model for the combined flow origin of hummocky cross stratification many shallow marine sandstones are characterized by gently dipping trough cross stratification there appear to be significant similarities between this low angle cross stratification and typical hummocky cross stratification and it is proposed that much of what is currently recognized as hummocky cross stratification is related to low relief megaripple bed forms that are equilibrium structures for fine grained sand under combined wave surge and unidirectional currents the geometry that obtains when these bed forms migrate is low angle trough cross stratification high rates of sedimentation from incipient suspension and limited lateral migration of individual bed forms promote preservation of convex upward depositional surfaces hummocks between troughs highly unsteady flow leads to abundant internal scour and drape surfaces neither random scour and drape nor oscillatory currents seem sufficient to generate hummocky bed forms

deposits from wave influenced turbidity currents pennsylvanian minturn formation colorado u s a turbidity currents generated nearshore have been suggested to be the source of some sandy marine event beds but in most cases the evidence is circumstantial such flows must commonly travel through a field of oscillatory flow caused by wind generated waves little is known however about the interactions between waves and turbidity currents we explore these interactions through detailed process oriented sedimentological analysis of sandstone event beds from the pennsylvanian minturn formation in north central colorado u s a the minturn formation exhibits a complex stratigraphic architecture of fan delta deposits that developed in association with high topographic relief in a tectonically active setting an 20 35 m thick unconformity bounded unit of prodelta deposits consists of dark green shale and turbidite like sandstone beds with tool marks produced by abundant plant debris some of the sandstone event beds most abundant at distal localities contain reverse to normal grading and sequences of sedimentary structures that indicate deposition from waxing to waning flows in contrast proximal deposits in some cases less than a kilometer away contain abundant beds with evidence for deposition by wave dominated combined flows including large scale hummocky cross stratification we interpret the majority of these event beds as a record of deposition from hyperpycnal flows i e turbidity currents generated directly from highly concentrated river plumes which accelerated and decelerated in response to a rising and falling flood discharge additional support for this interpretation includes the following 1 a variety of sole marks including flute and gutter casts as well as tool marks made by relatively large up to tens of centimeters across woody debris i e groove prod and chevron marks 2 consistent unimodal orientation of sole marks and abundant ripple cross stratification which indicate strong downslope directed flow 3 a well documented sedimentological framework for the formation of fan delta deposits adjacent to nearby highlands and 4 plant fossils typical of middle to high elevation habitats that are abundant in the turbidite beds but absent in underlying and overlying shoreline and marginal marine deposits which have a separate floral assemblage differences in grain sizes vertical stratification sequences and bed thicknesses between outcrops are interpreted to result from the spatial distribution of wave effects the time history of hyperpycnal flows and the interaction of these processes the latter varied both spatially and temporally and produced a wide range of bed types which are incorporated into a new conceptual model for storm influenced hyperpycnal flows

from transtension to transpression along the northern caribbean plate boundary off cuba implications for the recent motion of the caribbean plate abstract marine geophysical surveys using seabeam single channel seismic reflection gravimetric and magnetic measurements have been conducted along a segment of the northern caribbean transcurrent plate boundary seacarib ii cruise the data allow a better definition of the geometry and the tectonic regime of this major strike slip area they support the following results 1 1 along the southern cuban margin the oriente fault displays a discontinuous trace mainly composed of dextral offset en echelon segments some pull apart basins are located between fault segments cabo cruz basin chivirico and baitiquiri basins in the windward passage area the plate boundary enters into the tortue channel and is not connected with the subduction front off northern hispaniola 2 2 the eastern part of the oriente deep and the santiago promontory are characterised by active compressional tectonics they form the santiago deformed belt described here for the first time this deformed belt can be divided longitudinally into three main segments each one characterised by a particular tectonic style its development is related to a transpressional mechanism along the left lateral oriente strike slip fault our observations suggest that a tectonic and kinematic reorganisation occurred recently in this area probably in the late pliocene which may be compared with the recent geological events recorded on land in the northern caribbean domain the precise knowledge of both geometry and structures along the oriente strike slip fault south of cuba provides new constraints for the recent kinematic evolution along the northern caribbean transcurrent plate boundary it leads us to infer the existence of a convergence component associated with the slip component along the oriente transform fault

landward fining from multiple sources in a sand sheet deposited by the 1929 grand banks tsunami newfoundland abstract a sandy deposit from the 1929 grand banks tsunami in newfoundland contains sediment from two distinct sources one from an inferred gravel shoreline close to the deposit and one from a sandy dune some 200 m seaward of the deposit the deposit ranges from 0 to 15 cm thick and is composed of a bimodal mix of fine and coarse sand we took approximately 100 core samples of this deposit in an attempt to characterize lateral grain size trends within the sand although the coarse fraction does fine with distance inland the fine fraction does not change size over the study area and the aggregate grain size changes in no systematic way we interpret this deposit to represent the mixture of material picked up at the bar with material picked up at the gravel shoreline the bar material does not fine in part because it is already fairly well sorted but also because it is far from its source the shoreline material on the other hand is poorly sorted so that the tsunami took only those grains it was capable of moving and deposited them near their source we estimated the size of the tsunami by determining the flow depth flow velocity combinations required to advect sand from the bar to the back of the deposit and by estimating the shear velocity required for motion of the largest grain we found during our survey this modeling indicates an average flow depth of about 2 5 2 8 m over the area at a flow velocity of 1 9 2 2 m s this estimate compares well with eyewitness accounts of a maximum flow depth of 7 m at the shoreline if our estimate represents an average over the whole study area

whole flow field dynamics and velocity pulsing within natural sediment laden underflows sediment laden density underflows are important agents of erosion and deposition and are especially significant in the management of human made reservoirs pollutant dispersal and sediment deposition in the world9s oceans quantification of continuous sediment laden underflows in lillooet lake british columbia shows that the underflows descend along a distinct plunge line but although the input from the source is constant adopt a distinct pulsing in their velocity structure such velocity pulsing will produce temporally and spatially varying bed shear stresses sediment erosion and or deposition and fluid mixing and represents a central property of underflows that must be incorporated into models of density current behavior

attenuation of coda waves in new england abstract seismic wave attenuation was measured in new england from the time domain decay of coda wave amplitudes on narrow bandpass filtered seismograms of local earthquakes the frequency band of interest was 0 75 to 10 hz q c f was found to increase with frequency across this band but there was also a difference between the q c f values for short t t 100 sec lapse times of coda wave propagation for the short lapse time data corresponding to wave paths primarily in the upper crust q c f increases from 400 at 3 hz to 1300 at 10 hz for the large lapse time data corresponding to wave paths in the lower crust and upper mantle q c f was found to vary from 660 at 1 hz to 1500 at 10 hz if we interpret this dataset in terms of a model incorporating both scattering and anelastic attenuation we find that the mean free path of seismic waves over this frequency band is 80 km for short lapse times and 400 km for long lapse times if we assume that scattering is entirely responsible for the observed attenuation we find that the minimum mean free path for short lapse times is about 75 km over all frequencies whereas for long lapse times the minimum mean free path decreases from 400 km at 0 75 hz to 90 km at 10 hz

three dimensional modeling of negatively buoyant flow in diverging channels diverging channels also known as diffusers represent common natural and industrial outlets to lakes reservoirs and rivers if the outflow in a diffuser has a larger density than the ambient water the inflow may plunge and form a density underflow in this paper a three dimensional numerical study was conducted to gain insight into the mechanism of negatively buoyant flows in diffusers with a sloping bottom of particular interest is the formation of separated flows such as wall jet and free jet flows various cases of plunging and the associated density current in a diffuser with different divergence angles and inflow densimetric froude numbers are considered the model successfully simulates the formation of attached flow wall jets and free jets in a negatively buoyant environment the onset evolution and stabilization of a stall and the subsequent development of a wall jet in a negatively buoyant flow are investigated in detail computed results also show favorable agreement with some published experimental data on density current generated by the plunging of cold water in ambient warm water in a diverging channel

genesis and depositional model of subaqueous sediment gravity flow deposits in a lacustrine rift basin as exemplified by the eocene shahejie formation in the jiyang depression eastern china abstract subaqueous sediment gravity flow ssgf deposits have recently been recognized as a major reservoir plays in lacustrine basins research on their genesis and depositional models are crucial to understand gravity flow evolution and the distribution of their deposits this study analysed ssgf deposits in the third member of the shahejie formation in the jiyang depression eastern china based on 3 d seismic data well log data and core observations the types and origins of ssgf the distribution patterns and depositional models of ssgf deposits have been investigated the dominant types of ssgf include muddy debris flows sandy debris flows high density turbidity currents surge like low density turbidity currents and quasi steady low density turbidity currents which are mainly caused by river floods and sediment failures the overall distribution of ssgf deposits is controlled by the sediment supplies basin structure and sequence stratigraphic framework paleotopography flood energy triggering mechanism and the slope angle of the delta front are also important ssgf deposits associated with river floods can be further subdivided into elongated and fan shaped deposits elongated sandstones are composed predominantly of bedload dominated and suspension load dominated hyperpycnal flow deposits the fan shaped sandstones can be subdivided into an inner fan a middle fan and an outer fan inner fan deposits are dominated by gully filling debrites middle fan deposits are composed predominantly of channel fill deposits while outer fan deposits consist of thin bedded and laterally extensive lobe intervals ssgf deposits triggered by sediment failures can be further subdivided according to their proximity to slumping proximal intermediate and distal parts the proximal deposits are composed predominantly of slide and slump deposits the intermediate deposits are dominated by sandy debrite tongues while the distal deposits are composed predominantly of thin bedded surge like low density turbidites this study offers some insight into the types origin of gravity flow as well as the distribution of their deposits in a lacustrine rift basin

