one dimensional models because of computational costs for such models a relation describing the sediment division is necessary but only few relations are available and these remain poorly tested so far we study the division of sediment and the morphodynamics on a timescale of decades to centuries by idealized three dimensional modeling of bifurcations with upstream meanders and dominantly bed load transport an upstream meander favors one bifurcate with more sediment and the other with more water leading to destabilization the bifurcations commonly attain a highly asymmetrical division of discharge and sediment after a few decades to a few centuries depending on combinations of the relevant parameters although past work on avulsions focused on slope advantage we found that bifurcations can be quasibalanced by opposing factors such as a bifurcate connected to the inner bend with a downstream slope advantage nearly balanced bifurcations develop much slower than unbalanced bifurcations which explains the observed variation in avulsion duration in natural systems which branch becomes dominant and the timescale to attain model equilibrium are determined by the length of the downstream bifurcates the radius of the upstream bend a possible gradient advantage for one bifurcate and notably the width depth ratio the latter determines the character of the bars which may result in overdeepening and unstable bars the distance between the beginning of the upstream bend and the bifurcation determines the location of such bars and pools which may switch the dominant bifurcate in fact when the bifurcation is quasibalanced by opposing factors any minor disturbance or a different choice of roughness or sediment transport predictor may switch the dominant bifurcate the division of sediment is nearly the same as the division of flow discharge in most runs until the discharge division becomes very asymmetrical so that a bifurcate does not close off entirely this partly explains the sustained existence of residual channels and existence of anastomosing rivers and the potential for reoccupation of old channel courses we develop a new relation for sediment division at bifurcations in one dimensional models incorporating the effect of meandering the flow and sediment divisions predicted by two existing relations and the new relation for one dimensional models are in qualitative agreement with the three dimensional model these one dimensional relations are however of limited value for wider rivers because they lack the highly three dimensional bar dynamics that may switch the direction of bifurcation evolution the potential effects of bed sediment sorting bank erosion and levee formation on bifurcation stability and avulsion duration are discussed

avulsion flow path selection on rivers in foreland basins river avulsions help distribute sediment across the floodplain and the frequency and flow path selected by each avulsion determines flooding and sedimentation patterns however rivers avulse infrequently making it difficult to quantify flow path behavior and test hypotheses posed by experimental and numerical studies we used google earth engine to create a novel data set of 55 avulsions that occurred from a d 1984 to 2014 in the andean and himalayan foreland basins on each avulsion we measure hop length the displacement of the avulsion across the floodplain orthogonal to the parent channel and avulsion length the length of the parent channel reach without controlling for external factors such as climate or geology we find that both hop length and avulsion length scale with parent channel belt width avulsion channels tend to hop 2 5 channel belt widths away from their parent channel and avulsion length is 13 4 parent channel belt widths we use these data to test between end member scenarios where flow paths are randomly selected or steered by floodplain topography observed avulsions are inconsistent with flow path predictions derived from a directed random walk model and a scaling analysis of alluvial ridge size shows that hop lengths are set by alluvial ridge widths these results suggest that avulsion flow path selection in the andean and himalayan basins is driven by alluvial ridge topography which promotes evenly spaced or compensational channel sand bodies over decadal time scales these data and results are important for understanding controls on avulsion flow path selection and how avulsion processes are represented in the stratigraphic record of foreland basins

quaternary of the pantanal west central brazil abstract the pantanal is a tectonic depression located at the left margin of the upper paraguay river the paraguay is the trunk river of an alluvial depositional tract composed by several large marginal alluvial fans the taquari fan being the largest one the present landscape is a complex tropical wetland characterized by month long floods every year with geomorphic features derived from the present conditions and others inherited from successive pleistocene and holocene climates some areas containing ponds are landscape relicts generated by eolian deflation during the last glacial maximum many ponds closed depressions isolated from the superficial waters by vegetated crescent ridges of fine sands were interpreted as salt pans bordered by lunette sand dunes initiation of the modern wetland has occurred during the pleistocene holocene transition with the change to a more humid climate and the individualization of lacustrine systems active tectonics has been playing an important role in the development of the pantanal landscape nowadays the paraguay river meanders in a large flood plain with extensive swamp surfaces being structurally constrained by faults in the west border of the basin sedimentation within the pantanal wetland is also affected by tectonic activity especially along faults associated with the transbrasiliano lineament

upstream and downstream controls of recent avulsions on the taquari megafan pantanal south western brazil avulsion the natural relocation of a river is a key process in the evolution of subaerial fans river floodplains and deltas the causes of avulsion are poorly understood which is partly due to the scarcity of field studies of present avulsions at present two avulsions are occurring on the middle and lower taquari megafan pantanal basin south western brazil here we present an analysis of the causes of these avulsions based on field and remote sensing data and show that avulsions on megafans can be controlled by both upstream and downstream processes the middle fan avulsion started in 1997 1998 is a result of upstream control overbank aggradation was caused by the variable input of sandy sediment into the system which caused channel belt superelevation and also created an easily erodible subsurface favouring bank retreat crevassing and scour of deep floodplain channels the sandy subsurface in this area is inferred to have been a major factor in the causation of this avulsion under conditions of little gradient advantage the lower fan avulsion started c 1990 results from interplay of upstream and downstream controls the latter being related to the local base level the paraguay river floodplain at the toe of the fan channel and overbank aggradation on the lower fan was influenced by fan sub lobe progradation and channel backfilling fan sub lobe progradation caused a significant gradient advantage of the avulsion channel over the parent channel avulsions are commonly supposed to be preferentially triggered by high magnitude floods when there is considerable channel belt superelevation however both avulsions studied by us were triggered by small to average floods with modest channel belt superelevation we conclude that flood magnitude and channel belt superelevation have been overrated as causes of avulsion and demonstrate additional causes that influence the growth of crevasses into avulsions

a revised chronology for mississippi river subdeltas radiocarbon measurements by accelerator mass spectrometry relating to three of the four late holocene mississippi river subdeltas yielded consistent results and were found to differ by up to 2000 carbon 14 years from previously inferred ages these geological dataare in agreement with archaeological carbon 14 data and stratigraphic ages based on ceramic seriation and were used to develop a revised chronologic framework which has implications for prehistoric human settlement patterns coastal evolution and wetland loss and sequence stratigraphic interpretations

