a quantitative three dimensional depositional model of gravelly braided rivers a quantitative three dimensional depositional model of gravelly braided rivers has been developed based largely on the deposits of the sagavanirktok river in northern alaska these deposits were described using cores wireline logs trenches and ground penetrating radar profiles the origin of the deposits was inferred from observations of 1 channel and bar formation and migration and channel filling interpreted from aerial photographs 2 water flow during floods and 3 the topography and texture of the river bed at low flow stage this depositional model quantitatively represents the geometry of the different scales of strataset the spatial relationships among them and their sediment texture distribution porosity and permeability in the model are related to sediment texture the geometry of a particular type and scale of strataset is related to the geometry and migration of the bedform type e g ripples dunes bedload sheets bars associated with deposition of the strataset in particular the length to thickness ratio of stratasets is similar to the wavelength to height ratio of associated bedforms furthermore the wavelength and height of bedforms such as dunes and bars are related to channel depth and width therefore the thickness of a particular scale of strataset i e medium scale cross sets and large scale sets of inclined strata will vary with river dimensions these relationships between the dimensions of stratasets bedforms and channels mean that this depositional model can be applied to other gravelly fluvial deposits the depositional model can be used to interpret the origin of ancient gravelly fluvial deposits and to aid in the characterization of gravelly fluvial aquifers and hydrocarbon reservoirs

reservoir and aquifer characterization of fluvial architectural elements stubensandstein upper triassic southwest germany this paper aims at a quantitative sedimentological and petrophysical characterization of a terminal alluvial plain system exemplified by the stubensandstein south german keuper basin the study follows the outcrop analogue approach where information derived from outcrops is collected in order to enhance interpretation of comparable subsurface successions quantitative data on sandbody geometries porosities and permeabilities are presented in order to constrain modelling of subsurface sandbodies and permeability barriers for sedimentological characterization the method of architectural element analysis miall a d 1996 the geology of fluvial deposits springer berlin was used and modified to include poroperm facies a special photo technique with a precise theodolite survey was developed to create optically corrected photomosaics for outcrop wall maps from up to 20 000 m2 large outcrops nine architectural elements have been classified and quantified bedload mixed load and suspended load channel fills are separated the petrophysical characterization of the architectural elements integrated porosity and permeability measurements of core plugs with gamma ray measurements along representative sections it could be demonstrated that certain architectural elements show a characteristic poroperm facies four scales of sedimentary cycles have been recognized in the stubensandstein cyclic sedimentation causes changing lithofacies patterns within the architectural elements depending on their position in the sedimentary cycle stratigraphic position exerts only some paleogeographic position exerts significant influence on porosity and permeability of the sandbodies the highest poroperm values were found in proximal areas of the alluvial plain and in middle parts within sedimentary macrocycles the strong internal heterogeneity on the alluvial plain system is important for its reservoir and aquifer characteristics compartments of bedload channel sandstones in medial positions of a stratigraphic cycle represent very good reservoirs or aquifers the seals or aquicludes are formed by extensive floodplain claystones lacustrine sediments paleosols and suspended load deposits strongly cemented zones of sandstones represent aquitards

geochemical and geochronological constraints on the tectonic setting of the xinlin ophiolite northern great xing an range ne china abstract the xinlin ophiolite located in the great xing an range domain of the central asian orogenic belt caob is considered to represent a fragment of an oceanic plate the ophiolite consists of several discontinuous blocks composed dominantly of serpentinized ultramafic rocks with subordinate cumulate gabbros basalts and diabase dikes which are intruded by younger leucogranite dikes here we characterize the geochemical features and formation age of the ophiolite the ultramafic rocks are dominated by harzburgite with minor dunite and are enriched in large ion lithophile elements liles and light rare earth elements lrees consistent with remnants of oceanic mantle the mafic rocks show depletion in some high field strength elements hfses and have relatively low tio2 and mgo contents and likely originated from an e morb like source in a back arc setting this suggests that the ophiolite is of supra subduction zone ssz origin the cross cutting leucogranite dikes most of which are trondhjemitic have high al2o3 contents low k2o na2o ratios and negative zircon ehf t values 8 5 to 3 8 average 5 8 which are features distinct to other granites e g fractionation shear and subduction types within the ophiolite and to coeval granitoids in the northern great xing an range zircon u pb dating on samples of gabbro and a leucogranite dike yielded ages of ca 480 and 324 1 ma respectively suggesting that the ophiolite formed in the early ordovician and was emplaced at ca 320 ma together with the ages of ca 480 ma for the duobaoshan arc and ca 340 ma for subduction related high mg diorites we suggest that ophiolite formation was probably related to the opening of an ordovician back arc basin whereas formation of the younger leucogranite dikes was probably related to closure of the back arc basin

