clastic sedimentation in a late archaean accretionary terrain midlands greenstone belt zimbabwe abstract late archaean 2 68 ga clastic sedimentary rocks of the shamvaian group form a shear zone bounded lithotectonic unit in the midlands greenstone belt mgb of zimbabwe the sedimentary rocks are wedged in between mid archaean granitoid gneisses and late archaean mafic and bimodal volcanic rocks the 2000 m thick sedimentary sequence consists of marine turbidite deposits overlain by high energy shelf sandstone fluvial braid plain pebbly sandstone and alluvial fan conglomerate deposition took place in a northward deepening basin with fluvial systems draining a source dominated by granitoid gneiss and banded chert basin formation was associated with a west directed thrusting event and the basin fill was incorporated into the thrust stack soon after deposition the shamvaian group is similar to modern foreland and forearc successions and possibly accumulated during collision of a continental fragment with an island arc like complex

large scale dam removal on the elwha river washington usa river channel and floodplain geomorphic change abstract a substantial increase in fluvial sediment supply relative to transport capacity causes complex large magnitude changes in river and floodplain morphology downstream although sedimentary and geomorphic responses to sediment pulses are a fundamental part of landscape evolution few opportunities exist to quantify those processes over field scales we investigated the downstream effects of sediment released during the largest dam removal in history on the elwha river washington usa by measuring changes in riverbed elevation and topography bed sediment grain size and channel planform as two dams were removed in stages over two years as 10 5 million t 7 1 million m3 of sediment was released from two former reservoirs downstream dispersion of a sediment wave caused widespread bed aggradation of 1 m greater where pools filled changed the river from pool riffle to braided morphology and decreased the slope of the lowermost river the newly deposited sediment which was finer than most of the pre dam removal bed formed new bars largely pebble granule and sand material prompting aggradational channel avulsion that increased the channel braiding index by almost 50 as a result of mainstem bed aggradation floodplain channels received flow and accumulated new sediment even during low to moderate flow conditions the river system showed a two to tenfold greater geomorphic response to dam removal in terms of bed elevation change magnitude than it had to a 40 year flood event four years before dam removal two years after dam removal began as the river had started to incise through deposits of the initial sediment wave 1 2 million t of new sediment 10 of the amount released from the two reservoirs was stored along 18 river km of the mainstem channel and 25 km of floodplain channels the elwha river thus was able to transport most of the released sediment to the river mouth the geomorphic alterations and changing bed sediment grain size along the elwha river have important ecological implications affecting aquatic habitat structure benthic fauna salmonid fish spawning and rearing potential and riparian vegetation the response of the river to dam removal represents a unique opportunity to observe and quantify fundamental geomorphic processes associated with a massive sediment influx and also provides important lessons for future river restoration endeavors

high resolution magnetic stratigraphy across the oligocene miocene boundary in an alluvial lacustrine succession ebro basin northeast spain abstract the study of a late oligocene early miocene 300 m thick section of alluvial and shallow lacustrine sediments in the ebro basin has allowed a high resolution magnetic polarity stratigraphy for the oligocene miocene boundary to be established the samples studied include alluvial sandstones and mudstones as well as shallow lacustrine and paludine micritic and microsparitic limestones two hundred and fifty specimens were stepwise thermally or magnetically demagnetized following standard procedures magnetic imprints of the distinct lithologies present sharp contrasts especially for the stability of the secondary components the vgp data show 18 reversal events defining a sequence of 16 polarity intervals lateral and vertical continuity of the sequences studied has enabled a correspondence between the local paleomastological biozones and the magnetostratigraphic succession to be established the high resolution chronology of this magnetostratigraphic succession provides valuable information that allows the non marine european biochronological scales to be correlated to the global polarity time scale gpts the magnetic polarity zonation of the section has been correlated to the gpts using the distinct sequence and pattern polarity reversals as well as by considering the fossil mammal assemblages as a preliminary result chrons 8 7a 7 and 6c have been identified

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

tectonic systems and the deposition of iron formation abstract the iron formation group of chemically precipitated cherty iron rich sediments ranges in age from early precambrian to recent and is composed of a variety of lithological facies iron formations are classified as lake superior and algoma types to emphasize the contrasting conditions in the spectrum of sedimentary tectonic environments in which they formed thick stratigraphic units of lake superior type iron formation which form the most extensive iron ranges of the world are part of sedimentary tectonic systems that developed along the margins of cratons or continental platforms and were deposited over extended periods of time under relatively stable tectonic conditions the algoma type iron formations were deposited with volcanic rocks and greywacke in various tectonic volcanic systems formed under more dynamic tectonic conditions but are otherwise comparable to present day spreading ridges on the ocean floor the thick sequences of lake superior type iron formation deposited between 2 7 and 2 0 ga form part of major sedimentary tectonic systems that extended along the margins of continents direct biogenic factors and the composition of the atmosphere probably had only limited influence on the precipitation of these chemical sediments

morphology of red creek wyoming an arid region anastomosing channel system red creek in the red desert area of the great divide basin wyoming is an arid region anastomosing stream the narrow deep and sinuous main channel is flanked by anastomosing flood channels or anabranches most anabranches are initiated at meander bends the primary mechanism of anabranch initiation is avulsion during overbank floods anabranch enlargement occurs by headward erosion anabranches act as distributary channels during floods when water and sediment from overbank flows are transported to and deposited on the floodplain via the anabranches during periods of low discharges the anabranches act as tributaries to the main channel transporting runoff from the floodplain and surrounding hillslopes to the main channel of red creek aggradation is occurring in the main channel and on the floodplain throughout the study reach infilling of the main channel occurs primarily by lateral accretion while the floodplain accretes vertically through deposition of overbank sediment from the main channel and anabranches infilling of the main channel may cause avulsion of the main channel into an anabranch the abandoned main channel segment may then fill completely or act as an anabranch because lateral migration of channels is inhibited by the high cohesion of the silt and clay channel sediment periodic avulsion is the primary form of lateral mobility in the system

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

sediment storage partitioning in alluvial stratigraphy the influence of discharge variability abstract numerical models of formation of alluvial stratigraphy often specify either explicitly or implicitly the proportion of channel and overbank sediments that are deposited during a given interval of time however little is known about the factors that affect the partitioning of sediment between channels and the overbank environment over long time intervals and the fidelity with which that partition is preserved in the stratigraphic record here we use physical experiments to investigate the role that discharge variability plays in this partitioning in fluvial stratigraphy we find that channels formed under constant flow conditions have low lateral mobility and act mostly as conduits for sediments to reach the shoreline the bulk of the aggradation in this case is derived from sediment laden flow that escapes the main channels by contrast including floods increases channel lateral mobility and this change is recorded in stratigraphy as an increased proportion of channel deposits relative to overbank deposits when variable flow is included as an input condition a large volume of in channel deposition occurs rendering the channels substantial contributors to stratigraphic volume on their own the increase in channel deposit volume is driven mainly by a threefold increase in the average time that a location is subject to in channel aggradation other factors include a slight increase of in channel aggradation rates and an increase in erosion of the overbank environment that results from energetic overbank flows our study shows that the character of a river s hydrograph exerts a significant influence on the proportion of channel to overbank sediment bodies in alluvial successions which is an unexamined source of uncertainty in common stratigraphic models

neoarchean deep marine paleotemperature evidence from turbidite successions abstract numerous studies using various proxies have been conducted to ascertain the temperature of the archean ocean if as some of these investigations indicate the archean ocean was relatively hot with a cooling trend through time to the phanerozoic this would have had consequences for sediment erosion transport and deposition substantially higher surface temperatures would cause the viscosity of water an important variable in defining flow hydrodynamics to be lowered in particular less viscous water would have affected the development of current ripples these are dependent on the existence of the laminar sublayer as the sweeps of turbulent eddies impinging on this layer build small mounds that evolve into ripples the laminar sublayer thins at higher temperatures allowing sand grains of smaller sizes to destroy it at modern surface temperatures current ripples cannot form in coarse grained sands and if a hotter ocean existed in the archean it would have inhibited current ripple formation in medium grained sands and possibly even fine grained sands examination of numerous neoarchean deep water turbiditic successions in western superior province led to the realization that ripple cross lamination is rare in all these units as the formation of current ripples is dependent on the flow depositing sediment of the proper grainsize velocities in the appropriate range and existence of a viscous sublayer and the first two prerequisites were met during deposition from these decelerating turbidity currents the existence of the viscous sublayer during the majority of flows must be questioned the occurrence of current ripple cross lamination in well exposed turbidites of the neoarchean mckellar harbour formation was quantified and found to be substantially less than the abundance of ripple cross lamination present in phanerozoic turbidites indicating that the neoarchean deep ocean may have been considerably warmer than the deep ocean of today

accelerated river avulsion frequency on lowland deltas due to sea level rise sea level rise subsidence and reduced fluvial sediment supply are causing river deltas to drown worldwide affecting ecosystems and billions of people abrupt changes in river course called avulsions naturally nourish sinking land with sediment however they also create catastrophic flood hazards existing observations and models conflict on whether the occurrence of avulsions will change due to relative sea level rise hampering the ability to forecast delta response to global climate change here we combined theory numerical modeling and field observations to develop a mechanistic framework to predict avulsion frequency on deltas with multiple self formed lobes that scale with backwater hydrodynamics results show that avulsion frequency is controlled by the competition between relative sea level rise and sediment supply that drives lobe progradation we find that most large deltas are experiencing sufficiently low progradation rates such that relative sea level rise enhances aggradation rates accelerating avulsion frequency and associated hazards compared to preindustrial conditions some deltas may face even greater risk if relative sea level rise significantly outpaces sediment supply then avulsion frequency is maximized delta plains drown and avulsion locations shift inland posing new hazards to upstream communities results indicate that managed deltas can support more frequent engineered avulsions to recover sinking land however there is a threshold beyond which coastal land will be lost and mitigation efforts should shift upstream

