architecture of miocene overbank deposits in northern pakistan abstract understanding the geometry of sediment bodies and patterns of sediment disruption in ancient fluvial overbank successions is fundamental to interpretations of overbank deposition overbank deposits of the fluvial chinji formation siwalik group can be divided into sequences 1 20 m thick defined by the alternation of stratified sediments and paleosols preservation of stratified sequences does not support a gradual vertical aggradation of overbank sediments over the extent of the floodplain but instead indicates periods of rapid sediment deposition followed by long hiatuses and soil development rapid deposition of overbank sequences is predicted by models that hypothesize 1 episodic vertical aggradation of the entire floodplain 2 deposition controlled by the growth of alluvial ridges 3 incision and filling of local valleys 4 rapid filling of localized low areas or 5 rapid deposition associated with river channel avulsion these models also have important implications for understanding fossil preservation and floodplain paleoecology paleosol bounded sequences in the chinji formation can pinch out laterally over kilometers indicating that episodes of rapid deposition were restricted to local areas on the floodplain there is no evidence that bases of sequences are erosional and they do not appear to record valley incision and filling sequences generally do not thin and fine systematically away from the terminating margins of major channel deposits suggesting they are not related to the growth of alluvial ridges along active channels lithologic variations and patterns of sediment disruption within sequences appear to reflect the rapid tilling of low areas on the floodplain both adjacent to and distal from the major river channel this infilling of floodplain topography may reflect a continuous process whereby local areas were always being filled somewhere on the floodplain by sediments from minor tributary or crevasse channels alternatively the filling of local flood plain topography may have been caused by widespread but short lived events associated with river channel 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

late pleistocene alluvial plain sedimentation in lower narmada valley western india palaeoenvironmental implications abstract late pleistocene fluvial sediments that were deposited in a slowly sinking basin are now exposed as 30 50 m high incised vertical cliffs all along the lower narmada valley in western india the exposed fluvial deposits have been classified into two sediment packages alluvial fan sediments overlain by alluvial plain sediments the alluvial plain sequence has not been studied previously it consists mainly of sands and silts and is dominated by overbank deposits occurrence of large scale bedforms in the alluvial plain sequence points to the existence of a large sand bed river in an alluvial plain setting the major sedimentary facies in stratigraphic order include large channel fills giant epsilon cross bedded strata overbank fines occurring in horizontal massive and undulatory stratified forms associated with crevasse splay and backswamp deposits and a reddish brown palaeosol overlain by thinly stratified sands and silts at the top of the exposed sediment succession large sized channel fills occur at two stratigraphic levels which are morphologically similar and are indicative of high rates of deposition and avulsion the large channel fill structures and the giant epsilon cross bedded strata indicate a large single channel river that was consistently 10 15 m deep and about 70 80 m wide even during the dry seasons these dimensions are larger than those of the present day narmada river at low discharge levels the overbank sediments indicate rapid deposition through frequent overbank floods and floodplain aggradation by a laterally shifting river available chronologic data suggests that the reddish brown palaeosol correlates with a regional phase of pedogenesis in the alluvial plain of gujarat prior to the last glacial maximum lgm the thinly stratified sands and silts overlying the palaeosol were deposited by a considerably depleted but perennial river during the arid phase of the last glacial maximum overall the alluvial plain sediments of the lower narmada valley particularly those below the palaeosol have been attributed to a hyper avulsive large river with low sinuosity whose high discharge levels were determined primarily by a large catchment area further to the east and not by the semiarid climate prevailing in the gujarat alluvial plain during the upper part of the late pleistocene the study concludes that the narmada river has maintained a large catchment at least since the last 100 ka however the river was characterised by a much bigger channel during much of the late pleistocene with discharge levels higher than the present day

facies analysis of an ancient dismembered large caldera complex and implications for intra arc subsidence middle jurassic strata of cobre ridge southern arizona usa abstract the middle jurassic 170 ma cobre ridge caldera is an elongate caldera complex that formed during the eruption of the tuff of pajarito a crystal rich rhyodacite ignimbrite with an estimated volume 1000 km 3 the caldera subsided in two subequal blocks to the southeast caldera forming ignimbrite is as much as 3000 m thick whereas the northwest half of the caldera subsided to lesser depths during the initial eruption but was the locus of subsidence during subsequent eruptions providing a depocenter for a 1 5 km thick section of secondary and post collapse volcanic and sedimentary strata these strata record moat formation and filling and waning volcanism including 1 areally restricted ignimbrites up to several tens of meters thick that may have been ponded between caldera margins and or fault scarps 2 a 600 m thick ignimbrite whose eruption probably caused further collapse of the caldera or enlargement of the caldera margins 3 a localized debris apron deposit up to 500 m thick that represents material reworked from intra caldera pyroclastic deposits and possibly caldera walls and 4 eolian and fluvial sandstones and water lain tuffs megabreccia blocks as large as 0 5 km along strike probably demarcate approximate caldera margins suggesting that the composite caldera was approximately 50 km long by 25 km wide the preserved thickness of the strata of cobre ridge 4500 m is greater than many ancient continental volcanic sequences suggesting external i e tectonic controls on subsidence but preservation of this great thickness of strata is apparently due entirely to volcanic subsidence

fossil andisols identified with mass balance geochemistry oligocene john day formation oregon u s a mass balance geochemistry of thick sequences of fossil soils oligocene john day formation central oregon show trends similar to those produced by modern andisol soil forming processes these andic trends include phosphorus accumulation potassium retention and alumina and iron depletion identification of these trends benefited from the geochemical comparison of numerous fossil soil horizons from two paleosol sequences one andic and one non andic the comparison of two paleosol sequences is justified because both sequences developed from the same type of parent material in nearly identical tectonic depositional settings the two sequences are 1 the early oligocene big basin member dominated by clayey alfisol like paleosols and 2 the mid oligocene turtle cove member dominated by ash rich andisol like paleosols with calcareous horizons the change in paleosol type occurred circa 30 ma throughout the john day basin and is most likely due to global cooling which changed climatic conditions in central oregon from humid to subhumid drier climate is interpreted to have caused a decrease in weathering intensity so that amorphous weathering products of volcanic glass accumulated in soils to form andisols prior to this circa 30 ma transition weathering produced crystalline clays from volcaniclastic detritus to form alfisols rather than amorphous weathering products

