breaking the limit of seismic resolution a synthetic example based on tunu shallow gas development it is commonly admitted that resolution is limited to one fourth of the seismic bandwidth which is often too coarse to resolve thin layering inside a reservoir we present an application where a special form of seismic inversion has been able to retrieve layers as thin as 1 20th of the seismic bandwidth on synthetic data based on the real field of tunu indonesia

sediment budgeting a case study in the katiorin drainage basin kenya the primary objective of this study was to compute a detailed budget for a small semiarid tropical drainage basin in kenya results indicated that transfer of sediments inputs from primary source areas was minor in comparison to changes in storage the major sediment source area within the katiorin drainage basin was the colluvial hillslope zone the net change in storage within this zone was approximately 2100 mg yr 1 surface wash and rilling were the dominant transport processes responsible for the remobilization of colluvial sediments sediment storage within the in channel reservoir increased by 60 mg yr 1 which was minor when compared to the total store of sediment in this reservoir during 1986 the channel network stored only a small fraction 3 per cent of the sediment delivered from the hillslope subsystem therefore the in channel reservoir had limited influence on sediment conveyance to the basin outlet these data indicate that a static equilibrium condition cannot be assumed within the katiorin drainage basin such an assumption would result in erosion estimates of approximately 5 5 mm yr 1 for the entire basin based on a sediment output of 7430 mg km 2 yr 1 and a measured bulk density of 1 35 mg m 3 however this masked the actual rates of 1 2 to 7 1 mm yr 1 in subbasin primary source areas and rates of 0 6 to 17 mm yr 1 for colluvial material in the various subbasins the extreme accelerated erosion rates resulted from minimal ground vegetation steep slopes soil crust formation an erodible substrate and a well integrated drainage network for rapid conveyance of sediments from the hillslope subsystem to the basin outlet

stable isotope s c chemostratigraphy and hydrocarbon biomarkers in the ediacaran upper section of sierras bayas group argentina the neoproterozoic sedimentary successions of the sierras bayas group sbg are situated within the tandilia belt buenos aires province argentina the loma negra formation lnf from the upper part of the sbg has been investigated using a combination of inorganic organic and isotopic geochemical methods in order to determine the stable isotope composition of carbonates delta c 13 car delta o 18 car kerogen delta c 13 ker pyrite and carbonate associated sulfur delta s 34 py delta s 34 cas concentrations of major trace and rare earth elements and hydrocarbon distributions were also investigated this data set was used to further the understanding of the interactions between paleo biodiversity and paleoenvironmental conditions for terminal ediacaran post gaskiers shelf deposits the high delta s 34 py values exceeding the coeval delta s 34 cas values in green micritic limestones of the lower lnf may be explained by a combination of different events such as globally low seawater sulfate concentrations an increased rate of bacterial sulfate reduction and a decrease in sulfate levels in the near bottom deepest water layer inherited from glaciations negative ce anomalies in the dark grey micritic limestones of the upper lnf suggest they were deposited under oxidizing conditions however higher concentrations of fe mo zn and ree and the occurrence of authigenic s 34 enriched pyrite suggest that the sediments were maintained under reducing conditions the distributions of hydrocarbon biomarkers e g hopanes maximizing at c 29 are in line with this hypothesis and indicate a diverse microbial community including primary producers such as cyanobacteria e g terminally branched monomethyl alkanes hopanoid distribution phototrophic bacteria e g acyclic isoprenoids c phytane the unsteady delta c 13 car ker values reflect changes in primary biomass due to relative contributions of bacterial microorganisms using different photosynthetic carbon fixation pathways the lowest delta c 13 car ker values coincided with the latest biomarker signal interpreted as the signature of green non sulfur bacteria using a less c 13 fractionation pathway the combined biogeochemical features of the loma negra formation indicated a well stratified water column with oxygenated surface waters oxygen poor bottom waters and anoxic sediments which helped to refine a correlation with the polanco formation from the arroyo del soldado group in uruguay c 2013 elsevier b v all rights reserved

microbially induced sedimentary structures from the 1 64 ga chuanlinggou formation jixian north china abstract the present study documents abundant exceptionally preserved microbially induced sedimentary structures miss from the 1 64 ga chuanlinggou formation of the jixian area north china craton the chuanlinggou formation is dominated by black silty shale mudstone with fine sandstone of the lower part and was likely deposited in a relatively restricted shallow marine environment varying from fair weather wave action zone to below storm wave action zone both wrinkle structures and sand veins are very abundant in the lower and middle parts of the formation wrinkled layers are dark colored densely laminated and contain floating quartz grains and abundant mica grains both of which are aligned parallel to bedding plane such an arrangement pattern is attributed to the microbe s binding and trapping of miss sem imaging also shows that sticky networks interwoven by filamentous extracellular polymer substances eps and biofilms with distinct pores are very pronounced in clay matrix of wrinkled layers providing direct evidence of microbial activities during the formation of wrinkle structures in addition the circularly arranged quartz grains forming hollow cavities and vertical channels of quartz grains beneath the laminated layers are unique to the wrinkle structures showing initial forming stage of the sand veins sand veins similar to fills of sand cracks are usually preserved in silty shales and occasionally occur beneath the winkled surfaces they were formed due to methane degassing of decay of microbial mats and biofilms abundant nano particles of various epss isolated nanoglobules paired nanoglobules dumbbell shaped nanoglobules and polyhedron aggregates are detected in clay matrix close to sand veins indicating microbial participation in the formation of sand veins wide spatial distribution broadened environmental inhabitation and extremely high abundance and bed surface density of the misss indicate proliferation of miss forming microbial mats in mesoproterozoic times

the tandilia system of argentina as a southern extension of the rio de la plata craton an overview the southernmost outcrops of the rio de la plata cratonic region are exposed in the tandilia system in eastern argentina the geological evolution comprises mainly an igneous metamorphic paleoproterozoic basement named buenos aires complex which is covered by neoproterozoic to early paleozoic sedimentary units which display subhorizontal bedding the basement of calc alkaline signature consists mainly of granitic tonalitic gneisses migmatites amphibolites some ultramafic rocks and granitoid plutons subordinate rock types include schists marbles and dykes of acid and mafic composition tandilia was recognized as an important shear belt district with mylonite rocks derived mainly from granitoids the tectonic scenario seems related to juvenile accretion event 2 25 2 12 ga along an active continental margin followed by continental collision 2 1 2 08 ga after u pb zircon data the collisional tectonic setting caused thrusting and transcurrent faulting favouring the anatexis of the crustal rocks the tholeiitic dykes constrain the time of crustal extension associated with the last stages of the belt evolution the basement was preserved from younger orogenies such as those of the brasiliano cycle after a long paleoweathering process the sierras bayas group c 185 m thick represents a record of the first neoproterozoic sedimentary unit siliciclastic dolostones shales limestones superposed by cerro negro formation c 150 400 m thick siliciclastics assigned to upper neoproterozoic age the final sedimentary transgression during early paleozoic was the balcarce formation c 90 450 m thick deposited over all the mentioned precambrian units based on all the geological background a tectonic evolution is offered

