soft sediment deformation in upper proterozoic torridonian sandstones applecross formation at torridon northwest scotland abstract the applecross formation upper proterozoic torridon group northwest scotland was deposited as planar laminated and cross bedded coarse sands in braided streams soft sediment deformation is described from a 97 m thick study section most of which comprises contorted stratification the main structures are cusps and complex recumbent folded cross bedding formed by water escape during fluidization four assemblages of structures are recognized deformation as isolated structures in an otherwise undeformed unit may have been triggered by fluid turbulence discrete and chaotic units record widespread liquefaction and fluidization respectively of the most recently deposited sediment deformation may have been triggered by upwelling groundwater multilayer complexes formed when water escape from a buried layer generated fluidization in the overlying sediment this may have been triggered by seismically induced liquefaction some parts of the succession were disturbed on separate occasions by different processes sediment laden currents are unlikely to have been a major trigger for deformation it is concluded that specific soft sediment deformation structures result from the combination of driving forces deformation mechanism and initial geometry assemblages of structures relate to the timing of deformation and are of more value in identifying the trigger for deformation

strength of moist sand controlled by surface tension for tectonic analogue modelling abstract a technique has been developed to control the strength of moistened sand in a quantifiable accurate and reproducible way while other mechanical properties were maintained strength of dry sand was increased through adding a small amount of liquid in order to control the additional cohesion of moist sand the influence of the surface tension of the liquid was investigated direct shear experiments were performed on four granular materials at confining stress levels below 1 kpa it has been found that the surface tension of the added liquid controlled the additional apparent cohesion of sand with high accuracy the mechanical properties of moist sand show dynamic similarity towards natural brittle rock which enables analogue modelling of fault formation fault reactivation and tension fracture formation in the brittle regime with a controlled strength profile furthermore experimental results fit well to shear strength models from this followed the direct proportionality of the unsaturated shear strength parameter b relative to the matrix suction measured at low stress levels moreover shear strength turned out to be also a function of grain size and the grain shape

under what circumstances does a seismogenic patch produce aseismic transients in the later interseismic period whether aseismic transients occur as the next earthquake approaches or not is an important problem in the area of disaster mitigation by geophysical observation we conducted a series of rate state aging law earthquake sequence simulations with inertial effects and revealed that a b direct effect evolution effect is a key parameter controlling the complexity of interseismic behavior in a seismogenic patch interseismically a creep front invades a locked patch if a b 0 4 nucleation takes place as soon as the linear stability of the coherent creep is violated and no significant aseismic transient occurs if a b 0 4 nucleation size is given by the energy balance criterion and if a b 0 6 aseismic transients occur after the violation of linear stability and before the creeping region is able to host the nucleation thus not only a b but also a b must be selected carefully to obtain realistic results in numerical simulations

origin and evolution of polygonal cracks in hydrous sulphate sands white sands national monument new mexico the nucleation and propagation of polygonal cracks in hydrous sulphate dunes at white sands national monument new mexico are affected by water availability transport through the sand and exchange with the atmosphere these gypsum sands are cohesive enough to crack due to capillary forces and gypsum cements formed during evaporation surface cracks form five sided polygons with a variety of triple junction angles cracks extend down to variable depths and polygons increase in size with depth showing a maturation similar to experimentally produced polygonal columns in drying corn starch results from two years of monitoring crack geometries temperature and humidity demonstrate that cracks form when water is lost to the atmosphere through the transport of water vapour subsurface relative humidity below 5 to 10 cm is almost always maintained at 100 by the evaporation and condensation of water in thin films on grains the amplitude of daily temperature and thus absolute humidity changes decreases with depth consistent with lower evaporation and condensation rates with increasing depth changes in absolute humidity and the contrast between humidity in pore spaces versus the overlying atmosphere result in significant water loss from the dunes except during times of precipitation and frost dew condensation this water loss allows cracks to nucleate and propagate into the dunes this study hypothesizes that crack tips propagate into sand to the depth at which thin films of water on grains are drying and that this depth varies from the surface of the dune during precipitation events to depths greater than 45 cm when dunes are drier