river avulsions on the taquari megafan pantanal wetland brazil abstract the pantanal wetland is located in a tectonically active interior sedimentary basin in west central brazil the south flowing paraguay river is the trunk river of an alluvial constructional landform comprising several large alluvial fans the largest one of which is the taquari megafan the taquari river flows in two distinct geomorphologic zones within the megafan entrenched on sediments of pleistocene fan lobes the taquari river flows in a 3 to 5 km wide meander belt in the upper fan where avulsion is hindered by entrenchment downstream of the intersection point stream discharge progressively decreases and the taquari river becomes narrow and shallow toward the paraguay river plain within the distributary fan lobe the channel levee sandy complex is topographically higher than the adjacent floodplains and avulsion is a natural consequence of crevasses in the natural levees many channel avulsions have occurred during the last decades and documented cases show that significant channel changes may take place in a few years beginning with crevassing in 1988 and ending with the abandonment of the former channel in 1998 the river completely changed course in the lower fan presently a major avulsion is occurring in the upper portion of the growing fan lobe where many crevasses have appeared in the natural levees with associated splays onto the floodbasin new anastomosed channels have formed north of the taquari river but downstream of them the flow is unconfined and the water spreads into natural floodbasins this avulsion is still in process and allows observation of channel evolution the geomorphic features produced the sedimentary processes involved and resulting effects if the new channels do not rejoin the main channel the river mouth may abandon its present master channel and shift to a position a hundred kilometers north from its present position a large volume of sediment has been transferred to the floodbasin with progradation of crevasse splay deposits over fine overbank sediments many geomorphic features recognizable in satellite and radar images clearly show that avulsion has occurred many times before in the taquari river avulsion belt deposits and former diverted channels testify to ancient avulsion events within the fan lobe and show that progradation of splays onto the floodbasin is the most important infilling process within the taquari distributary fan lobe the avulsion process in the lower taquari river has accelerated in the last 30 years along with the magnitude of flooding pasture and intensive agriculture in the catchment area has increased the sediment supply to the wetland but larger floods are also a consequence of higher rainfall since 1973 avulsion and floods have been a cause of great concern among the local population and landowners before human intervention in attempting to control floods however a better understanding of the avulsive river system is needed especially because a major navigation project including the channelization of the paraguay river was recently proposed

human caused avulsion in the rhine meuse delta before historic embankment the netherlands although the shifting of deltaic river branches avulsion is a natural process that has become increasingly influenced by humans the impact of early human activities as a driver of avulsion success has remained poorly explored this study demonstrates how two important avulsions in the downstream part of the rhine meuse delta the netherlands were stimulated by human activities in the first millennium ce before historic embankment constrained the river courses peatland reclamation induced land subsidence in the lower delta this effect together with a human induced increase in suspended fluvial sediments and tidal backwater effects allowed for a gradual ingression of tidal creek channels and progradation of fluvial crevasse channels into human occupied and drained peatlands where they eventually connected we reconstructed the initial situation and identified the feedback loops among overbank sedimentation tidal incursion and land drainage subsidence that led to avulsion success the processes and feedbacks resulting from human activities are generic and hence relevant to many other deltas today where humaninduced subsidence results in tidal ingression potentially connecting to rivers and causing unexpected avulsions 2018 geological society of america

avulsion deposits in lower eocene alluvial rocks bighorn basin wyoming abstract eocene alluvial rocks in the northern bighorn basin can be subdivided into three major kinds fine grained deposits on which cumulative paleosols developed lithologically heterogeneous packages associated with immature paleosols and major sheet sandstone bodies the heterogeneous packages which alternate vertically with the cumulative paleosols are dominated by fine grained deposits and also contain ribbon sandstone bodies and thin 1 m sheet sandstone bodies most ribbon sandstones are less than 3 m thick and are more common in the middle and upper parts of the packages cut and fill deposits with fine grained fills are locally important because the lithologic packages are readily apparent in the field because of the immature or simple paleosols developed on the fine grain d facies they are referred to as simple paleosol packages the major sheet sandstones were deposited by meandering trunk rivers and locally overlie and truncate the other deposits contrary to classical models of meandering streams only a portion of the fine grained rocks in the study area were deposited by overbank flooding of a trunk channel on the basis of their stratigraphic position and few ribbon sandstones the upper parts of some simple paleosol packages are interpreted as ancient levee deposits associated with a trunk channel farther from the trunk channel overbank floods deposited alluvium on which the cumulative paleosols formed the greater parts of the simple paleosol packages including their fine grained deposits were deposited rapidly during channel avulsion ribbon sandstone bodies represent ancient splay channels on the avulsion belt simple paleosol packages in the willwood formation support the contention of smith et al 1989 that n many meandering river systems a significant portion of the fine grained deposits were deposited as the trunk channel avulsed deposits that resemble the simple paleosol packages may provide a means of recognizing avulsion episodes in other alluvial sequences

middle holocene avulsion belt deposits in the central rhine meuse delta the netherlands abstract the schoonrewoerd fluvial system in the central rhine meuse delta was active as a rhine distributary between 3900 and 3800 14c years bp based on planform patterns cross sectional facies architecture reconstructed paleodischarge and its short period of activity the schoonrewoerd fluvial system is interpreted to represent a failed avulsion the schoonrewoerd system consists of a complex of ribbon channel sand bodies encased in a lens of clayey deposits and it can be traced for more than 35 km in a downstream direction prominent features of the system are longitudinal facies architecture change with channel sand body width thickness ratio decreasing downstream from 40 to 5 and high lateral facies variability multiple anastomosing sand bodies occur in its upper and middle reaches paleodischarge reconstruction indicates that the schoonrewoerd system was only a minor distributary within the rhine meuse deltaic system specific stream power in the lower reach is estimated to have been low 0 5 to 1 8 w m2 the short period of activity of the schoonrewoerd system suggests that very high floodplain sedimentation rates were associated with the avulsion process with the formation of meters thick fine grained sedimentary successions in 100 years the facies architecture of the short lived schoonrewoerd system is interpreted to reflect the first stages of avulsion broadly analogous to the modern saskatchewan river avulsion in canada smith et al 1989 sedimentology v 36 p 1 23 having escaped from later erosion by a more mature channel belt many other systems often partly eroded with similar facies architecture exist in the west central rhine meuse delta suggesting that avulsion belt deposits are a major component of the holocene delta deposits the facies architecture model of the schoonrewoerd avulsion belt deposits presented in this paper may provide clues for the interpretation of fine grained sedimentary successions in ancient fluviodeltaic deposits