geostrophic circulation or shallow marine turbidity currents the dilemma of paleoflow patterns in storm influenced prograding shoreline systems abstract in ancient storm influenced prograding shoreline sequences sole marks mainly tool marks from hummocky crossstratified storm deposits are commonly oriented normal to paleoshoreline and the trend of paleobathymetric contours in the basin asymmetrical tool marks typically indicate flows directed offshore several workers have attributed their formation to storm generated shallow marine turbidity currents this interpretation conflicts with observations from modern shelves where storm driven circulation generally is geostrophically balanced and time averaged bottom currents approximately parallel bathymetric contours and the local shoreline the resolution of these apparently conflicting observations may lie in the realization that tool marks and many other small paleoflow indicators form almost instantly as the result of instantaneous flow conditions very near the bed beneath storm generated flows in the shallow ocean instantaneous and time averaged characteristics of the bottom boundary layer generally exhibit little similarity storm generated tool marks are formed by the movement of large tools within the thin less than 1 m inner boundary layer resulting from the superimposition of waves and currents the orientation of the peak instantaneous shear stress moving large tools under such combined flows mainly reflects wave orbital motions which typically are normal to shore the magnitude of stress is great y increased in the offshore direction and decreased in the onshore direction by superimposition of a steady current with an offshore component of flow but the direction of stress is only slightly affected in ancient storm influenced sequences therefore shore normal tool marks generally were not formed by turbidity currents rather their orientation is best attributed to shoaling waves approaching the coast at a very high angle asymmetrical tool marks are directed offshore due to enhanced shear stress on the offshore stroke of waves superimposed on a geostrophic current with an offshore flow component tool marks do not reflect the time averaged bottom flow direction in fact they provide almost no information concerning steady bottom currents in contrast high angle cross beds formed in coarser sediment by the migration of dunes and sandwaves although relatively rare in offshore storm deposits generally reveal approximately shore parallel flows in ancient systems cross beds closely reflect the time averaged flow direction in the outer boundary layer of a geostrophic current for three reasons 1 the net transport direction for sand moving as bed load beneath a combined flow lies between the directions of peak instantaneous shear stress and time averaged shear stress 2 large ripples disrupt the thin inner boundary layer and 3 long time intervals relative to wave induced velocity oscillations are required to form large ripples

fault reactivation during mesozoic extension in eastern offshore canada abstract new seismic data are presented that document reactivation of a thrust fault as a normal fault four deep seismic reflection profiles now straddle the hibernia oil field region of the grand banks off eastern canada the profiles intersect each other near the western edge of the 22 km deep jeanne d arc basin thereby giving a three dimensional picture of basement structures the basin bounding fault which marks the eastern edge of the bonavista platform appears to follow a pre existing fault system more precisely the fault that bounds the jeanne d arc basin just north of hibernia can be traced on the seismic data along strike to the south beneath the bonavista platform where it has not been reactivated the seismic data show that this feature was probably a palaeozoic thrust fault a laterally extensive palaeozoic basin occupies most of the bonavista platform the deeper part of the crust is quite reflective and is characterized by numerous dipping reflective packages this detailed study of the hibernia region suggests that the narrow elongated basins of the grand banks may have been formed within the reactivated hanging walls of older thrust sheets similar observations have been made for the mesozoic extensional basins of the u k continental shelf the pre mesozoic compressional structures documemed on the central grand banks lie along the appalachian orogen and may be of variscan age

mud rich density driven flows along an early ordovician storm dominated shoreline implications for shallow marine facies models abstract dynamic physical and biological seafloor processes shape the three dimensional stratal architecture of modern and ancient mud dominated continental shelves when evaluating a mudstone unit as a potential source reservoir or seal it is imperative to understand the physical biological and chemical seafloor processes that control mudstone depositional fabric and diagenesis this study presents sedimentological ichnological and geochemical data of exceptionally preserved mudstone originating from hyperpycnal and possible combined flow deposits in the lower ordovician tremadocian beach formation bell island group newfoundland seven mudstone facies are described based on textural compositional and ichnological characteristics mudstone deposits interpreted to originate from hyperpycnal flows are well cemented exhibit high chlorite illite ratios and contain sharp grain size changes commonly forming beds with tripartite subdivisions deposits of wave enhanced sediment gravity flows are in contrast poorly cemented illitic mudstone these mudstone facies exhibit decimeter thick wave or combined flow structures within siltstone and very fine sandstone beds as well as laterally discontinuous nonbioturbated mudstone layers beds of this facies have abundant mudstone on mudstone and mudstone on sandstone erosional contacts low organic carbon loading from the nonvegetated early paleozoic hinterland combined with a high reworking frequency on a shallow marine wave dominated shelf is inferred to have resulted in high remineralization efficiency and low preservation potential of reactive organic carbon burial efficiency and bioavailability of organic matter are considered to be the critical variables controlling infaunal colonization and bioturbation in mud dominated open coastline paleoenvironments this work demonstrates that ancient fine grained coastal systems are incompletely incorporated into sequence stratigraphic models owing to their atypical proximal to distal facies relationships it is proposed that the spatial organization of fine grained sediment and facies architecture in such systems are controlled by 1 frequency of sediment supply events 2 direction of mud transport 3 diagenetic reactivity of minerals and bioavailable organic carbon and 4 residence time of mineral grains and organic matter close to the sediment water interface

age and tectonic setting of neoproterozoic granitoid rocks antigonish highlands nova scotia canada implications for avalonia in the northern appalachian orogen the oldest rocks in the avalonian antigonish highlands of northern mainland nova scotia canada are late neoproterozoic 618 ma volcanic and sedimentary rocks of the georgeville group intruded

role of ponded turbidity currents in reservoir trap efficiency the capacity to store water in a reservoir declines as it traps sediment a river entering a reservoir forms a prograding delta coarse sediment e g sand deposits in the fluvial topset and avalanching foreset of the delta and is typically trapped with an efficiency near 100 the trap efficiency of fine sediment e g mud on the other hand may be below 100 because some of this sediment may pass out of the reservoir without settling out here a model of trap efficiency of mud is developed in terms of the mechanics of a turbidity current that plunges on the foreset the dam causes a sustained turbidity current to reflect and form a muddy pond bounded upstream by a hydraulic jump if the interface of this muddy pond rises above any vent or overflow point at the dam the trap efficiency of mud drops below 100 a model of the coevolution of topset foreset and bottomset in a reservoir that captures the dynamics of the internal muddy pond is presented numerical implementation comparison against an experiment and application to a field scale case provide the basis for a physical understanding of the processes that determine reservoir trap efficiency

the geological structure and distribution of paleozoic rocks on the avalon platform offshore newfoundland single channel seismic reflection profiles obtained across the avalon platform offshore avalon peninsula newfoundland have been studied for seismic reflections and interpreted in conjunction with lithologic and biostratigraphic data formline structural mapping revealed a 4000 m thick ordovician silurian marine shale sequence that is gently folded about north northwest south southeast axes and is unconformably overlain by a synclinal outlier of devonian redbeds approximately 700 m thick the avalon platform on the grand banks may represent a mildly deformed acadian terrane which is contiguous with onshore avalonian sequences or it may be part of a foreland zone adjacent to an overthrust belt or both

high density mud suspensions and cross shelf transport on the mechanism of gelling ignition abstract this study theoretically explores sediment fluxes of high density mud suspensions across the seafloor under the assumption that the dispersions received their entire sediment load from local resuspension of surface mud flocculation is ignored and a constant maximum floccule size is prescribed the resultant sediment flux equation which constitutes a bottom boundary condition for use in suspended sediment models has important physical implications first this study identifies a condition referred to as gelling ignition in which sediment concentrations will increase to extremely large values owing to a positive feedback between net erosion and hindered settling remarkably this work reveals that owing to boundary layer dynamics mud suspensions of both low and high concentrations are easier to keep in suspension than those of intermediate concentrations wave conditions in which high concentrations are produced in the model are within the realm of those typical of storms on shelves on a sloping shelf these highly concentrated suspended mud layers would be driven down slope by gravity and thus represent a possible mechanism for cross shelf transport

experimental study of reservoir turbidity current a series of experiments were conducted in a flume to study the hydraulic characteristics of the turbidity current in a reservoir kaolin was used as the suspended material the plunge points were found to be unstable initially as the experiment went on it moved downstream from the incipient plunge location and finally reached a stable location the densimetric froude number of the incipient and the stable plunge points are approximately equal to 1 0 and 0 6 respectively it is found that the length of the plunge region is 15 times the water depth at the stable plunge point the volumetric flow rates in the plunge region increase 17 during the plunging process about 70 of the water discharge and 80 of the sediment discharge are confined to the denser layer a layer close to the channel bed with approximately constant concentration the thickness of the turbidity current increases while the layer averaged velocity and concentration decrease in the longitudinal direction the layer averaged velocity has the smallest variation rates equations for the dimensionless velocity and concentration profiles are obtained

hyperpycnal plume formation from riverine outflows with small sediment concentrations a series of laboratory experiments has been conducted in order to elucidate the sediment induced mixing processes accompanying riverine outflows specifically the discharge of a warm fresh particle laden fluid over a relatively dense cool brine in a parameter regime analogous to recently acquired field measurements hypopycnal surface plumes were subject to a convective instability driven by some combination of heat diffusing out of the warm fresh sediment laden plume and particle settling within it convection was robust in the presence or absence of intense turbulence at sediment concentrations as low as 1 kg m 3 and took the form of millimetre scale sediment laden fingers descending from the base of the surface plume a consequence of the convective instability of the original hypopycnal plume is the generation of a hyperpycnal bottom riding flow the experiments presented here indicate that natural river outflows may thus generate hyperpycnal plumes when sediment concentrations are 40 times less than those required to render the outflow heavy relative to the oceanic ambient the resulting hyperpycnal plumes may play an important role in transporting substantial quantities of sediment to the continental slope and beyond

