quaternary blind thrusting in the southwestern sacramento valley california patterns of microearthquakes and quaternary surface deformation suggest that the tectonic setting of the sw sacramento valley is similar to areas of the western san joaquin valley known to be underlain by seismogenic blind thrust faults on the basis of previous work and analysis of geologic and seismic reflection data the following late cenozoic tectonic features and processes are identified 1 uplift of the northern coast ranges beginning approximately 3 4 ma and eastward propagation of uplift into the southwestern sacramento valley by 1 0 ma 2 uplift and homoclinal flexure of plio pleistocene strata at the eastern coast ranges mountain front 3 uplift and folding above blind thrusts approximately 15 km east of the mountain front in the southwestern sacramento valley similar associations of structures and processes have been observed in thrust belts in pakistan the peruvian andes and the canadian cordillera and are commonly attributed to thrusting within an intercutaneous wedge or triangle zone by using other thrust belts as analogs the propagation of an eastward tapering triangle zone is interpreted to be the principal mechanism for uplift and homoclinal flexure at the eastern coast ranges mountain front seismic reflection profiles reveal that 1 the triangle zone consists primarily of east vergent blind thrusts and 2 west vergent backthrusts exposed in the the eastern coast ranges and southwestern sacramento valley are rooted in the east vergent thrusts transfer of slip from the east vergent blind thrusts to the west vergent backthrusts occurs locally beneath the southwestern sacramento valley fault bend folding in the hanging walls of the backthrusts has created a north northwest striking chain of low hills approximately 15 km east of the mountain front the folds deform 3 4 1 0 ma fluvial sediments and thus are middle pleistocene in age or younger local variations in strike suggest that the fold chain is segmented like the new idria coalinga kettleman hills segmented fold chain in the southwestern san joaquin valley stein and ekstrom 1989 these data have implications for seismic hazard assessment anecdotal accounts indicate that two m 6 0 events of the 1892 winters vacaville earthquake sequence probably occurred beneath the eastern coast ranges dale 1977 toppozada et al 1981 ground cracking was observed following the main shocks along the mountain front in the southwestern sacramento valley we propose that the earthquakes were generated by slip on a blind thrust beneath the coast ranges and that the ground cracking in the valley represents propagation of the eastward tapering triangle zone the 1892 earthquake sequence suggests that blind thrusts beneath the southwestern sacramento valley are active and capable of generating moderate to large magnitude earthquakes

horizontal lamination and the sequence of bed phases and stratification under upper flow regime conditions two forms of horizontal laminae have been produced under upper flow regime conditions in a flume by deposition on a plane bed and on low in phase waves upper plane bed laminae consist of approximately equal numbers of fining upward fu and coarsening upward cu textural laminae heavy minerals when present are typically within or form cu laminae in horizontal laminae formed under low in phase waves fu laminae may predominate and are significantly thicker than cu laminae heavy minerals are associated with the tops of some fu laminae and are absent from cu laminae deposits aggraded on plane beds and low in phase waves both display low angle 13 14 upstream grain imbrication upper plane bed horizontal laminae are attributed to the effects of the burst sweep cycle on local bedload transport bursts form fu laminae and sweeps form cu laminae some fu laminae in in phase wave deposits are thought to form with downstream migration of bed waves on which sand size decreases from trough to crest heavy minerals are concentrated in the tops of these fu laminae due to their enrichment on the crests of the migrating bed waves other fu and cu laminae in in phase wave deposits probably formed by the action of bursts and sweeps as on upper plane beds a sequence of bed phases and stratification formed under upper flow regime conditions is described based on the observations here combined with descriptions from the literature with increasing flow strength the gradual sequence of bed phases is 1 plane bed 2 low relief downstream migrating inphase waves increasing in amplitude and wavelength with increasing flow strength 3 stationary in phase waves 4 upstream migrating including breaking wave antidunes the sequence of stratification formed under these bed phases is 1 upper plane bed horizontal lamination 2 in phase wave horizontal lamination to in phase wave foreset cross laminae 3 in phase wave drape laminae 4 antidune backset cross laminae

stream driven high density gravelly traction carpets possible deposits in the trabeg conglomerate formation sw ireland and some theoretical considerations of their origin within high density flood flows a prominent mechanism of gravel transport and deposition is by stream driven high density traction carpet with a rheology similar to grain flow these gravel carpets are envisaged to form the basal portion of a bipartite high density flood flow decoupled from an overlying sand and silt laden turbulent flow several examples already documented in the literature are reviewed and an additional case from the lower old red sandstone of southwest ireland is presented two mechanisms of traction carpet initiation are discussed by rapid entrainment of gravel into suspension on rising stage followed by settling into the gravel traction carpet at peak and falling stage and by overconcentration of a normal low density bedload gravel entrainment suspension and traction carpet development are significantly easier if the flood water already carries a high concentration of sand and silt in suspension theoretical consideration further shows that gravelly traction carpets can be maintained in channels of relatively low gradient by the shear stress exerted by the high density sand bearing turbulent flood flow above this tangential shear stress is converted to dispersive pressure which aids buoyancy and quasi static grain to grain contacts in the support of the clasts within the gravel carpet the carpet is thought to have a quasi plastic rheology but behave much like a viscous fluid at high shear rates stream driven gravelly traction carpets are expected to produce sheet like units of clast to matrix supported conglomerate characterized by a parallel or an a p a i clast fabric these units may be ungraded normally or inversely graded depending on the rate of shear the viscosity of the flow and the celerity of deposition

fluvial facies model of the ganga river sediments india abstract the river ganga flows from the northwest to the southeast following the lineaments developed in response to the himalayan tectonics ganga sediments show a large number of facies developments throughout the study area from hardwar to kachhla the active channel deposits are marked by the development of large scale facies in gravel and coarse sand small scale facies comprising fine sand silt and clay are developed in inactive channel deposits and overbank deposits the depositional model shows a progressive decrease in particle size thickness of units and scale of cross bedded sets from the lower to the upper parts of sequences as well as from upstream to downstream the sedimentation of the ganga basin is seasonally and tectonically controlled

the morphology and dynamics of low amplitude bedwaves upon upper stage plane beds and the preservation of planar laminae experimental studies of subcritical unidirectional flow over upper stage plane beds of medium grained sand reveal the ubiquitous presence of low amplitude bedwaves flow depth was 0 11 m mean flow velocities were 0 86 1 0 m s 1 shear velocities were 0 058 0 71 m s 1 and dimensionless shear stresses were 0 56 0 87 bedwaves are asymmetrical in profile and range from 0 75 to 11 mm in height mainly 2 6 mm from 0 7 to 1 3 m in wavelength and have mean celerities of 10 mm s 1 flow records suggest that the bedwaves are associated with accelerating flow over the bedwave crests and flow which decelerates and diverges laterally over the troughs high resolution bed profiling during aggradation of the bed combined with subsequent box coring illustrates that these bedwaves are responsible for the planar laminae characteristic of upper stage plane beds lamina preservation is dependent upon the mean aggradation rate and the sequence of bedwaves of different height crossing any point individual laminae are more readily preserved at higher aggradation rates where the possibility of reworking by later bedwaves is reduced laminae are recognized by small changes in grain size and commonly a fining upwards at the top of laminae which is generated by fine grained material infiltrating a lower lamina in the leeside of a bedwave

sedimentology paleoflow dynamics and flood history of jokulhlaup deposits paleohydrology of holocene sediment sequences in southern iceland sandur deposits abstract this paper examines the sedimentology of a series of deposits associated with catastrophic floods caused by subglacial volcanic eruptions in southern iceland a simple model is proposed for the interpretation of the sediments in terms of changes in flow characteristics of the fluid sediment mix a range of sediment types characteristic of these flood flows is identified the predominant one being thick massive homogeneous poorly structured black pumice fine gravels with 65 p rcent silt clay type e and single lithology boulder deposits type f these flood deposits alternate with or are channeled into more normal sandur deposits comprising heterogeneous poorly sorted clast supported imbricated rounded cobble and pebble gravels type g the dominant sediment sequences are interpreted as representing hyperconcentrated fluid sediment mixtures with dispersive stresses acting to create massive poorly sorted nongraded or inversely graded sediments the latter characterized by large surface boulders the vertical changes in sediment type reflect significant changes in flow conditions during transport and subsequent deposition with an initial massive flood surge followed by flow deceleration sediment freezing and dewatering producing a more fluid series of flows the more fluid phases were associated with extensive erosion and channelization cutting deep scour channels marked by residual streamlined hummocks boulder lags and fields of megaripples leading downstream to a washed sandur plain the paper examines some of the problems associated with the application of paleoflow models for determining former flow dynamics the models adopted suggest that most of the flows occurred as relatively mobile low viscosity flows with little internal resistance to shear while sediment concentrations are estimated to have reached about 50 percent by weight with maximum yield strengths between 2 9 103 and 4 2 105 dn cm 2 maximum viscosities ranged between only about 2 and 8 102 poises a range of paleovelocity models applied to the flood sediment indicates flow rates of between abo t 4 and 13 ms 1 with peak discharges reaching a maximum of 3 3 105 m3 s 1 dating of the deposits by 14c dating and tephrochronology indicates that at least 8 major sediment producing jokulhlaups occurred over a 4 000 year period between c 4500 yr bp and 1357 ad hence although relatively rare the effects of these floods still dominate the proglacial landscape probably producing 85 percent of the total thickness of proglacial sandur sediments

models of aggradation versus progradation in the himalayan foreland a frequent goal of decompaction analysis is to reconstruct histories of basin subsidence and tectonic loading in marine environments eustatic and paleobathymetric uncertainties limit the resolution of these reconstructions whereas in the terrestrial basins these ambiguities are absent it is still necessary to account for depositional slopes between localities in order to analyze three dimensional patterns of subsidence we define two end members for depositional surfaces aggradation and progradation the relative importance of either end member is a function of the interplay between the rate of net sediment accumulation and the rate of basin subsidence the models predict the patterns of major drainages transverse versus longitudinal and the way in which provenance should be reflected within different portions of a basin consequently paleocurrent and provenance data from the ancient stratigraphic record can be used to distinguish between these endmembers the subhorizontal depositional surfaces that dominate during times of aggradation provide a well defined reference frame for regional analysis of decompacted stratigraphies and related subsidence depositional slopes during progradation can not be as precisely specified and consequently yield greater uncertainties in reconstructions of subsidence these models are applied to the mio pliocene foreland basin of the northwestern himalaya where sequences of isochronous strata have been analyzed throughout the basin these time controlled data delineate a distinctive evolution from largely aggradational to largely progradational depositional geometries as deformation progressively encroaches on the foreland such a reconstruction of past depositional surfaces provides a well constrained reference frame for subsequent integration of subsidence histories from throughout the foreland

differential bedload transport rates in a gravel bed stream a grain size distribution approach the grain size distributions of bedload gravels in oak creek oregon follow the ideal rosin distribution at flow stages which exceed that necessary to initiate breakup of the pavement in the bed material the distributions systematically vary with flow discharge and bed stress such that at higher flow stages the grain sizes are coarser while the spread of the distribution decreases a differential bedload transport function for individual grain size fractions is formulated utilizing the dependence of the two parameters in the rosin distribution on the flow stress the total transport rate which is also a function of the flow stress is apportioned within the rosin grain size distribution to yield the fractional transport rates the derived bedload function has the advantage of yielding smooth continuous frequency distributions of transport rates for the grain size fractions in contrast to the discrete transport functions which predict rates for specified sieve fractions successful reproduction of the measured fractional transport rates and bedload grain size distributions in oak creek by this approach demonstrates its potential for evaluations of transport rates of size fractions in gravel bed streams the approach will be useful in investigations of downstream changes in bed material grain size distributions