spatial and temporal evolution of an avulsion on the taquari river distributive fluvial system from satellite image analysis avulsion processes have been observed and analyzed using field based studies however they have rarely been analyzed through use of satellite imagery even though such an approach allows documentation of spatial and temporal variations of the resulting deposits in order to document changes that are occurring at the caronal avulsion on the taquari river brazil a time series of landsat imagery from 1985 to present 2010 was compiled as of july 2010 the imagery shows that the avulsion is not yet complete with discharge being routed to both the parent channel and the avulsion channel in the floodbasin the avulsion was initiated as flow was diverted from the parent channel to the floodbasin through two small crevasses allowing the development and evolution of splays with a single splay stabilizing only since 1999 the avulsion area is marked by the growth of new splays the formation of an avulsion belt and progradation of avulsion deposits into the floodbasin the parent channel downstream of the avulsion site has experienced decreasing width through time associated with the higher volumes of water and sediment being captured by the avulsion channel because this avulsion is not yet complete we expect more morphological changes to occur as the avulsion belt evolves into the primary channel rapid sedimentation caused by this avulsion and the 24 year time series of imagery that captured this change allowed us to construct a 3d block representing the evolution of depositional environments associated with avulsion using time in the vertical dimension thus we could represent the evolution of geomorphological patterns including floodbasin proximal levee distal levee and channel deposits and project their distributions through time this block of evolving sedimentary settings can be used as a guide for future sedimentologic studies by offering visual clues regarding the way which depositional systems track avulsion initiated changes

the movement of hydrocarbons in shales oil and gas hydrocarbons are formed at temperatures above 100 c by the chemical breakdown of the organic remains of dead organisms preserved in fine grained rocks called source rocks the organic remains are known as kerogen if enough hydrocarbons are formed some are expelled from the source rocks we have analysed kimmeridge clay upper jurassic shales from the north sea which are currently expelling hydrocarbons into adjacent sandstones the results could be explained if most expulsion occurs by the pressure driven flow of a discrete hydrocarbon phase through the pores of shales and into the pores of the sandstones as suggested previously1 in addition capillary forces seem to increase the efficiency of expulsion close to the interfaces between the shales and interbedded sandstones it appears that molecular diffusion of hydrocarbons with less than about ten carbon atoms alters the composition of the hydrocarbon fluids remaining in and expelled from the shales

the impact of avulsion on groundwater level and peat formation in delta floodbasins during the middle holocene transgression in the rhine meuse delta the netherlands by redistributing water and sediment in delta plains avulsions of river branches have major environmental impacts notably in changing hydrological and peat forming conditions in floodbasins the central part of the rhine meuse delta with its extensive databases including detailed lithological data and high resolution age control offers a unique opportunity to study middle holocene avulsion impacts on floodbasin groundwater level and peat formation avulsion has caused local accelerations of rising groundwater tables to be superimposed on decelerating base level rise this is evident from comparing single site groundwater rise for multiple floodbasins in the river dominated part of the delta with regionally averaged groundwater rise reconstructions floodbasin type lacustrine versus terrestrial wetland size and openness partly through effects on discharge dispersal affect how strongly the floodbasin groundwater tables respond to avulsion diverted discharge cross sectional lithology repeatedly indicates a shift from high organic wood peat to low organic reed peat in the vicinity of the avulsed channel resulting from changes in water table regime and nutrient status avulsive impact on the floodbasin groundwater table was most pronounced during the transition from transgressive to high stand stage between ca 6000 and 4000 years ago owing to developing floodbasin compartmentalization size reduction confinement resulting from repeated avulsion by way of environmental impacts on groundwater tables and vegetation avulsions thus affect the heterogeneity of floodbasin facies 2017 the author s 2017

late quaternary cuiaba megafan brazilian pantanal channel patterns and paleoenvironmental changes the brazilian pantanal wetlands comprise some of the largest quaternary megafans in south america the origin of these megafans has been linked to climate changes during the late quaternary this study focuses on the cuiaba megafan the second largest megafan of the pantanal which has not been studied from the perspective of its basic geomorphology river channel patterns and sediment deposition ages analysis of the channel and floodplain morphology as well as optically stimulated luminescence dating enable us to establish the succession of events that built up the cuiaba megafan since the last glacial period abandoned lobes indicate that a phase of fan aggradation occurred between 48 and 19 ka with a network of distributary paleochannels suggesting that there was relatively low precipitation and reduced vegetation cover subsequently abrupt changes in discharge and sediment supply triggered the channel incision of the upper fan valley indicating a degradational phase after the last glacial maximum 19 ka a new phase of fan aggradation began at the early holocene when meandering and anabranching channels were constrained into the incised valley on the upper fan in the distal part of the fan the development of modern depositional lobes has been driven by numerous single sinuous channels that shift laterally due to successive avulsions regional tectonic features suggest that modern cuiaba lobes are in a subsiding area that provides accommodation space for sediment accumulation and prolonged flooding events changes in the channel patterns observed in the cuiaba megafan are related to changes in the south american monsoon strength which driven precipitation variation during the late quaternary periods of monsoon intensification and relatively high precipitation lead to channel incision while periods of fan aggradation correlate to times of fewer oscillations our findings highlighted the diversity of fluvial styles in the pantanal and its importance as a climate changes archives in the south american lowlands