distinct styles of fluvial deposition in a cambrian rift basin process based and facies models to account for the origin of pre vegetation i e pre silurian preserved fluvial sedimentary architectures remain poorly defined in terms of their ability to account for the nature of the fluvial conditions required to accumulate and preserve architectural elements in the absence of the stabilizing influence of vegetation in pre vegetation fluvial successions the repeated reworking of bars and minor channels that resulted in the generation and preservation of broad tabular stacked sandstone sheets has been previously regarded as the dominant sedimentary mechanism this situation is closely analogous to modern day poorly vegetated systems developed in arid climatic settings however this study demonstrates the widespread presence of substantially more complex stratigraphic architectures the guarda velha formation of southern brazil is a 500 m thick synrift fluvial succession of cambrian age that records the deposits and sedimentary architecture of three distinct fluvial successions i an early rift stage system characterized by coarse grained channel elements indicative of a distributive pattern with flow transverse to the basin axis and two coeval systems from the early to climax rift stages that represent ii an axially directed trunk fluvial system characterized by large scale amalgamated sandy braid bar elements and iii a distributive fluvial system characterized by multi storey sandy braided channel elements that flowed transverse to the basin axis integration of facies and architectural element analysis with regional stratigraphic basin analysis palaeocurrent and pebble provenance analysis demonstrates the mechanisms responsible for preserving the varied range of fluvial architectures present in this pre vegetation rift basin setting identified major controls that influenced pre vegetation fluvial sedimentary style include i spatial and temporal variation in discharge regime ii the varying sedimentological characteristics of distinct catchment areas iii the role of tectonic basin configuration and its direct role in influencing palaeoflow direction and fluvial style whereby both the axial and transverse fluvial systems undertook a distinctive response to syn depositional movement on basin bounding faults detailed architectural analyses of these deposits reveal significant variations in geometry with characteristics considerably more complex than that of simple laterally extensive stacked sandstone sheets predicted by most existing depositional models for pre vegetation fluvial systems these results suggest that the sheet braided style actually encompasses a varied number of different pre vegetation fluvial styles moreover this study demonstrates that contemporaneous axial and transverse fluvial systems with distinctive architectural expressions can be preserved in the same overall tectonic and climatic setting

resolving miss conceptions and misconceptions a geological approach to sedimentary surface textures generated by microbial and abiotic processes the rock record contains a rich variety of sedimentary surface textures on siliciclastic sandstone siltstone and mudstone bedding planes in recent years an increasing number of these textures have been attributed to surficial microbial mats at the time of deposition resulting in their classification as microbially induced sedimentary structures or miss research into miss has developed at a rapid rate resulting in a number of misconceptions in the literature here we attempt to rectify these miss misunderstandings the first part of this paper surveys the stratigraphic and environmental range of reported miss revealing that contrary to popular belief there are more reported miss bearing rock units of phanerozoic than precambrian age furthermore miss exhibit a pan environmental and almost continuous record since the archean claims for the stratigraphic restriction of miss to intervals prior to the evolution of grazing organisms or after mass extinction events as well as claims for the environmental restriction of miss appear to result from sampling bias in the second part of the paper we suggest that raised awareness of miss has come at the cost of a decreasing appreciation of abiotic processes that may create morphologically similar features by introducing the umbrella term sedimentary surface textures of which miss are one subset we suggest a practical methodology for classifying such structures in the geological record we illustrate how elucidating the formative mechanisms of ancient sedimentary surface textures usually requires consideration of a suite of sedimentological evidence from surrounding strata resultant interpretations microbial or non microbial should be couched within a reasonable degree of uncertainty this approach recognizes that morphological similarity alone does not constitute scientific proof of a common origin and reinstates a passive descriptive terminology for sedimentary surface textures that cannot be achieved with the current miss lexicon it is hoped that this new terminology will reduce the number of overly sensational and misleading claims of miss occurrence and permit the means to practically separate initial observation from interpretation furthermore this methodology offers a scientific approach that appreciates the low likelihood of conclusively identifying microbial structures from visual appearance alone informing the search for true miss in earth s geological record and potentially on other planetary bodies such as mars

