extensive felsic lavas and rheoignimbrites in the keweenawan midcontinent rift plateau volcanics minnesota petrographic and field recognition the 8 km thick sequence of plateau lavas north shore volcanic group in northeastern minnesota that was erupted during the formation of the 1 1 ga midcontinent rift system includes an unusually large proportion 10 25 of felsic flows rhyolites and icelandites individual flows are now recognized with estimated volumes up to 600 km3 and lateral extents up to 40 km some were probably originally much broader many are anomalously large compared to most silicic lava flows primary structures and textures of these essentially unmetamorphosed rocks indicate eruptive style for most of these large flows in particular the features of the basal meter or so and uppermost several meters show that some flows were emplaced as fluid lavas and others as strongly rheomorphic tuffs it may be essential in interpreting the origin of felsic flows elsewhere to examine petrographically the lowermost few decimeters and the top where the only evidence poorly to strongly welded shards may exist of the pyroclastic origin of a large otherwise lava like flow interior textures of these crystalline flows provide evidence of high temperature eruption especially in the form of abundant paramorphs of magmatic tridymite in the groundmass commonly arranged in a pilotaxitic flow structure based on structures textures field relations and geochemistry we conclude that a all the icelandites and some rhyolites were emplaced as large moderately to very fluid lava flows that spread widely while crystallizing tridymite and that b some large rhyolites which are only sparsely phyric were emplaced as pyroclastic flows subsequently remobilized as rheoignimbrites the mode of emplacement of one of the largest aphyric flows remains enigmatic vent areas or characteristics are unknown the low apparent viscosity of the erupted magmas is probably attributable to compositional factors f content high fe2 as well as to high temperatures and eruption rates estimated temperatures of about 1000 1100 c are associated with the low water content and perhaps great depth of origin of the magmas at the time of massive influx of hot matic melts from the mantle during continental rifting

empirical relationships among seismic velocity effective pressure porosity and clay content in sandstone we use a multivariate analysis to investigate the influence of effective pressure pe porosity and clay content c on the compressional velocity vp and shear velocity vs of sandstones laboratory measurements on water saturated samples of 64 different sandstones provide a large data set that was analyzed statistically for each sample relationships between effective pressure and vp and vs have been determined all samples were well fit by relationships that have an exponential increase in velocity at low pe tapering to a linear increase with pe for pe greater than 0 2 kbar there are differences in the pressure dependences of velocity for different rocks particularly at very low pressures however the differences cannot be attributed to or c for the combined set of measurements from all samples the best fitting formulations are vp 5 77 6 94 1 73c 0 446 pe e 16 7pe and vs 3 70 4 94 1 57c 0 361 pe e 16 7pe while this is admittedly a very simplified parameterization it is remarkable how well

new insights into deformation and fluid flow processes in the nankai trough accretionary prism results of ocean drilling program leg 190 moore g f taira a klaus a becker l boeckel b cragg b a dean a fergusson c l henry p hirano s hisamitsu t et al 2001 new insights into deformation and fluid flow processes in the nankai trough accretionary prism results of ocean drilling program leg 190 geochemistry geophysics geosystems 2 article no 2001gc000166

geomorphology and evolution of the late pleistocene to holocene fluvial system in the south eastern llanos de moxos bolivian amazon in the bolivian amazon several paleochannel generations are preserved their wide spectrum of morphologies clearly provides crucial information on the type and magnitude of geomorphic and hydrological changes within the drainage network of the andean foreland therefore in this study we mapped geomorphological characteristics of paleochannels and applied radiocarbon and optically stimulated luminescence dating seven paleochannel generations are identified significant changes in sinuosity channel widths and river pattern are observed for the successive paleochannel generations our results clearly reflect at least three different geomorphic and hydrological periods in the evolution of the fluvial system since the late pleistocene changes in discharge and sediment load may be controlled by combinations of two interrelated mechanisms i spatial changes and re organizations of the drainage network in the upper catchment and or ii climate changes with their associated local to catchment scale modifications in vegetation cover and changes in discharge inundation frequencies and magnitudes which have likely affected the evolution of the fluvial system in the llanos de moxos in summary our study has revealed the enormous potential which geomorphic mapping and analysis combined with luminescence based chronologies hold for the reconstruction of the late pleistocene to recent fluvial system in a large portion of amazonia

reply to wolff and turbeville s comment on a reappraisal of ignimbrite emplacement progressive aggradation and particulate to non particulate flow transitions during emplacement of high grade ignimbrite by mj branney and p kokelaar branney and kokelaar 1992 emphasized that many features of pyroclastic flow deposits largely record the last processes involved in their formation so wolff and turbeville s glib reminder of this philosophy at the end of their comment is quite unnecessary many structures in high grade ignimbrites record non particulate flow that occurred some considerable time after particulate aggradation ceased this is not in question and we pointed this out in our paper it is the earlier deposition and deformation history of rheomorphic ignimbrites that is at issue this is the aspect that bears more widely on the nature of pyroclastic flows and related eruptive phenomena in another paper branney and kokelaar 1994 we have documented evidence for hotstate remobilization and deformation of some stationary rheomorphic ignimbrites by post emplacement disturbance caldera collapse however in branney and kokelaar 1992 we provided evidence that shows that for most rheomorphic ignimbrites it is inappropriate to assume a twofold flow history of hot remobilization after an initial flow came to rest progressive aggradation and agglutination provide the most straightforward explanation for many of the markedly irregular vertical variations in welding intensity characteristic of high grade ignimbrites just as progressive aggradation best accounts for vertical variations in sedimentary lithofacies and or in chemical composition in other ignimbrites

elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the nankai trough subduction megathrust 1 recent seismic reflection and ocean bottom seismometer obs studies reveal broad regions of low seismic velocity along the nankai subduction plate boundary megathrust offshore sw japan these low velocity zones lvz s extend 55 km landward from the trench corresponding to depths of 10 km below sea floor here we estimate the in situ pore pressure and stress state within these lvz s by combining p wave velocities obtained from the geophysical surveys with new well constrained empirical relations between p wave velocity porosity and effective mean stress defined by laboratory deformation tests on drill core samples of the incoming oceanic sediment we document excess pore pressures of 17 87 mpa that extend 55 km into the subduction zone indicating that trapped pore fluids support 45 91 of the overburden stress along the base of the upper plate and surrounding major fault zones the resulting effective stresses in the lvz are limited to 1 3 of the values expected for non overpressured conditions these low effective stresses should lead to a mechanically weak and predominantly aseismic plate boundary the region of lowest effective stress coincides with precisely located very low frequency earthquakes providing the first quantitative evidence linking these anomalous slip events to low stress and high pore pressure

a quantitative approach to linking drainage area and distributive fluvial system area in modern and ancient endorheic basins abstract regression relationships are derived relating the areal extent of large distributive fluvial systems dfss 30 km in length formed in endorheic basins to contributing drainage area the resulting correlation coefficients vary between 79 and 98 derived correlation values are strikingly similar when stratified by climate large scale tectonic regime and basin context regression equation exponent values range from 0 41 to 0 75 indicating a strong positive relationship between drainage area and dfs area while coefficients account for the influence of external parameters on the derived relationships analysis of the dataset using planform morphology and drainage basin slope as additional variables reveals that regressions for braided bifurcating dfs produce a highly significant relationship the relationship suggests that drainage basin relief influences sediment supply to the sedimentary basin in terms of volume or caliber which in turn affects the depositional gradient of the dfs surface and resultant channel planform an evaluation of the predictive capability of the regression equations reveals that the relationships derived from braided bifurcating dfss perform best with 80 of the predicted values falling within 25 of the measured dfs area value with a third of the predicted values within 10 of the measured dfs area value application of the regression relationships derived here to rock record examples of fluvial deposits interpreted as large dfss show that dfs area can be used as a proxy to predict the surface area of fluvially transported sediment deposited in a sedimentary basin from the contributing drainage basin area the regression relationships for modern drainage areas and corresponding dfs areas from a range of tectonic and climatic settings suggest a measurable link between source and sink in the sedimentary rock record the results provide a potential tool for more accurate reconstruction and prediction of preserved fluvial deposits and basin fill architecture within basin scale climatic and tectonic contexts

seismic velocities from the barbados ridge complex indicators of high pore fluid pressures in an accretionary complex three wide aperture 0 5 km offset seismic profiles were shot perpendicular to the strike of the barbados ridge accretionary complex at 16 12 n line 465 14 23 n line 480 and 13 20 n line 484 to image the deep structure of the accretionary complex and obtain seismic velocity data velocity analyses on full fold common depth point cdp gathers were used to provide velocity depth functions at 1 km separation along each line to map the variation in velocity with depth and distance across the accretionary complex the sediments within the complex show consolidation increasing laterally with greater age of the complex as indicated by the increase in seismic velocity and the inferred decrease in porosity the velocity at the base of the wedge along line 480 is 2 4 km s at the toe the velocity at the wedge base at 60 km from the toe where the wedge thickness has increased from 1 8 km to 5 0 km has increased to greater than 3 0 km s however the velocities within the leading margins and within undeithrust sequences in each of the three sections are low as much as 1 0 km s less than velocities found in undeformed sedimentary sequences of equivalent thickness the low velocity regions are interpreted as sediments with high porosity where porosity is inferred to be high pore fluid pressures are also inferred to exceed hydrostatic and pore fluids actually support the increased lithostatic load produced by recent thrusting and thickening of the complex the correlation between the most rapidly thickened regions and the velocity anomalies suggests that a rapid rate of loading relative to the rate of pore fluid expulsion produced these overpressured sediments velocities in the underthrust sequences where the differences between loading histories of wedge sediments and the underthrust sequences are largest are sufficiently low to produce velocity inversions low velocities also occur in regions adjacent to the complex where the pore fluid pressures may be elevated by the pressure produced in the adjacent wedge west of the wedge in the forearc basin along line 480 and near a seismically imaged mud diapir velocities at a depth of 3 5 km below the seafloor are 0 5 km s less than what is expected in a more normal sedimentary section immediately seaward of the complex the deepest undeformed sequences have a velocity anomaly of 0 5 km s velocities remain low for at least 12 km seaward of the complex but they become closer to a more normal sediment section farther from the wedge

anabranching rivers their cause character and classification anabranching rivers consist of multiple channels separated by vegetated semi permanent alluvial islands excised from existing floodplain or formed by within channel or deltaic accretion these rivers occupy a wide range of environments from low to high energy however their existence has never been adequately explained they occur concurrently with other types of channel pattern although specific requirements include a flood dominated flow regime and banks that are resistant to erosion with some systems characterized by mechanisms to block or constrict channels thereby triggering avulsion the fundamental advantage of an anabranching river is that by constructing a semi permanent system of multiple channels it can concentrate stream flow and maximize bed sediment transport work per unit area of the bed under conditions where there is little or no opportunity to increase gradient on the basis of stream energy sediment size and morphological characteristics six types of anabranching river are recognized types 1 3 are lower energy and types 4 6 are higher energy systems type 1 are cohesive sediment rivers commonly termed anastomosing with low w d ratio channels that exhibit little or no lateral migration they are divisible into three subtypes based on vegetative and sedimentary environment type 2 are sand dominated island forming rivers and type 3 are mixed load laterally active meandering rivers type 4 are sand dominated ridge forming rivers characterized by long parallel channel dividing ridges type 5 are gravel dominated laterally active systems that interface between meandering and braiding in mountainous regions type 6 are gravel dominated stable systems that occur as non migrating channels in small relatively steep basins anabranching rivers represent a relatively uncommon but widespread and distinctive group that because of particular sedimentary energy gradient and other hydraulic conditions operate most effectively as a system of multiple channels separated by vegetated floodplain islands or alluvial ridges