lapse time and frequency dependent coda wave attenuation for delhi and its surrounding regions abstract attenuation of seismic wave energy of delhi and its surrounding regions has been estimated using coda of local earthquakes estimated quality factor qc values are strongly dependent on frequency and lapse time frequency dependence of qc has been estimated from the relationship qc f q0fn for different lapse time window lengths q0 and n values vary from 73 to 453 and 0 97 to 0 63 for lapse time window lengths of 15 s to 90 s respectively average estimated frequency dependent relation is qc f 135 8f0 96 0 02 for the entire region for a window length of 30 s where the average qc value varies from 200 at 1 5 hz to 1962 at 16 hz these values show that the region is seismically active and highly heterogeneous the entire study region is divided into two sub regions according to the geology of the area to investigate if there is a spatial variation in attenuation characteristics in this region it is observed that at smaller lapse time both regions have similar qc values however at larger lapse times the rate of increase of qc with frequency is larger for region 2 compared to region 1 this is understandable as it is closer to the tectonically more active himalayan ranges and seismically more active compared to region 1 the difference in variation of qc with frequencies for the two regions is such that at larger lapse time and higher frequencies region 2 shows higher qc compared to region 1 for lower frequencies the opposite situation is true this indicates that there is a systematic variation in attenuation characteristics from the south region 1 to the north region 2 in the deeper part of the study area this variation can be explained in terms of an increase in heat flow and a decrease in the age of the rocks from south to north

expanding the spectrum of shallow marine mixed carbonate siliciclastic systems processes facies distribution and depositional controls of a siliciclastic dominated example

storm driven bottom sediment transport on a high energy narrow shelf nw iberia and development of mud depocenters bottom sediment transport on the nw iberian shelf was monitored during a downwelling storm in september 2014 collected data was analyzed and fed into a 3d coastal ocean model to understand storm driven sediment transport on the shelf and its impact on mid shelf mud depocenters mdcs a significantly enhanced level of bottom sediment resuspension nearly two orders of magnitude higher than that in the pre storm period was recorded at the mooring site field data analysis reveals that it was induced by a short lasting strong bottom current in combination with enhanced wave current interaction simulation results indicate that this strong current was part of a coastal jet resulted from downwelling an across shelf horizontal density gradient as high as 0 32 g m4 occurred at the interface between the downwelling and the bottom waters forming a remarkable front due to buoyancy effect the downwelling water was mostly confined to the coast with a depth limit of 80 m in the south and 120 m in the north of the region resulting in a northward directed coastal jet simulation results suggest that during the storm local near bottom sediment suspensions with concentrations on the order of 10 kg m3 would be triggered by wave current interaction and flow convergence associated with the front direct impact on the development of mdcs by transport and deposition of concentrated sediment suspensions is indicated by model results the seaward limit of the front coincided with the shoreward edge of the mdc nucleus suggesting the front as a primary control on the deposition of fine grained sediment this article is protected by copyright all rights reserved

twentieth century floods recorded in the deep mediterranean sediments flood generated turbidity currents represent an important process of marine sedimentation however no deposit related to this process has yet been described at sea turbiditic sequences cored at 2000 m water depth in the mediterranean show tractive structures and superposition of reverse and normal grading they are related to floods triggered during the twentieth century deposits from flood generated turbidity currents have a great impact on the interpretation of deep ocean paleoclimate records obtained near continental margins by linking deep marine sedimentary records to continental climatic changes through flood frequency and magnitude implications of this study should help redefine the reservoir character of oil fields in fine grained turbidites

the importance of breaching as a mechanism of subaqueous slope failure in fine sand large bank failures comprising up to 10 6 m 3 of sediment are common features along steep channel banks in estuaries and large rivers that consist of clean fine sands and are mostly assumed to be generated by sudden liquefaction of large masses of very loosely packed sand another less commonly recognized type of failure is manifested by the gradual retrogression of a very steep wall steeper than the angle of repose instead of the voluminous surging plastic sediment water flow or hyperconcentrated density flow sensu mulder alexander 2001 generated by liquefaction this type of failure known as breaching by dredging companies and hydraulic engineers produces a sustained quasi steady turbidity current to date sedimentologists have not recognized the process of breaching as such in this paper it is suggested that breaching may be the origin of many thick massive sand layers known from ancient deposits from various environments notably in some turbidite successions possible differences in the sedimentary structure of the deposits produced by breach failures vs liquefaction slope failures liquefaction flow slides can be deduced from a knowledge of the sediment transport processes initiated by the failure a field study is presented on some poorly structured beds in the eocene vlierzele sands in belgium which are supposed to have originated from liquefaction failures but are reinterpreted to be the products of breaching it is postulated that the local steep slope disturbance required to initiate an active breach can be produced by a small liquefaction slope failure liquefaction flow slide failure or local erosion by river or tidal channel flow at the initial stage of the failure event

effects of ambient currents and waves on gravity driven sediment transport on continental shelves abstract observations from several shelf environments show that down slope gravity driven transport may constitute an important mode of suspended sediment dispersal across shelves and highlight the influence of ambient waves and currents on gravity induced sediment flux the phenomena discussed here involve high concentrations of suspended sediment mixed with seawater and thus differ in genesis from hyperpycnal plumes released directly from sediment laden rivers the field sites examined are the gulf of bohai off the mouth of the yellow river huanghe the northern california shelf off the mouth of the eel river and the louisiana shelf west of the mouths of the mississippi river off the yellow river rapid down slope transport over distances of a few km occurred when frictional resistance induced by strong along shelf currents was temporarily relaxed more prolonged down slope motion over longer distances occurred following floods of the eel river when wave and current agitation provided turbulence to sustain gravity driven transport of fluid mud on the louisiana inner shelf the down slope gravity force was much weaker but observations suggest that thin gravity flows may still have occurred in the presence of waves a simple analytical theory is developed that incorporates the influence of ambient shelf currents on gravity driven transport of suspended sediment this theory is quantitatively consistent with the observations from the three sites if the supply of easily suspended sediment is less than the capacity of ambient currents including waves to carry sediment then intense turbulence limits gravity induced sediment transport by increasing the drag at the base of the flow when ambient currents abruptly cease rapid down slope transport can then occur over short distances until the sediment settles such flows do not remain intensely turbulent because the slope of the continental shelf is too gentle to induce shear instability within the gravity flow the maximum sustained rate of gravity induced sediment transport occurs when ambient currents are strong but the supply of easily suspended sediment exceeds the resuspension capacity of the ambient currents feedback then leads to values of the gradient richardson number ri within the flow that are near the critical value of 1 4 this partially damps bottom drag but still allows the generation of sufficient turbulence to maintain sediment in suspension observations also indicate systematic relationships among ri the supply of easily suspended sediment and the bottom drag coefficient acting on the gravity flow

estimation of the intrinsic absorption and scattering attenuation in northeastern venezuela southeastern caribbean using coda waves northeastern venezuela has been studied in terms of coda wave attenuation using seismograms from local earthquakes recorded by a temporary short period seismic network the studied area has been separated into two subregions in order to investigate lateral variations in the attenuation parameters coda q 1 q c 1 has been obtained using the single scattering theory the contribution of the intrinsic absorption q i 1 and scattering q s 1 to total attenuation q t 1 has been estimated by means of a multiple lapse time window method based on the hypothesis of multiple isotropic scattering with uniform distribution of scatterers results show significant spatial variations of attenuation the estimates for intermediate depth events and for shallow events present major differences this fact may be related to different tectonic characteristics that may be due to the presence of the lesser antilles subduction zone because the intermediate depth seismic zone may be coincident with the southern continuation of the subducting slab under the arc

late holocene sediment accumulation on the northern california shelf oceanic fluvial and anthropogenic influences the late holocene sedimentary record of the northern california continental shelf archives the combined influences of fluvial and oceanic mechanisms of land to ocean sediment flux and provides perspective on the cumulative impacts of major floods and land use change in the drainage basin during historical time piston cores collected on the shelf 50 150 m water depths off the eel and klamath rivers were analyzed to identify lithologies indicative of depositional processes over the past 5 k y and dated using 137cs and 14c methods to resolve spatial patterns in sediment accumulation rates sediment accumulation rates averaged over the past several millennia display along and across shelf gradients that reflect interactions between the shelf flow field and antecedent tectonically produced topography the highest rates 3 6 mm yr 0 4 0 8 g cm2 yr are associated with structural lows on the middle shelf whereas the lowest rates 0 2 0 6 mm yr 0 03 0 05 g cm2 yr occur at structural highs and at the shelf edge this accumulation pattern is spatially coincident with postglacial sediment thicknesses on the shelf and with depocenters that form on shorter time scales in response to oceanic flood events the sediment column exhibits an upward fining sequence consistent with increased bypass of river silt and clay as delta plains progressively filled during the holocene transgression sedimentary event beds indicative of major river floods including two probable hyperpycnal discharges of the eel river within the past 1 k y are preserved stratigraphic evidence of accelerated sediment delivery to the shelf during historical time in the form of increased burial of fluvial mud relative to fine grained sand is superimposed on this natural record this change is concordant with the regional history of forest clearing and related land use practices in the watershed after the early 1800s and appears to record the combined impacts of timber harvesting and several extreme floods after ca a d 1950