a multi isotope b sr o h and c and age dating 3h 3he and 14c study of groundwater from salinas valley california hydrochemistry dynamics and contamination processes 1 the chemical and isotope 11b 10b 87sr 86sr 18o 16o 2h h 13c 12c 14c and 3he 3h compositions of groundwater from the upper aquifer system of the salinas valley in coastal central california were investigated in order to delineate the origin and processes of groundwater contamination in this complex system the salinas valley has a relatively deep confined 400 foot aquifer overlain by a 180 foot aquifer and a shallower perched aquifer all made up of alluvial sand gravel and clay deposits groundwater from the aquifers have different 14c ages fossil 14c 21 3 percent modern carbon pmc for the 400 foot aquifer and modern 14c 72 2 98 2 pmc for the 180 foot aquifer fresh groundwater in all aquifers is recharged naturally and artificially through the salinas river the two modes of recharge can be distinguished chemically we identified several different saline components with distinguishable chemical and isotopic fingerprints 1 saltwater intrusion in the northern basin has c1 concentrations up to 1700 mg l a na cl ratio less than seawater a marine br cl ratio a ca cl ratio greater than seawater 11b between 17 and 38 and 87sr 86sr between 0 7088 and 0 7096 excess dissolved ca relative to the expected concentration for simple dilution of seawater correlates with 87sr 86sr ratios suggesting base exchange reaction with clay materials 2 agriculture return flow is high in no3 and so4 with a 87sr 86sr 0 7082 11b 19 and 13c between 23 and 17 the 3h 3he ages 5 17 years and 14c data suggest vertical infiltration rates of irrigation water of 3 10 m yr 3 nonmarine saline water in the southern part of the valley has high total dissolved solids up to 3800 mg l high so4 na cl ratio 1 11b between 24 and 30 and 87sr 86sr 0 70852 this groundwater may have acquired its geochemical signature from leaching of sedimentary rocks associated with the coast range marine deposits of mesozoic to early cenozoic age the combination of different geochemical and isotopic fingerprints enables us to delineate the impact of salt sources in different areas of the valley and to reconstruct the origin of the so4 enriched no3 depleted saline plume that is located west of the city of salinas we suggest that the latter is derived from a mixture of different natural saline waters rather than from anthropogenic contamination

concentration of magnetite and gold at bar and reach scales in a gravel bed stream british columbia canada abstract a model is presented for placer formation at the grain bar 10 100 m and reach scales 1000 m in gravel bed streams the model is based on the processes of erosion and re deposition of the bed during annual flood events and uses einstein s bedload function to estimate changes in equilibrium transport rates heavy mineral enrichment in a gravel bed is postulated to occur in the voids between framework clasts it is demonstrated that increases in bed roughness represented by the d65 diameter of material in the bed result in preferential accumulation of heavy minerals with enrichment increasing as their density increases at the reach scale decreasing channel slope from 0 04 to 0 02 also results in enrichment of heavy minerals distribution of gold and magnetite predicted by the model is in good agreement with field data for a gravel bed stream in british columbia samples were collected from counter current eddies and other low velocity areas containing sands at gravel bar mils and from typical bar gravel deposits for the purpose of comparison with the model the sands were assumed to represent the bulk composition of sediment transported by the stream under conditions of falling discharge after the seasonal snowmelt flood trends in magnetite and gold distribution along a 5 km reach can be directly related to the model gravels are sites of heavy mineral accumulation with gold being enriched to a much greater extent than magnetite as predicted by the model density sorting is most obvious for the coars r sand fractions changes in channel slope along the reach also result in predicted changes in heavy mineral distribution

high energy ephemeral stream deltas an example from the upper silurian holmestrand formation of the oslo region norway abstract the late orogenic deposits of the upper silurian holmestrand formation of the oslo region norway comprises sharply alternating ephemeral stream and sandy beach face deposits the formation is divided into three units reflecting the down slope change on an extensive ephemeral stream delta system the lower unit of the formation comprises ephemeral stream sediments as stacked sheet sandstone bodies deposited on a broad coastal floodplain each sheet is bounded by a lower intraclast strewn scoured surface overlain by massive horizontally stratified low angle and trough cross bedded sandstones in the medial and upper units of the formation the ephemeral stream sediments alternate with beach face deposits forming ephemeral stream deltas the ephemeral stream deltas are marked by intraclast strewn scoured surfaces and dominated by trough cross bedded sandstones they may contain escapement burrows marine bioturbation and wave working of the delta surfaces the beach face deposits are composed of alternating irregular horizontally stratified sandstones with current and wave ripple cross lamination and tabular cross bedded sandstones deposition took place in a coastal non to micro tidal shallow water environment influenced by longshore currents and low palaeo wave energy seven ichnotaxa are recognized and support the sedimentary record of a transitional environment between a broad coastal floodplain and a shallow marine environment the presence of calcretes indicate an arid to semi arid palaeo climate

facies relationships alluvial architecture and paleohydrology of a paleogene humid tropical alluvial fan system chumstick formation washington state u s a the middle to late eocene chumstick formation represents a thick sequence of fluvial and lacustrine rocks that filled a tectonically active basin under humid tropical paleoclimatic conditions four facies associations are recognized in the chumstick formation gravel bedload sand bedload and mixed load stream deposits and lacustrine deposits the depositional system is interpreted as a humid tropical alluvial fan system five sets of basinal characteristics are believed to result from the combination of humid tropical paleoclimate active basin margin faulting and high 0 2 to 1 2 m k y basin subsidence rates 1 fluvial architecture was strongly affected by basin margin faulting proximal regions of the fan consist of broadly lenticular laterally stacked i e offlapping ribbons gravel bedload stream deposits representing fanhead channels distal regions of the fan consist of gravel and sand bedload stream deposits with a sheet like geometry the basin fill is dominated by multistory vertically stacked sand bedload and mixed load stream deposits and lacustrine deposits 2 rapid subsidence has resulted in the preservation of compositionally immature sediments sublithic feldspathic arenites with thin shallow paleosols 3 paleohydraulic reconstructions indicate that streams carried coarse grained bedload through channel reaches with paleoslopes 5 m km 4 rapid vegetation growth resulted in the bank stabilization of sandy sediments development of paleosols leaf litter layers on emergent bar tops and the introduction of woody debris into channels even coarse grained proximal fluvial deposits have a significant overbank component and show evidence for bank stability cutbanks and cohesive bank failures 5 debris flow deposits are relatively minor except for local talus cone deposits the rarity of debris flows throughout the basin may be due to hillslope stability imparted by vegetation

the trollheim alluvial fan and facies model revisited widely held fundamental alluvial fan facies concepts generated in past literature based on the trollheim fan deep springs valley california are inconsistent with the actual characteristics of this fan previous publications have established mostly without field study that the trollheim fan consists of debris flow sieve lobe sheetflood and channel facies debris flow and sieve deposits comprise the proximal area giving way to channel or sheetflood deposits distally contrary to these views clayey and gravelly matrix supported debris flow deposits dominate all parts of the trollheim exposures suggesting that this fan has been built almost exclusively by mass wasting events channel fill deposits constitute only a small component of the fan and most are restricted to the proximal area where they have an inset relationship with debris flow sequences the channel facies consists of laterally restricted clast supported gravel lenses formed primarily through the winnowing of debris flow fines by streamflow the most widespread facies on the surface of the fan is a mantle of varnished matrix free pebbles cobbles and boulders that commonly are distributed in radially oriented lobes or levees characteristic of debris flow deposits this mantle was produced by the surficial winnowing of the fine fraction from debris flow deposits primarily by overland water flow in contrast to previous conclusions sheetflood and sieve deposits were not identified anywhere on the fan additionally many of the facies depicted in the widely cited trollheim vertical profile including ripple bedded sand cross bedded sand and overbank mud are not present on or in stratigraphic cuts of the fan after which the profile was named

characterization of fluvial and aeolian reservoirs problems and approaches sedimentological research has traditionally produced geologic models primarily relevant to hydrocarbon exploration focusing predominantly on location and external geometry of sediment bodies be they clastic or carbonate and underplaying the importance of the internal reservoir framework for example it has been estimated that geological heterogeneity traps up to 40 of moveable oil reserves in fluvial reservoirs tyler finley 1991 this failure severely inhibits the application of new and sophisticated technologies to improving recovery efficiencies which are lamentably low in far too many cases efficiencies less than 25 being common tyler finley 1991 it is reservoir architecture the internal fabric and structure of a reservoir that ultimately controls the paths of fluid migration during oil and gas emplacement and subsequent extraction this architecture is in turn the product of the depositional and diagenetic processes which created the reservoir if an understanding of the origin of the reservoir is developed reservoir architecture and hence fluid flow paths become predictable the task for the geologist then is to locate and describe this heterogeneity so that the trapped hydrocarbons may be liberated in the past few years much more emphasis has been placed by reservoir geologists on elucidating the nature of geological heterogeneity and developing predictive models for use by engineers however in geologically complex reservoirs and those composed of sediments deposited in continental settings almost invariably come into this category the rate of change of lithology and petrophysical properties is usually very high making subsurface geologicar prediction extremely difficult indeed for such sequences it can be remarkably demanding just to correlate major units from one well to another to establish gross connectivity pathways let alone establish the more subtle detail many of the current depositional models encompass too large a scale for the needs of reservoir exploitation and do not provide the quantitative component critical to the engineering of hydrocarbon extraction at the interwell scale ability to predict the distribution of rock fabrics is related to knowledge of the physical and chemical processes of sedimentation and diagenesis sadly our knowledge of these factors has proved to be too sketchy and qualitative for the demands being made of it recognition of this situation has spawned a resurgence in studies of analogue outcrops of the ancient and reexamination of equivalent modern depositional environments what is more the geologists are now being joined in the field by petroleum engineers in an attempt to improve cross discipline communication and to ensure the data relevant to the engineering needs are collected given that the deposits of continental settings are generally more accessible than those of marine settings it might be expected that our knowledge of these would be the most advanced which ought to be reflected in the sophistication and accuracy of predictive reservoir models for such environments the starting point for this volume of papers was our desire to test if this was indeed the case and if it were not then to attempt to document the current status and to highlight the areas needing additional work based on our reading of the literature and contacts with the industry we had several areas for concern i are the geologists collecting the quantitative data needed by the engineers if not what is preventing them from doing so the geology or the lack of appreciation of the requirements ii are the results from outcrop and modern environment studies being translated into predictive tools suitable for modelling subsurface geology iii is the knowledge being gained having any significant impact on exploitation of hydrocarbon reserves iv is there a pre occupation with small scale heterogeneity when in many cases still the primary problem is large scale sand distribution and connectivity

light stable isotope systematics of large scale hydrologic regimes in california and nevada surface waters shallow groundwaters and precipitation samples were collected along three traverses which begin at the california coast and are oriented in an east west manner roughly parallel to the atmospheric flow path of meteoric water the hydrogen isotope compositions from short term precipitation records varied as much as 40 at one location and were not representative of the average annual isotopic composition of precipitation the hydrogen isotope ratios of surface waters and shallow groundwaters distinguish segments with isotopic variations that correlate with measured vertical fluxes of meteoric water one coastal segment appears to closely approximate an open system in which precipitation dominates over evapotranspiration most segments are described by rather regular variations in d 3 45 per 100 km which represent different degrees of closure of the hydrologic system caused by variable partitioning of precipitation between runoff and evapotranspiration all partially closed systems imply terrestrial recycling of water a simple model estimates this amount to be about 20 across northern and central california three segments representing the great basin show virtually no geographic variation and appear to represent isotopically closed systems

small scale hummocky cross stratification in turbidites a form of antidune stratification small scale hummocky cross stratification occurs in upper cretaceous calciclastic turbidites exposed in the western basque pyrenees facies associations and microfossil assemblages indicate slope to base of slope bathyal depositional environments it is developed in the fine grained portion of beds and displays spacings mostly between 0 2 and 0 7 m the beds fine upward with no sharp grain size breaks or mud partings suggesting that deposition occurred during a single flow event hummocky intervals are 0 1 0 8 m thick and consistently grade laterally and vertically into flat planar laminations of bouma b divisions suggesting that deposition occurred under upper flow regime conditions they have wave like geometries with laminae continuous across crests and troughs and display a ratio of wavelength to estimated underflow thickness of 11 3 12 8 combining the above observations and inferences these examples of small scale hummocky cross stratification are interpreted as a form of antidune stratification generated by standing waves along the interface of a thinner denser underflow main body tail of the turbidity current and an overlying thicker low density layer this occurrence is further evidence that small scale hummocky cross stratification is multigenetic and therefore not indicative of a particular flow condition or depositional environment

palaeoenvironmental reconstruction of the early anisian from sedimentology and plant remains in the se iberian range e spain abstract the end of the permian period is characterized by the largest mass extinction ever recorded in marine and terrestrial sedimentary rocks the oldest plant macro remains after the p t boundary recorded in the castilian branch of the iberian range after this mass extinction belong to the eslida formation of anisian age these associations have been recovered at four localities and include sphenophytes equisetites perhaps neocalamites or schizoneura conifers volzia albertia pelourdea lycophytes pleuromeia and a possible seed fern peltaspermum facies associations and sedimentological analysis of the eslida formation have allowed for distinguishing four architectural elements ch channel deposits la lateral accretion cs crevasse splay and ff overbank fines the largest fossil remains are associated with ch and la architectural elements and high energy events and they form part of the basal lag or reactivation surfaces smaller and well preserved fragments are associated with the cs architectural elements a sedimentologic taphonomic and palaeoclimatic analysis reveals gradation from riparian hygrophytic vegetation to semi arid xerophitic vegetation across the alluvial complexes and a progressive aridification through time a comparison with other areas shows that this association is similar to coeval floras of iberia and of the gres a voltzia formation of northeastern france