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

unconfined alluvial flow processes recognition and interpretation of their deposits and the significance for palaeogeographic reconstruction abstract palaeogeographic interpretation of the sedimentary rock record depends on correct recognition from the preserved evidence of the processes responsible for transporting and depositing the sediment this in turn depends on robust knowledge transfer from previous workers and the successful exchange of ideas between workers requires consistent use of a well defined vocabulary we have identified serious breakdowns in all these interpretation steps in the case of terrestrial unconfined flow and its deposits and these failures are leading to unreliable environmental and climatic interpretation this is significant because such alluvial deposits commonly form a majority of the rock record of continental environments working from the basic principles of geomorphology and fluid dynamics we have undertaken a wide ranging analysis of the nature of out of channel flow and from this make predictions about the characteristics of its deposits we identify the range of possible locations and conditions that lead to the development of unconfined flow review the processes operating in each case and examine the range of lithological features that can be produced by these processes this allows us to evaluate the reliability of the criteria claimed for identification of out of channel flow deposits and examine how our new insights might alter palaeoclimatic and palaeogeographic reconstructions published previously by others the sedimentary record of unconfined flows is much more diverse and complex than usually portrayed the received wisdom that the record of unconfined flow consists solely of upwards fining thin beds produced from shallow waning flows is shown to be flawed a wide range of lithofacies are possible and the variation in both flow steadiness and uniformity needs to be taken into account the previously published criteria for recognition of flows of this type are not diagnostic of process or location unconfined flow deposits cannot reliably be identified from grain size or bed thickness similar lithofacies may develop in a wide range of geomorphic and climatic settings because the deposits solely reflect the local flow conditions and sediment availability we recommend that the terms sandflat and sheetflood should not be used in sedimentological accounts because there is no longer a safe informal usage for either contradictory application of these terms and lack of robust definitions is leading to significant misunderstanding of palaeogeography and process our analysis should improve reconstruction of past terrestrial environments because it reveals more clearly the true variety of possibilities for the occurrence of unconfined flow and the resultant deposits enhanced understanding of the inherent uncertainties and realisation of the wider range of plausible alternative explanations should help resolve apparent contradictions with independent indicators of climate or geographic position

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

remarkably preserved prokaryote and eukaryote microfossils within 1ga old lake phosphates of the torridon group nw scotland abstract the 1000 ma torridon group of northwest scotland are here shown to contain a rich diversity of organic walled microfossils preserved in exceptional detail within sedimentary phosphate the phosphatic nodules and bands in which they occur are autochthonous as are the organic fossils they contain the associated sedimentology of this phosphate is shown to be consistent with a lacustrine setting informal taxonomic treatment of the microfossils allows statistical assessment of the relative abundances of different morphotypes across a range of lacustrine environments exceptional preservation of pristine organic walled eukaryotes and prokaryotes together with a taphonomic spectrum in both physicochemical and biological degradational conditions allows a picture to be built of the earliest lacustrine ecological communities