estimation of coda wave attenuation in the east anatolia fault zone turkey in this study the attenuation of coda waves qc f has been estimated for different lapse times and frequencies in the east anatolian fault zone eafz in turkey using a single back scattering model of s coda envelopes the data include 255 earthquakes recorded by ten stations the frequency dependent qc values are estimated at central frequencies of 1 5 3 6 8 12 and 18 hz using 20 30 40 s lapse time windows along this fault zone the frequency dependent qc obtained for all data and lapse time is qc 57 5f0 82 the entire study area is divided into six subregions according to the magnitude of the earthquakes and the location of the fault segments the estimated average frequency dependent relation for all lapse times are q text c left text i right 3 4 3f 0 9 3 for bingol karliova erzincan triple junction q text c left text ii right 56 3f 0 71 for bingol lake hazar segment q text c left text iii right 68 5f 0 75 for lake hazar sincik segment q text c left text iv right 72 5f 0 78 hazar sincik and celikhan golbasi faults q text c left text v right 59 7f 0 87 for kahramanmaras triple junction and q text c left text vi right 67 4f 0 94 for amanos range and karasu basin the lowest qc is determined between bingol and malatya the highest qc value is along amanos range in addition qc values are calculated for each regions at different lapse times the average qc value of the study region varies from 53 11 at 1 5 hz to 498 41 at 18 hz for 20 s lapse time window as its variation is from 116 11 at 1 5 hz to 749 75 at 18 hz of central frequency for 40 s lapse time window the increase of qc with lapse times changes from one subregion to another along the fault zone the rate of increment is significantly higher in bingol karliova erzincan triple junction than in the other subregions this rapid increase of low qc values in the junction reaches the general attenuation characteristics of the fault at 40 s lapse time finally the low q0 and high n values can be attributed to the energy loss as a result of the heterogeneity and activity along the fault zone the increase of qc with frequency lapse time may be related to heterogeneity decreases with depth the rapid increase of qc with depth in bingol karliova erzincan triple junction may be interpreted that the effect of the northern anatolian fault zone is effective in the upper crust is not so deep compared to eafz

predicting the geometry of channelized deep sea turbidites analytical relationships between the dynamic properties of a deposit forming turbidity current propagating across a sea floor with small slope and the geometry of the resulting deposit have been confirmed experimentally for a wide range of particle sizes initial concentrations and volumes of the driving suspension these simple expressions provide a basis for inferring the dynamics of natural flows from their deposits we surmise that the turbidity current responsible for depositing the black shell turbidite in the northwestern atlantic ocean had an initial sediment concentration on the order of 100 g of silt per litre and a volume on the order of 1000 km 3

internal waves vs surface storm waves a review on the origin of hummocky cross stratification hummocky cross stratification hcs is considered a diagnos tic structure of surface storm activity at the shoreface offshore transition however the origin of hcs is still debated laboratory experiments have not yet reproduced it and direct observations on the continental shelves do not exist most hydrodynamic interpretations invoke pure oscilla tory flows unidirectional dominated combined flows and oscillatory dominated flows but they all share the assump tion of hcs to reflect the combined action of surface storm waves and related currents within this context of uncertain ties internal waves gravity waves propagating along the pycnocline provide an alternative mechanism to explain the origin of hcs internal waves breaking on the shelf create episodic high turbulence events and induce upslope and downslope currents as well as oscillatory flows at the depth where the pycnocline intersects the sea floor in this scenario both the oscillatory and the unidirectional components needed for hcs to form are not necessarily linked to surface storm waves but can occur at various depth as far and near there is a pycnocline where internal waves can form

evidence for an earthquake triggered basin collapse in saguenay fjord canada abstract on february 5 1663 perhaps the largest earthquake ever witnessed in eastern north america struck the saguenay fjord basin with such force that landslides and submarine slides were triggered over a wide region over 3 km 3 of clayey holocene sediments collapsed from the margins of the fjord resulting in amalgamated debris flow and slide deposits that covered much of the basin floor these deposits reached thicknesses of up to 100 m and extended over an area of 100 km 2 the ambient seafloor was compressed under the impact and load of these debris masses and or liquefied under the cyclic stress of the seismic wave a 0 2 km 3 landslide also occurred at this time blocking the saguenay river and possibly facilitating an extra large spring freshet discharge biotracers planktonic and benthic foraminifera arcellaceans pollen and wood in conjuction with mass physical properties of sediment cores and high resolution seismic and sidescan profiler data have been used to identify local and distal sediment sources including their original water depth of deposition and their subsequent mode of mass gravitational transport the landslide material was eroded by the river and super elevated sediment concentrations are considered responsible for a long lasting 28 days turbidity current that entered the saguenay fjord depositing a single 0 3 km 3 turbidite the 2 to 10 m thick turbidite is considered to have been self igniting having eroded into the surface of the collapsed basin debris paleo hydraulic calculations suggest that this turbid flow averaged 0 45 m s and reached a flow thickness of 30 m

the extraterrestrial impact evidence at the palaeocene eocene boundary and sequence of environmental change on the continental shelf we have identified clear evidence of an extraterrestrial impact within the onset of the carbon isotope excursion cie that defines the palaeocene eocene p e boundary hyperthermal event approx 56 ma from several sites on the eastern atlantic coastal plain and offshore we review and update the state of the evidence for an impact at the p e boundary including a k ar cooling age of the ejecta that is indistinguishable from the depositional age at the p e which establishes the ejecta horizon as an isochronous stratigraphic indicator at the p e immediately above the ejecta peak at the base of the coastal plain marlboro clay unit we identify a sharp increase in charcoal abundance coincident with the previously observed dramatic increase in magnetic nanoparticles of soil pyrogenic origin we therefore revisit the observed sequence of events through the p e boundary on the western atlantic coastal plain showing that an extraterrestrial impact led to wildfires landscape denudation and deposition of the thick marlboro clay whose base coincides with the spherule horizon and cie onset the sr ca ratio of the spherules indicates that the carbon responsible for the onset may be vaporized caco 3 target rock mixed with isotopically light carbon from the impactor or elsewhere crucially we do not argue that the impact was responsible for the full manifestation of the cie observed globally onset to recovery approx 170 kyr rather that a rapid onset was triggered by the impact and followed by additional carbon from other processes such as the eruption of the north atlantic igneous province such a scenario agrees well with recent modelling work though it should be revisited more explicitly this article is part of a discussion meeting issue hyperthermals rapid and extreme global warming in our geological past

colorado river sediment transport 2 systematic bed elevation and grain size effects of sand supply limitation the colorado river in marble and grand canyons displays evidence of annual supply limitation with respect to sand both prior to topping et al this issue and after the closure of glen canyon dam in 1963 systematic changes in bed elevation and systematic coupled changes in suspended sand concentration and grain size result from this supply limitation during floods sand supply limitation either causes or modifies a lag between the time of maximum discharge and the time of either maximum or minimum depending on reach geometry bed elevation if at a cross section where the bed aggrades with increasing flow the maximum bed elevation is observed to lead the peak or the receding limb of a flood then this observed response of the bed is due to sand supply limitation sand supply limitation also leads to the systematic evolution of sand grain size both on the bed and in suspension in the colorado river sand input during a tributary flood travels down the colorado river as an elongating sediment wave with the finest sizes because of their lower settling velocities traveling the fastest as the fine front of a sediment wave arrives at a given location the bed fines and suspended sand concentrations increase in response to the enhanced upstream supply of finer sand then as the front of the sediment wave passes that location the bed is winnowed and suspended sand concentrations decrease in response to the depletion of the upstream supply of finer sand the grain size effects of depletion of the upstream sand supply are most obvious during periods of higher dam releases e g the 1996 flood experiment and the 1997 test flow because of substantial changes in the grain size distribution of the bed stable relationships between the discharge of water and sand transport rates i e stable sand rating curves are precluded sand budgets in a supply limited river like the colorado river can only be constructed through inclusion of the physical processes that couple changes in bed sediment grain size to changes in sand transport rates in some rivers the upstream supply of sediment is in equi librium with the upstream supply of water whereas in others the upstream supply of sediment is decoupled either com pletely or partially from the upstream supply of water in the first type of river changes in sediment transport are controlled by changes in the discharge of water whereas in the second and perhaps more common type of river changes in sedi ment transport are also coupled to changes in sediment grain size in this paper we investigate the systematic changes in bed elevation sediment transport and sediment grain size that occur in response to changes in the upstream supply of sand in a river with an intermittent limited supply of sand specifically the colorado river in marble and grand canyons figure 1 to develop an intuitive understanding of the linkage be tween sediment grain size and the upstream supply of sediment in a river it is informative to first examine sediment transport

relative importance of processes that govern the generation of particulate hyperpycnal flows simple mathematical approaches are used to investigate the relative importance of salinity temperature dissolved load stratification of suspended load and estuarine mixing on the direct formation of particulate hyperpycnal currents from river outflow specifically the critical concentration of suspended sediment for hyperpycnal flow formation is related to a reference concentration that does not take any of these processes into account the results show that the most important process in reducing this critical concentration is estuarine mixing which can reduce it by several orders of magnitude the other processes are relatively secondary but can all reduce the critical concentration by about one order of magnitude examples from rivers where hyperpycnal flows are known to form illustrate the influence of the described processes

sediment flux and the anthropocene data and computer simulations are reviewed to help better define the timing and magnitude of human influence on sediment flux the anthropocene epoch impacts on the earth surface processes are not spatially or temporally homogeneous human influences on this sediment flux have a secondary effect on floodplain and delta plain functions and sediment dispersal into the coastal ocean human impact on sediment production began 3000 years ago but accelerated more widely 1000 years ago by the sixteenth century societies were already engineering their environment early twentieth century mechanization has led to global signals of increased sediment flux in most large rivers by the 1950s this sediment disturbance signal reversed for many rivers owing to the proliferation of dams and sediment load reduction below pristine conditions is the dominant signal today a delta subsidence signal began in the 1930s and is now a dominant signal in terms of sea level for many coastal environments overwhelming even the global warming imprint on sea level humans have engineered how most water and sediment are discharged into the coastal ocean hyperpycnal flow events have become more common for some rivers and less common for other rivers bottom trawling is now widespread suggesting that even continental shelves have received a significant but as yet quantified anthropocene impact the anthropocene attains the level of a geological climate event such as that seen in the transition between the pleistocene and the holocene