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

using sediment geochemistry and detrital zircon geochronology to categorize eroded igneous units an example from the mesoarchean birch uchi greenstone belt superior province abstract sediments deposited in volcanically active areas preserve a record of the geochemistry of eroded igneous units in some instances this record can be more complete than the remaining volcanic rocks whole rock geochemistry and detrital zircon geochronology were performed on sedimentary rocks of the balmer assemblage in the birch uchi greenstone belt western superior province to investigate the types and ages of igneous rocks that were present in the source area at approximately 2 9 ga spatially juxtaposed mesoarchean volcanic rocks were also analyzed for comparative purposes the detrital zircon geochronology revealed that the sandstones studied are significantly younger than the adjacent volcanic rocks inferring that their stratigraphic position is the result of tectonic interleaving and they do not belong in the balmer assemblage as well the majority of the detrital zircons were older than the adjacent volcanic rocks severely limiting the potential role of these volcanic units as a source terrain extreme geochemical heterogeneity within the sandstone sample set implies the depositional environment was supplied by multiple sediment fed systems which probably delivered detritus to a submarine ramp environment a subset of the sandstone samples are hfse enriched and had in part an alkalic igneous source this infers that the ocean plateau tholeiitic basalts which are abundant in northwest superior province may have been associated with alkalic eruptive units similar to modern intra plate igneous activity either erosive or tectonic processes have removed these alkalic igneous units from the preserved rock record but their detritus is present and identifiable in the sedimentary succession

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

scale dependent compensational stacking an estimate of autogenic time scales in channelized sedimentary deposits recent studies show that paleoenvironmental allogenic signals preserved in the stratigraphic record may be contaminated or overprinted by internally generated autogenic sedimentation this is problematic but it is unclear over what temporal and spatial scales autogenic patterns are most prevalent we propose that scale breaks in basin fi lling trends can be used to identify the transition between allogenic and autogenic stratigraphy using data from numerical and physical experiments and an ancient outcrop we explore how compensation the tendency for sediment transport systems to preferentially fi ll topographic lows varies with stratigraphic scale object based models demonstrate the temporal scales at which stratigraphy changes from being partially infl uenced by autogenic processes to being completely determined by allo genic forcings and suggest that this transition occurs at a time scale set by the maximum scale of surface roughness in a transport system divided by the long term aggradation rate this hypothesis is validated in a physical experiment where delta topography was monitored along fl ow perpendicular transects at a high temporal resolution relative to channel kine matics the strength of compensation in the experiment changes at the predicted time scale where the maxi mum surface roughness is equal to the depth of the experimental channels above this compensation time scale deposits stack purely compensationally but below this time scale deposits stack somewhere between randomly and deterministically similar scale dependent stacking is also observed in the ferris formation cretaceous paleogene hanna basin wyoming united states this study demonstrates that scale dependent compensational stack ing may be useful for isolating allogenic and autogenic stratigraphy in sedimentary basins

natural oscillations in coupled geomorphic systems an alternative origin for cyclic sedimentation internally oscillating cycles of cutting and filling occur naturally in many coupled geomorphic systems as a demonstration we have modeled stream entrenchment and backfilling where alluvial systems couple to uplifting mountain blocks this simple model tracksbedrockstreamerosionandtransportofalluviumtobaselevel internaloscillations arise from coupling of a kinematic wave model for mountain erosion to a diffusion model for the alluvial basin decaying periodic cutting andfilling occur on time scales of 10 5 yr the largest magnitudes are found at the coupling point model results suggest that cyclic sedimentation may result from the tintinnabulation of single perturbations applied to natural systems and need not record cyclic changes in climate tectonics or base level

facies and architectural analysis to interpret avulsion style and variability upper cretaceous blackhawk formation wasatch plateau central utah u s a abstract sequence stratigraphic models emphasize the role of external allogenic controls such as relative sea level tectonic subsidence and climate on alluvial to coastal plain stratigraphic architecture at the expense of internal autogenic behaviors such as avulsion these models are tested via sedimentologic analysis of fine grained floodplain deposits and their relationship to channelized fluvial sandbodies in a well exposed alluvial to coastal plain succession developed under progressively increasing distance from the coeval shoreline and an accompanying decrease in the rate of accommodation creation late cretaceous blackhawk formation wasatch plateau central utah u s a the studied alluvial to coastal plain succession contains fourteen lithofacies which are grouped into four associations and eight architectural elements facies associations consist of 1 channelized fluvial sandstone bodies 2 nonchannelized fluvial sandstone and siltstone bodies 3 fine grained floodplain deposits and 4 brackish floodplain deposits paleosols are immature and are classified as entisols inceptisols and histosols integrated analysis of paleosol character facies analysis and architectural element analysis indicates deposition on a rapidly aggrading floodplain that was subject to frequent crevassing and channel belt switching the character relative abundance and vertical stacking of the paleosol types architectural elements and the first three facies associations exhibit little variation stratigraphically from base to top of the blackhawk formation c 250 m or paleogeographically between the northern and southern limits of the outcrop belt c 100 km local stratigraphic architectures record three styles of avulsion representing avulsion by reoccupation avulsion by progradation and avulsion by incision avulsion style displays no consistent pattern with stratigraphic position or paleogeographic location in combination these results suggest that floodplain sedimentation and local stratigraphic architecture are dominated by autogenic avulsion the three observed avulsion mechanisms operated in apparent uniformity over a range of tectonic subsidence rates c 80 700 m ma and distances from the coeval shoreline c 0 100 km

predictable patterns in stacking and distribution of channelized fluvial sand bodies linked to channel mobility and avulsion processes despite the importance of channel avulsion in constructing fluvial stratigraphy it is unclear how contrasting avulsion processes are reflected in stratigraphic stacking patterns of channelized fluvial sand bodies as a proxy for how river depocenters shifted in time and space using an integrated geospatially referenced three dimensional data set that includes outcrop core and lidar data we identify for the first time in an outcrop study a predictive relationship between channelized sand body architecture paleochannel mobility and stratigraphic stacking pattern single story sand bodies tend to occur in vertically stacked clusters that are capped by a multilateral sand body indicating an upward change from a fixed channel system to a mobile channel system in each cluster vertical sand body stacking in the clusters implies reoccupation of abandoned channels after local avulsion reoccupational avulsion may reflect channel confinement location downstream of a nodal avulsion point that maintained its position during development of the sand body cluster and or aggradation and progradation of a backwater mediated channel downstream of a nodal avulsion point sand body clusters and additional multilateral sand bodies are laterally offset or isolated from each other implying compensational stacking due to regional switching of a nodal avulsion point to a new topographically lower site on the floodplain the predictive links between avulsion mechanisms channel mobility and resultant sand body distributions and stacking patterns shown in our findings have important implications for exploring and interpreting spatiotemporal patterns of stratigraphic organization in alluvial basins

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

riverine mixing and fluvial iron formation a new type of precambrian biochemical sediment precambrian iron formations are biochemical sediments that record ocean chemistry and circulation on the early earth the appearance of large economically important continental margin iron formation reflects the creation of extensive continental shelves and oxygenation of the ocean atmosphere system near the end of the archean exhalative iron formation contains a record of hydrothermal vent chemistry through time we introduce here fluvial iron formation a new type of fe rich microbial biochemical sediment that formed by mixing river discharge and seawater in coastal environments the paleoproterozoic chiall formation ca 1 8 ga earaheedy basin western australia contains laminated and granular hematitic iron formation in delta channel deposits where mixing occurred in adjacent peritidal settings laminated iron formation and hematitic oncoids formed because fluvial iron formations precipitated at the interface between terrestrial and marine realms the locus of known fe precipitation processes is shifted landward into paleoestuarine settings and reflects fe derived from both terrestrial weathering and coastal upwelling providing a new window into ocean atmosphere evolution

the waning stage of a greenstone belt the mesoarchaean mosquito creek basin of the east pilbara western australia abstract the mosquito creek basin stands apart in the archaean granite greenstone terrane of the east pilbara craton because of the rectangular outcrop but crescent shaped subcrop and its composition of low grade metamorphic mainly turbiditic clastic sedimentary rocks these sediments belong to the c 2 9 ga mosquito creek formation tectonic interpretations of the basin have varied from a simple synclinorium to an accretionary prism an extensional passive margin and more recently into a rift basin separating the kurrana terrane from the east pilbara terrane ept combined sedimentological and structural geological investigations show coarse clastic fan delta marginal facies with stacked unconformities rimming an e w basin centre with predominantly finer grained mass flow dominated sediments a n s traverse therefore contains a complete cross section of the mosquito creek basin shortened to about 50 of its original width by tight folding and thrusting the basin fill shows one major fu sequence from fan delta sandstone to distal turbidites unconformably overlying an older partially preserved clastic sequence of comparable facies a large slump unit occupies the basin axis the unconformities along the margins interfere with folding of the sediments which also influenced basin centre turbiditic palaeocurrents the basin is interpreted as a near symmetrical underfilled compressional intramontane clastic sedimentary basin unconformably superposed on a volcanic greenstone belt rock assemblage with enclosed low angle shear zones compression started during sedimentation under conditions of net subsidence within a rim syncline between adjacent domes this occurred after phases of low angle extensional and compressional deformation of the substrate not related to the present day geometry of the granitoid domes the geological setting of the mosquito creek basin does not differ fundamentally from that of other adjacent e woriented greenstone belts of the ept the basin is inferred to represent the last stratigraphically uppermost and because of subsequent erosion rarely preserved stage of greenstone belt evolution