archean to recent aeolian sand systems and their sedimentary record current understanding and future prospects the sedimentary record of aeolian sand systems extends from the archean to the quaternary yet current understanding of aeolian sedimentary processes and product remains limited most preserved aeolian successions represent inland sand sea or dunefield erg deposits whereas coastal systems are primarily known from the cenozoic the complexity of aeolian sedimentary processes and facies variability are under represented and excessively simplified in current facies models which are not sufficiently refined to reliably account for the complexity inherent in bedform morphology and migratory behaviour and therefore cannot be used to consistently account for and predict the nature of the preserved sedimentary record in terms of formative processes archean and neoproterozoic aeolian successions remain poorly constrained palaeozoic ergs developed and accumulated in relation to the palaeogeographical location of land masses and desert belts during the triassic widespread desert conditions prevailed across much of europe during the jurassic extensive ergs developed in north america and gave rise to anomalously thick aeolian successions cretaceous aeolian successions are widespread in south america africa asia and locally in europe spain and the usa several eocene to pliocene successions represent the direct precursors to the present day systems quaternary systems include major sand seas ergs in low lattitude and mid latitude arid regions pleistocene carbonate and holocene modern siliciclastic coastal systems the sedimentary record of most modern aeolian systems remains largely unknown the majority of palaeoenvironmental reconstructions of aeolian systems envisage transverse dunes whereas successions representing linear and star dunes remain under recognized research questions that remain to be answered include i what factors control the preservation potential of different types of aeolian bedforms and what are the characteristics of the deposits of different bedform types that can be used for effective reconstruction of original bedform morphology ii what specific set of controlling conditions allow for sustained bedform climb versus episodic sequence accumulation and preservation iii can sophisticated four dimensional models be developed for complex patterns of spatial and temporal transition between different mechanisms of accumulation and preservation and iv is it reasonable to assume that the deposits of preserved aeolian successions necessarily represent an unbiased record of the conditions that prevailed during episodes of earth history when large scale aeolian systems were active or has the evidence to support the existence of other major desert basins been lost for many periods throughout earth history

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

microbially induced sedimentary structures a new category within the classification of primary sedimentary structures abstract cyanobacterial films and mats syndepositonally influence erosion deposition and deformation of sediments the biomass levels surface morphologies and microbial mats stabilize depositional surfaces and shelter the sediment against erosion or degassing growing microbial mats dredge grains from their substrate upwards whereas cyanobacterial filaments that are oriented perpendicular to the mat surface reach into the supernatant water and baffle trap and bind suspended particles these and similar biotic physical interactions are reflected in syndepositional formation of microbially induced sedimentary structures we distinguish structures on bedding planes leveled bedding surfaces wrinkle structures microbial mat chips erosional remnants and pockets multidirectional ripple marks and mat curls and internal bedding structures sponge pore fabrics gas domes fenestrae structures sinoidal laminae oriented grains benthic ooids biolaminites mat layer bound grain sizes we propose to place this group of microbially mediated structures as a fifth category bedding modified by microbial mats and biofilms in pettijohn and potter s 1964 existing classification of primary sedimentary structures

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

three dimensional sedimentary architecture of a large mid channel sand braid bar jamuna river bangladesh the three dimensional subsurface alluvial architecture of a large approximately 3 km long 1 km wide 12 m high mid channel sand braid bar in the jamuna river bangladesh is described evolution of the bar and its depositional characteristics are assessed from a unique combination of ground penetrating radar surveys vibracoring and trenching that are allied to a series of bathymetric surveys taken during growth of the bar over a 29 month period this methodology permits identification of the formative processes of different packages of braid bar sedimentation and provides a facies model for deposition within the entire bar mid channel bar growth occurred in a region of flow expansion and was probably initiated by the stalling and amalgamation of large dunes these dunes created a bar core that grew by i propagation of a downstream accreting slipface ii vertical accretion through stacking of dunes on both bar stoss and top and iii lateral accretion on the bar margins during recession of the flood hydrograph braid bar sedimentation is dominated by four radar facies 1 large scale predominantly planar dipping reflections interpreted as cross stratification up to 8 m in height and greater than 100 m in width that is produced by the cross channel migration of bar margins 2 medium scale trough shaped and planar discontinuous reflections interpreted as cross stratification up to 4 m in height and 300 m wide that is deposited from large sinuous crested sand dunes 3 discontinuous reflections up to 2 m high and 30 m wide interpreted as small scale trough cross stratification that is the product of smaller sinuous crested dunes and 4 high amplitude undulating reflections interpreted as mud drapes deposited in regions of slow flow often in the bar tail region at low stage dune size decreases vertically within the bar in response to the progressively shallower flows on the emerging bar top later evolution of the bar as one anabranch channel became dominant created a 1 5 km extension to the bar tail with an 8 m high angle of repose bar margin slipface formed by flow transverse to the long axis of the bar seven styles of deposition can be defined that constitute the alluvial architecture bar margin slipface vertical accretion in channel bar top vertical accretion upstream accretion lateral accretion downstream accretion and low stage mud drapes a model of braid bar sedimentation is presented that shares many similarities with previous studies of smaller sand bed braid bars with the dominance of dune scale cross stratification the presence of large scale bar margin cross stratification and the occurrence of lateral vertical upstream and downstream accretion however the contribution of the bar margin facies to the preserved stratigraphy highlighted herein may have been underestimated in previous models of braided rivers in which the braid bars were migrating slowly this study suggests a scale invariance in several aspects of mid channel bar sedimentation in sand bed rivers and proposes a model of braid bar sedimentation that may be applied widely within studies of braided alluvial architecture