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

ground penetrating radar for high resolution mapping of soil and rock stratigraphy ground penetrating radar is a technique which offers a new way of viewing shallow soil and rock conditions the need to better understanding overburden conditions for activities such as geochemical sampling geotechnical investigations and placer exploration as well as the factors controlling groundwater flow has generated an increasing demand for techniques which can image the subsurface with higher resolution than previously possible the areas of application for ground penetrating radar are diverse the method has been used successfully to map ice thickness water depth in lakes bedrock depth soil stratigraphy and water table depth it is also used to delineate rock fabric detect voids and identify karst features the effective application of the radar for the high resolution definition of soil stratigraphy and fractures in bedrock is highlighted the basic principles and practices involved in acquiring high quality radar data in the field are illustrated by selected case histories one example demonstrates how radar has been used to map the bedrock and delineate soil horizons to a depth of more than 20 m two case histories show how radar has been used to map fractures and changes of rock type to 40 m range from inside a mine another case history demonstrates how radar has also been used to detect and map the extent of groundwater contamination the corroboration of the radar results by borehole investigations demonstrates the power and utility of the high resolution radar method as an aid for interpolation and extrapolation of the information obtained with conventional coring programmes with the advent of new instrumentation and field procedures the routine application of the radar method is becoming economically viable and the method will see expanded use in the future

architectural elements and bounding surfaces in fluvial deposits anatomy of the kayenta formation lower jurassic southwest colorado abstract three well exposed outcrops in the kayenta formation lower jurassic near dove creek in southwestern colorado were studied using lateral profiles in order to test recent regarding architectural element analysis and the classification and interpretation of internal bounding surfaces examination of bounding surfaces within and between elements in the kayenta outcrops raises problems in applying the three fold classification of allen 1983 enlarging this classification to a six fold hierarchy permits the discrimination of surfaces intermediate between allen s second and third order types corresponding to the upper bounding surfaces of macroforms and internal erosional reactivation surfaces within the macroforms examples of the first five types of surface occur in the kayenta outcrops at dove creek the new classifications is offered as a general solution to the problem of description of complex three dimensional fluvial sandstone bodies the kayenta formation at dove creek consists of a multistorey sandstone body including the deposits of lateral and downstream accreted macroforms the storeys show no internal cyclicity neither within individual elements nor through the overall vertical thickness of the formation low paleocurrent variance indicates low sinuosity flow whereas macroform geometry and orientation suggest low to moderate sinuosity the many internal minor erosion surfaces draped with mud and followed by intraclast breccias imply frequent rapid stage fluctuation consistent with variable seasonal monsonal ephemmeral flow the results suggest a fluvial architecture similar to that of the south saskatchewan river through with a three dimensional geometry unlike that interpreted from surface studies of that river

ice jam caused fluvial gullies and scour holes on northern river flood plains abstract two anomalous fluvial landforms gullies and scour holes eroded into flood plains bordering meandering and braiding river channels have not been previously reported we observed these features along the milk river in southern alberta canada and northern montana usa which has a history of frequent 50 probability of recurrence and high magnitude 12 probability of recurrence greater than bankfull ice jam floods gullies have palmate and narrow linear shapes with open ends downvalley and measure up to 208 m long 139 m wide 3 5 m deep below bankfull channel ice jams reroute river water across meander lobes and cause headward gully erosion where flow returns to the main channel erosion of the most recent gully was observed during the record 1996 ice breakup flood and ice jams scour holes bowl shaped closed depressions eroded by water vortices beneath and between grounded ice jam blocks measure up to 91 m long 22 m wide 4 5 m deep ice jam caused gullies may be precursors to the formation of u shaped oxbow lakes and multiple channels common in many northern rivers