4d seismic to well tying a key step towards 4d inversion abstractcalibrating the 4d signal at the well with information obtained from production data is essential for it to be used quantitatively we have developed a model based inversion method to estimate the changes of elastic parameters in the reservoir due to production at the well our scheme is based on the observation that flow behavior is constrained by the dynamic properties of the layer i e permeability and therefore a layered model should be used to parameterize the inversion the inversion scheme considers traveltime inside and below the reservoir but not in the overburden and impedance effects implied by the change of elastic parameters inside the reservoir therefore even at zero offset we can separate changes in density from changes in p velocity when using multiple offset data we can use an exact formulation for the reflectivity if the base logs density p velocity and s velocity are available otherwise an approximation to the exact form can be used theoretical and practic

complex trace fossils from the terminal proterozoic of namibia we document for the first time the occurrence of the complex trace fossil treptichnus from the terminal proterozoic nama group namibia these traces differ in detail from the cambrian index fossil treptichnus pedum but document precursors with the same basic behavior the producer of the trace is not known in its complexity however this trace fossil provides strong evidence for advanced latest proterozoic bilaterians

sediment yield and erosion rate estimation in the mountain catchments of the camastra artificial reservoir southern italy a comparison between different empirical methods abstract the sedimentary budget assessment is an important topic for the scientific and social community because it is crucial to understand the dynamics of orogenic belts and cope with a number of practical problems such as soil conservation and sediment accumulation in reservoir sediment yield or denudation rate estimates in southern central italy are generally obtained either by applying simple empirical relationships based on the statistical regression between the geomorphic parameters of the drainage network and the suspended sediment yield measured at the drainage basins outlets or by using qualitative quantitative sediment delivery ratio or erosion models in this work we carry out a study of catchment dynamics and a sedimentary yield computation of several mountain catchments of the central western sector of the basilicata region southern italy which are located upstream of an artificial reservoir the sediment yield and erosion rate have been computed through both an indirect assessment of the suspended sediment yield based on the tu index mean annual suspension sediment yield and the application of the rusle and usped empirical methods the results obtained have been compared with the historical data of sediment accumulation measured in the artificial reservoir of the camastra dam where a detailed evaluation of the volumes of historical i e about 40 years sediment storage was collected the collected dataset represents a basic tool both for the investigation of the morpho dynamics of a typical mountain catchment of the mediterranean area and the evaluation of sediment budget related to fluvial and hillslope processes the 38 year long record of the sediment storage in the camastra artificial reservoir located at the outlet of the studied mountain catchments permits the validation of empirical relationships based on the geomorphic and climatic parameters of the drainage basin among the three different methods of sediment yield evaluation tu index rusle and usped the tu index showed the best prediction ability although usped erosion model also furnished a good estimation as also confirmed by other works the indirect estimation of sediment yield based on ciccacci s empirical relationships can represent a good proxy of short term denudation rates in the mediterranean areas with geological and geomorphological features similar to the study area but the use of empirical models with increasing complexity such as the usped erosion model can help to explore the spatial distribution of the sediment yield sectors of the drainage basin the erosion hot spots and the role of landslides in sediment mobilization and hillslope channel connectivity processes

large scale mapping of gully affected areas an approach integrating google earth images and terrain skeleton information abstract the gully erosion process poses a threat to the ecological environment land management and agronomic productivity although current studies on gully feature mapping conducted at hill slope or catchment scales have yielded good performances the approach for large scale gully affected areas mapping is still challenging this study demonstrates a novel approach inspired by the relationship between gully distribution and surface runoff the google earth ge images and aster gdem were applied to explore the feasibility of open data source in large scale gully mapping the ge images were used for the initial classification based on object based image analysis and random forest model the drainage network obtained from the digital elevation model was generated as the terrain skeleton which controls the gully bodies the rule sets for removing commission and omission errors were developed based on the spatial relationship between the segments and drainage networks the experiment was conducted at one of the most serious erosion regions in the loess plateau with an area of 154 000 km2 owing to the novel integration strategy of image and topographic features the mapping results prove the approach extracts gully affected areas efficiently with acceptable accuracy overall accuracy is 78 80 even within the complex gully zone compared with visual interoperation the proposed approach provides a potential tool to erosion research community that could also be of interest to geomorphologists and policy makers

a spatially distributed model for the long term suspended sediment discharge and delivery ratio of drainage basins 1 understanding and quantifying the long term suspended sediment discharge of drainage basins is a key goal of geomorphology with important implications for the study of water quality agricultural sustainability and the evolution of landscapes and sedimentary basins over geologic timescales previous studies have highlighted the importance of relief slope precipitation temperature vegetation and soil texture in controlling suspended sediment discharge in natural undisturbed landscapes however globally applicable models currently used to predict suspended sediment discharges are limited because they are based on basin averaged versions of these properties and do not incorporate all of the controlling variables into a single model in this paper i propose a spatially distributed globally applicable model for the long term suspended sediment discharge of drainage basins that includes all of the principal controls on suspended sediment discharge previously documented in the geomorphic literature the model explicitly distinguishes the detachment of sediment on hillslopes and in low order valleys from the transport of sediment in higher order alluvial channels the model uses slope soil texture mean monthly rainfall and mean monthly leaf area index as controlling parameters for the detachment component the transport component is modeled using a rouse number dependent transport criterion that explicitly includes the effects of slope and soil texture the model is capable of reproducing the long term sediment yield of 128 global rivers with a pearson correlation coefficient r value of 0 79 using just two free parameters the model also predicts sediment delivery ratios consistent with those measured in natural drainage basins