characteristics and origin of carbon isotopes of n alkanes in crude oils from the western pearl river mouth basin south china sea abstract the quantitative characterization of carbon isotopes of n alkanes is commonly carried out in organic geochemical studies possible controls on carbon isotopes include source organic matter maturity fractionation during oil expulsion and migration and the mixing of different oils in this study of the origin of crude oils in the western pearl river mouth basin the influences of all of these factors have been considered in reaching a conclusion carbon isotopes of n alkanes in the crude oils and the extracts of the two effective source rocks the wenchang and enping formations in the basin exhibit clear differences the wenchang source rocks have heavy 13 c values that remain almost constant or become slightly heavier with increasing carbon number the enping source rocks have light 13 c values that become lighter with increasing carbon number two groups of oils in this area were identified based on the carbon isotopes of the n alkanes groupioils are similar to extracts of the wenchang source rocks however the groupiioils are different from both the wenchang and enping source rocks and the carbon isotopic profiles of their n alkanes exhibit a v feature with increasing carbon number the results of artificial thermal maturation experiments indicate that from the early stage to the peak stage of oil generation with easyro between 0 64 and 1 02 the 13 c values of n alkanes in the pyrolysis oils become heavier by about 3 with increasing thermal maturity but the shape of the carbon isotopic profiles are not significantly changed calculated 13 c values of n alkanes in mixed artificial pyrolysis oils indicate that the mixture of oils generated from the same source rocks with different maturities could not change the carbon isotopic profile of the n alkanes however a mixing of the wenchang and enping oils could give the v feature in the profiles similar to the groupiioils in this area the groupiioils appear to be mixed wenchang and enping oils the latter being the dominant component in the mixture we conclude that the source organic matter and the degree of mixing are the main factors controlling the carbon isotopic characteristics of n alkanes in crude oils in the western pearl river mouth basin

three dimensional model of alluvial stratigraphy theory and applications abstract a three dimensional model of alluvial stratigraphy has been developed to simulate the spatial distribution proportion and connectedness of coarse grained channel belt deposits in alluvial strata as a function of channel belt width floodplain width bankfull channel depth channel belt and overbank sedimentation rates avulsion location and period compaction and tectonism tilting and faulting in this model a floodplain surface of variable width and length is occupied by a single channel belt changes in floodplain topography are produced by spatial and temporal variation of channel belt and floodplain deposition rates and by compaction and local or regional tectonism the location and timing of avulsions are determined by local changes in floodplain slope relative to channel b lt slope and by flood magnitude and frequency the diverted channel belt follows the locus of maximum floodplain slope at the end of each simulation architectural parameters are calculated including channel deposit proportion and connectedness and the dimensions of channel belt sandstone bodies three dimensional perspective diagrams mesh surfaces and two dimensional stratigraphic sections can be plotted to illustrate depositional surfaces time planes and the location and geometry of coarse grained channel belt deposits within finer grained overbank deposits the model predicts that channel belt proportion and connectedness and dimensions of sandstone bodies vary as a function of distance from avulsion points and cross section orientation upstream from avulsion points sandstone bodies have low width thickness ratios because of aggradation in a fixed channel belt immediately downstream from avulsion points channel belts tend to be connected resulting in sandstone bodies with high width thickness ratios avulsion sequences develop where points of avulsion shift up valley with a progressive decrease in avulsion period such sequences may produce successions in which channel belt proportion and connectedness vary vertically with a cyclic period of 103 to 105 years down valley increases in aggradation rate or down valley decreases in floodplain slope for example associated with a rise in base level may result in an increase in channel belt proportion and connectedness because of high avulsion frequencies in down valley regions of the floodplain down valley decreases in aggradation rate as in alluvial fans foreland basins and during base level fall may result in high avulsion frequencies in up valley parts of the floodplain tectonic tilting and faulting locally increase avulsion probabilities and channel belts generally shift toward areas of maximum subsidence under certain conditions however depositional topography may cause channels to shift away from areas of maximum subsidence channel deposit proportion and connectedness are gen rally high near downthrown areas of the floodplain but distribution clustering of channel belts may not be a reliable indicator of fault geometry or displacement models of alluvial architecture that consider only sediment accumulation rate as the main controlling factor are oversimplified the three dimensional model presented here predicts many of the features of channel behavior observed in modern rivers but there is a pressing need for better models and adequate natural data to test them

salt affected soils on elevated landforms of an alluvial megafan northern pantanal brazil abstract salt affected soils play an important role in the pantanal wetlands ecosystem in brazil but their occurrence on higher landforms is not well understood in order to investigate the major processes involved in salt affected soil formation a trench was opened in a representative transition between the floodplain and a paleochannel paleolevee complex not currently reached by floodwater the trench was 6 meter long and 2 meter deep and 121 soil samples were collected in a 0 25 0 25 m regular grid and submitted for soil particle size ph exchangeable cations and solution extract major cation and anions analyses analytical results were subjected to geostatistical and principal components analyses pca for selected horizons an undisturbed sample was collected to perform thin sections and micromorphology description the presence of degraded carbonates in the soils indicated an ancient formation the regional environmental settings of the alluvial fan and particle size variation along the soil profiles suggested that the sediments of the study site were deposited during the late pleistocene and holocene in a river channel levee complex and an abandoned channel by avulsion processes the depressed topography of the abandoned channel accompanied by moderately drier climates during early holocene likely favored low leaching of salts and higher evapotranspiration enabling the development of salt affected soils later erosion of the region which is expected in older lobes of alluvial fans triggered a relief inversion in the area preserving the more clayey areas as a slightly higher landform with remaining salt affected soils currently the seasonal flooding mainly reaches the footslope of this landform triggering redoximorphic processes expressed as redox concentrations and depletions and solodization processes expressed by lower values of ph cation exchange capacity cec base saturation and exchangeable sodium percentage esp than at the summit thus the occurrence of salt affected soils on higher landforms is the reflection of a complex sedimentological history in this way the genesis of salt affected soils on higher landforms which seems to be poorly understood in many situations worldwide should be carefully investigated in order to address formation mechanisms especially in extremely dynamic environments of tropical wetlands

