the bagdad reflection sequence as tabular mafic intrusions evidence from seismic modeling of mapped exposures two dimensional seismic modeling of exposed rocks implicates subhorizontal tabular mafic intrusions as the likely cause of the unusually prominent reflections that constitute the bagdad reflection sequence brs these basement reflections from depths of 3 15 km are observed on cocorp and other data to extend for some 100 km across the basin and range colorado plateau transition zone in west central arizona the nearby buck mountains fortuitously expose an upended upper crustal block which is used in this study as the basis for a 12 x 6 km seismic model ray tracing through this model produces synthetic reflections from the numerous proterozoic diabase intrusions occurring in the buck mountains these synthetic reflections approximate the scale geometry and character of reflections observed on the cocorp data indicating that the intrusions in the buck mountains furnish a viable geologic analogy for the brs several lines of evidence argue that the major competing hypothesis that tertiary detachment surfaces cause the brs is less satisfactory although we cannot rule out the possibility that the brs is related to hypothetical tertiary intrusions of similar distribution and structure geologic evidence more directly implies that these reflectors are proterozoic in age if so the bagdad reflections serve as strain markers and can be used to constrain subsequent deformation in the arizona transition zone their relatively continuous nature thus argues against widespread disruption of the upper crust in the transition zone during tertiary extension moreover identification of these reflectors as subhorizontal mafic intrusions may have profound consequences for interpretation of similar reflections observed elsewhere by bolstering the contention that mafic intrusions are an important cause of basement reflections in the continental crust

paleodrainage and paleochannel morphology of a barakar river early permian in the rajmahal gondwana basin bihar india abstract undisturbed sediments belonging to the barakar formation of the rajmahal basin are commonly coarse to medium grained and feldspathic they display both tabular and trough cross bedding and are commonly interbedded with thin arenaceous and carbonaceous shale and coal the average thickness of crossbedding sets is 28 15 cm with an inclination of 16 5 these features and other evidence suggest that the crossbedded sandstone units are subaqueous in origin the paleodrainage in the basin is deduced by circular directional statistics using a vectorial parameter of 217 foreset planes of crossbedded units from 16 widely separated outcrops dip azimuth distribution is generally unimodal fan shaped and displays a high magnitude of resultant r 0 6448 and a mean paleoflow of 15 48 paleochannel results indicate that the barakar river was about 235 415 m wide and 3 7 5 5 m deep and that it was moderately sinuous p 1 39 and flowed on a slope of about 35 cm km 0 00035 with an average flow velocity of 60 150 cm sec the azimuthal variability smaller value of circular variance together with a high width depth ratio steep slope and paucity of fine clastics suggest fast deposition by an anastomosing stream with bed load channels a possible extension of the rajmahal gondwana basin is discussed in the subsurface into the purnea and darjeeling basins to the north and northern part of bangladesh in the east with a northerly outlet of the barakar river towards a cratonic shore zone

seismic models of a prograding carbonate platform vercors south east france abstract the southern vercors area south east france displays superb exposures of a lower cretaceous prograding carbonate platform these exposures allow a geometrical sequence analysis to be combined with detailed outcrop observations two dimensional seismic modelling techniques were used to determine the seismic response of three of these exposures the montagnette the archiane valley and the rocher de combau the outcrops display prograding and retreating series of clinoforms interfingering with fine grained basinal sediments photographs from different angles of view combined with detailed field observations were used to construct lithological profiles with a resolution of f few metres p wave velocities and bulk densities were measured from cored hand specimens and selected values were assigned to lithostratigraphic units in each profile the vertical incidence modelling technique was then used to compute migrated time sections of reflectivity these were convolved with source wavelets of different frequencies to produce the final synthetic seismic profiles none of the modelled outcrop geometries is correctly portrayed seismically at 25 50 hz frequencies not only do the synthetic seismic profiles show less detail they also show misleading geometries in most seismic models for instance pseudo unconformities are present that correspond to rapid changes in dip and in facies in outcrop furthermore the seismic tool is not able to resolve between stratal patterns punctuated by unconformities and those with gradual shifts in facies belts accompanied by complex interfingering relationships this implies that an unconformity recognized on a real seismic line should not automatically be assumed to represent a true stratal unconformity large outcrops are often used to show the validity of sequence stratigraphy a concept mainly based on the analysis of seismic lines this study however shows that there is no one to one relationship between geometries observed in outcrop and in seismic lines seismic modelling of outcrops therefore provides a means for evaluating outcrop geometries in terms of seismic stratigraphy

sediment trend models fail to reproduce small scale sediment transport patterns on an intertidal beach a rigorous test is presented of the application of sediment trend models to an intertidal beach environment characterized by bar morphology sediment samples were collected during low tide from a regular grid and their sediment fall velocity distributions obtained using a settling tube were analysed using moment analysis the net sediment transport direction determined from beach surveys hydrodynamic measurements wave ripple observations and sediment transport modelling was compared with predictions by sediment trend models based on the spatial distribution of sediment parameters it was found that the sediment transport pathways and patterns of sedimentation predicted using sediment trend models were at odds with field observations and varied significantly depending on whether surface or sub surface sediment samples were used the sediment trend models are thought to fail because in energetic and morphologically variable beach environments spatial patterns in sediment characteristics are mainly attributed to the presence of different hydrodynamic regions and associated morphology rather than sediment pathways the use of sediment trend models cannot replace the collection of morphological hydrodynamic and sediment transport data in the field to define relationships between flows forms and sedimentation patterns on a dynamic intertidal beach

topographic slope as an estimator of acoustic impedance abstract seismic models of large outcrops have provided important insights into the nature of seismic unconformities and other elements of sequence stratigraphy further advances in outcrop modelling critically depend on more detailed acoustic impedance functions as a basis for the seismic models this is difficult because continuous logs of sonic velocity and density are lacking and the impedance function has to be generated from spot samples combined with proxy indicators that can be observed continuously hill slope angle has been identified as a possible proxy of impedance in the past we have tested and largely confirmed this notion in a case study using laser technology to quantify topography this opens the possibility to extrapolate the impedance function to other areas within the same outcrop and thus make a prediction for the total acoustic impedance distribution

bed load grain velocities and sediment transport rates theoretical consideration of the dynamics of bed load sediment transport leads to expressions for bed grain velocity and sediment transport rate as respectively ub a u u c and ib a tan u u c 0 c agreeing in form with many previous analyses the grain velocity equation agrees well with all experimentally determined velocities of single grains moving over fixed rough beds if a is allowed to vary with bed roughness and saltation height values of u c for such overriding grains are substantially less than normally accepted values the sediment transport rate equation predicts plane bed experimental data well over five orders of magnitude of ib values of a tan increase significantly from lower to upper stage plane beds due to an increase in a with suspended sediment concentration but also due to a decrease in tan with increasing transport stage and grain size as predicted by bagnold

the continental margin of the mesozoic tethys in the western alps abstract most of the structural units of the western alps were derived from the european continental margin of the ligurian ocean a segment of the mesozoic tethys ocean their mesozoic palaeotectonic and stratigraphic evolution bears witness of the following main stages 1 deposition of the triassic platform carbonates essentially prerift but nevertheless bearing the imprint of some extensional movements 2 the liassic middle jurassic rifting corresponds to the creation of a horst and graben system with especially tilted blocks and hence a shoal and basin palaeogeography but without a preliminary doming event 3 the late jurassic early cretaceous opening and spreading of the ligurian ocean began with a general collapse of the continental margin thermal subsidence latest middle jurassic and earliest late jurassic in this paper emphasis is given to the refting derived structures more especially to the major tilted crustal blocks a few tens of kilometres wide which can be either reconstructed or directly observed the rifting stage lasted roughly 40 m y with the alternation of extensional tectonic phases and of relatively quiet periods during the extensional phases movements along fault planes and related episodes of tilting were followed by sudden and rapid episodes of subsidence the latter may be interpreted as resulting from the isostatic and thermal readjustment that follows a phase of stretching of the crust and of the lithosphere the tectonic evolution of the margin continued but decreased during the late jurassic early cretaceous oceanspreading stage tectonic activity resumed in the late cretaceous this probably resulting from the beginning of contraction of both the ocean and the continental margin leading progressively to the continental collision in the tertiary