channel trenching and climatic change in the southern u s great plains fifteen alluvial sequences in texas and oklahoma exhibit the same late holocene record of channel trenching at 1 ka the erosion was preceded by slow alluvial sedimentation in most stream valleys resulting in the formation of a cumulic organic rich flood plain soil previously named the copan soil the soil formed during a period of regionally moister climate as indicated by pollen spectra molluscan faunas vertebrate faunas sedimentary structures and high alluvial water tables at 1 ka the regional climate changed from moist to dry coinciding with an episode of channel incision of valley floors throughout the southern great plains channel trenching occurred simultaneously in both small and large streams in drainage basins of the arkansas red trinity brazos and colorado rivers the sequences are the first documented example of widespread holocene incision accompanied by firm evidence for a synchronous change in regional climate

three dimensional antidunes as hcs mimics in a fluvial sandstone the pennsylvanian south bar formation near sydney nova scotia abstract an array of three dimensional bedforms is present on bedding planes within a 20 35 cm thick medium grained sandstone unit in the braided fluvial south bar formation pennsylvanian near sydney nova scotia the unit is near the top of a 4 7 m thick channel sandstone and is attributed to a late stage of fill prior to the abrupt abandonment of the channel the bedforms comprise radially symmetric mounds and intervening curvilinear hollows and they contain concordant strata they closely resemble hummocks and swales but are identified as three dimensional antidunes based on their fluvial setting and the ubiquitous presence of well defined current lineation oriented parallel to unidirectional paleocurrent indicators within the channel fill the antidunes secreted vertically in paleoflo s with mean depth and velocity of about 0 12 m and 1 0 m s antidune bedforms are rare in the stratigraphic record because such conditions are seldom maintained long enough to accrete a preservable set of strata three dimensional antidunes such as these could easily be mistaken for hummocks and swales and the environment misidentified as storm influenced if the nature and relationships of current lineation were not observed or if incomplete exposure did not permit recognition of shallow water origin

distal alluvial deposits in a foreland basin setting the lower freshwater molasse lower miocene switzerland sedimentology architecture and palaeosols the lower freshwater molasse untere susswasser molasse crops out over a wide area of the swiss molasse basin coarse grained alluvial fan conglomerates dominate in proximal basin areas along the alpine front these conglomerates pass northwards into sandstones and mudstones of an extensive northeastward draining meandering river system which ran parallel to the basin axis sedimentological study of outcrops quarry exposures and boreholes in the basal miocene aquitanian has permitted detailed facies analysis of this distal alluvial sequence the distal aquitanian is made up of distinct architectural elements characterized by their geometries and sedimentary structures each may be assigned to a particular depositional setting meander belt levees crevasse channels and splays overbank fines and palaeosols and lacustrine meander belt sandstones were deposited in mixed load channels with a dominant bedload component sandstones commonly comprise amalgamated and locally stacked ribbon bodies 2 15 m thick and 150 1500 m wide interbedded rippled laminated and mottled fine grained levee sandstones and siltstones form lenticular packages up to 3 m thick and 30 100 m across small scale crevasse channel sandstones 2 4 m thick and 5 10 m across pass laterally into metre scale medium to fine grained crevasse sandstone sheets rare laminated lacustrine siltstones occur only in the north east part of the basin floodplain mudstones and marls make up the remainder of the succession these display a variety of pedogenic features recording cyclical palaeosol development palaeosols show strong variations in morphology and maturity both laterally across the floodplain and downstream along the basin axis reflecting local variation in aggradation rate associated with proximity to alluvial channel courses as well as regional variation in subsidence and floodplain drainage analysis of the organization and distribution of the various sediment bodies permits reconstruction of the fluvial system and allows development of a model for the sedimentary architecture of the lower freshwater molasse in the study area integration of palaeosol studies into a well defined architectural framework assists recognition of areal facies belts and may aid location of sand prone sequences in the subsurface

alluvial response to differential subsidence sedimentological analysis aided by remote sensing willwood formation eocene bighorn basin wyoming usa a colour landsat thematic mapper image was used to identify and to map on a regional scale alluvial facies in the lower eocene willwood formation each facies covers a distinct geographical area of several hundred square kilometres in the bighorn basin of wyoming usa comparison of the image with laboratory spectra from willwood formation rocks and field exposures shows that the image provides information regarding the colour of mudstones their relative abundances and the relative abundance of sandstones facies were further characterized from field study and published descriptions on the basis of sandbody geometry the types of nodules present in the mudstones and the abundance and geometry of carbonaceous mudstones this information helps to interpret the fluvial environments represented and types of alluvial palaeosols found in the willwood formation four facies have been distinguished the first facies is dominated by sheet sandbodies associated with immature palaeosols the sheets mark the locations of major channels which tend to occur in topographic lows the second facies is characterized by more mature moderately drained soils that required relatively lengthy periods of non deposition facies three and four dominantly consist of drab mudstones and abundant carbonaceous units both formed in poorly drained areas although minor red mudstones demonstrate that drainage conditions periodically improved in facies three facies four underwent prolonged saturation the geographical facies distribution suggests that east west faults previously mapped in the bighorn mountains extend westward into the bighorn basin and were active during early eocene time the lithological heterogeneity of the willwood formation rocks is attributed to warping associated with movement along these basement controlled faults fault activity created topographic gradients which influenced the position of major channel systems and helped produce variable drainage conditions that affected eocene soil development

contamination of groundwater under cultivated fields in an arid environment central arava valley israel the purpose of this study is to obtain a better understanding of groundwater contamination processes in an arid environment precipitation of 50 mm year due to cultivation additional aims were to study the fate of n k and other ions along the whole hydrological system including the soil and vadose zone and to compare groundwater in its natural state with contaminated groundwater through the drilling of several wells a combination of physical chemical and isotopic analyses was used to describe the hydrogeological system and the recharge trends of water and salts to the aquifers the results indicate that intensive irrigation and fertilization substantially affected the quantity and quality of groundwater recharge low irrigation efficiency of about 50 contributes approximately 3 5 4 million m 3 year to the hydrological system which corresponds to 0 65 m per year of recharge in the irrigated area by far the

variations in fluvial style in the westwater canyon member morrison formation jurassic san juan basin colorado plateau abstract techniques of architectural element analysis and lateral profiling have been applied to the fluvial westwater canyon member of the morrison formation jurassic in southern san juan basin on a large scale the sandstone body architecture consists mainly of a series of tabular sandstone sheets 5 15 m thick and hundreds of meters wide separated by thin fine grained units internally these sheets contain lateral accretion surfaces and are cut by channels 10 20 m deep and at least 250 m wide on a more detailed scale interpretations made from large scale photomosaics show a complex of architectural elements and bounding surfaces typical indicators of moderate to high sinuosity channels lateral accretion deposits coexist in the same outcrop with downstream accreted macroform deposits that are typical of sand flats of low sinuosity multiple channel rivers broad deep channels with gently to steeply dipping margins were mapped in several of the outcrops by carefully tracing major bounding surfaces locally thick accumulations of plane laminated and low angle cross laminated sandstone lithofacies suggest rapid flow probably transitional to upper flow regime conditions such a depositional style is most typical of ephemeral rivers or those periodically undergoing major seasonal or more erratic stage fluctuations an interpretation consistent with independent mineralogical evidence of aridity fining upward sequences are rare in the project area contrary to the descriptions of campbell 1976 the humid alluvial fan model of galloway 1978 cannot be substantiated and similarly the architectural model of campbell 1976 requires major revision comparisons with the depositional architecture of the large indian rivers such as the ganges and brahmaputra still seem reasonable as originally proposed by campbell 1976 although there is now convincing evidence for aridity and for major stage fluctuations which differs both from those modern rivers and campbell s interpretation

reconstructing random topography from preserved stratification one of the fundamental problems of interpreting the sedimentary record is reconstructing the original heights of palaeotopographical features such as bedforms or river channels this requires an understanding of the relationship between topography and set thickness but at present an exact theory exists only for periodic topography of uniform height the applicability of this simple theory is severely limited by the random variability characteristic of many sedimentary systems in this paper we develop an exact theory for the probability density function pdf of sets generated from topography of random height we focus on the limiting case of zero net deposition in order to provide a lower bound for the set thickness and derive an analytical set thickness pdf that is determined by one parameter of the pdf for topographical height this parameter measures the breadth of the tail of the topographical pdf the mean set thickness is 0 8225 for bedforms and 1 645 for river channels if the topographical height is gamma distributed the preservation ratio defined as mean thickness of preserved sets mean topographical height is 0 8225 r2 for bedforms and 1 645 r2 for river channels where r is the coefficient of variation standard deviation mean of the generating topography in a comparison with data from laboratory current ripples our analytical predictions compare well with observations of both mean set thickness and thickness distribution the preservation ratio for the ripples is about 0 5 whilst measured dune heights give a preservation ratio of about 0 12 depth data from two modern braided streams yield preservation ratios ranging from 0 4 to 0 75 as more data on the distributions of topographical height in modern environments become available calculations such as these should help provide reliable error bounds for quantitative topographical reconstruction

fluvial aeolian interactions part i modern systems two modern fluvial aeolian depositional systems great sand dunes national monument colorado and the mojave river wash california are remarkably similar in spite of different climates sizes fluvial sediment textures and relative directions of aeolian and fluvial transport dune growth and migration and deflation of blowouts create 8 10 m of local relief in unflooded aeolian landscapes there are six prominent fluvial aeolian interactions 1 fluvial flow extends into the aeolian system until it is dammed by aeolian landforms 2 interdune areas overbank interdunes upstream of aeolian dams and alongside channels are flooded 3 water erodes dunes alongside channels and interdunes 4 flood waters deposit sediment in interdune areas 5 fluvially derived groundwater floods interdunes interdune playas 6 wind erodes fluvial sediment and redeposits it in the aeolian system unique and characteristic sediments are deposited in overbank interdunes and in interdune playas reflecting alternate fluvial and aeolian processes and rapidly changing flow and salinity conditions these fluvial aeolian interdune deposits are characterized by irregular concave up bases and flat upper surfaces containing mudcracks or evaporite cement characteristic low relief surfaces form in aeolian systems as an effect of flooding fluvial deposits are resistant to aeolian deflation aeolian sand is preserved when flood sediments are deposited around the bases of the dunes thus repetitive fluvial and aeolian aggradation tends to be stepwise as interdune floors are suddenly raised during floods the effects of flooding should be easy to recognize in ancient aeolianites even beyond the area covered with overbank muds

geochemistry of tectonically expelled fluids from the northern coast ranges rumsey hills california usa tectonic compression has created abnormally high pressure on deep basinal fluids causing their expulsion from areally exposed upper cretaceous rock along the eastern margin of the california coast ranges the fluids emerge as near neutral perennial sodium chloride springs at high elevations with flow rates as high as 10 l per min higher spring discharges are more common around the exposure of a west vergent fault propagation fold axis spring waters range from 1000 to 27 000 mg l tds the least saline water 18o 7 5 closely represents local meteoric water that mixes with saline fluid 18o 5 3 and forms a slope of 3 5 on a d vs 18o plot a na 125 to 8000 mg l vs cl 150 to 17 000 mg l plot shows a linear dilution trend that extends close to but below the values for modern seawater calcium 75 3000 mg l is considerably enriched relative to seawater and forms a nonlinear trend with chloride in detail the na deficit defined by the difference between the measured na content and the na concentration on a hypothetical seawater dilution line is approximately balanced by the ca excess similarly defined by the seawater dilution line this relationship strongly suggests that the fluid is diluted seawater that is being modified by active albitization of plagioclase at different depths simultaneous b and 18o enrichment of the fluids accompanied by deuterium depletion further suggest that the seawater modification is influenced by clay diagenesis bicarbonate and sio2 concentrations show an inverse correlation with cl with most waters being saturated or slightly oversaturated with calcite and quartz at the discharge temperatures some freshwater springs with near meteoric stable isotope values may represent mixing of young groundwater from perched aquifers but in many cases the freshwater springs emerge along the same structures and have the same perennial nature as the saline fluids and expulsion of an older fresh groundwater component that is under abnormal fluid pressures cannot be ruled out basinal fluids elsewhere commonly show dilution trends with local meteoric water and in the case of the rumsey hills some of the dilute saline waters may indicate deep penetration of meteoric water 1 km in the pleistocene before the latest tectonic uplift geothermometry of the spring waters maximum 90 c suggest an origin from as deep as 4 0 km this depth is consistent with the depth of the core of a fault propagation anticline below the surface of the rumsey hills developed by active internal deformation of an east tapering wedge beneath the southwestern sacramento valley active tectonic compression causes near lithostatic fluid pressures in the shallow subsurface below the rumsey hills and volume strain within the core of the anticline that results in upward expulsion of the saline fluids from the indicated depths