elasticity of marine sediments rock physics modeling we offer an effective medium model for the elastic moduli of high porosity ocean bottom sediments the elastic constants of the dry sediment frame depend on porosity elastic moduli of the solid phase and effective pressure the model connects two end points in the elastic modulus porosity plane the hertz mindlin modulus of a dense elastic sphere pack at critical porosity and zero at 100 porosity the elastic moduli of saturated sediment are calculated from those of the dry frame using gassmann s equation unlike the suspension model our model assigns non zero elastic constants to the dry sediment frame and can predict the shear wave velocity unlike various modifications of the travel time average equation it is first principle based and contains only physical parameters we justify this model by matching sonic data in shallow marine sediments and in an odp well

unconfined flow deposits in distal sectors of fluvial distributary systems examples from the miocene luna and huesca systems northern spain abstract thin sheet sandstone beds in a continental succession may be the products of unconfined overbank flow or deposition in lacustrine environments in the distal parts of fluvial distributary systems these two settings may be intercalated recording fluctuations in lake level in response to climatic changes field studies of the luna and huesca fluvial distributary systems in the miocene of the ebro basin spain are here used to characterise sheet sandstone deposited in distal fluvial and lake margin environments in this study we document the facies and relationships between 322 individual sandbodies deposited in both alluvial settings as sheetfloods and in lacustrine settings as deltaic lobes the alluvial sandstone sheets were deposited from lateral and frontal sheetflood events when unconfined flow replaced channelised flow as the main transport mechanism the term sheetflood is used here to refer to sub aerial unconfined turbulent flow events that undergo expansion thinning and deceleration with increasing radial distance from source the depositional process is similar to modern floodouts and terminal splays of the channel country and lake eyre central australia such a process was thought to be the dominant depositional mechanism during lake lowstands the sheets formed in the lacustrine setting are interpreted as deltaic sediments deposited in a shallow gradient lake similar to the modern day volga delta caspian sea shallow water friction dominated deltaic processes are thought to have been the primary depositional mechanism in the distal sectors of the fluvial systems during lake highstands

sedimentology and progressive tectonic unconformities of the sheetflood dominated hell s gate alluvial fan death valley california abstract the hell s gate alluvial fan of northern death valley has an area of 49 5 km2 a radius of 11 8 km and a smooth 5 3 sloping surface interrupted by shallow

characterization of excess pore pressures at the toe of the nankai accretionary complex ocean drilling program sites 1173 1174 and 808 results of one dimensional modeling 1 elevated fluid pore pressures play a critical role in the development of accretionary complexes including the development of the decollement zone in this study we used measured permeabilities of core samples from ocean drilling program odp leg 190 to develop a permeability porosity relationship for hemipelagic sediments at the toe of the nankai accretionary complex this permeability porosity relationship was used in a one dimensional loading and fluid flow model to simulate excess pore pressures and porosities simulated excess pore pressure ratios as a fraction of lithostatic pressure hydrostatic pressure using the best fit permeability porosity relationship were lower than predicted from previous studies we then tested sensitivity of excess pore pressure ratios in the underthrust sediments to bulk permeability lateral stress in the prism and a hypothetical low permeability barrier at the decollement our results demonstrated significant increase in pore pressures below the decollement with lower bulk permeability such as obtained by using the lower boundary of permeability porosity data or when a low permeability barrier is added at the decollement in contrast pore pressures in the underthrust sediments demonstrated less sensitivity to added lateral stresses in the prism although the profile of the excess pore pressure ratio is affected both simulations with lateral stress and a low permeability barrier at the decollement resulted in sharp increases in porosity at the decollement similar to that observed in measured porosities furthermore in both scenarios maximum excess pore pressure ratios were found at the decollement suggesting that either of these factors would contribute to stable sliding along the decollement

channel avulsion on alluvial fans in southern arizona historical aerial photographs and field observations on five fluvially dominated alluvial fans in southern arizona demonstrate that channel avulsion invariably occurs where bank heights are low and often at channel bends channel abandonment occurs through stream capture when overland flow from the main channel accelerates and directs headward erosion of smaller channels heading on the fan surface five distinct channel morphologies observed on the fans are related to different stages of the avulsion process and can be used to identify areas on a fan surface that are prone to avulsion a descriptive model of channel avulsion illustrates how the morphology of a single channel reach will evolve through time as it captures the main flow path and is itself eventually abandoned immediately following avulsions small preexisting channels that capture flow from the main channel will typically experience three fold or greater increases in channel width subsequent large floods can be stably conveyed through these high capacity reaches an uninterrupted sequence of sediment charged small flows however will eventually begin to back fill the wide channels as vegetation growth stabilizes the banks the stabilized and back filled channels are now prone to abandonment during large floods because the decrease in the channel s capacity leads to the generation of overland flow beyond the margins of the shallowed channels the action of the small aggrading floods is critical in the avulsion process since the greatest amount of overland flow is generated where bank heights are lowest as a result both small and large floods are effective agents of landscape change on the fans channel avulsions on the five fans are not completely random events in space and time because their occurrence is controlled by the relative positioning of low banks along the main channel and smaller channels draining the fan surface consequently the location and timing of future channel avulsions can potentially be anticipated in an effort to improve flood hazard assessment on fluvial fans in the rapidly urbanizing southwestern united states

facies analysis of a probable large fluvial fan depositional system the upper triassic chinle formation at petrified forest national park arizona u s a abstract the upper triassic blue mesa member and the overlying sonsela member of the chinle formation consist of paleosol bearing alluvial strata whose characteristics vary markedly strata exposed within a 4 km2 study area at petrified forest national park were evaluated using sedimentologic stratigraphic paleopedologic and petrographic criteria to formulate potential depositional models that may account for varying alluvial deposition the study succession records a progressive up section increase in grain size increase in channel depth and width increase in lateral and vertical connectivity of channel deposits decrease in overbank preservation and crevasse splay and or sheetflood deposition and increase in overbank drainage mean annual precipitation from the calmag paleosol weathering proxy suggests precipitation averages approximately 1300 mm yr 1 3 m yr throughout the succession despite changing paleosol drainage these features are consistent with deposition in a progradational fluvial system such as a large fan or splay complex sediment accumulation rates in the study interval suggest decreased subsidence in the upper blue mesa member reduced rates of accommodation gain may have promoted progradation of the fluvial system an up section decrease in sandstone mineralogical maturity may be related to increasing sediment bypass in proximal fan positions or increased rates of erosion and sedimentation increasingly better drained paleosols are attributed to deposition in more upland positions during fan or avulsion complex progradation rather than changing climate

elevated fluid pressure and extreme mechanical weakness of a plate boundary thrust nankai trough subduction zone pore fluid pressure is a key parameter governing both the shear strength of faults and the conditions for seismic and tsunamigenic slip on subduction zone megathrusts however quantitative constraints on this parameter based on in situ data in these or any faults have proven elusive using seismic interval velocities derived from a three dimensional seismic reflection survey we compute porosity fluid pressure and effective stress at the plate interface in the nankai trough subduction zone offshore southwestern japan to 20 km down the fault dip using well constrained locally calibrated empirical relationships between velocity porosity and effective stress we show that the fault and immediately subjacent footwall are nearly undrained implying that the subduction megathrust slips under a shear stress of

ancient ephemeral stream deposits a local terminal fan model from the bunter sandstone formation l triassic in the tonder 3 4 and 5 wells denmark

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

significance of interdune deposits and bounding surfaces in aeolian dune sands bounding surfaces and interdune deposits provide keys for detailed interpretations of the development shape type wavelength and angle of climb of aeolian bedforms as well as overall sand sea conditions current alternate interpretations of bounding surfaces require very different but testable models for sand sea deposition two perpendicular traverses of jurassic entrada sandstone utah reveal relations among cross strata first order bounding surfaces and horizontal strata these field relations seem explicable only as the deposits of downwind migrating climbing enclosed interdune basins horizontal strata and dune bodies consisting of superimposed smaller crescentic dunes cross stratified deposits a 1 7 km traverse parallel to the palaeowind direction provides a time transgressive view showing continuous cosets of cross strata first order bounding surfaces and interdune deposits climbing downwind at an angle of a few tenths of a degree changes occur in the angle of climb cross strata structure and interdune deposits these reflect changes in depositional conditions through time a 1 5 km traverse perpendicular to the palaeowind direction provides a view at an instant in geological time showing first order bounding surfaces and interdune deposits forming flat laterally discontinuous lenticular bodies the distribution of interdune sedimentary structures in this traverse is very similar to that of some modern interdune basins such as those on padre island texas hierarchies of bounding surfaces in an aeolian deposit reflect the bedform development on an erg the presence of three orders of bounding surfaces indicates dune bodies consisting of smaller super imposed dunes the geometry of first order bounding surfaces is a reflection of the shape of the inter dune basins second order bounding surfaces originate by the migration of the superimposed dunes over the larger dune body and reflect individual dune shape and type third order bounding surfaces are reactivation surfaces showing stages in the advance of individual dunes the presence of only two orders of bounding surfaces indicates simple dunes modern and entrada interdune deposits show a wide variety of sediment types and structures reflecting deposition under wet damp and dry conditions interdune deposits are probably the best indicators of overall erg conditions and commonly show complex vertical sequences reflecting changes in specific depositional conditions

welding and rheomorphism of phonolitic fallout deposits from the las canadas caldera tenerife canary islands the las canadas caldera is a nested collapse caldera formed by the successive migration and collapse of shallow magmatic chambers among the pyroclastic products of this caldera are phonolitic fallout deposits that crop out in the caldera wall and on the extracaldera slopes these deposits exhibit an uninterrupted facies gradation from nonwelded to lava like and record continuous volcanic deposition densely welded and lava like facies result from the extreme attenuation and complete homogenization of juvenile clasts that destroy original clast outlines and any evidence of fallout deposition agglutination contributes significantly to the final degree of flattening observed in the welded facies after deposition rheomorphic flowage occurs emplacement temperatures for one of the welding sequences are calculated from magmatic temperatures and a model of tephra cooling during fallout results are 486 c for the nonwelded facies and 740 c for the moderately welded facies for the same welding sequence a cooling time between 25 and 54 days is estimated from published experimental and computational data as the possible duration of welding and rheomorphism following deposition and agglutination the lava like pyroclastic facies had the rheological properties of viscous lavas and flowed down the outer slopes away from the caldera some lava like masses detached from proximal areas to more distal regions during deposition the eruptive style evolved from plinian fallout to fountain fed spatter deposition this evolution was accompanied by a decrease in explosive power and a lower height of the eruptive column which produce higher emplacement temperatures and more effective heat retention of pyroclasts

stratigraphic evolution and preservation of aeolian dune and damp wet interdune strata an example from the triassic helsby sandstone formation cheshire basin uk new and previously published models of wet aeolian system evolution form a spectrum of types that may be explained in terms of aeolian dune dynamics rate of water table rise and or periodicity of interdune flooding this is illustrated with an example from the mid triassic anisian helsby sandstone formation cheshire uk lenses of damp and wet interdune strata exhibit an intertonguing transitional relationship with the toe sets of overlying aeolian dune units this signifies dune migration that was contemporaneous with water table controlled accumulation in adjacent interdunes downwind changes in the geometry and facies of the interdune units indicate periodic expansion and contraction of the interdunes in response to changes in the elevation of the groundwater table and episodic flooding during which accumulation of dune strata continued relatively uninterrupted this contrasts with other models for accumulation in wet aeolian systems where interdune flooding is associated with a cessation in aeolian bedform climbing and the formation of a bypass or erosional supersurface architectural panels document the detailed stratigraphy in orientations both parallel and perpendicular to aeolian transport direction enabling a quantitative three dimensional reconstruction of genetically related aeolian dune and interdune elements sets of aeolian dune strata are composed of grainflow and translatent wind ripple strata and are divided by a hierarchy of bounding surfaces originating from oblique migration of superimposed dunes over slipfaceless sinuous crested parent bedforms together with lee slope reactivation under non equilibrium flow conditions silty mudstone and sandstone interdune units are characterized by wind ripple wavy and subaqueous wave ripple laminae desiccation cracks mud flakes raindrop imprints load casts flutes intraformational rip up clasts and vertebrate and invertebrate footprint impressions and trackways these units result from accumulation on a substrate that varied from dry through damp to wet surface conditions interdune ponds were flooded by either fluvial incursions or rises in groundwater table and were periodically subject to gradual desiccation and reflooding red silty mudstone beds of subaqueous origin pass laterally into horizontally laminated wind ripple beds indicating a progressive transition from wet through damp to dry surface conditions within a single interdune