progradation geometries of carbonate platforms examples from the triassic of the dolomites northern italy various types of progradation of triassic carbonate platforms are described from the dolomites of the southern alps the internal and external geometric relationships are exposed in spectacular natural sections and moreover their scale 500 1000 m of thickness is such that they can be compared with features found in seismic profiles the different types of progradation are controlled by a number of factors which normally interact with each other these factors include rate of basinal sedimentation rate of subsidence width of the platform depth of the surrounding basin and eustatic variations of sea level progradation is not a continuous process but episodic moments of massive debris input during which the platform advances alternate with long periods of negligible progradation during which basinal sediments accrete and onlap the toe of slope upper boundary relationships of the prograding platforms include offlap toplap and erosional truncation lower boundary relationships are horizontal climbing and descending progradations a variety of phenomena and circumstances have caused the cessation of progradation of the triassic platforms they include volcanism collapse of margins drowning rapid relative rise of sea level subaerial exposure relative fall of sea level and probably a natural decay of the system in the triassic of the dolomites two main progradation models can be put forward in the ladinian model progradation took place simultaneously with aggradation relative rise of sea level whereas the characteristic feature of the carnian model is toplap relative stillstand of sea level

sortable silt and fine sediment size composition slicing parameters for palaeocurrent speed and palaeoceanography fine sediment size 63 ixm is best measured by a sedimentation technique which records the whole size distribution repeated size measurement with intermediate steps of removal of components by dissolution allows inference of the size distribution of the removed component as well as the residue in this way the size of the biogenic and lithogenic noncarbonate fractions can be determined observations of many size distributions suggest a minimum in grain size frequency curves at 8 to 10 ixm the dynamics of sediment erosion deposition and aggregate breakup suggest that fine sediment behavior is dominantly cohesive below 10 1xm grain size and noncohesive above that size thus silt coarser than 10 ixm displays size sorting in response to hydrodynamlc processes and its properties may be used to infer current speed silt that is f mer than 10 ixm behaves in the same way as clay 2 xm useful parameters of the distribution are the 10 63 ixm mean size and the percentage 10 63 ixm in the fine fraction we cannot use size distributions to distinguish the nature of the currents therefore to infer water mass advection speeds i e the mean kinetic energy of the flow ku regions of high eddy kinetic energy ke must be avoided at the present such abyssal regions lie under the high surface k e of major current systems gulf stream kuroshio agulhas antarctic circmpolar current and brazil falldand currents in the argenthe basin this is probably a satisfactory guide for the pleistocene with regard to the carbonate subfraction of the size spectrum size modes due to both cocco iths and foramlnlferal fragments can be recognized and analyzed with the boundary between them again at about 10 lm the flux of less than 10 lm carbonate at pelagic sites above the lysocline is another candidate for a productivity indicator

routing of heterogeneous sediments over movable bed model development a one dimensional numerical model of sediment routing is derived to simulate erosion transport and deposition of individual size density fractions in the bed material within a relatively straight nonbifurcating alluvial channel the reach of interest is subdivided into a number of longitudinal elements of varying width averaged properties during each time step flow depths and velocities in each element are determined from the gradually varied flow equation using the standard step method for backwater calculations the bedload transport rate of each size fraction is calculated from a modified bagnold equation implementing a novel approach that takes into consideration the effects of turbulent fluctuations in the bed shear stress critical shear stresses for entrainment of particles from the mixed bed are determined using relationships that treat grain protrusion and hiding the transport of particles in suspension is modeled using a convection diffusion sediment continuity equation either explicitly

seismic modeling of an early jurassic drowned carbonate platform djebel bou dahar high atlas morocco 1 along the southern margin of the high atlas of morocco early jurassic carbonate platforms developed in a transtensional rifted basin one of these platforms was the djebel bou dahar platform which grew to a vertical relief of 500 m drowned and was onlapped by deep marine sediments exceptional outcrops allowed detailed lithologic models of the platform to be made with a resolution of a few meters we constructed a geometric model from building blocks comprising particular rock types we estimated p wave velocities and bulk densities of these lithologies from hand specimens and we assumed these properties to be homogeneous and invariant throughout the building blocks we constructed time sections of reflectivity from vertical incidence and image ray migration techniques these time sections were convolved with 25 to 200 hz ricker wavelets to produce synthetic seismic sections large scale features such as the platform and the drowning unconformity were resolved regardless of the frequencies and modeling techniques used broad low frequency wavelets resulted in profiles with apparent truncation of bedding and basinward shifts in the point of onlap smaller scale features such as reef knolls and sand bodies at the platform margin were poorly resolved at frequencies up to 50 hz however we inferred the existence and location of these smaller features from the disturbance and distortion of other reflections this distortion is most obvious in image ray migration modeling because of the refraction of rays through the small scale features geometric effects caused lateral amplitude variations due to wavelet interference along the unconformity reflection where no petrophysical changes were modeled the seismic models of the djebel bou dahar in its present stage may be good analogs for seismic lines of deeply buried carbonate platforms to make a comparison with real seismic lines of less deeply buried platforms possible we reconstructed both the geometry and petrophysical properties and the subsequent seismic modeling of the djebel bou dahar carbonate platform as it was prior to the onset of deep burial at this stage impedance contrasts were at their maximum this study clearly demonstrates the significance of modeling the seismic response of a detailed outcrop based stratigraphic model and shows that seismic modeling of specific sequence stratigraphy can reveal what features are likely to be resolved under ideal conditions and what pitfalls may await the interpreter

sea level changes an integrated approach sea level changes an integrated approach in october 1985 sepm sponsored a four day conference entitled sea level changes an integrated approach the purpose of the conference was to provide a forum for an interdisciplinary exchange of ideas on sea level changes and to provide an opportunity for integrating various types of evidence in approaching unresolved issues the conference was successful in bringing together scientists from industry academia and government representing all of the major geosciences disciplines presentations of many new papers plus significant releases of data that were previously held proprietary provided fertile ground for discussion this much cited volume represents the best of the material presented at the conference includes the early vail chart

sample site variation in settling velocity distribution subpopulations using curve dissection analysis it is generally believed that grain size distributions can be broken down into subpopulations which may be characteristic of a specific mode of transport some source signature or the nature of sediment packing in the depositional process this paper evaluates the variation at a sample site of subpopulation parameters of settling velocity distributions derived using a non linear least squares algorithm for curve dissection results incorporate artificial samples constructed using spherical glass beads in the 125 840 m range and natural samples collected as close as possible from a single sedimentation unit in marine fluvial and aeolian environments if subpopulation distributions from the same sample site are compared then 90 of artificial bead samples and 65 of natural sands have cumulative curves diverging from each other by less than 20 similarity in means and standard deviations of subpopulations collected at the same site was not easily obtained however if it is important to be able to identify a specific process or source signature correctly then 86 of all subpopulations from the same natural sample site grouped together if subpopulations of grain size distributions are to be used for process identification it is advisable to analyse more than one sample from a site

ground penetrating radar imaging of carbonate mound structures and implications for interpretation of marine seismic data images of carbonate mound structures in denmark and sweden are obtained from ground penetrating radar gpr reflections and outcrop analysis the gpr data are collected in limestone quarries and provide a depth penetration of about 10 m 33 ft and a vertical resolution of about 0 5 m 1 6 ft the mounds are part of the northwest european upper cretaceous danian lower paleocene chalk group and they are similar to each other in terms of architecture spatial distribution and size with widths and lengths of 30 60 m 100 200 ft and heights of 5 10 m 16 33 ft seismic reflection images from the sea of kattegat between sweden and denmark and the north sea show large 500 1000 m 1640 3280 ft wide and 50 100 m 160 330 ft high moundlike carbonate structures interpretations of such large mounds conflict with the gpr and outcrop observations we address these conflicting observations we construct realistic reference models of carbonate mound complex geometries based on the results of the gpr measurements and outcrop analysis and calculate synthetic seismic sections for the models the modeling results show that the size of individual mounds is below the seismic resolution of 10 25 m 33 82 ft and that interference effects caused by stacks of mounds may explain the observed large moundlike structures our findings are important for the interpretation of seismic images of carbonate mound structures carbonate mound buildups may form traps and correct seismic interpretation of mound complex geometry may be essential for evaluation of the nature and reservoir potential of such structures