sedimentology of pennsylvanian quartzose sandstones of the lee formation central appalachian basin fluvial interpretation based on lateral profile analysis abstract lateral profiling including bounding surface and architectural element analyses are utilized to demonstrate a fluvial origin for the quartzose sandstones of the lee formation internal architecture of the sandstones consists primarily of multistorey and multilateral channel bodies individual channel elements up to 20 m thick contain a complex hierarchy of bedform deposits the primary depositional elements within the channels were downstream accreting mid channel macroforms channel elements frequently contain deposits of more than one macroform reconstruction of the macroforms reveals accretion primarily by superposed bedforms that migrated down a low angle front steeper giant foresets transitional along flow with the low angle facies indicate that the macroform episodically developed a steep slipface the uppermost channels within the rockcastle member contain macroform elements with components of lateral accretion interpreted as deposits of alternate bank attached macroforms other depositional elements recognized within channel fill deposits are minor channel sandy bedform gravity flow attributed to bank slumping and channel bottom elements the latter element is contained within a facies sequence that suggests rising to flood to waning stage deposition in general deposition probably occurred during relatively high stage little evidence of low stage flow was recognized subordinate fine grained facies are interpreted as levee and overbank deposits the greater part of the lee formation was deposited in deep low sinuosity bedload dominated fluvial channels strongly unimodal paleocurrents lack of facies that suggest low stage reworking and paucity of lateral accretion features indicate deposition in a single channel system i e a low braiding index

a discussion of alluvial sandstone body characteristics related to variations in marine influence middle jurassic of the cleveland basin uk and the implications for analogous brent group strata in the north sea basin

channel fill and sheet flood facies sequences in the ephemeral terminal river gash kassala sudan abstract detailed facies analysis of the river gash fluvial sediments on two dimensional profiles across and along the main channel has revealed a number of facies and two distinct types of sequences channel fill and sheet flood sequences it seems that each sequence type is produced by specific depositional event and both sequences represent two end members for suite of mixed sequences the facies and sequences show rapid lateral variability and they merge and interfinger with each other they also show partial to complete modification due to subsequent floods and stage variation it seems that the thick sediments of the river gash in the kassala area were produced in two phases first by braided channel aggradation and lateral migration second by both channelized and unchannelized sheet flood deposition deposits of these two phases contribute to the sediments of the present day gash fluvial basin in the kassala area the gash sediments show some similarities and differences to those of ephemeral and low sinuosity braided streams the predominance of sheet flood deposits is consistent with the ephemeral flashy high flow regime in the river gash the non conformity of the river gash depositional style with the braided model of fluvial deposition may be due to this semi arid terminal flashy character which has control on the depositional processes and patterns

upper regime parallel lamination as the result of turbulent sediment transport and low amplitude bed forms a series of laboratory experiments has been carried out in which parallel laminated deposits were produced from an upper regime plane bed the laminae had thicknesses of a few mm and could be traced continuously over distances up to the length and width of the depositional area 0 3 m by 1 5m fluctuations in bed elevation were measured both during deposition and at equilibrium much of the bed fluctuation occurs at time scales that are too long to be due directly to turbulence as most theories for lamina formation would require we suggest instead that extremely low amplitude bed forms are present even on apparently flat beds and that the migration of these bed forms produces laterally continuous lamination all the lamination produced in the laboratory experiments was normally graded using high speed photography it was observed that the normal grading results from rapid deposition of a layer of loosely packed coarse sand several grain diameters thick followed by the slow sieving out of a well packed surface layer of finer sand the initial deposition is the result of small scale turbulent fluctuations in boundary shear stress the sieving out that follows results in a smooth surface whose low friction coefficient temporarily inhibits further deposition we term this process glazing the alignment of small scale turbulent scour and fill structures along the paths traced by migrating bedform troughs produces laterally continuous parallel lamination

episodes of vertical accretion and catastrophic stripping a model of disequilibrium flood plain development this paper describes flood plains formed episodically by vertical accretion along high energy laterally stable channels in southeastern australia overbank deposition gradually builds a flood plain of fine textured alluvium over a period of hundreds or thousands of years following which catastrophic erosion by a single large flood or a series of more moderate floods strips the flood plain to a basal lag deposit from which it slowly reforms this periodic destruction appears due to the progressive development of large levee banks and flood plain surfaces of highly variable relief as the levees and flood plain grow overbank flow is gradually displaced from the broad flood plain into the main channel and flood plain backchannels with a resulting concentration of erosional energy eventually high flows greatly exceed erosional thresholds and wholesale scour of the channel boundary and flood plain occurs vertical accretion flood plains at different stages of development result in a wide range of bankfull recurrence intervals even along the same river some of these flood plains are so infrequently flooded that they can be mistaken for terraces formed under a prior flow regime the almost random but catastrophic nature of this flood plain erosion means that sediment supply and transport are highly variable and probably impossible to predict this model of flood plain formation is seen as only part of a continuum of alluvial environments ranging from vestigial coarsegrained traction load flood plains along high energy rivers in narrow gorges to extensive low gradient flood plains in which alluvial stratigraphy is dominated by fine grained overbank deposition

palaeosols in westphalian coal bearing and red bed sequences central and northern england westphalian a c coal bearing strata and red beds in the pennine basin of central and northern england contain a wide variety of palaeosols in coal bearing facies associations alluvial gley semi gley organic and rare well drained palaeosols reflect the predominance of poor drainage conditions during deposition in red beds more evolved ferruginous and ferallitic palaeosols testify to free drainage conditions during deposition catenary relationships can be inferred between most of the palaeosols in both facies associations providing further information on the palaeogeomorphology of the depositional systems and assisting in the understanding of the environments and conditions of coal seam formation

evolving alluvial stratigraphy and thrust front development in the west jaca piggyback basin spanish pyrenees in the southwestern pyrenees distinctive alluvial mega sequences correlate with the development of a shallow mountain front thrust structure the pena flexure the flexure evolved during the oligocene to early miocene and is interpreted as a duplex marking the limit of thrust front advance three main depositional facies preserved completely across the pena flexure are described a pre thrust emergence high sinuosity fluvial sequence the c 2300 m thick sheet sandstone facies of the campodarbe formation a syn thrust emergence conglomeratic alluvial fan sequence the c 1100 m thick conglomeratic facies of the bernues formation and a post thrust emergence low sinuosity fluvial sequence the 700 m thick ribbon sandstone facies of the uncastillo formation collectively their continuous exposure across the pena flexure provides a more or less complete record of the sedimentary evolution of a mountain front syn deformational features in the alluvial fan sequence of the bernues formation and marked contrasts in palaeochannel morphology sandstone body geometry and palaeocurrent patterns between the two fluvial sequences are best understood by invoking a strong tectonic control over their deposition an increased rate of vertical sediment accretion was a key process in determining fluvial style the onset of major thrusting led to more rapid basin floor subsidence thereby promoting a change from lateral to vertical accretion as channels responded to the rising base level

size selective entrainment of bed load in gravel bed streams entrainment of mixed size gravel bed material was studied in nine reaches of three high power streams in scotland and norway paired measurements of at a point shear stress estimated from velocity profiles and bed load transport by hand held sampler were made they extended to very high values 400 n m 2 3 5 kg m x s x analyses of maximum bed load diameter mean bed load diameter transport rates of individual size fractions and tracer pebble movements all show some dependence of threshold shear stress for entrainment on absolute particle size despite strong relative size effects precise equal mobility of all sizes was approached in the data set with the highest shear stresses and transport rates size selective transport in the streams studied is also indicated by clear downstream and downbar reductions in surface sediment size over distances too short for abrasion to be significant

debris flow deposits in alluvial fans on the west flank of the white mountains owens valley california u s a abstract sections measured in incised channels on 10 alluvial fans show that debris flow beds mostly 30 200 cm thick are more important than stream flow deposits in construction of fans along the west flank of the white mountains the debris flow beds have a matrix supported fabric with a sandy mud matrix that comprises about 40 of individual beds complete grain size analyses of 19 debris flow beds show that they average 50 gravel 25 sand 11 silt and 4 clay inverse grading at the base of most beds is interpreted as due to a layer of high shear stress beneath an overlying semi rigid high strength plug that supported cobbles and boulders sticks and logs embedded in the plugs are oriented parallel with flow directions reflecting the laminar viscous motion of the plugs in the shear layer the average inclination of discoidal clasts is subhorizontal varying from 2 to 7 in the up flow direction in the plugs discoidal clasts have a subhorizontal fabric with average dips of 5 13 compared to the shear layer plugs have more scatter in clast orientation with numerous clasts dipping at 60 90 levees along the margins of debris flow lobes contain concentrations of the larger clasts in the flows in the leeves discoidal clasts have an average up flow dip of 21 31 with substantial variability in orientation similar to the plugs maximum clast size in individual debris flow beds is fairly constant down fan until the flows spread and thinned on the sandflat at the fan toe clast lithologies show that debris flows originate on steep slopes underlain by granitic and metavolcanic rocks at high elevations in the canyons in contrast stream flows obtained most of their clasts from talus slopes on metasedimentary rocks near the apices of the fans debris flows are generated during intense rainstorms in the spring and summer when landslides in the water saturated regolith move down slope shear dilate and by adding water are transformed into debris flows which then move with surging laminar motion along canyon flows to the fans the recurrence interval for debris flows is about 320 years as evidenced by 14c dates on plant material buried beneath debris flow beds

controls on the accumulation of coal and on the development of anastomosed fluvial systems in the cretaceous dakota formation of southern utah alluvial strata of the cretaceous dakota formation of southern utah are part of a transgressive systems tract associated with a foreland basin developed adjacent to the sevier orogenic belt these strata contain valley fill deposits anastomosed channel systems and widespread coals the coals constitute a relatively minor part of the dakota formation in terms of sediment volume but may represent a substantial amount of the time represented by the formation the coals are separated by clastic units up to 20 m thick the stratigraphically lowest clastic unit of the dakota formation lies above an unconformity cut into jurassic rocks incised valleys associated with the unconformity are up to 12 m deep two discrete episodes of valley fill sedimentation are recognized including a lower sandstone unit with conglomerate layers and an upper discontinuous coal bearing unit after the valleys filled the area became one of low relief where extensive mires formed peat accumulation was interrupted at least three times by deposition of clastic sediment derived from the west the clastic units consist of sandstone mudstone or heterolithic ribbon bodies stacked tabular sandstones and laminated mudstones and contain minor coal beds less than 0 35 m thick ribbon bodies are 1 9 m thick and 15 160 m wide have pronounced basal scours and are filled with both lateral and vertical accretion deposits an anastomosed channel complex is suggested by the large number of coeval channels of varying dimensions the variation in the structure and grain size of channel fills and the presence of abundant tabular sandstones interpreted as crevasse splays although some sandstone bodies have well developed lateral accretion surfaces the overall ribbon geometry indicates that mature meandering streams were not well developed this is in contrast to modern anastomosed systems which are commonly thought to be a transitional morphology caused by avulsion of a meander belt to a new position on its floodplain rather than being a transitional channel pattern related to river avulsion the anastomosed channels of the dakota formation may have formed part of a large inland delta that episodically invaded widespread mires the mires developed during periods when clastic influx was reduced either by high rates of subsidence close to the thrust belt or by deflection of rivers by emergent thrusts