icelandic jokulhlaup impacts implications for ice sheet hydrology sediment transfer and geomorphology abstract glaciers and ice sheets erode entrain and deposit massive quantities of debris fluxes of subglacial meltwater exert a fundamental control on ice dynamics and sediment transport budgets within many glacial systems outburst floods jokulhlaups constitute high magnitude high frequency fluxes of meltwater relative to normal ablation controlled discharge this paper presents a synthesis of research on recent icelandic jokulhlaups and their geomorphological and sedimentary impact we identify jokulhlaup impacts within subglacial englacial and proglacial settings and discuss their wider significance for ice sheet hydrology sediment transfer and geomorphology because jokulhlaups erode deposit and re work sediment simultaneously they usually cause significant glaciological and sedimentological impacts jokulhlaups that propagate as subglacial flood waves often produce widespread hydromechanical disruption at the glacier base recent icelandic jokulhlaups have been recognised as highly efficient agents of reworking subglacial sediment and glacial sediment entrainment models of jokulhlaup impact therefore need to encompass the sub and englacial environment in addition to the proglacial zone where research has traditionally been focussed most jokulhlaups transport sediment to proglacial sandar and often directly to oceans where preservation potential of the impact is greater proglacial jokulhlaup deposits form distinctive sedimentary assemblages coupled with suites of high energy erosional landforms this study of modern jokulhlaup processes and sedimentary products may be useful for the interpretation of meltwater processes associated with quaternary ice sheets

depositional controls in a bedload dominated fluvial system internal architecture of the lee formation kentucky abstract sandstones of the early pennsylvanian lee formation central appalachian foreland basin were deposited in a low sinuosity bedload dominated fluvial system the internal architecture of the sandstones consists of a six fold hierarchy of bounding surfaces which are evaluated with respect to depositional controls minor first second and third order surfaces generally less than a few hundred meters in extent separate deposits of small to intermediate scale bedforms third order surfaces which dip gently in the downflow direction represent reactivation surfaces of downstream accreting macroforms major surfaces fourth fifth and sixth order are hundreds of meters to kilometers in extent fourth order surfaces separate deposits of individual macroforms fifth order bounding surfaces are erosional boundaries of major system scale channels individual sandstone members of the lee formation are bound by sixth order surfaces erosional at the base and gradational at the top deposits overlying major surfaces contain fining upward sequences the trends are more apparent with increased scale of the deposit these sequences represent responses of the fluvial system to various mechanisms that controlled sedimentation both fourth and fifth order surfaces are interpreted as results of autocyclic controls the nature of the lee formation fluvial system was such that a solitary major channel was relatively stable in one position within the alluvial plain for a significant period of time during this time the channel aggraded with the passage of one or more downstream accreting macroforms creating a fourth order erosional surface the channel eventually diverted its flow to another portion of the alluvial plain by avulsion creating a fifth order surface a gross fining upward trend and upward change in fluvial style for individual sandstone members both characteristic of paleovalley fills indicate that sixth order surfaces were controlled by allocyclic processes these characteristics and the stair step positioning of the sandstone members younger away from the alleghenian mountains to the east suggest that tectonic processes were the ultimate control on the system

bar top hollows a new element in the architecture of sandy braided rivers abstract discrete hollows in the bar tops of the south saskatchewan river are described that form a newly recognized morphological element of sandy braided rivers these bar top hollows which are up to 1 7 m deep and may extend for 10 30 m down and across flow have a circular to ovoid planform and are shown through use of ground penetrating radar to be filled by a series of distinct often angle of repose foresets the hollows form by both erosion and bar top deposition and may be generated by bar tail accretion cross bar channel cutoff and subsequent fill or lateral accretion at the bar head bar top hollows occur in the upper part of the bar depositional sequence and may thus prove useful indicators for braid bar reconstruction in ancient sediments and should not be confused with channel scour