simplified process modeling of river avulsion and alluvial architecture connecting models and field data abstract modeling is an invaluable tool for studying sedimentary basin filling and for understanding depositional processes with long recurrence intervals including channel avulsion simplified modeling approaches such as cellular models and process analogue experiments are particularly useful for efficiently exploring alternative hypotheses and evaluating first order controls on river avulsion and alluvial architecture here we review the history and current state of the art in simplified avulsion and alluvial architecture models with a particular focus on how results and insights from these models can be incorporated into field and subsurface studies and vice versa simplified avulsion and alluvial architecture models have proliferated in the past decade providing a wide variety of models to serve as a basis for future coupled field modeling studies we compare features of leading models and discuss avenues for effectively pairing model capabilities with hypotheses and field data outstanding questions highlighted by recent modeling efforts include 1 what thresholds control avulsion initiation in different systems 2 how do floodplain processes and topography influence avulsion dynamics and alluvial architecture 3 what factors determine where avulsion channels stabilize answering these questions will require targeted modeling efforts coupled to data from ancient systems hence our model comparison emphasizes features that can be used to choose or design fit for purpose models and we outline how quantitative data useful for model selection and validation can be obtained from modern systems and ancient deposits matching model goals with targeted questions and model parameters and predictions with quantitative field data will help tighten communication between field and model oriented sedimentary geologists facilitating advances in our understanding of river avulsion and alluvial architecture

the structure and rate of late miocene expansion of c4 plants evidence from lateral variation in stable isotopes in paleosols of the siwalik group northern pakistan this study uses stable isotope variation within individual mio pliocene paleosols to investigate subkilometer scale phytogeography of late miocene vegetation change in southeast asia between ca 8 1 and 5 ma a time interval that coincides with dramatic global vegetation change we examine trends through time in the distribution of low latitude grasses c4 plants and forest c3 plants on indo gangetic floodplains using carbon 13c and oxygen isotopic 18o values in buried soil carbonates in siwalik series sediments exposed in the rohtas anticline north central pakistan revised high resolution magnetostratigraphy and a new 40ar 39ar date provide improved age control for the 2020 m rohtas section carbon isotope results capture lateral variability of c3 versus c4 plants at five stratigraphic levels r11 8 0 ma r15 6 74 6 78 ma r23 5 78 ma r29 4 8 4 9 ma and upper boundary tuff ubt 2 4 ma using detailed sampling of paleosols traceable laterally over hundreds of meters paleosols and the contained isotopic results can be assigned to three different depositional contexts within the fluvial sediments channel fill crevassesplay and floodplain environments 13c results show that near the beginning 8 0 ma and after 4 0 ma the period of major ecological change vegetation was homogeneously c3 or c4 respectively regardless of paleo landscape position in the intervening period there is a wide range of values overall with c4 grasses first invading the drier portions of the system floodplain surfaces and c3 plants persisting in moister settings such as topographically lower channel swales although abrupt on a geologic timescale changes in abundance of c4 plants are modest 2 per 100 000 yr compared to rates of vegetation turnover in response to glacial and interglacial climate changes in the quaternary earlier research documented a sharply defined c3 to c4 transition in pakistan between 8 1 and 5 0 ma based on vertical sampling but this higher resolution study reveals a more gradual transition between 8 0 and 4 5 ma in which c3 and c4 plants occupied different subenvironments of the siwalik alluvial plain 18o values as well as 13c values of soil carbonate increase up section at rohtas similar to isotope trends in other paleosol records from the region spatially however there is no correlation between 13c and 18o values at most stratigraphic levels this implies that the changes in soil hydrology brought about by the shift from forest to grassland i e an increase in average soil evaporation did not produce the shift through time in 18o values we interpret the trend toward heavier soil carbonate 18o values as a response to changes in external climatic factors such as a net decrease in rainfall over the past 9 ma

high resolution sequence stratigraphy of fluvio deltaic systems prospects of system wide chronostratigraphic correlation a basin scale numerical model with a sub grid parameterization of fluvio deltaic processes and stratigraphy was used to study the relation between alluvial sedimentation and marine deltaic deposition under conditions of time invariant forcing the experiments show that delta evolution is governed by a robust morphodynamic feedback loop which provides a link between major avulsions delta lobe switches and sequestration of sediments on the delta plain major avulsions driven by local superelevation result in abandonment of delta lobes and initiation of new lobes progradation of the delta front lengthens the fluvial profile and reduces its gradient which induces aggradation upstream the aggradation in turn causes local superelevation of the channel belt each major avulsion causes a wave of incision to migrate upstream whereas downstream of the avulsion point the rate of aggradation temporarily increases until a new equilibrium situation has been established the feedback loop explains storage and release of fluvial sediments without the need to invoke changes in upstream or downstream controls and provides a plausible mechanism for the generation of high frequency incision aggradation cycles as the sole result of compensational stacking the stratigraphic expression of a depocentre shift is an essentially isochronous surface hence the stratigraphic record of fluvio deltaic systems may be subdivided into a series of units representing intervals during which a channel belt and delta lobe were forming at a fixed location in the basin so called chronosomes fluvio deltaic chronosomes are bounded by abandonment surfaces which are clearly expressed in the marine as well as the fluvial domain the surface marking the abandonment of a particular channel belt and delta lobe correlates with the surface at the base of a new delta lobe landward this surface forms the base of an aggradational package of fluvial sediments downstream of an avulsion site associated with a lobe switch upstream of this site the conformable surface passes into a surface of fluvial incision and terrace formation onlapped by aggradational channel belt deposits identification of the isochronous bounding surfaces of chronosomes in the field offers the attractive prospect of high resolution chronostratigraphic correlation throughout fluvio deltaic systems