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

regional scale modeling of hillslope sediment delivery a case study in the barasona reservoir watershed spain using watem sedem summary soil erosion and sediment delivery to streams are important environmental problems and major concerns in sustainable development in recent years several tools have been proposed for assessing the delivery of sediment from hillslopes to stream networks but there are still few examples of their application to large basins and studies include a discussion of calibration and validation issues in this study a spatially distributed soil erosion and sediment delivery model watem sedem was applied to the watershed of the barasona reservoir 1504 km 2 central spanish pyrenees which is drained by the esera and isabena rivers model calibration and validation was based on the depositional history of the barasona reservoir and suspended sediment records over 3 years may 2005 may 2008 at the outlet of the isabena river despite interannual differences in precipitation and runoff it was possible to establish an optimum value 7 23 for the ktc parameter this enabled estimation of the long term average annual sediment yield to the reservoir as well as the relative contribution of each river large spatial variability in hillslope sediment delivery was found and the major sediment sources were in the lower part of the watershed agricultural fields and in the badlands developed on eocene marls in the middle part of the watershed pyrenean inner ranges the relative importance of sediment source areas was assessed in relation to land use and the relationship between the sediment delivery ratio and the catchment area was studied as a function of the geomorphological units for the moment watem sedem remains mainly as a researcher s tool until either the problem of the scarcity of soil erosion data or the need for calibration of the transport parameters will be solved

modelling the runoff sediment yield relationship using a proportional function in hilly areas of the loess plateau north china based on data observed at the 12 small watersheds in hilly areas of the loess plateau north china the relationship between event runoff volume and sediment yield is examined the results reveal that the runoff sediment yield relationship at the inter event timescale is mainly determined by the runoff sediment concentration relationship at the intra event timescale in the study area the sediment concentration tends to be stable when the flow discharge exceeds a certain critical value many factors that are important for determining the characteristics of low magnitude events such as flow discharge particle size of fluvial sediment and accumulation of loose material on land surfaces prior to a rainstorm appear to have little importance for high magnitude events consequently mean sediment concentration tends to be stable for large flood events suggesting a strong similarity between the two flow sediment relationships at inter and intra event temporal scales furthermore a proportional function is proposed to predict event sediment yield and the correspondence between the predicted and observed sediment yields is examined the performance of the model is good for high magnitude events especially extreme events the applicability of the proposed model at the annual timescale is also discussed

scale effects on modelling the seismic signature of gas results from an outcrop analogue numerous examples of reservoir fields from continental and marine environments involve thin bedded geology yet the inter relationship between thin bedded geology fluid flow and seismic wave propagation is poorly understood in this paper we explore the 4d seismic signature due to saturation changes of gas within thin layers and address the challenge of identifying the relevant scales and properties which correctly define the geology fluid flow and seismic wave propagation in the field based on the study of an outcrop analogue for a thin bedded turbidite we model the time lapse seismic response to fluid saturation changes for different levels of model scale and explore discrepancies in quantitative seismic attributes caused by upscaling our model reflects the geological complexity associated with thin bedded turbidites and its coupling to fluid flow which in turn affects the gas saturation distribution in space and its time lapse seismic imprint rock matrix and fluid properties are modelled after selected fields to reproduce representative field models with realistic impedance contrasts in addition seismic modelling includes multiples in order to assess their contribution in seismic propagation through thin gas layers our results show that multiples could contribute significantly to the measured amplitudes in the case of thin bedded geology this suggests that forward inverse modelling involving the flow simulation and seismic domains used in time lapse seismic interpretation should account for thin layers when these are present in the geological setting

geomorphic considerations for erosion prediction current soil erosion prediction technology addresses processes of rainsplash overland flow sediment transport and rill erosion in small watersheds the effects of factors determining sediment yield from larger scale drainage basins in which sediment movement is controlled by the combined small scale processes and a complex set of channel and other basin scale sediment delivery processes such as soil creep bioturbation and accelerated erosion due to denudation of vegetation have been poorly evaluated general suggestions are provided for the development of erosion prediction technology at the geomorphic or drainage basin scale based on the separation of sediment yield data for channel and geomorphic processes from those of field scale soil loss an emerging technology must consider 1 the effects on sediment yield of climate geology and soils topography biotic interactions with other soil processes and land use practices 2 all processes of sediment delivery to a channel system and 3 the general tendency in most drainage basins for progressively greater sediment storage in the downstream direction

soil erosion assessment tools from point to regional scales the role of geomorphologists in land management research and implementation geomorphological research has played an important role in the development and implementation of soil erosion assessment tools because policy and management approaches include the use of soil erosion assessment tools soil erosion research directly affects the public in terms of providing information on natural hazards and human impacts and also as the basis for regulatory policy on land management for example soil loss calculations and geomorphological expertise are used to support soil conservation planning both through agricultural legislation that defines maximum tolerable soil loss rates and through federal and local legislation that requires soil erosion controls on many construction sites to be useful for decision makers soil erosion models must have simple data requirements must consider spatial and temporal variability in hydrological and soil erosion processes and must be applicable to a variety of regions with minimum calibration the growing use of erosion models and geographic information systems gis in local to regional scale soil and water conservation raises concerns about how models are used this has prompted interest in methods to assess how models function at management scales and with the types of data that are commonly available to users a case study of a gis based soil erosion assessment tool using the process based water erosion prediction project wepp shows that using commonly available data rather than research grade data can have predictably a significant impact on model results if model results are then used in management decisions it is critical to assess whether the scale and direction of variation in results will affect management and policy decisions geomorphologists provide unique perspectives on soil erosion and can continue to affect policy through soil erosion research this research should focus on fundamental processes but equally important is continued development and evaluation of models that are matched to real world data availability geomorphic settings and information needs d 2002 elsevier science b v all rights reserved

floodplain evolution in a small tectonically active basin of northern california sediment cores were collected along oodplains in the navarro river basin of coastal northern california to examine the controls on oodplain evolution in a tectonically active setting sedimentary strata were subsampled for organic content bulk density and grain size measurements organic samples were analysed for 14c age which yielded net averaged sedimentation rates for all cores overbank deposition rates decreased at all study sites through time and declined in the downstream direction the ability of intermediary order streams to store sediment in oodplains decreased the ability of highest order streams to record sediment pulse events the effects of anthropogenic disturbance primarily logging on long term overbank deposition rates were minimal climatic variability by affecting sediment loading in the channel network is the principal control on oodplain evolution through the holocene a hypothetical model is proposed to explain overbank deposition rates in the navarro basin which may be extrapolated to the northern coastal california region during the late pleistocene and holocene the complexities observed in sediment storage and routing in this study imply that caution should be made when extrapolating sediment yield measurements obtained at river mouths or coastal shelves to geomorphic events within small tectonically active basins

