age and origin of the ardal dike complex sw norway false isochrons incomplete mixing and the origin of caledonian granites in basement nappes 1 granitic dikes intruding the upper thrust sheets of the jotun nappe complex southwest norway previously assigned a sveconorwegian age based on rb sr data are redated to 427 1 ma by u pb the erroneous rb sr age can be explained by mixing of magmas with different isotopic compositions similar geochemical signatures evidence of sr mixing inherited proterozoic zircons and synkinematic modes of emplacement are also seen in silurian granites in the lifjorden complex as well as in the lindas nappe and the totland unit of the bergen arc complex we propose that these granites are related and derived from anatexis of rb poor sediments of baltica affinity overridden by the nappes during scandian thrusting this interpretation is compatible with the presence of the granites in nappes of both oceanic and of continental affinities and their absence in the autochthonous basement

facies and facies architecture of paleogene floodplain deposits willwood formation bighorn basin wyoming usa abstract paleogene deposits of the willwood formation were analyzed in two areas of the bighorn basin to provide a better understanding of the facies and facies arrangement of floodplain deposits and the various processes that influence floodplain construction and facies variability despite similar facies and facies organization floodplain deposits in the two areas differ in grain size hydromorphy and maturity of the paleosols and scale and organization of ribbon sandstones in the avulsion deposits these differences appear to be controlled by basin position of the study areas and differences in avulsion between the areas two kinds of cumulative floodplain paleosols red and purple paleosols formed on overbank deposits the red paleosols were better drained and formed on less clay rich parent material intense mottling and iron oxide nodules indicate that the purple paleosols which formed on more clay rich parent material were poorly drained the grain size differences reflect basin position in particular distance from a local sediment source avulsion deposits are volumetrically important comprising about half of the floodplain deposits in both areas the avulsion deposits resemble the stage iii splays described by smith et al 1989 smith n d cross ia dufficy j p clough s r 1989 anatomy of an avulsion sedimentology 36 1 24 in terms of characteristics of their ribbon sandstones and the presence of thin sheet sandstones or siltstones paleosols that formed on the fine grained facies show morphologic differences interpreted as developmental differences and attributed to differing rates of avulsion in the two study areas this study shows that avulsion belt paleosols provide a key to understanding avulsion rate in the stratigraphic record other differences between the willwood avulsion deposits in the two areas and between these ancient deposits and the modern avulsion deposits described by smith et al may reflect 1 problems in clearly recognizing ancient avulsion deposits in the field and 2 sampling bias resulting from the areally restricted view provided by the stratigraphic record of an areally extensive modern depositional feature at the same time because exposures are three dimensional the willwood strata reveal aspects of the facies and facies arrangement of avulsion belts not readily observed in the modern ones

shredding of environmental signals by sediment transport 1 landscapes respond to climate tectonic motions and sea level but this response is mediated by sediment transport understanding transmission of environmental signals is crucial for predicting landscape response to climate change and interpreting paleo climate and tectonics from stratigraphy here we propose that sediment transport can act as a nonlinear filter that completely destroys shreds environmental signals this results from ubiquitous thresholds in sediment transport systems e g landsliding bed load transport and river avulsion this morphodynamic turbulence is analogous to turbulence in fluid flows where energy injected at one frequency is smeared across a range of scales we show with a numerical model that external signals are shredded when their time and amplitude scales fall within the ranges of morphodynamic turbulence as signal frequency increases signal preservation becomes the exception rather than the rule suggesting a critical re examination of purported sedimentary signals of external forcing citation jerolmack d j and c paola 2010 shredding of environmental signals by sediment transport geophys res lett 37 l19401 doi 10 1029 2010gl044638

early cambrian braid delta deposits mackenzie mountains north western canada latest neoproterozoic to earliest cambrian strata in north western canada provide an example of a pre vegetation braid delta depositional system depositional environments represented in the succession include braided fluvial and braid delta distributary channels aeolian dune fields and interdistributary lagoons bays as well as mouth bar beach to shoreface and prodelta to distal shelf settings three formations have been investigated the ingta formation formed in wave dominated nearshore to offshore shelf environments with little or no apparent deltaic influence whereas the overlying backbone ranges and vampire formations contain an extensive record of braid delta deposits ranging from braidplain to distal prodelta facies on the braid plain river channels reached widths of up to several kilometres such channels terminated seaward in braid deltas that showed some shoreline protuberance and were characterized by fluvial dominated mouth bar deposition with lesser wave influence wave dominated deltaic successions are rare in the succession interdeltaic areas were characterized by wave dominated prograding shorelines interdistributary lagoons probably formed primarily in abandoned distributary channels delta front prodelta deposits are silt rich and contain abundant soft sediment deformation including slumps the deposits in these formations illustrate the significantly different nature of sedimentation prior to the advent of land plants this is illustrated in the dominance of braided fluvial deposition and of silt rich offshore facies that may have resulted from enhanced aeolian transport of loess the non actualistic effects of limited bioturbation and extensive microbial binding apparently exerted relatively little control on the distribution of facies however the absence of extensive bioturbation is manifest in pristine preservation of primary sedimentary structures while the hypothesized latest proterozoic earliest cambrian decline in microbial binding may be reflected in the upward increase in the abundance of sole marks in the succession

horodyskia williamsii new species a mesoproterozoic macrofossil from western australia abstract a new species of abundant macrofossil previously described informally as a string of beads is here assigned to the genus horodyskia yochelson and fedonkin 2000 and formally named horodyskia williamsii it is known from mesoproterozoic siliciclastic rocks that are older than 1070 ma and younger than 1465 ma the backdoor formation of the collier group and the poorly time constrained but probably correlative stag arrow formation of the manganese group both part of the bangemall supergroup of the ashburton region of western australia the fossils are most commonly found as impressions on the sole of thin frequently ripple marked sandstones very rarely they are present as traces on the tops of sandstone beds at some localities there are thin silty partings separating the sandstone beds some strings are current oriented but commonly most strings crosscut current directions much of the observed variation appears to be taphonomic

fluvial bars reconstructed from a deep straight channel upper carboniferous coalfield of northeast england the seaton sluice sandstone is typical of these ancient rivers deposition was dominated by interdigitating lobate crested sandwaves at least 20 m wide which descended the leeside of a single 10 m high 200 m wide bar and deposited planar cross beds lunate megaripples and dunes transported sediment along the top of these sandwaves and deposited trough cross beds paleocurrents became more complex up through the bar as low stage component flows directed towards the river bank influenced the topographically high areas of the bar winnowing of the bar head top produced a coarsening up of sand grain size this seaton sluice river is interpreted as an ancient medial bar which migrated westward along a 1 9 km wide channel and accreted onto the northern river bank to form a lateral bank attached bar there is no evidence of bar emergence and flow stage flunctuations were probably not pronounced this type of detailed facies reconstruction enables accurate estimates to be made of channel depth width and flow variation channel processes were not related to local climate but to bank stability flow character and sediment grain size modified journal abstract

mid neoproterozoic 830 750 ma isotope stratigraphy of australia and global correlation abstract carbon and strontium isotope studies of numerous drillcore and outcrop sections within individual australian neoproterozoic basins in a tectonic and sedimentologic framework have allowed the construction of isotopic records for the interval 830 750 ma this combined with the discovery of a restricted stratigraphic range for the acritarch cerebrosphaera buickii in mid neoproterozoic strata of australia makes possible correlation with successions in canada namibia and spitsbergen that part of the mid neoproterozoic succession in spitsbergen which contains c buickii was previously considered younger than the sturtian glacial epoch but this study has indicated that the akademikerbreen group is older than that a correlation scheme is proposed along five tie lines and is based on a correlation between australia and canada at 830 ma and australia spitsbergen and namibia at 760 ma the lowest ever recorded seawater 87 sr 86 sr ratios in the upper shaler supergroup of canada 0 70561 0 70670 compare with ratios in evaporites of the 830 ma bitter springs formation amadeus basin australia 0 70568 0 70720 comparable 13 c carb records support a correlation at about 760 ma a correlation is made between the heaviest mid neoproterozoic 13 c carb values of 7 2 upper burra group adelaide rift complex and 8 1 kanpa formation western officer basin in australia 8 5 in spitsbergen backlundtoppen formation akademikerbreen group and 8 5 in namibia ombombo subgroup congo craton the stratigraphic range of c buickii in australia and spitsbergen supports this correlation a compilation of 13 c carb and 87 sr 86 sr records between about 830 and 750 ma from australia canada namibia and spitsbergen reveals numerous negative excursions which are not associated with preserved glacial deposits it is necessary therefore to consider additional models to the snowball earth to explain the large and rapid carbon isotopic excursions

sedimentology of a proterozoic erg the venkatpur sandstone pranhita godavari valley south india reappraisal of the late proterozoic venkatpur sandstone indicates that the bulk of the sandstone is aeolian in origin aeolian stratification types namely i inverse graded translatent strata ii adhesion laminae iii grainflow strata and iv grainfall strata are present throughout the outcrop belt nine facies have been identified that represent both aeolian and related aqueous environments within a well developed erg cosets of large cross beds at the bellampalli section in the nw of the study area record dune fields in the interior of the sand sea to the se at the godavari river and ramgundam sections a progressive increase in the relative proportion of the flat bedded to cross bedded facies and intercalated non aeolian facies delineates the transition from the dune field to sand sheet environment an alternating sequence of aeolian and marine sediments at laknavaram in the extreme se marks the termination of the sand sea palaeocurrent data suggest that the nw se trend of the sections represents a transect across the sand sea in a direction normal to the resultant primary palaeowind direction abundant horizontally stratified units in the vankatpur sandstone do not always represent the interdune sediments on the basis of the thickness and geometry of the units nature of bounding surfaces and associated facies sequence the facies is variously interpreted to represent interdune inland sabkha sand sheet and coastal sand flat deposits

evolution of atchafalaya lacustrine deltas south central louisiana abstract the entrenched pleistocene holocene mississippi alluvial valley was cut during the last low sea level stand and alluviation of the entrenched valley occurred during the holocene sea level rise regional gulf coast structures salt domes and faults have strongly influenced the morphology of the valley valley paleotopography has controlled depositional patterns and locations of major river courses as well as partially isolated large interdistributary basins within the alluvial deltaic plain interdistributary basin formation is further enhanced by distributary channel progradation switching and abandonment in conjunction with compactional subsidence of the alluvial plain surface geomorphology and sediment cores from the atchafalaya basin louisiana revealed that this basin has alternately accumulated fine grained organic rich sediments and terrigenous clastic dominated meander belt and lacustrine deposits basin subsidence and the presence of an active distributary channel in the basin are major determinants of the lithology of the sedimentary fill prograding lacustrine deltas of the atchafalaya river are the primary mechanism for filling the atchafalaya basin these upward coarsening deltas are relatively thin 3 m but are areally extensive tens to hundreds of kilometers and because they occur so frequently during the infilling process they account for a significant volume of the basin sequence the atchafalaya basin sequence documents lacustrine deltaic episodes dating from the early holocene maringouin and teche deltas to the present atchafalaya delta continued development of the atchafalaya river will rework portions of the backswamp and lacustrine deposits into meander belt deposits upon complete filling of the atchafalaya basin sediment will mostly bypass the basin and become deposited in the marine delta in atchafalaya bay