a recent turbidity current event hueneme fan california reconstruction of flow properties a recent 100 yr old turbidite is described from hueneme fan california continental borderland dense sampling over the fan surface has allowed excellent delineation of the characteristics of this deposit it exhibits bouna de sequences and has a distinctly bimodal sandy silt grain size distribution through the use of generalized fluid dynamics equations it is possible to reconstruct original flow properties of the current which deposited this material the calculated velocities ranged from 10 90 cm s 1 and excess density above ambient seawater from 0 001 0 005 g cm 3 in the lower midfan and upper fan channel regions respectively height of the current ranged from 5 15 m on slopes from 1 5 to 0 15 a total of 107 m3 of sediment was deposited during 10 days the turbidity current is conjectured to have originated from direct river input during the floods of 1884 an older event is also described which has distinctly different properties and origins the grain sizes of this older deposit are much coarser and sedimentary structures suggest higher flow regimes this turbidite is conjectured to have been deposited from a higher density faster current thought to have been generated by slumping the need for a better understanding of the controls on the characteristics of turbidity currents and their effect on fan morphology is emphasized

toe of slope of a cretaceous carbonate platform in outcrop seismic model and offshore seismic data apulia italy synthetic seismic models of outcrops in the early cretaceous slope of a carbonate platform on the gargano promontory southern italy were compared to an offshore seismic section south of the promontory outcrops of the same age on the promontory have the same sequence stratigraphic characteristics as their offshore equivalent and are the only areas where the transition from platform to basin of early cretaceous is exposed on land two adjacent outcrop areas were combined into one seismic scale lithologic model with the aid of photo mosaics measured sections and biostratigraphic data velocity density and porosity measurements on spot samples were used to construct the impedance model seismic models were generated by vertical incidence and finite difference programs the results indicate that the reflections in the seismic model are controlled by the impedance contrast between low porous intervals rich in debris from the platform and highly porous intervals of pelagic lime mudstone nearly devoid of debris finite difference seismic display showed best resemblance with the real seismic data especially by mapping a drowning unconformity

automatic grain sizing of vertical exposures of gravelly deposits abstract the grain size distribution gsd is a key indicator in stratigraphy and morphology it is a basic tool used by a variety of disciplines geology geomorphology archeology ecology and engineering automatic grain sizing ags has been developed to estimate the gsd of surfaces in a rapid and effortless manner yet the traditional bulk method time consuming and laborious is still in use for the determination of the gsd of coarse grained sedimentary deposits and of the near subsurface of landforms principally river beds the ags technique is examined here in its application to the stratigraphic record and to the subsurface images processed by ags and manual samples of coarse grained deposits in section were compared for evaluation of accuracy and determination of the extent of validation of the ags technique the ags often achieves results very similar to field counting especially when the largest grains are accounted for in the size of the sampling area and when photographs are taken afar without flash best results are provided when two images of the same sediment are taken at close range and also from afar to combine their gsds hence overcoming errors generated by resolution sampling area and grain size the computed errors are larger than those in prior studies yet remain reasonably small even though results are not bias corrected axes exposed in an outcrop very likely differ from those exposed on the surface and sediment properties and lighting conditions vary although results indicate a 25 burial effect the performance of the ags for sections is high unlike prior application of ags for riverbed surface openwork gravels conglomerate sections and the near subsurface include a matrix resulting in a somewhat larger mean irreducible error the technique may be utilized in studies of conglomerate texture allowing the evaluation of entire gsds under difficult sampling conditions an example use of ags to subsurface gsd evaluation is provided among others enabling an easy and fast method to calculate extent of armoring

stratal patterns sediment composition and sequence stratigraphy at the margin of the vercors carbonate platform lower cretaceous se france abstract conventional sequence stratigraphic concepts relate the large scale stratal architecture of sediment bodies to changes in relative sea level this paper evaluates the relationship between stratal geometry and sea level stand based on a study of large semicontinuous outcrops of cretaceous carbonate platform strata in the vercors southeastern france multiple lines of evidence for sea level change are combined including stratal geometry detailed quantitative microfacies analyses and diagenetic patterns at platform top hardground surfaces the studied outcrops of the cirque d archiane show two main prograding platform tongues both over 100 m thick the stratal geometries at the boundary between these platform tongues including an apparent pinchout of a wedge of slope sediments suggest the presence of a major lowstand unconformity however this stratal boundary does not coincide with the horizon containing the most extensive meteoric alteration furthermore detailed platform to basin correlation shows that the wedge of slope sediments is not basin restricted but makes a thin drape over the platform top the sedimentologic and diagenetic evidence suggest incipient drowning of the platform at the boundary between the two main platform tongues preceded by a minor exposure event only internally the main platform tongues consist of smaller 10 30 m thick units that prograde towards the basin and aggrade on the platform top and which are interpreted as stacked highstand wedges these wedges are usually topped by hardground surfaces with minor evidence for subaerial exposure the platform top horizon with the most extensive subaerial diagenesis and erosion correlates with a distinct but relatively thin unit of lithoclastic debris on the slope a pronounced scour in the clinoforms of the vercors does not correlate with a major exposure surface on the platform top this study shows that stratal geometries alone are a rather ambiguous guide to sea level history without the accompanying sedimentologic and diagenetic evidence for sea level change most of the stratal architecture of the vercors platform can be explained by either changes in accommodation or changes in carbonate production

depositional sequences and correlation of middle to late miocene carbonate complexes las negras and nijar areas southeastern spain during serravallian through messinian time marine carbonates flanked topographic highs that rimmed neogene basins in the western mediterranean middle to upper miocene carbonate strata in the las negras and nijar areas southeastern spain are 50 150 m thick and display 50 200 m of shelf to basin relief over 1 2 km detailed studies in those areas document the effects of relative sea level change on sedimentation biotic composition and reef development we identify three previously unrecognized regionally correlatable depositional sequences ds1 ds2 ds3 that occur between the underlying basement and the overlying terminal carbonate complex the lower depositional sequences ds1 ds2 are mostly normal marine shelf ramp carbonates deposited on the flanks of basement highs the basal part of ds2 locally contains some megabreccia reef blocks composed of tarbellastraea and porites these blocks are the first evidence of reef growth in the area and represent a previously unrecognized period of reef development prior to the fringing reef development the reef blocks probably formed as upslope patch reefs that were eroded and transported to distal slope locations the upper sequence ds3 is characterized by clinoform strata of a porites dominated fringing reef complex that prograded basinward in a downstepping style with successively younger reefs forming in a topographically lower and more basinward position as a result of a net sea level drop regional correlation of miocene shallow marine strata between basins in spain and elsewhere in the western mediterranean is complicated because basins were semi isolated from adjacent basins making physical correlation impossible in addition age definitive biostratigraphic markers are poorly preserved in most of the miocene shallow water strata basinal sediments that are more easily dated by microfossils do not typically interfinger with the shallow marine strata in outcrop even where datable microfossils are found resolution of dating is poor our studies in the las negras and nijar areas illustrate the usefulness of integrating sedimentological geometric and biotic data with locally derived relative sea level accommodation space curves for correlation the relative sea level curves for each area show remarkable similarities in shape and magnitude of sea level changes these curves indicate several relative sea level fluctuations during miocene carbonate deposition prior to the major sea level drop at the end of ds3 deposition that culminated in the exposure of the basin margin deposits and the deposition of evaporites in basinal areas during the messinian the depositional sequences in the las negras and nijar areas may correlate with depositional sequences of similar age throughout the southern cabo de gata area in mallorca some 600 km to the northeast and possibly in other mediterranean locations the widespread occurrence and possible correlation of the depositional sequences suggest regional processes such as eustacy or tectonism for their formation the integration of sedimentological palaeontological and sequence stratigraphic studies and the construction of relative sea level accommodation space curves may help in the interpretation of depositional histories of shallow marine carbonate complexes and correlation of these strata between isolated areas other dating methods in addition to microfossil dating may allow for better age determination of the sequences and aid in identifying the importance of eustacy and tectonism in sequence development