is there evidence for geostrophic currents preserved in the sedimentary record of inner to middle shelf deposits abstract unidirectional oscillatory and combined flow paleocurrent data from many ancient wave dominated coastlines indicate that sediment transport was directed offshore at angles of 70 to 90 from the inferred local paleoshoreline solemarks and parting lineations from turbidite like storm beds and solemarks from hummocky beds indicate that flows transporting sediment offshore were orthogonal to the local shoreline the shore normal orientations of solemarks parting lineation and asymmetric ripples on hummocky cross stratification and their orthogonal relationship with respect to wave ripple crest orientation indicate that the dominant direction of wave approach at the sediment water interface was also orthogonal to the shoreline the formation of hummocky cross stratification by an al ng shore flow component superimposed on oscillatory wave motion is not supported by paleocurrent data rather much of the evidence from the ancient record suggests that hummocky cross stratification forms under high energy oscillatory flow with a weak shore normal combined flow component the orientation of wave ripple crests from the lower shoreface to offshore transitional zone of wave dominated coasts can be used to approximate the local paleoshoreline trend the paleocurrent data from the ancient examples suggest a strong predominance of offshore directed sediment transport as opposed to the nearly shore parallel orientation of currents observed in modern geostrophic flows

a decadal record of underflows from a coastal river into the deep sea four periods of extreme turbidity were measured in monterey submarine canyon over the past 12 yr these turbid events occurred simultaneously with the four largest flood events of the nearby salinas river they filled the canyon with fresher warmer and apparently buoyant water that extended to depths below 1 km the low salinity signature must reflect underflow of the salinas river plume that was driven to depth by negative buoyancy contributed from the suspended sediment a flux estimate for the 1995 event suggests that one half or more of the river s suspended sediment load was carried down the canyon in the underflow underflows may contribute significantly to the carbon budget of the continental slope

the avalon terrane of newfoundland geophysical correlations from onshore to offshore as evidence for precambrian to tertiary structural evolution abstract the avalon terrane in eastern canada has a complex geological and structural history from the precambrian to the present on land the terrane is typified by faulted precambrian clastic basins bounded by arcuate belts of mafic and felsic volcanics these basins are overlain by spatially limited exposures of cambro ordovician sediments there is structural evidence of paleozoic events after the cambro ordovician including the juxtaposition of the avalon against the rest of newfoundland in devonian carboniferous times but no significant deposition offshore there are siluro devonian basins extensive areas of carboniferous deposition and the mesozoic petroliferous basins of the grand banks each having characteristic trends traditional studies of the avalon have focused on either the onshore portion or the mesozoic basins this paper presents an attempt to understand the relationship between the two using published geophysical data gravity and magnetic data were used to produce basic and derived maps to interpret the orientation of geophysical features of the terrane directional filtering techniques were used to identify and highlight particular trend orientations and directional arrow plots were used to identify the spatial distribution of trends various age events in the avalon terrane are correlated with specific geophysical trend directions the dominant arcuate and northerly directed patterns are considered to be precambrian with some hercynian reworking east west trends are probably related to the emplacement of the collector anomaly in the devonian carboniferous and to the continuation into the avalon of south armorican trends from europe the nw oriented features correlate with major transfer faults of jurassic middle cretaceous age in the mesozoic basins and can be followed to the land area providing evidence for similar aged events onshore there is also evidence for nne tertiary trends offshore the mesozoic features are closely related to the atlantic opening history the nw oriented features appear to cross the dover hermitage fault suggesting that more of newfoundland than the avalon terrane may have been affected by mesozoic events

coda q estimates in the andaman islands using local earthquakes the attenuation property of andaman island has been investigated analyzing coda waves from 57 local earthquakes in the magnitude range of 2 0 4 9 using the single backscattering model these earthquakes waveforms recorded on five broadband seismographs sited over the island from north to south during nov 2003 to march 2004 have been used to calculate the frequency dependent coda q q c applying the time domain coda decay method the coda q computed at central frequencies from 0 5 12 hz and five lapse time windows from 40 to 80 s progressively increases from 105 f 0 88 in the north andaman to 135 f 0 79 in the south andaman with an average of 119 f 0 80 the average q c values vary from 75 42 at 0 5 hz to 697 54 at 12 hz central frequency for 40 s lapse time window while for 80 s lapse time window its variation is from 117 38 at 0 5 hz to 1256 115 at 12 hz the q c estimated at different lapse times manifests a significant variation from 122f 0 75 to 174f 0 73 corresponding to lapse time window lengths of 40 and 80 s respectively the variation of q c with frequency lapse time and also with the location of seismograph reflects the marked structural and compositional inhomogeneity with depth along the andaman islands these observations are well correlated with the seismicity pattern and distinct high angle subduction beneath the island

occurrence and regional geological setting of paleozoic rocks on the grand banks of newfoundland analyses of seismic reflection profiles supported by lithological and palynological studies of core samples from submarine outcrops indicate that the lower paleozoic succession of the avalon terrane southeast newfoundland is continuous offshore the succession crops out over an area greater than 30 000 km2 and is approximately 8 km thick the sequence is dominantly siltstone and is of late cambrian to devonian or younger age it is relatively unmetamorphosed underlain by hadrynian acoustic basement and overlain along its eastern and southern margins by a mesozoic cenozoic succession that is economically important from an oil and gas perspective lithofacies studies indicate that in early ordovician time restricted shallow marine conditions probably prevailed over a vast area of the avalon terrane upper ordovician and silurian siltstones show evidence of deposition under more dynamic and well oxygenated conditions and probably represent a normal shallow marine environment redbeds of possible devonian

seismic attenuation of coda waves in the eastern region of cuba abstract cuba s seismic attenuation had never been studied in detail in this paper we present the results of the research on the seismic attenuation of cuba s eastern zone based upon the information collected by the seismological cuban network from 1998 to 2003 581 earthquakes were selected from the cuban catalogue to make this study all of them recorded by at least three seismic stations had their epicenters located in the eastern cuban region 19 3 22 n 79 73 w epicentral distances between 15 km and 213 km their coda duration magnitudes ranging from 2 and 4 1 and their focal depths reaching up to 30 km the seismic wave attenuation was studied using coda waves the single scattering method proposed by sato in 1977 was applied the attenuation and frequency dependency for different paths and the correlation of the results with the geotectonics of the region are presented in this paper the mean q c value calculated was q c 64 2 f 0 84 0 01 the relatively low q 0 and the high frequency dependency agree with the values of a region characterized by a high tectonic activity the q c values of seven subregions of eastern cuba were calculated and correlated with the geology and tectonics of the area

sediment laden gravity currents with reversing buoyancy there are many natural occurrences of sediment laden gravity currents in which the density of the interstitial fluid is less than that of the ambient fluid although the bulk density of the current is greater such currents are driven by the excess density of suspended particles however after sufficient particles have sedimented the current will become buoyant cease its lateral motion and ascend to form a plume examples of such currents include brackish underflows in deltas turbidity currents and pyroclastic flows experimental studies are described which show that due to sedimentation sediment laden gravity currents decelerate more rapidly than saline currents of the same density there is little difference in the experiments between a sediment laden current with neutrally buoyant interstitial fluid and one with buoyant interstial fluid until sufficient sediment has been lost to cause the latter kind of current to lift off a marked deceleration is then observed and a plume is generated with lift off occurring along the length of the current the resulting buoyant plume then generates a gravity current below the upper surface of the fluid in the tank the deposit from a current with buoyant fluid shows a fairly abrupt decrease in thickness beyond the lift off distance and has a flatter profile than that from a simple sediment current a theoretical model is presented which is based on the two layer shallow water equations and incorporates a model of the sedimentation in which particles are assumed to be uniformly suspended by the turbulence of the current the model shows good agreement with the observed lengths of the experimental currents as a function of time and predicts the lift off distance reasonably well these processes have implications for the behaviour of turbidity currents the interpretation of turbidites mixing processes in the oceans and the lift off of pyroclastic flows

overbank and channelfill deposits of the modern yellow river delta abstract the huanghe is noted for its high transport rate of silt and clay which may reach depth averaged values of 200 kg m 3 during peak discharge the sediment load transported through the river on entering the delta plain amounts to 1012 kg per year in contrast to most other large deltas only one distributary channel is active at any one time the high sediment load causes the rivermouth to prograde at a yearly rate of 1 4 km into the shallow less than 20 m deep bohai gulf the vertical aggradation of the channel belt and mouth bar complex is also rapid decimetres per year on average so that after a normal average of twelve years increasing channel instability and avulsion create the start of a new delta lobe a series of satellite images covering the last fifteen years has provided insight in the evolution of the river pattern as well as the progradation of the delta front a newly developed distributary passes from a multichannel to a single straight channel system and ends with the formation of meanders the protruding mature delta lobe shows a radiating pattern of crevasse channels overbank crevasse deposits are made of vertically stacked dm scale waning flow sequences structurally characterized by from bottom to top small scour and fills even parallel lamination and climbing ripple crosslamination accumulation rates on crevasse splays can be predicted on the basis of estimated river sediment discharge it can be concluded that each sequence has been deposited within a few hours and that tidal waterlevel fluctuations may have played a role in the generation of a single sequence

storm deposition in marine sand sheets wall creek member frontier formation powder river basin wyoming abstract the turonian cretaceous wall creek member frontier formation and equivalent turner sandy member carlile shale powder river basin wyoming were deposited on the western margin of the north american seaway the seaway occupied the foreland of the sevier orogenic belt the wall creek turner interval is up to 67 m thick and consists of shale sandstone and minor conglomerate and bentonite deposition occurred during rising sea level following an approximately 90 ma lowstand sandstone and mudstone of the wall creek were deposited in thin shelf sand sheets 6 15 m thick up to approximately 120 km from equivalent shorelines shelf sand sheets dip gently seaward core and outcrop show shelf sandstone of the wall creek in the powder river basin to consist of burrowed to bioturbated medium scale cross beds near horizontal laminated beds and ripple laminated beds reactivation surfaces and mud drapes are common the lack of consistent periodicity across cross sets in foreset angle and thickness and unperiodic spacing of reactivation surfaces mud drapes and burrowed intervals suggest that tides were not an important transport process over most of the depositional area the dominance of storm currents over tides is also indicated by variable bed thickness structure type grain size and degree of burrowing vertically much of the mud separating beds and draping foresets likely represents sedimentation from river floods following storm events p