how much subsidence is enough to change the morphology of river deltas understanding the effect of subsidence on fluviodeltaic morphology is important not only to maintain sustainable coastal cities and habitats but also to interpret the information contained in the stratigraphic record while tectonic steering in alluvial environments has been investigated similar studies in fluviodeltaic environments are limited to physical experiments and field observations we perform numerical experiments with a parcel based cellular model to analyze the deltaic surface and subsurface responses to regional subsidence we quantify model results using robust metrics and show that while sediment partitioning and shoreline pattern vary gradually with increasing subsidence rate channel mobility and stratigraphic connectivity of channel deposits show a threshold transition conditions for this transition are captured with a dimensionless filling index defined as the ratio between the rates of accommodation creation and sediment supply a channel locking mechanism activates when exceeds 0 6 and is responsible for the threshold transition

piping coarse grained sediment to a deep water fan through a shelf edge delta bypass channel tank experiments 1 it is now generally accepted that deltas that prograde to the shelf edge are able to transport coarse sediment to deep water either with or without sea level changes however it is still unclear how feeder rivers behave differently in the shelf edge delta case to rivers found in a delta that progrades over the shelf a series of nine shelf edge delta experiments are presented to investigate the lateral mobility of the feeder channel at the shelf edge and the associated deep water depositional system under a range of sediment supply rates and shelf front depths in the experiments constant sediment supply from an upstream point source under static sea level led the fluviodeltaic system to prograde over the shallow shelf surface and advance beyond the shelf edge into deep water the feeder river of the fluviodeltaic system became a bypass system once the toe of the delta front reached the shelf edge after the delta front was perched at the shelf edge a submarine fan developed in deep water although remaining disconnected from the delta in this bypass stage no regional avulsion or lateral migration of the feeder river occurred and all sediment from the upstream source bypassed the river delta front and shelf front slope the duration of the bypass stage is proportional to shelf front depth and inversely proportional to sediment discharge the combined duration of the shelf transit phase of the fluviodeltaic system and the bypass phase is the characteristic time scale for the continental margin to anneal transgression inducing perturbation due to high frequency and or high amplitude relative sea level rise the sequential evolution in the experiment compares favorably to the eocene sobrarbe formation a shelf edge delta in spain although natural variations are noted this comparison justifies the application of concepts proposed herein to natural systems and provides insight into interpreting processes from ancient shelf edge delta systems

composition and architecture of braided and sheetflood dominated ephemeral fluvial strata in the cambrian ordovician potsdam group a case example of the morphodynamics of early phanerozoic fluvial systems and climate change abstract two fluvial facies associations braided and ephemeral fluvial are recognized in strata of the cambrian ordovician potsdam group in the ottawa embayment and quebec basin in northeastern north america braided fluvial sedimentation was dominated by the migration of coarse grained sandy dunes and rare unit bars and local gravels that built up low relief 0 4 5 m thick downstream accreting compound bars in very wide and shallow braided channels rare meter scale confluence scours incision and filling of decimeter thick bypass channels and thin 7 cm overbank deposition also occurred ephemeral fluvial sedimentation was dominated by the progradation of lobate terminal splays most of which were deflated and or reworked into thin 10 95 cm sheets by eolian processes during the long recurrence intervals between floods these strata were locally incised by rare 0 3 1 9 m thick scour filling high energy supercritical bedform strata and decimeter thick bypass channels both braided and ephemeral fluvial deposits consist of meter scale stacked high order stratal units separated by horizontal laterally extensive surfaces recording the recurring buildup avulsion and finally abandonment of channel belts in braided fluvial strata of the potsdam the dominance of thin 2 m elongated compound bars with low relief 5 bar margins and internal accretion surfaces 15 suggest that braided channels and in channel compound bars expanded laterally with negligible increase in flow depth during periods of increased discharge in the absence of bank stabilizing vegetation similar to other examples of the pre vegetated sheet braided style however in areas where bedrock topography inhibited the lateral expansion of channels thicker compound braid bars 1 2 3 5 m with higher relief accretion surfaces up to 35 like those observed in post silurian and modern systems preferentially formed in relatively deep channels ephemeral fluvial deposits here are similar to ephemeral examples throughout geologic history in that they mostly comprise poorly confined sheet like terminal splays that accumulated in topographic lows however compared to several post silurian examples strata in the potsdam exhibit considerably more eolian reworking of splays and preservation of supercritical bedform strata a manifestation of greater rates of surface water infiltration due to a lack of sheltering by rooted plants and propensity for unconfined flooding finally in the upper part of the potsdam two regional ephemeral fluvial units are interstratified with two braided fluvial units providing evidence for shifts in regional climate the sharp areally extensive surfaces that separate these fluvial systems are interpreted to approximate time lines and therein provide a means for relative and absolute chronostratigraphic correlation these changes are correlated with documented late cambrian to earliest ordovician global climate fluctuations with semiarid conditions and related ephemeral fluvial systems corresponding to global cooling events at ca 491 and 487 ma

sedimentology of archean greenstone belts signatures of tectonic evolution abstract stratigraphic styles in archean greenstone belts are compared to those of modern and phanerozoic depositional basins in order to test the conformity of tectonic style through time six lithological associations in greenstone belts are recognized 1 mafic ultramafic volcanic 2 calc alkaline volcanic 3 bimodal volcanic 4 quartz arenite iron formation or carbonate 5 conglomerate wacke and 6 conglomerate arenite examples of the associations are described from the kaapvaal superior and zimbabwe provinces and the pilbara block each association differs only slightly between the four regions thereby emphasizing a common tectonostratigraphic theme sedimentary rocks are a minor component of the mafic ultramafic volcanic association they were deposited in two types of volcanic basin environments one similar to barred lagoons and bays around oceanic volcanic islands and the other similar to sediment starved platforms adjacent to coalesced volcanoes in inter arc intra arc and back are basins in contrast sedimentary rocks in the calc alkaline volcanic association comprise thick wedges of epiclastic and volcaniclastic detritus deposited in elongate sedimentary basins and thick wedges of pyroclastic and volcaniclastic detritus deposited adjacent to volcanic centers sedimentary facies were identical to those of forearc trench and marine volcanoplutonic arc settings chemogenic lithofacies in both volcanic associations were deposited in shallow to deep marine waters although shallow water profiles were selectively preserved in some terrains sedimentary rocks of the bimodal volcanic association are thick wedges of siliciclastic and volcaniclastic deposits interbedded with subaerial to submarine erupted flows of komatiite basalt and rhyolite depositional environments ranged from braid plain and braid delta to coastal marine and submarine fan starved basin deposits include banded iron formation and sulfidic black shale the association is identical to that of cratonic extensional basins in arc continent and intracontinental rifts sedimentary rocks of the quartz arenite iron formation or carbonate association onlap weathered basement or bimodal volcanic association rocks depositional environments were either transitional from fluvial to siliciclastic and carbonate marginal marine to offshore pelagic hemipelagic or from inshore intrabasinal clastic and volcaniclastic to offshore pelagic hemipelagic siliciclastic deposits are supermature two tectonic sites are represented those with siliciclastic facies represent continent adjacent syn to post rift stable shelves and those dominated by banded iron formation with volcaniclastic deposits represent arc adjacent post rift stable shelves sedimentary rocks of the conglomerate wacke association are represented by thick wedges of siliciclastic or volcaniclastic sandstone with minor volcanic components these were deposited in braid plain braid delta and submarine fan environments within elongate sedimentary basins coastal plain and pelagic basin facies are also present siliciclastic detritus was derived from syndepositional magmatic and metasedimentary thrust belts two stratigraphic styles are preserved one that records a flysch like stage of deep to marginal marine environments and the other that records a molasse like stage of marginal marine to fluvial environments the association is identical to that of compressional foreland basins of arc continent collisional and compressional arc tectonic affinities the conglomerate arenite association is preserved in elongate structural basins that transgress crustal components and all other supracrustal units it is dominated by thick wedges of siliciclastic sandstone although bimodal volcanic and volcaniclastic rocks can also be present depositional environments ranged from talus and alluvial fan and braid plain to either lacustrine or coastal marine local basin margins often show lateral offsets from source terrains and vertically skewed facies patterns longitudinal facies geometries are typical the association is similar to that of strike slip collisional graben in hinterland tectonic escape and terrane accretion orogens the examples show that despite all the ramifications of secular geophysical geothermal and geochemical global change the stratigraphic style of sedimentary rocks in archean greenstone belts can be matched with modern tectonic analogues thereby emphasizing the conformity of stratigraphic style through time the inseparable link between stratigraphic and tectonic styles implies that the tectonic style of greenstone belts was not temporally unique major differences exist with regards to the chemical composition of certain magmatic and sedimentary products but these are compensated for by their constant stratigraphic function in terms of depositional processes depositional environments and tectonic sites