evolution of the holocene mississippi river floodplain ferriday louisiana insights on the origin of fine grained floodplains abstract the alluvial architecture and soil characteristics of holo cene mississippi river floodplain deposits in the southern lower mississippi valley provide evidence for significant changes in floodplain development in response to sea level rise floodplain cores acquired near ferriday louisiana show that holocene deposits consist of 15 30 m ave 20 m of sands silts and clays which overlie late wisconsin sands and gravels on the basis of differences in sediment grain size sediment body geometry and the abundance of soil features the holocene deposits are subdivided into lower and upper holocene units lower holocene deposits 5000 yr b p consist of lacustrine and poorly drained backswamp muds that contain authigenic siderite pyrite and vivianite and show little evidence of soil formation muds encase crevasse splay and floodplain channel sand bodies 1 km wide and collectively these deposits represent a mosaic of shallow lakes poorly drained backswamps and multichannel streams similar to modern examples in the atchafalaya basin 100 km south of ferriday upper holocene deposits 5000 yr b p are represented by large mississippi river meander belt sand bodies that are up to 15 km wide and 30 m thick natural levee silts and sands and well drained backswamp muds are present between meander belt sands upper holocene deposits contain abundant soil features and sandy and silty soils are entisols inceptisols and alfisols whereas clayey soils are vertisols the presence of isolated sand bodies surrounded by mud and the scarcity of soil features suggest that lower holocene sediments reflect a period of rapid floodplain aggradation during which crevassing lacustrine sedimentation and avulsion dominated floodplain construction no evidence of large meandering mississippi river channels represented by buried thick tabular sands exists near ferriday and discharge in lower mississippi valley flow was probably conveyed by a network of small multichannel floodplain streams upper holocene sediments record a dramatic change ca 5000 yr b p from rapid to slower floodplain aggradation which was accompanied by extensive lateral channel migration overbank deposition and soil formation on the basis of differences in meander belt dimensions and numbers of abandoned channels upper holocene meander belts are subdivided into simple and complex forms relative age relationships suggest that the smaller and older simple meander belts represent periods of divided mississippi river flow and early attempts to establish a large single channel meandering regime this type of meandering regime is represented by the larger and younger complex meander belts and includes the modern meander belt similarities in the timing of changes in floodplain processes and fluvial style and decreasing rates of holocene sediment accumulation in the southern lower mississippi valley strongly suggest that decelerating holocene sea level rise in the gulf of mexico affected floodplain development at least 300 km inland from the present day coast the alluvial architecture of the lower holocene deposits and the absence of large meandering mississippi river channel deposits older than 5000 yr b p near ferriday indicates that most of the floodplain muds were deposited by avulsion related crevassing and lacustrine sedimentation rather than by overbank flooding of large mississippi river channels similarities between the floodplain history of the mississippi river and those of modern and ancient rivers elsewhere further suggest that avulsion rather than simple overbank deposition contributes to the construction of fine grained floodplains to a greater degree than generally recognized

luminescence chronology of river adjustment and incision of quaternary sediments in the alluvial plain of the sabarmati river north gujarat india river adjustment and incision in the sabarmati basin gujarat india have been examined at a site near mahudi towards this the morphostratigraphy and depositional chronometry of the middle alluvial plains were investigated the upper fluvial sequence along with the overlying aeolian sand and riverbed scroll plains provide clues to the evolution of the present sabarmati river sedimentological analyses of the upper fluvial sequence indicate its deposition by a meandering river system during what is believed to be a persistent wetter phase luminescence chronology bracketed this sequence to between 54 and 30 ka which corresponds to oxygen isotope stage 3 during which the sw monsoon was enhanced the overlying aeolian sand has been dated to 12 ka indicating that dune accretion occurred simultaneously with the strengthening of the sw monsoon during the early holocene adjustment of the sabarmati along a n s transect is placed around 12 ka and the incision is bracketed between 12 and 4 5 ka river adjustment could have been tectonic however the incision was facilitated by the availability of continuous flow in the river caused by the sw monsoon the basin experienced two tectonic events at about 3 and 0 3 ka as demonstrated by the morphology of the scroll plains

high frequency controls on alluvial successions an integrated sedimentological and palaeopedological approach to the plio pleistocene of argentina abstract plio pleistocene palaeosol bearing alluvial strata are continuously exposed along the marine cliffs of southeastern buenos aires province argentina the study interval was deposited by a mixed predominantly suspended load fluvial system outcrops are dominated by floodplain siltstones and mudstones that exhibit a cyclic alternation between calcic and vertic palaeosols this cyclicity is represented by meter scale alluvial cycles that show as a general trend a gradual increase in the proportion of channel fill deposits up section cyclic variations in palaeosols facies and architectural elements can be related to allogenic drivers such as climate and eustasy for the lower punta san andres alloformation the stratigraphic arrangement of the recognised palaeosols and the clay mineral distribution indicate that the palaeoclimate was subhumid average mean annual precipitation map 840 mm seasonal and temperate average mean annual temperature mat 7 85 c for at least 1 ma from ca 2 9 ma to ca 1 8 ma late pliocene early pleistocene with several intervals in which conditions became drier and probably colder even though climate was a major control on palaeosol development it is suggested here that it was not the main control on high frequency cycles variations in the orbital parameters more specifically in insolation were interpreted as having controlled the sedimentation and architectural evolution of the lower punta san andres alloformation depositional systems

the fluvial evolution of the holocene nile delta abstract the evolution of the nile delta the largest delta system in the mediterranean sea has both high palaeoenvironmental and archaeological significance a dynamic model of the landscape evolution of this delta system is presented for the period c 8000 4500 cal bp analysis of sedimentary data and chronostratigraphic information contained within 1640 borehole records has allowed for a redefinition of the internal stratigraphy of the holocene delta and the construction of a four dimensional landscape model for the delta s evolution through time the mid holocene environmental evolution is characterised by a transition from an earlier set of spatially varied landscapes dominated by swampy marshland to better drained more uniform floodplain environments archaeologically important pleistocene inliers in the form of sandy hills protruding above the delta plain surface known as turtlebacks also became smaller as the delta plain continued to aggrade while the shoreline and coastal zone prograded north these changes were forced by a decrease in the rate of relative sea level rise under high rates of sediment supply this dynamic environmental evolution needs to be integrated within any discussion of the contemporary developments in the social sphere which culminated in the emergence of the ancient egyptian state c 5050 cal bp