determining the causes of pleistocene stream aggradation in the central coastal areas of western australia summary late pleistocene stream aggradation was widespread in the moresby ranges western australia the general geomorphological characteristics of the drainage basins and the details of the sedimentology paleohydraulics suggest that stream aggradation was due to sediment yields exceeding stream capacity however from a consideration of the mechanics of stream aggradation it is demonstrated that stream aggradation was a response to both sediment supply and base level controls with both processes operating against a background of high sediment yields the study demonstrates that it can be relatively straightforward to establish the causes of stream aggradation

the coral reef of nijar messinian uppermost miocene almeria province s e spain abstract the nijar reef is a thoroughly exposed and representative example of the messinian coral reefs outcropping along the periphery of the western mediterranean facies patterns in the nijar reef complex are presented in this paper as a generalized guide or model for the study of other messinian reefs messinian paleogeography of this area was characterized by an archipelago whose islands are present day sierras at the beginning of the messinian and before development of the reefs marine terrigenous sediments marginal terrigenous complex were deposited on an irregular erosion surface on the metamorphic basement the reef complex developed as a belt fringing the positive areas islands when terrigenous deposition decreased the reef complex consists of 1 reef core constructed by a framework of vertical branching colonies of porites coated by laminated micrite probably submarine cements which form pinnacles as well as thickets and 2 fore reef made of sloping beds of broken skeletons and debris the fore reef is subdivided into i reef talus slope ii proximal slope and iii distal slope the reef talus forms massive strata with dips of 20 30 and abundant pebble to boulder size slumped blocks of reef core and coral breccia the proximal slope shows dips of 10 20 of well bedded thick strata with internal parallel layering of granule to sand size skeletal calcarenites the distal slope is made of predominant bioturbated skeletal calcisiltites and fine grained calcarenites with ngles lower than 10 the distal slope grades into the basinal pelagic marls of the nijar plains the original reef morphology is well preserved in present day topography with relicts of pinnacles reef valleys and slopes messinian volcanic highlands located to seaward complicated the local paleogeography with atoll like features reef growth was concomitant with a gradual fall of sea level and occurred before the messinian gypsum was deposited on top of the basinal marls in what is now the plains area the reef complex is partially truncated by a major erosion surface and it is overlain by oolitic shoals and stromatolites of the so called terminal carbonate complex the terminal complex is interpreted as an upslope equivalent in age to the messinian upper evaporite and it implies a new cycle of sea level rise

sedimentary architecture of a plio pleistocene proto back arc basin wanganui basin new zealand the sedimentary architecture of active margin basins including back arc basins is known only from a few end members that barely illustrate the natural diversity of such basins documenting more of these basins types is the key to refining our understanding of the tectonic evolution of continental margins this paper documents the sedimentary architecture of an incipient back arc basin 200 km behind the active hikurangi subduction margin north island new zealand the wanganui basin wb is a rapidly subsiding plio pleistocene sedimentary basin located at the southern termination of the extensional back arc basin of the active central volcanic region tvz the wb is asymmetric with a steep thrust faulted outer arc ward margin and a gentle inner craton ward margin it contains a 4 km thick succession of plio pleistocene sediments mostly lying offshore composed of shelf platform sediments it lacks the late molasse like deposits derived from erosion of a subaerial volcanic arc and basement observed in classical back arc basins detailed seismic stratigraphic interpretations from an extensive offshore seismic reflection data grid show that the sediment fill comprises two basin scale mega sequences 1 a pliocene 3 8 to 1 35 ma sub parallel regressive bpre growthq sequence that overtops the uplifted cratonward margin above the reverse taranaki fault and 2 a pleistocene 1 35 ma to present divergent transgressive bsyn growthq sequence that onlaps i the craton ward high to the west and ii uplifted basement blocks associated with the high angle reverse faults of the arc ward margin to the east along strike the sediments offlap first progressively southward mega sequence 1 and then southeastward mega sequence 2 with sediment transport funnelled between the craton and arc ward highs towards the hikurangi trough through the cook strait the change in offlap direction corresponds to the onset of arc ward thrust faulting and the rise of the axial ranges at ca 1 75 ma resulting in 5100 5700 m of differential subsidence across the fault system sedimentation has propagated south to southeast ward over the last 4 myrs at the tip of successive back arc graben volcanic arcs and the associated thermally uplifted parts of the north island following the southward migration of the hikurangi subduction margin subsidence occurred by mantle flow driven flexure the result of active down drag of the lithosphere by locking of the hikurangi subduction interface and sediment loading the wb is considered to be a proto back arc basin that represents the intermediate stage of evolution of an epicratonic shelf platform impacted by active margin processes

seismic expressions of a miocene prograding carbonate margin mut basin turkey the detailed stratigraphic architecture of a miocene intraplatform prograding carbonate margin outcropping in mut basin turkey was used to build two dimensional synthetic seismograms by normal ray tracing and finite difference exploding reflector methods the synthetic seismic models at various frequencies display the overall prograding architecture of the 1 5 km 0 95 mi 250 m 820 ft prograding margin even at 40 hz frequency however most of the complex stratigraphic architecture is below standard seismic resolution sigmoidal outcrop geometries create complex shingled seismic reflections that contain pseudodownlap pseudotoplap and various truncations at 40 hz frequency in addition reflections that do not correspond to any impedance contrast are generated by a tuning effect within the clinoform interference patterns created by the complex impedance distribution create reflections that change phase laterally and cross stratigraphic timelines several high impedance coral buildups at the slope break and on the platform top cause reflections to change phase and amplitude along the timeline in addition lateral impedance variations generate mounded reflections that in places cross timelines and resemble seismic images for buildups or anticlinal structures several of these resolution problems and interference patterns are reduced by an increase of the frequency for example at 80 hz peak frequency the sigmoidal geometries are better imaged improved resolution was also achieved on the synthetic seismic data simply by dividing trace spacing from 25 to 12 5 m 82 to 39 ft but keeping the modeling frequency constant indicating that for stratigraphic interpretation horizontal seismic resolution is at least as important as vertical seismic resolution this implies that adjusting the acquisition geometries would improve the seismic resolution without compromising depth by increased frequency for seismic interpretation this study shows how little of a complex stratigraphic architecture is truly imaged on seismic data and how complex impedance distribution can create false seismic reflection geometries that impede a correct seismic facies and sequence stratigraphic interpretation

use of sedimentological information for geometric simulation of natural porous media structure a geometric simulation method was used to develop a three dimensional highly detailed synthetic representation of point bar sediments in the wabash river system geometric simulation methods in comparison to well known second order stochastic methods offer the advantage of being more closely related to depositional processes which are often similarly conceptualized i e described in terms of shapes of discrete bed forms trends in grain size and spatial relationships of defined geologic facies multiple scales of geometric variation were defined within a sedimentologically prescribed framework and shapes of discrete geometric elements were established at each scale the selected shapes were based on published field studies including sedimentological bed form studies and trench studies in active point bar sediments the parameterization of the shapes allowed for random variability of the shape descriptors discrete shapes were then generated and assimilated by computer hydraulic conductivity values were assigned to the discrete elements based on reports of observed variations in grain size and field measurements of hydraulic conductivity the synthetic model referred to as a numerical aquifer is being used as the basis for extensive numerical experimentation to study the relationship between natural spatial structure and subsurface flow and transport

depositional mechanisms as revealed from grain size measures of the palaeoproterozoic kolhan siliciclastics keonjhar district orissa india abstract the depositional mechanisms of the palaeoproterozoic kolhan siliciclastics have been examined using grain size analytical approaches grain size distributions of different lithofacies reveal a somewhat diversified paralic depositional environment including nearshore beach macrotidal estuary and inland dunes these inferences arise from different ways of analysing grain size data such as cumulative curves bivariant plots linear and multigroup discriminant functions and sediment trend matrix analyses sediment trend matrix analysis of various lithofacies taken together appears to be most effective for synthesizing the relationship between facies and sub environments taking into account the regional and local variations of lithofacies distribution and primary sedimentary structures grain size distributions and sediment transport paths as revealed from the above statistical parameters suggest overall hydraulic control of the grain size distribution of the kolhan siliciclastics in a fluvio beach shallow marine depositional setup