sedimentology of the paleoproterozoic kungarra formation turee creek group western australia a conformable record of the transition from early to modern earth abstract this paper presents the first detailed sedimentological analysis of the paleoproterozoic kungarra formation the lowermost of three formations comprising the turee creek group in western australia which was deposited across the rise in atmospheric oxygen the great oxidation event or goe and the transition from early to modern earth the data show that the kungarra formation has a gradational conformable lower contact with underlying banded iron formation of the hamersley group and predominantly comprises an upward shallowing succession from deepwater shales and siltstones through rippled fine grained sandstones and stromatolitic carbonates to tidal flat deposits that immediately underlie coastal fluvial deposits of the overlying koolbye formation at the base of the kungarra formation is a gradual transition from alternating units of magnetic green shale and thin units of banded iron formation that pass upsection to units of non magnetic shale and ferruginous chert and grey chert reflecting a gradual loss of iron from the world s oceans accompanying the rise of atmospheric oxygen a falling stage systems tract is recognised above this transition in the hardey syncline area capped by stromatolitic carbonates and a period of exposure marked by an erosional unconformity and carbonate beachrock two glacio eustatic cycles are recognised within the middle to upper parts of the kungarra formation each of which is marked by the rapid onset of falling systems tracts and characterised by falling systems tracts during and following diamictite deposition stratigraphic data are used to infer a depobasin filled by a sediment wedge prograding from southeast to northwest in contrast to previous models of a north northeastward deepening foreland basin the lack of seismites or internal unconformities within the formation precludes a foredeep setting rather deposition is interpreted as having occurred within an intracratonic basin with detritus sourced from erosion of uplifted bedrock to the southeast

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

determining the causes of pleistocene stream aggradation in the central coastal areas of western australia summary late pleistocene stream aggradation was widespread in the moresby ranges western australia the general geomorphological characteristics of the drainage basins and the details of the sedimentology paleohydraulics suggest that stream aggradation was due to sediment yields exceeding stream capacity however from a consideration of the mechanics of stream aggradation it is demonstrated that stream aggradation was a response to both sediment supply and base level controls with both processes operating against a background of high sediment yields the study demonstrates that it can be relatively straightforward to establish the causes of stream aggradation

on the origin of crevasse splay amalgamation in the huesca fluvial fan ebro basin spain implications for connectivity in low net to gross fluvial deposits abstract floodplain deposits are abundant in low gradient dryland river systems but their contribution to connected reservoir volumes has not yet been fully acknowledged due to their poor detectability with typical wireline log suites and relatively lower reservoir quality this study presents an analysis of stacked crevasse splays in the distal part of the miocene huesca fluvial fan ebro basin spain vertical stacking of crevasse splays implies local aggradation of the active channel belt lateral amalgamation of crevasse splays created an elevated rim around their feeder channel raising its bankfull height subsequent crevasse splays were deposited on top of their predecessors creating sand on sand contact through incision and further raising the active channel belt this process of channel belt super elevation repeated until an upstream avulsion occurred amalgamated crevasse splays constitute connected reservoir volumes up to 107 m3 despite their lower reservoir quality they effectively connect channel deposits in low net to gross fluvial stratigraphy and hence their contribution to producible volumes should be considered unswept intervals of amalgamated crevasse splays may constitute a secondary source of natural gas their interval thickness can serve as a proxy for feeder channel dimensions which can in turn be used to estimate the degree of stratigraphic connectivity

determination of widths of meander belt sandstone reservoirs from vertical downhole data mesaverde group piceance creek basin colorado paleohydrologic reconstructions have been applied to downhole measurements made in the u s department of energy s multi well experiment mwx wells in order to derive sand body widths for the nonmarine part of the mesaverde group cretaceous in the east central part of the piceance creek basin of northwestern colorado these sediments were deposited in a predominantly meandering fluvial system paleochannel depths recognized in the wells from the heights of fining upward trends in core can be converted to channel widths using a published formula the resulting channel widths are used to calculate meander belt amplitudes sand body widths from relationships derived from published empirical data the estimate of width derived from this technique can be compared with sa d body widths derived by three other methods these various techniques give compatible results indicating the validity of the downhole technique

indicators of relative completeness of the glacial record of the port askaig formation garvellach islands scotland abstract the port askaig formation paf is a diamictite bearing succession in the dalradian supergroup of scotland that provides an excellent archive of a cryogenian glaciation in the garvellach islands and islay argyll the formation is 1100 m thick comprises 5 members and includes 47 diamictite beds interbedded with siltstones dolostones and sandstones here we document seven features of the paf that indicate its relative stratigraphic completeness there are gradual progressive changes up section in the lithologies of the diamictites their interbeds and clast lithologies the sharp basal surfaces of the diamictites each show the same repeated pattern of environmental change from non glacial to glacial many of the top surfaces of the diamictites show evidence of periglacial conditions the succession in the paf records a total of 76 climatically related stratigraphic episodes 28 glacial episodes 25 periglacial episodes and 23 non glacial episodes parts of member 1 diamictites 1 12 and diamictites 16 18 and member 2 diamictite 31 to the base of member 3 are most compete on the east coast of garbh eileach the paf in the garvellach islands occurs within a succession that is several kilometres thick as newly revealed by sea floor mapping compared with other cryogenian and phanerozoic glacial successions the paf is exceptional in its combination of formation thickness the number of climatically related stratigraphic episodes and the considerable thickness of its host supergroup furthermore these indicators of relative stratigraphic completeness provide evidence that the base of the paf on the east coast of garbh eileach is a succession without a major break in deposition supporting the account of the strata at and below the base of the paf in the companion article by fairchild et al 2018

neoarchean trans subprovince sediment transport in southwestern superior province sedimentological geochemical and geochronological evidence at 2700 ma volcanism spawned by northward directed subduction under wabigoon subprovince canadian shield fed sediment into an adjacent fore arc basin trench system volcanically active island arcs to the south wawa subprovince were transported northward during this interval and at 2690 ma collided with the accretionary complex quetico subprovince metamorphosed sedimentary units intercalated with the ocean floor and island arc volcanic rocks of northern wawa subprovince provided the opportunity to amass data useful in extending the paleogeographic interpretation of the area this study examines the mckellar harbour formation a several kilometre thick assemblage of graded metasandstone beds outcropping to the north of lake superior its sedimentology geochemistry and detrital u pb zircon geochronology were investigated in conjunction with the same attributes of possible correlative units in the region similarities between the mckellar harbour formation and sedimentary rocks composing the fore

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

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

stratigraphy sedimentology and physical volcanology of the henik group central ennadai rankin greenstone belt northwest territories canada late archean paleogeography of the hearne province and tectonic implications abstract extensive areas of the hearne and rae provinces are underlain by archean metavolcanic and metasedimentary rocks in the rae province supracrustal sequences with prominent continental to shallow water quartz arenites have been interpreted by others as rift and passive margin deposits in the hearne province to the southeast the ennadai rankin greenstone belt is exposed in an area ca 700 200 km forming the second largest greenstone belt in canada it comprises mainly greenschist facies grade mafic and felsic volcanic siliciclastic chemogenic and volcaniclastic rocks stratigraphic sedimentologic and physical volcanologic data based on 1 50 000 scale mapping in the central part of the belt indicate deposition in a subaqueous mafic volcanic plateau slope basin system that was well removed from a continental influence water depth was below storm wave base as indicated by preservation of delicate lamination in pelitic and iron formation units absence of features suggestive of subaerial exposure oscillatory flow or tidal currents and lack of interfingering rocks that could represent laterally adjacent shelf coastal or continental environments reconnaissance level regional geologic and geochronologic data indicate an overall pattern of supracrustal deposition in the hearne province consisting of a large mafic volcanic plain isolated felsic volcanic centres with lenses and belts of deep water sedimentary rocks at least two speculative yet testable tectonic models are consistent with the inferred paleogeography for both the rae province is considered to be the western continental hinterland to the ensimatic hearne province the rae and hearne provinces may record late archean continental extension sea floor spreading oceanic volcanic arc growth diachronous back arc spreading and collapse and arc continent collision with thrusting of oceanic rocks on to attenuated rae crust alternatively the hearne province may represent northwestward accretion of multiple volcanic arc trench pairs

characteristics of two pleistocene channel fill deposits and their implication on the interpretation of megasequences in ancient sediments two pleistocene channel fills located in two different geological settings on guadeloupe in the lesser antilles and along the coppermine river in the northwest territories of canada have different clast compositions scale and origin of fragmentation but have similar depositional characteristics massive bedding or absence of structures caused by traction sharp non erosive contacts reverse coarse tail grading matrix supported clasts together indicate a debris flow mechanism in which mixtures of fine and coarse sediments moved downslope by laminar flow field evidence suggests subaqueous deposition for both deposits both bed thickness bth maximum grain size mgs ratios and correlations appear characteristic of subaqueous masses capable of flowing on slopes less than 1 the two channel fills have random bth and mgs variations they do not thin and fine up section it is suggested that fills originating from laminar mass transport could normally have random bth and mgs up section variations in flysch and volcaniclastic sequences where coarse sediments interpreted as laminar flow deposits are common up section bth and mgs variations may result from processes related to source slope and flow characteristics rather than from the environment in which the sediments accumulate channel