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

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

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

considering river structure and stability in the light of evolution feedbacks between riparian vegetation and hydrogeomorphology river ecological functioning can be conceptualized according to a four dimensional framework based on the responses of aquatic and riparian communities to hydrogeomorphic constraints along the longitudinal transverse vertical and temporal dimensions of rivers contemporary riparian vegetation responds to river dynamics at ecological timescales but riparian vegetation in one form or another has existed on earth since at least the middle ordovician c 450 ma and has been a significant controlling factor on river geomorphology since the late silurian c 420 ma on such evolutionary timescales plant adaptations to the fluvial environment and the subsequent effects of these adaptations on fluvial sediment and landform dynamics resulted in the emergence from the silurian to the carboniferous of a variety of contrasted fluvial biogeomorphic types where water flow morphodynamics and vegetation interacted to different degrees here we identify several of these types and describe the consequences for biogeomorphic structure and stability i e resistance and resilience along the four river dimensions of feedbacks between riparian plants and hydrogeomorphic processes on contrasting ecological and evolutionary timescales

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

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

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

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

paleohydraulics of chute and pool structures in a paleoproterozoic fluvial sandstone abstract chute and pool structures consist of an inclined surface leading into a pool where a hydraulic jump causes deposition of backset laminae structures fitting this description are present at the base of a very fine grained sandstone bed in the fluvial matinenda formation of northern ontario reconstruction of the paleohydraulic system indicates that a depth of approximately 1 cm and a velocity of approximately 65 cm s formed these bedforms during the initial stage of a flow with increasing depth and decreasing velocity this type of deposit should be most common at the base of sandy beds deposited as emergent bars are overtopped during increasing discharge events

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

stokes surfaces and the effects of near surface groundwater table on aeolian deposition abstract stokes surfaces in aeolian deposits are caused by wind scour of unconsolidated material to a roughly planar horizon controlled by near surface water tables stokes 1968 a water table forms a downward limit of scour through the cohesion of damp or wet sand near water table and through early cementation by evaporites precipitated in the sediments as water evaporates near the sand air interface study of modern analogues reveals that stokes surfaces exist in a variety of depositional settings including a coastal offshore prograding sand sea jafurah saudi arabia a coastal onshore prograding sand sea guerrero negro mexico and a continental sand sea white sands new mexico usa these modern analogues indicate that our concept of stokes surfaces must be broadened to include the following i modern analogues for stokes surfaces described here cover areas on the order of 25 km2 these may be as representative of similar surfaces in ancient rocks as hypothesized plains of deflation requiring removal of entire sand seas ii stokes surfaces occupy a continuum in scale from local to extensive and erosional surfaces of different magnitude may be stacked closely in the sediments iii stokes surfaces although erosional in nature are commonly associated with deposits both above and below the stokes bounding surface which plainly reveal the influence of a near surface groundwater control on wind sedimentation moreover the erosional relief of the bounding surface itself as well as other features reveals the influence of a groundwater table iv stokes surfaces may be diachronous representing the lateral shift of a zone of scour within a sand sea rather than simultaneous removal of all dunes from the area encompassed by the erosional surface v stokes surfaces and associated deposits are often laterally transitional to surfaces and deposits of adjacent depositional environments including interdunes tidal flats lagoons beaches lakes and non aeolian sabkhas finally modern examples from different depositional settings suggest that while most stokes surfaces have many features in common such as erosional ridges due to early cementation there are some features which may with further study be revealed to be distinctive of an individual depositional setting