note on the analysis of plunging of density flows this note is devoted to the correction of an error in a calculation contained in a classical published paper on the plunging of river flows as they enter lakes or reservoirs the correction of the error provides a felicitous result according to which not only the ratio of underflow thickness just after plunging to depth just before plunging but also the densimetric froude numbers both just before and just after plunging are all specified as a function of a single parameter this single parameter characterizes the tendency of plunging to entrain ambient water into the density underflow

structure and dynamics of fluid muds on the amazon continental shelf thick layers of fluid mud occur in strong tidal flows over the inner portion of the amazon continental shelf in regions of strong salinity fronts associated with the plume discharged from the amazon river detailed shipboard profile measurements obtained in this region during a multidisciplinary amazon shelf sediment study amasseds provide an unprecedented opportunity to examine the structure and dynamics of fluid muds under natural conditions the analysis focuses on flows in which the motion is fully turbulent and suspended sediment dominates the stratification under these conditions a comparison of measurements and one dimensional model calculations indicates that vertical transport is controlled by suppression of turbulent mixing at gradient richardson numbers near 1 4 this constraint produces a distinctive vertical structure and leads to an upper bound on the total amount of suspended sediment that may be carried in a turbulent suspension by a tidal flow

late paleozoic orogenies in western africa and eastern north america the diachronous closure of theic ocean the carboniferous evolution in western morocco southeastern canada and new england was dominated by intracontinental processes the subsidence of pull apart sedimentary basins and their subsequent deformation during middle carboniferous to early permian times with the development of autochthonous structures and mainly low to medium grade metamorphism on the other hand the structural style of the late carboniferous deformation in the southern appalachians and to a substantial degree also in new england that is related to a continental collision is characterized by deep seated northwest vergent thrusts and medium to high grade metamorphism the mauritanides structures are symmetrically east vergent thrusts this pattern is explained by the diachronous closure of the theic ocean that separated the avalon and carolina microcontinental blocks from paleo gondwana the closure of the theic ocean occurred during devonian times between the northern appalachians and the northwestern edge of paleo gondwana morocco whereas it was achieved only at the end of carboniferous times between the southern appalachians and western africa mauritania senegal the late devonian to early carboniferous counterclockwise rotation of gondwana induced the opening of the pull apart basins of canada new england and morocco as well as later the oblique collision in the southern appalachians and the generalized late paleozoic dextral transcurrent faulting along the northern edge of paleo gondwana

linking river flood dynamics to hyperpycnal plume deposits experiments theory and geological implications turbid river plumes entering ocean or lake water of lesser density i e hyperpycnal plumes can plunge to form turbidity currents providing an important link between terrestrial sediment sources and marine depositional sinks a leading hypothesis suggests that hyperpycnal plume deposits accurately record the rising and falling discharge of a flooding river in terms of sediment size grading bedform sequence and deposit thickness which if correct has significant implications for unraveling river dynamics reservoir potential and earth history from marine event beds herein we present one of the first experimental flume studies aimed at testing this hypothesis results indicate that depth averaged hyperpycnal plume velocities can be uncorrelated or even anti correlated with river discharge at certain seabed locations because of translation of the plunge point resulting from temporal variations in discharge and sediment concentration through the duration of a river flood an advection length scale of settling sediment is found to be an important control on hyperpycnal plume deposits where coarse sediment sand is most likely to record multiple flow accelerations and decelerations related to plunge point translation even for a river flood with a single peaked hydrograph in contrast fine sediment mud is relatively insensitive to local plunge point dynamics and is most likely to preserve directly rising and falling river discharge finally it was found that the necessary fluvial sediment concentration to form a plunging plume can be much larger than the concentration typically used assuming density equivalence because of deposition upstream of the plunge point

coda wave seismic structure beneath the indian ocean region and its implications to seismotectonics and structural heterogeneity abstract the indian ocean region is one of the challenging areas for seismological research because of its complex seismotectonic settings and intricate structural heterogeneity prevails beneath the indian ocean region in order to address the problem we attempted to determine the attenuation structure for the indian ocean region using coda wave generation by single backscattering model in this study the central frequency of interest has been considered between 1 5 and 12 0 hz with lapse time window ltw of 20 30 40 50 and 60 s measured from the origin time of the earthquake we obtained the attenuation relation for different seismotectonic zones associated with varying estimate of attenuation parameters spreading zone qc spz 171f1 10 subduction zone qc sz 220f0 961 triple point junction qc tpj 156f1 15 prominent faults of indian ocean region qc pfioz 197f0 99 and volcanic and hotspot qc vz hs 209f0 97 we estimated average frequency dependent attenuation parameters of q0 that varies as 156 32 q0 220 33 with corresponding variability in frequency parameter as 0 96 0 05 1 15 0 09 at 40 s ltw for the entire indian ocean region suggesting that the entire indian ocean is seismo tectonically highly active and is attributed to the nature and extent of attenuation due to varying amount of structural heterogeneity in different tectonic zones characterized by the occurrence of earthquakes of varying strengths in the seismically active tectonic zones of the region we found that the triple junction rtj is associated with low qc and high attenuation because of deposition of a variety of sediments at the rtj which is in unison to the geological mapping of the region we observed that our estimated attenuation model is comparable that of the aleutian islands the adak seismic zone at low frequency 5 hz and to the philippine sea subduction zone petukhin at higher frequency range 5 hz which is very much correlative to seismo tectonic settings we infer that our assimilated attenuation model has potential to provide information on intricate seismotectonics and seismogenesis for computing earthquake source parameters and earthquake hazards that may be useful for assimilating detailed seismic velocity structure beneath the indian ocean region

colorado river sediment transport 1 natural sediment supply limitation and the influence of glen canyon dam analyses of flow sediment transport bed topographic and sedimentologic data suggest that before the closure of glen canyon dam in 1963 the colorado river in marble and grand canyons was annually supply limited with respect to fine sediment i e sand and finer material furthermore these analyses suggest that the predam river in glen canyon was not supply limited to the same degree and that the degree of annual supply limitation increased near the head of marble canyon the predam colorado river in grand canyon displays evidence of four effects of supply limitation 1 seasonal hysteresis in sediment concentration 2 seasonal hysteresis in sediment grain size coupled to the seasonal hysteresis in sediment concentration 3 production of inversely graded flood deposits and 4 development or modification of a lag between the time of a flood peak and the time of either maximum or minimum depending on reach geometry bed elevation analyses of sediment budgets provide additional support for the interpretation that the predam river was annually supply limited with respect to fine sediment but it was not supply limited with respect to fine sediment during all seasons in the average predam year sand would accumulate and be stored in marble canyon and upper grand canyon for 9 months of the year from july through march when flows were dominantly below 200 300 m3 s this stored sand was then eroded during april through june when flows were typically higher after closure of glen canyon dam because of the large magnitudes of the uncertainties in the sediment budget no season of substantial sand accumulation is evident because most flows in the postdam river exceed 200 300 m3 s substantial sand accumulation in the postdam river is unlikely

offshore extensions of the avalon zone of newfoundland gravity and magnetic data from the nearshore region of the avalon zone of newfoundland provide evidence of the nature of a sedimentary basin immediately east of the avalon peninsula the data also suggest a strong correlation between the arcuate magnetic patterns of the offshore portion of the avalon zone and the precambrian mafic volcanics mapped onshore the offshore basin is interpreted to have been formed by precambrian block faulting and subsequently filled by sediments derived from surrounding paleotopographic highs two dimensional gravity and magnetic modelling results suggest the presence of mafic material beneath the modelled 9 km of sediment in the basin

extension of continental crust at the margin of the eastern grand banks newfoundland seismic and gravity observations from the rifted margin of the eastern grand banks newfoundland support a new model for extension of the continental crust from the shelf edge to odp site 1277 where mantle rocks are exhumed we find that the largest decrease in crustal thickness from about 28 km to 6 km occurs beneath the continental slope of the grand banks over a distance of just 20 km this rapid decrease in crustal thickness coincides with anomalously high seismic velocities 7 0 7 2 km s 1 in the lower crust of the shelf edge the thin crust of the continent ocean transition cot in this area has a smooth basement surface void of upper crustal blocks and prerift sediments we compare our geophysical results with a geodynamical model that represents rifting of a relatively hot continental lithosphere and with another numerical model that represents rifting of a cold lithosphere both geodynamic models suggest that crustal thinning beneath the continental slope was achieved by extensional faulting in the upper crust and ductile shear zones in the middle crust the geodynamic models provide an explanation for the formation of distinct continental slopes at rifted margins beneath the continental shelf of the grand banks the moho and the strong lower crust rotated upwards toward to a 50 dip without visible internal deformation the presence of these strong lower crustal rocks at shallow depth in the rift flank subsequently helped to localize the extension farther seaward with ongoing extension some high angle normal faults may have rotated to a sub horizontal orientation which would explain the lack of brittle deformation visible in the seismic reflection data the two geodynamic models produce different amounts of extension of continental crust in the distal margins the hot rifting model localizes strain much more rapidly leaving narrow zones of extended continental crust and it produces a relatively large amount of melt 30 in the final stages of rifting continental breakup may occur rapidly in hot lithosphere 5 myr on the other hand a cold extension model extends the continental crust to a thickness smaller than 10 km over a width of 50 km in the distal margin similar to what we inferred at the eastern grand banks the cold lithospheric model requires about 23 myr of extension before continental breakup and it predicts much less melting in the mantle 13 the long rift duration wide zones of thinned continental crust and small amount of magmatism make the cold rifting model the most applicable to newfoundland iberia rift