steep rock lake sedimentology and geochemistry of an archean carbonate platform abstract the 2800 ma mosher carbonate formation at steep rock lake in south central canada is one of earth s oldest limestone deposits it is both thick up to 500 m and relatively well preserved and was one of the first precambrian successions to be noted for evidence of early life it continues to provide information on the antiquity of such processes as photosynthesis the carbonate contains a variety of fabrics and facies from apparently fine grained bedded limestones with domal and columnar stromatolites atikokania radial fans and sheet cracks to closely packed metric scale hybrid domes of originally aragonite seafloor crust interlayered with fenestral fabric here we review previous work provide new observations concerning these sediments and their geochemistry and discuss the inception overall development and ultimate demise of the steep rock platform we divide the mosher carbonate into the lower hogarth member and upper elbow point member these units constitute a shallow water carbonate platform succession the hogarth member overlies eroded 3000 ma tonalite and is at least 120 m thick thin horizons of domal pseudocolumnar columnar and fenestral stratiform stromatolites and occasional layers of atikokania radial crystal fans occur within relatively fine grained thin bedded limestone with iron carbonate horizons in the upper part meter scale domes occur locally with sheet cracks and stromatactis the irregular laminae that build the small domal and columnar stromatolites are interpreted to be lithified mats and biofilms presumably cyanobacteria dominated fine grained well bedded facies of the lower hogarth member and its varied association of small stromatolites is interpreted to have accumulated in a relatively protected inner platform environment numerous very thin horizons of iron carbonate in the lower and upper parts of the hogarth member suggest temporally limited influence of iron rich offshore waters vertical trends in fe mn ba sr ce and 13 c probably reflect progressive restriction of circulation with at least one abrupt but short lived change back to more open seawater conditions the upper hogarth member with large banded domes and fenestral crusts and stromatolites is interpreted as back margin facies the overlying elbow point member is at least 70 m thick and appears to be entirely composed of innumerable juxtaposed elongate smooth and thickly layered domes lacking intervening sediment each dome is typically several meters across and composed of centimetric alternations of cuspate fenestral fabric possibly microbial and crystal fan fabric that is probably abiotic seafloor aragonite precipitate adjacent cuspate fenestral and crystal fan fabric show significant differences in concentration of 13 c sr ba fe and mn this giant dome facies is interpreted as a platform margin deposit in which alternating calcite aragonite mineralogies within each dome reflect a laterally fluctuating offshore redox boundary negative ce anomalies positive gd anomalies and reduced positive eu anomalies relative to laterally correlative deep water iron formation occur in the majority of crystal fan samples and in some cuspate fenestral fabric samples ca carbonate precipitation at the margin of an anoxic iron rich sea would have been promoted by oxygenic removal of iron from seawater otherwise fe carbonate would have been favored over ca carbonate precipitation the mosher carbonate formation is interpreted as an early marine oxygen oasis a shallow platform that favored cyanobacterial productivity was sufficiently isolated from open marine circulation for the seawater to become relatively oxygenated and where carbonate sediment aggradation was a positive feedback that sustained these shallow water conditions limestone accumulation terminated as transgressive suboxic and anoxic seawater deposited first manganese oxide rich and then manganese oxide poor iron formation sediments

experimental constraints on bar migration through bends implications for meander wavelength selection the bed of an initially straight channel often deforms into a series of migrating alternate bars that can cause bank erosion and the development of periodic planform curvature which is often expressed as a meandering channel trace the planform curvature in turn leads to topography similar to alternate bars but stationary with respect to the meander the migration of alternate bars over this steady topography may lead to interactions influencing meander initiation and wavelength selection we studied these interactions in flume channels with bends of various lengths and angles and having various width to depth ratios in channels whose wavelength was equal to the length of bars in straight channels nearly eight channel widths bars migrated through sine generated channels with angles up to 10 the critical bend angle was smaller for channels of longer wavelength and smaller width to depth ratio migration was nonuniform and bars temporarily stalled when in phase with the curvature induced topography leading to amplification of the topography flow against an opposing bank can lead to such a strong boundary shear stress divergence that migration of the bars is prevented because sediment that otherwise would have led to bar migration is scoured from the bar front we propose that the amplification of bar pool topography in bends where free alternate bars are stalled enhances scour of the bank which leads to selection of a meander wavelength equal to that of the alternate bars

avulsion the relative roles of autogenic and allogenic processes abstract avulsion the abandonment of all or part of a channel belt in favor of a new course is controlled by both autogenic intrabasinal and allogenic extrabasinal processes however their relative roles are not known well the rhine meuse delta is without doubt the best studied delta regarding holocene avulsion processes based on studies of this delta an overview is given of 1 allogenic and autogenic processes influencing different aspects of avulsion 2 the relative importance of these processes in time and space and 3 observations that can be used to differentiate autogenic and allogenic causes of avulsion the following avulsion parameters are quantified and their allogenic and or autogenic nature are discussed period of activity of channel belts period between beginning and end of overbank sedimentation by the river channel interavulsion period time period between successive avulsions of a channel belt avulsion frequency number of avulsions per time interval in a given area and avulsion location in the rhine meuse delta the period of activity of the holocene channel belts varies considerably but shows no significant trend over time whereas extrabasinal factors climate base level and tectonics changed this suggests that this parameter is mainly autogenic controlled the average interavulsion period increased from 8000 cal yr bp to 2800 cal yr bp and decreased since then significant fluctuations occurred on a shorter time scale a maximum variability in the interavulsion period occurred between 3200 and 1800 cal yr bp this was a time when large channel belts gradually came into existence during this period the avulsion frequency also reached a maximum the long term increasing trend in interavulsion period is related to the decreasing rate of sea level rise the decreasing interavulsion period since 2800 cal yr bp can be explained by increased discharge and sedimentation resulting in an increase in the number of avulsions seven avulsion sequences as predicted by the mackey and bridge mackey s d bridge j s 1995 three dimensional model of alluvial stratigraphy theory and application journal of sedimentary research b65 1 pp 7 31 model may be recorded in the holocene dataset of the rhine meuse suggesting a periodicity of 500 600 yr each avulsion sequence coincides with a peak in the avulsion frequency the periodicity of 500 yr in the avulsion frequency is proposed to be autogenic however there is also a long term trend in the avulsion frequency that can be related to sea level rise and after 3000 14c yr bp to climate change and human influence avulsion locations are not randomly distributed over the delta and are related to the allogenic factors relative sea level rise local tectonics and changes in discharge and sediment load

trench and slope basin deposits in an archean metasedimentary belt superior province canadian shield the identification of a late archean arc trench assemblage in northwestern ontario provides the opportunity to compare depositional systems developed in a precambrian convergent setting with cenozoic examples two types of sedimentary associations exist in the accretionary complex medium to thick bedded bouma a ab and abc felsic turbidites dominate the belt these are primarily organized into unstructured sequences and reflect deposition in a ramp like environment with multiple feed points supplying sediment from a forearc basin mafic turbidites with possible shallow water reworked intervals form isolated pods within the metasedimentary belt erosion of upthrust blocks of sedimentary strata containing ultramafic masses supplied this sediment to elevated slope basins these types of depositional systems are similar in many respects to those developed in cenozoic and holocene arc trench settings

avulsion cycles and their stratigraphic signature on an experimental backwater controlled delta river deltas grow in large part through repeated cycles of lobe construction and channel avulsion understanding avulsion cycles is important for coastal restoration and ecology land management and flood hazard mitigation emerging theories suggest that river avulsions on lowland deltas are controlled by backwater hydrodynamics however our knowledge of backwater controlled avulsion cycles is limited here we present results from an experimental delta that evolved under persistent backwater hydrodynamics achieved through variable flood discharges shallow bed slopes and subcritical flows the experimental avulsion cycles consisted of an initial phase of avulsion setup an avulsion trigger selection of a new flow path and abandonment of the parent channel avulsions were triggered during the largest floods 78 of avulsions after the channel was filled by a fraction 0 3 0 13 of its characteristic flow depth at the avulsion site which occurred in the upstream part of the backwater zone the new flow path following avulsion was consistently one of the shortest paths to the shoreline and channel abandonment occurred through temporal decline in water flow and sediment delivery to the parent channel experimental synthetic stratigraphy indicates that bed thicknesses were maximum at the avulsion sites consistent with our morphologic measurements of avulsion setup and the idea that there is a record of avulsion locations and thresholds in sedimentary rocks finally we discuss the implications of our findings within the context of sustainable management of deltas their stratigraphic record and predicting avulsions on deltas