eolian action and the distribution of cambrian shales in north america prolonged eolian action on the barren prevegetation craton provides a better explanation for the absence of shale in the cambrian and lower ordovician orthoquartzite carbonate suite of east central and arctic north america than does marine bypassing it is hypothesized that paleo trade winds transported much of the missing silt and clay to the southern and western united states and southwestern canada while the remainder diffused radially off the craton under the influence of variably directed storm winds the distribution of shale in the sauk sequence supports this and indicates that the orthoquartzite carbonate suite of upwind regions is replaced by a shale carbonate suite in downwind areas

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

a model for the moist seismic reflection profile n scotland an interpretation of the moine and outer isles seismic traverse reflection profile along the north coast of scotland is developed here by the construction of a model crustal cross section from which by ray tracing a synthetic seismic section has been prepared for comparison with the observed record section faults are modelled by narrow low velocity zones and the moho by narrow high and low velocity zones the geometry of the faults shown in the model is believed to imply that during the caledonian orogeny crustal thickening was achieved by imbrication of thrust flakes from the se in which the thrust front propagated northwestward into the foreland and at the same time cut down so that the basal decollement deepened relative to the moho eventually into the mantle concomitant erosion and uplift initiated extension and the development of sedimentary basins in half grabens

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

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

the ancestors of meandering rivers modern alluvial plains in contrast to their pre silurian counterparts are characterized by the presence of meandering rivers in low downstream gradient areas constituting efficient transport systems that maintain high bottom shear stresses in deep channels which are made possible by bank stabilization most commonly provided by vegetation in earth s recent history here we show through numerical modeling and field based description of large scale exposures in mesoproterozoic successions that prevegetation rivers in low downstream gradient areas were markedly different from both younger meandering rivers and the common prevegetation sheet braided rivers showing deeper braided channels and greater floodplain preservation than the latter these systems were less frequent and had lower transport efficiency than modern meandering rivers implying differences in global scale earth surface dynamics from the weathering of silicate minerals in floodplains to the grain size distribution in all clastic depositional systems

diverse microbially induced sedimentary structures from 1 ga lakes of the diabaig formation torridon group northwest scotland abstract the siliciclastic lacustrine rocks of the 1000 ma diabaig formation northwest scotland contain a remarkable diversity of macroscopic structures on bedding planes that can be compared with various kinds of microbially induced sedimentary structures miss field sedimentological investigations combined with laboratory analysis of bedding planes and petrographic study of thin sections have allowed us to characterise a range of depositional environments and document the spectrum of biological structures miss are reported from frequently subaerial environments through commonly submerged facies and down to permanently sub wavebase settings palaeoenvironmental conditions water depth exposure hydrodynamic energy control the distribution of miss within these facies this demonstrates that mat forming microbial communities were arguably well adapted to low light levels or periodic exposure some miss from the diabaig formation are typical of precambrian microbial mats including reticulate fabrics and old elephant skin textures in addition to these a number of new and unusual fabrics of putative microbial origin are described including linear arrays of ridges and grooves cf arumberia and discoidal structures that are comparable with younger ediacaran fossils such as beltanelliformis these observations indicate that benthic microbial ecosystems were thriving in freshwater lake systems 1000 ma and indicate how microbially induced sedimentary structures may be applied as facies indicators for proterozoic lacustrine environments the discovery of structures closely resembling ediacaran fossils cf beltanelliformis also serves to highlight the difficulty of interpreting simple discoidal bedding plane structures as metazoan fossils