siwalik mammals of the jammu sub himalaya india an appraisal of their diversity and habitats abstract the red bed sequence of the lower siwaliks around ramnagar and the nagrota formation of the upper siwaliks in the uttarbeni parmandal section are the two highly fossiliferous intervals of the siwaliks of jammu sub himalaya india the lower siwalik mammalian fauna of ramnagar is best represented in its upper 350 m interval small and medium sized forms mostly belonging to browsing species dominate the mammalian assemblage the ramnagar fauna represents the chinji mammalian fauna middle miocene of the potwar plateau pakistan the assemblage indicates a woodland type ecosystem having substantial forest component and subordinately developed grassy subareas the mammalian fauna of the nagrota formation in the uttarbeni parmandal section in contrast exhibits relatively low species diversity but considerable increase in the mean body size and progressive hypsodonty of the cheek teeth in the herbivores this becomes more pronounced above the 2 5 ma old volcanic tuff layers occurring at the middle of the formation broadly a savanna type ecosystem is indicated by the mammalian communities of nagrota formation which appears to have been dominated by grassland communities above the tuff zone the above tuff faunal zone of the nagrota formation can be correlated with the pinjor faunal zone plio pleistocene of the siwalik hills punjab

lower eocene alluvial paleosols pedogenic development stratigraphic relationships and paleosol landscape associations abstract two kinds of cumulative floodplain paleosols red and grey paleosols formed on overbank deposits of the willwood formation in the sand coulee area of the bighorn basin wyoming although both kinds experienced down profile iron movement due to seasonal saturation of a clay rich a horizon the red paleosols were better drained and their b horizons were rubified various redoximorphic features indicate that the grey paleosols were poorly drained the red paleosols show progressive increases in the degree of profile development away from an associated channel sandstone a paleosol landscape relationship termed pedofacies by bown and kraus 1987 although the grey paleosols show relatively systematic changes in hydromorphy consistent pedofacies changes were not recognized furthermore no lateral relationship between the red and grey paleosols was observed these features suggest that because of the retarding effects of poor drainage on soil weathering poorly drained soils are not amenable to pedofacies modelling and that the landscape associations of well drained and poorly drained soils may be difficult to document without unusually extensive exposures results of this study also show that the pedofacies model is limited by sediment accumulation rates and the kind of fine grained facies on which paleosols developed although readily observable in sand coulee pedofacies are difficult to recognize in the elk creek area where accumulation rates were approximately half as rapid as in sand coulee the attainment of steady state conditions is believed to have obscured pedofacies in the elk creek area finally pedofacies are only developed on true overbank deposits which in the willwood formation and probably many other ancient alluvial sequences make up only a fraction of the fine grained deposits only immature soils formed on the remainder of the fine grained facies because they were deposited very rapidly as a result of channel avulsion

geochemistry of the floodplain sediments of the kaveri river southern india abstract the floodplain sediments of the kaveri river southern india derived from archean gneissic and charnockitic source regions show interbedding of silty 4 4 7 and sandy units 1 4 3 7 the geochemistry of silty beds is remarkably uniform at a given location and over a lateral distance of nearly 250 km the sandy beds have more variable chemical compositions yet are comparable to those of silty beds except for the diluting effect of quartz silty sediments retain the geochemical signature of prominently exposed source rocks for almost all elements and provide evidence of tectonic instability in the source region the floodplain sediments contain all size grades sand silt and clay which may have resulted in minimizing the biases imposed on suspended and bedload sediments due to sorting the low chemical index of alteration cia the dominance of unweathered primary minerals and the minor proportion of smectitic clay all suggest that the region has been subjected to little chemical weathering this is possible if the region has undergone recent uplift exposing fresh archean rock to surface denudation the formation of fertile farmland along the kaveri river course and its delta is related to these recent geological processes

pedosedimentary history and floodplain dynamics of the lower cretaceous upper blairmore group southwestern alberta canada thick alluvial successions in the upper blairmore group in the foothills of southwestern alberta contain few well developed paleosol profiles but abundant evidence of pedogenic modification the pedogenically modified successions are characterized by vertical root traces compound illuvial clay coatings and ferruginous coatings and nodules five representative paleosol successions composed of nine microfacies are analyzed the paleosol successions indicate soil development on more or less continuously aggrading floodplains however locally rates of sedimentation and pedogenesis were highly variable the paleosol successions contain features similar to modern alluvial soils brunisols and luvisols vertical trends within individual paleosol successions preserve the changing paleoenvironmental record at each site the dynamic nature of the floodplain topography is reflected in alternating drainage conditions represented by quasi regular colour banding compound clay coatings and the type and degree of pedogenic development upwards within the paleosol successions overall the paleosols indicate soil development under a warm temperate paleoclimate with seasonal precipitation although representing minor diastems the paleosols formed contemporaneously with sedimentation and are an integral part of the alluvial successions local variations in paleosol development are attributed to variations in sediment supply and water table conditions related to the overall floodplain geomorphology

sequence stratigraphic distribution of coaly rocks fundamental controls and paralic examples significant volumes of terrigenous organic matter can be preserved to form coals only when and where the overall increase in accommodation approximately equals the production rate of peat accommodation is a function of subsidence and base level for mires base level is very specifically the groundwater table in paralic settings the groundwater table is strongly controlled by sea level and the precipitation evaporation ratio peat accumulates over a range of rates but always with a definite maximum rate set by original organic productivity and space available below depositional base level groundwater table below a threshold accommodation rate nonzero no continuous peats accumulate due to falling or low groundwater table sedimentary bypass and extensive erosion by fluvial channels this is typical of upper highstand lowstand fan and basal lowstand wedge systems tracts higher accommodation rates provide relatively stable conditions with rising groundwater tables mires initiate and copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received september 7 1995 revised manuscript received november 25 1996 final acceptance may 5 1997 2exxon production research company 3120 buffalo speedway houston texas 77096 3exxon production research company 3120 buffalo speedway houston texas 77096 present address conoco inc p o box 2197 houston texas 77252 we benefited from the input and assistance of many people of special assistance were the teams involved in collaborative studies with esso australia and esso malaysia p moore m sloan j emmett b burns a partridge s creaney hanif hussein r hill r lovell and m feeley we also thank the rock springs team r beauboeuf p mclaughlin w devlin a carroll y y chen g grabowski jr k miskell gerhardt m farley r webster and j schwalbach group members d curry and j yeakel were always helpful we also enjoyed and profited from many discussions of these concepts with our colleagues outside exxon c diessel r boyd k shanley b zaitlin p mccabe m hendricks a cohen and m kirschbaum we thank the reviewers of company reports whose careful comments on several generations of this work improved it s creaney m feeley j van wagoner f wehr j yeakel and a young f weber and j zullig provided extensive management support k linke translated our sketches into the fine figures herein we value all the help