middle and late holocene avulsion history of the river rhine rhine meuse delta netherlands at present little evidence is available on avulsion rates avulsion frequencies and interavulsion periods of aggrading fluvial systems over time scales of millenia the large number of high quality 14c ages from the holocene rhine meuse delta central netherlands enables a detailed assessment of such avulsion parameters and an evaluation of the factors controlling avulsion frequency 14c histogram analysis indicates that instantaneous avulsion coexistence of successive channel belts for less than a few centuries has dominated in this area interavulsion periods were typically 1000 yr although values of 500 or 5000 yr have also been found avulsion frequency was especially large prior to 4300 cal yr b p indicating a strong control by aggradation rate in turn controlled by sea level rise in addition periods with high avulsion frequency appear to correlate with time intervals characterized by large scale transgressive tendencies the time interval with the largest number of coexisting distributaries corresponds to the period with dominance of anastomosing fluvial style

shifting of the kosi river northern india the kosi river shifts laterally over the himalaya foreland plain by continual minor cutoffs and bank cutting and by episodic major shifts across watersheds by moving into and then out of preexisting adjacent less actively aggrading streams migration is unidirectional because after a channel is filled to instability floodwater will drain preferentially into a new adjacent low rather than across it to the next watershed or back to the last abandoned channel major shifts seem stochastic and autocyclic they do not correlate with the many severe quakes and floods that undoubtedly helped prime the system for shifts

is history of rivers important in restoration projects the example of human impact on a lowland river valley the obra river poland abstract palaeoenvironmental and palaeohydrological reconstructions can provide important guidance for river restoration projects this paper reconstructs the trajectory of river pattern changes of a lowland river the obra river western poland as a basis to inform realistic restoration activities the history of river changes is reconstructed for the last 12 700 radiocarbon years bp the sequence and timing of change differ from widely recognized conventional assessments of transitions from braided to meandering planforms in the polish lowlands traces of a laterally migrating anabranching system were found in the middle obra valley in its lower course a transition from a braided to meandering planform took place later than in other rivers of the polish lowlands sediment delivery from parts of the catchment situated downstream of lakes in the river course could have been the main reason for maintenance of the braided pattern restoration scenarios that take into account the trajectory of river planform changes are hypothesized in relation to variations of sediment supply and degree of anthropogenic impact

further evidence of changes in the hydrological regime of the river paraguay part of a wider phenomenon of climate change abstract analysis of flow measured at 20 sites rainfall measured at 36 sites and a 95 year record of water level at one site in the basin of the river paraguay area 1095 10 3 km 2 showed that the flow regime during the approximate period 1960 1970 differed substantially from the flow regime both before and after the long record of ladario water levels suggested that the changes between one period and the next were considerably abrupt and that the periods differed not only in terms of mean water levels but also in terms of the year to year correlational structure within the record despite the fragmentary nature of rainfall records from 36 sites an explanation for the increased flows since 1970 was found in the increases of rainfall as assessed in terms of the frequency of annual rainfalls more than the long term mean rainfall there was some degree of consistency in the change of rainfall pattern across the paraguay basin as a whole a detailed examination of daily rainfall characteristics at two gauges where records were fairly complete showed that during the 1960 1970 period when river flows were low dry spells were more persistent and on days when rain did fall the amounts of rain were generally smaller the results obtained were compared with results obtained by other researchers using flow records from the rivers paraguay parana negro and uruguay in the la plata basin and rainfall records from other parts of south america there is now strong evidence of changes in the runoff regime of the la plata basin during the last 40 years not all of which can be attributed to land use change as there is complementary evidence of change in rainfall regime the results were also compared with findings from the congo basin which appears to exhibit changes in flow regime that are a mirror image of those found for the paraguay at ladario

on the use of δ18oatm for ice core dating abstract deep ice core chronologies have been improved over the past years through the addition of new age constraints however dating methods are still associated with large uncertainties for ice cores from the east antarctic plateau where layer counting is not possible indeed an uncertainty up to 6 ka is associated with aicc2012 chronology of epica dome c edc ice core which mostly arises from uncertainty on the delay between changes recorded in 18oatm and in june 21st insolation variations at 65 n used for ice core orbital dating consequently we need to enhance the knowledge of this delay to improve ice core chronologies we present new high resolution edc 18oatm record 153 374 ka and o2 n2 measurements 163 332 ka performed on well stored ice to provide continuous records of 18oatm and o2 n2 between 100 and 800 ka the comparison of 18oatm with the 18ocalcite from east asian speleothems shows that both signals present similar orbital and millennial variabilities which may represent shifts in the intertropical convergence zone position themselves associated with heinrich events we thus propose to use the 18ocalcite as target for 18oatm orbital dating such a tuning method improves the ice core chronology of the last glacial inception compared to aicc2012 by reconciling ngrip and mid latitude climatic records it is especially marked during dansgaard oeschger 25 where the proposed chronology is 2 2 ka older than aicc2012 this 18oatm 18ocalcite alignment method applied between 100 and 640 ka improves the edc ice core chronology especially over mis 11 and leads to lower ice age uncertainties compared to aicc2012

multistage supercritical fluid extraction of petroleum source rocks application to samples from kimmeridge clay and posidonia shale formations abstract a study to determine whether supercritical fluid extraction can replace liquid extraction of source rocks has been initiated the extractions have been performed with plain carbon dioxide with small amounts of 2 propanol mixed with carbon dioxide and with carbon disulfide and tetrahydrofuran added to carbon dioxide to increase extraction yields multistage extractions were performed in order to obtain a compromise between rapid extraction and selective extraction the extracts were collected either in a solvent or on a sorbent cartridge in a stream of water the yields were similar to or higher than those obtained by liquid extraction with a 3 stage extraction the first extract was dominated by saturates the second by aromatics and the third by nso compounds in preliminary results obtained with 1 2 g samples of posidonia shale 0 8 soluble organic matter was extracted with multistage sfe and each stage was completed in 30 min the extracts could be used for the determination of biomakers