high energy flood events recorded in the mesoproterozoic meall dearg formation nw scotland their recognition and implications for the study of pre vegetation alluvium the mesoproterozoic neoproterozoic torridonian sandstone of nw scotland has renewed sedimentological significance following recent advances in the understanding of pre vegetation alluvium as it is one of the most extensive and easily accessible successions of such strata worldwide this paper presents the first modern sedimentological analysis of the unit9s constituent meall dearg formation late mesoproterozoic recognizing a dominance of alluvial facies with subordinate aeolian facies alluvial strata within the meall dearg formation contain direct evidence for event deposition by high energy floods including the following 1 widespread upper and transitional upper flow regime elements 2 frequent stacking of successively lower flow regime elements 3 common subcritical subaqueous dune fields with superimposed ripple marks 4 occasional thin desiccated mudstones 5 evidence that microbial mats colonized substrates during intervals of sedimentary stasis together these strands of primary sedimentary geological evidence indicate that the alluvial deposition of the meall dearg formation was typified by supercritical flows during high energy floods punctuated by prolonged intervals of sedimentary stasis the preservation potential of all of the features was boosted by highly aggradational sedimentary conditions these primary observations have implications for the norm in pre vegetation alluvium and confirm that despite increasing recognition of diversity within precambrian fluvial systems classical pre vegetation motifs of high energy alluvial flood deposits preserved as sheet braided alluvium are still an archetypal sedimentary signature in some instances using supportive evidence from the meall dearg formation we recommend that the term sheet braided be used in inverted commas in future studies this emphasizes the polygenetic depositional nature of the 20 1 width thickness fluvial style while maintaining the value of the term in the isolation of a key characteristic of pre vegetation sedimentary architecture

palaeomagnetic dating of sub torridon group weathering profiles nw scotland verification of neoproterozoic palaeosols a palaeoplain that cuts across lewisian gneiss in the cape wrath area northwest scotland is weathered to depths of 1 6 m beneath fluvial red conglomerates and sandstones of the applecross formation of the early neoproterozoic torridon group weathered lewisian rocks are enriched in k 2 o and fe 2 o 3 and contain hematite as an alteration product of biotite and magnetite palaeomagnetic analysis of the weathering profiles has been carried out to constrain the age of alteration thermal step demagnetization of 87 oriented core specimens of biotite gneiss and amphibolite spanning four profiles at poll a mhurain and sheigra reveal a stable high temperature component with a mean direction of remanence of d 149 1 i 57 2 95 5 0 corrected for dip of the overlying applecross strata that gives a pole at 9 8 s 199 2 e dp 5 3 dm 7 3 this direction differs significantly from that for little weathered lewisian at those two localities and also from directions for the late mesoproterozoic stoer group the remanence direction for the weathered rocks is however near the dip corrected high temperature remanence direction d 122 3 i 50 5 95 10 8 obtained by us from the thermal step demagnetization of 39 sandstone specimens of the applecross formation in the cape wrath area furthermore the direction for the weathered rocks agrees closely with other dip corrected results for the torridon group such as the detrital remanent magnetization drm mean direction d 148 3 i 59 9 95 13 8 for the applecross formation on rum and that for the main outcrop of the aultbea formation d 139 7 i 52 6 95 14 5 the concordance of palaeomagnetic results for the sediments and weathered rocks implies that the weathering profiles in the cape wrath area acquired their remanence penecontemporaneously with deposition of the applecross and aultbea formations the palaeomagnetic results therefore argue strongly that the weathering profiles are palaeosols of applecross age c 980 ma that formed at a palaeolatitude of 38 and constitute the oldest known palaeosols in the british isles views that the weathering is substantially older or younger than applecross deposition cannot be sustained our findings carry implications for applecross palaeoclimate and source and strengthen the conclusion that atmospheric oxygen was a substantial fraction of its present abundance by early neoproterozoic time