crevasse splay processes and deposits in an ancient distributive fluvial system the lower beaufort group south africa abstract up to 12 of the mud prone ephemeral distributive fluvial system stratigraphy in the permo triassic lower beaufort group south africa comprises tabular fine grained sandstone to coarse grained siltstone bodies which are interpreted as proximal to distal crevasse splay deposits crevasse splay sandstones predominantly exhibit ripple to climbing ripple cross lamination with some structureless and planar laminated beds a hierarchical architectural scheme is adopted in which 1 m thick crevasse splay elements extend for tens to several hundreds of meters laterally and stack with other splay elements to form crevasse splay sets up to 4 m thick and several kilometers in width and length paleosols and nodular horizons developed during periods or in areas of reduced overbank flooding are used to subdivide the stratigraphy separating crevasse splay sets deposits from crevasse splays differ from frontal splays as their proximal deposits are much thinner and narrower with paleocurrents oblique to the main paleochannel in order for crevasse splay sets to develop the parent channel belt and the location where crevasse splays form must stay relatively fixed during a period of multiple flood events beaufort group splays have similar geometries to those of contemporary perennial rivers but exhibit more lateral variability in facies which is interpreted to be the result of more extreme fluctuations in discharge regime sharp based crevasse splay packages are associated with channel avulsion but most are characterized by a gradual coarsening upward interpreted to represent progradation the dominance of progradational splays beneath channel belt deposits may be more characteristic of progradational stratigraphy in a distributive fluvial system rather than dominated by avulsion processes in a trunk river system this stratigraphic motif may therefore be an additional criterion for recognition of distributive fluvial systems in the ancient record

evolution of miocene fluvial environments eastern potwar plateau northern pakistan the miocene pliocene siwalik group records changing fluvial environments in the himalayan foreland basin the nagri and dhok pathan formations of this group in the eastern potwar plateau northern pakistan comprise relatively thick tens of metres sandstone bodies and mudstones that contain thinner sandstone bodies metres thick and palaeosols thick sandstone bodies extend for kilometres normal to palaeoflow and are composed of large scale stratasets storeys stacked laterally and vertically adjacent to each other sandstone bodies represent single or superimposed braided channel belts and large scale stratasets represent channel bars and fills channel belts had widths of km bankfull discharges on the order of 103 cumecs and braiding parameter up to about 3 individual channel segments had bankfull widths maximum depths and slopes on the order of 102 m 101 m and 10 4 respectively and sinuosities around 1 1 these rivers are comparable to many of those flowing over the megafans of the modern indo gangetic basin and a similar depositional setting is likely thin sandstone bodies within mudstone sequences extend laterally for on the order of 102 m and have lobe wedge sheet and channel form geometries they represent crevasse splays levees and floodplain channels mudstones are relatively bioturbated disrupted and represent mainly floodbasin and lacustrine deposition mudstones and sandstones are extremely disrupted in places showing evidence of prolonged pedogenesis these mature palaeosols are m thick and extend laterally for km lateral and vertical variations in the nature of their horizons apparently depend mainly on deposition rate the 500 m thick nagri formation has a greater proportion and thicker sandstone bodies than the overlying 700 m thick dhok pathan formation the thick sandstone bodies and their large scale stratasets thicken and coarsen through the nagri formation then thin and fine at the base of the dhok pathan formation compacted deposition rates increase with sandstone proportion 0 53 mm year for nagri 0 24 mm year for dhok pathan and palaeosols are not as well developed where deposition rates are high within both formations there are 100 m scale variations representing on the order of 105 years in the proportion and thickness of thick sandstone bodies and tens of m scale alternations of thick sandstone bodies and mudstone sandstone strata that represent on the order of 104 years formation scale stratal variations extend across the potwar plateau for at least 100 km although they may be diachronous however 100 m and smaller scale variations can only be traced laterally for up to tens of km alluvial architecture models indicate that increases in the proportion and thickness of thick sandstone bodies can be explained by increasing channel belt sizes mainly average deposition rate and avulsion frequency on a megafan comparable in size to modern examples 100 m scale variations in thick sandstone body proportion and thickness could result from regional shifts in the position of major channels possibly associated with fan lobes on a single megafan or with separate megafans however such variations could also be related to local changes in subsidence rate or changes in sediment supply to the megafan system formation scale and 100 m scale stratal variations are probably associated with interelated changes in tectonic uplift sediment supply and basin subsidence increased rates of hinterland uplift sediment supply and basin subsidence recorded by the nagri formation may have resulted in diversion of a relatively large river to the area alternatively changing river sizes and sediment supply rates may be related to climate changes affecting the hinterland possibly linked to tectonic uplift climate during deposition of the siwalik group was monsoonal although the deposits contain no direct evidence for climate change independent evidence indicates global cooling throughout the miocene and the possibility of glacial periods e g around 10 8 ma corresponding to base of nagri formation if the higher himalayas were periodically glaciated a mechanism would exist for varying sediment supply to megafans on time scales of 104 105 years although eustatic sea level changes are related to global climatic change they are not directly related to siwalik stratigraphic changes because the shoreline was many 100 km away during the miocene