a three dimensional numerical model of sediment transport erosion and deposition within a network of channel belts floodplain and hill slope extrinsic and intrinsic controls on floodplain dynamics and alluvial architecture a three dimensional numerical model of sediment transport erosion and deposition within a network of channel belts and associated floodplain is described sediment and water supply are defined at the upstream entry point and base level is defined at the downstream edge of the model sediment and water are transported through a network of channels according to the diffusion equation and each channel has a channel belt with a width that increases in time the network of channels evolves as a result of channel bifurcation and abandonment avulsion the timing and location of channel bifurcation is controlled stochastically as a function of the cross valley slope of the floodplain adjacent to the channel belt relative to the down valley slope and of annual flood discharge a bifurcation develops into an avulsion when the discharge of one of the distributaries falls below a threshold value the floodplain aggradation rate decreases with distance from the nearest active channel belt channel belt degradation results in floodplain incision extrinsic extrabasinal allogenic and intrinsic intrabasinal autogenic controls on floodplain dynamics and alluvial architecture were modelled and sequence stratigraphy models were assessed input parameters were chosen based on data from the rhine meuse delta to examine how the model responds to extrinsic controls the model was run under conditions of changing base level and increasing sediment supply rises and falls in base level and increases in sediment supply occurred over 10 000 years rising base level caused a wave of aggradation to move up valley until aggradation occurred over the entire valley frequency of bifurcations and avulsions increased with rate of base level rise and aggradation rate channel belt width varied with water discharge and the lifespan of the channel belt wide connected channel belts and high channel deposit proportion occurred around the upstream inflow point because of their high discharge and longevity less connected smaller channel belts occurred further down valley such alluvial behaviour and architecture is also found in the rhine meuse delta during base level fall valley erosion occurred and the incised valley contained a single wide channel belt during subsequent base level rise a wave of aggradation moved up valley filling the incised valley bifurcation and avulsion sites progressively moved upstream relatively thin narrow channel belts bordered and cut into the valley fill these results differ substantially from existing sequence stratigraphy models the increase in sediment supply from upstream resulted in an alluvial fan most bifurcations and avulsions occurred at the fan apex nodal avulsion and channel belts were the widest and the thickest here giving high channel deposit proportion due to their high discharge and longevity the width and thickness of channel belts decreased down valley due to decreased discharge longevity and aggradation rate this behaviour occurs in modern alluvial fans intrinsic controls also affect floodplain dynamics and alluvial architecture variation of aggradation rate bifurcation frequency and number of coexisting channel belts occurred over periods of 500 to 2000 years compared with 10 000 years for extrinsic controls this variation is partly related to local aggradation and degradation of channel belts around bifurcation points channel belts were preferentially clustered near floodplain margins because of low floodplain aggradation rate and topography there

holocene stratigraphic evolution of saline lakes in nhecolandia southern pantanal wetlands brazil nhecol ndia is a fossil lobe of the taquari river megafan and a prominent geomorphic subunit of the pantanal wetlands because of the presence of 10 000 small lakes we investigated the stratigraphic records of three saline lakes from nhecol ndia to explore their potential as quaternary hydroclimate archives radiocarbon data indicate that accumulation at two lakes was approximately continuous in the late holocene and chemostratigraphic variability suggests sensitivity to environmental change with multicentennial resolution a basal sandy unit and an upper muddy unit comprise the shallow stratigraphy of each lake a pronounced change in depositional environment from freshwater wetlands to saline lakes at 3300 3200 cal yr bp best explains the lithofacies transition ephemeral freshwater wetlands formed on the abandoned megafan lobe which was molded by deflation in the arid early holocene wind scouring of the megafan lobe generated topographically closed depressions with complex marginal sand ridges which allowed permanent lakes to evolve when rainfall increased in the late holocene the lakes became highly saline and alkaline after 910 cal yr bp which influences biogeochemistry and aquatic ecology the results hold implications for understanding the response of the southern pantanal to climate change as well as the development of pans in tropical megafan settings

depositional development of the muskeg lake crevasse splay in the cumberland marshes canada crevasse splays are common geomorphological features in alluvial and deltaic floodplains although crevasse splays can develop into full avulsions thereby transforming large areas of floodbasins little is known about their sedimentary and geomorphological development at the decadal scale and their avulsion potential we used aerial photography and lithological cross sections to reconstruct crevasse splay formation in the largely unmanaged floodplain of the saskatchewan river in the cumberland marshes saskatchewan canada based on surface geomorphology and subsurface deposits various stages of crevasse splay development were described which were linked to both external forcing and internal morphodynamics initial splay deposition following a levee breach during a large flood occurred as a broad but relatively thin sandy sheet in a down basin direction in the receiving backswamp area in a next phase these primary crevasse splay deposits blocked local down basin flow thereby forcing the crevasse splay channel in a direction perpendicular to the parent channel and original floodbasin gradient this created an asymmetrical splay sequence composition which differs in appearance from more commonly observed dendritic crevasse splays it is concluded that sedimentation patterns in the splay have been influenced by inherited effects of previously formed deposits feedbacks of the original floodbasin gradient and earlier stages of splay formation are suggested as prominent mechanisms in creating the current morphology orientation and architecture of its deposits

simple kinetic models of petroleum formation part iii modelling an open system abstract the movement of petroleum from organic rich source beds to potential carrier beds is the net result of two processes expulsion the release of generated petroleum from kerogen within the source bed and primary migration flow of a petroleum phase through the inorganic void networks of the source bed and all other intervening fine grained rocks expulsion is by far the more important process within the source bed orgas a simple zero dimensional model of the source rock as an open system explicitly models expulsion concurrent with oil and gas generation and oi lgas cracking as a storage threshold problem oil and gas are retained adsorbed absorbed within kerogen until their concentrations exceed the respective sorptive capacity of the residual organic carbon 0 1 and 0 02 g g ck 1 the initial hydrogen index hl 0 controls the important balance between potential sorbate petroleum and sorbant residual carbon using simple geochemical measurements the initial organic carbon is divided into initial oil inert kerogen and oil and gas generative reactive kerogen components reactive kerogens are assigned to one of five global kinetic organofacies with pre determined kinetic parameter sets governing separately rates of oil and gas generation kinetic parameters governing the rates of oil cracking within the source rock are predicted from hl 0 the two most important determinants of source rock behaviour are organofacies and hl 0 at a reference constant heating rate of 2 c ma 1 the projected oil expulsion threshold for typical organofacies members increases in the order a de 100 110 120 135 c f typically expels no oil an order of magnitude increase decrease in heating rate increases decreases these thresholds by ca 15 c typical cumulative gas mass fractions in their ultimate expulsion products are a f 0 25 0 3 0 25 0 65 and 1 0 g g 1 a value of 200 mg hc g c 1 is an approximate minimum hl 0 prerequisite for oil expulsion irrespective of absolute petroleum potential p 0 as a family coals vary dramatically in their expulsion behaviour as their hl 0 ranges both above and velow this value primary migration is efficient for expulsive source rocks movement within the source bed is limited only at low p 0 a source rocks total genetic potential p 0 thickness becomes important during migration outside the source bed it potentially restricts the primary migration distance and the excess of petroleum available for secondary migration and entrapment we evaluate primary migration approximately using a small migration loss factor no more than about 10 saturation of porosity or 1 mg g 1 coupled with an understanding of fluid potential gradients which determine migration direction within the stratigraphic architecture ultimately kerogen degradation and oil to gas cracking in the source rock are the rate limiting processes in petroleum accumulation