morphological impacts of flow events of varying magnitude on ephemeral channels in a semiarid region abstract flows in ephemeral channels in semiarid areas are only occasional and channel changes are episodic but the flash floods can be devastating and have major geomorphological impacts data on morphological impacts of flows in semiarid areas are needed to increase understanding of the dynamics and variability of geomorphic responses in such channels for this purpose nine reaches of river channel in three catchments in se spain the nogalte torrealvilla and salada have been sites for measurement of flows and their effects over the period 1997 2012 the sites encompass a range of channel size channel morphology substrate vegetation and position within the catchments a major difference is between schist and marl bedrock areas peak flow stage has been recorded and topography surveyed at frequent intervals and after major flow events over the 16 year period an average of 0 5 flow events per year has been recorded at the schist sites and an average of one per year at the marl sites but occurrence has been highly variable from year to year threshold daily rainfall for channel flow is mostly 15 20 mm but higher rainfalls do not always produce flow one to two major floods have occurred in each of the catchments in the period including the extreme flood event of september 2012 in the nogalte catchment measured morphological changes have occurred between 2 and 10 times at the monitored sites the same size flow can have differing effects depending on the state of the system low flow can mobilise sediment without producing much morphological change the long term trajectory of the reaches and the sediment substrate has a major influence on response to events channel change is governed by threshold values of hydraulic conditions the measurements provide an indication of the scale of maximum erosion and deposition that occurs within the channels and on the floodplains over a range of flow magnitudes and the flow impacts that need to be considered in management

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

a meta analysis of soil erosion rates across the world abstract over the last century extraordinary efforts have been devoted to determining soil erosion rates in units of mass per area and time under a large range of climatic conditions and land uses and involving various measurement methods we undertook a meta analysis of published data from more than 4000 sites worldwide the results show that there is extraordinarily high variability in erosion rates with almost any rate apparently possible irrespective of slope climate scale land use land cover and other environmental characteristics however detailed analysis revealed a number of general features including positive relationships of erosion rate with slope and annual precipitation and a significant effect of land use with agricultural lands yielding the highest erosion rates and forest and shrublands yielding the lowest despite these general trends there is much variability that is not explained by this combination of factors but is related at least partially to the experimental conditions our analysis revealed a negative relationship between erosion rate and the size of the study area involved significant differences associated with differing measurement methods with direct sediment measurement yielding the lowest erosion rates and bathymetric radioisotope and modeling methods yielding the highest rates and a very important effect of the duration of the experiment our results highlight that when interpreting erosion rates the experimental conditions involved must be taken into account even so the data suggest that only order of magnitude approximations of erosion rates are possible and these retain a very large degree of uncertainty consequently for practical purposes such as calculation of global sediment budgets empirical equations are not a substitute for direct measurements our results also show that a large proportion of the experiments have been short term less than 3 years which reduces dramatically the reliability of the estimated erosion rates given the highly non normal behavior of soil erosion time dependency despite the efforts already made more long term measurement experiments need to be performed especially in regions of the world that are under represented in global datasets in addition protocols need to be established for standardizing the measurement methods and reporting the results to enable data to be compared among diverse sites

variability of sediment removal in a semiarid watershed field and documentary data from walnut gulch watershed an instrumented semiarid drainage basin of approximately 150 km 2 57 mi 2 in southeastern arizona show that 83 of the alluvium removed from the basin during a 15 year erosion episode beginning about 1930 was excavated from the highest order stream the amount of alluvium removed in the erosion episode would have been equal to a covering of about 4 cm 1 6 in over the entire basin the rate of sediment removal during the erosion episode was 18 times greater than the rate of present channel sediment transport production of sediment from slopes and channel throughput at present rates are approximately equal and refilling will not occur under present conditions the channel forms left by the massive evacuation of sediment impose controls on the spatial distribution of tractive force and total stream power that make renewed storage of sediment likely in only a few restricted locations modern instrumented records of a decade or more provide an inadequate perspective on long term sediment movement

trace fossil preservation and the early evolution of animals abstract the trace fossil record is an important element in discussions of the timing of appearance of bilaterian animals a conservative approach does not extend this record beyond about 560 555 ma crucial to the utility of trace fossils in detecting early benthic activity is the preservational potential of traces made close to the sediment water interface our studies on the earliest cambrian sediments suggest that shallow tiers were preserved to a greater extent than typical for most of the phanerozoic this can be attributed both directly and indirectly to the low levels of sediment mixing the low levels of sediment mixing meant that thin event beds were preserved the shallow depth of sediment mixing also meant that muddy sediments were firm close to the sediment water interface increasing the likelihood of recording shallow tier trace fossils in muddy sediments preservation of surficial trace fossils in this type of setting remains problematic but the above factors suggest that also these can be expected to have left a reasonable record overall the trace fossil record can be expected to provide a sound record of the onset of bilaterian benthic activity the lack of convincing trace fossils significantly before the cambrian supports models of late appearance of macroscopic benthic bilaterians

microbially induced sedimentary structures in archean sandstones a new window into early life abstract until now the most valuable information on the early life on the archean earth derived from bacterial fossils and stromatolites preserved in precipitated lithologies such as chert or carbonates also shales contain complex biomarker molecules and specific isotopes constitute an important evidence for biogeneicity in contrast because of their low potential of fossil preservation sandstones have been less investigated but recent studies revealed a variety of microbially induced sedimentary structures miss that differ greatly from any other fossils or sedimentary structures wrinkle structures multidirected ripple marks biolaminites and other macrostructures indicate the former presence of photoautotrophic microbial mats in shallow marine to tidal paleoenvironments the miss form by the mechanical interaction of microbial mats with physical sediment dynamics that is the erosion and deposition by water agitation the structures occur not only in archean tidal flats but in equivalent settings throughout earth history until today miss are not identified alone by their macroscopic morphologies in thin sections the structures display the carpet like fabrics of intertwined filaments of the ancient mat constructing microorganisms geochemical analyses of the filaments proof their composition of iron minerals associated with organic carbon in conclusion microbial mats colonize sandy tidal settings at least for 3 2 ga years therefore archean sandstones constitute an important archive for the exploration of early life