spatial and temporal evolution of a terminal fluvial fan system the permian organ rock formation south east utah usa the recognition of terminal fluvial systems otherwise termed terminal fans or distributary fluvial fan systems preserved in the ancient rock record is based primarily on the recognition of facies characteristics indicative of a progressive downstream decrease in i fluvial discharge ii channel depth and width iii lateral and vertical connectivity of channel fill elements and iv evidence for channellized flow and a systematic increase in i evidence for sheetflood deposition ii aeolian and or playa deposits and iii channel bifurcation however despite these criteria having been applied previously to a variety of outcrop successions there is still no unifying facies model that adequately accounts for the complex stratigraphic architectural relationships expected for such systems based on the varied styles of fluvial activity and system interaction known from modern examples moreover few previous studies have given significant consideration to the long term temporal evolution of terminal fluvial fans these issues are addressed by this study of the permian leonardian artinskian organ rock formation of the paradox basin south east utah a detailed stratigraphic framework based on 84 sedimentary logs demonstrates proximal to distal variations in sedimentary style integration of these data with high resolution architectural panels depicting the geometry and facies characteristics of individual fluvial elements has enabled the development of a series of depositional models that account for both the spatial and temporal evolution of the system and which are representative of i initial progradation of the fluvial system into the paradox foreland basin ii retreat of the fluvial system and expansion of a distal aeolian dune system iii the final phase of fluvial progradation following aeolian dune deflation and iv the final retrogradation of the fluvial system back towards the hinterland

anatomy and facies distribution of terminal lobes in ephemeral fluvial successions jurassic tordillo formation neuquen basin argentina in terminal fluvial fan systems characteristic proximal to distal variations in sedimentary architectures are recognized to arise from progressive downstream loss of water discharge related to both infiltration and evaporation this work aims to elucidate downstream trends in facies and architecture across the medial and distal zones of terminal fan systems which record transitions from deposits of channel elements to lobe like and sheet like elements this is achieved via a detailed characterization of ancient ephemeral fluvial deposits of the well exposed kimmeridgian tordillo formation neuquen basin argentina the fine sand prone and silt prone succession associated with the medial to distal sectors of the system has been studied to understand relationships between depositional processes and resulting architectures facies and architectural element analyses and quantification of resulting sedimentological data at multiple scales have been undertaken to characterize sedimentary facies facies transitions bed types architectural elements and larger scale architectural styles eight bed types with distinct internal facies transitions are defined and interpreted in terms of different types of flood events channelized and non channelized architectural elements are defined based on their constituent bed types and their external geometry the most common elements are terminal lobes which are composite bodies within which largely unconfined sandy deposits are stacked in a compensational manner a hierarchical arrangement of internal components is recognized proximal feeder channel avulsion events likely controlled the evolution of terminal lobe elements and their spatiotemporal shifts stratigraphic relations between architectural elements record system wide trends whereby a proximal sector dominated by channel elements passes downstream via a gradational transition to a medial sector dominated by sandy terminal lobe elements which in turn passes further downstream to a distal sector dominated by silty terminal lobe margin and fringing deposits this work enhances current understanding of the stratigraphic record of terminal fluvial systems at multiple scales and provides insight that can be applied to predict the facies and architectural complexity of terminal fluvial successions

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

effective stress and pore pressure in the nankai accretionary prism off the muroto peninsula southwestern japan 1 we developed a theoretical method for predicting effective stress and pore pressure based on rock physics model we applied the method to reveal the pore pressure distribution within the nankai accretionary prism off southwestern japan and to investigate variations in pore pressure associated with evolution of the plate boundary decollement from the crack aspect ratio spectrum estimated from laboratory and well log data we calculated a theoretical relationship between acoustic velocity and mean effective stress by using differential effective medium theory by iteratively fitting the theoretically calculated velocity to the seismic velocities derived from 3d tomographic inversion we estimated in situ mean effective stress within the accretionary prism pore pressure is then the difference between the effective stress and the confining stress when we calculated pore pressure we considered compressive state of stress in the accretionary prism our results confirm that pore fluid pressure is high within the subducting sedimentary sequence below the decollement we determined a normalized pore pressure ratio of 0 4 0 7 abnormal pore pressures develop in the under thrust sequence as a result of the increase in overburden load because of the thickened overlying prism and a low permeability barrier across the decollement overpressuring within the accreted sequence is initiated at the deformation front and proceeds landward the increase in horizontal compaction within the accreted sequence may raise pore pressures within the accreted sequence and the pore pressure mean effective stress contrast at the decollement becomes smaller landward of the deformation front

a quantitative model for deposition of thin fluvial sand sheets sheetfloods are typically invoked as the mechanism responsible for the kilometre scale transport of sand sized sediment grains in shallow gradient fluvial systems this concept is based on the lateral extent of ancient thin sheet sandstone deposits rather than on fluid dynamics which has resulted in a loosely constrained model for sheetfloods this study tested the conceptual mechanism by developing a depth averaged 2d computational fluid dynamics model the model results compare well against observations from modern deposits at lake eyre to provide a quantitative physically sound basis for sheetfloods that can be applied in ancient and modern settings to constrain otherwise qualitative interpretations

landscape and depositional controls on palaeosols of a distributive fluvial system upper cretaceous brazil abstract the stratigraphic record of distributive fluvial systems is commonly characterised by frequent and complex interstratification of palaeosols among channel and overbank deposits however current models focus primarily on sedimentation and pay only limited attention to palaeopedogenesis thereby failing to incorporate important palaeoenvironmental and stratigraphic information this study proposes a pedosedimentary model for distributive fluvial systems that depicts and accounts for two palaeopedogenetic trends one downdip in relation to distality from the fan apex and one along strike in relation to distance from active channel belts palaeosols are reported in detail from an upper cretaceous succession of the bauru basin southeastern brazil through the application of macro micromorphological and geochemical studies combined with facies and architectural element analyses of sediments in the downdip palaeopedogenetic trend the proximal zone of the depositional system is characterised by a dominance of well drained inceptisols that develop on amalgamated channel fills in the medial zone inceptisols occur interlayered with overbank deposits containing entisols and poorly drained vertisols the distal zone preserves more mature and poorly drained inceptisols developed on deposits of overbank and sporadic distal channel fills these pedotypes show an increase in maturity and hydromorphism moving away from the apex to the fan toe this is likely linked to i the progressive approach of the topographic surface to the water table and ii the average increase in distance to an active channel belt in distal locations the along strike palaeopedogenetic trend culminates in poorly developed palaeosols in floodplain regions that correspond to topographic depressions located between channel belts and which were subject to recurrent floods because palaeopedogenesis in the floodplain region is penecontemporaneous to sedimentation pedotypes show an increase in maturity bioinduced calcification and hydromorphism with distance from the active channels they pass laterally from entisols and inceptisols near active channels to vertisols away from active channels conversely following avulsion abandoned channel belts remain as topographically elevated alluvial ridges located at some distance from the newly active channels and positioned above the water table and this leads to the development of better drained and better developed inceptisols relative to pedotypes of the floodplain region overall both palaeopedogenetic trends demonstrate the overriding controls of topography sedimentation rate and parent material on pedogenesis with only minor climatic influence this work offers a novel pedosedimentary model for distributive fluvial systems and highlights the palaeoenvironmental significance of palaeosol trends providing new constraints for the recognition of distributive fluvial systems in the rock record

distinguishing strongly rheomorphic tuffs from extensive silicic lavas high temperature silicic volcanic rocks including strongly rheomorphic tuffs and extensive silicic lavas have recently been recognized to be abundant in the geologic record however their mechanisms of eruption and emplacement are still controversial and traditional criteria used to distinguish conventional ash flow tuffs from silicic lavas largely fail to distinguish the high temperature versions we suggest the following criteria ordered in decreasing ease of identification to distinguish strongly rheomorphic tuffs from extensive silicic lavas 1 the character of basal deposits 2 the nature of distal parts of flows 3 the relationship of units to pre existing topography and 4 the type of source as a result of quenching against the ground basal deposits best preserve primary features can be observed in single outcrops and do not require knowing the full extent of a unit lavas commonly develop basal breccias composed of a variety of textural types of the flow in a finer clastic matrix such deposits are unique to lavas because the chilled base of an ashflow tuff generally does not participate in secondary flow primary pyroclastic features are best preserved there massive flow banded bases are more consistent with a lava than a pyroclastic origin lavas are thick to their margins and have steep abrupt flow fronts ashflow tuffs thin to no more than a few meters at their distal ends where they generally do not show any secondary flow features lavas are stopped by topographic barriers unless the flow is much thicker than the barrier ash flow tuffs moving at even relatively slow velocities can climb over barriers much higher than the resulting deposit lavas dominantly erupt from fissures and maintain fairly uniform thicknesses throughout their extents tuffs commonly erupt from calderas where they can pond to thicknesses many times those of their outflow deposits these criteria may also prove effective in distinguishing extensive silicic lavas from a postulated rock type termed lava like ignimbrite the latter have characteristics of lavas except for great areal extents up to many tens of kilometers these rocks have been interpreted as ash flow tuffs that formed from low boiling over eruption columns based almost entirely on their great extents and the belief that silicic lavas could not flow such distances however we interpret the best known examples of lava like ignimbrites to be lavas this interpretation should be tested through additional documentation of their characteristics and research on the boiling over eruption mechanism and the kinds of deposits it can produce flow bands flow folds ramps elongate vesicles and probably upper breccias occur in both lavas and strongly rheomorphic tuffs and are therefore not diagnostic pumice and shards also occur in both tuffs and lavas although they occur throughout ash flow tuffs and generally only in marginal breccias of lavas dense welding secondary flow and intense alteration accompanying crystallization at high temperature commonly obliterate primary textures in both thick rheomorphic tuffs and thick lavas high temperature silicic volcanic rocks are dominantly associated with tholeiitic flood basalts extensive silicic lavas could be appropriately termed flood rhyolites

root zone of the late proterozoic salma caldera northeastern arabian shield kingdom of saudi arabia the eroded root of the late proterozoic salma caldera crops out in a striking roughly elliptical feature about 27 km long and 22 km wide near the northeastern edge of the arabian shield the caldera is genetically part of an elongate alkalic granitic massif jabal salma that extends 35 km from the caldera to the southwest comenditic ash flow tuff and lava of the caldera fill probably more than 1 km thick are the oldest recognized rocks of the caldera complex these rocks were erupted during caldera collapse associated with the rapid evacuation of the upper mildly peralkalic part of a zoned magma reservoir within the caldera fill a massive lithic rich intracaldera rhyolite probably a lava in excess of 1 km thick is overlain by a layered ash flow sequence numerous megabreccia blocks probably derived from the caldera wall occur in the massive rhyolite open folds in the layered volcanic rocks may be due to high temperature slumping of the rocks toward the center of the caldera following collapse later peralkalic granite that intruded the caldera ring fracture zone occurs in an arcuate pattern outside the area of exposed caldera fill after caldera collapse metaluminous to peraluminous magma rose beneath the caldera at approximately 580 ma and solidified as biotite alkali granite rim syenogranite and late high level granophyre rare earth element abundances indicate that the layered rhyolite tuff peralkalic granite and granophyre are chemically more evolved than the biotite alkali granite and rim syenogranite the granophyre intruded the caldera fill as a dome shaped body composed of numerous sheetlike masses granophyric texture resulted from rapid pressure release and quenching accompanying the intrusion of each sheet maximum penetration of the granophyre into overlying rocks occurred in the central region and along the west side of the caldera where the caldera fill volcanic rocks have been removed by erosion no apparent structural doming of the exposed volcanic rocks along the east side of the caldera took place the layered ash flows commonly dip steeply toward the center of the caldera postemplacement deformation and metamorphism of the caldera are minimal small displacement strike slip faults cut the complex which is tilted to the northeast by no more than about 2