differential bed sedimentology and the maintenance of riffle pool sequences abstract riffle pools are the characteristic reach scale bedforms of gravel and mixed bedded channels of low to moderate slope the purpose of this paper is to clarify several aspects of the sedimentology of riffle pool sequences and to suggest how these both reflect and control riffle pool stability in the presence of spatial differences in the turbulent near bed flow field high frequency velocity measurements obtained using an array of electromagnetic current meters are combined with results from grid and transect sampling of bed sediments from 9 sites on 3 small upland rivers in n exmoor u k riffle sediments are shown to possess greater development of microtopographic structuring than adjacent pools which enhances riffle stability and which may reflect greater turbulent kinetic energy there over most of the flow range data on surface sediment arrangement and turbulent flow characteristics can be combined with different transport threshold equations to explain the maintenance of riffle pool sequences with respect to a particle queuing kinematic wave model without recourse to a competence reversal as conventionally assumed

suspended sediment distribution and residual transport in the coastal ocean off the ganges brahmaputra river mouth abstract an examination of suspended sediment dispersal in the coastal ocean off the ganges brahmaputra river system using shipboard and satellite data reveal distinct patterns of sediment distribution and sediment motion which help explain the dynamics of the world s largest sediment dispersal system during the low discharge period suspended sediment concentration distribution is mainly a function of tidal energy and the sediments are primarily transported as a suspension population satellite and sea bed data indicate that transport of river sediments is largely restricted to an area landward of about the 20 isobath during the low discharge period a simplistic estimate of residual transport and its separation into tidal and non tidal components indicate that non tidal transport contributes about 47 of the total transport and has transport directions mostly southwestward during the northeast monsoon the tidal residual transport which contributes about 53 of the total transport however does not show any particular direction for most of the stations although some landward stations show shoreward transport although few water column measurements were made during the high discharge period data from this and previous studies suggest that the dominant transport direction is to the southwest for most of the year

a currents of removal approach for interpreting carbonate sedimentary processes abstract this study develops a currents of removal methodology to examine and quantify the nature of physical transport processes affecting the formation of bioclastic deposits in the cocos keeling islands an indian ocean atoll this approach is based on the hydraulic settling and threshold of entrainment characteristics of bioclastic deposits and measured current energy potential mobility pm analysis of 157 samples quantifies the proportion of deposits able to be transported in each geomorphic zone results show that under mean energy conditions wave induced currents entrain and transport sediment within the atoll and show that transport of sediment is not solely reliant on storm energy conditions gradients of pm values are used to reconstruct sediment transport pathways from the reef flat 85 100 pm through shallow passages 75 100 pm to sand aprons 50 75 pm and shallow 0 20 pm and deep lagoon 0 pm comparison of settling velocity distributions of bed material and sediment retained in sediment trap experiments show that actual mobility levels correspond with pm estimates potential mobility analysis also identifies the immobile portions of deposits which increase lagoonward constituent analysis of immobile fractions and analysis of settling frequency distributions are used to differentiate the importance of physical and biological processes in the formation of deposits throughout cocos and identify the role of each geomorphic zone in the transport system the pattern of sediment is controlled by physical processes between the reef flat the primary sediment production zone with sediment transported through the shallow passage conduits to the sand aprons the formation of shallow and deep lagoon deposits is controlled by autochthonous sediment production and storm deposition potential mobility analysis is a powerful tool enabling physical transport processes within bioclastic sedimentary environments to be quantified the ability to examine an individual deposit s hydraulic behaviour and development also enables depositional processes to be examined at a much finer resolution than previously attempted using conventional textural approaches

discussion bulk sampling of coarse clastic sediments for particle size analysis gale and hoare 1992 have provided a figure relating maximum particle diameter in a coarse clastic sediment to the size of sample required to generate a reproducible measure of the particle size distribution of that sediment however they fail adequately to justify the basis for the criterion of sample adequacy adopted in their proposal additionally there is a range of separate issues neglected by gale and hoare 1992 which makes their conclusion inapplicable in many situations a fundamental issue that they neglect is the purpose lying behind a grain size analysis this issue determines what grain size measures are appropriate rather than the empirical seemingly purpose independent approach of gale and hoare 1992

acoustic properties of carbonates effects of rock texture and implications for fluid substitution more than 250 plugs from outcrops and three nearby boreholes in an undisturbed reef of miocene tortonian age were quantitatively analyzed for texture mineralogy and acoustic properties we measured the p and s waves of carbonate rocks under dry humidified and brine saturated conditions at 10 mpa effective pressure with an ultrasonic pulse transmission technique 1 mhz the data set was compared with an extensive database of petrophysical measurements of a variety of rock types encountered in carbonate sedimentary sequences two major textural groups were distinguished on the basis of trends in plots of compressional wave velocity versus poisson s ratio a specific ratio of p wave over s wave velocity in granular rocks the framework of depositional grains is the main medium for acoustic wave propagation in crystalline rocks this medium is provided by a framework of interlocking crystals formed during diagenesis rock textures are connected to primary depositionalparameters and a diagenetic ov

geochemical variability induced by entrainment sorting quantified signals for provenance analysis abstract a substantial part of the mineralogical and chemical heterogeneity of clay poor sands at the bed scale results from tractive redistribution of the grains in the bedload since coarse and light particles are more easily entrained than denser and smaller heavy minerals the latter commonly concentrate in grain scale lags that alternate with sand depleted in these minerals when sampled and analysed at the centimetre scale this primary heterogeneity translates into nice geochemical covariations between the minor trace elements that concentrate in some heavy minerals zr ti p cr v y ce among others regressing these variations allow us to derive two chemical fingerprints of the sand one records the proportions between some heavy minerals the other the average composition of framework grains i e a sand composition corrected from the contribution of heavy minerals both relate to provenance and allow different sand beds to be compared in appropriate spider diagrams the construction of these diagrams their potential use and their limitations are discussed using two contrasting examples continental channel sandstones from the ravenscar group middle jurassic yorkshire uk illustrate how these geochemical fingerprints vary from the local to the group scales and how robust they are against diagenetic overprint at ravenscar the zr vs ti v cr covariations record the presence of ti oxides and magnetite together with zircon and of volcanic lithoclasts at the deposition stage despite a high degree of diagenetic alteration overbank sandstones lateral to channels from the deep marine gres d annot formation eocene oligocene alps illustrates how apatite and monazite delivered by a dominantly granitic source generate covariations between zr and p y ce and th contents with characteristically steep slopes

occurrence and formation of water laid placers a placer is a deposit of residual or detrital mineral grains in which a valuable mineral has been concentrated by a mechanical agent the agent is usually running water and the valuable mineral is usually denser than quartz for example gold diamond cassiterite ilmenite or chromite it is difficult to overemphasize the importance of placers as sources of mineral wealth the witwatersrand south africa paleoplacers discovered in 1886 alone have provided over half of all the gold ever mined in the world pretorius 1976 and alluvial placer deposits of cassiterite in southeast asia presently are the world s major source of tin toh 1978 the earliest evidence of placer mining comes from stone carvings in egypt dated at 2500 b c macdonald 1983 and as early as 7 b c strabo pointed out that in turdetania spain more gold was procured from washing sand in the rivers than by digging in the mines given the economic importance of water laid placers and the long history oftheir exploitation it is remarkable that they have received so little attention in sedimentary research mining and exploration practices for the most part have been guided by empirical rules of thumb that are locally useful but lack any firm physical basis of understanding see bateman