proterozoic fluvial styles response to changes in accommodation space rivieradal sandstones eastern north greenland abstract fluvial styles recorded by the uppermost part of the neoproterozoic rivieradal sandstones succession of eastern north greenland reflect variations in rate of generated accommodation space and possibly climatic changes three facies associations arranged in two genetic sequences are recognised within the succession the lower sequence initially records little available accommodation space a high degree of reworking results in sheet like high energy bed load dominated braided river deposits lacking recurrent facies patterns as accommodation space increases upwards through the sequence reduction in reworking is recorded by the development of fining and thickening upward muddy fluvial cycles evidence of desiccation or prolonged periods of drought are absent within the deposits and climate was probably relatively humid channel deposits in the lower sequence reflect mixed load braided fluvial systems with stable channel banks and floodplains and the gradient appears to have been low to moderate these features are generally considered favourable for the establishment of meandering river systems but channels nevertheless retained an overall braided character and their deposits show no evidence of meandering despite indications of a climatic setting without significant periods of drought sediments indicate that large fluctuations in discharge occurred within the mixed load streams and this is suggested to be the main cause for the absence of meandering the swift and rather dramatic response of the fluvial systems to changes in precipitation probably resulted from rapid runoff rates caused by the absence of vegetation the upper sequence shows an initial return to shallow sandy braided river deposition recording little available accommodation space a subsequent increase in the rate of generated accommodation space is indicated by the presence of alternating sheet sandstones and sand streaked mudstones with abundant desiccation cracks the sheet sandstones show evidence of high energy unconfined ephemeral fluvial flash flood deposition while the mudstones are interpreted to represent muddy floodplain deposits the change in fluvial style combined with the widespread evidence of desiccation suggest an evolution towards a more semi arid climate in the upper sequence this climatic change could account for the reduced clastic input seen in the overlying marine succession which culminated in carbonate platform deposition the present study suggests that even under conditions considered favourable for the formation of meandering streams these will rarely occur in proterozoic deposits due to the lacking influence of vegetation although meandering deposits cannot be ruled out as having formed in pre vegetational times the conditions for their formation appear to have been even more restricted than previously realised

pin stripe lamination a distinctive feature of modern and ancient eolian sediments abstract pin stripe laminations are a distinctive feature of modern and ancient eolian sediments in sets of eolian ripple or translatent strata they represent deposition of silt and very fine sand in the troughs of the advancing wind ripples in sets of avalanche strata they probably result from the downward settling of fine sand and silt within the moving avalanche to the interface of moving and unmoving sands wind tunnel experiments suggest that pin stripe laminations can also form in grainfall deposits the textural segregation associated with deposition of the fine layers in most cases leads to early cementation along and near the finest sand and silt comprising the pin stripe lamination the pin stripe effect seen in outcrops is usually due to resistance to weathering along such cemented zones the cementation of the pin stripe laminations can occur early in the history of diagenesis and thus may provide clues to the post depositional history of the rock pin stripe laminations in many instances represent the sequestering of the small population of ultrafine sediment present in most eolian depositional systems they may prove useful in the recognition of ancient eolian sediments

geometry and origin of large scale deformation structures in some ancient wind blown sand deposits large scale deformational features that disrupt the cross bedded strata of some ancient wind blown dune sand deposits jurassic aztec and navajo sandstones u s a can betraced laterally for at least tens of metres information from four exposures leads to an idealized deformation style characterized by 1 a head portion marked by collapse features 2 a middle portion marked by a thrust ramp or a large recumbent fold and 3 a toe portion marked by a planar shear zone with small recumbent folds drag folds and possibly small horizontal shear faults an hypothesis involving earthquake induced liquefaction and collapse of the dunes is proposed sands liquefied during earthquakes cannot support an unequal surface load resulting in the collapse of surface dunes above the liquefied substrate compensatory lateral squeezing of liquefied sand and buried strata produces the shear zone in the toe portion engineering studies suggest that buried strata underlying interdunal lows where over burden stress is least will be most susceptible to liquefaction other considerations suggest that it will be the steeper lee sides of advancing surface dunes that collapse squeezing liquefied sand forward or downwind i e away from the lee slope of surface dunes if the liquefied condition persists load structures can be formed when unliquefied layers founder into the liquefied substrate

sedimentology of cold climate aeolian sandsheet deposits in the askja region of northeast iceland active aeolian sandsheets cover an area of 270 km2 in the askja region of ne iceland the sandsheet is divided into i a deflationary upwind part that is characterised by deflation lags ventifacted boulder fields and restricted aeolian sand patches in the lee of obstacles ii an accumulating central part that is characterised by continuous aeolian sand cover up to 10 m thick where three scales of aeolian ripples are evident along with larger scale forms including longitudinal zibars and iii a downwind part that is currently subject to aeolian bypass with only localised accumulation occurring in topographic hollows between basaltic lava fields trains of aeolian granule megaripples are a characteristic landform of the downwind part of the sandsheet these sinuous crested oblique bedforms attain heights of 0 35 m and downwind spacings of 6 m they are composed principally of low density pebble grade pumice clasts and other reworked volcaniclastic detritus derived from the nearby askja volcanic centre the distribution of landforms across the sandsheet reflects a regional aeolian sediment budget that is controlled by dynamic interactions between glacial ice margin fluvial aeolian and volcaniclastic processes under the influence of a distinctive climatic regime sediment supply at the upwind sandsheet margin adjacent to the dyngjujokull glacier increases during summer periods when rates of glacial ablation are at a maximum longer term controls on sediment supply rate are determined by episodes of glacier surging and retreat and by periodic eruptions of volcaniclastic debris from the askja volcanic centre the availability of sediment for aeolian transport is controlled by the level of the groundwater table the depth and continuity of the permafrost layer in the proglacial area the degree of development of armoured pebble lags and freezing of the sandsheet surface during winter months the transport capacity of the wind varies on a seasonal basis with mean wind velocities and wind gustiness reaching a maximum during winter periods both winter and summer mean wind velocities have been steadily increasing since formal records began in 1969 the suite of aeolian landforms developed within the askja region as defined by sediment texture and composition and bedform scale morphology and spatial distribution reflects a special set of controlling circumstances which mean that the askja sandsheet deposits differ significantly from those described from elsewhere

architecture of channel belt deposits in an aggrading shallow sandbed braided river the lower niobrara river northeast nebraska abstract architecture of recent channel belt deposits of the niobrara river northeast nebraska usa records the response of a sandy braided river to rapid base level rise up to 3 m of aggradation has occurred within the lower 14 km of the niobrara river since the mid 1950s as a result of base level rise at the confluence of the niobrara and missouri rivers aerial photographs and channel surveys indicate that the lower niobrara has evolved from a relatively deep stable channel with large bank attached braid bars to a relatively shallow aggrading channel with braid bars and smaller secondary channels architecture of channel belt deposits associated with the recent aggradation has been defined using ground penetrating radar gpr and vibracores the channel belt deposits exhibit a series of amalgamated channel fills and braid bar complexes i e macroforms radar facies identified in the gpr data represent architectural elements of the braid bar complexes large and small bedforms two dimensional 2 d and three dimensional 3 d dunes and channels individual braid bars appear to consist of basal high flow and upper low flow components preservation of the complete high flow bar geometry is generally incomplete due to frequent migration of smaller scale secondary channels within the channel belt i e braided channel network at low discharges the large scale stratification of the braid bar deposits is dominated by cross channel and upstream accretion elements of downstream accretion are also recognized these accretion geometries have not been documented previously in similar sandy braided rivers braid bar deposits with low flow modification e g incision by secondary channels are recognized in the deeper portions of the deposits imaged by gpr preservation of braid bars with both high and low flow components is a result of the rapid base level rise and channel bed aggradation experienced by the niobrara river over the past 45 years recent avulsion of the river channel allowed preservation of the upper low flow component of the braid bar deposits i e bar top sequences the relative abundance and stratigraphic position of the amalgamated channels and braid bar complexes within the channel belt deposits constitute a signature of the recent base level rise

proterozoic aeolian quartz arenites from the hornby bay group northwest territories canada implications for precambrian aeolian processes abstract the hornby bay group is a middle proterozoic 2 5 km thick succession of terrestrial siliciclastics overlain by marine siliciclastics and carbonates a sequence of conglomeratic and arenaceous rocks at the base of the group contains more than 500 m of mature hematitic quartz arenite interpreted to have been deposited by migrating aeolian bedforms bedforms and facies patterns of modern aeolian deposits provided a basis for recognizing two sequences of aeolian arenite both sequences interfinger with alluvial wadi fan conglomerates and arenites deposited by braided streams depositional processes facies patterns and paleotopographic position of the arenites are consistent with modern sand sea dynamics distal aeolian facies in both sequences are composed of trough crossbed megasets deposited by climbing sinuous crested transverse dunes megasets comprise a gradational assemblage of tabular to wedge planar cosets formed by deflation reactivation of dune lee slopes and migration of smaller superposed aeolian bedforms small dunes and wind ripples megasets in the proximal facies are thinner display composite internal stratification and have a tabular planar geometry which suggests that they were formed by smaller straight crested transverse dunes most stratification within the crossbeds is inferred to have formed by the downwind climbing of aeolian ripples across the lee slopes of dunes remarkably few precambrian aeolian deposits have been reported previously this seems anomalous because most precambrian fluvial sediments appear to have been deposited by low sinuosity braided streams the emergent parts of which are prime areas for aeolian deflation frequent floods and rapid lateral migration of precambrian humid climate fluvial systems probably restricted aeolianite deposition to arid paleoclimates thus the apparent anomaly may reflect non recognition and or non preservation of aeolianites and or variations in some aspect of sand sea formation and migration unique to the precambrian reconstruction of the hornby bay group aeolianites using recently developed criteria for their recognition suggests that the latter reason did not exert a strong influence