principles of geochemical prospect appraisal our current understanding of the geochemical processes responsible for the creation of commercial accumulations of oil and gas can be reduced to simple rules and equations to allow the petroleum geologist to use geochemistry to estimate before drilling the likely volumes and compositions of oil and gas delivered to the trap in most sedimentary basins oil is expelled from source rocks between 120 and 150 c whereas most gas and gas condensate are released between 150 and 230 c when the initial potential of source rocks exceeds 10kg mt oil expulsion efficiencies are between 60 and 90 expelled oil and gas migrate as petroleum rich phases driven by fluid potential gradients most flow is laterally updip in beds with effective horizontal permeabilities greater than 1 md in lower permeability rocks the petroleum fluids move vertically up or down along the path of least resistance i e the most permeable or thinnest beds separating them from a high permeability lateral carrier bed the residual saturations of petroleum left behind along the migration pathway are of the order of a few percent of the porosity geochemical analysis of source rocks helps determine the yields and compositions of petroleum fluids expelled in the catchment area of a prospect these fluid masses are converted to volumes from knowledge of the phase relations and densities of petroleum fluids evaluation of the total volume of rock through which the petroleum potentially can migrate and the mean porosity of the rocks that constitute the migration pathway allow migration losses to be assessed these losses constrain both the range of migration of petroleum fluids away from the mature source rock and the volumes of petroleum remaining for entrapment in the prospect

spatial variation of overbank aggradation rate and its influence on avulsion frequency river avulsions are commonly considered to be driven by the aggradation and growth of alluvial ridges and the associated increase in cross valley slope relative to either the down channel slope or the down valley slope the latter is termed the slope ratio in the present paper therefore spatial patterns of overbank aggradation rate over stratigraphically relevant time scales are critical in avulsion dominated models of alluvial architecture detailed evidence on centennial to millennial scale floodplain deposition has to date been largely unavailable new data on such long term overbank aggradation rates from the rhine meuse and mississippi deltas demonstrate that the rate of decrease of overbank deposition away from the channel belt is much larger than has been supposed hitherto and can be similar to observations for single overbank floods this leads to more rapid growth of alluvial ridges and more rapid increase in slope ratios potentially resulting in increased avulsion frequencies a revised input parameter for overbank aggradation rate was used in a three dimensional model of alluvial architecture to study its effect on avulsion frequency realistic patterns of avulsion and interavulsion periods 1000 years were simulated with input data from the holocene rhine river with avulsions occurring when the slope ratio is in the range 3 5 however caution should be practised with respect to uncritical use of these numbers in different settings evidence from the two study areas suggests that the avulsion threshold cannot be represented by one single value irrespective of whether critical slope ratios are used as in the present study or superelevation as has been proposed by other investigators

anastomosis and the continuum of channel pattern anastomosing rivers are characterized by multiple channels separated by islands excised from the floodplain their status relative to the continuum concept of channel pattern is assessed with channel pattern defined in terms of three variables flow strength bank erodibility and relative sediment supply using an ordinal scaling l ow m oderate h igh the traditional forms of straight meandering and braided have respective representations of l l l m l m l m and h h m h in terms of those variables the anastomosing pattern is on average represented by l l m h but not so definitively as other forms specification of the third element sediment supply is particularly hampered by the paucity of data but aggradation a characteristic of many anastomosing rivers can be thought of as symptomatic of a moderately high rate of supply relative to the ability for onward transport a sufficiently high rate of supply to a channel with low flow strength and resistant banks would induce shoaling and or lateral constriction that locally forces flow out of the main channel and ultimately leads to the cutting of anabranches a flow regime characterized by concentrated floods of relatively large magnitude is also regarded as highly conducive to the formation of new channels where low bank erodibility constrains channel capacity anastomosis may in certain cases represent a transitional form of channel pattern but there is no denying the longevity of some anastomosing systems

the role of long term human impact on avulsion and fan development this study aims to understand mainly qualitatively the long term role of human impact on avulsion processes and the development of fluvial mega fans in semi arid environments in this paper we refer to human impact as the direct influences of actions on the river s hydraulics i e flow regulation flow diversion and channel engineering in five case studies drawn from the khuzestan plains in southwest iran we have analysed the setup and triggering conditions of specific avulsions that occurred in the past timescale of millennia and identified the role of human interference in their causation our analysis is based on the integration of historical archaeological geomorphological and geological data through this study we demonstrate that avulsions in the khuzestan plains are the result of long term and complex interplay between multiple human induced and natural causes in similar ways human induced actions may play important roles during different phases of avulsion development the success of an avulsion in the post triggering phase may be defined by human induced setup causes as well as morphodynamic processes we suggest that present day flood events may be partly inherited from long term human alterations of the natural processes these finding could have implications for any fluvial system e g distributive fluvial systems deltas where avulsion plays a major role in their development and research tends to emphasize on natural mechanisms copyright 2016 john wiley sons ltd