erosion storage and transport of sediment in two subbasins of the rio puerco new mexico arroyos in the american southwest proceed through cut and fill cycles that operate at centennial to millennial time scales the geomorphic community has put much effort into understanding the causes of arroyo cutting in the late quaternary and in the modern record late 1800s while little effort has gone into understanding how arroyos fill and the sources of this fill here we successfully develop a geographic information system gis modeled sediment budget that is based on detailed field measurements of hillslope and channel erosion and deposition field measurements were made in two arroyo basins draining different lithologies and undergoing different land disturbance volcano hill wash 9 30 km 2 arroyo chavez 2 11 km 2 over a 3 yr period both basins have incised channels that formed in response to the late nineteenth century incision of the rio puerco large volumes of sediment were generated during arroyo incision equal to more than 100 yr of the current annual total sediment load bed load suspended load in each basin downstream reaches in both arroyos are presently aggrading and the main source of the sediment is from channel erosion in upstream reaches and first and second order tributaries the sediment budget shows that channel erosion is the largest source of sediment in the current stage of the arroyo cycle 98 and 80 of the sediment exported out of volcano hill wash and arroyo chavez respectively the geomorphic surface most affected by arroyo incision and one of the most important sediment sources is the valley alluvium where channel erosion gullying soil piping and grazing all occur erosion rates calculated for the entire volcano hill wash 0 26 mm yr and arroyo chavez 0 53 mm yr basins are higher than the modeled upland erosion rates in each basin reflecting the large contributions from channel erosion erosion rates in each basin are affected by a combination of land disturbance grazing and lithology erodible sandstones and shales in arroyo chavez compared with basalt for volcano hill wash despite these differences hillslope sediment yields are similar to long term denudation rates as the arroyo fills over time from mouth to headwaters hillslope sediment becomes a more significant sediment source

the application of semi quantitative methods and reservoir sedimentation rates for the prediction of basin sediment yield in spain traditionally the problems in predicting sediment yield at the basin scale are related to model s high data requirements the focus on a limited amount of erosion and sediment transport processes and a general lack of systems knowledge here we explore the performance of semi quantitative approaches to predict mean annual area specific sediment yield ssy at the scale of small to medium sized basins the factorial scoring model fsm verstraeten g poesen j de vente j koninckx x 2003 sediment yield variability in spain a quantitative and semiqualitative analysis using reservoir sedimentation rates geomorphology 50 4 327 348 the psiac model psiac 1968 pacific southwest inter agency committee report of the water management subcommittee on factors affecting sediment yield in the pacific southwest area and selection and evaluation of measures for reduction of erosion and sediment yield and some variations of these models were applied to predict ssy for the 60 reservoirs using the reservoir sedimentation rates as published by avendano salas et al avendano salas c sanz montero e gomez montana j l 1997 sediment yield at spanish reservoirs and its relationship with the drainage basin area dix neuvieme congres des grands barrages commission internationale de grands barrages florence pp 863 874 the fsm explained 72 of the variability found in reservoir sedimentation rates which is significantly better than estimates based on basin area alone explaining only 22 the fsm uses five factors i e topography lithology vegetation cover gullies basin shape to characterise a drainage basin in the vicinity 5 km of the reservoir and the main tributaries feeding the reservoir by providing a score an index is calculated by multiplying the scores per factor together with basin area this index is used to predict ssy the psiac model works in a comparable way though some other factors are used i e topography climate runoff lithology soil vegetation land use upland erosion channel erosion basin area is not part of the relation and the index is calculated by summation of all scores psiac explained 58 of the variation in ssy advantages of both models over physically based and other empirical models are that less data are required gully erosion is included and the importance of connectivity in the landscape is recognised this study thus demonstrates that semi quantitative approaches can provide fairly accurate and reliable estimates of ssy at the scale of small to medium sized basins with relatively low data requirements

trends in depth and extent of bioturbation in cambrian carbonate marine environments western united states cambrian carbonate strata of the great basin deposited below fairweather wave base but above mean storm wave base inner shelf and below mean storm wave base but above maximum storm wave base middle shelf were examined for extent and depth of bioturbation data indicate that initial extensive colonization of marine infaunal habitats occurred between tommotian and atdabanian time throughout the remaining 65 m y of cambrian time bioturbation increased although depth of bioturbation remained relatively constant at less than 6 cm during this time the amount of bioturbation in the inner shelf was greater than that in the middle shelf

low erosion rates measured for steep sparsely vegetated catchments in southeast spain abstract in order to reduce widespread degradation in desertification prone areas there is an urgent need to understand the mechanisms controlling human induced degradation processes in semi arid ecosystems southeast spain is known as one of the most arid regions of europe and its landscape is marked by sparsely vegetated degraded hillsides large areas of dry cultivation have been abandoned since the early part of the twentieth century and irrigated cultivation is now expanding rapidly in this study modern erosion rates for two mountain ranges belonging to the betic cordillera were assessed by direct measurements of the accumulated sediment volumes behind 20 checkdams the volume of sediment deposited behind the checkdams ranges between 4 and 920 m 3 for catchments with a drainage area varying between 1 5 and 317 ha our measurements indicate that mean annual catchment wide erosion rates in these mountain ranges are generally low the observed erosion rates are well below maximum tolerable annual soil loss rates for the mediterranean region as 90 of the catchments have mean annual erosion rates below 2 t ha 1 y 1 our erosion data from 20 small catchments in the betic range are lower than the results of previous erosion studies in southeast spain that were conducted in the neogene intramontane basins this study deals with erosion rates on thin soils developed on metamorphic rocks which are not often the subject of study in the mediterranean region in the ephemeral stream systems in the betic range the spatial pattern of the vegetation cover within the catchment in relation to the concentrated flow lines appears to be crucial our data question the direct association of steep sparsely vegetated hillsides with enhanced soil erosion rates and suggest that the main erosion problems are currently not located in these steep sparsely vegetated environments of the betic mountain ranges

benthic cyanobacteria and their influence on the sedimentary dynamics of peritidal depositional systems siliciclastic evaporitic salty and evaporitic carbonatic abstract peritidal sedimentary systems are widely colonized by benthic cyanobacteria that form biofilms and microbial mats the bacterial communities interfere with physical and chemical sedimentary dynamics which is documented by the formation of microbially induced sedimentary structures miss the structures form a new fifth category in the existing classification of primary sedimentary structures siliciclastic depositional systems are dominated by physical dynamics by biostabilization cyanobacteria shelter their substrata against erosion during periods of intensive hydraulic reworking or they permit flexible deformation of sandy sediments during low hydrodynamic disturbance the bacteria enhance deposition of sediments by baffling trapping and binding such biotic physical interference is recorded by miss such as erosional remnants and pockets or planar stromatolites chemical depositional systems include i evaporitic salty environments characterized by evaporation and dissolution and ii evaporitic carbonatic environments that include evaporation dissolution and in situ lithification of organic matter here cyanobacterial mats experience periodical desiccation or evaporation of crystals and mat related structures such as petees and polygonal patterns of cracks are formed cyanobacteria and heterotrophic bacteria provide a chemical microenvironment that supports in situ lithification of organic matter in thin sections carbonate precipitates as ooids or lines of decaying filaments are visible miss occur in modern and ancient depositional systems they record i biological abilities of benthic cyanobacteria to cope with sedimentary dynamics and ii paleoclimate and paleoenvironmental conditions during earth history similar structures are also expected in extraterrestrial paleo environments