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

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

evolution of a rift basin dominated by subaerial deposits the guaritas rift early cambrian southern brazil abstract most existing models for the evolution of rift basins predict the development of deep water depositional systems during the stage of greatest tectonic subsidence when accommodation generation potentially outpaces sedimentation despite this some rift basins do not present deep water systems instead being dominated by subaerial deposits this paper focuses on one of these particular rift basins the cambrian guaritas rift southern brazil characterized by more than 1500 m of alluvial and aeolian strata deposited in a 50 km wide basin the deposits of the guaritas rift can be ascribed to four depositional systems basin border alluvial fans bed load dominated ephemeral rivers mixed load ephemeral rivers and aeolian dune fields these four systems are in part coeval and in part succeed each other forming three stages of basin evolution i rift initiation to early rift climax stage ii mid to late rift climax stage and iii early post rift stage the first stage comprises most of the guaritas group and is characterized by homogeneous bed load dominated river deposits which do not clearly record the evolution of subsidence rates the onset of sedimentation of finer grained deposits occurred as a consequence of a reactivation event that changed the outline of the basin and the distribution of the nearby highlands this strongly suggests that the capture of the main river system to another depression decreased the sediment supply to the basin the study of the guaritas rift indicates that rift basins in which the sediment supply exceeds the accommodation generation occur as a consequence of moderate subsidence combined with the capture of a major river system to the basin during the initial stages of basin evolution in these basins changes in the average discharge of the river system or tectonic modification of the drainage network may be the major control on the stratigraphic architecture

ripple marks in quartz arenites of the hurwitz group northwest territories canada evidence for sedimentation in a vast early proterozoic shallow fresh water lake abstract supermature quartz arenites of the whiterock member deposited in the early proterozoic intracratonic hurwitz basin have a maximum thickness of 400 m and a minimum areal extent of 100 000 km2 the unit is unusual in that it is virtually devoid of other rock types and displays sedimentary structures that consist almost entirely of short spaced ripple marks and parallel stratification both formed by oscillatory flow the ripples have spacings that are consistently small average

a new phase diagram for combined flow bedforms abstract combined flows which involve a combination of unidirectional and wave induced oscillatory flow are omnipresent in coastal and lacustrine environments despite the extensive progress made in understanding the bedforms generated by such flows in past studies there still remains a wide range of unexplored flow conditions where the bedform geometry and hence consequent sedimentary structures have not been explored especially for strong unidirectional flows greater than 0 30 m s 1 and intermediate oscillation periods between 2 and 8 seconds to address this gap in our knowledge the stability of bedforms in a 0 25 mm diameter sand bed was studied under pure unidirectional pure oscillatory and combined flow conditions with oscillation periods of 4 5 and 6 s the maximum orbital velocity uo was varied from 0 00 to 1 00 m s 1 while the unidirectional component uu was varied from 0 to 0 50 m s 1 the experimental data collected under unidirectional flows stronger than 0 30 m s 1 allows expansion of current understanding of the bed configurations for current dominated combined flows where the phase boundary between combined flow bedforms and current ripples was uncertain under these flow conditions ten distinctive bedform states can be recognized no motion nm 2d symmetric ripples 2d sr 3d symmetric ripples 3d sr 3d symmetric dunes 3d sr 3d asymmetric ripples 3d ar 3d quasi asymmetric ripples 3d qar 3d asymmetric dunes 3d ad 3d current ripples 3d cr 3d current dunes 3d cd and upper stage plane bed uspb each of these bedform stages is described quantified and characterized in dimensional phase diagrams a complete re evaluation of the nomenclature for combined flow bedforms is proposed which includes their planform and cross sectional geometry in order to better represent the bed morphologies this new nomenclature also unifies past research on bedforms in both unidirectional and oscillatory flows and thus presents a new synopsis of bedforms developed under such flows one of the main changes proposed that allows integration with the nomenclature used in unidirectional flows is the reclassification of large ripples as dunes furthermore the introduction of the planform and cross sectional geometries as properties by which to classify bedforms leads to the definition of a stable phase space for two dimensional symmetrical ripples and three dimensional quasi asymmetrical ripples these new data and analysis allow proposition of a new unified phase diagram for combined flows

some aspects of the geochemistry provenance and palaeoclimatology of the torridonian of nw scotland geochemical data mostly from mudstones are used in an attempt to investigate some aspects of the palaeoclimatology and provenance of the torridonian succession in nw scotland the basal part of the stoer group which has been interpreted as glaciogenic is thought to have formed in a warm arid setting the presence of oscillation ripples and desiccation cracks in mudstones containing isolated clasts is thought to preclude their interpretation as ice rafted debris some aspects of the major element geochemistry of the stoer group mudstones suggest deposition in an environment with little weathering but chemical weathering is inhibited in both frigid and arid settings and sedimentological evidence favours the latter the stoer group mudstones are enriched in mg ca and na relative to the torridon group samples which are much closer in composition to an average shale estimate paas the cr and ni contents of the stoer mudstones are significantly higher than those of the torridon group samples there is however no evidence that enrichment in these elements was due to contamination with volcanic material like that of the stac fada member the scourian basement appears to be an adequate source of the ni and cr in the stoer group sediments th sc ratios are higher in the torridon than in the stoer mudstones this difference is interpreted as indicating that the stoer mudstones were largely derived from local archaean sources but that the torridon group mudstones incorporated materials from a much wider provenance including younger and recycled materials evidence from rare earth elements also provides evidence for different source terrains for the stoer and torridon groups the torridon group mudstones are more weathered have a higher ree content and display a more prominent eu anomaly lower value for eu eu than do the stoer samples these results support evidence from th sc ratios that the torridon group was derived from a more varied recycled and granite rich terrain than the stoer much of which appears to have relatively local provenance the geochemistry of mudstones can shed considerable light on problems of provenance and palaeoclimatology because many useful trace elements tend to be concentrated in such fine grained rocks and some aspects of their major element chemistry may be related to climatic factors

kosi megafan historical records geomorphology and the recent avulsion of the kosi river abstract it has been proposed that the kosi river continuously migrated 113 km westward across the surface of the megafan over the last two centuries examination of a number of old maps published between 1760 and 1960 shows that during most of this period the kosi river occupied a position slightly east of the megafan axis the apparent channel movement shown in these maps is oscillating in nature and not unidirectional instead of encountering deposits left behind by a sweeping braided kosi like stream a preliminary study of the uppermost 2 5 m succession in the north central part of the megafan reveals overwhelming dominance of meandering stream deposits assuming the existing notion of kosi river migration the rate of deposition averaged over 100 200 years for the uppermost 5 10 m of megafan deposits works out to be unusually high 50 mm y all these observations question the soundness of the hypothesis of rapid westward migration of the kosi river over the last two centuries the existing facies model for the uppermost 8 10 m of the megafan deposits also appears untenable the three dimensional geometry of the kosi megafan is similar to those of typical alluvial fans but with much gentler gradient 0 05 0 01 and with larger area 10 000 km 2 based mainly on the patterns of paleo and modern channels recognised in the satellite images three major accretionary lobes can be identified on the kosi megafan relative age of the lobes determined from the truncating relationship of the paleochannels indicates a random shift of the trunk channel forming these lobes similar multilobate form and evidences of random switching of the loci of lobe aggradation are also found to be common in the tista and taquari megafans the factors known to favour avulsion and the results of the recent simulation studies of alluvial deposits are inconsistent with the notion of unidirectional shift of the channels for more than 100 km across the entire megafan surface this study suggests that the relocation of the kosi river in the past was through random nodal avulsion rather than systematic unidirectional shift the recent avulsion of the kosi channel by a large distance to the east follows this expected pattern further study and age dating is required for a comprehensive understanding of the depositional dynamics of the megafan and the pattern of channel movement on it future flood predictions and disaster management plans should be based on such comprehensive understanding

aeolian to marine transition in cambro ordovician cratonic sheet sandstones of the northern mississippi valley u s a cratonic quartz sandstones have presented several intractable problems besides their extreme textural and compositional maturity and paucity of shale their sheet like geometry is particularly notable if the sandstones were entirely marine as long supposed such geometry is difficult to explain in terms of modern shelf sediments which are generally held to be either relict or only slightly reworked by the holocene transgression palimpsest re study of two quartz sandstones in the northern mississippi valley region reveals evidence for significant non marine deposition followed by varying degrees of marine reworking during transgressions facies patterns are similar in the cambrian wonewoc and ordovician st peter sandstones both of which overlie unconformities in both a large scale cross stratified facies believed to represent aeolian ergs passes laterally into a planar and channelled facies inferred to represent sand plains composed of braided fluvial and aeolian sand sheet deposits criteria of aeolian deposition in both facies include adhesion structures large ripple index fine climbing translatent lamination grainfall and grainflow stratification criteria of braided fluvial deposition include shallow channels containing sequences of thinning upward sets of trough cross stratification reactivation surfaces low index ripples and polygonal cracks probable aeolian sand sheets contain flat bedding punctuated by small channels adhesion structures and coarse sand ripples with large index there is a conspicuous absence of trace and body fossils from these inferred non marine deposits in contrast is a burrowed and trough cross stratified facies characterized by medium scale cross bedding alternating with bioturbated intervals and rare brachiopod or trilobite mould coquinas which is interpreted as shallow marine in both formations this last facies replaces laterally and overlaps the other two reflecting transgression and variable reworking the main areas of non marine deposits in both formations are capped by a thin burrowed subfacies that represents the culmination of each transgression that is a stillstand during which sediment influx ceased and both physical winnowing and bioturbation were intense it is suggested that the sheet like geometry of many cratonic quartz sandstones is due primarily to initial sand dispersal by aeolian and fluvial processes that such processes must have been orders of magnitude more important on pre devonian vegetation free landscapes than they have been since not only helps to explain the sheet like character but also the exceptional maturity of the older cratonic sandstones

fluvial channel belts floodbasins and aeolian ergs in the precambrian meall dearg formation torridonian of scotland inferring climate regimes from pre vegetation clastic rock records abstract the interpretation of climate regimes from facies analysis of precambrian clastic rocks has been challenging thus far hindering full reconstructions of landscape dynamics in pre vegetation environments yet comparisons between different and co active sedimentary realms including fluvial channelised floodplain and aeolian hold the potential to shed further light on this thematic this research discusses a fluvial aeolian record from the 1 2 ga meall dearg formation part of the classic torridonian succession of scotland tentatively considered to date as a braided fluvial deposit this unit is here reappraised as the record of fluvial channel belts floodbasins and aeolian ergs fluvial deposits with abundant transitional to upper flow regime structures mostly cross beds with tangential sets and plane antidunal beds and simple low relief sediment bars indicate a low sinuosity ephemeral style floodbasin deposits consist of plane and cross beds ubiquitously bounded by symmetrical ripples and rare sediment bars related to the progradation of splay complexes in temporary flooded depressions aeolian deposits occur nearby basement topography and are dominated by large scale pin stripe laminated cross beds indicative of intermountain ergs neither ephemeral fluvial nor intermountain aeolian systems can be considered as reliable indicators of local climate since their sedimentary style is respectively controlled by catchment size and shape and basin topography relative to groundwater tables contrarily the occurrence of purely clastic rather than carbonate or evaporitic floodplain strata can be more confidently related to humid regimes in brief this study provides new insight into an overlooked portion of the torridonian succession of scotland and discusses climate inferences for precambrian clastic terrestrial rocks