indications of glaciation at the base of the proterozoic stoer group torridonian nw scotland the stoer group of nw scotland has long been regarded as the product of ephemeral flood events the resulting facies have been interpreted as fanglomerates braided river deposits and lake sediments new observations of the lewisian basement palaeotopography and basal stoer group facies at enard bay clachtoll and achiltibuie suggest that earliest torridonian time was characterized by glaciation lewisian basement displays rochr rnoutonnee like forms and evidence of subglacial fracturing possibly by water under high pressure breccias associated with rough craggy basement are interpreted as fractured bedrock formed subglacially on the downglacier sides of the roches moutonnees diamictites draping the basement are regarded as basal tillite stratified diamictites and laminated mudstone sandstone breccia conglomerate units contain outsized clasts displaying drop stone structures and are therefore inferred to be ice rafted deposits associated microbial carbonates are inferred to be analogous to contemporary antarctic lake deposits limited palaeo iceflow indicators suggest a source of the ice mass to the n or ne

polygonal sandcracks unique sedimentary desiccation structures in bahamian ooid grainstone sandcracks which are ubiquitous in holocene eolian and beach backshore carbonate grainstone on alligator point cat island bahamas resemble polygonal mudcracks but formed in ooid sand without muddy matrix in experiments on cat island beach sand sediment surfaces cracked polygonally in the absence of mud or biofilms while drying at room temperature due to contraction generated by capillary effects related to surface tension attraction of interstitial water gravitational collapse of irregular open pores and repacking of sand grains due to loss of cohesion between particles caused by evaporation of water enhance the cracking process and appearance of polygons by providing space for cracks to expand the polygons are held together by any remaining capillary moisture and associated meniscus cement which precipitates as the sand dries polygonal sandcracks can be preserved by rapid lithification of carbonate sand but have been documented only rarely from other localities because their formation requires well sorted well rounded spherical grains rather than those making up the more common heterogeneous skeletal and peloidal sediment in carbonate settings interpretation of this primary sedimentary desiccation structure provides new insights into sedimentation and diagenesis of ooid rich deposits and can aid in recognizing ancient subaerial exposure horizons

isolating the roles of different forcing agents in global stratospheric temperature changes using model integrations with incrementally added single forcings satellite instruments show a cooling of global stratospheric temperatures over the whole data record 1979 2014 this cooling is not linear and includes two descending steps in the early 1980s and mid 1990s the 1979 1995 period is characterized by increasing concentrations of ozone depleting substances odss and by the two major volcanic eruptions of el chichon 1982 and mount pinatubo 1991 the 1995 present period is characterized by decreasing ods concentrations and by the absence of major volcanic eruptions greenhouse gas ghg concentrations increase over the whole time period in order to isolate the roles of different forcing agents in the global stratospheric temperature changes we performed a set of simulations using the nasa goddard earth observing system chemistry climate model with prescribed sea surface temperatures we find that in our model simulations the cooling of the stratosphere from 1979 to present is mostly driven by changes in ghg concentrations in the middle and upper stratosphere and by ghg and ods changes in the lower stratosphere while the cooling trend caused by increasing ghgs is roughly constant over the satellite era changing ods concentrations cause a significant stratospheric cooling only up to the mid 1990s when they start to decrease because of the implementation of the montreal protocol sporadic volcanic events and the solar cycle have a distinct signature in the time series of stratospheric temperature anomalies but do not play a statistically significant role in the long term trends from 1979 to 2014 several factors combine to produce the step like behavior in the stratospheric temperatures in the lower stratosphere the flattening starting in the mid 1990s is due to the decrease in ozone depleting substances mount pinatubo and the solar cycle cause the abrupt steps through the aerosol associated warming and the volcanically induced ozone depletion in the middle and upper stratosphere changes in solar irradiance are largely responsible for the step like behavior of global temperature anomalies together with volcanically induced ozone depletion and water vapor increases in the post pinatubo years