sedimentology geochemistry and osl dating of the alluvial succession in the northern gujarat alluvial plain western india a record to evaluate the sensitivity of a semiarid fluvial system to the climatic and tectonic forcing since the late marine isotopic stage 3 abstract the alluvial successions in the northern gujarat alluvial plain western india have been investigated for reconstructing the climatic fluctuations during the last 40 ka alluvial architecture and geochemical proxies indicate prevalence of a strengthened indian summer monsoon ism with fluctuations between the late marine isotopic stage 3 mis 3 37 ka to the early mis 2 27 ka a gradual onset of aridity declining ism after 27 ka with peak aridity at 22 ka is observed a gradual strengthening of ism at around 18 and 12 ka followed by a short reversal in ism intensity between 12 and 11 ka is attributed to the younger dryas yd cooling event the aeolian sand sheet dated to 6 and 3 5 ka represents the onset of regional aridity following this a short lived humid phase was observed after 2 ka which includes the medieval warm period mwp the study suggests that the variability in the ism to the latitudinal migration of the inter tropical convergence zone was caused by insolation driven cooling and warming events in the north atlantic the incision of the valley fill alluvium occurred in two distinct phases the older incision phase occurred after 11 ka and before 6 ka whereas the younger incision phase that led to the development of present day topography is bracketed between 3 5 ka and before 1 ka the older incision phase is ascribed to the early to mid holocene enhanced ism climatically driven whereas the younger incision seems to be modulated by the activation of basement faults tectonically driven

micromorphology and sequence stratigraphy of the interfluve paleosols from the ganga plains a record of alluvial cyclicity and paleoclimate during the late quaternary

lacustrine records of holocene flood pulse dynamics in the upper paraguay river watershed pantanal wetlands brazil abstract the pantanal is the world s largest tropical wetland and a biodiversity hotspot yet its response to quaternary environmental change is unclear to address this problem sediment cores from shallow lakes connected to the upper paraguay river pr were analyzed and radiocarbon dated to track changes in sedimentary environments stratal relations detrital particle size multiple biogeochemical indicators and sponge spicules suggest fluctuating lake level lowstand conditions between 11 000 and 5300 cal yr bp punctuated by sporadic and in some cases erosive flood flows a hiatus has been recorded from 5300 to 2600 cal yr bp spurred by confinement of the pr within its channel during an episode of profound regional drought sustained pr flooding caused a transgression after 2600 cal yr bp with lake level highstand conditions appearing during the little ice age holocene pr flood pulse dynamics are best explained by variability in effective precipitation likely driven by insolation and tropical sea surface temperature gradients our results provide novel support for hypotheses on 1 stratigraphic discontinuity of floodplain sedimentary archives 2 late holocene methane flux from southern hemisphere wetlands and 3 pre colonial indigenous ceramics traditions in western brazil

palaeogeography of neogene red bed sequences along the aksa ata river in the parkent nurekata intermontane basin tien shan mountains uzbekistan with special reference to the magnetic susceptibility of siliciclastic rocks abstract siliciclastic red beds formed by stream action in the parkent nurekata intermontane basin of the tien shan mountains e uzbekistan the sedimentary rocks were cross sectioned in the field and subject to heavy mineral clay mineral and thin section analyses in the laboratory in addition to these conventional methods a survey into the variation of the magnetic susceptibility was conducted during the paleogene calcareous rocks unit a developed in a subtidal through supratidal environment of deposition subsequently tectonic disturbances tilted the paleogene carbonate rocks and triggered the formation of an unconformity during the oligocene the miocene to pliocene red beds have the hallmarks of a delta like wedge actively prograding over near shore marine deposits two progradational stages units b and c were substituted for by a more regressive event unit d during deposition and burial the siliciclastics were percolated by strongly alkaline pore fluids under semi arid climatic conditions the joint action of rapid uplift in the hinterland and the afore mentioned hydrological processes were instrumental for heavy minerals such as epidote and staurolite to survive and some peculiar phyllosilicates such as smectite and palygorskite to precipitate debris from metamorphic and igneous rocks were delivered from the rising hinterland the petrographic and magnetic records of red beds are a mirror image of the unroofing of a magmatic edifice in the provenance area between 11 and 3 ma

spatial and temporal distribution of sand containing basin fills in the holocene rhine meuse delta the netherlands the quantitative significance of coarse grained deposits in the overbank realm such as crevasse splay deposits has not been studied at the delta scale or at the holocene timescale such knowledge would be beneficial for understanding and explaining sediment distribution in delta plains this study addresses delta scale distribution of sand containing basin fills and their sand body proportion variability based on eight valleywide cross sections in the holocene rhine meuse delta in the netherlands we found that sand containing basin fills form 7 1 of the fluviodeltaic wedge of which splay deposits are most frequently observed midway between the delta apex and the coast organic clastic lake fills and bay head delta deposits in contrast are limited to the distal delta plain over four successive periods between 9000 and 800 cal yr bp the largest proportions of splay deposits remain at 50 150 km downstream of the upstream shifting delta apex we show that intermediate floodbasin widths between 3 1 and 3 6 km in the holocene rhine meuse delta yield the highest proportions of splay deposits high rates of base level rise and wide floodplains both facilitate the creation of accommodation which in turn provides conditions for peat forming wetlands in which organic clastic lake fills can develop the results show that sand bodies form 26 30 of sand containing basin fills this proportion is shown to be controlled by among other variables channel planform and superelevation of the trunk channel and substrate composition we conclude that potentially large volumes of nonchannel sand bodies exist in distal delta plains they constitute up to 39 of the reservoir volume in the distal rhine meuse delta and yield relatively high connectedness ratios 2011 by the university of chicago all rights reserved

a hierarchical approach for evaluating fluvial systems architectural analysis and sequential evolution of the high net sand content middle wasatch formation uinta basin utah well exposed three dimensional fluvial outcrops of the high net sand content middle wasatch formation in three canyon uinta basin utah were used to create and develop a new methodology for describing the architecture of fluvial systems the methodology builds on the works of campbell jackson allen and miall and addresses sedimentary processes scale and temporal context for reservoir and non reservoir bodies the methodology developed herein is a three level hierarchical framework that classifies meso and macroscale architecture of fluvial systems the three level hierarchy contains from smallest to largest stories elements and archetypes eight story types provide the foundational building blocks of this framework and account for sedimentation in both channel belt and floodplain belt elements including 1 downstream accreting 2 laterally accreting 3 erosionally based fine grained fill 4 fine grained fill associated with laterally accreting 5 levee 6 splay 7 crevasse or overbank channels and 8 floodplain fines two types of elements are recognized 1 channel belt and 2 floodplain belt an archetype consists of a channel belt element and its genetically related floodplain belt elements two distinct upward stacking patterns differentiate braided and meandering archetypes in deconstructing the evolution of archetypes three distinct associations between channel belt elements and their adjacent splays are documented 1 unassociated splays 2 associated coeval splays and 3 associated non coeval splays width and thickness for stories channel belt elements and archetypes are documented providing dimensional constraints for analog high net sand content fluvial systems additionally this methodology provides object based models with shape defined reservoir and nonreservoir geobodies that realistically compare to fluvial systems