on the transport modes of fine sediment in the wave boundary layer due to resuspension deposition a turbulence resolving numerical investigation previous field observations revealed that the wave boundary layer is one of the main conduits delivering fine sediments from the nearshore to continental shelves recently a series of turbulence resolving simulations further demonstrated the existence of a range of flow regimes due to different degrees of sediment induced density stratification controlled by the sediment availability in this study we investigate the scenario in which sediment availability is governed by the resuspension deposition from to the bed specifically we focus on how the critical shear stress of erosion and the settling velocity can determine the modes of transport simulations reveal that at a given wave intensity which is associated with more energetic muddy shelves and a settling velocity of about 0 5 mm s three transport modes ranging from the well mixed transport mode i two layer like transport with the formation of lutocline mode ii and laminarized transport mode iii are obtained as the critical shear stress of erosion reduces moreover reductions in the settling velocity also yield similar transitions of transport modes we also demonstrate that the onset of laminarization can be well explained by the reduction of wave averaged bottom stress to about 0 39 pa due to attenuated turbulence by sediments a 2 d parametric map is proposed to characterize the transition from one transport mode to another as a function of the critical shear stress and the settling velocity at a fixed wave intensity

behaviour of particle laden flows into the ocean experimental simulation and geological implications the behaviour of subaerial particle laden gravity currents e g pyroclastic flows lahars debris flows sediment bearing floods and jokulhlaups flowing into the sea has been simulated with analogue experiments flows of either saline solution simple suspensions of silicon carbide sic in water or complex suspensions of sic and plastic particles in methanol were released down a slope into a tank of water the excess momentum between subaerial and subaqueous flow is dissipated by a surface wave at relatively low density contrasts between the tank water and the saline or simple suspensions the flow mixture enters the water and forms a turbulent cloud involving extensive entrainment of water the cloud then collapses gravitationally to form an underwater gravity current which progresses along the tank floor at higher density contrasts the subaerial flow develops directly into a subaqueous flow the flow slows and thickens in response to the reduced density contrast which is driving motion and then continues in the typical gravity current manner complex suspensions become dense flows along the tank floor or buoyant flows along the water surface if the mixtures are sufficiently denser or lighter than water respectively flows of initially intermediate density are strongly influenced by the internal stratification of the subaerial flow material from the particulate depleted upper sections of the subaerial flow becomes a buoyant gravity current along the water surface whereas material from the particulate enriched lower sections forms a dense flow along the tank floor sedimentation from the dense flow results in a reduction in bulk density until the mixture attains buoyancy lifts off and becomes a secondary buoyant flow along the water surface jokulhlaups lahars and debris flows are typically much denser than seawater and thus will usually form dense flows along the seabed after sufficient sedimentation the freshwater particulate mixture can lift off to form a buoyant flow at the sea surface leading to a decoupling of the fine and coarse particles flood waters with low particulate concentrations 2 may form buoyant flows immediately upon entering the ocean subaerial pyroclastic flows develop a pronounced internal stratification during subaerial run out and thus a flow splitting behaviour is probable which agrees with evidence for sea surface and underwater flows from historic eruptions of krakatau and mont pelee a pyroclastic flow with a bulk density closer to that of sea water may form a turbulent cloud resulting in the deposition of much of the pyroclasts close to the shore dense subaqueous pyroclastic flows will eventually lift off and form secondary buoyant flows either before or after the transformation to a water supported nature

event suspended sediment characteristics and the generation of hyperpycnal plumes at river mouths east coast continental margin north island new zealand abstract steepland rivers draining small coastal watersheds often have very high suspended sediment loads but the event characteristics of suspended sediment concentration and yield in this class of river is not well documented continuous monitoring at four sites in the waipaoa river basin new zealand demonstrates that during individual and composite events suspended sediment concentration versus water discharge relations typically show clockwise hysteresis and that event maximum concentration is poorly related to event peak discharge the signature of different erosion processes is also imprinted on the event yield magnitude frequency distributions gully dominated tributary basins produce relatively high yields at all frequencies reflecting greater sediment availability whereas in tributary basins where shallow landsliding is the dominant erosion process there is a steep increase in yields in relation to return period we estimate that flood discharges from the waipaoa river approach or exceed

backwater and river plume controls on scour upstream of river mouths implications for fluvio deltaic morphodynamics sediment flux from rivers to oceans is the fundamental driver of fluvio deltaic morphodynamics and continental margin sedimentation yet sediment transport across the river to marine boundary is poorly understood coastal rivers typically are affected by backwater a zone of spatially decelerating flow that is transitional between normal flow upstream and the offshore river plume flow deceleration in the backwater zone as well as spreading of the offshore plume should render rivers highly depositional near their mouths leading to sedimentation and eventual elimination of the backwater zone at steady state this reasoning is counter to observations of riverbed scour erosional bed forms and long lived backwater zones near the mouths of some coastal rivers e g mississippi river united states to explain these observations we present a quasi 2 d model of a coupled fluvial backwater and offshore river plume system and apply it to the mississippi river results show that during high discharge events the normal flow depth can become larger than the water depth at the river mouth resulting in drawdown of the water surface spatial acceleration of flow and erosion of the riverbed as proposed by lane 1957 the transition to drawdown and erosion is ultimately forced by spreading of the offshore river plume this points to the need to model coupled river and river plume systems with a dynamic backwater zone under a suite of discharges to accurately capture fluvio deltaic morphodynamics and connectivity between fluvial sediment sources and marine depositional sinks

conditions for the ignition of catastrophically erosive turbidity currents abstract the basic impediment to a clear understanding of eroding and depositing turbidity currents has been the lack of a proper formulation of bed sediment entrainment this problem is addressed herein a detailed analysis indicates that the bagnold criterion is necessary but insufficient for self sustaining turbidity currents the analysis reveals two possible equilibrium states a relatively low velocity ignitive state and a high velocity dense catastrophic state a stability analysis indicates that the ignitive state is unstable flows below it invariably die out and flows above it ignite i e accelerate and entrain sediment to the catastrophic state which is stable the ignitive state thus defines the criterion for a self sustaining turbidity current estimates of the catastrophic state suggest that it is highly erosive and competent to scour out submarine canyons

wave modified turbidites combined flow shoreline and shelf deposits cambrian antarctica sandstone tempestite beds in the starshot formation cen tral transantarctic mountains were deposited in a range of shoreline to shelf environments detailed sedimentological analysis indicates that these beds were largely deposited by wave modified turbidity currents these currents are types of combined flows in which storm generated waves overprint flows driven by excess weight forces the interpreta tion of the tempestites of the starshot formation as wave dominated turbidites rests on multiple criteria first the beds are generally well graded and contain bouma like sequences like many turbidites the soles display abundant well developed flutes they also contain thick divisions of climbing ripple lamination the lamination however is dominated by convex up and sigmoidal foresets which are geometries identical to those produced experimentally in current dominated com bined flows in clear water finally paleocurrent data support a tur bidity current component of flow asymmetric folds in abundant con volute bedding reflect liquefaction and gravity driven movement and hence their orientations indicate the downslope direction at the time of deposition the vergence direction of these folds parallels paleocur rent readings of flute marks combined flow ripples and a number of other current generated features in the starshot event beds indicating that the flows were driven down slope by gravity the wave component of flow in these beds is indicated by the presence of small to large scale hummocky cross stratification and rare small two dimensional ripples wave modified turbidity currents differ from deep sea turbidity cur rents in that they may not be autosuspending and some proportion of the turbulence that maintains these flows comes from storm waves such currents are formed in modern shoreline environments by a com bination of storm waves and downwelling sediment laden currents they may also be formed as a result of oceanic floods events in which intense sediment laden fluvial discharge creates a hyperpycnal flow event beds in the starshot formation may have formed from such a mechanism oceanic floods are formed in rivers of small to medium size in areas of high relief commonly on active margins the starshot formation and the coeval douglas conglomerate are clastic units that formed in response to uplift associated with active tectonism sedi mentological and stratigraphic data suggest that coarse alluvial fans formed directly adjacent to a marine basin the geomorphic conditions were therefore likely conducive to rapid fluvial discharge events asso ciated with storms the abundance of current dominated combined flow ripples at the tops of many starshot beds indicates that excess weight forces were dominant throughout deposition of many of these beds

sediment transport events on the northern california continental shelf abstract a long term monitoring tripod has been maintained in 60 m water depth at the northern end of the strataform study site on the northern california continental shelf as part of this ongoing study tripod data for 1 year beginning 24 september 1995 are used to provide a sediment transport analysis on an event by event basis the objective of this paper is to highlight the energetic nature of this shelf region in terms of the frequency duration and magnitude of sediment suspension events and the associated particle flux analyses are based on measurements from two current meters and two optical backscatterance sensors located at 30 and 100 cm above the bed data from these instruments and a pressure sensor were averaged over 7 5 min every hour during the year of record 41 distinct sediment suspension events occurred i e sustained suspended sediment concentrations greater than 20 mg l above background level at 100 cm above bed suspended sediment events were associated with significant wave activity tidal currents and river discharge the average duration of a sediment suspension event was 3 1 days and varied from 0 7 to 8 days during events mean suspended sediment concentrations of 110 mg l were observed with peak hourly observations exceeding 1000 mg l concentrations between events averaged 35 mg l the overall sediment flux for the period of record was directed seaward and southward the distribution and magnitude of sediment suspension events and particle flux showed a strong seasonality for example of the sediment transport during events 98 of the net along shelf and 73 of the net across shelf sediment transport occurred in the winter three major winter storms could account for 72 of the total along shelf transport but only 10 of the across shelf transport the across shelf flux was more evenly distributed over the year and occurred as a result of mean currents and very low frequency seaward flows associated with mesoscale circulation patterns