4d propagated layer based inversion abstractwe developed a new model based 4d inversion algorithm that propagated through a 4d data set from a well location minimizing the traveltime and amplitude differences of the data the model consisted of a finite number of layers in which the thickness and change in elastic parameters were solved in each layer the inversion used a local optimization strategy at each trace in which the solution of a previously inverted neighboring trace was used as the initial guess it started from a 4d layer based inversion solution found at a well location information from the well was therefore propagated into the data set whereas it remained consistent with the data when several wells were used simultaneously they propagated in a concurrent manner to determine each well s area of influence by using the time shift and amplitude differences it became possible to decouple density and p wave velocity changes however even with this additional information solutions were seldom consistent enough to provide

measuring medium term sheet erosion in gullies from trees a case study using dendrogeomorphological analysis of exposed pine roots in central iberia the assessment of gully erosion poses a great challenge because of the complexity and connectivity of the geomorphic processes involved this study focuses on the quantification of sheet erosion rates in a set of slope gullies located on the northern piedmont of the guadarrama mountains spanish central system in order to delineate accurately the gully areas in which sheet erosion was predominant the hydrologic erosion response unit hru eru approach was used and a dendrogeomorphological analysis of exposed tree roots was carried out to quantify sheet erosion rates in one selected hru eru identification of the first year of exposure by erosion from anatomical criteria was therefore critical the 29 samples taken were prepared for anatomical analysis and cross dated anatomical analysis of the samples showed a reduction in the lumen area of earlywood tracheids following root exposure and also in most cases a slight increase in growth rings moreover at the end of the ring latewood tissue and visible annual borders were very clearly defined by several rows of thick walled tracheids a non parametric test was used on the findings derived from this qualitative analysis to objectify determination of the first year of exposure estimates of sheet erosion were obtained by dividing the height of eroded soil by the number of years that each root was exposed the mean value of soil erosion for the entire study site was then determined from statistical inference using this procedure a range of sheet erosion rates between 6 2 and 8 8 mm y 1 125 2 and 177 8 t ha 1 year 1 was obtained for the dominant hru eru of these gullies in central iberia these estimates of eroded soil thickness were adjusted based on the recent finding that root anatomical changes occur prior to their exposure by erosion

determining soil erosion rates on semi arid watersheds using radioisotope derived sedimentation chronology this study investigates erosion dynamics of the past 90 years on three small semi arid watersheds with histories of grazing and vegetation change activity of 137cs and excess 210pb from 18 cores collected from sedimentation ponds were measured using a gamma spectrometer the sediment was dated using a constant rate of supply crs model this study represents the first time that reservoir sediment accumulation rates determined from fallout isotopes have been verified by direct volumetric measurements of aggradation based on topographic surveys measured sedimentation in the ponds ranged between 1 9 and 2 3 cm y 1 representing average sediment delivery rates from the watersheds of between 0 6 and 2 0 t ha 1 y 1 these sediment delivery rates were in agreement with those established by other methods for similar catchments in the region past variations in sedimentation rates were identified and correlated with recorded history of anthropogenic disturbance 137cs and 210pb methods are suitable for use in arid environments and can complement each other to increase reliability of erosion rate estimates the abundance of stock ponds in southwestern us presents an opportunity to quantify historic erosion and sediment transfer dynamics in areas that have not been well studied or instrumented this article is protected by copyright all rights reserved

proliferation of miss related microbial mats following the end permian mass extinction in terrestrial ecosystems evidence from the lower triassic of the yiyang area henan province north china microbially induced sedimentary structures misss are commonly present in siliciclastic shallow marine settings following the end permian mass extinction but have been rarely reported in the post extinction terrestrial ecosystems here we present six types of well preserved misss from the upper sunjiagou formation and lower liujiagou formation of induan early triassic age in the yiyang area henan province north china these misss include polygonal sand cracks worm like structures wrinkle structures sponge pore fabrics gas domes and leveled ripple marks microanalysis shows that these misss are characterized by thin clayey laminae and filamentous mica grains arranged parallel to bedding plane as well as oriented matrix supported quartz grains which are indicative of biogenic origin facies analysis suggests that the miss hosting sediments were deposited in a fluvial sedimentary system during the early triassic including lake delta riverbeds point bars and flood plain paleoenvironments abundant misss from yiyang indicate that microbes also proliferated in terrestrial ecosystems in the aftermath of the permian triassic p tr biocrisis like they behaved in marine ecosystems microbial blooms together with dramatic loss of metazoans may reflect environmental stress and degradation of terrestrial ecosystems or arid climate immediately after the severe permian triassic ecologic crisis

application of a grid consistent inversion to a 4d reservoir model extraction of the 4d signal from time lapse seismic is an efficient way to evaluate the lateral evolution of changes in fluid pressure and saturation during hydrocarbon production this 4d seismic signal can be used in two ways castro et al 2009 qualitatively from a static perspective thus evaluating whether a compartment has been drained or not or if a fault is permeable or not or quantitatively from a dynamic perspective by trying to exactly match changes in dynamic properties predicted from fluid flow simulation with 4d seismic signal itself in both approaches the main difficulty is to link the seismic information with a description of the reservoir grid especially with regard to scaling problems relative to the simulation model the 4d seismic signal is obtained with a higher lateral resolution 12 5 or 25 m and a poorer vertical resolution 10 20 m indeed the reservoir grid consists of a set of layers with a very high vertical resolution 1 5 m and limited lateral resolution 50