the influence of debris flow rheology on fan morphology owens valley california we have investigated factors controlling the surface morphology of debris flow fans comprising the bajada along the western slope of owens valley california these fans have average slopes of 4 an extensive network of abandoned boulder lined channels rough undulatory surfaces near the range front and smooth distal surfaces field relationships and mechanical considerations indicate that the channels of the bajada are products of fluvial incision and not debris flow scour this is significant because detailed geomorphic maps indicate that the channels strongly influence the pattern of debris flow deposition the locus of debris flow deposition on a channelized fan surface is set by the interaction of debris flows with the channel system and is controlled by channel size channel gradient flow volume flow hydrograph and flow rheology debris flow behavior is most directly controlled by variations in bulk sediment concentration and its influence on flow rheology whereas low sediment concentration debris flows tend to smooth the surface of the lower fan spreading into thin sheets and filling channels and surface undulations repeated deposition of high sediment concentration debris flows produces the rugged topography of the upper fan the texture of the fan surface rough or smooth is determined by the relative volumetric importance of these two types of debris flow in addition channel avulsions and the associated long term shifting of depositional loci are driven by in channel deposition of debris flows with the highest sediment concentrations these debris flows therefore play a critical role in determining both the structure of the channel network and the long term pattern of deposition on the fan surface as a whole we infer that the frequency distribution of debris flow rheologies set by source terrain geology and hydrology is an important control on fan morphology

alluvial paleosols and pedofacies sequences in the permian lower beaufort of the southwestern karoo basin south africa abstract three types of paleosol profiles are described as representative of the range of soil types that developed on the ancient alluvial plains of the southwestern karoo basin during the late permian continuous 3 dimensional exposures of the lower beaufort adelaide subgroup strata allow detailed sedimentological taphonomic and paleopedological observation of thick floodplain sequences that contain numerous stacked paleosol profiles various floodplain strata contain characteristic paleosol profiles that are regulated by the periodicity and competence of overbank flooding both of which decrease with increasing distance from the contributing channel paleosols developed in levee or channel bank deposits are immature calcic entisols displaying evidence of early carbonate translocation but without well developed horizonation proximal floodbasin paleosols at the foot of the meanderbelt slope are mature calcic vertisols with more fully developed profiles containing zones of both carbonate and clay illuviation as well as some groundwater calcretes slickensided stress argillans in the b horizons are indicative of multiple cycles of wetting and drying distal floodbasin paleosols are dominated by hydromorphic and evaporitic processes as a result of their axial position in the floodbasin sheet like networks of mudcrack casts composed of calcretized aeolian sand and silt suggest that dust was a major source of calcium carbonate to the e soils the paleosols are interpreted as having formed under warm to hot semi arid climatic conditions with a strongly seasonal rainfall of 500 to 800 mm per annum and a mean annual moisture deficiency the geomorphic setting was similar to the holocene indo gangetic alluvial plain vertebrate burrow casts in the proximal floodbasin facies indicate a minimum depth to watertable of 150 cm the distribution of rooted horizons reflects dense vegetation flanking major drainage channels and sparse cover in the distal floodbasin a simple stratigraphic sequence of once laterally continuous paleosols is termed a pedofacies stacked sequences of pedofacies within approximately 20 50 m thick and 100 250 m thick stratigraphic successions reflect second and first order sedimentary cycles respectively both scales of cyclicity are attributed to autocyclically controlled avulsions of differentially aggrading channels in the first instance and of complete drainage networks in the second

tectonic and climatic control of triassic sedimentation in the beryl basin northern north sea the beryl basin embayment occupies a central position along the viking graben of the northern north sea and has been a hydrocarbon play since the early 1970s detailed analysis reveals a complex half graben that was established during a rift phase of development that occurred in the early triassic teist formation after which a long period of thermal subsidence lomvi and lunde formations provided accommodation for in excess of 1000 m of sediment the teist formation sediments are complex they include shales sands and conglomerates and their superimposition on zechstein salts is indicative of both uplift and the development of a moderate relief they give way to the comparatively clean and occasionally pebbly sands of the lomvi formation for which an analysis of both bedding and texture suggests deposition by a westerly directed river system draining the hanging wall ramp the lunde formation records the spreading influence of a bajada playa complex and its eventual conversion to a more persistent lake the growing importance and eventual dominance of the succession by lacustrine sediments gives clear indication of a change in climate and this has been attributed to the rapid northward drift of pangea during the triassic period

depositional model of a macrotidal estuary and floodplain south alligator river northern australia the south alligator river northern territory of australia has a macrotidal estuary tidal influence spring tidal range 5 6 m at the mouth extends 105 km up the channel it is dominated by freshwater in the wet season december april with a salt wedge near the mouth but is well mixed and becomes saline throughout the dry season the tidal channel can be divided into four different channel types an estuarine funnel a sinuous meandering segment a cuspate meandering segment in which the inside of bends are pointed and an upstream tidal channel the distribution of morphologically defined land classes and morphological units within each land class on the floodplain flanking the estuary differs from one channel type to another several stratigraphic and morphostratigraphic units have been recognized from drill holes on the coastal and deltaic estuarine plains and a model of development is proposed on the basis of extensive radiocarbon chronology and palynology the coastal plain has prograded with most rapid sedimentation between 5000 and 3000 yr bp a similar pattern of progradation is identified in the estuarine funnel in the sinuous segment of the estuary the channel has migrated laterally across the floodplain previous channel positions are indicated by palaeochannels and the meander tract is occupied by laminated channel sediments within the cuspate segment there are numerous sinuous palaeochannels on the plains in the upstream segment the channel and palaeochannels have long straight reaches with irregular bends and discontinuous levees and channel avulsion is indicated mangrove mud is a widespread stratigraphic unit throughout the plains the initial phase of development is a transgressive phase 8000 6800 yr bp when mangrove forests extended landwards into a pre existing valley as sea level rose as sea level stabilized the transgressive phase was followed by a widespread mangrove phase termed the big swamp 6800 5300 yr bp the mangrove forests disappeared from most of the plains as vertical accretion continued and were replaced by grass and sedge covered floodplains during the sinuous phase about 5300 2500 yr bp the channel migrated laterally and eroded the deltaicestuarine plain and deposited lateral accretion deposits laminated channel sediments part of the channel of the south alligator river has then progressed from sinuous to cuspate in form and erosion of river banks has occurred transgressive and big swamp phases occurred under rising and stabilizing sea level respectively later morphodynamic channel adjustments occurred under conditions of stable sea level the depositional model has direct application to other estuaries in northern australia and may be applied to other areas where sea level change has been similar

grain sorting in gravel bed streams and the choice of particle sizes for flow competence evaluations flow competence assessments of floods have been based on the largest particle sizes transported and yield either the mean flow stress mean velocity or discharge per unit flow width the use of extreme particle sizes has potential problems in that they may have been transported by debris flows rather than by the flood it may be difficult to locate the largest particles within the flood deposits and there are questions concerning how representative one or a few large particles might be of the transported sediments and therefore of the flood hydraulics such problems would be eliminated for the most part if competence evaluations are based on median grain sizes of transported sediments or perhaps on some coarse percentile that is established by a reasonable number of grains in order to examine such issues the gravel transport data of milhous from oak creek oregon and of carling from great eggleshope beck england have been analysed in terms of changing grain size percentiles with varying flow stresses a comparison between these two data sets is of added interest because the bed material in oak creek is segregated into well developed pavement and subpavement layers while such a layering of bed materials is largely absent in great eggleshope beck the analyses show that the trend of increasing sizes of the largest particles in the bedload samples diameter dm with increasing flow stresses is consistent with similar dependencies based on sieve percentiles ranging from the medians d50 to the 95th percentiles d95 this indicates that the largest particles are an integral part of the overall distributions of bedload grain sizes and respond to changing flow hydraulics along with the rest of the size distribution in oak creek the median grain size shows the largest change with increasing flow stresses followed by d60 and so on to d95 which shows the smallest change the variations in dm continue this trend and are similar to those for d95 this systematic variation of grain size percentiles in oak creek is consistent with changes in the overall distributions which tend to be symmetrical and gaussian for low discharges but become skewed rosin distributions for high discharges in contrast in great eggleshope beck the several percentiles and dm show the same rate of shift to coarser sizes as flow stresses increase this results in part from differences in sampling techniques wherein the bedload samples from great eggleshope beck represent a complete flood event while shorterterm samples at a specific flow stage were obtained in oak creek as a result of the integrated sampling in great eggleshope beck the bedload grain size distributions are more complex commonly with a bimodal pattern however after accounting for differences in sampling schemes in the two streams contrasting patterns in changing grain size distributions remain and these are concluded to reflect grain sorting differences as the bedload grain size distributions approach the distributions of the bed materials it is surprising that if criteria commonly employed to demonstrate the equal mobility of different grain sizes are used in the comparison then great eggleshope beck is far closer to this condition in spite of its minimal development of a pavement it is concluded that the respective shapes of the bed material grain size distributions in particular their degrees of skewness are more important to the observed sorting patterns than are the effects of a pavement layer regulating grain entrapment to produce an equal mobility of different grain sizes therefore the comparison has established that flow competence relationships will differ from one stream to another depending on the pattern of grain sorting which is a function of the bedmaterial grain size distribution

bedrock benches and boulder bars floods in the burdekin gorge of australia the burdekin gorge of northeastern australia lies within the seasonal tropics and is characterized by high discharge variability slackwater sediments and paleostage indicators in the gorge record seven large floods that have occurred during the past 1 200 yrs these floods range in magnitude from 11 000 to 30 000 m 3 s 1 and are characterized by large downstream variations in hydraulics the floods generate high values of velocity boundary shear stress and stream power per unit area downstream fluctuations in these variables help to explain the location of boulder bars high flood levees small scale erosional features in the bedrock and the formation of inner channels boulder bars and flood levees form where shear stress and stream power decrease due to channel widening small scale erosional features such as potholes and troughs are best developed at sites of channel constriction and associated increases in shear stress and stream power the development of the inner channel appears to be controlled by complex interactions between bedrock lithology and structure and by flow hydraulics all of the channel features are related to the hydraulics of large floods which therefore are the dominant controls on many aspects of channel morphology in this bedrock gorge

facies evolution and cyclicity of alluvial coal deposits in the lower permian of east africa tanzania the ruhuhu basin in sw tanzania contains several small coal basins i e mchuchuma ngaka north mbalawala lumecha consisting of fault controlled half grabens submitted to several stages of tectonic activation all basins underwent fragmentation in the middle jurassic and late miocene to pliocene palaeotopography of pre karoo basement was partly responsible for the development of coal seam thickness distribution facies characteristics of the lower middle and upper mchuchuma formation and the scarp sandstone of the overlying ketewaka formation exhibit synsedimentary basin subsidence vitrinite reflectance data suggest similar temperature gradients and burial history for the mchuchuma and ngaka subbasins

stratigraphic effects of change from internal to external drainage in an extending basin southeastern nevada observations from southern white river valley southeastern nevada shed light on sediment dispersal and geomorphic development during and after extensional faulting that formed the valley fine grained lacustrine and alluvial sediments of the late neogene white river narrows formation were deposited in a subsiding half graben while coeval coarse grained deposits were confined to within 1 km of the active fault the syntectonic deposits are overlain along an erosional unconformity by post tectonic uniformly coarse grained alluvial plain and fluvial terrace deposits this sequence conforms to a recent proposal that fine grained intervals in extensional basins are deposited syntectonically whereas coarse grained intervals are deposited during post tectonic progradation of marginal clastic wedges the stratigraphy and geomorphology of the study area suggest that post tectonic progradation of coarse clastic sediment resulted from incorporation of the white river valley into the colorado river drainage system meadow valley wash also links previously closed young but presently quiescent extensional basins to the colorado river system by contrast most nearby basins that contain quaternary scarps remain internally drained maintenance of internal drainage by active subsidence therefore may have been important in ponding of coarse grained sediments next to the fault after tectonic subsidence ceased the basin drainage became southward this action resulted in post tectonic progradation of coarse grained clastic deposits as sediment dispersal systems responded to the new external base level regional comparisons however indicate that depending on local rates of tectonic subsidence and erosion a basin may become externally drained syntectonically which can cause syntectonic deposits to be coarse grained in addition a basin may remain closed long after tectonic activity ceases causing fine grained deposits to continue to accumulate in a quiescent basin we conclude that the interplay between internal and external drainage which depends not only on the rate of local fault activity but also regional tectonics climate and other factors is a significant factor in the stratigraphic evolution of continental extensional basins