the genesis of the hope downs iron ore deposit hamersley province western australia the banded iron formation bif hosted hope downs high grade hematite ore deposits are situated within the marra mamba iron formation with subsidiary deposits in the brockman iron formation of the archean to proterozoic hamersley group of western australia the main orebody extends to 260 m below the surface and is unusually rich in martite pseudomorphous hematite after magnetite and poor in limonite and goethite compared to other ore deposits of the marra mamba iron formation the high grade hematite ore is mainly within the newman member but also occurs in parts of the nammuldi member together with low grade limonitic ore that becomes high grade after calcining karst erosion of the overlying wittenoom formation has produced steep sided buried valleys adjacent to the in situ orebodies that contain thick deposits 160 m of goethitic and sideritic sediments including remnants of robe pisolite formation bedded siderite hematite gravels red ochreous detrital material and enriched scree deposits that are additional sources of ore the ore consists of low phosphorous martite limonite goethite derived from chert free bif by supergene weathering no evidence of the complete carbonate replacement of chert has been found at hope downs nor were any traces of preexisting chert bands seen in the ore despite the abundance of chert bands in bif elsewhere a variety of textures and composition shown by cherty bif adjacent to the orebodies is described from which the origin of the chert free bif is inferred including sedimentary structures consistent with density current deposition a model is presented for the origin of the host iron formation and the ore deposits in which density currents transported reworked iron silicates and hydroxides in colloidal suspension onto an unstable sea floor the amorphous silica produced during diagenesis of al poor iron silicates formed the characteristic chert bands of bif but some of the hydrous amorphous silica was lost prior to lithification to form chert free bif weathering of the chert free bif produced the high grade hematite ore that is exposed today

overbank and channelfill deposits of the modern yellow river delta abstract the huanghe is noted for its high transport rate of silt and clay which may reach depth averaged values of 200 kg m 3 during peak discharge the sediment load transported through the river on entering the delta plain amounts to 1012 kg per year in contrast to most other large deltas only one distributary channel is active at any one time the high sediment load causes the rivermouth to prograde at a yearly rate of 1 4 km into the shallow less than 20 m deep bohai gulf the vertical aggradation of the channel belt and mouth bar complex is also rapid decimetres per year on average so that after a normal average of twelve years increasing channel instability and avulsion create the start of a new delta lobe a series of satellite images covering the last fifteen years has provided insight in the evolution of the river pattern as well as the progradation of the delta front a newly developed distributary passes from a multichannel to a single straight channel system and ends with the formation of meanders the protruding mature delta lobe shows a radiating pattern of crevasse channels overbank crevasse deposits are made of vertically stacked dm scale waning flow sequences structurally characterized by from bottom to top small scour and fills even parallel lamination and climbing ripple crosslamination accumulation rates on crevasse splays can be predicted on the basis of estimated river sediment discharge it can be concluded that each sequence has been deposited within a few hours and that tidal waterlevel fluctuations may have played a role in the generation of a single sequence

neoarchaean banded iron formation within a 2620 ma turbidite dominated deep water basin slave craton nw canada abstract banded iron formations bifs are iron and silica rich chemical metasedimentary rocks that span much of the archaean and early palaeoproterozoic rock record given that bifs precipitated from ambient seawater they provide an important archive of earth s ancient marine environments within the slave craton occurs one of the largest and well preserved achaean turbidite basins in the world interstratified within the greywacke mudstone turbidites is bif that is commonly associated with ferruginous sediments and locally with a felsic tuff bed field investigations were conducted at four bif locations from west to east across the slave craton these bifs are composed of micro and mesobands of quartz and magnetite with various amounts of metamorphic grunerite ferro actinolite ferro hornblende as well as greenalite and stilpnomelane the latter two are interpreted as retrograde metamorphic phases as the bif was deposited in a deep water turbidite dominated basin s the chemical component was occasionally diluted by clastic detritus as illustrated by highly variable al 2 o 3 content and al 2 o 3 tio 2 and cr ni that are similar to the clastic sediments on a layer by layer basis the iron content in the bif ranges from as low as 4 wt and up to 75 wt fe 2 o 3 whereas the sio 2 content ranges from 20 wt to as high as 94 wt this together with a hyperbolic germanium silicon distribution suggests that continentally sourced silica comprised the background precipitation which was periodically interrupted by pulses of hydrothermally sourced iron high temperature hydrothermal fluids contributed with an average shale normalized positive eu anomaly of 1 92 which indicates input of reduced eu 2 to the ambient seawater the bif shows characteristic aqueous fluid derived rare earths yttrium fractionation pr yb 3 to ce 4 the age and trace element signature of the felsic tuff is similar to arc type volcanic rocks potentially linking the bif and turbidite basin s to the defeat suite plutons ca 2630 2620 ma the slow chemical rainout behind bif deposition presumably took place on the basin plain adjacent to a steep submarine ramp where turbidite currents and slump generated debris flows sourced and triggered by the tectonically active arc terrain episodically delivered clastic sediment

turbidites and iron formations beardmore geraldton ontario application of a combined ramp fan model to archaean clastic and chemical sedimentation sandy turbidites grain flows conglomeratic mass flows and oxide facies iron formation are present in the late archaean beardmore geraldton terrain a metasedimentary belt which extends for at least 80 km in an e w direction the marine portion of this basin contains four lithofacies associations la 1 thinbedded iron formation clastic sediment association this association represents a continuum of deposit types containing iron formation subtypes are defined on the basis of bedding attributes and the proportion of iron formation to sand silt 2 thin bedded turbidite dominated association these sediments consist mostly of silt sand beds which either show no vertical trends or thin and fine upwards over a few metres 3 medium bedded turbidite dominated association most of these sediments are medium to coarsegrained vertically unstructured sand sequences with occasional structured intervals 4 thick bedded association this is dominated by poorly graded sands up to 7 8 m in thickness sand beds are characterised by a thin basal zone of coarse sand and pebbles a large central interval containing a mixture of medium and coarse sand and a thin upper zone of fine sand silt the overall depositional system was initiated by transport of sediment by braided streams to the strand area where it accumulated in distributary mouth bars we infer a nearshore break in slope locally with large channels la 4 extending from close to the strand line across deltaic surfaces to the deeper portions of submarine fans structured portions of la 3 however many deltaic surfaces probably were not tapped by major channels but merged downslope into a submarine ramp sediment was transported across the ramp by slump events and sheet like grain flows unstructured portions of la 3 iron formation and la 2 sediments probably accumulated both in upper mid ramp areas with low sediment delivery rates and distal to fan ramp successions as major streams on the braid plain changed position associated submarine channels and slump fed ramp deposits also would have shifted laterally this produced overlap of different facies associations in both the fan and ramp environments which may explain why observed vertical trends in bedding are limited to several metres we suggest that on narrow active archaean cratonic margins additional non regular processes such as variations in sediment supply related to periods of heightened pyroclastic activity and seismic activity associated with the arc also contributed to the lack of vertically structured sequences turbidite sequences in such environments in general may contain important contributions from both submarine ramp and submarine fan sedimentation

from bif to red beds sedimentology and sequence stratigraphy of the paleoproterozoic koegas subgroup south africa abstract the 2 4 ga old koegas subgroup of the transvaal supergroup contains siliciclastics and iron formations it occupies a transitional stratigraphic position between the major iron formations on the kaapvaal craton and strata with evidence for oxygenation of the environment koegas siliciclastics reflect a new phase of uplift and erosion on the craton after a period of chemical deposition paleocurrent data indicate transport to the west and northwest away from the craton however no precise source area can as yet be defined rocks were deposited in foreshore to offshore environments of a prograding delta or submarine fan system immature arkosic wackes and subarkoses occur in proximal parts and pass to terrigenous mudstones and chemical iron formations in offshore environments with low detrital input lateral facies transitions and the hierarchical stratigraphic stacking pattern of the koegas subgroup allow a sequence stratigraphic interpretation in which regressive coarse siliciclastics prograded repeatedly over transgressive units of chemical iron formations metals in the transgressive units have an intrabasinal hydrothermal source similar to underlying iron formations localized iron stained detrital units resemble red beds they reflect additional diagenetic fe remobilization from deeper water iron formations or possibly a continental fe source the second possibility would imply the presence of some free oxygen in the environment

geochemical discrimination of clastic sedimentary rock sources abstract factors controlling the geochemistry of a clastic sedimentary rock can include 1 composition of source terrain 2 chemical weathering 3 hydraulic sorting 4 diagenesis 5 metamorphism and 6 hydrothermal alteration a linear solution inferring source terrain composition from geochemistry of the sediment is impossible in this multivariate system as several unknowns will commonly be present the use of graphical analysis of element pairs circumvents the problem chemically immobile elements will maintain invariant ratios during rock mass change caused by either addition or depletion of mobile elements this results in chemically immobile element scattergrams exhibiting linear trends along radians extending from the origin if the major mineral phases of the immobile elements have behaved in a hydrodynamically similar manner as chemically mobile element plots will produce a scatter of points this relationship can be used to test chemical immobility and similarity in hydrodynamic sorting history the constraint on analysis is that the source area must be compositionally uniform or the sediment well mixed prior to delivery to the basin a second technique using sio2 plots has also been developed to investigate element mobility and hydrodynamic behaviour of the mineral phases containing the elements sio2 immobile element plots result in a linear arrangement of points extending towards either 0 or 100 sio2 the 0 intercept position occurs for elements with major mineral phases concentrated in sand the 100 intercept for those concentrated in clays plotting chemically mobile elements produces different patterns and this can be used to gain information on alteration and sorting history elemental ratios for chemically immobile elements with similar hydrodynamic behaviour will be the same as those for the weighted average composition of the source material this provides a powerful tool for deducing source terrain from sediment geochemistry techniques outlined above were tested on archaean metasandstones from superior province canada immobile element ratio diagrams for nb al ti and zr al ti indicate that a calc alkaline extrusive intrusive suite lying to the north of the study area was the source and not five other volcanic suites in the region this conclusion agrees with previous clast lithology studies and accentuates the applicability of the geochemical techniques