periodic accumulation and destruction of aeolian erg sequences in the permian cedar mesa sandstone white canyon southern utah usa the permian cedar mesa sandstone represents the product of at least 12 separate aeolian erg sequences each bounded by regionally extensive deflationary supersurfaces facies analysis of strata in the white canyon area of southern utah indicates that the preserved sequences represent erg centre accumulations of mostly dry though occasionally water table influenced aeolian systems each sequence records a systematic sedimentary evolution enabling phases of aeolian sand sea construction accumulation deflation and destruction to be discerned and related to a series of underlying controls sand sea construction is signalled by a transition from damp sandsheet ephemeral lake and palaeosol deposition through a phase of dry sandsheet deposition to the development of thin chaotically arranged aeolian dune sets the onset of the main phase of sand sea accumulation is reflected by an upward transition to larger scale ordered sets which represent the preserved product of climbing trains of sinuous crested transverse dunes with original downwind wavelengths of 300 400 m regularly spaced reactivation surfaces indicate periodic shifts in wind direction which probably occurred seasonally compound co sets of cross strata record the oblique migration of superimposed slipfaced dunes over larger slipfaceless draa each aeolian sequence is capped by a regionally extensive supersurface characterized by abundant calcified rhizoliths and bioturbation and which represents the end product of a widespread deflation episode whereby the accumulation surface was lowered close to the level of the water table as the sand sea was progressively cannibalized by winds that were undersaturated with respect to their potential carrying capacity aeolian sequence generation is considered to be directly attributable to cyclical changes in climate and related changes in sea level of probable glacio eustatic origin that characterize many permo carboniferous age successions sand sea construction and accumulation occurred during phases of increased aridity and lowered sea level the main sand supply being former shallow marine shelf sediments that lay to the north west sand sea deflation and destruction would have commenced at or shortly after the time of maximum aridity as the available sand supply became exhausted restricted episodes of non aeolian accumulation would have occurred during humid interglacial phases accumulation and preservation being enabled by slow rises in the relative water table subsidence analysis within the paradox basin together with comparisons to other similar age successions suggests that the climatic cycles responsible for generating the cedar mesa erg sequences could be the product of 413 000 years so called long eccentricity cycles by contrast annual advance cycles within the aeolian dune sets indicate that the sequences themselves could have accumulated in just a few hundred years and therefore imply that the vast majority of time represented by the cedar mesa succession was reserved for supersurface development

porosity corrections for smectite rich sediments impact on studies of compaction fluid generation and tectonic history porosity is a fundamental parameter that must be correctly determined in order to relate physical property hydrologic and chemical flux studies to natural systems traditional porosity determinations generated from physical property or seismic data can greatly overestimate the true porosity of sediments in which hydrated minerals such as smectite are abundant to produce a true porosity distribution such data must be corrected to account for the h 2 o residing in smectite interlayers which can make up to 25 of the total hydrated mineral mass such h 2 o is easily removed from the mineral by oven drying and or exposure to low humidities standard physical property measurements can be corrected provided the weight percent smectite in the sediment is known and the interlayer h 2 o content of the mineral can be estimated we illustrate the significant consequences of this correction by comparing profiles of reported and corrected porosities for smectite rich barbados abyssal plain and accretionary wedge sediments from ocean drilling program leg 110 sites 671 and 672

variable style of transition between palaeogene fluvial fan and lacustrine systems southern pyrenean foreland ne spain two palaeogene fluvial fan systems linked to the south pyrenean margin are recognized in the eastern ebro basin the cardona su ria and solsona sanau ja fans these had radii of 40 and 35 km and were 800 and 600 km2 in area respectively during the priabonian to the middle rupelian the fluvial fans built into a hydrologically closed foreland basin and shallow lacustrine systems persisted in the basin centre in the studied area both fans are part of the same upward coarsening megasequence up to 800 m thick driven by hinterland drainage expansion and foreland propagation of pyrenean thrusts fourteen sedimentary facies have been grouped into seven facies associations corresponding to medial fluvial fan channelized terminal lobe nonchannelized terminal lobe mudflat deltaic evaporitic playa lake and carbonate rich shallow lacustrine environments lateral correlations define two styles of alluvial lacustrine transition during low lake level stages terminal lobes developed whereas during lake highstands fluvial dominated deltas and interdistributary bays were formed terminal lobe deposits are characterized by extensive 100 600 m wide sheet like fine sandstone beds formed by sub aqueous quasi steady hyperpycnal turbidity currents sedimentary structures and trace fossils indicate rapid desiccation and subaerial exposure of the lobe deposits these deposits are arranged in coarsening fining sequences metres to tens of metres in thickness controlled by a combination of tectonics climatic oscillations and autocyclic sedimentary processes the presence of anomalously deeply incised distributary channels associated with distal terminal lobe or mudflat deposits indicates rapid lake level falls deltaic deposits form progradational coarsening upward sequences several metres thick characterized by channel and friction dominated mouth bar facies overlying white grey offshore lacustrine facies deltaic bar deposits are less extensive 50 300 m wide than the terminal lobes and were also deposited by hyperpycnal currents although they lack evidence of emergence sandy deltaic deposits accumulated locally at the mouths of main feeder distal fan streams and were separated by muddy interdistributary bays whereas the terminal lobe sheets expand from a series of mid fan intersection points and coalesced to form a more continuous sandy fan fringe

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

fluvial processes and cyclicity in terminal fan deposits an example from the late devonian of southwest ireland abstract the gun point formation is a sandstone dominated sequence approximately 1 5 2 5 km thick which forms part of a major 6 km late devonian alluvial basin fill succession in southwest ireland it is interpreted as the deposits of a southerly flowing terminal fan system which operated under a semi arid climate the proximal zone was characterized by low sinuosity gravelly feeder channels which passed downstream into a medial zone dominated by a sandy braidplain further south in the distal zone phases of high energy flood activity were centred on transient low sinuosity ephemeral channels and were separated by more quiescent periods of shallow sheetflooding subaerial exposure and non deposition sedimentary parameters such as set thickness coset thickness channel sand body density and maximum sand body thickness all exhibit regular cyclic fluctuations with a periodicity of 160 180 m the cycles are interpreted as evidence of regular discharge variations which may have been a response to the 412 ka orbital eccentricity cycle a second periodicity of approximately 700 m has also been detected and is thought to reflect long term variations in basin subsidence

multi scale classification of fluvial architecture an example from the palaeocene eocene bighorn basin wyoming this work was supported by phase 2 of the fluvial systems research group bp bg chevron conocophillips and total mgms thanks the sao paulo research foundation fapesp 2014 13937 3 the authors also wish to thank numerous residents of the bighorn basin for their kind hospitality and access to land we also thank john howell for discussions on fluvial geometries and nomenclature and isobel buchanan and alistair swan for assistance in the field comments from reviewers andrew miall luca colombera and an anonymous reviewer greatly improved this manuscript

a discrete episode of seismic and aseismic deformation of the nankai trough subduction zone accretionary prism and incoming philippine sea plate new insights into episodic deformation at the nankai trough subduction zone are provided by data from two ocean drilling program borehole hydrologic observatories drilled into the philippine sea plate site 1173 and the seaward part of the nankai accretionary prism off southwestern japan site 808 and from an array of high sensitivity borehole accelerometer and velocity seismometers on shikoku island hi net fluid pressures monitored at multiple levels in each of the offshore boreholes document steady secular trends that indicate contraction of the crust and sedimentary section at the incoming plate site and relaxation of the accretionary prism toe the rate of strain ca 10 6 yr 1 inferred from the rates of pressure rise at site 1173 up to 5 kpa yr 1 is similar to that which would be produced by plate convergence if convergent elastic strain were distributed over a region a few tens of km wide opposing these trends are transient pressure anomalies observed in late june early july 2003 that indicate an episode of rapid relaxation of the incoming plate and contraction of the outer prism concurrent with these transients a swarm of very low frequency earthquakes occurred farther landward in the prism above the estimated seaward limit of the currently locked seismogenic portion of the subduction thrust reverse fault mechanisms determined for one event of this swarm and several of another also indicate transient contraction of the prism we suggest that both the earthquakes and the pressure transients are the consequences of an aseismic slip dislocation that initiated on about june 26 2003 at or near the up dip limit of the locked portion of the subduction thrust and propagated seaward over the course of about 10 days to the accretionary prism toe along the decollement separating the prism and underthrust section deformational events like this may serve incrementally to relieve stress locally along the subduction thrust and to load neighboring areas in addition to demonstrating that the prism is far from inactive during interseismic intervals the observations may also provide a small amplitude analog for strain and hydrologic response at the time of great subduction earthquakes

episodic deposition and preservation of eolian sands a late paleozoic example from southeastern utah the upper pennsylvanian and lower permian stratigraphic sequence in canyonlands national park contains about 40 flat topped eolian sand bodies rhizoliths burrows and traces of former evaporites abound at the tops of these tabular genetic units upper surfaces of units were created by eolian deflation to the level of the groundwater table analysis of cross stratification reveals that sands accumulated via migration and climb of relatively small dunes brief episodes of sand accumulation were triggered by regression and were terminated when upwind sand supplies were depleted upper surfaces of genetic units are diastems and represent much more geologic time than the rocks themselves

characteristics of gravel bars in ephemeral streams abstract bars are major sediment storage places within channels and much is known about their character from proglacial and temperate regions little is know however about bar characteristics from ephemeral streams this study describes the morphology and composition of simple and compound gravel bars in nahal zin israel an ephemeral desert stream a hierarchy of microscale mesoscale and macroscale structures and bed forms was identified in nahal zin that is similar to the hierarchy described for perennial streams while the hierarchy is similar between arid and perennial streams bar composition patterns are distinct compound bars and unit bars in humid areas are known to have armored surfaces composed of structured gravels whereas the bar surfaces of both unit bars and compound bars in nahal zin were found to lack surface structures and to contain a large amount of sand in ephemeral streams both accreted and dissected compound bars do not display any downstream fining in contrast downstream fining along bars in humid areas is well documented the flashy nature of floods high sediment supply high sediment transport rates and large amount of sand associated with the bars are likely reasons the surfaces were not armored in nahal zin erosion and accretion dominate the bar edges and accretion alone dominates the platform part of the bars in summary sediments in ephemeral streams of the nahal zin type are well mixed unsorted and unlikely to leave a well defined stratigraphic record these characteristics are distinct from their counterparts in proglacial and temperate regions and may be useful when investigating paleoenvironmental conditions

welded tuffs deformed into megarheomorphic folds during collapse of the mcdermitt caldera nevada oregon the montana mountains contain part of a resurgent dome complex within the southwestern sector of the mcdermitt caldera exposed intracaldera units consist of welded tuffs lavas domes and epiclastic deposits the majority of volcanic units have a comenditic composition but a few units are nonperalkaline and biotite bearing a deformed series of comenditic welded tuffs occur in a strongly folded rheomorphic zone more than 500 m thick the geometry of vitrophyres flow bands foliations and lineations causes the rheomorphic zone to superficially resemble a series of intrusive domes however the contacts between units are conformable the vitrophyres are continuous from one fold to the next and the units display pumice shards and other features common in welded tuffs the deformed tuff sequence lies on top of biotite tuffs and epiclastic deposits that are tilted and faulted but not folded and beneath relatively undeformed tuffs and lake deposits this relationship suggests that their large scale folds in the deformed sequence developed shortly after caldera collapse

critical porosity a key to relating physical properties to porosity in rocks discovering accurate relations between porosity and seismic wave velocities in porous rocks has been an important topic of rock physics for decades such relations if they actually exist are critical when seismic and or sonic data are to be used to infer porosity and porosity related properties e g permeability strength saturation in situ these inferences became especially important in recent years with the rapid development of seismic and sonic methods which are applied to the characterization of heterogeneous reservoirs and recovery monitoring