rate limited mass transfer or macrodispersion which dominates plume evolution at the macrodispersion experiment made site we present a model of solute transport that explains the large scale behavior of the solute tracer test plumes at the macrodispersion experiment made site as the result of advection and rate limited mass transfer between mobile and small scale immobile domains this model does not consider the process of dispersion and yet provides an alternative explanation of the evolution of the observed concentration profiles compared to the macrodispersion model the mass transfer model better represents the change in mobile dissolved mass with time the peak of the concentration profile and the profile asymmetry specifically unlike the macrodispersion model the mass transfer model explains the facts that the observed mass of the plume was greater than the injected mass in early snap shots of the plume and less than the injected mass at late times we suggest that the injected mass advects through the mobile domain and diffuses into and out of the immobile domain the immobile domain consists of a combination of low permeability zones on the scale of centimeters to decimeters the darcy scale immobile domain and intragranular porosity dead end pores and surface sorption the pore scale immobile domain we suggest that the mobile domain was sampled preferentially when water was extracted therefore at early times relatively clean water in the immobile domain was not sampled and incorrectly assumed to contain high solute concentrations similarly the mass at late times was underestimated because solute trapped in the pore scale immobile domain was not extracted during sampling and therefore ignored the combination of advection and slow mass transfer is consistent with the fact that the peak of the plume migrated only 5 m by the termination of the experiment as well as the different behavior of bromide and tritium tracers

the chronostratigraphic significance of seismic reflections along the bahamas transect abstract continuous cores drilled during the bahamas drilling project bdp and the ocean drilling program odp leg 166 along a transect from the top of great bahama bank to the basin in the straits of florida provide a unique data set to test the assumption in seismic stratigraphy that seismic reflections are time lines and thus have a chronostratigraphic significance seismic reflections that are identified as seismic sequence boundaries ssbs were dated by means of biostratigraphy in the five odp sites and by a combination of biostratigraphy magnetostratigraphy and sr isotope stratigraphy in the two bdp sites the seismic reflection horizons are carried across a variety of facies belts from shallow water carbonates over slope carbonates to drift deposits in the straits of florida within this system 17 ssbs were identified and dated despite the fact that the seismic reflections cross several facies belts their ages remain remarkably constant the average offset in all sites is 0 38 myr in no cases do the seismic reflections cut across time lines the age differences are the combined result of the biostratigraphic sampling frequency the spacing of marker species that required extrapolation of ages and the resolution of the seismic data in addition uncertainties of age determination in the proximal sites where age diagnostic fauna are rare add to the age differences between sites therefore it can be concluded that the seismic reflections which mark the ssbs along the bahamas transect are time lines and can be used as stratigraphic markers this finding implies that depositional surfaces are preferentially imaged by reflected seismic waves and that an impedance contrast exists across these surfaces facies successions across the sequence boundaries indicate that the sequence boundaries coincide with the change of deposition from times of high to low sea level in the carbonate setting of great bahama bank sea level changes produce changes in sediment composition sedimentation rate and diagenesis from the platform top to the basin the combination of these factors generates differences in sonic velocity and thus in impedance that cause the seismic reflection the impedance contrasts decrease from the proximal to the distal sites which is reflected in the seismic data by a decrease of the seismic amplitude in the basinal area

the hydraulics of conduit flow in carbonate aquifers abstract analysis of the morphology of dissolution bedform assemblages and hydraulically transported sediments found within conduits in carbonate aquifers in north america and the british isles has allowed the hydraulic conditions under which conduit flows occur to be established mean values of flow velocity boundary shear stress conduit reynolds number conduit froude number boundary friction factor boundary roughness and flow power have been calculated the values obtained are in agreement with other evidence in the conduits and are comparable to those obtained from other similar hydraulic systems

seismic model of a progradational carbonate platform picco di vallandro the dolomites northern italy we present a seismic model of a well exposed carbonate platform to basin transition from the triassic succession at picco di vallandro the dolomites northern italy the core of this model is a detailed lithologic cross section that represents 1500 m of stratigraphic section over a distance of 3 5 km the cross section is based on detailed measured sections photo interpretation and careful projection of surface mapping into the plane of section samples from the measured sections provide control on velocity and density distribution and these data were used to generate an impedance model from the cross section the impedance model was convolved with a zero phase 25 hz peak frequency wavelet to produce the seismic model the results illustrate how the platform to basin transition might be imaged by conventional seismic reflection data how the reflections relate to the distribution of lithology and through going geological surfaces and how constructive and destructive interference and seismic resolution combine to produce individual reflections the picco di vallandro section displays several scales of progradation and retrogradation ranging from stacked buildups that are several hundreds of meters thick to the interfingering of individual beds the seismic model displays this complexity with varying degrees of success the individual buildups are well imaged but as expected smaller features are not as clearly reproduced by the model intermediate scale sections that represent platform retrogradation are shown as surfaces of onlap in the seismic model but field work shows that the platform was still shedding sediment while retreating and that no simple onlap surface exists in the outcropping section a similar lack of stratigraphic resolution is shown by the model at the downlapping clinoforms which do not correspond to iscrete stratal surfaces but to the toe of slope position where major bedding units thin below seismic resolution smaller features such as the position of downslope terminations of carbonate breccia wedges are not resolved by the model toe of slope packages of limestone turbidites are represented only by reflection segments of higher amplitude the decreasing seismic resolution is caused by limited seismic bandwidths and by the effects of constructive and destructive interference interference has the largest effects at the complexly interfingering buildup to basin transition or at the most detailed part of our model an understanding of these types of seismic limitations and their controls is important as we try to extract more and more stratigraphic information from seismic data p

seismic models of a carbonate foreslope to basin transition picco di vallandro dolomite alps northern italy detailed geologic cross sections augmented by laboratory calibration of lithology and acoustic impedance have been used to produce synthetic reflection seismic sections of a carbonate foreslope to basin transition two areas from the picco di vallandro region of the dolomite alps were modeled a progradational section and a retrogradational backstepping section the resulting models show how these complex areas of strata interfingering might be displayed on conventional reflection seismic lines in the area of progradation rapid stratigraphic thinning below seismic detectibility coupled with abrupt impedance changes produces a reflection discontinuity between steeply dipping reflections of the foreslope and gently dipping parallel reflections of the basin section this apparent downlap surface marks the toe of slope for successive clinoforms but does not correspond to a discrete stratigraphic surface in the backstepping example similar stratigraphic thinning and impedance changes create an apparent onlap surface wavelet interference causes complications in both examples these models indicate how stratigraphic complexity can be simplified by the seismic reflection process and suggest that caution should be exercised when using seismic data to construct general models in areas of complex depositional geometries and rapidly changing facies

traditional and multivariate techniques in the interpretation of floodplain sediment grain size variations the vertical and horizontal variation of sedimentary fades is the raw data for the interpretation of flood plain history from which palaeohydrological inferences are frequently drawn mixed and fine floodplain sediments present problems of interpretation because of a large grain size range and frequent polymodality caused by the mixing of process associated grain size components this paper discusses the use of traditional grain size statistics and the use of the mode and multivariate statistics the mode although much neglected is indicative of up profile grain size changes and has practical advantages over the mean for mixed and fine floodplain sediments constrained cluster analysis and principal components analysis are used directly on coulter counter results these techniques can rapidly divide a floodplain profile into grain size units and indicate the principal vectors of grain size variation which will be related to the changing processes of deposition principal components analysis reveals the importance of the medium to fine silt category in accounting for grain size variations suggesting that a critical factor in determining the type of alluvial unit deposited is the degree to which it has received fine suspended material grain size data from the lower severn are used to construct a cm diagram which is compared with a texture triangle from both the cm and multivariate analysis a generalized backswamp profile is constructed which shows the existence of a coarser top unit caused by the addition of a fine to medium sand component to the underlying sediment during the late holocene