sediment concentrations in run off varying with spatial scale in an agricultural subwatershed of the chinese loess plateau information is scarce on the spatial scale effect on sediment concentrations in run off this study addressed this issue within an agricultural subwatershed of the chinese loess plateau using data observed at a hilltop plot three nested hillslope plots two entire slope plots a combination of hillslope and valley side slope and the subwatershed outlet dominated by the splash and sheet erosions the hilltop plot has a minimum cae mean sediment concentration for all recorded events of 45 kg m 3 unexpectedly the high sediment concentrations at the hilltop do not occur at high rainfall intensities or large run off events because of the protection of surface soils by relatively thick sheet flows because of the emergence of rills cae is as high as 310 kg m 3 even on the most upper hillslope downslope both cae and esc extreme large values of recorded sediment concentrations increase such a slope length effect attenuates with increasing slope length and event magnitude as a result of insufficient sediment availability associated with rill development active mass wastings ensure sufficient sediment supply and thus a spatially invariant cae approximately 700 kg m 3 and esc approximately 1000 kg m 3 at the scale of the entire slope and subwatershed detailed examination shows that most small events experience a decrease in sediment concentrations when moving from the entire slope to the subwatershed indicating that the spatially invariant sediment concentration is valid only for large run off events this study highlights the control of the spatial scale which determines the dominant erosional process on erosional regime copyright 2015 john wiley sons ltd

the natural and human structuring of rivers and other geomorphological systems a tribute to william l graf this special issue honors the contributions of william l graf to geomorphology and river science a hallmark of will s work over the course of his career has been a focus on the natural and human structuring of river systems more broadly will has been an innovator and leader who has shaped the way in which geomorphologists conduct research through his work he has made fundamental contributions to basic fluvial theory to the understanding of human impacts on river systems and to policy relevant science he has demonstrated by example how to pursue policy relevant science and to participate in science based policy formulation his contributions to river science can be classified into several themes 1 the hydrology and geomorphology of suburban drainage systems 2 riparian vegetation and river systems 3 the spatial structure and dynamics of incised channels 4 the dynamics of dryland river systems 5 heavy metals in river systems 6 dams and dam removal and 7 water and public policy the papers in this special issue reflect many aspects of these themes and address topics related to 1 the understanding of rivers and other geomorphic systems in the midst of dynamic physical change 2 human influences on geomorphic processes 3 the intersection of geomorphology and public policy and 4 the fusion of geomorphic analysis and giscience

sediment yield and runoff frequency of small drainage basins in the mojave desert u s a sediment yield from small arid basins particularly in the mojave desert is largely unknown owing to the ephemeral nature of these fluvial systems and long recurrence interval of flow events we examined 27 reservoirs in the northern and eastern mojave desert that trapped sediment from small b 1k m 2 drainage basins on alluvial fans over the past 100 yr calculated annual sediment yield and estimated the average recurrence interval ri of sediment depositing flow events these reservoirs formed where railbeds crossed and blocked channels causing sediment to be trapped and stored upslope deposits are temporally constrained by the date of railway construction 1906 1910 the presence of 137 cs in the reservoir profile post1952 sediment and either 1993 when some basins breached during regional flooding or 2000 2001 when stratigraphic analyses were performed reservoir deposits are well stratified at most sites and have distinct fining upward couplets indicative of discrete episodes of sediment bearing runoff average ri of runoff events for these basins ranges from 2 6 to 7 3 yr and reflects the incidence of either intense or prolonged rainfall more than half the runoff events occurred before 1963 a period of above normal precipitation from 1905 to 1941 may have increased runoff frequency in these basins mean sediment yield 9 to 48 tons km 2 yr 1 is an order of magnitude smaller than sediment yields calculated elsewhere and may be limited by reduced storm intensity the presence of desert pavement and shallow gradient of fan surfaces sediment yield decreases as drainage area increases a trend typical of much larger drainage basins where sediment transport processes constrain sediment yield coarse substrate and low angle slopes of these alluvial fan surfaces likely limit sediment transport capacity through transmission losses and channel storage d 2005 elsevier b v all rights reserved

the spatio temporal invariability of sediment concentration and the flow sediment relationship for hilly areas of the chinese loess plateau abstract the spatio temporal variation underlies and complicates studies related to earth surface processes subsequent to our previous studies reporting the temporal invariability of sediment concentration this study further reports the spatial invariability of sediment concentration or the flow sediment relationship in the hilly part of the chinese loess plateau the dense channel network dissects this area into numerous sub watersheds which are composed of entire slopes a combination of the hill slope and the valley side slope our data mainly involves ten experimental sites of entire slope and sub watershed and 580 flood events at three locations our analysis at the intra event time scale shows that the instantaneous sediment concentrations in runoff can generally approach the detach limited maximum sediment concentration since the active massing wasting ensures an abundant sediment supply consequently the event mean sediment concentration does not only remain fairly constant among flow events but also among the experimental sites despite the possible differences in topography soil management or land cover water source soil surface properties etc this observation also holds at the annual time scale our result implies that a single proportional function is adequate to fit the flow sediment relationship for any entire slope or sub watershed where eroded sediments are primarily sourced over a considerable areal extent unless soil property has changed or check dams are installed moreover the proportionality coefficient which is indicative of the maximum detachment capacity can be approximated through flume experiments rather than necessarily through long term field observations our finding can facilitate the obtainment of spatially distributed information on soil loss in the chinese loess plateau and is also indicative of other locations where sediment supply is sufficiently available

characteristics controlling factors and importance of deep gullies under cropland on loess derived soils abstract several studies describe the formation and importance of shallow ephemeral gullies under cropland depth comparison of a dataset with 28 deep gullies formed in the period 1985 2003 and 123 shallow ephemeral gullies formed in the period 1994 1999 in central belgium indicates that the deep gullies have clearly different morphological characteristics compared to shallow ephemeral gullies several factors were analyzed to understand the formation of deep gullies plotting runoff contributing area versus slope of the soil surface at the gully head indicates that the topographical threshold for deep gully formation is significantly larger compared to ephemeral gully formation deep gullies form on short steep valley sides and their position is strongly affected by the presence of linear landscape elements all deep gullies incised on landscape positions with a very erodible soil horizon at shallow depth analysis of causative rainfall showed no significant differences in incision thresholds between rills and shallow ephemeral gullies on the one hand and deep gullies on the other hand the relation between area specific sediment yield in central belgium and drainage area indicates that the development of deep gullies contributes significantly to the sediment output of small rural catchments and causes peaks in the mean area specific sediment yield that are up to a factor of three higher compared with catchments where shallow ephemeral gullying occurs