factors affecting the accumulation of organic rich deposits in a modern avulsive floodplain examples from the cumberland marshes saskatchewan canada since the 1870s the saskatchewan river at the cumberland marshes east central saskatchewan canada has been undergoing progradational avulsion resulting in development of a 500 km2 avulsion belt the avulsion belt consists of an unstable network of active and abandoned channels splays and floodbasins some containing small shallow lakes fine grained deposits of varying total organic content toc are currently accumulating within floodbasins inside adjacent to and outside the avulsion belt the toc of these deposits depends on factors that control the amount of siliciclastic sediment received by floodbasins five factors that affect toc content are identified distance from an active channel position relative to the avulsion belt presence of alluvial ridges changes in local channel activity and developmental stage of avulsion a channel to basin trend of increasing toc is present at many locations reflecting greater suspended sediment loads received by near channel areas and locations within the avulsion belt generally accumulate sediment with lower toc than those adjacent to or outside the belt alluvial ridges locally act as barriers to floodbasin deposition and channel abandonment reduces suspended sediment to adjacent floodbasins both of which increase toc of sediment finally as avulsion wanes less siliciclastic sediment is distributed throughout the avulsion belt resulting in increased toc in the geological record such fine grained avulsion deposits are likely to appear as intervals of mudrock between thin beds of coal or carbonaceous shale formed from organic rich sediment deposited between avulsion events a succession that is common in many ancient alluvial strata

sedimentary products of avulsions in the holocene rhine meuse delta the netherlands abstract the main objective of this study was to describe the characteristics of deposits related to avulsion to determine if avulsion locations can be recognized by characteristic sediment architecture in the rhine meuse delta the majority of avulsion sites is accompanied by crevasse splay deposits with a relatively large areal extent these crevasse splay complexes occur near the base of the overbank deposits of the newly formed channel belt and are similar to avulsion belt deposits described by smith et al sedimentology 36 1989 1 can j earth sci 35 1998 453 the lithology of all investigated crevasse splay deposits ranges from coarse sand to silty clay sand is concentrated in crevasse splay channels and silty and sandy clay occurs as distal deposits and as levee deposits between the channels the total volume of sand is small relative to clay and silt of a total of 91 avulsion sites three sites were studied in detail one each representing the sedimentary architecture of a full nodal avulsion a failed avulsion and a crevasse splay complex that did not lead to avulsion it is concluded that dating evidence of the abandoned and newly formed paleo channel is always required to verify if avulsion really occurred although the presence of large scale sandy crevasse splay deposits in combination with two main meandering channels often indicates avulsion based on a 2 d outcrop or just one cross section it is impossible to determine whether a splay complex was associated with an avulsion or not because the architecture and lithology in both cases can be identical not all avulsion locations are characterized by crevasse splay deposits there are two possible explanations for this 1 splay deposits were eroded by the newly formed channel or 2 splay deposits were never formed this may be a result of reoccupation of an old channel in this case avulsive flow is immediately appropriated by the pre existing channel and no large scale splay formation occurs alternatively splay deposits were not formed because avulsion resulted from headward erosion

fluvial systems in the siwalik miocene and wyoming paleogene abstract the 3 km thick miocene siwalik group himalayan foredeep in northern pakistan and the 2 km thick paleogene fort union willwood formations bighorn basin in wyoming both preserve long records of fluvial deposition adjacent to rising mountain belts depositional environments and associated habitats change across large basins along with changing physiography and with the location of different river systems that may have varied greatly in patterns of channel deposition and the drainage of adjacent floodplain areas deposits exposed in these two basins provide very different records of shifting paleoenvironments and patterns of basin filling these differences reflect distinct patterns and scales of depositional environments the nature of the exposures and the types of sedimentologic studies that have been carried out in each basin the siwalik group fills a basin that extended at least 1000 km along its axis and 150 250 km away from the mountain front comparison of siwalik deposits and modern drainages in the himalayan foredeep suggests the ancient siwalik basin was filled by large rivers that deposited low gradient sediment fans covering areas on the order of 1000 km 2 and by smaller intrafan rivers with more poorly drained floodplains despite the scale of these river systems relative to siwalik exposures in pakistan transitions between different systems have been recognized deposits of coeval river systems in the siwalik basin show pronounced differences in alluvial architecture the character of overbank deposits and the abundance and taphonomy of organic remains in contrast the bighorn basin in wyoming is a relatively small intermontane foreland basin extending 200 km along its axis and about 80 km across bighorn basin strata were deposited by a river that flowed south to north along the basin axis and by smaller rivers that flowed transverse to the basin axis much of this basin is exposed and thus it is possible to reconstruct changing patterns of deposition and environments through time in more detail than in the siwalik basin these patterns indicate changes in basin wide drainage conditions and environments through time but there are also important differences among coeval strata upsection shifts in environments and vertebrate faunas within both the siwalik and bighorn basins may reflect tectonic or climatic forcing but this comparison emphasizes the importance of recognizing deposits from different contemporaneous river systems before inferring such large scale controls on paleoecological change through time

mid holocene extreme hydrological events in the eastern great rann of kachchh western india abstract the present study is an attempt to reconstruct the paleohydrological conditions during the mid holocene in the hyper arid terrain of the northern great rann of kachchh remote sensing detailed field geomorphology sedimentology geochemistry and optical chronology suggests that the fluvial sediments fringing the khadir island were transported locally and deposited during the extreme hydrological events corresponding to the mid to late holocene indian summer monsoon ism variability overall five extreme hydrological events are deciphered during the last 5 ka which broadly correlates with periods of strengthened ism our data indicate that the emergence and decline of the harappan civilization was unrelated to any major fluvial system in the region