sediment characteristics and transportation dynamics of the ganga river abstract understanding of river systems that have experienced various forcing mechanisms such as climate tectonics sea level fluctuations and their linkages is a major concern for fluvial scientists the 2525 km long ganga river derives its fluvial flux from northern part of the indian subcontinent and drops in the ganga brahmaputra delta and the bengal fan regions this paper presents a study of the ganga river sediments for their textural properties grainsize characteristics and transportation dynamics a suite of recently deposited sediments 189 bedload samples and 27 suspended load samples of the river and its tributaries was collected from 63 locations dry and wet sieve methods of grainsize analysis were performed and folk and ward s parameters were calculated transportation dynamics of the sediment load was assessed by means of channel hydrology flow sediment rating curves bedform mechanics grainsize images and cumulative curves textural properties of the bedload sediments of the ganga river tributaries originating from the himalaya orogenic belt the northern indian craton and the ganga alluvial plain regions are characterised by the predominance of fine to very fine sand medium to fine sand and very fine sand to clay respectively downstream textural variations in the bedload and suspended load sediments of the ganga river are therefore complex and are strongly influenced by lateral sediment inputs by the tributaries and channel slope at the base of the himalaya a very sharp gravel sand transition is present in which median grainsize of bedload sediments decreases from over 0 16 to 2 46 within a distance of 35 km downstream decline in mean grainsize of bedload sediments in the upper ganga river within the alluvial plain can be expressed by an exponential formula as mean grainsize in 0 0024 distance in kilometres from the himalayan front 1 29 it is a result of selective transport phenomena rather than of abrasion the condition attributed to channel competence with low gradient of 70 to 25 cm km coarsening of the bedload sediments in the middle segment of the river develops due to the coarse bedload sediments mean grainsize 1 8 2 2 supply from the tributaries of the northern indian craton region and increased unit stream power the ganga river sediments show a wide range of grainsize parameters bedload sediments mean grainsize 1 5 3 8 standard deviation 0 3 0 7 skewness 0 2 0 5 and kurtosis 0 8 2 4 suspended load sediments mean grainsize 2 9 5 0 standard deviation 0 6 1 6 skewness 0 6 0 4 and kurtosis 0 7 4 6 mean grainsize of the bedload sediments is 2 6 which is 1 3 units coarser than the suspended load sediments the cumulative grainsize curves of the bedload sediments show a distinct break in slope representing nearly 80 of the bedload sediment transported as graded suspension during the peak monsoon discharge most of the bedload sediments are transported as the discontinuous fine sandy graded suspension by the unidirectional unsteady shooting flow at a high transport rate about 8 106 tons d a model of sedimentation transportation at river channel is proposed the ganga river mobilises a total of 729 106 tons of sediments annually through a narrow zone within its river valley under the present hydrogeological conditions the river sedimentation is climatically controlled and is predicted to produce a 2000 km long 2 to 40 km wide and 25 to 50 m thick ribbon shaped well sorted symmetrically skewed fine sand body

seismic models of a mixed carbonate siliciclastic shelf margin permian upper san andres formation last chance canyon new mexico seismic models based on randomly distributed samples from the permian upper san andres formation last chance canyon new mexico verify that the most prominent seismic reflections are related to stratal geometry however at least some reflections arise from lateral facies transitions that are commonly associated with highly prograding mixed carbonate siliciclastic sediments understanding seismic reflections and reflection terminations in sedimentary rocks requires simulation of at least 2 d cross sections of the impedance distribution to investigate the cause of reflections in strongly prograding mixed carbonate siliciclastic shelf margins acoustic velocity and bulk density were measured on more than 60 plugs that spatially cover one higher order genetic sequence the resulting impedance values were gridded and contoured and the genetic sequence was repeated to create a cross section averaging impedance values for each lithofacies zone generated a second impedance cross section seismic models of both impedance cross sections revealed the following observations 1 reflections associated with the sequence boundaries are subject to amplitude and polarity phase changes and 2 at least one reflection within the high order sequences is related to subhorizontal facies changes and is associated with two pseudounconformities the contoured impedance model is suggested to closer resemble the true impedance function in outcrop and shows subtle vertical shifts and significantly higher impedance contrasts

muddled or mixed inferring palaeoclimate from size distributions of deep sea clastics abstract one of the outstanding problems of palaeoclimate reconstruction from physico chemical properties of terrigenous deep sea sediments stems from the fact that most basin fills are mixtures of sediment populations derived from different sources and transported to the site of deposition by different mechanisms conventional approaches to palaeoclimate reconstruction from deep sea sediments do not distinguish between provenance and dispersal related variations and therefore often fail to recognise the true significance of variations in sediment properties we formulate a set of requirements that each proposed palaeo environmental indicator should fulfil and focus on the intrinsic coupling between grain size and chemical composition a critical review of past achievements in grain size analysis serves to introduce a conceptual model of spatio temporal grain size variation based on dynamic populations each dynamic population results from a characteristic combination of production and transport mechanisms that corresponds to a distinct subpopulation in the data analysed the mathematical statistical representation of the conceptual model can be obtained by means of the end member modelling algorithm emma applications of the model to several ocean basins are discussed as well as methods to examine the validity of grain size based palaeoclimate reconstructions palaeoclimate reconstructions of a high and low latitude basin illustrate the common degree of complexity of deep sea grain size records

significance of porosity for stratigraphy and textural composition in subsurface coarse fluvial deposits boise hydrogeophysical research site recognition and quantitative characterization of subsurface stratigraphic units in coarse unconsolidated fluvial deposits are difficult because large grain size and the large scale of sedimentary structures make direct interpretation from core difficult or impossible in this paper we use porosity data from well logs and grain size distribution gsd data from core to investigate four pebble and cobble dominated units that have been identified in porosity logs in deposits at a research well field boise hydrogeophysical research site lacking direct observation at an appropriate scale questions about distribution of parameters and textural composition in these units are analyzed with statistical tests the four pebble and cobble dominated porosity stratigraphic units may be grouped into two types 1 units 1 and 3 have low porosity mean 0 17 0 18 and low porosity variance and 2 units 2 and 4 have higher porosity mean 0 23 0 24 and higher porosity variance based on gsd data core samples are subdivided into five lithotypes the five lithotypes occur in different proportions and have different vertical transition probability characteristics in the two types of units 1 units 1 and 3 have only framework gravel dominated lithotypes and have random vertical transition probability between these two lithotypes and 2 units 2 and 4 consist of both framework gravel dominated and sand or matrix dominated lithotypes and have structured vertical transition probability the two framework gravel dominated lithotypes occur in all four stratigraphic units but have distinctly lower porosity in units 1 and 3 i e tighter packing than in units 2 and 4 looser packing considering the repeated stratigraphic occurrence of and the statistical significance of differences between the two types of units both the individual unit distinctions and the two unit groupings appear to be valid it is reasonable to interpret that the observed packing differences associated with units 1 and 3 compared with units 2 and 4 are related to different sedimentary processes that produce different bedforms or grain fabrics perhaps under different bedload transport rates

grain size analysis of the lower cambrian lower cretaceous clastic sequence of jordan sedimentological and paleo hydrodynamical implications abstract 356 sand sized samples ranging in age from early cambrian epoch to late cretaceous epoch of known depositional environments were analyzed mechanically to obtain the grain size parameters of mode median mean and sorting the constructed grain size distribution curves for the various paleozoic and mesozoic formations were interpreted in terms of the paleo hydrodynamic processes prevailing through transportation and deposition the study proved that the grain size parameters of sand sized sediments most sensitive to paleo depositional environmental recognition were the graphic mean grain size and the inclusive graphic sorting mean grain size values below 1 77 of a sand sized sample could indicate a fluvial channel paleo depositional environment whereas the mean grain size values above 2 8 are indicative to a shallow marine paleo depositional environment sorting values above 1 1 may indicate a fluvial channel paleo depositional environment a bivariate discriminatory plot of mean grain size versus sorting yields two fields that can successfully discriminate between the fluvial channel paleo depositional environment and the shallow marine paleo depositional environment a new grain size parameter termed the mean sorting index was introduced in this study and calculated by dividing the mean grain size value over the sorting value it is recorded here that the mean sorting index is more sensitive to paleo depositional environment recognition than the individual mean grain size and sorting parameters

hydraulic interpretation of grain size distributions using a physical model for bedload transport abstract grain size distributions of bedload sediment are modelled by considering the time variation of sediment transport in a unidirectional turbulent flow a frequency distribution of instantaneous bed shear stress is divided into a large number of shear stress ranges with mean values of oi each oi is capable of transporting a range of sediment sizes as bedload the maximum size is just at its entrainment threshold the minimum size is just too large to be suspended rates of bedload transport are calculated for each size range using a theoretical bedload function and the proportion of time that these rates occur is obtained by integrating the frequency distribut on between the boundaries of each shear stress range the relative weights of sediment transported in particular size ranges determined for all oi values constitutes the grain size distribution obtained from bedload sampling at a fixed point on the bed cumulative grain size curves have been simulated using the model in order to demonstrate the effects of the various model parameters e g mean bed shear stress viscosity all curves show a distinct break in slope which represents the maximum grain size transportable in suspension when the shear stress attains its maximum fluctuating value examination of many natural bedload distributions shows close qualitative and quantitative agreement with the model and reasons for discrepancies can be fully explained typical grain size distributions of deposited sediments can be obtained by mixing of different flow related bedload distributions in various proportions thus although deposited sediments may primarily result from bedload transport during a steady flood flow flow unsteadiness also plays a part the effects of some deposition directly from suspended load can also be surmised