avulsions channel evolution and floodplain sedimentation rates of the anastomosing upper columbia river british columbia canada ages of channels of the anastomosing upper columbia river south eastern british columbia canada were investigated in a cross valley transect by c 14 dating of subsurface floodplain organic material from beneath levees the avulsion history within the transect was deduced from these data and morphological stages in channel development were recognized additionally floodplain sedimentation rates were established the new data demonstrate that the upper columbia river is a long lived dynamic anastomosing system results show that anastomosis at the study location has persisted since at least 2700 cal years bp with avulsions occurring frequently at least nine channels have formed in the studied cross valley transect within the past 3000 years channel lifetimes from formation to abandonment appear to be highly variable ranging from approximately 800 to 3000 years log jams provoking avulsions and or silting up of old channels are proposed as reasons for this variability long term average floodplain sedimentation rates appear to be significantly lower than previously proposed by smith 1983 int assoc sedimentol spec publ 6 155 168 a long term 4550 years average of 1 75 mm per year after compaction was based on c 14 dates whilea short term sedimentation rate of 0 8 mm was determined for a single relatively small seasonal flood in 1994 using sediment traps however short term sedimentation rates vary considerably over the floodplain with levees aggrading up to four times faster than floodbasins channels of the upper columbia river anastomosed reach follow a consistent pattern in their development with each stage being characterized by different morphology and processes channel evolution comprises the following succession 1 avulsion stage in which a crevasse splay channel deepens by scour and levee sedimentation 2 widening and deepening stage in which bank slumping and bed scouring dominates 3 infilling stage in which either channel narrowing bank accretion or channel shallowing bed accretion takes place and 4 abandonment stage in which the residual remnant channel is filled exclusively by silt clay and organic material vertical stacking super imposition of active channels on recent channel fill sand bodies is a notable feature of the upper columbia river which suggests that reoccupation of residual channels is a common process

avulsion frequency avulsion duration and interavulsion period of holocene channel belts in the rhine meuse delta the netherlands abstract avulsion parameters greatly influence alluvial architecture because they determine channel density and interconnectedness reliable data on avulsion parameters is still scarce from a detailed reconstruction of the holocene avulsion history of the whole rhine meuse delta we determined quantitative values for the avulsion parameters avulsion frequency avulsion duration and interavulsion period in the rhine meuse delta the number of coeval channels is related to the avulsion frequency calibrated 14c intensity histograms are used to analyze 14c dates of beginning and ending river activity the histograms show that instantaneous and gradual avulsions were almost equally important in the rhine meuse delta a high avulsion frequency occurred from 8000 to 7300 cal yr bp a total of 17 avulsions i e 2 43 avulsions 100 years during this period the avulsion frequency was related to the high rate of sea level rise which induced fluvial sedimentation in the present near coastal area after 7300 cal yr bp avulsion frequency decreased as a result of a continuously decreasing rate of sea level rise from 7300 to 3200 cal yr bp avulsion frequency was low 35 avulsions took place within 4100 years 0 85 avulsions 100 years approximately 5000 cal yr bp the coastal barriers became closed and large scale peat formation occurred this resulted in fixation of the river channels and low cross valley gradients reducing the chances for avulsion between 3200 and 1400 cal yr bp avulsion frequency was high again a total of 34 avulsions i e 1 89 avulsions 100 years as a result of increased discharge and or within channel sedimentation which enhanced chances for avulsion the period of activity of channel belts shows no significant trend on the time scale of the holocene it is highly variable and averages 1280 820 cal yr the avulsion duration fluctuates between less than 200 and 1250 cal years and averages 335 cal years the avulsion duration shows no significant trend through time and remained constant until at least 1900 cal yr bp avulsion duration seems not to be influenced by aggradation rate average interavulsion period is shorter than the period of activity of channel belts and is approximately 945 cal years in the rhine meuse delta on the time scale of the holocene interavulsion period and avulsion duration are on average constant therefore the number of coeval channels is directly related to avulsion frequency available evidence from other rivers and deltas around the world however suggests that the relationship between these avulsion parameters is not everywhere the same

morphodynamic adjustments in the yichang chenglingji reach of the middle yangtze river since the operation of the three gorges project abstract to investigate the evolution characteristics of the yichang chenglingji reach ycr of the middle yangtze river and develop simulation methods the temporal and spatial morphodynamic adjustments of the ycr were analysed comprehensively on the basis of runoff sediment load and cross sectional profiles data at 220 cross sections from 2002 to 2016 results showed that significant channel degradation has occurred in the ycr especially in the low flow channels due to a drastic reduction in sediment yield since the operation of the three gorges project tgp the variations of upstream flow and sediment regimes would drive equilibrium channels to become increasingly narrow and deep and the current bankfull width and depth were less than their equilibrium counterparts in the recent period however the recent channel evolution was mainly characterized by the prominent channel deepening via a one way deformation due to the large scale bank revetments within the ycr where the reach scale bankfull depth increased by 1 6 m in the span of 15 years a semi empirical method for determining the equilibrium values of channel degradation and aggradation volume at the ycr was proposed on the basis of a comprehensive analysis of the effects of flow sediment concentration sediment gradation and water depth moreover the equilibrium values of the channel width and depth were estimated with the least action principle method the calculation methods for the equilibrium width depth width depth ratio and aggradation degradation volume were combined with the delayed response model drm then the methods for calculating the variations in channel geometry and aggradation degradation volume were derived results showed that the proposed methods effectively simulated the channel geometry and cumulative erosion volume in the ycr from 2002 to 2016 in addition the response of the channel evolution in the ycr was closely related to the previous four year hydrological conditions implying that channel evolution may lag behind the variations of flow and sediment discharges

calculation of petroleum masses generated and expelled from source rocks abstract an algebraic scheme for calculating the masses of petroleum formed in mature source rocks has been constructed the scheme exploits routine geochemical measurements and a basic understanding of the behaviour of sedimentary organic matter at elevated temperatures under both laboratory and subsurface conditions application of these techniques to data from a wide range of source rock types has led to the following general conclusions 1 1 major oil and gas generation from kerogen occurs between 120 and 150 c in the subsurface gas generation continues to temperatures 160 c 2 2 oil prone source rocks with average initial potential 5 kgt 1 of rock and average organic carbon contents 1 5 expel 60 90 of the total oil generated 3 3 oil expulsion from leaner source rocks is less efficient most of the oil generated remains in the source rock it is subsequently converted to lighter components at higher temperatures and expelled in the vapour state 4 4 the major loss of organic carbon from rich oil prone source rocks during generation and expulsion means that the petroleum potential and organic carbon content of mature source rocks are much less than their initial immature values hence the algebraic scheme must be used to calculate the initial values before expelled petroleum masses can be determined