pore pressure estimation from mudrock porosities in tertiary basins southeast asia porosity reduction during mechanical compaction of a sediment generally has been assumed to be controlled by the increase in vertical effective stress which is convenient because vertical stress profiles may be readily calculated from density logs poroelasticity theory shows however that mean effective stress controls porosity reduction according to published data horizontal stresses increase with overpressure as well as with depth so mean stress and vertical stress profiles are poorly correlated in overpressured sections we have used wireline logs to compare the pore pressures estimated in mudrocks by relating porosity to mean effective stress and to vertical effective stress for overpressured tertiary sections in southeast asia wells from three different basins were studied mudrock porosities were estimated from the sonic log response and sorted by lithology according to the natural gamma log response two sets of normal compaction curves relating porosity to mean effective stress and to vertical effective stress were determined empirically by fitting data points where the pore pressure was thought to be hydrostatic these curves were then used to estimate the minimum pore pressure corresponding to mudrock porosity values in the overpressured sections the pore pressures inferred using the mean effective stress are consistent with direct measurements of pore pressure in the adjacent sands in contrast pore pressures inferred in mudrocks using the vertical effective stress are significantly lower for the overpressured sections implying discontinuities in the pore pressure profiles at lithological boundaries which cannot readily be explained we conclude that the pore pressures estimated using the vertical effective stress are wrong and that empirical relationships between porosity and vertical effective stress should not be used for estimating pore pressures porosity should be empirically related to mean effective stress instead

a stratigraphic model to account for complexity in aeolian dune and interdune successions wet aeolian systems in which the water table or its capillary fringe are in contact with the accumulation surface such that moisture influences sedimentation are well known from modern aeolian systems and several ancient preserved successions are recognized from outcrop one common mechanism by which accumulation of wet aeolian system deposits occurs is via a progressive rise in the relative water table level that is coincident with ongoing dune and interdune migration the angle of dune climb being determined by the ratio between the rate of relative water table rise and the rate of downwind migration of the bedforms accumulations of wet aeolian system deposits tend to be characterized by units of climbing dune strata separated by units of damp or wet interdune strata for simple geometric configurations where the size of the dune and interdune units the rate of bedform migration and the rate of aggradation all remain constant over space and time the resulting accumulation has a simple architecture characterized by sets of uniform thickness inclined at a constant angle however the dynamic nature of most aeolian dune systems means that such simple configurations are unlikely in nature the complexity inherent in these systems is accounted for here by a numerical model in which key controlling parameters including dune and interdune wavelength and spacing migration rate and aggradation rate are allowed to vary systematically both spatially from a dune field centre to its margin and temporally in response to changes in sediment availability or water table level the range of synthetic stratigraphic architectures generated by the model accounts for all the best known examples of aeolian dune and interdune stratigraphic configurations documented from the stratigraphic record modelling results have enabled the erection of a scheme for the classification of dune system type whereby the many elaborate stratal architectures known to exist in nature can effectively be accounted for by only four parameters that are allowed to vary over space and time dune and interdune wavelength and spacing rate of bedform migration and rate of accumulation results have applied implications including the modelling of reservoir heterogeneity and the prediction of fluid flow pathways of hydrocarbons water co2 and contaminants in subsurface reservoirs and aquifers in which low permeability interdune units might act as baffles or barriers

processes facies and architecture of fluvial distributary system deposits abstract there is evidence from the stratigraphic record of examples of fluvial deposits that were the products of deposition from river systems which had decreasing discharge down flow and transitions from proximal channelised to distal unconfined flow these deposits form fan shaped bodies several tens of kilometres in radius and their stratigraphic architecture is aggradational with no evidence of deep incision driven by base level fall the fluvial systems that generated these deposits formed under conditions for which there is no complete analogue today an endorheic basin with a relatively arid climate adjacent to an uplifted area with higher precipitation a conceptual model for fluvial systems of this type has therefore been built on the basis of outcrop examples and a consideration of the controls on sedimentation proximal areas are characterised by amalgamated coarse pebbly and sandy channel deposits with little preservation of overbank facies channel dimensions are generally smaller in the medial areas but sizes are variable deposits are of braided meandering and simple channels which show varying degrees of lateral migration the channel fills may be mud or sand with overbank flow processes playing an important role in filling channels abandoned on the floodplain after avulsion the proportion of overbank deposits increases distally with sheets of sand deposited as lateral and terminal splays by unconfined flow interconnection of sandstone bodies is poor in the distal areas because channel fill bodies are sparse small and are not deeply incised the radial pattern of the sediment body forms by the repeated avulsion of channels active channels build up lobes on the alluvial plain and rivers switch position to follow courses on lower lying areas the term fluvial distributary system is here used to describe a river system which has a downstream decrease in discharge and has a distal zone which is characterised either by terminal splays on to a dry alluvial plain or a lake delta during periods of lake highstand

rheomorphism of welded tuffs abstract peralkaline welded tuffs from the islands of gran canaria canary islands and pantelleria italy show abundant evidence for post depositional flow it is demonstrated that rheomorphism or secondary mass flowage can occur in welded tuffs of ignimbrite and air fall origin the presence of a linear fabric is taken as the diagnostic criterion for the recognition of the process deposition on a slope is an essential condition for the development of rheomorphism after compaction and welding internal structures produced during rheomorphic flow can be studied by the methods of structural geology and show similar dispositions to comparable features in sedimentary slump sheets it is shown that secondary flowage can occur in welded tuffs emplaced on gentle slopes provided that the apparent viscosity of the magma is sufficiently low compositional factors favor the development of rheomorphism in densely welded tuffs of peralkaline type

calcretes in semi arid alluvial systems formative pathways and sinks late quaternary deposits in gujarat western india show an abundant development of calcretes three major sinks of carbonate in the alluvial deposits are recognized 1 groundwater calcretes 2 pedogenic calcretes and 3 calcrete conglomerates groundwater calcretes originate from carbonate saturated waters travelling preferentially along stratification planes pedogenic calcretes form through soil forming processes typically in extra channel areas calcrete conglomerates occur as ribbons sheets and lenses due to the reworking of both pedogenic as well as groundwater calcretes as a result a pathway of calcretization develops that has the route groundwater calcrete to pedogenic calcrete to calcrete conglomerate the formation of pedogenic calcretes over sediments containing groundwater calcretes demonstrates that 1 apart from aeolian dust river waters are also a major source of carbonate and 2 pedogenic carbonates may attain large sizes at accelerated rates due to the presence of pre existing groundwater calcretes consequently the maturity of a soil may be overestimated if determined by following established morphogenetic sequences

fluvial aeolian interactions part ii ancient systems an understanding of fluvial aeolian deposition derived from modern case examples in a previous study is applied to the permian cutler formation and cedar mesa sandstone on the colorado plateau these formations supply an excellent three dimensional exposure of intertonguing fluvial and aeolian strata four distinct facies associations form the bulk of the cutler formation and cedar mesa sandstone 1 aeolian dune deposits 2 wet interdune deposits 3 fluvial channel deposits and 4 overbank interdune deposits in addition two distinctive types of erosion surfaces are found within the cutler formation and cedar mesa sandstone pebble to granule rich erosion surfaces aeolian deflation surfaces and flood surfaces fluvial and aeolian intertonguing result in extensive tabular sheets of aeolian sandstone separated by flood surfaces and overbank interdune deposits fluvial channels are associated with the deposits overlying flood surfaces and are incised into the underlying aeolian sandstones overbank interdune deposits and wet interdune deposits cover flood surfaces and intertongue with overlying aeolian sandstones the primary characteristics of ancient fluvial aeolian deposition are overbank interdune deposits and pronounced extensive erosion surfaces flood surfaces which are parallel to underlying fluvial sandstones and thus trend parallel to the palaeoslope and palaeohydrological gradient

barbados ridge hydrogeologic tests implications for fluid migration along an active decollement hydrogeologic tests were conducted at a sealed borehole penetrating the decollement of the barbados ridge accretionary complex at low excess pore pressures p p p h p l p h 0 0 to 0 36 where p p pore pressure p h hydrostatic pressure and p l lithostatic pressure estimated permeabilities were comparable to those of similar unfractured sediment these tests complement shipboard packer tests completed at higher fluid pressures 0 5 to 1 0 during ocean drilling program odp leg 156 together the test results suggest a 4 to 5 order of magnitude permeability increase as fluid pressure varied from hydrostatic 0 to lithostatic 1 however unlike the results of the shipboard packer tests the test results presented here exhibit no evidence of a relationship between permeability and pore pressure the combined findings from the two sets of hydrogeologic tests indicate that significant permeability increases can occur within the decollement at pore pressures below lithostatic pressure

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

overpressure detection from compressional and shear wave data by analyzing experimental data we show that in many room dry rocks the poisson s ratio pr decreases with decreasing differential pressure confining minus pore pressure this means that in gas saturated rocks pr decreases with increasing pore pressure we confirm the generality of the observed effect by theoretically reproducing it via effective medium modeling this effect can be used as a new tool for overpressure detection from surface seismic cross well sonic logs and measurements ahead of the drill bit

flood surfaces and deflation surfaces within the cutler formation and cedar mesa sandstone permian southeastern utah areally extensive erosion surfaces in eolian deposits have been interpreted as long hiatuses in eolian deposition such erosion surfaces form during deflationary episodes or during periods of erg stabilization or contraction they are called super surfaces and can be useful for stratigraphic correlation because of their regional extent the permian cedar mesa sandstone and cutler formation contain a type off super surface herein termed flood surfaces 10 400 km2 flood surfaces form as a result of fluvial floods into active dune seas the surfaces may expand through nonclimbing migration of eolian dunes but do not imply long hiatuses in eolian deposition as do other types of super surfaces cutler and cedar mesa flood surfaces are overlain by shales and sandstones which thicken laterally and merge with fluvial channel fill deposits flooding of active dune fields in the cedar mesa sandstone is suggested by intertonguing of eolian dune and fluvial deposits flood surfaces can easily be mistaken for deflationary super surfaces but are distinguished by evidence of dune migration coincident with flood events and by an increase in the number of surfaces adjacent to associated aqueous deposits

geology geochemistry and origin of volcanic rock hosted uranium deposits in northwestern nevada and southeastern oregon usa abstract northwestern nevada and southeastern oregon have the largest uranium u deposits in tertiary volcanic rocks in the us most deposits are in or adjacent to calderas or rhyolite lava dome fields and are hydrothermal almost all are associated with rhyolites that have high primary u concentrations 9 to 20 ppm but the rhyolites range from peralkaline to peraluminous caldera related occurrences are hosted by peralkaline rocks in the mcdermitt and virgin valley calderas at mcdermitt major deposits are along or just inboard of the southwestern kings river and northern ring fractures bretz opalite of the caldera complex the kings river deposits are strongly enriched in trace elements associated with epithermal precious metal deposits e g as au ag mo and sb much of the u occurs as hydrothermal uraniferous zircon and whole rock samples contain as much as 5 zr gangue minerals include quartz adularia fluorite and numerous sulfides 40 ar 39 ar dating of adjacent volcanic rocks and adularia indicates that mineralization was contemporaneous with igneous activity the bretz opalite deposits which include the large aurora resource 7 5 106 kg u3o8 are less enriched in epithermal trace elements and contain uraninite coffinite chalcedony opal and numerous sulfides at virgin valley deposits are mostly in tuffaceous sediments adjacent to ring fracture rhyolites trace element enrichment is minor u is hexavalent in all identified u minerals and associated phases include opal organic material and pyrite deposits in the lakeview u district oregon are hosted by weakly peraluminous rhyolite domes and tuffs and are highly enriched in epithermal trace elements the mineral assemblage includes coffinite opal and numerous sulfides minor u occurrences at buff peak nevada the newly recognized westernmost topaz peraluminous rhyolite in the us are also trace element enriched gangue minerals include quartz adularia and sulfides on the basis of their geologic setting geochemistry age and mineralogy the mcdermitt lakeview and buff peak deposits are hydrothermal sparse published fluid inclusion data indicate temperatures between 200 c and 330 c virgin valley deposits most likely are hydrothermal minor deposits in outflow ash flow tuffs are distant from any intrusive heat source and probably formed from low temperature groundwater