12 000 year long record of 10 to 13 paleoearthquakes on the yammouneh fault levant fault system lebanon we present results of the first paleoseismic study of the yammouneh fault the main on land segment of the levant fault system within the lebanese restraining bend a trench was excavated in the yammouneh paleolake where the fault cuts through finely laminated sequences of marls and clays first order variations throughout this outstanding stratigraphic record appear to reflect climate change at centennial and millennial scales the lake beds are offset and deformed in a 2 m wide zone coinciding with the mapped fault trace ten to thirteen events are identified extending back more than 12 kyr reliable age bounds on seven of these events constrain the mean seismic return time to 1127 135 yr between 12 ka and 6 4 ka implying that this fault slips in infrequent but large m 7 5 earthquakes our results also provide conclusive evidence that the latest event at this site was the great a d 1202 historical earthquake and suggest that the yammouneh fault might have been the source of a less well known event circa a d 350 these findings combined with previous paleoseismic data from the zebadani valley imply that the parallel faults bounding the beqaa release strain in events with comparable recurrence intervals but significantly different magnitudes our results contribute to document the clustering of large events on the levant fault into centennial episodes such as that during the eleventh through twelfth centuries separated by millennial periods of quiescence and raise the possibility of a m 7 event occurring on the yammouneh fault in the coming century such a scenario should be taken into account in regional seismic hazard assessments and planned for accordingly

controls on acoustic properties of upper jurassic siliciclastic rocks boulonnais northern france more than 200 plugs from outcrop and a nearby borehole in a carbonaceous siliciclastic interval of late jurassic kimmeridgian to tithonian age were quantitatively analyzed for texture mineralogy and acoustic properties our primary goal was to study the effect of clay fraction smaller than 8 m silt sand fraction larger than 8 m and carbonate on the acoustic properties the quantitative nature and volume of the data made it possible to observe four dimensional relationships in contoured ternary diagrams primary control on the acoustic properties is exerted by porosity but the trend of this relationship significantly deviates from popular velocity transforms the contribution of clay silt sand and carbonate particular material and cement explains the remaining variation in acoustic properties although no clear linear thresholds are defined a general trend is that clay and carbonate content have opposite and overlapping effects on acoustic properties the influence of clay content progressively increases with decreasing carbonate content and visa versa with increasing carbonate content the variation of acoustic velocity at a given porosity value increases to nearly twice of that in the clay dominated sediment traditional classification boundaries are present but strongly overprinted by this interplay between clay and carbonate this study may have important implications for porosity and lithofacies prediction from wireline logs in similar mixtures of sediment in addition the quantitative character of the textural and mineralogical data may provide a direct link from acoustic properties to the primary depositional system and sequence stratigraphy

distribution of grain sizes across a transgressive shoreface abstract distributions of seventeen grain sizes in the sand fraction of the shoreface sediments south of long island new york can be categorized into middle and lower shoreface species very fine sand upper shoreface species fine sand shoreface depleted species medium sand and swash zone species coarse sand the long term fair weather wave base of the study area at a depth of about 16 m is determined on the basis of the characteristic response of grain sizes to the average shoreface hydrodynamics the crosshore deviations from the mean distribution for each grain size were examined by using empirical orthogonal function eof analysis these results show differential associations of grain size deviations with various dynamic zones on the shoreface the hydrodynamic processes of the modern shoreface are speculated to winnow out fine and very fine sands and transport them onshore leaving behind on the lower shoreface and inner continental shelf a lag deposit of transgressive sand sheet composed mainly of medium sand

environments of deposition paleocurrents and provenance of tertiary deposits along kachemak bay kenai peninsula alaska abstract grain size distributions and sedimentary structures of tertiary strata along kachemak bay indicate a variety of inchannel and overbank fluvial facies a markov analysis indicates cyclicity of these facies probably caused by repeated and regular channel migration during subsidence fossils indicate a cool temperate climate and probable freshwater deposition paleocurrent directions and sandstone composition suggest a metamorphic and sedimentary clastic provenance during homerian time with the addition of pyroclastics during clamgulchian time the homerian stage beds represent facies of braided meandering streams whereas the clamgulchian stage beds represent facies of meandering streams these streams flowed westward and northwestward from the chugach kenai mountains onto wooded low gradient alluvial plains

systems tracts and depositional sequences in a carbonate setting a study of continuous outcrops from platform to basin at the scale of seismic lines abstract the application of sequence stratigraphy concepts to carbonates is feasible and this paper presents an example of its successful application the continuity and lateral extent of outcropping sedimentary sections observed at the scale of a seismic line from platform to basin in the southern vercors plateau western alps france confirm the validity of sequence stratigraphy concepts when applied to carbonate depositional systems from the stratal patterns displayed in outcrop we determined that carbonate lowstand systems tracts are major components in the basin even in the deep pelagic realm transgressive and highstand systems tracts are very thin a response to starvation in contrast on the shelf transgressive and highstand systems tracts are the major components carbonate lowstand and shelf margin systems tracts pinch out in the vicinity of the offlap break it clearly appears that the stratal patterns and stratal termination patterns of carbonate systems tracts especially of carbonate lowstand systems tracts are basically similar to those of siliciclastic systems tracts this study documents that carbonate platforms may provide the basin with carbonate clastics during periods of relative sea level falls and lowstand this is one of the most controversial aspects of the application of sequence stratigraphy concepts in carbonate environments a second controversial aspect of the application of sequence stratigraphy concepts to carbonate rocks is the distinction between transgressive and regressive facies cycles and depositional sequences and systems tracts this paper demonstrates that the shoreline position for carbonates which indicates the facies transgression or regression is independent of prograding or retrograding shelf breaks the stratal pattern and facies within systems tracts vary depending on the overall evolution of the accommodation space i e on their position within the transgressive and regressive cycles

event based stratigraphic simulation of wave dominated shallow marine environments abstract the presence of event beds in the shallow marine stratigraphic record indicates formation is governed by high magnitude low frequency processes rather than low magnitude high frequency processes in this paper a 2 d multiple grain size process response model barsim is presented which uses event deposition to simulate stratigraphic response to changes in sea level and sediment supply barsim uses variable time steps to simulate individual storm event and fair weather periods deposition during storm conditions solely consists of reworked shoreface sediment while fair weather deposits consist of reworked shoreface sediment combined with sediment supplied by littoral drift and suspension simulations using variations in sea level sediment supply both amount and grain size and wave height regime result in distinct depositional patterns stratal geometry and bed characteristics model results indicate that both wave height regime and grain size of supplied sediment have considerable effect on shoreface morphology and stratal characteristics unraveling coastal evolution from the shallow marine stratigraphic record may therefore be more difficult than previously assumed as both variables are expected to vary over geologic time scales

visualization of the turbulent flow structure in a gravel bed river a novel technique for visualizing turbulent flow data from a gravel bed river is presented the time development of flow velocity and shear stress at three heights is displayed using a computer program this can be used to observe how the fluctuations of velocity and shear stress interact both spatially and temporally we highlight examples of flow events which are important for the understanding of flow dynamics the visualization suggests that the turbulent flow field is characterized not only by coherence over time at a point but also by spatial interdependence between points we suggest that this new visualization approach will assist further interpretation of statistical analyses of turbulent signals as well as focusing future measurement strategies by providing a clearer spatio temporal picture of the flow structure