neoproterozoic trace fossils vs microbial mat structures examples from the tandilia belt of argentina abstract the tandilia belt in northeast argentina includes a neoproterozoic sequence of sediments sierras bayas group in which the cerro largo formation ca 750 ma in age forms a siliciclastic shallowing upward succession of subtidal nearshore to tidal flat deposits trace fossils palaeophycus isp and didymaulichnus isp have been described from the upper part of this succession specific sedimentary structures consisting of round crested bulges arranged in a reticulate pattern and networks of curved cracks are associated with the trace fossils these structures are considered to be related to epibenthic microbial mats that once colonized the sediment surface they reflect stages of mat growth and mat destruction if compared to analogous structures in modern cyanobacterial mats of peritidal siliciclastic depositional systems also the trace fossils are interpreted as mat related structures partly forming components of networks of shrinkage cracks partly representing the upturned and involute margins of shrinkage cracks or circular openings in desiccating and shrinking thin microbial mats the definition of didymaulichnus miettensis young as a terminal proterozoic trace fossil is questioned and it may be considered to interpret the bilobate structure as the upturned opposite margins of microbial shrinkage cracks which have been brought back into contact by compaction after burial

microbial signatures in peritidal siliciclastic sediments a catalogue a catalogue of microbial structural signatures is presented based upon the coupling of fundamental biogeochemical microbial processes and local morphogenetic determinants it summarizes a collection of sedimentary structures obtained from two modern siliciclastic peritidal environments in different climatic zones temperate humid mellum island southern north sea subtropical arid coast of southern tunisia textural geometries reveal a high structural diversity but their determinants are primarily based upon six major parameters 1 intrinsic biofactors structural diversification of sedimentary microbial films and mats inherent in the organisms i e their construction morphology growth taxis and behaviour and local abundance of specific morphotypes most prominent are the ensheathed filamentous cyanobacteria microcoleus chthonoplastes and lyngbya aestuarii and the sheathless filamentous cyanobacterium oscillatoria limosa 2 biological response to physical disturbances sediment supply erosion and fracturing of surface layers resulting from desiccation cause growth responses of biofilms and microbial mats 3 trapping binding effects physicobiological processes give rise to grain orientations and wavy to lenticular lamina lamina specific grain arrangements and sucrose calcium carbonate accumulations 4 secondary physical deformation of biogenic build ups mechanical stresses acting upon sediments overgrown and biostabilized by biofilms and mats produce erosional and overthrust structures 5 post burial processes textural fabrics that evolve from mechanical effects of gas formation from decaying mats and features related to the formation of authigenic minerals calcium carbonates calcium sulphates pyrite 6 bioturbation and grazing post depositional structures such as skolithos type dwellings traces of burrowing insects gastropod grazing traces and faecal pellets in synopsis the catalogue firstly comprises a sound set of ubiquitous signatures this uniformity in architectural characteristics is attributed to the presence and local dominance of certain microbes throughout the different settings the catalogue secondly documents signatures that are extremely sensitive to tidal position hydrodynamic regime and overall climatic conditions these kinds of signature indicate narrow facies zones which often coincide with the activity or dominance zones of certain organisms an overview of structures of microbial origin from the fossil record underlines the potential of many of the signatures included in this catalogue to become fossilized and provide strong indicators of former siliciclastic tidal settings

mat related sedimentary structures in neoproterozoic peritidal passive margin deposits of the west african craton anti atlas morocco abstract proterozoic inliers in the central anti atlas mountains expose predominantly siliciclastic sedimentary successions deposited in peritidal zones along the neoproterozoic continental margin of the west african craton wac the low grade metamorphic and modestly deformed sediments contain a wealth of sedimentary structures related to the former presence and activities of microbial mats and respective physicobiological processes the well preserved structures include wrinkle structures erosion marks microbial sand chips spindle shaped and subcircular microbial shrinkage cracks and possibly gas domes and cabbage head structures thin sections exhibit mat fragments and dispersed grains of hematite limonite after pyrite in fine grained quartzitic storm deposits post storm layers frequently consist of matrix supported sand sized to silt sized grains and are overlain by argillaceous veneers including isolated silt sized grains and black carbonaceous laminae the muddy veneers are considered to represent compacted stacks of microbial mats biolaminites which colonized and biostabilized storm and post storm layers and thus prevented them from eroding in the absence of grazing and burrowing organisms and at suitable depositional and hydrodynamic conditions it may be expected that proterozoic microbial mats extensively grew from the supratidal to the intertidal zones and occasionally e g behind protective barriers in the subtidal zone and beyond mat related structures however need specific conditions for their formation and preservation wrinkle structures erosion marks and microbial sand chips require tractional currents and soon deposition and burial respectively such conditions are preferably met in intertidal and supratidal zones spindle shaped and subcircular cracks require mat shrinkage due to either desiccation or syneresis crack propagation implies progressive shrinkage while superposition of crack generations indicates repeated alternation between mat exposure and flooding respective conditions prevail in the upper intertidal zone gas domes and cabbage head structures are related to the production of gas from decaying organic material beneath a sealing cohesive mat they may be some of the first features formed during genesis of petee structures in the intertidal to lower supratidal zones mat related structures may serve as sensitive facies indicators once their modes of formation are revealed

a comprehensive avo classification in this paper a new classification for avo responses is proposed this classification covers all possible avo responses and is independent of the fluid content of the beds in 1989 rutherford and williams introduced a three fold classification of avo amplitude versus offset characteristics for seismic reflections from the interface between shales and underlying gas sands the classification scheme they proposed has become the industry standard and has proven its validity and usefulness in countless exploration efforts the rutherford and williams classification is primarily based on the sign and magnitude of the reflection coefficient r at the top of the sand these authors went on to observe that owing to the high poisson s ratio values for the typical encasing shales relative to the gas sands the avo response of all of these gas sand classes would be characterized by an increasingly negative reflection coefficient with increasing offset their classification is explicitly defined for gas sands in 1997 castagna and swan proposed avo crossplotting wherein an estimate of the normal incidence reflectivity is plotted against a measure of the offset dependent reflectivity using this approach castagna and swan graphically illustrated the continuum between the classes and defined the characteristics of the classes using what they termed avo intercept and avo gradient they also added a class 4 which they believed was originally included in rutherford and williams class 3 definition the authors of the present paper propose to expand and partly redefine the currently used classifications in order to distinguish among the schemes the term types as opposed to classes is used to name the different categories and in order to preserve congruity where the schemes overlap the sands which rutherford and williams would call class 1 2 or 3 are referred to as types 1 2 or 3 type 4 of the proposed