pre vegetation fluvial floodplains and channel belts in the late neoproterozoic cambrian santa barbara group southern brazil abstract one key element to the understanding of the dynamics of pre vegetation fluvial systems is the reconstruction of processes operating on their floodplains given that in modern systems channel banks and floodplains are the environments most affected by plant colonization notwithstanding few pre vegetation floodplains have been described and major questions regarding their most basic characteristics are still unresolved in order to address these questions detailed analysis of coeval channel belt fluvial floodplain and alluvial fan deposits from the santa barbara group late neoproterozoic to early cambrian southern brazil was performed while floodplain facies resemble ephemeral stream deposits being coarser grained than modern floodplains and marked by the stacking of flood event cycles channel belt deposits show composite bars which do not present conclusive evidence for high water discharge variation the floodplain deposits show particular features common to other pre vegetation fluvial systems such as better preserved small scale structures lack of bioturbation and abundance of cross laminated sandstones while other features differ from previous depositional models namely abundant mudcracks and evidence of soil formation the lateral variation of depositional systems recorded in the santa barbara group shows contrasting signatures of water discharge variation in sand dominated coeval environments and offers an example of the relation between different alluvial environments before the evolution of land plants

architecture of pre vegetation sandy braided perennial and ephemeral river deposits in the paleoproterozoic athabasca group northern saskatchewan canada as indicators of precambrian fluvial style abstract the late paleoproterozoic athabasca basin contains more than 1 5 km of predominantly sandy strata most of which are of braided fluvial origin in the eastern part of the basin at mcclean lake sandstones and minor conglomerates of the read formation at the base of the succession are preserved within a steep walled valley system they consist predominantly of meter scale sheet elements characterized by massive and flat laminated fine to very coarse grained sandstone with minor discontinuous cobble and boulder conglomerate along lower set boundaries these are interpreted as sheet flood and stream flood deposits of a terminal dry land system deposited in an arid climatic setting analysis of closely spaced drill core indicates that late stage run off was confined to small shallow channels typically only a few tens of meters wide and a few meters deep overlying strata of the bird member of the manitou falls formation are predominantly coarse to very coarse grained sandstones with abundant small scale cross stratification and minor granule and small pebble conglomerate at mcclean lake these appear to have been deposited as sheet elements by semi ephemeral to perennial braided rivers under more humid conditions at key lake 160 km to the southwest architectural analysis of a 184 m wide section indicates that at least part of the bird member was deposited in deeper sandy braided rivers characterized by seasonally varied flow the closest modern analogue is the south saskatchewan river in which large simple flow transverse bedforms become exposed and dissected during falling stage and act as nuclei for sand flat development the presence of numerous low relief 4th order surfaces suggests continued reworking of bar tops during rising stage the incremental character of downstream accretionary elements suggests periodic migration of barforms during peak flood stages separated by periods of low flow this indicates similarities with modern perennial braided systems and is counter to the idea that all pre devonian fluvial systems should consist of stacked sheets formed by individual flood events thin gravel layers accumulated as lags on fourth order surfaces with discontinuous mud layers suggesting deposition within temporary ponds in channel thalwegs after major floods strata in overlying units indicate a return to semi perennial conditions

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

preservation of pre vegetational mixed fluvio aeolian deposits in a humid climatic setting an example from the middle proterozoic eriksfjord formation southwest greenland abstract sedimentological studies of a 30 m thick coastal cliff section within the middle proterozoic eriksfjord formation in western south greenland reveals three distinct types of fluvial sand sheet deposits that reflect perennial streams type i semi perennial streams type ii and ephemeral flash floods type iii perennial river sand sheets are characterised by co sets of medium scale trough cross beds interbedded with isolated medium and large scale high angle tabular cross beds indications of desiccation or subaerial exposure are absent semi perennial fluvial sand sheets consist predominantly of low angle cross beds interbedded with isolated sets of high angle tabular cross beds with common reactivation surfaces horizontal lamination and climbing ripple lamination form subordinate structures associated with the sand sheets are adhesion structures and 0 05 0 4 m thick sets of wind ripple lamination indicating periods of subaerial exposure and aeolian reworking high energy ephemeral flash flood sand sheets consist almost exclusively of planar parallel lamination and climbing ripple lamination with some isolated sets of low angle cross bedding scouring and internal truncation surfaces are common the three types of sand sheets are considered to reflect deposition under changing climatic conditions varying from humid to arid or semi arid aeolian deposits are preserved within the sand sheets showing characteristics of dominantly perennial flow punctuated by shorter periods of desiccation type ii while sand sheets showing features typical of arid and or semi arid flow conditions type iii contain no preserved aeolian deposits this selective preservation is interpreted to be a result of the combined effect of groundwater table level and fluvial style which in turn are inferred to have been controlled by the climatic regime the deposits show that during pre vegetational times the preservation of aeolian deposits under certain conditions may be more optimal in fluvial systems formed in a humid climate than in fluvial systems formed under semi arid or arid circumstances the occurrence of aeolian deposits within a precambrian succession of fluvial deposits therefore need not be an indication of the most arid environmental conditions

initiation of 2 45 2 1 ga intracratonic basin sedimentation of the hurwitz group keewatin hinterland northwest territories canada continental to shallow marine rocks of the hurwitz group exposed as outliers across the hearne province in northern canada were deposited between 2 45 and 2 11 ga on a 3 3 2 6 ga granite greenstone basement initiation of the hurwitz basin was characterized by low relief and rates of subsidence that never progressed beyond the intensity of regional sagging during stage 1 polymictic conglomerate lower noomut formation was deposited in palaeovalleys cut into archaean basement and subarkose to quartz arenite quartz pebble conglomerate upper noomut formation was deposited on a low relief sand plain abundant first cycle quartz rich sandstones are attributed to intense humid climate weathering in low relief source areas and at sites of temporary storage as well as to repeated fluvial and eolian reworking interbedding of pyritic palaeoplacers and continental redbeds denotes fluctuating levels of atmospheric oxygen disappearance of pyritic palaeoplacers upsection signifies ultimate changeover to an atmosphere with sustained free oxygen stage 2 padlei formation was marked by a climatic shift toward cold glacial conditions as indicated by dropstones greywacke till pellets and possible ikaite pseudomorphs in lacustrine rhythmites stage 3 kinga formation represents a return to wet warm conditions with fluvial eolian deposition of subarkose and quartz arenite on a low relief sand plain maguse member and deposition of supermature quartz arenite in a vast 100 000 km2 minimum shallow water lake or series of lakes whiterock member local chert beds at the top of the kinga formation hawk hill member formed by spring discharge due to regional gravity driven convective groundwater flow an intracratonic setting is implied by 1 a symmetric basin centred fill 2 absence of features indicating a shelf slope break 3 absence of rift rocks 4 a prolonged slow subsidence history 5 likely correlation with other continental to shallow marine deposits scattered across the keewatin hinterland and 6 positioning 1000 2000 km away from closest coeval passive margin sequences we speculate that hurwitz basin formed due to lithospheric stretching related to a large scale mantle upwelling during the protracted breakup of an earliest palaeoproterozoic supercontinent kenorland

a horodyskia nenoxites dominated fossil assemblage from the ediacaran cambrian transition liuchapo formation hunan province its paleontological implications and stratigraphic potential abstract because of the scarcity of fossil data life evolution in deep water settings through the ediacaran cambrian transitional interval is still poorly understood this study describes a horodyskia nenoxites dominated fossil assemblage from the chert of the liuchapo formation in liujiata village western hunan province which was deposited in a slope to basinal setting at the terminal ediacaran and earliest cambrian in this assemblage horodyskia covers the longest range possibly from the upper part of the lower ediacaran to the base of the cambrian other members including three types of nenoxites helanoichnus shaanxilithes and two affinity undetermined forms appear in the upper part of the liuchapo formation the whole assemblage exhibits an upward increasing complexity and fossil abundance which reaches its acme at a few meters below the cambrian xiaoyanxi formation regardless of the lithology of the host rock these fossils are exclusively preserved with clay minerals and devoid of any detectable concentration of carbon such a preservation mode is dominant among the known records of these fossils worldwide and possibly related to an unknown biological commonality of these organisms microscope observations support a body fossil origin of nenoxites and contradict the trace fossil interpretation meanwhile some large horodyskia show features implying a close relation to testate amoebae however despite some resemblances the differences between these fossils and any suggested relatives are considerable taking all the information together we tentatively interpret this fossil assemblage as a diverse and complex deep water community of protist grade organisms the worldwide occurrence of some members in this assemblage endows them with a biostratigraphic potential nevertheless to establish such a tool more quality data of fossil occurrences and referential radiometric ages and a better understanding of the interplay between these organisms and sedimentary environments are required

alluvial facies evolution during the palaeozoic greening of the continents case studies conceptual models and modern analogues the palaeozoic greening of the continents the appearance and expansion of embryophytes land plants in terrestrial environments was arguably the most fundamental phanerozoic change to the earth system thirteen case studies of cambrian to devonian fluvial deposits from north america and europe are documented here to illustrate the evolution of fluvial style during this period during the cambro ordovician prior to the advent of terrestrial vegetation fluvial systems laid down relatively coarse sands with little mud resulting in self formed channels and an architecture dominated by broad sheets of trough cross beds sheet braided style similar deposits formed across a wide range of latitudes and passed basinward into sandy coastal deposits from the mid ordovician onwards an increase in floodplain mudstone corresponds broadly with the appearance of embryophytes which would have progressively enhanced upland weathering mud production and floodplain storage of fines during the late silurian small heterolithic channel bodies with lateral accretion sets provide the first evidence of meandering channels and floodplain mudstones contain more varied palaeosols especially calcretes which appear abundantly for the first time by the early devonian channel deposits comprise sandstone lenses channelled braided style probably due to the increased bank strength and cohesion and reduced potential for sediment sorting imparted by sand mud mixtures muddy coastal deposits are prominent by the upper devonian fluvial deposits commonly contain fossil trees and large mainstem meandering channels with lateral accretion sets indicating that rooted vegetation stabilized channels the advent of stable floodplains with levees crevasse splays and organic litter would have encouraged the diversification of terrestrial invertebrates which left alluvial ichnological signatures from the late silurian onwards