structures and textures in triassic braided stream conglomerates bunter pebble beds in the sherwood sandstone group north staffordshire england in the northern parts of the needwood and stafford eccleshall basins england the pebble beds of the sherwood sandstone group contain thick successions of texturally mature fluvial pebble cobble conglomerates which are organized into either horizontal or cross stratified sets the horizontally lying sets generally coarser grained and more poorly sorted than the cross bedded sets are usually disorganized and either matrix or clast supported although thin lenses of well sorted occasionally openwork units interpreted as falling stage phenomena are often present the cross stratified conglomerates have foresets exhibiting remarkable textural organization with a coarse bimodal sometimes matrix supported part grading upwards or being abruptly overlain by a finer well sorted occasionally openwork part and finally capped by sandstone these rhythmic textural changes are attributed partly to an avalanche process at high stage and partly to falling stage conditions the most common types of vertical association are thick successions of horizontally bedded conglomerates up to 20 m and sequences of an upwards coarsening nature 2 12 m in which cross stratified sets are overlain by flat lying sets the environment of deposition of the gravels is interpreted as one in which water depths at high stage were greater than depths in most modern braided stream plains proglacial or alluvial fan but shallower than depths associated with the pleistocene catastrophic floods from which texturally mature giant gravel bars have been recorded recent braided streams with relatively confined channels and considerable bar channel relief are better analogues in particular medial or mid channel bars with a two tier structure subaqueous and partly emergent portions may explain the upward coarsening sequences in which horizontally lying conglomerates overlie cross stratified conglomerates the thicker sequences of horizontally stratified conglomerates represent proximal longitudinal bar deposits sheets of pebbly sandstone and argillaceous sandstone lying between the conglomerates and commonly occurring towards the top of the succession largely represent deposition from sandwaves and dunes finer interbedded argillaceous sandstones siltstones and mudstones are interpreted as overbank and waning flood deposits basin forming tectonism of increasing intensity probably caused the initial coarsening upwards of the lower part of the succession whilst more stable tectonic conditions and decreasing relief on the margins of the basins and in the areas of provenance in the midlands and the hercynides account for the upwards fining of the upper part of the succession

eolian origin of upper paleozoic sandstones southeastern utah abstract throughout its outcrop in southeastern utah the permian cedar mesa sandstone contains large scale crossbedding and is devoid of marine fossils except for sand size skeletal fragments although this rock unit has been interpreted by several recent workers as shallow marine stratification identical to that documented in modern eolian dunes is ubiquitous in the cedar mesa thin inversegraded laminations within crossbeds represent the unique product of climbing wind ripples this type of stratification is the best evidence for an eolian origin of the cedar mesa such an interpretation is strongly supported by other evidence vertebrate trackways and simple small cylindrical burrows constitute a distinctively nonmarine ichnofauna rhizoliths root casts are abundant at many stratig aphic levels in the underlying rico and hermosa formations sandstones with physical and biogenic sedimentary structures identical to those in the cedar mesa are interbedded with crossbedded limestones with abundant megafossils and marine traces contacts between sandstones and limestones are sharp gradational relationships would be expected if both sandstones and limestones accumulated subaqueously siliciclastic pebbles are present in some of these carbonate rocks suggesting that the well sorted nature of the sandstones cannot be attributed to the unavailability of coarse clasts palcocurrent data from the sandstones and limestones show significantly different vector means and variance southward pinchout of marine carbonates indicates that winds were onshore winnowing of silicicla tics from coarser carbonate detritus created a strong dichotomy in bulk mineralogy between marine and eolian sediments reinterpretation of the sandstones necessitates reappraisal of ideas concerning the mineralogical maturity of these sediments the relative importance of eolian and shallow marine transport processes to the destruction of labile siliciclastics is unknown

aeolian system sediment state theory and mojave desert kelso dune field example the sediment state of aeolian dune fields and sand seas at a basinal scale is defined by the separate components of sediment supply sediment availability and the transport capacity of the wind the sediment supply for aeolian systems is the sediment that contemporaneously or at some later point serves as the source material for the aeolian system numerous factors impact the susceptibility of grains on a surface to transport but these are cumulatively manifested by the actual transport rate which serves as a proxy for sediment availability transport capacity is the potential sediment transport rate of the wind because the three aspects of sediment state can be given as a volumetric rate they are directly comparable plotted simultaneously against time the generated curves define nine possible classes of sediment state sediment supply that is stored occurs because it is transport or availability limited or generated at a rate greater than the potential or actual transport rates respectively contemporaneous or lagged influx to an aeolian system may be limited by sediment availability but cannot exceed the transport capacity of the wind for the kelso dune field in the mojave desert of california a variety of stratigraphic and geomorphic evidence is used to approximate the sediment state of the system the sediment supply was generated during the latest pleistocene and earliest holocene during humid periods of enhanced discharge by the mojave river to form the lake mojave fan delta or terminal fan and has been calculated over time from the sedimentation rate and the frequency of floods estimation of transport capacity over time was based upon modern wind data with an allowance for greater winds during the pleistocene based upon climatic models sediment availability was approximated by calculation of a modern dune mobility index with variation over time based upon climatic inferences while quantifying the kelso or any natural system is subject to numerous uncertainties the sediment state approach reflects the temporal and spatial disjointed nature of accumulations at kelso as well as illuminating questions for future research

large scale rheomorphic shear deformation in miocene peralkaline ignimbrite e gran canaria the single ignimbrite cooling unit e average thickness 28 m volume ca 30 km3 forms the uppermost member of the miocene upper mogan formation on gran canaria it is strongly chemically zoned from basal first erupted comendite peralkaline rhyolite to late erupted trachyte and apart from an upper trachytic zone it is densely welded e was emplaced onto a surface inclined ca 2 5 from the source caldera detailed mapping of key sections up to 300 m long exposed in barranco walls ca 10 km from the caldera margin reveals structures that are interpreted to have been produced by rheomorphic deformation of the ignimbrite along shear zones the shear zones formed within the lower viscosity comenditic tuff extensional structures include mega boudinage and decapitated sequences and compression resulted in sequence repitition by overthrusting mechanisms traditionally thought to be important during rheomorphic deformation of welded tuffs compaction lateral creep folding vertical density driven diapirism cannot account for these features which reflect lateral post compactional rheomorphic movement locally in excess of 800 m we suggest the following sequence of events emplacement of the several flow units compaction with little lateral movement rheomorphic deformation during and after compaction layers of secondary porosity developed within the comenditic tuff possibly where upward escape of gas was prevented by overlying relatively impermeable layers of densely compacted ignimbrite these structurally weak layers of high porosity subsequently acted as shear zones

anabranching rivers on the northern plains of arid central australia anabranching rivers are a widespread feature of the northern plains in the alice springs region of central australia but their unusual characteristics previously have not been described on the northern plains anabranching occurs on rivers transporting bedloads of coarse sand and gravel and is characterised by channels of variable size and shape which occur within a broader typically well defined channel train channels are separated by channel train ridges narrow flow aligned vegetated features or by wider islands ridges and islands are either depositional features formed in situ by accretionary processes or erosional features formed by excision from once continuous areas of floodplain vegetation plays a key role in the initiation survival and growth of depositional forms through its influence on flow sediment transport and ridge and island stability anabranching is also related to the influence of tributaries for some large rivers alternate from single thread to anabranching along their length in response to tributary inputs of water and sediment tributary inputs occur during flow events that are either independent from or in concert with floods in the trunk channel ridges and islands form in association with tributaries as a result of various hydrological depositional and erosional processes including irrigation of enhanced numbers of in channel trees and resulting lee side sediment accretion floodplain scour and the formation and maintenance of deferred junction tributaries the change from single thread to anabranching downstream of tributary junctions occurs in the absence of any significant change in channel gradient or degree of channel confinement on the northern plains anabranching appears to be a stable river pattern that helps to maintain the throughput of relatively coarse sediment in low gradient typically 0 0005 0 002 channels characterised by an abundance of within channel vegetation and subject to declining downstream discharges

distribution of palaeosols and deposits in the temporal evolution of a semiarid fluvial distributary system bauru group upper cretaceous se brazil abstract the stratigraphic and sedimentological knowledge of the bauru group upper cretaceous se brazil is still generally insufficient and controversial a sedimentological and palaeopedological study allowed to interpret the south eastern portion of the bauru group according to the model of a fluvial distributary system this work has two objectives 1 to include palaeosols in the interpretation of a fluvial distributary system and 2 to give detailed information on the sedimentological and stratigraphic features of the se portion of the bauru group in order to support biostratigraphical taphonomic and palaeoecological studies in the south eastern portion of the bauru group three genetic stratigraphic units were described and interpreted here informally called lower intermediate and upper units the lower unit is constituted of muddy sandstone salt flat deposits and sandstone sheet deltas deposits and is interpreted as a basinal part of a fluvial distributary system the intermediate unit is formed of very fine to fine grained sandstone filled ribbon channel and sandy sheet shaped beds suggesting a distal or medial portion of a fluvial distributary system the upper unit does not match with the present models of the fluvial distributary system because mostly constituted of moderately developed well drained medium to fine grained sandstone palaeosols which testify pauses of sedimentation to the order of 104 years preserved features of sedimentary structures suggest that the parent material was formed by occasional catastrophic unconfined flows this unit may represent the most distal portion of a fluvial distributary system generated by retrogradation of the alluvial system due to aridification of the climate the upper unit may be interpreted also as proximal portion of fluvial distributary system if considering the coarser grained and the well drained palaeosols however the absence of channel deposits makes this interpretation unconvincing

alluvial fan sedimentation of the cutler formation permo pennsylvanian near gateway colorado the permo pennsylvanian cutler formation near gateway colorado is the most complete 1 334 m proximal section of alluvial fan sediments deposited along the western flank of the ancestral uncompahgre uplift cutler facies can be correlated with depositional processes observed on modern dry alluvial fans proximal cutler facies include matrix supported bouldery debris flow and channel form streamflood conglomerates midfan sedimentation in the cutler is represented by trough cross bedded granular braided stream sandstones laterally continuous streamflood conglomerates and sheetflood deposits laterally continuous streamflood conglomerate was deposited at the mouth of large channels near the intersection point and consists of a cross bed set as much as 2 m thick with a basal boulder cobble lag rippled and laminated siltstone with gravel channels represents distal sheetflood sedimentation pedogenic features include rhizocretions and calcareous nodules vertical changes in facies and maximum clast size delineate three megasequences on the scale of hundreds of metres thick each mega sequence is composed of a coarsening upward sequence of proximal facies overlain by a fining upward sequence of more distal facies coarsening upward sequences record periods of tectonic uplift and fan progradation whereas fining upward sequences result from tectonic quiescence and weathering back of the mountain front small scale cycles on the scale of ten metres occur within the larger megasequences and represent changes inherent to the alluvial fan system autocyclic sedimentologic data on cutler alluvial fan sediments at gateway support previous interpretations of semiarid or arid paleoclimate during permian time along the western flank of uncompahgria and may act as a standard of comparison for tests of the role of tectonism on sedimentation trends in the paradox basin

abnormal formation pressure and shale porosity an abnormally pressured jurassic shale from the north sea viking graben has a fluid pressure over burden ration close to 0 8 at a depth of more than 4 000 m and is associated with low porosity and high density the porosity and fluid pressure history of the shale involves three successive stages 1 normal gravitational compaction prevailed during the first few hundred meters of burial 2 the shale porosity was severely reduced by carbonate cementation in conjunction with a major break in sedimentation but the fluid pressure remained hydrostatic and finally 3 the porosity was further reduced by compaction and possibly cementation and the fluid pressure increased to above hydrostatic in response to renewed subsidence and the cretaceous and tertiary overburden lo d