climate variations in britain during the last glacial holocene transition 15 0 11 5 cal ka bp comparison with the grip ice core record stratigraphic records from four sequences in britain that span the last glacial holocene transition c 15 0 11 5 cal ka bp are summarized and compared using a calibrated timescale palaeoclimatic changes are defined using temperature estimates based on coleopteran data at two of the sites and chironomid assemblages at the other sites an attempt is made to distingish between the timing of a climate shifts and b ecosystem responses at all four sites the british data are compared with the grip ice core record recently advocated by bjorck et al as the type sequence for the last termination in the north atlantic region there is a degree of compatibility between the british and grip palaeoclimatic records the clearest climatic signals in the british lake records are a a period of warm conditions a thermal maximum between 15 0 and 14 5 cal ka bp which equates approximately with grip zone gi le b the onset of climatic cooling which appears to have commenced during grip zone gi le at c 14 5 cal ka bp c a period of marked climatic instability from 14 4 to 12 7 cal ka bp the regional effects of which are difficult to resolve using the evidence presently available d further climatic cooling which commenced at around 13 0 cal ka bp after which the lake biota catchment soils and vegetation cover at least in some parts of britain did not recover until the onset of the holocene e continued climatic cooling with the most severe impact on lake ecosystems occurring during the period c 12 6 to 11 5 cal ka bp

lower cretaceous paleo vertisols and sedimentary interrelationships in stacked alluvial sequences utah usa the yellow cat member of the cedar mountain formation in poison strip utah usa consists of stacked erosionally bounded alluvial sequences dominated by massive mudstones lithofacies fm with paleo vertisols sediment bodies within these sequences grade vertically and laterally into each other at pedogenic boundaries across which color texture and structures sedimentary vs pedogenic change slickensides unfilled sealed cracks carbonate filled cracks and deeper cracks filled with sandstone the latter features suggest thorough desiccation during aridification thin sandstones sms in some sequences typically as well as laminated to massive mudstones flm with which they are interbedded in some cases are interpreted as avulsion deposits the termini of many beds of these lithofacies curve upward parallel to nearby pedogenic slickensides as the features we call turnups turnups are overlain or surrounded by paleosols but strata sheltered underneath beds with turnups retain primary sedimentary fabrics turnups were produced by movement along slickensides during pedogenesis by differential compaction alongside pre existing gilgai microhighs or by a combination of both palustrine carbonates lithofacies c appear only in the highest or next highest alluvial sequences along with a deep paleo vertisol that exhibits partially preserved microrelief at the base of the overlying poison strip member the attributes of the yellow cat member suggest comparatively low accommodation slow accumulation long hiatuses in clastic sedimentation and substantial time intervals of subaerial exposure and pedogenesis it appears to be distinct among the members of the cedar mountain formation in these respects

facies characteristics and cyclicity of lower siwalik sediments jammu area a new perspective the lower siwalik succession of the jammu area has been distinguished into three major lithofacies associations a sand dominant association a sandy mud dominant association and a siltyheterolithic association the sand dominant association is made up of three lithofacies cross bedded sandstone rippled silty sandstone and bioturbated sandy siltstone which are organized in multistoreyed sandbodies representing deposition in major river channels the sandy mud dominant association is made up of two lithofacies mottled clayey siltstone and interbedded sandstone siltstone and mudstone representing deposition in overbank areas of flood plain and natural levee crevasse splays the sand dominant association and sandy mud dominant association are grouped together as a channel related succession and are products of processes in the river channel the siltyheterolithic association consists of four lithofacies mottled siltstone mottled silty sandstone bedded calcrete and mottled mudstone they are considered to be deposits of doab upland interfluve areas operating independently of present day major river channels these deposits have been formed in minor channels sloping surfaces and lakes and ponds of the interfluve regions the cyclicity of both successions channel related and doab related has been determined using a partial independence statistical model

necessary conditions for a meandering river avulsion there is as yet no rational basis for predicting the conditions leading to avulsion of a meandering river here we present a conceptual model and quantify it under simplifying assumptions as a first step toward the construction of a stability diagram for avulsion initiation it is assumed initially that a rectangular crevasse channel of arbitrary depth is cut into the levee of a meandering river because the water entering the crevasse channel is derived from relatively high in the main flow it contains low concentrations of suspended solids consequently the crevasse flow is under capacity and the entrance is eroded deepening of the entrance increases the crevasse channel discharge and the concentration of suspended solids supplied because more sediment laden waters are tapped from the deeper flow in the main channel the crevasse entrance is predicted to deepen until its sediment carrying capacity is satisfied by the suspended solids entering from the main channel whether an avulsion will occur for a particular combination of initial conditions depends upon whether there exists steady state hydraulic and sediment transport conditions for crevasse channel depths that are equal to or less than the main channel depth this conceptual model is quantified by writing the unsteady gradually varied one dimensional conservation of mass and momentum equations for water and sediment transport through a main and crevasse channel sediment transport is computed as the integral of a cross sectional mean velocity and a rouse concentration profile solutions show that stable crevasse channels exist only for particular combinations of the initial height of the crevasse bed relative to the water depth in the main channel and the ratio of initial crevasse bed slope to main channel slope

formation of paleovalleys in the central himalaya during valley aggradation abstract the formation of paleovalleys in mountainous regions is considered to result from extreme events such as landslides and glacial or landslide lake outburst floods according to ipcc 2012 the extreme events are rarest of the rare weather climatic events when the climate weather variable is significantly above or below the defined threshold value the present study suggests that paleovalleys can also form during years long periods of valley aggradation a series of paleovalleys thus formed runs parallel to the present river course in this study we suggest that paleovalleys in the alaknanda valley of the central himalaya have formed in two ways 1 major valley aggradation and 2 local events of landsliding and lake breaching most of the paleovalleys in the alaknanda valley formed during a major valley aggradation phase between 15 and 8 ka paleovalleys formed due to local landsliding also formed around 8 ka local landslides were triggered due to high rainfall in lower valley reaches during unstable climatic conditions therefore the formation of paleovalleys both by regional and local mode within 15 8 ka indicates that the valley was receiving excess sediment from upper catchment as well as from lower reaches during this period this phase of excess sediment supply and valley aggradation coincides well with post glacial climatic amelioration therefore the study underlines the role of climate in the time scales of 10 3 years in shaping the landscape of an active mountain like the himalaya the role of other landscape changing agencies such as tectonics and erosion is not accounted in the present study