life on land in the proterozoic evidence from the torridonian rocks of northwest scotland the stoer group and diabaig formation of the torridonian succession in northwest scotland are late mesoproterozoic to early neoproterozoic ca 1200 1000 ma features preserved on the top surfaces of fine to medium grained sandstone beds in a number of stratigraphically and geographically separated localities are attributable to microbially induced sedimentary structures these include wrinkle structures remnants of apparent microbial crusts and indications of original cohesiveness and pliancy in sand sized sediment the surfaces on which the microbial structures formed were exposed subaerially abundant deep desiccation cracks and locally pedogenic structures in alluvial interfluve and lacustrine margin settings and many of the structures developed in areas well away from the perennially wetted regions adjacent to shorelines and fluvial channels thus these features indicate that earth9s biosphere had adapted to and colonized land surfaces many hundreds of millions of years before the dawn of the phanerozoic

tectonics and sedimentation in a paleo mesoproterozoic rift sag basin espinhaco basin southeastern brazil abstract sedimentologic paleogeographic stratigraphic structural and tectonic studies in a paleo mesoproterozoic metasedimentary succession espinhaco megasequence southeastern brazil indicates deposition in a rift sag basin four basin evolution stages are recognized prerift rift transitional and flexural the four stages can be represented by six unconformity bounded tectonosequences the unconformities are recognized in the field and mappable even on a regional scale the prerift and rift stages of the espinhaco basin were filled by products of continental depositional systems the prerift stage probably represents the first product of the rifting process before the development of the half grabens that characterize the rift stage during the rift stage mechanical subsidence due to lithospheric stretching was predominant and led to episodic rising of the depositional base level as a result the basin fill is characterized by coarsening upward intervals paleocurrent patterns indicate that block tilting and half graben subsidence uplift controlled sediment dispersion the first marine incursion within the espinhaco basin marks the change in the subsidence regime of the basin the evolution of the transitional and flexural stages was probably controlled by thermal subsidence due to thermal contraction of the lithosphere during cooling the transitional stage was characterized by relatively low subsidence rates higher subsidence rates and a consequent sea level rise characterize the flexural stage of the espinhaco basin in which three second order transgressive progradational sequences can be recognized

facies architecture and stratigraphic evolution of a river dominated delta front turonian ferron sandstone utah u s a abstract facies architectural analysis of a river dominated deltaic parasequence in the turonian ferron sandstone member near factory butte utah was conducted to determine the hierarchical spatial arrangement of mouth bars and their role in constructing the delta five architectural elements were identified using 21 measured sections paleocurrent measurements and bedding diagrams they are 1 prodelta pf 2 frontal splay fs 3 detached distal bars dbs 4 distributary mouth bars dmbs and 5 terminal distributary channels tdcs lower in the parasequence detached bars consist of proximally located cross bedded and quasi parallel laminated subaqueous bars that onlap the delta front slope and grade distally into finer grained distal delta front splays these bars translate downstream in a progradational aggradational degradational trajectory and are overlapped by adjacent bars forming a bar assemblage these bars are interpreted to have been deposited by descending hyperpycnal flows as a decelerating axial jet deposition of these bars occurs at the delta front to prodelta chokepoint presumably due to flow deceleration the distal bar assemblage is capped by an extensive but discontinuous shale drape suggesting local abandonment upper facies assemblages show a distinct switch to predominantly cross bedded distributary mouth bars corresponding to a decrease in water depth an increase in friction and a likely transition into planar versus axial jets this is indicated by aggrading cross bedded distributary mouth bars that overlie the distal bars laterally the mouth bar deposits are interstratified with terminal distributary channel deposits with prominent scour surfaces at the base separate distributary channels and their bars coalesce ultimately to build the entire delta lobe the detached distal bars prograde locally west to northwest and lack fair weather wave formed sedimentary structures suggesting a local embayment along the regionally northeast prograding shoreline low levels of ichnofaunal diversity indicate a stressed river dominated setting at the base of the section frontal splays and detached bars dominated by storm driven river flood deposits are deposited over muddy prodelta facies detached bars are likely initiated about 500 meters distal of the distributary channel in water depths of 10 30 meters based on the thickness of the parasequence once this subaqueous platform is constructed by the distal bar assemblage more proximal distributary mouth bars fill the remaining shallow water accommodation as the system continues to shallow distributary channels override and scour the bars reflecting reduced accommodation

how important and different are tropical rivers an overview abstract tropical river systems wherein much of the drainage basin experiences tropical climate are strongly influenced by the annual and inter annual variations of the inter tropical convergence zone itcz and its derivative monsoonal winds rivers draining rainforests and those subjected to tropical monsoons typically demonstrate high runoff but with notable exceptions high rainfall intensities from burst weather events are common in the tropics the release of rain forming aerosols also appears to uniquely increase regional rainfall but its geomorphic manifestation is hard to detect compared to other more temperate river systems climate driven tropical rivers do not appear to transport a disproportionate amount of particulate load to the world s oceans and their warmer less viscous waters are less competent tropical biogeochemical environments do appear to influence the sedimentary environment multiple year hydrographs reveal that seasonality is a dominant feature of most tropical rivers but the rivers of papua new guinea are somewhat unique being less seasonally modulated modeled riverine suspended sediment flux through global catchments is used in conjunction with observational data for 35 tropical basins to highlight key basin scaling relationships a 50 year daily model simulation illuminates how precipitation relief lithology and drainage basin area affect sediment load yield and concentration local sediment yield within the amazon is highest near the andes but decreases towards the ocean as the river s discharge is diluted by water influxes from sediment deprived rainforest tributaries bedload is strongly affected by the hydraulic gradient and discharge and the interplay of these two parameters predicts foci of net bedload deposition or erosion rivers of the tropics have comparatively low inter annual variation in sediment yield

the importance of air escape processes in the formation of dish and pillar and teepee structures within modern and precambrian fluvial deposits abstract attributed to escape of water from unconsolidated deposits teepee and dish and pillar structures are common features in the late proterozoic torridonian red bed succession of nw scotland study of the first annual flood on a modern depositional analogue in wadi baysh saudi arabia revealed that a layer of mud initially deposited across the alluvium restricted the escape of air being displaced through the normally unconfined aquifer by waters entering the aquifer from the river channels as air bubbled through high level pool floor muds it generated dish and pillar structures the formation of teepee structures by similarly combined air and water escapes nearer to the channel later in the annual cycle of flooding is foreseen it is suggested that the two sediment structures are generated in response to different parts of the aquifer recharge spectrum the role of air displacement by influent waters is important in the initial phases of structure formation and combined air and water escape creates greater disturbance to the sediments the sedimentary structures formed are not therefore solely due to water escape as has been previously suggested ephemeral seasonal flooding appears to have characterized the deposition of at least the stoer group and diabaig formation sediments in the torridonian

sedimentology of a wet pre vegetation floodplain assemblage descriptions of fluvial systems operating prior to significant terrestrial macrophyte vegetation have concentrated on assessing the impact of plant evolution on channel style this is probably in part due to the scarcity of well developed floodplain successions in fluvial assemblages of these ages this study describes wet pre vegetation floodplain deposits and processes observed and inferred from a continuous succession of drill core through an extensive 1 4 ga delta top channel floodplain assemblage forming a portion of the sibley group ontario canada sub aerial deposits are dominated by flaser wavy and lenticular bedded fine grained sandstones siltstones and mudstones with abundant small mudstone rip up clasts soft sediment deformation of these units is ubiquitous with loading and injection features being the most prominent thicker medium grained sandstone beds representing crevasse splays commonly have poorly developed protosols in their upper portions well laminated sediments with wave ripples and only rarely containing rip up clasts and soft sediment deformation were deposited in floodplain ponds these deposits differ from post vegetation floodplain sediments in having i better preservation of layering without rootlet bioturbation ii dominance of rippled sand on the floodplain probably due to lack of vegetation induced baffling iii large scale generation of small intraformational clasts iv desiccation crack fills consisting of peds and locally derived intraclasts probably delivered during horton overland flow during rainfall events v ubiquitous soft sediment deformation features in sub aerial deposits and vi well laminated commonly oxidized sediment that accumulated in floodplain ponds these attributes are the direct result of the lack of macrophyte vegetation and produce floodplain assemblages that are distinctly different from those presently forming in similar climatic settings

new aspects of deformed cross strata in fluvial sandstones examples from neoproterozoic formations in northern norway abstract extensive 20 200 m long exposures of tabular cross sets in neoproterozoic fluvial sandstone in northern norway demonstrate that deformed cross strata in the form of recumbently folded cross strata with associated massive sand are localized features passing in both up and down current direction into undeformed concave upward or sigmoidal cross strata the deformation occurs in down current inclined tangential wedge shaped zones beneath reactivation surfaces and less commonly as flat topped lenticular zones the localized nature of the sediment deformation is attributed to local liquefaction below the top of the bed in the case of the flat topped lenses and at the dune front in the case of the more common tangential wedges the position of the flat topped lenses suggests deformation by the shear stress of high velocity suspension laden currents although liquefaction of the dune front implies the action of gravity forces it is argued that the fluvial currents were the main driving force at the instant of bed liquefaction post folding gravitational shearing probably enhanced the deformation within the upper part of the wedges with their long flat lying toeset resulting from redeposition of downslope moving liquefied sand the down current alternation of deformed tangential wedges and undeformed cross strata suggests that the mechanism that triggered the liquefaction of the dune lee side was related to the fluvial system itself and hence was of autokinetic origin the tabular cross sets have previously been interpreted as a product of the dune upper stage plane bed flow regime in this flow context it can be speculated that the liquefaction and deformation occurred when the flow conditions approached the plane bed phase probably inducing a highly differential turbulent pattern and pressure fluctuations sufficient to liquefy the fine medium sand the small flat topped deformation lenses also suggest liquefaction by cyclic loading whereas the solitary nature of the large lenses may alternatively be the result of impulsive loading from bank collapse in less well exposed fluvial successions the auto or allokinetic origin of recumbently folded cross strata may be difficult to determine however the common occurrence of recumbent folds in cross stratified fluvial sandstones and their virtual absence in marine cross stratified deposits suggests strongly that in the majority of cases the deformation was autokinetic resulting from flow phenomena typical of river channels this kind of deformation structure should thus not be attributed to an allokinetic seismic triggering mechanism unless independent evidence of fault activity can be documented

detecting ancient life investigating the nature and origin of possible stromatolites and associated calcite from a one billion year old lake atb acknowledges the hospitality of the north west highlands geopark in july 2017 dw acknowledges funding from the australian research council via the future fellowship scheme ft 140100321