morphology and sediment dynamics of ephemeral stream terminal distributary systems in the kobo basin northern welo ethiopia channel morphology bedforms and sedimentary structures of the distributary systems of ephemeral streams in the structural basin of kobo northern ethiopia are analysed the basin is characterised by a semi arid monsoon affected climate most of the rivers originate in the western margin range and dry up in the middle of the basin where they form large distributary systems they show characteristics different from those of the terminal fans and floodouts previously reported in the literature no flow data are available but bankfull discharge for several study reaches has been obtained by field survey of a number of cross sections hydraulic geometry streambed gradient and bed material grain size distribution in both the main stems and the distributary channels two different types of terminal distributary systems have been identified bird foot and lobate the first type by far predominates in the kobo basin bird foot distributary systems were recognized to consist of five distinctive reach units while the lobate splays have a simpler morphology and are thought to evolve into the former more complex type a description of channel morphology bedforms and sedimentary structure distribution is given and these characteristics are compared to those of other similar depositional systems reported in the literature other features such as the lack of scour pockets around boulder particles scattered on the streambed surface and the interaction of individual large particles with massive or horizontal laminated sand on top of typical upward fining channel sequences are also highlighted

conditions favourable for the formation of warm climate aeolian sand sheets aeolian sand sheets are areas of aeolian sand where dunes with slipfaces are generally absent sand sheets are ubiquitous to modern warm climate sand seas generally occurring marginal to dune fields although they may exist within the interior of a sand sea or independent of a dune field sand sheet deposits are recognized in ancient aeolian sequences where they may account for significant accumulations of low angle aeolian stratification we suggest that the occurrence of sand sheets instead of dunes indicates that conditions are outside the range within which dunes form or that one or more factors interfere with dune development while also favouring the accumulation of sand sheets a study of six modern sand sheets in north america located at great sand dunes gran desierto dumont algodones padre island and colorado river delta indicates that the factors favourable for sand sheet development are 1 a high water table 2 surface cementation or binding 3 periodic flooding 4 a significant coarse grained sediment population and 5 vegetation these factors are reflected in the nature of stratification and the accessory features of sand sheet accumulations within the areas of modern sand sheets as well as in their ancient counterparts in the triassic dolores and pennsylvanian permian rico formations

splay formation along the lower reaches of ephemeral rivers on the northern plains of arid central australia abstract splays are common features of many rivers and deltas forming as a result of overbank flooding and sediment deposition but they have rarely been described from modern dryland settings this paper investigates splay formation along the lower reaches of ephemeral rivers on the northern plains of arid central australia here splays vary from small lobate or tongue shaped features 1 km long to larger elongate features up to 4 km long and are supplied with rare floodwater and sediment through well defined breaches in the upper parts of the main channel banks in proximal and medial parts splay channels are incised up to 2 m into pre splay fine grained floodplain sediments but remain elevated between 0 5 m and 2 m above the main channels only in the very distal parts are splay channels less deeply incised either grading into the floodplain surface or terminating as sediment lobes that are deposited upon the floodplain aerial photographs and field observations indicate that many splays develop rapidly during large floods and are modified during infrequent subsequent floods four main stages of splay development are proposed 1 aggressive overbank flow results in bank line breaching and channel scour on the floodplain but little sediment deposition 2 coarse bed sediment is diverted from the main channel preventing further deepening at the bank line breach and slowing channel scour 3 increasing coarse sediment transport through the bank line breach and scoured channels leads to the formation of distal sediment lobes 4 subsequent floods divert around these distal lobes sometimes leading to further channel scour and lobe formation on the northern plains splays contribute to downstream decreases in the discharge and size of the main channels and splay deposits form local accumulations of gravelly sand in the predominantly fine grained floodplain successions some splays ultimately may evolve into distributaries or anabranches the dominantly erosional bases of these splay channels contrast with typical descriptions of many humid region splays but they do not display any geomorphological or sedimentological features that are necessarily diagnostic of formation in a dryland setting

stratigraphic evolution of an aeolian erg margin system the permian cedar mesa sandstone se utah usa the permian cedar mesa sandstone of south east utah is a predominantly aeolian succession that exhibits a complex spatial variation in sedimentary architecture which in terms of palaeogeographic setting reflects a transition from a dry erg centre through a water table controlled aeolian dominated erg margin to an outer erg margin subject to periodic fluvial incursion the erg margin succession represents a wet aeolian system accumulation of which was controlled by progressive water table rise coupled with ongoing dune migration and associated changes in the supply and availability of sediment for aeolian transport variation in the level of the water table relative to the depositional surface determined the nature of interdune sedimentary processes and a range of dry damp and wet flooded interdune elements is recognized variations in the geometry of these units reflect the original morphology and the migratory behaviour of spatially isolated dry interdune hollows in the erg centre locally interconnected damp and or wet interdune ponds in the aeolian dominated erg margin and fully interconnected fluvially flooded interdune corridors in the outer erg margin relationships between aeolian dune and interdune units indicate that dry damp and wet interdune sedimentation occurred synchronously with aeolian bedform migration temporal variation in the rates of water table rise and bedform migration determined the angle of climb of the erg margin succession such that accumulation rates increased during periods of rapidly rising water table whereas sediment bypassing zero angle of climb occurred in the aftermath of flood events in response to periods of elevated but temporarily static water table during these periods in the outer erg margin the expansion of fluvially flooded interdunes in front of non climbing but migrating dunes resulted in the amalgamation of laterally adjacent interdunes and the generation of regionally extensive bypass flood supersurfaces a spectrum of genetic depositional models is envisaged that accounts for the complex spatial and temporal evolution of the cedar mesa erg margin succession

elastic wave velocity in marine sediments with gas hydrates effective medium modeling we offer a first principle based effective medium model for elastic wave velocity in unconsolidated high porosity ocean bottom sediments containing gas hydrate the dry sediment frame elastic constants depend on porosity elastic moduli of the solid phase and effective pressure elastic moduli of saturated sediment are calculated from those of the dry frame using gassmann s equation to model the effect of gas hydrate on sediment elastic moduli we use two separate assumptions a hydrate modifies the pore fluid elastic properties without affecting the frame b hydrate becomes a component of the solid phase modifying the elasticity of the frame the goal of the modeling is to predict the amount of hydrate in sediments from sonic or seismic velocity data we apply the model to sonic and vsp data from odp hole 995 and obtain hydrate concentration estimates from assumption b consistent with estimates obtained from resistivity chlorinity and evolved gas data

the ability of rock physics models to infer marine in situ pore pressure pore fluid pressure is an important parameter defining the mechanical strength of marine sediments obtaining high spatial resolution in situ pore pressure measurements in marine sediments however is a challenge and as a result only a handful of in situ pore pressure measurements exist at scientific drill sites integrating rock physics models with standard iodp odp measurements provides a potentially widely applicable approach for calculating in situ pore pressure here we use a rock physics approach to estimate in situ pore pressure at two scientific ocean drill sites where in situ pressure is well constrained odp site 1173 used as reference for normal hydrostatic fluid pressures and odp site 948 where previous studies infer high fluid pressures 0 45 0 95 where the pore pressure ratio is defined as the pore pressure above hydrostatic divided by the difference between the largest principal stress and hydrostatic stress our analysis indicates that the rock physics method provides an accurate low precision method for estimating in situ pore pressure at these drill sites and sensitivity analysis indicates this method can detect modestly high 0 6 pore pressure at the 95 confidence level this approach has broad applicability because it provides an inexpensive high resolution meter scale method for retrospectively detecting and quantifying high pore pressure at any drill site where quality wireline logs and ocean drilling data exist

recognition of the sedimentary architecture of dryland anabranching anastomosing rivers abstract the application of rigorous criteria for identifying fluvial style from sedimentary deposits is crucial if environmental reconstructions from stratigraphy are to be reliable from modern observations it appears that anabranching anastomosing rivers can be divided into separate humid and dryland classes each class has dissimilar characteristics to the other necessitating different criteria for their recognition in the rock record the criteria being used by sedimentologists to recognize deposits of anabranching rivers are derived solely from modern examples in humid regions heavily biased to a few specific cases in canada these criteria are dominated by the concept of ribbon sandstone bodies encased in fine grained floodplain sediment with an overall low sand to mud ratio in the succession there has been a general failure to appreciate that dryland anabranching rivers have characteristics significantly different from these and almost certainly leave a sedimentary record quite dissimilar to that previously expected avulsion or obtrusion the gradual formation of an alternative channel are the characteristic dynamic of anabranching rivers forming and maintaining coexisting multiple anabranch channels avulsion or obtrusion require stream banks to have a high resistance to erosion usually achieved by cohesive bank sediments or the reinforcement of banks by vegetative root mats in some drylands riparian vegetation can be sparse and bank resistance may depend in large part on a high clay content here channel fills are mud dominated and floodplains have a uniform fine grained character which is usually structureless because of intense pedoturbation and some bioturbation the evidence from such present day dryland anabranching rivers suggests that their sedimentary record is likely to be one of laterally extensive mud rich beds with weak or enigmatic fluvial indicators elsewhere dryland anabranching systems are sand dominated their banks reinforced with riparian gallery forests sustained by infrequent flow and a shallow water table though the stratigraphy of these has barely been investigated they are clearly different from humid region anabranching systems for their channels and islands are predominantly sandy anabranching rivers with prominent channel sandbodies separated by muddy islands have never been described from contemporary drylands the interpretation of a dryland origin for ancient stratigraphic successions with this form is made therefore without evidence that they can actually exist in the light of this such interpretations should be reevaluated

reworked calcretes their significance in the reconstruction of alluvial sequences permian and triassic minorca balearic islands spain the permian and triassic of minorca balearic islands consists of a 670 m thick red alluvial succession that includes in situ calcrete profiles and reworked calcrete material in the permian succession the calcretes vary from laminar forms developed on the carboniferous basement to weakly developed nodular calcretes in fluvial sediments the palaeosols in the triassic are mostly dolomitic and the profiles reach up to stage iii of soil development spec pap geol surv am 203 1995 1 the clasts formed through reworking of the palaeosol profiles are about 0 5 10 cm across and include mosaics of calcite dolomite crystals brecciated clasts rhizolith fragments and aggregates of clay and or silt these clasts appear in three different types of deposits type 1 corresponds to lenticular bodies that fill small scour surfaces and consists only of intraformational conglomerates these deposits are interpreted as ephemeral channels and sheet floods that represent the interfluvial drainage systems that captured only the precipitation falling on the alluvial plain type 2 includes sand dune 3 d bodies with flat bottoms and convex tops these bodies are about 20 cm high and 2 m wide and were formed by floodwaters that flowed down the levees of the major streams type 3 channel deposits contain reworked calcretes and extrabasinal clasts which overlie erosive surfaces and are found in layers within cross bedded sandstones and conglomerates these are interpreted as channel floor lag deposits of major channels that entered from distant uplands and drained the alluvial plain variations in the aggradation rates of the floodplain resulted in five different infill stages in the lowstand to early transgressive interval as in stages i p1 and iv b1 the fluvial deposits filled palaeovalleys calcretes and reworked calcrete deposits were of difficult formation apart from terraces and preservation accommodation space was at its greatest in the transgressive stages ii p2 and v b2 this caused the greatest aggradation of the floodplains which are formed of thick sequences of fine grained sediments isolated meandering channels weakly developed calcretes compound and reworked calcrete deposits mostly of types 1 and 2 the density of channels notably increased in stage iii p3 highstand interval because of the reduction of accommodation space this could favour the formation of composite or even cumulative palaeosols but of difficult preservation reworked calcrete deposits are mostly of type 3 but types 1 and 2 are also recognised the reworked calcrete deposits are an important part of the permian and triassic fluvial sediments and their occurrence and characteristics are important in order to interpret the infill of terrestrial basins and the construction of floodplains