trough cross bedded conglomerate facies abstract gravel bed river deposits contain a range of facies that includes planar cross bedded gravel gp massive gravel gm and trough cross bedded gravel gt of these the gt facies is commonly interpreted to represent deposits of channel fills and or three dimensional 3d gravel dunes and or scours or pools in this study a calcrete conglomerate sheet built up of gt facies is found to consist of trough cross beds of 3d dune origin gtd and of bedload sheet origin gtb whereas gtd occurs as sets gtb is found to occur as isolated large troughs the gtd facies is characterised by high dips low width height ratios and foreset bed thickness of about 100 140 mm in contrast the gtb facies has low dips high width height ratios and foreset bed thickness usually less than 50 mm the bivariate plot of width height of troughs vs maximum dip effectively discriminates between these varieties of troughs a third variety of gt may be recognised by the presence of a scoop or spoon shaped lower bounding surface as seen in the depositional dip direction this facies has been demonstrated in other works to represent filling of scours or pools on the streambed and is designated as gtc for the preservation of 3d gravel dunes in the stratigraphic record special flow conditions are necessitated of prime importance is the rapid recession of flow subsequent to a short duration of peak discharge such discharge patterns are critical to the preservation of gravel dunes in the stratigraphic record and appear to have been met with in the case of glacial outburst floods or floods in the seasonal tropics

sediment transport and bed morphology at river channel confluences river channel confluences form important morphological elements of every river system being points at which rapid changes in flow sediment discharge and hydraulic geometry must be accommodated this article presents results of a quantitative investigation of sediment transport at channel confluences accomplished through both scaled laboratory flume simulation and complementary monitoring of a natural channel confluence bed morphology at channel confluences is characterized by three distinct elements avalanche faces at the mouth of each confluent channel a deep central scour and a bar within the separation zone formed at the downstream junction corner these elements are controlled predominantly by the confluence angle and the ratio of discharges between the tributary and mainstream channels as confluence angle and discharge ratio increase the sediment contributions from the confluent channels are progressively segregated in their paths through the junction with sediment being transported around rather than through the centre of the confluence this segregation of sediment loads is accompanied by the retreat of the main channel avalanche face from the confluence an increase in the scour depth a change in the orientation of the scour and an increase in the size of the separation zone bar field measurements closely replicate the flume simulation a model of sediment transport and bed morphology links these features to the fluid dynamics of these sites an understanding of confluence dynamics is important not only in considerations of channel morphology and design criteria but must form the basis for the interpretation of confluence sediments in the ancient record

characteristics of co rich ferromanganese nodules and crusts sampled in french polynesia abstract co rich ferromanganese nodules and crusts were sampled at water depths of about 1000 m on the southern flanks of the tuamotu archipelago in french polynesia crusts developed on a hard sedimentary substrate form a continuous ferromanganese encrustation about 2 5 cm in average thickness on the plateaus and slopes of seamounts nodule clusters occur within the crusts on the tops of the plateaus a detailed study of the structure mineralogy and geochemistry of the nodules and crusts established the following 1 the nodule and crust cores are made up of phosphatized breccia with brecciated biogenous limestone elements which reflects the gradual subsidence of the archipelago since the middle eocene 2 two discontinuous growth episodes of ferromanganese oxide zones were separated by a deposition of carbonate apatite 3 co increases according to the morphological oxide layers from the inner layer 0 5 co to the peripheral layer where co concentrations reach up to 2 and 4 co enrichment appears to be related to the oxygen minimum layer the characteristics of samples from french polynesia are similar to those of co rich crusts from other regions of the north pacific co rich nodules and crusts from the tuamotu archipelago should be considered as a potential metal resource similarly to those from the hawaiian emperor archipelago and the line islands mid pacific mountains system