geology geography and humans battle for dominance over the delivery of fluvial sediment to the coastal ocean abstract sediment flux to the coastal zone is conditioned by geomorphic and tectonic influences basin area and relief geography temperature runoff geology lithology ice cover and human activities reservoir trapping soil erosion a new model termed bqart in recognition of those factors accounts for these varied influences when applied to a database of 488 rivers the bqart model showed no ensemble over or underprediction had a bias of just 3 across six orders of magnitude in observational values and accounted for 96 of the between river variation in the long term 30 years sediment load or yield of these rivers the geographical range of the 488 rivers covers 63 of the global land surface and is highly representative of global geology climate and socioeconomic conditions based strictly on geological parameters basin area relief lithology ice erosion 65 of the between river sediment load is explained climatic factors precipitation and temperature account for an additi

modeling fluvial incision and transient landscape evolution influence of dynamic channel adjustment 1 channel geometry exerts a fundamental control on fluvial processes recent work has shown that bedrock channel width depends on a number of parameters including channel slope and is not solely a function of drainage area as is commonly assumed the present work represents the first attempt to investigate the consequences of dynamic gradient sensitive channel adjustment for drainage basin evolution we use the channel hillslope integrated landscape development child model to analyze the response of a catchment to a given tectonic perturbation using as a template the topography of a well documented catchment in the footwall of an active normal fault in the apennines italy that is known to be undergoing a transient response to tectonic forcing we show that the observed transient response can be reproduced to first order with a simple detachment limited fluvial incision law transient landscape is characterized by gentler gradients and a shorter response time when dynamic channel adjustment is allowed the differences in predicted channel geometry between the static case width dependent solely on upstream area and dynamic case width dependent on both drainage area and channel slope lead to contrasting landscape morphologies when integrated at the scale of a whole catchment particularly in presence of strong tilting and or pronounced slip rate acceleration our results emphasize the importance of channel width in controlling fluvial processes and landscape evolution they stress the need for using a dynamic hydraulic scaling law when modeling landscape evolution particularly when the relative uplift field is nonuniform

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

predicting the terrestrial flux of sediment to the global ocean a planetary perspective a new model for predicting the long term flux of sediment from river basins to the coastal ocean is applied to a global data set of 340 river basins the model is based on relief basin area or averaged discharge and basin averaged temperature basinaveraged temperature is determined from basin location latitude longitude and the lapse rate across the basin relief hypsometric approximation the sediment flux model incorporates climate through basin temperature and hydrologic runoff solutions are provided for each of the major hemispheric climate regions polar temperate and tropic the model successfully predicts the pre anthropogenic flux of sediment to within the uncertainties associated with the global observations within a factor of two for 75 of rivers that range across five orders of magnitude in basin area and discharge most of the problem rivers are associated with low observational loads often smaller rivers where anthropogenic impacts are often magnified and temporal variability is high model predictions provide a baseline for researchers 1 to question the quality of observational data where available and disagreement is greatest 2 to examine a river basin for unusually large anthropogenic influences i e causes of erosion or causes of hinterland sediment retention and 3 to uncover secondary factors not addressed by our model lithology lakes the model provides a powerful tool to address the impact of paleo climate fluctuations warmer colder wetter drier on the impact of sediment flux to the coastal ocean d 2003 elsevier b v all rights reserved

orbital climate forcing in mudflat to marginal lacustrine deposits in the miocene teruel basin northeast spain abstract upper miocene mudflat and marginal lacustrine sediments were deposited on the fringes of an alluvial fan system in the low gradient endorheic southern teruel basin northeast spain the 147 m thick prado succession consists of 55 meter scale sedimentary cycles displaying a clastic rich and a carbonate rich interval the clastic rich parts are weakly pedogenically modified red to yellow orange mudstones and are interpreted as deposits of a well drained mudflat the carbonate rich sediments consist of calcretes marls and muddy limestones the calcretes range from dispersed nodules to massive nodular and are interpreted to be of pedogenic or phreatic origin the marls and muddy limestones are light to dark gray and rich in organic matter shell fragments and gastropods they are interpreted as ephemeral pond to marginal lacustrine deposits a composite sedimentary cycle model is presented summarizing the different facies and their vertical distribution throughout the prado area on a 10 m scale distinct stratigraphic intervals show lithologic trends representing alternating wetter and drier paleoenvironments cycle characteristics are laterally consistent whereby limestone beds pinch out towards the basin margin and thicken towards the central parts of the 10 km wide and 20 km long subbasin the high resolution magnetostratigraphy resolves four normal polarity and four reversed polarity intervals that are straightforwardly correlated to chron interval c5n 2n to c5ar 1r in the atnts04 time scale the resulting age control indicates a duration of 19 to 23 kyr for the meter scale lithofacies cycles suggesting control by precession driven climate change an astronomical tuning is established by correlating each cycle to the la2004 precession curve using recently recalibrated ages for the polarity reversals as a starting point the resulting astronomical ages for the reversals in the prado section are consistent with the astronomically calibrated magnetostratigraphy and cyclostratigraphy of the marine monte dei corvi section in italy at prado three intervals with thicker than average cycle thickness most likely reflect obliquity influence during 405 kyr eccentricity minima when precession amplitudes are low this corroborates the orbital forcing hypothesis of the lithofacies cycles and the astronomical tuning of the prado succession the combined magnetostratigraphy and cyclostratigraphy method applied in this study shows the high potential of this method for detecting orbital climate forcing in continental successions

3 2ga eolian deposits from the moodies group barberton greenstone belt south africa implications for the origin of first cycle quartz sandstones abstract the 3 2 ga moodies group in barberton greenstone belt south africa contains the oldest recognized evidence for eolian processes on earth these eolian facies indicate that significant wind regimes existed in the archean atmosphere eolian deposits are developed in the lower moodies group in the central part of the belt and consist of stacked inversely graded strata ranging in thickness from 2 to 10 mm individual strata consist of fine to medium grained quartz sandstone at the base that passes vertically into very coarse to coarse grained quartz sandstone strata make up cross bed sets up to 3 5 m thick sets of wind ripple strata are either tabular tangential or in the form of broad troughs this type of inversely graded stratification is the product of wind ripple migration the coarse grain size of the sediment combined with the low angle of inclination of the strata is best interpreted as the product of sand sheet or dune plinth preservation based on the geometry of eolian set boundaries dunes were probably simple barchan form the tens of meters thick eolian deposits are underlain by braided stream deposits composed of subfeldspathic pebbly sandstones the vertical change from braided stream subfeldspathic sandstones to eolian quartz arenites highlights the underappreciated capacity of eolian processes to generate first cycle quartz sandstones on landscapes pre dating the advent of land vegetation

the pseudofossil horodyskia flocs and flakes on microbial mats in a shallow mesoproterozoic sea appekunny formation belt supergroup western north america abstract horodyskia moniliformis yochelson and fedonkin 2000 is found abundantly in its type area the lower appekunny formation belt supergroup ca 1 45 ga of northwestern montana and adjacent southwestern alberta it has a distinct curving to meandering string of beads appearance on bedding planes of thin bedded laminated argillaceous siltstone and has been regarded by some as one of the oldest eukaryotic organisms in the fossil record the beads are disc shaped to lenticular 1 5 mm in diameter circular to elliptical to polygonal in outline and composed of variably silty clay they are interpreted as mud flocs and small intraclasts the beads in individual specimens are fairly uniform in size and spacing but string length varies from about one to 15 cm depending in part on bead size the radius of curvature varies from tight to open but strings never form loops isolated beads are also present and smaller ones become indistinguishable from pustules which along with mud cored wrinkles and blisters are associated on many bedding surfaces these are regarded as evidence for a benthic microbial mat with variable topography ranging from crudely linear or stellate ridges to small domes and pinnacles rather than representing a semi infaunal colonial eukaryote fungal bladders or macroalgae horodyskia is here interpreted to be large mud particles that were trapped on small protrusions of the microbial mat probably consisting of filament tufts commonly the tufts were oriented in rows of similar sized and generally evenly spaced elevations and these bound suspended flocs and flakes of a more or less uniform size resulting in the features that appear organized as strings of beads horodyskia is therefore considered to be a microbially induced sedimentary structure specific to a muddy relatively low energy subtidal marine setting with just the right combination of sediment type and depositional factors

a precambrian proximal ejecta blanket from scotland ejecta blankets around impact craters are rarely preserved on earth although impact craters are ubiquitous on solid bodies throughout the solar system on earth they are rapidly effaced and few records exist of the processes that occur during emplacement of ejecta the stac fada member of the precambrian stoer group in scotland has previously been described as volcanic in origin however shocked quartz and biotite provide evidence for high pressure shock metamorphism while chromium isotope values and elevated abundances of platinum group metals and siderophile elements indicate addition of meteoritic material thus the unit is reinterpreted here as having an impact origin the ejecta blanket reaches 20 m in thickness and contains abundant dark green vesicular devitrified glass fragments field observations suggest that the deposit was emplaced as a single fluidized flow that formed as a result of an impact into water saturated sedimentary strata the continental geological setting and presence of groundwater make this deposit an analogue for martian fluidized ejecta blankets

alluvial eolian and lacustrine sedimentology of a paleoproterozoic half graben baker lake basin nunavut canada the northeast trending baker lake sub basin was a volcanically active half graben during deposition of ca 1 85 1 76 ga baker lake group drainage was oriented along transverse and axial directions with flow to playa lake and deeper perennial lacustrine depocentres basin marginal streamflow dominated alluvial fans were concentrated along the southern margin and provided sediment from archean crystalline basement rocks these fed transverse gravel and sand bed braided streams alluvial dynamics were characterized by channel aggradation and abandonment abandoned channel belts were sites of floodplain and eolian deposition basin axial braided streams fed northeast and southwest to a depocentre near christopher island where eolian playa and lacustrine environments were intimately linked felsic minette flows were initially erupted from localized centres contemporaneous sedimentary deposits typically contain minor volcaniclastic components that increase in abundance basinward voluminous and widespread younger minette flows prograded outward from volcanic centres contributing significant additional basin infill