fluvial fans myths misconceptions and the end of the terminal fan model we propose that the so called terminal fan facies model should be abandoned since it is flawed on several counts and it is leading to misunderstanding and poor communication rivers in drylands may experience excessive down stream discharge reduction such that they terminate subaerially rather than reach the sea or a lake the facies model predicts that the distal reaches of such rivers form a network of bifurcating distributary channels producing a fan shaped sediment body with downstream thinning and fining of sedimentary units ending in sand filled ribbons encased in mud extensive review of modern rivers has failed to turn up convincing examples that fit the model rivers in drylands do not ubiquitously end in fans fan shaped fluvial bodies are common wherever rivers are released from confinement and the discharge conditions promote frequent avulsion channels on such fans generally do not repeatedly bifurcate downstream where they are seen to do so it can usually be shown they are lacustrine deltas inherited from wetter times the term distributary is being used carelessly and is conveying incorrect understanding of sediment geometry and architecture the proposed synonym of fluvial distributary systems is unsatisfactory as it perpetuates the same misunderstandings reliance on planform alone in analogue selection is highly risky the fluvial fan is a composite sediment body resulting from frequent nodal avulsions in a setting without horizontal constraints channels on fans range in planform as much as any other river the resultant sedimentary record differs little from that expected from non fan fluvial systems except having a regionally radiating orientation when viewed over geological time scales contrary to the implications of the facies model there is no distinctive terminal fan sedimentary succession

the emplacement history of a remarkable heterogeneous chemically zoned rheomorphic and locally lava like ignimbrite tl on gran canaria abstract ignimbrite tl on gran canaria is a complex compositionally zoned rheomorphic tuff that locally exhibits features previously considered to be diagnostic of lavas it is made up of two locally overlapping lobes of ignimbrite that were emplaced during a single eruptive episode the eastern lobe is high grade with rheomorphic zones and localised patches that are lava like the western lobe is extremely high grade more extensively lava like and welded to its top surface both parts are zoned with a basal comendite rich zone grading up through a mixed zone into an upper trachyte rich zone lithic contents and the relative proportions of comendite and trachyte pyroclasts vary with height each comendite rich zone is vitroclastic whereas each trachyte rich zone is partly lava like with local gradations into vitroclastic ignimbrite mixed zones are intermediate in character and locally show compositional banding gradational zoning in massive ignimbrite best seen in lower strain zones and welding fabrics that are pervasively lineated and oblique to bedding suggest that deposition was sustained agglutination was rapid and rheomorphic deformation began during the sustained deposition the viscosity and porosity of the agglutinate varied with height because successively deposited pyroclast populations varied in grainsize composition and temperature the hot agglutinate continued to compact and shear downslope after the density currents had dissipated causing further rheomorphic folding thrusting attenuation and autobrecciation the western lobe locally overlies the partly welded top of the eastern lobe in part because it advanced rheomorphically across it for at least 300 m hot state loading and auto intrusion occurred due to unstable density layering in the chemically zoned agglutinate deformation behaviour changed during cooling and degassing and because of heat transfer between juxtaposed agglutinates and localised retention of dissolved volatiles where there was an overlying impermeable cap

climatic and geomorphologic cycles in a semiarid distributive fluvial system upper cretaceous bauru group se brazil abstract studies of distributive fluvial systems and their preserved successions envisage the distribution and orientation of architectural elements to be primarily controlled by channels radiating outward from fan apices in many cases along an elongate basin margin conceptual models for such systems account for the downstream dynamics of the fluvial network but with limited consideration of temporal geomorphic variations resulting vertical organisation of architectural elements or of the interplay of factors controlling system dynamics to understand the external and internal architecture of distributive fluvial systems and the factors that influence their sequential facies organisation a sedimentary succession of the proximal portion of an upper cretaceous semiarid distributive fluvial system located at the north eastern margin of the bauru basin southeast brazil has been analysed in detail three fining and thinning upward fluvial sequences are identified forming an interval separated at the top and the bottom by two palaeosol profiles each sequence is formed of channel and floodplain deposits two types of channel deposits are identified one is composed of stacked sets of small scale dune deposits suggesting perennial and steady fluvial regime associated with more humid climate periods the other is composed of large scale sets indicative of flattened dunes associated with unsteady and fast changing fluvial flow formed in quasi supercritical flow regime conditions associated with drier climate periods the vertical alternation of these two types of channel deposits records the accumulation of a fluvial succession that responded to high frequency climate induced cyclic change in boundary conditions two palaeosol profiles at the top and at the bottom of the succession indicate temporary interruptions and cessation of the fluvial sedimentation likely related to avulsion of the fluvial belt thus the studied succession reveals high frequency climate induced allogenic sedimentary cycles that occur within a long period autogenic geomorphologic induced sedimentary cycle this work suggests that the internal architecture of the channel deposits can be used as a climate proxy and that climate and geomorphology act jointly as notable factors to control the vertical organisation of distributive fluvial systems

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

late quaternary paleoenvironments and paleoclimatic conditions in the distal andean piedmont southern mendoza argentina abstract the andean piedmont of mendoza is a semiarid region covered by extensive and partially vegetated dune fields consisting of mostly inactive aeolian landforms of diverse size and morphology this paper is focused on the san rafael plain srp environment situated in the distal andean piedmont of mendoza 34 30 s and reports the sedimentology and osl chronology of two representative exposures of late quaternary deposits including their paleoenvironmental and paleoclimatic significance eleven facies including channel floodplain fluvio aeolian interaction and reworked pyroclastic and aeolian deposits were described and grouped into two facies associations fa1 and fa2 fa1 was formed by unconfined sheet flows minor channelized streams and fluvial aeolian interaction processes fa2 was interpreted as aeolian dune and sand sheet deposits osl chronology from the srp sedimentary record indicates that between ca 58 39 ka and ca 36 24 ka mis 3 aggradation was governed by ephemeral fluvial processes fa1 under generally semiarid conditions during mis 2 the last glacial maximum ca 24 12 ka a major climatic shift to more arid conditions is documented by significant aeolian activity fa2 that became the dominant sedimentation process north of the diamante atuel fluvial system the inferred paleoenvironmental conditions from the srp sections are in broad agreement with regional evidence

deformation of the wineglass welded tuff and the timing of caldera collapse at crater lake oregon abstract four types of deformation occur in the wineglass welded tuff on the northeast caldera rim of crater lake a vertical tension fractures b ooze outs of fiamme c squeeze outs of fiamme and d horizontal pull apart structures the three types of plastic deformation b d developed in the lower part of the wineglass welded tuff where degree of welding and density are maximum deformation originated from concentric normal faulting and landsliding as the caldera collapsed the degree of deformation of the wineglass welded tuff increases toward the northeast part of the caldera where plastic deformation occurred more easily because of a higher emplacement temperature probably due to proximity to the vent the probable glass transition temperature of the wineglass welded tuff suggests that its emplacement temperature was 750 c where the tuff is densely welded calculation of the conductive cooling history of the wineglass welded tuff and the preclimactic cleetwood lava flow under assumptions of a initially isothermal sheet and uniform properties suggests that a caldera collapse occurred a maximum of 9 days after emplacement of the wineglass welded tuff and that b the period between effusion of the cleetwood lava flow and onset of the climactic eruption was

rheomorphic structures in a high grade ignimbrite the nuraxi tuff sulcis volcanic district sw sardinia italy deposits of the 15 8 ma nuraxi explosive eruption crop out in the sulcis volcanic district southwestern sardinia they consist of a decimeter thick pumice fallout layer overlain with no apparent temporal break by several tens of meters thick high grade ignimbrite the eruption was from a vent probably located 20 30 km north of s pietro island and it tapped a compositionally uniform rhyolite magma reservoir sio2 70 72 wt bearing about 20 vol crystals the ignimbrite consists of a a lower ignimbrite li 2 m thick fine grained and glassy grading upward into a partly devitrified lapilli tuff and b an upper ignimbrite ui that is an intensely welded to lava like lapilli tuff the deposit has an inferred maximum runout of about 80 km and overall dispersal thickness characteristic of a low aspect ratio ignimbrite over most of its outcrop the tuff rests on a flat horizontal paleosurface and forms an ignimbrite plateau with uniform thickness and structural characteristics however in some areas it shows extensive folding and over thickening as a result of rheomorphic flow into topographic depressions structural analysis of the ignimbrite reveals the presence of a pervasive foliation and lineation as well as macroscopic and microscopic flow structures occurrence of abundant widely distributed stretched cavities in the tuff indicates that a substantial amount of gas was trapped in the tuff during aggradation the observed pattern of structures of the ignimbrite plateau accords well with a conceptual model in which syn depositional structures are formed during non particulate basal laminar flow due to agglutination of juvenile particles within the lower part of the pyroclastic density current the flow was controlled by the substantial transfer of momentum from the high speed particulate upper part of the current to the top of the non particulate part of the current and the onset of a complex relationship between welding flow and sedimentation processes it is proposed in particular that elongated gas pockets played a major role in guiding the laminar shear within the non particulate part of the current in flat horizontal areas eventually promoting brittle failure of the deforming tuff mass along shear planes

downstream changes in dryland river channels the northern plains of arid central australia abstract many dryland rivers undergo marked downstream changes owing to factors such as infrequent floods flow transmission losses and typically few tributary inflows beyond the headwaters along the sandover bundey sandover bundey and woodforde rivers on the northern plains of arid central australia downstream channel changes are broadly similar in upland zones small rocky channels transporting sand and gravel gradually increase in size before entering piedmont zones where channels and narrow floodplains are confined by bedrock alluvial terraces or aeolian dunes in lower gradient lowland zones channels and floodplains remain confined and in the absence of tributary inflows channel cross sectional areas and discharges decrease downstream confining landforms are not present in floodout zones which results in splay formation increased floodplain widths and marked overall downstream decreases in cross sectional areas eventually channelised flow and bedload transport terminate although occasional large floods continue across extensive unchanneled alluvial surfaces termed floodouts these broad similarities apart downstream changes along the three rivers differ in detail the sandover is largely a single thread channel whereas many reaches of the sandover bundey and woodforde are anabranched on the small woodforde river downstream decreases in parameters such as cross sectional area and width are roughly linear on the larger sandover and sandover bundey downstream changes are more irregular particularly through the floodout zones where there are marked fluctuations in widths depths and bed slopes the irregular downstream changes typical of the lower reaches of these large rivers may be due to the reduced influence of vegetation on bankline stability and width adjustment relative to that of smaller rivers on the northern plains as in other drylands complex interactions between discharge sediment transport slope patterns of tributary drainage bank sediment type and vegetation result in variable patterns of downstream channel change

consolidation state and strength of underthrust sediments and evolution of the decollement at the nankai accretionary margin results of uniaxial reconsolidation experiments 1 uniaxial reconsolidation experiments conducted on ocean drilling program drill cores along the muroto transect of the nankai accretionary margin demonstrate complex yield and postyield behavior and provide evidence for enhanced strengths within sediments beneath the decollement zone tests were conducted on samples collected from similar stratigraphic levels below the decollement and its seaward projection consolidation state of the samples increased landward and with depth and was tracked closely by sediment yield stress for all but one of the samples the sediments however exhibited substantial postyield strength up to 2 8 times the predicted in situ effective vertical stress beneath the protothrust zone this enhanced strength results from diagenesis that leads to matrix cementation during stable effective stress conditions within the underthrust section the close correspondence between yield stress and predicted in situ effective stress suggests that despite the cemented state the sediment matrix remains sensitive to in situ stress conditions the low yield stress of one sample collected within 40 m of the decollement fault at site 808 may reflect diagenesis under reduced effective stress conditions due to postconsolidation increases in pore pressure along the decollement due to their cemented state the strong underthrust sediments resist decollement downcutting beneath the toe of the prism leading to preferential incorporation of weaker continuously deforming accreted sediments during shear seismogenic slip along the decollement at depth may create stresses in excess of sediment strength causing shear failure and rapid release of trapped pore fluids feeding high pore fluid pressures along the decollement zone