iron formation in neoarchean deltaic successions and the microbially mediated deposition of transgressive systems tracts abstract neoarchean deltaic deposits present in superior province canada contain an excellent record of shallow water siliciclastic and iron formation if deposition the deltas were fed by braided fluvial systems which commonly resulted in the development of small active lobes that repeatedly cycled through progradation and abandonment although a thick progradational deltaic assemblage was also investigated magnetite rich chemical sediments interbedded with siltstones and slates are present on flooding surfaces overlying shore proximal fluvial channels and distributary mouth bars of the braid delta deposits magnetite laminae are also interbedded with some distributary mouth sediments draping reactivation surfaces in ripples to barforms thicker accumulations of if up to 73 m form portions of transgressive systems tracts where chemical sedimentation dominated offshore equivalents of these successions do not contain chemical sediments sedimentologic data imply that if deposition in this well mixed nearshore water mass was not compatible with slow rates of constant iron compound precipitation in the world ocean combined with siliciclastic sediment starvation the model presented for iron precipitation relies on an elevated flux of continentally derived nutrients in the nearshore stimulating photosynthetically induced oxidation of fe2 under these conditions elevated accumulation rates precluded the development of condensed intervals characteristic of transgressive systems tracts

sedimentation in the chile trench depositional morphologies lithofacies and stratigraphy the depositional bodies of the chile trench trench fans the axial channel sheeted basins ponded basins and axial sediment lobes control the spatial distribution of modem lithofacies in the basin sheeted basins south of 41 s are presumably fed by closely spaced submarine gullies that approximate a line of source of sediment supply along the base of the slope trench fans 41 s 33 s are built at the mouths of major submarine canyon systems which act as point sources of sediment supply the axial channel follows the northward gravitational gradient draining the distributary networks of trench fans into the longitudinal transport system down gradient northern fan lobes are severely dissected by erosional processes evidently periods of proximal deposition alternate with periods of massive sediment remobilization and progradation of the axial dispersal system into more distal environments channelized basins in the canyon mouth areas yield to sheeted basins depositional surface maintains an axial gradient or ponded basins depositional surface is strictly flat in inter canyon areas tectonic disruption of the oceanic basement can locally augment the axial gradient and stimulate flow channelization or reverse the gradient and induce sediment ponding a large margin parallel sediment lobe is built at the base of a high axial escarpment near 33 s where the axial channel crosses a transverse discontinuity at the convergent plate boundary five lithofacies are defined by q mode factor analysis of sediment textures and hydro dynamic structures in 27 cores from the chile trench the channel facies thick amalgamated sand massive to laminated or cross bedded is deposited by high energy processes within the coarse grained bedload of turbidity currents it forms in distributary and axial channels the levee facies rhythmically bedded internally structureless graded sand and graded silt is deposited from concentrated sediment suspensions that quickly lose momentum during channel spillover it forms on channel flanks although constructional levees are not always present the basin 1 facies more complete bouma sequences both upper and lower flow regime structures forms in ponded basins where flows are confined by a high relief seaward trench wall the basin 2 facies graded and laminated silt lower flow regime structures forms in low energy environments such as interchannel areas distal basins trench walls and elevated topographic features the contourite facies silt and sand laminations winnowed from hemipelagic muds and distal turbidites is best developed in sediment starved basins where geostrophic currents are constricted and accelerated between the steep inner and outer trench walls the trench wedge records a coarsening upward sequence as the oceanic plate migrates toward and into the trench during plate convergence and becomes more proximal to sediment sources along the base of the continental margin near canyon mouths prograding trench fans drive the axial channel seaward into the trench wedge and the coarsening upward sequence is truncated by a time transgressive erosional unconformity abandoned axial channel deposits are carried landward beneath prograding fans to record a fining upward sequence above the basal unconformity channel migration and lobe aggradation may produce fining and coarsening upward sequences on depositional fan lobes but sequences on the erosional lobes are fragmented by numerous truncation surfaces

palaeogene evolution of deformation in the northeastern iberian peninsula during the palaeogene alpine compression three units were differentiated in the studied area south of the ebro basin the catalan coastal range dominated by ne sw major basement faults with a sinistral movement the iberian range where the important basement faults have a nw se direction and a reverse movement often with a dextral component the linking zone between these two ranges where an e w dominant structural direction is marked by an array of folds and thrusts with a northward vergence in the mesozoic cover from the analysis of both these major structures and the small scale structures it can be deduced that the compression in the studied area has evolved from a nw se direction lower middle eocene to a n s direction and to a ne sw one uppermost oligocene the major structures are due to the n s compression later the stress field progressively changed to a distensive regime we suggest a relation between the compressive phases and the displacement direction of the iberian plate with reference to the european plate

evolution of miocene fluvial systems in the himalayan foredeep through a two kilometer thick succession in northern pakistan abstract sedimentological variations through a kilometer thick coarsening upward succession in the miocene himalayan foredeep basin fluvial chinji and nagri formations of the siwalik group are documented in the chinji village area on the potwar plateau of northern pakistan from the chinji formation upwards into the nagri formation 1 the proportion of major sandstone bodies increase relative to mudstone dominated beds 2 average thickness and mean grainsize of channel deposits increase 3 sediment aggradation rates increase 4 there is a reported change in sediment providence 5 paleocurrents are generally to the southeast and vary little upsection and 6 paleosols are more distinct lower in the section where there is also more evidence of ponding of water on the floodplain smaller scale 100 m thick cyclic variations in the proportion of major channel sandstone bodies also occur in both formations in the modern himalayan basin sediment dispersal away from the mountain belt is associated with 1 major rivers that flow along the basin axis 2 large rivers that drain substantial areas of the mountain belt and deposit low relief sediment fans extending hundreds of kilometers into the basin and 3 smaller rivers in fan and interfan areas that drain local areas and carry finer sediment loads sediment variations through the chinji nagri formation boundary can be related to a shifting of depositional environments within such fan and interfan areas major basin axial rivers are not exposed in this area it appears that kilometer thick upsection variations in these deposits record the progradation of the sediment fan formed by a large river system river discharge on the order of 104 m3 s over the floodplain of a smaller river system river discharge on the order of 103 m3 s large scale vertical sediment variations over hundreds of meters to kilometers appear to reflect changes in the basin subsidence and the rate of sediment input to the basin over time scales on the order of 105 106 years upsection variations can not easily be related to climatic change or eustatic variations in sea level the most likely explanation for upsection changes is tectonism deposition rates and shifting positions of river systems within the basin responded more quickly to changes in basin subsidence rate whereas increases in the proportion of sandstone bodies and increases in floodplain slopes lagged behind deposit variations in the siwalik group clearly record variations between different scale river systems delivering sediment to the basin as well as the overall basin evolution

coal deposition in carbonate rich shallow lacustrine systems the calaf and mequinenza sequences oligocene eastern ebro basin ne spain two main coal bearing sequences developed during the oligocene in the tertiary ebro basin the calaf early oligocene and mequinenza late oligocene coal basins coal deposition took place in shallow marsh swamp lake complexes which sometimes became closed and evolved under warm climatic conditions with fluctuating humidity these shallow lacustrine systems are closely interrelated with the terminal parts of the distributive fluvial systems which spread from the tectonically active ebro basin margins laterally extensive lignite bearing sequences including rather thin lenticular autochthonous and or hypautochthonous coal seams with high ash and sulphur contents characterized coal deposition in the shallow lacustrine systems coal seam geometry which makes them nearly subeconomic resulted from the tectonic instability during basin margin evolution and the sometimes closed arid conditions under which the lacustrine systems evolved high ash and sulphur contents resulted from the inadequate isolation of peat forming environments from clastic influx and from the very low acidity and sometimes high sulphate contents of the lacustrine waters coal exploration in shallow lacustrine sequences similar to those described here must take into account that the spread of coal forming environments and maxima of coal deposition are usually coincident with lake expansions and retraction or shifting of the terminal fluvial zones interrelated with the lacustrine areas