bed load transport of mixed size sediment fractional transport rates bed forms and the development of a coarse bed surface layer fractional transport rates bed surface texture and bed configuration were measured after a mixed size sediment had reached an equilibrium transport state for seven different flow strengths in a recirculating laboratory flume fractional transport rates were also measured at the beginning of each run when the bed was well mixed and planar the start up observations allow us to describe the variation of fractional transport rates with bed shear stress for a constant bed surface texture and bed configuration the start up and equilibrium observations together allow for the first time an unambiguous description of the mutual adjustment among the transport the bed configuration and the bed surface as the transport system moves toward equilibrium we find that a substantial interaction exists among the transport bed surface and bed configuration bed forms and a coarse surface layer coexist over a range of bed shear stress under some flow conditions the size and shape of the bed forms are controlled by the presence of the coarse surface layer at higher flows the coarse surface layer is eliminated by scour in the lee of the bed forms if the bed surface is defined as that over which the bed forms move a coherent relation between the bed surface texture and the transport grain size distribution may be defined at equilibrium the transport rates of all fractions were not equally mobile defined as identical transport and bed grain size distributions although equal mobility was approached for runs in which the bed shear stress was more than twice that for initial motion of the mixture under some flow conditions the transport was observed to adjust away from equal mobility as the bed adjusted from a well mixed start up condition to an equilibrium state development of a partial static armor wherein some individual grains become essentially immobile even though other grains in the same fraction remain in transport is suggested to explain these adjustments between the transport and bed surface grain size distributions constraints on equilibrium mixed size sediment transport are defined the special conditions for which equal mobility must hold and the relevance to natural conditions of flume results and the equal mobility concept are discussed

laboratory results indicating complex and potentially unstable frictional behavior of smectite clay a central problem in explaining the apparent weakness of the san andreas and other plate boundary faults has been identifying candidate fault zone materials that are both weak and capable of hosting earthquake like unstable rupture our results demonstrate that smectite clay can be both weak and velocity weakening at low normal stress 30 mpa our data are consistent with previous work which has focused on higher normal stress conditions 50 mpa and greater and found only velocity strengthening if natural fault zones contain significant smectite one key implication of our results is that localized zones of high pore pressure which reduce effective normal stress could be important in controlling potential sites of earthquake nucleation our experiments indicate that friction of smectite is complex and depends upon both sliding velocity and normal stress this complexity highlights the need for detailed experiments that reflect in situ conditions for fault gouges

the sedimentary dynamics of active fluvial channels on the okavango fan botswana the 1800 km2 okavango alluvial fan of northern botswana represents an unusual depositional setting in which peat forming perennial swamps 6000 km2 occur in a region of aeolian and semi arid sedimentation within an incipient graben of the east african rift a channel system distributes water and sediment on the fan surface but cannot contain seasonal flood water which spreads laterally from the channels through permeable channel margins sustaining the flanking swamps all sediment introduced is deposited on the fan a detailed study of sediment movement and associated hydrological conditions in the channels was undertaken to examine sediment dispersal bedload greatly exceeds suspended load at least by a factor of four vegetation and peat form permeable levees which confine the channels in the upper reaches two way exchange of water occurs between channel and swamp depending on the season but on the fan itself channels lose water to the swamp bedload measurements reveal that the channel system is in a state of grade disequilibrium with interspersed depositional and erosional reaches deposition of most of the incoming bedload occurs on the upper portion of the fan in a meandering and anastomosed channel system but on the midfan deposition of bedload occurs by channel bed aggradation at a rate of up to 5 cm yr 1 further down slope the channel enters a large lake where all remaining bedload is deposited the presently observed sedimentation patterns may be due to a recent disturbance of the fluvial system either by avulsion or neotectonics

noble gases stable isotopes and radiocarbon as tracers of flow in the dakota aquifer colorado and kansas abstract a suite of chemical and isotope tracers dissolved noble gases stable isotopes of water radiocarbon and cl have been analyzed along a flow path in the dakota aquifer system to determine likely recharge sources ground water residence times and the extent of mixing between local and intermediate flow systems presumably caused by large well screens three water types were distinguished with the tracers each having a very different history two of the water types were found in south eastern colorado where the dakota is poorly confined the tracer data suggest that the first group recharged locally during the last few thousand years and the second group was composed of ground water that recharged earlier during a cooler climate presumably during the last glacial period lgp and mixed aged water the paleotemperature record archived in this groundwater system indicates that south eastern colorado was about 5 c cooler during the lgp than during the late holocene similar temperature changes derived from dissolved noble gases in other aquifer systems have been reported earlier for the south western united states the third water type was located down gradient of the first two in the confined dakota in western and central kansas groundwater residence time of this water mass is on the order of 10 4 10 5 yrs and its recharge location is near the colorado and kansas border down gradient of the other water types the study shows the importance of using multiple tracers when investigating ground water systems

channel planform as a non controlling factor in fluvial sedimentology the case of the squamish river floodplain british columbia abstract channel planform styles demonstrate a continuum of variability whereas end member situations may exhibit diagnostic sedimentologic characteristics non end member situations may be exceedingly difficult to reliably differentiate the relevance of channel planform based sedimentologic differentiation of fluvial depositional units has been questioned for some time e g jackson 1978 bridge 1985 brierley 1989a despite reservations outlined in these papers facies models and interpretations of depositional suites in the rock record continue to be applied in terms of channel planform style this paper describes a test of this principle in one particular depositional setting the squamish river in southwestern british columbia is a high energy gravel based river in a fjord setting in a 20 km reach this river displays a distinct downstream gradation in channel planform type from braided through wandering gravel bed to meandering assemblages of bedform scale facies units on bar surfaces in this river do not differ by planform type brierley 1989a rather facies associations relate directly to the pattern of morphostratigraphic units on bar surfaces brierley 1991a when analysed in floodplain exposures sediment sequences which are equivalent to morphostratigraphic units are termed elements these elements are defined in terms of their geometry bounding surface and sediment characteristics detailed analysis of floodplain deposits in the braided wandering gravel bed and meandering reaches of the squamish river demonstrates that just as assemblages of bedform scale facies cannot be differentiated by planform style neither can assemblages of floodplain elements indeed for the squamish river the type character and spatial association of these elements cannot be differentiated reliably at the channel planform scale this indicates the limited relevance of planform differentiation of sedimentary environments under transitional planform situations such as those of high energy gravel bed rivers in confined valleys in such situations sediment inventory at the element scale provides greater insight into mechanisms of floodplain evolution than can be provided by interpretation at either the channel bedform or channel planform scales

controls on synorogenic alluvial fan architecture beartooth conglomerate palaeocene wyoming and montana late palaeocene uplift of the beartooth range in northwestern wyoming and southwestern montana generated the beartooth conglomerate along the eastern and northeastern flanks of the range systematic unroofing sequences and intraformational unconformities folds and faults in the conglomerate attest to deposition during uplift along the eastern flank at least three ancient alluvial fan systems and a braidplain system can be distinguished on the bases of petrofacies and lithofacies the two southern fans consist of 700 m of sedimentary clast conglomerate and subordinate sandstone dominated by hyperconcentratedflow and stream flow facies the next fan to the north is dominated by plutonic and metamorphic clasts and contains abundant mud matrix supported debris flow facies as well as stream flow facies the northernmost depositional system consists of arkosic channellized fluvial conglomerate and sandstone overbank mudstone and crevasse splay sandstone units palaeocurrent data indicate eastward dispersal away from the beartooth range outstanding exposure of the beartooth conglomerate allows facies to be mapped on lateral photographic mosaics a seven fold hierarchy of bounding surfaces and enclosed lithosomes exists in the beartooth conglomerate first through fourth order surfaces are analogous to first through fourth order surfaces that recently have been documented in sandy fluvial facies with one exception sediment gravity flows are bounded by first order surfaces fifth order surfaces are either erosional e g lateral migration of fanhead trench or accretionary e g aggradation of fan surface during backfilling of trench and construction of lobes on lower fan during entrenchment on upper fan some fifth order surfaces coincide with intraformational angular unconformities and are thus the result of long term fanhead entrenchment following uplift of the upper part of the fan sixth order surfaces bound individual fan packages that are several hundred metres thick and 10 km2 in area the enclosed sixth order lithosomes are distinguishable in terms of petrofacies and lithofacies a single seventh order surface bounds the entire beartooth conglomerate lower order lithosomes are produced by intrinsic processes of fan construction fifth order lithosomes can be attributed to both extrinsic and intrinsic controls sixth and seventh order lithosomes are generated by extrinsic controls

basic building blocks and process variability of a cretaceous delta internal facies architecture reveals a more dynamic interaction of river wave and tidal processes than is indicated by external shape abstract although delta building processes and the resultant facies and facies associations have been studied for over a century there remains a lack of well documented facies architectural studies of ancient deltas in this study architectural element analysis is applied to determine the basic building blocks of a cretaceous delta at the top of the upper turonian wall creek member exposed along the western flanks of the powder river basin wyoming we document the bed scale basic building blocks of this ancient delta and show them in the context of their facies and stratal variability and their hierarchical spatial arrangement within a single deltaic parasequence we also test the idea that external sand body shape may not reflect the internal facies complexity as has recently been suggested in studies of several modern deltas including the burdekin in australia the brazos in the gulf of mexico and the baram trusan in borneo five orders of bounding surfaces separate six facies architectural elements in the prodelta and delta front deposits these elements are prodelta fines pf frontal splay fs channel ch storm sheet ss tidally modulated deposit tm and bar accretion ba seasonal to decadal river floods are thought to represent the main building phases of the delta producing channels bars and frontal splay elements during intervening periods the delta was reworked by waves storms and tides producing tidally modulated and storm sheet elements due to the complex interactions of river effluents with waves storms and tides the delta is interpreted as mixed influenced regional sandstone isolith facies and paleocurrent maps help to reconstruct the paleogeography and show how the various architectural elements varied across the delta front the channels bars frontal splays and tidally modulated elements are found only near the distributary mouth whereas the storm sheets occur extensively away from the distributary mouth the plan view morphology of the studied delta shows a smooth fronted arcuate to cuspate shape which should indicate wave dominance in contrast our analysis of the internal facies architecture shows that the delta was constructed rapidly during major river floods with significant tidal reworking additional outcrop mapping shows that storm wave reworked sands are attached to the flanks of the system previously published delta models predict incorrectly that the tidal reworking of a river flood dominated system should result in a more bird foot shape versus the lobate geometry that is actually mapped our analysis of internal facies versus external shape matches similar observations on several modern deltas and suggests that external shape is a poor indicator of internal facies complexity

braidplain evolution in the pennsylvanian south bar formation sydney basin nova scotia canada abstract the south bar formation is a 1 km thick alluvial succession in the pennsylvanian morien group of the sydney basin nova scotia that fines upward from conglomerate to pebbly sandstone to sandstone facies assemblages the conglomerate assemblage comprises mainly massive to horizontally stratified conglomerate and planar cross stratified sandstone attributed to deposition in proximal braided rivers confined between bedrock ridges the sandstone assemblage shows uniformity of facies and paleocurrents across the basin and is interpreted as a distal braidplain deposit the pebbly sandstone assemblage is transitional between the other two both sandstone assemblages are dominated by trough cross stratified sandstone formed by dune migration in channel tracts about 4 m deep this succession differs markedly from platte river and icelandic sandy braidplain deposits which are mainly planar cross stratified sands formed by broad shallow flow the south bar sandstones are more like south saskatchewan river deposits but differ in 1 greater abundance of trough sets and 2 rarity of planar sets and other components of shallow water channel fill these features are ascribed to 1 inheritance of relatively deep channels from confined upstream reaches 2 maintenance of deep flow in active tracts by local confinement between temporary banks and 3 rapid avulsive abandonment of active tracts which inhibited development of channel fill sequences evidence for local temporary confinement is preserved as in situ and slumped channel margin mudrock units lateral and vertical transitions from the upper south bar formation to the mud rich waddens cove and sydney mines formations are ascribed to changes in channel pattern from braided to meandering the former is attributed to decreased slope the latter to eustatic rise in sea level