morphological evolution and dynamics of a large sand braid bar jamuna river bangladesh the initiation and evolution of a kilometre scale sand braid bar was monitored during a 28 month survey period from 1993 to 1996 in one of the world s largest braided rivers the jamuna river bangladesh repeated bathymetric surveys through two monsoon flood seasons combined with bar top surveys during exposure of the bar at low flow provide the most detailed chronology of braid bar growth yet compiled for a large sand bed river during rising and peak flow of the 1994 monsoon flood a 1 5 km long 0 5 km wide 12 m high symmetrical mid channel bar was deposited in the centre of a major channel downstream of a zone of flow convergence and significant bank erosion initial deposition and growth of the bar core were probably caused by amalgamation of dunes that are present in the jamuna channels at all flow stages bar top aggradation continued through downstream migration of an accretionary dune front a 3 m high angle of repose slipface that was composed of amalgamated 0 5 to 1 m high dunes at waning and low flow the mid channel bar widened by up to 1 km through the lateral accretion of dunes onto the margins of the initial bar core a low velocity zone in the sheltered wake region of the bar tail led to the accumulation of substantial volumes of silts and clays during the rising and peak flows of the next monsoon flood the mid channel bar extended its bar tail by up to 1 5 km as one of the anabranches became dominant and flow was deflected across the bar tail accretion at the bar tail generated a lobate transverse bar front with a 10 m high angle of repose avalanche face emergence of several smaller bars along this depositional front produced an overall reach morphology that more closely resembled an alternate bar rather than several mid channel bars the conversion of a mid channel bar to an alternate bar is contrary to many previous descriptions of the braiding process

using two and three dimensional georadar methods to characterize glaciofluvial architecture abstract the threat of pollution in the shallow subsurface has led to an increasing need to understand how complex heterogeneities in gravel aquifers influence groundwater transport to characterize these heterogeneities we have conducted extensive two dimensional 2 d and three dimensional 3 d ground penetrating radar gpr or georadar surveys in two quarries within the rhine valley of northeastern switzerland the quarries comprised gravel and sand deposited in a proglacial braided river system that followed the maximum wurm stage glaciation after the surveys the terrace walls beneath the two study sites were photographed as they were being excavated by combining information extracted from the 2 and 3 d georadar images with the outcrop photographs it was possible to correlate georadar facies with the various glaciofluvial architectural elements the dominant elements were scour pools formed at or near the confluences of two stream channels and horizontally bedded and massive gravel sheets deposited during moderate to high water flow conditions and exposed during low flow conditions architectural elements were generally elongated parallel to the mean flow directions of the ancient river system variations in the strike of their long axes reflected lateral and vertical changes in the local flow direction time slices showed structural trends not evident on 2 d georadar images and photographs data interpretation was quicker more complete and less ambiguous when georadar facies analyses were based on a combination of georadar vertical profiles and time slices than when based on georadar vertical profiles alone

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

obstacle marks produced by flow around stranded ice blocks during a glacier outburst flood jokulhlaup in west greenland the effects of glacier ice block grounding on the morphology and sedimentology of proglacial fluvial outwash were examined during a glacier outburst flood or jokulhlaup near sondre stromfjord west greenland observations made during and after the 1987 jokulhlaup both on the surface of an ice contact delta and within a confined valley sandur plain provided information about the formation of ice block obstacle marks and the significance of these bedforms for sandur morphology and sedimentology flow directions determined from obstacle mark morphology have been used successfully to chart flow direction changes on the falling limb of the jokulhlaup maximum flow depths for scour around stranded ice blocks may be given by 0 5 0 9 times the diameter of the ice block as estimated from the depth of scour the height of the obstacle shadow or the extent of ice block meltout sediments minimum flow depths can be represented by the height of the obstacle shadow above the mean bed level the internal composition of the shadow indicates the ability of the flow to transport various sizes of material into the lee of obstacles ice block obstacle marks within the distal portion of the sandur initiated waning stage channel change proximal and lateral erosion around stranded ice blocks extended downstream from the ice block forming chute channels which then captured waning stage flows resulting in significant bar incision with associated deposition of lobate or deltaic deposits it is suggested that ice block obstacle marks are important in terms of channel morphology channel morphological change and their usefulness as palaeohydrological indicators