the string of beads fossil horodyskia in the mid proterozoic of tasmania abstract horodyskia has been found at a single tasmanian locality in the cassiterite creek quartzite ca 1300 800 ma part of a thick proterozoic mildly deformed low greenschist facies marine shelfal siliciclastic succession known as the rocky cape group the rock hosting the fossils is thinly interbedded and interlaminated dark grey slaty shale and quartzose siltstone the sharp based graded siltstone layers are interpreted as distal storm surge deposits on an outer marine shelf the strings of beads are mostly preserved at the base of the siltstone layers in concave hyporelief external moulds on the soles of the event beds and as convex epirelief casts on the tops of the underlying shale beds the casts comprise shale identical to the underlying bed the beads average 1 7 mm in diameter and the gap between the borders of adjacent beads tends to be approximately equal to the bead diameter occasionally the fossils are preserved within shale as wholly flattened beads delineated by a subtle darkened halo the tasmanian strings of beads have most of the morphological attributes of previously described horodyskia including regularity of size and spacing of beads in any one string lack of branching and in some instances haloes and casts with apical depressions the strings on at least one bedding plane have a strong n s preferred orientation of unknown origin tectonic deformation has resulted in 30 shortening in a sw ne direction the morphologic similarity but differing mode of preservation of the tasmanian horodyskia to the two previously described mesoproterozoic species is strong evidence for a biologic origin for the string of beads phenomenon after morphological and morphometric comparisons with other species of horodyskia the tasmanian specimens are assigned to horodyskia williamsii

facies model for fluvial systems in the seasonal tropics and subtropics facies models that summarize the deposits of fluvial systems are well established for humid climate settings and for desert environments but the deposits of rivers in the sub humid and semiarid seasonal tropics have been largely ignored our observations and data from modern streams and recent deposits in northeastern australia show how such rivers with extremely variable discharge have distinctive deposit characteristics that are substantially different from conventional fluvial facies models these properties include 1 erosionally based channel fill lithosomes that exhibit complex lateral facies changes with 2 abundant pedogenically modified mud partings 3 complex internal architecture that may lack the macroform elements typical of other fluvial sediment bodies 4 an abundance of sedimentary structures formed under high flow stage and 5 an abundance of in situ trees that colonize channel floors and are adapted to inundation by fast flowing water we illustrate examples of this fluvial style from the rock record and set out a new facies model the recognition of such a distinctive fluvial character and of changes in character through vertical successions will aid paleoclimate and reservoir analysis

towards understanding the early gondwanan margin in southeastern australia abstract this review synthesizes the proterozoic and early paleozoic geology of tasmania bass strait and western and central victoria we examine the many different conflicting hypotheses that have been proposed to solve the paradoxical relationships between tasmanian geology and that of mainland australia most notably the prevalence of proterozoic basement of western and central tasmania while immediately across bass strait evidence of proterozoic rocks is much more cryptic we conclude that the selwyn block model is the most satisfactory hypothesis to date since it fits best with the obvious patterns in the magnetic and gravity data this model proposes that the central victorian melbourne zone is underlain by the northern extension of thin tasmanian proterozoic and cambrian crust under bass strait and that the silurian to middle devonian melbourne zone was shortened along a decollement during the tabberabberan orogeny the ordovician rocks of eastern tasmania correlate more closely with the tabberabbera zone than the melbourne zone in victoria however the silurian and devonian correlations are less certain major unresolved issues are the origins of the proterozoic and early cambrian lithostratigraphic packages tectonic models for their assembly during the tyennan orogeny and how these models fit with those for mainland australia

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

sedimentary facies in an extensional and deep lacustrine depositional system the pliocene tiberino basin central italy abstract the tiberino basin was formed during plio pleistocene extensional tectonic activity in central italy the pliocene sediments that fill this structure consist predominately of fine grained clastic lithofacies deposited in a lacustrine system four facies associations have been distinguished facies association a was deposited in a deep offshore lacustrine environment and consists of massive laminated bluish gray marly clays that lack benthonic fauna but have preserved organic matter leaves wood this facies association is the main lacustrine deposit whereas the other three represent marginal facies facies association b is interpreted as a delta system deposit it has gilbert type sandy gravel bodies with foresets up to 50 m high and prodelta bodies consisting of laminated to massive marly clays that alternate with sandy and gravelly mud strata association c corresponds to a coastal environment where two sub environments are recognized 1 a wave dominated coastline characterized by interbedded muddy and sandy strata with sedimentary structures attributed to hummocky cross stratification and 2 clayey silts and lignites interpreted as a coastal wetland a spectacular fossil forest is preserved in situ within this unit association d formed on the distal part of a muddy alluvial fan consists of sheet flood clayey sandy silts that alternate with incipient paleosols rare ribbon channel fills are also present paleoenvironmental reconstruction indicates that the tiberino basin was occupied by a narrow deep meromictic lake during the pliocene the size shape depth and sub environment distribution of this lake were controlled by tectonic phenomena such as the high subsidence rate which caused the lake to be large and deep only in the few coastal areas that had high sedimentary input were wetland or alluvial fan deposits formed

aeolian architecture bedform climbing and preservation space in the cretaceous etjo formation nw namibia sets of aeolian cross strata within the cretaceous etjo formation of nw namibia are bounded by a hierarchy of surfaces the origin of which are ascribed to one of four processes related to aeolian bedform and erg behaviour the base of the main aeolian succession is characterized by a basin wide erosional supersurface that formed in response to a period of aeolian deflation before the onset of the main phase of erg building interdune migration surfaces formed by draa migration are planar in sections parallel to the palaeowind and are inclined at up to 5 in an upwind direction sw perpendicular to the palaeowind interdune surfaces form 500 m wide troughs signifying crestline sinuosity within the original bedforms superimposition surfaces are inclined at 5 10 in a downwind direction and indicate the migration of crescentic oblique dunes over larger slipfaceless transverse draa reactivation surfaces associated with minor changes in dune slipface orientation are distinct from other bounding surface types because overlying cross strata lie parallel to them rather than downlap onto them analysis of the geometry of these bounding surfaces together with the orientation of the cross strata within the sets that they bound has enabled the detailed morphology of the original bedforms to be reconstructed the maximum preserved thickness of individual aeolian sets varies systematically across the basin from 52 m in the basin depocentre to only 8 m at the basin margin the set architecture indicates that this spatial variation is primarily the result of decreased angles of bedform climb at the basin margin rather than the presence of smaller bedforms similarly a temporal reduction in the angle of climb rather than a reduction in bedform size is considered to be responsible for an upward decrease in preserved set thickness reductions in bedform climb angle reflect progressive loss of accommodation space as the accumulating erg filled the basin

sedimentation patterns during the precambrian a unique record abstract although the similarities between depositional processes and products as well as the analogous controls on basin filling and evolution appear to have enjoyed great uniformity throughout the sedimentary rock record a noticeable distinction exists in the rates and intensities of a broad range of geological processes in the precambrian epoch this paper searches for distinctiveness in the precambrian sedimentary record both siliciclastic and carbonate through an extensive though not exhaustive review of the relevant literature augmented by new observations while differences in precambrian deltaic aeolian glacial and possibly also lacustrine deposits and settings appear to have been small their large scale development was controlled largely by a combination of temporal and geodynamic influences essentially of global compass in this regard the onset of the supercontinent cycle and major perturbations in palaeo atmospheric composition appear to have been significant marine environments provide a poor platform for precambrian phanerozoic comparisons of sedimentation patterns as those from the former period are preserved almost exclusively in epeiric settings an environment essentially lacking on modern earth for the shallow marine carbonates biological mediation of chemical sediment deposition changed radically from dominance by microbial biota in the precambrian to a combination of metazons protozoans and algae for the skeletal carbonates of the phanerozoic despite it being widely recognized that precambrian channel systems were braided in all environments deltaic tidal alluvial fluvial as a consequence of the lack of vegetation and poor development of soils the fluvial setting has some enigmatic aspects amongst these is evidence for ponding of muddy detritus in apparently sandstone bed load dominated braided systems with effects on local palaeoslopes which have resulted in unusual palaeohydraulic parameters for precambrian fluvial systems this is perhaps a field of research which holds greater promise when investigating sedimentation patterns prior to the phanerozoic

the oldest record of ediacaran macrofossils in gondwana 563 ma itajai basin brazil abstract the avalon biota ediacaran period 570 559 ma marks the first appearance of macroscopic and complex benthic communities in the fossil record this assemblage is known from a few localities worldwide mainly in canada and england here we report for the first time the presence of ediacaran macrofossils in deposits of similar age from gondwana itajai basin southern brazil our new radiometric date 563 ma indicates that the itajai basin can be chronocorrelated with the classic avalonian deposits and thus represents one of the oldest records of the ediacaran biota in gondwana we describe the presence of the ediacaran genus palaeopascichnus as well as discs aspidella and nimbia and other problematic forms contrary to the deep marine macroorganisms of the avalon assemblage the itajai fossils are associated with abundant and exceptionally preserved three dimensional microbial mats and microbially induced sedimentary structures miss in relatively shallow settings upper slope and distal delta front deposits in this sense the itajai biota could represent early adaptations of benthic macrobiota to the shallower and more photic environments that characterize the later white sea assemblage

problematic bedding plane markings from the middle proterozoic manganese subgroup bangemall basin western australia abstract enigmatic bedding plane markings impressions resembling isolated strings of beads occur in sandy and argillaceous strata of the stag arrow formation in the middle proterozoic manganese subgroup bangemall group of the bangemall basin east of newman western australia markings are abundant at two localities and traces are found at a third structures similar in both morphology and dimension have previously been reported from the appekunny formation in the middle proterozoic belt supergroup of u s a the manganese subgroup structures provide further details of morphology enabling comparison with a variety of biogenic and non biogenic structures a source for the markings still cannot be determined with certainty but a biogenic origin is favoured and affinities with the green or brown algae suggested such an origin is consistent with records of megascopic algae from rocks of similar age

stratigraphic classification of ancient lakes balancing tectonic and climatic controls lakes and lake deposits present two fundamental paradoxes 1 modern lakes are vastly complicated but the rock records of lakes are relatively simple extensive observations reveal three distinct facies associations of common and widespread occurrence these are referred to here as fluvial lacustrine fluctuating profundal and evaporative facies associations 2 most explanations of modern and ancient lakes attribute their nature to climate but neither modern lake parameters lake size depth and salinity nor the character of ancient lake strata thickness extent lithology correlate with measured or inferred climatic humidity we propose that it is the relative balance of rates of potential accommodation mostly tectonic with sediment water fill mostly a function of climate that controls lake occurrence distribution and character lake basins may be termed overfilled balanced fill or underfilled depending on the balance between these rates we conclude that climate and tectonics exert coequal influence on lake deposits at both mesoscales 1 m to hundreds of meters and macroscales hundreds to thousands of meters