the sedimentology and accretionary styles of an ancient gravel bed stream the budleigh salterton pebble beds lower triassic southwest england abstract three accretionary styles are present in the fluvial conglomerates and pebbly sandstones of the lower triassic budleigh salterton pebble beds bspb of east devon the lower portions of this 20 30 m sequence are dominated by extensive 50 m sheets or narrow lenses of planar cross bedded conglomerate sets 1 3 m thick these abruptly pass both downstream and laterally into horizontally bedded conglomerates cross bedded sandstones and muddy sandstones this accretionary style reflects deposition from linguoid shaped macroforms whose distal margins were bounded by slipfaces they accreted at anabranch confluences and as bank attached bars in relatively confined and channelised gravel bed streams lateral facies changes represent macroform evolution resulting from changing discharges or abandonment of a channel reach followed by accretion of a new macroform the upper portions of the bspb mainly comprise couplets of horizontally bedded conglomerate overlain by large scale trough cross bedded sandstone the conglomerate shows local channel macroform relief and lateral and downstream transitions into sets of gently dipping conglomerate this style of accretion represents deposition in less confined and less channelised fluvial systems by relatively low relief gravel macroforms and large sand dunes the third style recognised in the bspb is much more localised it consists of wedge like sets of large scale trough cross bedded conglomerate the sets are formed by erosion and subsequent fill of scours cut into a thick sandy substrate techniques of lateral profile analysis and hierarchical ordering of bounding surfaces can usefully be applied to some parts of the bspb this is important as the approach was originally developed for the deposits of sand bed streams however this approach is more difficult to apply to fluvial conglomerates because of the cryptic nature of some gravel stratification the appearance and character of some hierarchies of bounding surfaces may differ from their equivalents in fluvial sandstones

flood geomorphology of the katherine gorge northern territory australia the katherine gorge is a narrow deep canyon system developed in resistant sandstone the tropical monsoonal climate results in rare flood events of very large magnitude these floods flush sand sized sediment as washload through the gorge but local accumulations are preserved as slack water deposits at tributary mouths paleoflood hydrologic analysis of the deposits employing step backwater flow modeling allows quantitative estimates to be made of geomorphically significant flows for an annual exceedence probability of 10 2 the gorge may experience discharges of 6 000 m 3 s mean velocities of 7 5 m s flow depths of 15 to 45 m stream powers per unit area as great as 1 10 4 watts m 2 and bed shear stresses of 1 5 10 3 n m 2 such flows are required to transport boulders 3 m or more in intermediate diameter that occur at large scale riffles at maximum stage pools reach depths of 45 m and are preferentially developed at vertical joint intersections in the rock probably through intense hydraulic action pool and riffle development thus reflects the channel boundary characteristics for extreme flow conditions additional indicators of especially intense flow phenomena include potholes flutes abraded facets on rock bar surfaces and scabland development on upland bedrock surfaces

gravel antidunes in the tropical burdekin river queensland australia the geological record is punctuated by the deposits of extreme event phenomena the identification and interpretation of which are hindered by a lack of data on contemporary examples it is impossible to directly observe sedimentary bedforms and grain fabrics forming under natural particle transporting high velocity currents and therefore their characteristics are poorly documented the deposits of such flows are exposed however in the dry bed of the burdekin river queensland australia following tropical cyclone induced floods long wave length up to 19 m gravel antidunes develop during short days high discharge flows in the upper burdekin river maximum recorded discharge near the study reach over 25 600 m s in february 1927 flood water levels fall quickly metres in a day and flow is diverted away from raised areas of the river bed into subchannels exposing many of the high stage bedforms with little reworking by falling stage currents gravel bedforms were observed on the dry river bed after the moderate flows of february 1994 max 7700 m s and january 1996 max 3200 m s the bedforms had wave lengths in the range 8 19 m amplitudes of up to 1 m with steeper stoss than lee faces and crest lines generally transverse to local peak discharge flow direction the gravel fabric and size sorting change systematically up the stoss and down the lee faces the antidune deposits form erosive based lenses of sandy gravel with low angle downstream dipping lamination and generally steep upstream dipping a b planes the internal form and fabric of the antidune gravel lenses are distinctly different from those of dune lee gravel lenses the erosive based lenses of low angle cross bedded gravel with steep upstream dipping a b planes are relatively easy to recognize and may be diagnostic of downstream migrating antidunes the antidune gravel lenses are associated with thick to 1 m high angle cross bed sets ancient antidune gravel lenses may be diagnostic of episodic high discharge conditions and particularly when they are associated with high angle cross bedded gravelly sand they may be useful for palaeoenvironmental interpretation

volcanic and tectonic controls of lacustrine and alluvial sedimentation in the stephanian coal bearing sequence of the malpas sort basin catalonian pyrenees the infill of a small variscan basin shows a complex succession of lacustrine and alluvial sediments whose pattern of distribution through time appears to have changed in response to contemporaneous volcanism following initial deposition of pyroclastic breccias and extrusion of lavas small lakes were established on the floor of the basin their infills were probably locally controlled by relief on top of the lavas but the pattern was disrupted by phases of mass movement from the basin margin which emplaced debris flows and a sedimentary melange containing large blocks of limestone and led to extensive soft sediment deformation this complex sequence of events culminated in the emplacement of a widespread ash flow bed an alluvial plain established during a period of rather uniform subsidence was rapidly drowned to give an extensive lake whose progradational infill led to re establishment of an alluvial plain the deposition of a widespread airfall tuff coincided with the onset of differential subsidence during which major channel sandstones were stacked in association with poorly drained palaeosols in the east of the area whilst a mudstone dominated sequence characterized by well drained palaeosols developed in the west a return to more uniform subsidence appears to have coincided with the cessation of volcanic activity

bed configurations in steady unidirectional water flows part 3 effects of temperature and gravity abstract a change in bed configuration caused by a change in water temperature without any change in flow and sediment variables in a unidirectional flow is analyzed in the context of the dimensionless depth velocity size diagram discussed in part 2 bed states that differ only in water temperature are termed temperature related states these are attainable from any given state solely by a change in water temperature such a set of bed states forms a curve in the dimensionless depth velocity size diagram this curve a straight line in a log log log plot is termed a temperature change line to predict the change in bed state consequent upon a change in water temperature alone one finds the temperature related state lying along the temperature change line passing through the ori inal bed state in the dimensionless depth velocity size diagram three examples from the literature are analyzed in this way to show how such changes in bed configuration can be put into the context of existing stability relationships among the various bed phases without however addressing the general dynamical problem of why those relationships are as they are the effect of the acceleration of gravity on the bed configuration is also examined from the standpoint of the dimensionless depth velocity size diagram the results show that in a hypothetical open channel water flow on mars a given transition between two adjacent bed phases with increasing flow velocity e g ripples to dunes would take place at a lower velocity on mars than on earth by a factor of 0 74

paleokarst breccias calcretes silcretes and fault talus breccias at the base of upper proterozoic windermere strata northern canadian cordillera in the ogilvie mountains west central yukon the upper proterozoic lower mount harper group informal name is stratigraphically equivalent to basal windermere supergroup rocks that occur in other areas of the north american cordillera breccias at the base of the lower mount harper group are of two different but in part coeval origins a normal fault that was active during late proterozoic time forms the southern boundary of the lower mount harper group and is genetically related to a wedge of paleotalus breccia that thins rapidly northward from the fault a different breccia layer is preserved on a disconformity surface that separates lower harper group strata from a thick succession of underlying dolostones this disconformity breccia contains silcretes and calcretes that provide evidence of several episodes of subaerial exposure rare interbedded debris flows display features that suggest coincidence of the last exposure event with initiation of deposition of the mount harper group sediments silcretes and calcretes in this succession suggest that at the onset of windermere deposition in this area a temperate to equatorial probably semi arid to arid climate regime existed basal breccias in windermere equivalent strata of other areas in the cordillera generally have been interpreted as fault related but some of these breccias also contain features compatible with paleokarst origin

a review of recent research in fluvial sedimentology this volume has been compiled to represent the current status of research into fluvial sedimentology as part of the proceedings for the 5th international conference on fluvial sedimentology 5icfs brisbane july 1993 the volume contains papers on a wide variety of topics representing the diversity of interests covered by the icfs this review serves to introduce these papers within the context of recent research post1988 in fluvial sedimentology icfs papers may be somewhat arbitrarily divided into those which report fundamental studies of sediment transport studies of modern alluvial and coastal systems studies of environmental management of fluvial systems facies analyses of ancient fluvial sequences studies of controls on alluvial stratigraphy and basin fill and applied sedimentological studies of fluvialhosted mineral resources it is interesting to compare the balance of papers in this volume with that of previous icfs volumes miall 1978 collinson and lewin 1983 ethridge et al 1987 marzo and puigdefabregas 1993 as a guide to changing emphasis in fluvial sedimentology figure 1 illustrates the distribution of papers over the five icfs meetings categorised as above among various trends two are worthy of comment the first being the marked peak in interest in controls on fluvial sequence and basin fill at the 4icfs and the second the influx of papers on environmental management in this volume although there has been a steady level of activity in fundamental aspects of sediment transport and bedform generation the rela

constraining permeabilities in a large scale groundwater system through model calibration abstract the range of field estimates of permeabilities in rocks of the atikokan research area ara canada has been reduced by calibrating a groundwater flow model of the ara the flow model was calibrated to the recharge component of the water balance and to measured freshwater heads in boreholes a good match between field and simulated recharge and between measured and simulated heads was obtained with two different permeability depth distributions that differed by up to two orders of magnitude in places solute transport simulation of the distributions of total dissolved solids tds chlorides cl and oxygen 18 18 o in groundwater in the area was performed to further refine the permeability estimates vertical average linear groundwater velocities from the calibrated flow model were used in one dimensional solute transport models of the system to generate depth concentration profiles of tds cl and 18 o recharge midline and discharge area models of both the fracture zones and the rock mass were employed the midline area is the narrow zone that separates the recharge downward flow area from the discharge upward flow area groundwater flow is essentially horizontal in the midline area envelopes formed by the simulated profiles that were created with the velocities of one simulation case more tightly enclosed the field data tds cl 18 o than those of another simulation case indicating that velocities of the former simulation case better represent the hydrodynamics of the real system the models demonstrate that the scatter of tds cl and 18 o field data for the ara is consistent with the groundwater flow model predictions and can be explained by the complexity arising from different hydraulic regimes recharge midline discharge and hydrogeologic environments fracture zones rock mass of the regional flow system solute transport simulation of hydrochemical constituents can aid in refining and constraining permeabilities in large scale groundwater systems

architectural analysis of fluvial sequences in the northwestern part of kyongsang basin early cretaceous se korea abstract the northwestern part of kyongsang basin early cretaceous largely comprises interbedded sandstones and mudstones of fluvial depositional environments with local basin margin conglomeratic deposits of alluvial fan systems these non marine successions are divided into 5 successive stratigraphic units each of which is unique in facies assemblages and architecture of sandstone bodies one stratigraphic unit sinpyong anpyong unit is documented in this paper with detailed architectural analysis of sandstone bodies the deposits of the sinpyong anpyong unit are divided into three major components thick sandstone thin sandstone and mudstone dominated bodies the thick sandstone bodies 2 47 m thick comprise stacked element groups bounded by laterally persistent erosional surfaces probably representing superposition of channel belts within individual element groups elements are laterally superposed indicating growth and superposition of bars and movement of associated channels the thickness of bar deposits and dip angles of laterally inclined strata suggest that channels were a few to 10 m in depth and 9 m to several tens of meters in width paleocurrent data indicate southward or southeastward draining channel systems the thin sandstone bodies generally the thick sandstone bodies formed in major channels and channel belts are characterized by lateral superposition of numerous bar and channel deposits limited extent of individual lateral accretion sets and the presence of mid channel bar deposits suggesting deposition in braided rivers detailed reconstruction of bars and channels reveals a range of processes including growth coalescence and erosion of bars and migration switching and filling of channels lateral accretion of bars and the associated channel migration might have been principal processes in the sinpyong anpyong river systems bars were commonly coalesced by lateral superposition forming large bar complexes

recent and ancient fluvial deposition systems in the amazonian foreland basin peru still active sub andean foreland deformation is suggested to have syndepositionally modified the fluvial depositional environments in the peruvian amazonian foreland basin throughout neogene quaternary time modern fluvial aggradation continues to proceed on a large scale c 120 000 km 2 in two differing depositional systems firstly various multistoried floodbasin deposits are derived from the meandering and anastomosing rivers within the subsiding intraforeland basins secondly in the northern part of the pastaza maranon basin the largest known holocene alluvial fan like formation c 60 000 km 2 composed of reworked volcaniclastic debris derived from active ecuadorian volcanoes has been identified the widespread poorly known dissected surface alluvium terra firme which covers the main part of the peruvian amazonian foreland basin shows further evidence of long term foreland deformation and terraces indicate both the effects of tectonism and pleistocene climatic oscillations in northern peru the surface alluvium was deposited by a tertiary fluvial system with palaeocurrents to the west and northwest into the andean foreland basin in southern peru the respective surficial alluvium was part of a post miocene fluvial system flowing northeast into the main amazon basin both systems were gradually abandoned when the eastward migrating andean foreland deformation led to the more distinctive partitioning of the intraforeland basins and the modern drainage system was created