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

the 1870s avulsion of the saskatchewan river several major avulsions of the saskatchewan river have occurred in the cumberland marshes east central saskatchewan during the past few thousand years the most recent avulsion occurred in about the 1870s converting over 500 km2 of floodplain into a belt of anastomosing channels splay complexes and small lakes a region that is still evolving today the avulsion began near the tip of a large meander bend by following a small outflowing creek sturgeon river which in turn followed an abandoned former channel of the saskatchewan river flow began to permanently divert out of the saskatchewan when a narrow strip of floodplain separating the sturgeon river from the nearby torch river became breached diversion into the connected sturgeon torch began to increase sometime in the 1870s and probably culminated around 1882 the triggering event for the avulsion may have been a chute cutoff of the meander bend shown by numerical modeling experiments to have significantly raised water surface elevations at th

experimental study of avulsion frequency and rate of deposition in existing models of alluvial architecture it is typically assumed that mean avulsion frequency is independent of sedimentation rate however if avulsion is driven by superelevation of a river bed above its surrounding flood plain one might expect avulsion rate to increase with sedimentation rate we have carried out a series of experiments with laboratory scale fluvial fans in which we measured the frequency of apical avulsions as a function of mean sedimentation rate on the fan avulsion frequency increased strongly with increasing sedimentation rate and then stabilized as mass flows began to influence deposition in the regime of increasing avulsion frequency the added volume of sediment needed to trigger an avulsion decreased with increasing sedimentation rate although our experimental results cannot simply be scaled up to natural rivers they suggest the possibility of coupling between avulsion frequency and sedimentation rate that would be strong enough to change qualitatively the results of existing models of alluvial architecture these models should be applied with caution until avulsion mechanics are better understood

subaqueous sediment gravity flow deposits practical criteria for their field description and classification a new method for the description and classification of subaqueous sediment gravity flow deposits is proposed the classification scheme employs a convenient letter code and divides deposits individual beds into descriptive categories of two hierarchical levels facies and subfacies facies as the higher rank categories are distinguished chiefly on the basis of sediment type i e bed grain size texture a total of 13 facies have been distinguished g gravel gs gravel sand couplet gys gravelly sand s sand sm sand mud couplet ms mud sand couplet tm silt mud couplet mt mud silt couplet m mud mys muddy sand sym sandy mud myg muddy gravel gym gravelly mud subfacies as the lower rank categories are distinguished within the individual facies on the basis of the bed s internal structures the number of subfacies is unlimited and their labelling code includes particular facies symbols see above preceded by lower case letters denoting specific sedimentary structures and their vertical arrangement subfacies thus refer to the bed s intervals or divisions which are labelled as follows m massive unstratified and ungraded g graded unstratified and graded s plane stratified x cross stratified 1 parallel and or cross laminated q liquefied for example subfacies gsg graded to plane stratified gravel are gravel beds that have a lower graded interval and an upper plane stratified interval subfacies xg cross stratified gravel are gravel beds that are cross stratified throughout subfacies sls plane stratified to laminated sand are sand beds that have a lower plane stratified interval and an upper laminated parallel and or cross laminated interval

a multiproxy study of younger dryas and early holocene climatic conditions from the grabia river paleo oxbow lake central poland abstract a multi proxy reconstruction of water depth temperature and precipitation inferred from cladocera chironomidae and pollen assemblages has been obtained from wierczyna paleo oxbow central poland during the younger dryas yd and early holocene results suggest that the yd was relatively cold and comprised two main phases the first ca 12 500 12 000 cal yrs bp is characterized by a continental climatic regime and a decrease in winter temperatures and precipitation but an increase in spring summer precipitation the second phase ca 12 000 11 500 cal yrs bp was more mild with a variable continental climate an increase in summer and winter temperature a lengthening of the growing season and increased annual precipitation the reconstructed water level generally follows changes in spring and summer precipitation and length of growing season the frequency and timing of hydroclimatic oscillations at wierczyna show strong similarities to records from other sites in europe this confirms that oxbows and valley mire ecosystems respond to rapid climate change during the yd and early holocene this study therefore brings new insights into the effects of climate changes on river environments especially during the yd we also discuss the limitations of water depth temperature and precipitation reconstructions inferred from the studied biotic assemblages

sequence stratigraphy of the archaean supracrustal belts of the pilbara block western australia abstract the stratigraphic succession of the 3580 2770 m y old granite greenstone terrain of the pilbara block is re assessed by the first and second order sequence stratigraphic techniques that are the standard for phanerozic terrains with the view to testing the conformity of stratigraphic cause and response a holistic tectonostratigraphic framework is presented that pairs the supracrustal and crustal rock records the resulting sequence scheme is tested against the cause of first order sequence cycles viz global sea level responses to supercontinent breakup and assembly three first order inter regional unconformities divide the stratigraphic succession of the supracrustal belts into four units of first order megasequence status in order of decreasing age these are named the warrawoona gorge creek roebourne and mount negri megasequences a total of 17 second order supersequences are also named although some may represent supersequence sets the megasequences and or their genetic crustal equivalents represent the preserved rock record of four megacycles of forearc arc and or back arc geotectonic evolution associated with the convergent margins of a pilbara continent individual supersequences are the rock record of separate second order basins or basin phases and reflect the different geotectonic components of each megasequence the four megacycles had respective longevities from oldest to youngest of 150 m y 230 m y 160 m y and 180 m y they are identical in their stratigraphic successions and predicted first order sea level cycles to phanerozoic first order tectono eustatic cycles and are interpreted as the same response to global tectonic change the warrawoona and gorge creek megacycles and the roebourne and mount negri megacycles are tectonogenetic pairs of the east pilbara and the west pilbara megacycle sets respectively the east pilbara megacycle set evolved over a period of 380 m y from 3490 ma to 3110 ma whereas the west pilbara megacycle set evolved over a period of 340 m y from 3110 ma to 2770 ma the two megacycle sets are identical in their longevity geotectonic components and predicted sea level cycles to convergent margin wilson and supercontinent cycles and are thereby interpreted individually as the oldest established records of the supercontinent cycle the implication of this interpretation is that a steady state global tectonic regime has been in operation from early in earth s history which has important consequences for theoretical modelling of the earth s early tectonic style and secular changes in its geochemical regime

holocene evolution of the assiniboine river paleochannels and portage la prairie alluvial fan the portage la prairie alluvial fan was constructed by numerous successive paleochannels of the assiniboine river along the western side of the lake agassiz basin as the level of the lake rapidly declined beginning 9500 years ago the history of the paleochannels during the first several thousand years is not known paleochannel morphologies and cross cutting relations soil maturity and radiocarbon dates however indicate that by 6000 7000 years ago flow was northward into lake manitoba this direction was maintained until about 3000 years ago when avulsion redirected the assiniboine eastward to the red river near winnipeg the morphologies of the paleochannels suggest that channel forming discharges and sediment loads of the ancestral rivers have not differed significantly from the modern values despite palynological evidence that the climate was warmer and drier during much of the holocene

patterns of sedimentation in the precambrian abstract the principle of uniformitarianism may be applied to precambrian basin evolution and to the sedimentary record as a whole the major difference in the precambrian eon lay in variability of rates and intensities of processes controlling weathering erosion transport deposition lithification and diagenesis this paper examines precambrian sedimentation patterns within the larger framework of earth evolution pre rock record sedimentation probably comprised deep water oceanic realms within which meteoritic and cometary impact events generated very large tsunamis resulting in very coarse volcaniclastic detritus combined with fine dust settling out of suspension all reworked by marine current systems and localised turbidites from c 4 to 3 2 ga greenstone belts provided the predominant settings for the thin passive margin carbonates bif stromatolitic evaporites pelites and quartzites and lesser synorogenic turbidites conglomerates and sandstones that accompanied the volcanic and volcaniclastic rocks typical of these settings common palaeoenvironments were high gradient alluvial fans low sinuosity braided rivers and relatively shallow marine settings subject to wave and tidal action and turbidity currents although continental crustal growth continued largely through greenstone belts until c 2 7 ga the witwatersrand basin c 3 0 2 7 ga kaapvaal craton south africa reflects initial stabilisation of the oldest craton with an epeiric sea accumulating largely fluvial detritus subject to tidal inland and storm wave craton marginal reworking within a retroarc foreland basin setting neoarchaean palaeoproterozoic sedimentation is discussed within a framework of two global superevents at c 2 7 ga and 2 2 1 8 ga each encompassing major changes in earth s evolution related to the supercontinent cycle mantle superplumes peaks in crustal growth rates and significant biochemical changes within the atmosphere hydrosphere system concomitant globally raised sea levels led to chemical and clastic epeiric seas within which the first giant carbonate platforms developed and deposition of iron formation peaked globally at c 2 5 ga the first global glaciation at c 2 4 2 2 ga provides little support for the snowball earth theory but does suggest a negative feedback loop model whereby intraglacial co 2 related warming and synglacial decreases in weathering alternated up to three times models for palaeo atmospheric and oceanic evolution are mutually exclusive but by c 1 8 ga the existence of large landmasses and free oxygen in earth s atmosphere enabled erg development and red bed sedimentation globally the full spectrum of phanerozoic modern sedimentary environments was thus present on earth a third postulated superevent at c 0 8 0 6 ga essentially recreated conditions experienced at c 2 2 1 8 ga with additionally at least three global refrigeration events

paleoslope reconstruction in sandy suspended load dominant rivers constraining the depositional gradient of ancient alluvial river systems can aid in reconstructing landscapes and estimating paleodischarge by establishing boundaries on the climatic and tectonic history of continental sequences we present three methods for estimating ancient depositional gradients based on the interpreted mode of sediment transport for the range of particle sizes found on the beds of modern rivers or preserved in channel fills for sandy rivers with suspension as the dominant mode of sediment transport these methods take advantage of observations that can be directly obtained from preserved strata including measurements of paleo flow depth and grain size the first method relates river slope to shields number at bankfull flow the second method is similar to the first but allows for variation in shields number at bankfull flow with grain diameter the final method relies on criteria required for both suspended and bed material load sediment transport in the same system and is the most comprehensive and physically justified method we present each method and test them with modern river datasets to verify accuracy and help constrain uncertainty the first step to adapting them to ancient systems results indicate that all methods estimate slope within a factor of two these methods are potentially very powerful for interpreting sandy fluvial deposits because they can provide reasonably accurate quantitative estimates of paleoslope an elusive yet important environmental variable