quantitative geomorphology of the mars eberswalde delta the eberswalde delta is composed of six separate depositional lobes that have prograded some 17 km from their apex crosscutting distributary relationships and compensated depositional lobes are clearly visible in images acquired by the mars global surveyor mars orbiter camera here several lobe systems have been examined for sinuosity radius of channel curvature meander bend width and channel width parameters channel sinuosities of between 1 2 and 1 8 defi ne low to moderate sinuosity systems typical of the type transporting bedload or mixed grain size loads channel systems increase in sinuosity as they get older however some of the younger systems show specifi c reaches of increased sinuosity these localized changes may be due to either abutment against resistant beds of older lobes or rise in base level at the channel system terminus if the former such an effect of older deposits on the morphology of the channels suggests that these older lobes were fairly well indurated prior to the deposition of the youngest progradational lobes eighty six percent of distributaries in the mars eberswalde delta are 100 240 m wide comparatively 62 and 44 of distributaries in the atchafalaya and wax lake deltas of louisiana respectively are of similar size small distributaries may indicate lower average and shorter duration fl ows or coarser sediment in mars distributaries than those typical of the gulf coast systems the volume of the material in the mars deposit suggests long periods of sediment deposition sinuosity indexes meander bend migration and ridge and swale point bar topography suggest periods of stable discharge on the delta surface

methodology for reconstructing wind direction wind speed and duration of wind events from aeolian cross strata 1 a methodology for reconstructing wind direction speed and event duration from aeolian dune cross strata was developed from analysis of crescentic dunes at white sands new mexico during wind events dune lee faces were surveyed lee face deposits mapped deposition rates measured grain size sampled by stratification type and winds characterized from meteorological and field data the spatial distribution of lee face stratification styles is a function of the incidence angle formed between the wind and the brinkline with secondary controls by wind speed and dune sinuosity and height sets of wind ripple strata form at incidence angles of 25 40 grainfall grainflow foresets over wind ripple bottomsets at 40 70 and grainflow grainfall foresets at 70 90 erosional reactivation surfaces form at incidence angles up to 15 bypass surfaces up to 25 the total sediment load is fractionated within lee face stratification types wind speed can be reconstructed from relationships between grain size transport mode shear velocity and grain settling velocity where the full range of grain transport modes occurs and grain size is limited by shear stress the shear velocity and grain size range in each transport mode can be estimated by assuming the coarse fraction in grainflow strata traveled in creep and the coarse fraction in grainfall traveled in saltation the minimum duration of a wind event can be estimated using measures of shear velocity dune height and dune forward migration method limitations arise with source area control on grain size extremes in wind events and severe truncation of sets of cross strata

silicified horodyskia and palaeopascichnus from upper ediacaran cherts in south china tentative phylogenetic interpretation and implications for evolutionary stasis horodyskia is one of the earliest known macroscopic life forms with a fossil record dating from c 1 4 ga palaeopascichnus represents a key ediacaran element with world wide distribution however their body constructions and affinities are poorly understood partly because previously described species are mostly preserved as casts and moulds in siliciclastic rocks silicified specimens from the upper ediacaran liuchapo formation in eastern guizhou south china are described as horodyskia minor sp nov and palaeopascichnus jiumenensis sp nov their taxonomic assignments are based on their uniserial arrangement of spheroidal or discoidal units which are connected by a filament and surrounded by a quartz halo they are unlikely to be brown algae animal traces faecal pellets colonial metazoans or giant sulphide oxidizing bacteria instead we propose that horodyskia and palaeopascichnus may be phylogenetically related and their collective morphologies allow tentative comparison with agglutinated foraminifers the segments can be compared with cytoplasm filled chambers connecting filament with small passage between chambers and quartz haloes with agglutinated tests however their ontogeny appears to be distinct from that of modern foraminifers the occurrences of horodyskia fossils in mesoproterozoic and ediacaran rocks indicate an extremely long range c 900 ma and echoes the proposition of extended evolutionary stasis in the proterozoic

fluvial features on titan insights from morphology and modeling fluvial features on titan have been identified in synthetic aperture radar sar data taken during spacecraft flybys by the cassini titan radar mapper radar and in descent imager spectral radiometer disr images taken during descent of the huygens probe to the surface interpretations using terrestrial analogs and process mechanics extend our perspective on fluvial geomorphology to another world and offer insight into their formative processes at the landscape scale the varied morphologies of titan s fluvial networks imply a variety of mechanical controls including structural influence on channelized flows at the reach scale the various morphologies of individual fluvial features implying a broad range of fluvial processes suggest that paleo flows did not occupy the entire observed width of the features disr images provide a spatially limited view of uplands dissected by valley networks also likely formed by overland flows which are not visible in lower resolution sar data this high resolution snapshot suggests that some fluvial features observed in sar data may be river valleys rather than channels and that uplands elsewhere on titan may also have fine scale fluvial dissection that is not resolved in sar data radar bright terrain with crenulated bright and dark bands is hypothesized here to be a signature of fine scale fluvial dissection fluvial deposition is inferred to occur in braided channels in paleo lake basins and on sar dark plains and disr images at the surface indicate the presence of fluvial sediment flow sufficient to move sediment is inferred from observations and modeling of atmospheric processes which support the inference from surface morphology of precipitation fed fluvial processes with material properties appropriate for titan terrestrial hydraulic equations are applicable to flow on titan for fully turbulent flow and rough boundaries for low reynolds number flow over smooth boundaries however knowledge of fluid kinematic viscosity is necessary sediment movement and bed form development should occur at lower bed shear stress on titan than on earth scaling bedrock erosion however is hampered by uncertainties regarding titan material properties overall observations of titan point to a world pervasively influenced by fluvial processes for which appropriate terrestrial analogs and formulations may provide insight

sedimentary cycles in a mesoproterozoic aeolian erg margin succession mangabeira formation espinhaco supergroup brazil aeolian systems were abundant and widespread in the early proterozoic post 2 2 ga however the majority of aeolian successions of such great age are intensely deformed and are preserved only in a fragmentary state meaning that hitherto few attempts have been made to apply a sequence stratigraphic approach to determine mechanisms of aeolian construction accumulation and preservation in such systems the mangabeira formation is a well preserved mesoproterozoic erg successions covering part of the sao francisco craton northeastern brazil the lower unit of the mangabeira formation 500 m thick comprises aeolian deposits of dune interdune and sand sheet origin as well as some of waterlain origin these deposits are organized into vertically stacked depositional cycles each 6 to 20 m thick and characterized by aeolian sandsheet and waterlain deposits succeeded by aeolian dune and interdune deposits indicative of a drying upward trend aeolian cross strata exhibit a mean dip direction to the north each of these cycles likely arose in response to climatic oscillation from relatively humid to arid conditions possibly related to orbital forcing the lower unit of the mangabeira formation comprises up to 14 erg sequences the accumulation and preservation of each was determined by the relative rate of water table rise and the availability of sand for aeolian transport both of which changed through time resulting in the preservation of a succession of repeated drying upward cycles

sedimentology and architecture of the douglas creek terminal splay lake eyre central australia douglas creek terminal splay sited on the western shoreline of lake eyre north central australia covers a surface area of approximately 4 km2 with a down system length of 2 5 km from the distributary channels terminus to the splay fringe two distributary channels feed two sediment lobes which have amalgamated to form the terminal splay three primary facies associations have been identified sub dividing the creek terminus into distributary channel proximal and distal splay sections proximal splay sediments are characterized by erosionally based relatively thick 100 mm stacked sheets of coarse to medium sand which commonly display trough and planar cross bedding whereas the distal splay is characterized by thin generally 50 mm massive beds of very fine sand silt and clay the change in splay sedimentology is interpreted as reflecting the transition from bedload dominated deposition to suspended load dominated deposition from decelerating sheetfloods as they spread out from the channel onto the dry lake bed a proximal to distal splay transition zone is also noted where deposits of both facies associations interfinger laterally and vertically in scale geometry and facies associations the douglas creek terminal splay is very different to the often cited neales terminal splay complex located 70 km to the north it is suggested that these architectural differences reflect variations in discharge volume input sediment distribution and the degree of vegetation cover understanding the variation in terminal splay architecture has very significant implications for the modelling of analogous subsurface petroleum systems which at present relies on few modern day analogues

mesoproterozoic erg and sand sheet system architecture and controlling factors galho do miguel formation se brazil abstract aeolian systems of the precambrian time were established in different palaeoclimatic and palaeogeographic conditions than the present different morphologies of dunes draas as linear oblique and star forms are widespread in the recent aeolian systems however the ancient aeolian deposits are rarely interpreted as these types of bedforms and usually referred to as transverse dunes the nature and complexity of bedform morphology in precambrian aeolian systems as well as the factors that allow the construction accumulation and preservation of these environments are still not well defined extensive exposures of galho do miguel formation in southeastern brazil represent a mesoproterozoic erg succession and exhibit a distinct facies architecture which contributed to the understanding of these deposits three architectural elements were identified i tangential cross stratified sandstone with superimposed trough cross bedded sandstone ii planar laminated sandstone and iii extensive flat bedded sandstone dominated by wrinkly laminae these elements are organized into three succession groups representative of simple transverse dune oblique compound draa and sand sheet systems the simple to compound dune draa system corresponds to the most central parts of the erg associated with a higher sediment availability and lower water table level conversely the sand sheet system occurs under low sediment availability and higher water table level in the marginal portions of the erg precambrian climatic oscillations may have changed the aeolian system morphology allowing the alternation of simple and complex aeolian dunes resulting in high frequency changes in sand sheet system and the remobilization of previously accumulated sediments over large geographic areas

pre vegetation alluvial fan facies and processes an example from the cambro ordovician rozel conglomerate formation jersey channel islands prior to the silurian a lack of land vegetation is expected to have influenced the processes of sedimentation on alluvial fans principally by causing increased rates of run off and erosion in the fan catchments in the cambro ordovician rozel conglomerate formation this effect was central to the generation of alluvial fan deposits that are unusually deficient in sand and clay despite being sourced from a catchment dominated by sandstone and mudstone seven facies are identified interpreted as representing the deposits of i shallow stream and sheetfloods ii channelized non cohesive debrisflows iii sub aerial mud rich debrisflows iv sub aqueous mud rich debrisflows v low energy streams that reworked abandoned fan sectors vi a sandflat playa lake system and vii talus slopes the first two facies are both clast supported conglomerate comprise 98 of the deposit and represent deposition on active depositional lobes and in the fan head trench the remaining facies are the products of infrequent sedimentary processes fan abandonment processes and marginal sub environments the facies assemblage in many ways mimics that of a modern day water lain arid region fan however the palaeolatitude of these fans was high and the climate is inferred to have been cool and wet the near absence of sandstone and mudstone beds with few mudflows is ascribed to rapid hinterland uplift and high rates of erosion resulting in minimal chemical breakdown of source rocks in the catchments such high rates of erosion are in turn ascribed to a combination of frequent rainstorms and an absence of vegetation cover