avulsion mechanisms on the okavango fan botswana the control of a fluvial system by vegetation a study of the avulsion of a major distributory channel on the alluvial fan 22 000 km2 in area of the okavango river in northern botswana has revealed that channels serve as arterial systems distributing water which sustains large areas of permanent swamp the channels are vegetatively confined a primary channel defined here as a channel which receives water and sediment directly from the fan apex aggrades vertically as a result of bedload deposition the rate of aggradation increases downchannel and may exceed 5 cm yr 1 in the distal reaches rapid aggradation is associated with a decline in flow velocity this initiates a series of feedback mechanisms involving invasion of the channel by aquatic plants which trap floating plant debris further reducing flow rate and causing the channel water surface to become elevated thereby increasing rate of water loss from the channel accelerating blockage and aggradation the channel ultimately fails enhanced water loss from the channel promotes the growth of flanking swamp vegetation which confines the failing channel increased flow through the swamp erodes pre existing hippopotamus trails producing a secondary channel system which overlaps but does not connect directly to the failing reach of the primary channel the region of failure of the primary channel migrates upstream accompanied by headward propagation of the secondary channel system the swamp distal to the failed primary channel dessicates and is destroyed by peat fires secondary channels are stable and not prone to blockage comparison with avulsions described in other river systems indicates that the influence of plants in the okavango river system is exceptionally strong

fluvial architecture as a response to two layer lithospheric subsidence during the permian and triassic in the iberian basin eastern spain the stratigraphy of a sedimentary basin is mainly the result of the long term response of a depositional surface to prolonged subsidence however the real nature of interrelations between fluvial architecture and subsidence is still unknown herein we present new data on these relationships by combining the results of detailed sedimentological field work with data acquired through automated forward modelling and backstripping for the alluvial permian and triassic sediments of the se iberian ranges using this methodology we determined tectonic subsidence of the basin by means of backstripping analysis and crust and lithospheric mantle stretching factors and respectively using forward modelling technique results indicated that a configuration of two individual and independent layers during lithospheric subsidence for each tectonic phase fit better for this time of the studied basin evolution than the assumption of subsidence due to a single layer spanning the whole lithosphere for this study we simplified fluvial geometries as two main types isolated i and amalgamated a with subtypes in each case different order bounding surfaces b s were distinguished in the field although we only selected those affecting the whole basin under study these included those b s of clear tectonic origin ranging from individual basin boundary fault pulses produced over periods of approximately 1 my to those arising from major tectonic events such as the beginning of extension in the basin causing major changes in basin geometry over periods of 3 5 my the comparison of and values and fluvial geometries for each identified tectonic phase in the basin evolution revealed some possible relationship between subsidence and fluvial geometry sections showing the most varied fluvial architectural geometries including ribbon and nested forms were related to higher and stretching factors values indicating tectonic phases of greater stretching and subsidence when both stretching factors were similar and close to 1 fluvial geometry was basically reduced to amalgamated geometry type wider ranging of fluvial geometries was associated with stages of basin development in which crust and upper mantle activities differed that is showing larger differences of and stretching factors values the related slope changes are proposed as the main surface control of fluvial styles combination of subsidence with other possible controlling factors such as avulsion rate climate or budget of sediments gives rise to the definitive alluvial architecture of a basin

hydrochemical and stable isotope evidence for the extent and nature of the effective chalk aquifer of north norfolk uk in eastern england the chalk aquifer is covered by extensive pleistocene deposits which influence the hydraulic conditions and hydrochemical nature of the underlying aquifer in this study the results of geophysical borehole logging of groundwater temperature and electrical conductivity and depth sampling for major ion concentrations and stable isotope compositions 18o and 2h are interpreted to reveal the extent and nature of the effective chalk aquifer of north norfolk it is found that the chalk aquifer can be divided into an upper region of fresh groundwater with a cl concentration of typically less than 100 mg l 1 and a lower region of increasingly saline water the transition between the two regions is approximately 50 m below sea level and results in an effective aquifer thickness of 50 60 m in the west of the area but less than 25 m where the eocene london clay boundary is met in the east of the area hydrochemical variations in the effective aquifer are related to different hydraulic conditions developed in the chalk where the chalk is confined by low permeability chalky boulder clay isotopically depleted groundwater 18o less than 7 5 is present in contrast to those areas of unconfined chalk where glacial deposits are thin or absent 18o about 7 0 the isotopically depleted groundwater is evidence for groundwater recharge during the late pleistocene under conditions when mean surface air temperatures are estimated to have been 4 5 c cooler than at the present day and suggests long groundwater residence times in the confined aquifer elevated molar mg ca ratios of more than 0 2 resulting from progressive rock water interaction in the confined aquifer also indicate long residence times a conceptual hydrochemical model for the present situation proposes that isotopically depleted groundwater occupying areas where confined groundwater dates from the late pleistocene is being slowly modified by both diffusion and downward infiltration of modem meteoric water and diffusive mixing from below with an old saline water body

geospatial drivers of the groundwater δ18o isoscape in a temperate maritime climate republic of ireland abstract in recent years the concept of isoscapes has been used to describe spatiotemporal stable isotope distributions within natural environments including groundwater systems at multiple scales this study presents an updated groundwater 18 o isoscape for the republic of ireland and describes the climatic and geological drivers influencing 18 o composition in all 142 geographically and geologically representative groundwater installations were sampled and analysed in addition to 35 samples from six nested boreholes geospatially irish groundwater exhibits high 18 o values in southern and western coastal regions becoming progressively depleted inland before reaching lowest measured values along the eastern coast equating to a national groundwater 18 o profile range of approximately 3 groundwater 18 o composition is primarily driven by location with respect to orographically influenced rainfall deposition and annual precipitation volume with a bias towards winter recharge results also demonstrate that local regional hydro geological setting exerts a secondary influence on 18 o composition via infiltration and recharge mechanisms flashy groundwater systems e g karst limestones are more likely to exhibit seasonal groundwater 18 o patterns whereas low productivity systems with high residence times e g granites tend to display a dampened composition to groundwater recharge and exhibit more temporally constant 18 o values the derived 18 o isoscape enhances current understanding of what is a geologically distinct groundwater setting whilst also potentially serving as a 18 o sentinel for continental europe with respect to both groundwater and precipitation due to ireland s geographic location

evolving fluvial architecture during a marine transgression upper buntsandstein triassic central spain abstract the sedimentation in central spain during the late anisian middle triassic limos y areniscas de rillo formation recorded the evolution from an alluvial environment into a coastal one with a diversity of environments this evolution was the result of one of the episodes of the tethys sea transgression over iberia during the first and second stages of this evolution mixed load low sinuosity channels evolved upwards into a distal braidplain with ephemeral run off and poorly defined channels during a third stage the characteristics of the sediments indicate the existence of both continental and marine waters in a transitional zone with seasonal hydrological fluctuations the fourth stage records the sedimentation in an upper intertidal to supratidal environment in a semiarid coastline with the presence of thin cryptalgal sheets and chicken wire textures the final evolution consists of an environment with a high percentage of fluvial channel deposits probably related to a meandering pattern this evolution has been correlated with the depositional sequences established for the ne of spain where the anisian transgressive event resulted in the formation of an inter to supratidal carbonate ramp so lowstand transgressive and highstand system tracts from depositional sequence 1 have been recognized in the centre of spain

stable isotope imaging of a dynamic groundwater system in the southwestern sacramento valley california usa abstract a dynamic image of a shallow 45 160 m below the surface aquifer in a 25km2 area in the southwestern sacramento valley california has been obtained by 18 o 16 o and d h determinations of groundwater from municipal wells the regular summertime drawdown of the water table is strongly correlated with municipal wide increases in the 18o values particularly in wells with shallow perforation levels thus demonstrating that the high 18o water resides in the upper reaches of the aquifer and penetrates more deeply with increased discharge rates these 1 5 enrichments in the 18o values are primarily attributed to infiltration of irrigation water that has penetrated to depths of at least 80 m the increased 18o values correlate with increased deviation from the meteoric water line and with increased nitrate levels indicating that the high 18o groundwater component represents local meteoric water that has been enriched by soil evaporation processes in nitrogen fertilized flood irrigated cropland isotopic gradients provide a new method for qualitatively determining subsurface permeability zones of low isotopic gradients interpreted as high 18o plumes probably follow coarse textured deposits of ancient stream channels the 18o gradients are primarily controlled by subsurface permeability although additional influences may include 1 the heterogeneous distribution of the high 18o groundwater 2 isotopic contributions from deeper groundwater 3 variable perforation depths of the wells 4 local differences in the water table level deeper sodium bicarbonate groundwaters 300 650 m below the surface have lower 18o values to 9 4 than the more shallow magnesium bicarbonate groundwaters suggesting derivation from 18o depleted meteoric groundwaters of the sierra nevada minor mixing of the deeper groundwater into the municipal discharge occurs even though previous workers considered the deeper groundwater to be confined

particle over passing on depth limited gravel bars an experimental channel is used to examine the transport of mixed sand and gravel bedload over the crestal platform of hump back bars and along the top of planar gravel sheets hydraulic processes result in the simultaneous transport of cobbles and pebbles over a static closely packed bed consisting of like sized and finer particles for prescribed conditions flat upper stage plane sand beds develop over the crestal location with pebbles rolling easily over the sandy bed at the brinkpoint flow separation ensures effective segregation of the gravel from the sand over the slip face the deposition rate of the sand is insufficient to fill fully the interstices within the gravel foresets before rapid deposition of gravel further advances the bed form consequently distinctive vertical assemblages of open work and closed contact framework gravels could be generated as another bar migrates over and preserves the initial structure in respect to the observed mechanisms of sorting over the bars a mathematical expression is developed to explain the critical conditions allowing coarse particle mobility over planar sand or gravel beds under upper stage plane bed conditions on the crestal platform the model then is used to ascertain whether the depositional environment ascribed to certain facies in the bunter pebble beds described in a recent publication is appropriate given the distinctive facies assemblages generated in this experiment and the known hydrodynamic control of the particle segregation process

sandstone body structures and ephemeral stream processes in the dinosaur canyon member moenave formation lower jurassic utah u s a abstract studies of fluvial sandstone body structures in the lower jurassic dinosaur canyon member suggest a threefold subdivision of the ephemeral stream deposits sandstone sheets with interbedded siltstones are less than 1 m thick and laterally extensive for hundreds of metres they are interpreted as sheetflood deposits simple channel sandstone bodies are a few metres thick and a few tens of metres wide they reflect solitary channel incision episodic migration and plugging multistorey channel sandstone bodies are a few metres thick and laterally extensive for hundreds of metres they are composed of several channel shaped storeys and exhibit only local incision the multistorey sandstone bodies are interpreted as braided ephemeral stream deposits two sandstone sheet subtypes with grooves and mounds respectively are interpreted as intermediate between the sheetflood deposits and solitary incised channel deposits on one hand and between sheetflood deposits and braided stream deposits on the other hand the solitary channels and braided streams are accordingly interpreted to be initiated from sheetfloods through differential erosion and differential deposition respectively this model of channel evolution from sheetfloods is probably applicable to other semiarid and arid fluvial environments dominated by surface runoff

bed configurations in steady unidirectional water flows part 1 scale model study using fine sands abstract reynolds froude scale modeling using hot water flows to model cold water flows was applied to the study of unidirectional flow bed configurations in a large thermally insulated laboratory flume three different water temperatures ranging from 14 to 75 c provided three different scale ratios from 1 1 to 2 3 effective sand sizes scaled to 10 c water were about 0 12 mm 0 18 mm and 0 28 mm relative to 10 c water use of hot water multiplied the effective flow depth and all flume dimensions by factors of up to 2 3 the flow velocity by factors of up to 1 5 and the water discharge by factors of up to 8 the effective dimensions of the flume scaled to 10 c water temperature ranged up to over 26 m long and 2 m wide with a flow depth of up to 0 5 m in the coarser sand sizes 0 17 0 26 mm three bed phases were observed ripples dunes and upper regime plane bed dunes tended to be two dimensional crests straight and continuous at low flow velocities and three dimensional crests sinuous and discontinuous at relatively high flow velocities except that at flow velocities transitional between ripples and dunes the bed showed a complex intermingling of irregular larger dunelike bed forms and smaller irregular ripples in the dune range ripples were prominently superimposed at relatively low flow velocities but nearly absent at relatively high flow velocities