initiation of turbidity currents outcrop evidence for eocene hyperpycnal flow turbidites abstract the most widely used although not as widely documented explanation for initiation of turbidites is the conversion of catastrophic sediment failures into turbidites such instantaneous sediment failures generate short lived surge type flows that deposit turbidite beds described as bouma sequences in this communication we present data from the eocene central basin of spitsbergen that shows turbidite systems that are volumetrically dominated by thick ungraded and laminated sandy turbidite beds deposited by gradual aggradation from sustained flows sustained flows are long lived and more or less continuous they can be generated by a variety of mechanisms e g 1 instability during volcanic eruptions and the consequent remobilisation of unconsolidated material 2 seismically triggered subaerial sliding within the drainage 3 storm surges 4 retrogressive slope failure 5 breaching and 6 hyperpycnal flows our database shows that in the eocene central basin the sustained flows were generated by hyperpycnal flows i e by direct river effluent the hyperpycnal flow turbidites have been recognized on the basis of 1 physical connection between fluvial and turbidite channels at the shelf edge 2 abundance of thick turbidite sandstone beds 3 sand prone nature of the turbidite systems 4 downslope changes of individual thick sandy turbidite beds and their collapsed pinch out segments 5 great abundance of continental material leaves coal fragments in turbidite beds 6 low abundance of associated slumped or debris flow beds and 7 the occurrence of turbidites in systematically accreted shelf margins these features strongly suggest that hyperpycnal flow generated by direct river effluent deposited significant number of turbidite beds in the documented slope and basin floor turbidite successions we also argue that the hyperpycnal flows are preferentially fed into the deepwater slopes beyond the shelf edge during the falling stage and lowstand of relative sea level the turbidite beds were studied in the context of seismic scale 1 15 km dip oriented outcrops by measuring detailed vertical sections following individual beds and packages of beds downslope for distances over 5 km lateral mapping and helicopter taken photomosaics

hyperpycnal sediment discharge from semiarid southern california rivers implications for coastal sediment budgets southern california rivers discharge hyperpycnal river density greater than ocean density concentrations of suspended sediment 40 g l according to buoyancy theory during flood events mostly during el nino southern oscillation enso conditions because hyperpycnal river discharge commonly occurs during brief periods hours to occasionally days mean daily flow statistics often do not reveal the magnitude of these events hyperpycnal events are particularly important in rivers draining the transverse range and account for 75 of the cumulative sediment load discharged by the santa clara river over the past 50 yr these events are highly pulsed totaling only 30 days 0 15 of the total 50 yr period observations of the fate of sediment discharge although rare are consistent with hyperpycnal river dynamics and the high likelihood of turbidity currents during these events we suggest that much of the sediment load initially bypasses the littoral circulation cells and is directly deposited on the adjacent continental shelf thus potentially representing a loss of immediate beach sand supply during particularly exceptional events 100 yr recurrence intervals flood underflows may extend past the shelf and escape to offshore basins

regionalization of crustal coda q in the continental united states a contour map of crustal q sub 0 values for the entire united states is presented based on a scattering model to explain the coda waves of local and near regional earthquakes these coda q sub 0 values are in good agreement with q of lg waves data are obtained from over 250 local earthquakes with most magnitudes between 3 0 and 5 0 recorded by 25 wwssn and 27 lrsm stations throughout the continental united states these two sets of data provide a range of frequencies from 0 5 to 3 5 hz a frequency dependence of q is observed in the range of frequencies considered a power law dependence of the form q q sub 0 f f sub 0 sup eta is assumed the value of frequency dependence eta is found to be maximum in the tectonically active western united states and least in the stable regions of the central and south central united states the lowest q sub 0 values are obtained in the western united states with values ranging from 140 to 200 in the coastal plains average crustal q sub 0 values for the basin and range province vary from 200 to 300 increasing gradually to 400 in the colorado plateau region q sub 0 valuesmore in the columbia plateau vary from 200 to 400 q sub 0 values increase very rapidly along the central and southern rocky mountains from 400 to 800 east of the rocky mountains q sub 0 values increase gradually in the interior plains to a maximum value of around 1300 in the mississippi embayment region farther east q sub 0 values decrease gradually to an almost constant value of 1000 along the appalachian mountains coastal regions of eastern and northeastern united states have a crustal q sub 0 value between 900 to 700 regions of north central and gulf coastal plains show low q sub 0 values ranging from 600 to 400 and less the source factor b f sub p a measure of the intensity of scattering was obtained for several different regions of the continental united states regions of high q sub 0 values exhibit low intensity of scattering less

frequency dependent attenuation of coda waves in the crust in southwest anatolia turkey the attenuation of coda waves in the earth s crust in southwest sw anatolia is estimated by using the coda wave method which is based on the decrease of coda wave amplitude in time and distance a total of 159 earthquakes were recorded between 1997 and 2010 by 11 stations belonging to the koeri array the coda quality factor q c is determined from the properties of scattered coda waves in a heterogeneous medium firstly the quality factor q 0 the value of q c at 1 hz and its frequency dependency are determined from this method depending on the attenuation properties of scattered coda waves for frequencies of 1 5 3 0 6 0 8 0 12 and 20 hz secondly the attenuation coefficients are estimated the shape of the curve is controlled by the scattering and attenuation in the crustal volume sampled by the coda waves the average q c values vary from 110 15 to 1 436 202 for the frequencies above the q 0 and values vary from 63 7 to 95 10 and from 0 87 0 03 to 1 04 0 09 respectively for sw anatolia in this region the average coda q f relation is described by q c 78 9 f 0 98 0 07 and 0 012 km 1 the low q 0 and high are consistent with a region characterized by high tectonic activity the q c values were correlated with the tectonic pattern in sw anatolia

coda q in the eastern caribbean west indies summary coda q q has been estimated from seismograms of local earthquakes recorded on short period seismographs on some of the eastern caribbean islands of the lesser antilles arc west indies using the s s single scattering model the region was subdivided into four distinct subregions in order to investigate lateral variations in q the qc values exhibit a strong frequency dependence in the frequency band 1 5 to 16 hz with calculated coda q at 1 hz qo ranging from a minimum of 97 in the dominica area to a maximum of 145 in the leewards the degree of frequency dependence is also largest in the dominica area n 1 09 and smallest in the leewards n 0 82 although there appears to be some lateral and depth variation of both q and n in the region this variation does not seem to be very significant

late paleozoic strike slip faults in maritime canada and their role in the reconfiguration of the northern appalachian orogen major late paleozoic faults many with documented strike slip motion have dissected the ordovician devonian appalachian orogen in the maritime provinces of atlantic canada activity alternated between east west faults minas trend and ne sw faults appalachian trend nw se faults canso trend were probably conjugate to minas trend faults major dextral movement on faults with appalachian trend in total between 200 and 300 km began in the late devonian this movement initiated the maritimes basin in a transtensional environment at a releasing bend formed around a promontory in the laurentian margin and thinned the crust accounting for the major subsidence of the basin appalachian trend strike slip continued in the mississippian but was accompanied by major movement on e w minas trend faults culminating around the mississippian pennsylvanian boundary juxtaposing the meguma and avalon terranes of the appalachians close to their present day configuration however strike slip continued during the pennsylvanian permian interval resulting in transpressional deformation that reactivated and inverted earlier extensional faults a final major episode of transtension mainly sinistral occurred during the mesozoic opening of the atlantic ocean restoration of movements on these faults amounting to several hundred kilometers of slip explains anomalies in the present day distribution of terranes amalgamated during early paleozoic appalachian tectonism in the restored geometry the nashoba and ellsworth terranes of ganderia are adjacent to one another and the meguma terrane lies clearly outboard of avalonia a restored post acadian paleogeography not the present day geometry of the orogen should be used as a basis for reconstructions of its earlier paleozoic history

predicting the migration rates of subaqueous dunes in this paper we develop a simple physically based method for predicting what fraction of sediment moving over the crests of dunes will bypass their lee faces the bypass fraction is found by calculating a characteristic excursion length for every grain size making up a particular train of dunes all particles with excursion lengths greater than the downstream span of the lee face of the average dune are assumed not to contribute to dune propagation bypass fractions based on distributions of excursion lengths account for the discrepancies between total sediment discharge and dune migration rate measured by stein 1965 and guy et al 1966 in laboratory flumes as well as by us in the north loup river of nebraska calculations and these data agree over sediment transporting conditions associated with the entire stability field for dunes composed of medium sand the two laboratory studies show that commonly 30 60 of all sand moving over fully developed dunes is not deposited on slip faces measurements from the north loup river reveal that of all sediment moving over the dune crests there roughly 45 is not being captured on lee faces even though 99 of sediment is transported within 2 cm of the bed the method developed herein successfully estimates measured values for the bypass fraction from 0 to 80 our analysis indicates that the division between grains that are deposited versus those that are bypassing falls within the range of sand sizes making up the suspended load at dune crests

variation of coda wave attenuation in the alborz region and central iran summary more than 340 earthquakes recorded by the institute of geophysics university of tehran igut short period stations from 1996 to 2004 were analysed to estimate the s coda attenuation in the alborz region the northern part of the alpine himalayan orogen in western asia and in central iran which is the foreland of this orogen the coda quality factor qc was estimated using the single backscattering model in frequency bands of 1 25 hz in this research lateral and depth variation of qc in the alborz region and central iran are studied it is observed that in the alborz region there is absence of significant lateral variation in qc the average frequency relation for this region is qc 79 2f1 07 0 08 two anomalous high attenuation areas in central iran are recognized around the stations las and raz the average frequency relation for central iran excluding the values of these two stations is qc 94 2f0 97 0 12 to investigate the attenuation variation with depth qc value was calculated for 14 lapse times 25 30 35 90s for two data sets having epicentral distance range r 100 km data set 1 and 100 r 200 km data set 2 in each area it is observed that qc increases with depth however the rate of increase of qc with depth is not uniform in our study area beneath central iran the rate of increase of qc is greater at depths less than 100 km compared to that at larger depths indicating the existence of a high attenuation anomalous structure under the lithosphere of central iran in addition below 180 km the qc value does not vary much with depth under both study areas indicating the presence of a transparent mantle under them

geostrophic circulation or shallow marine turbidity currents the dilemma of paleoflow patterns in strom influenced prograding shoreline systems in ancient storm influenced prograding shoreline sequences sole marks mainly tool marks from hummocky crossstratified storm deposits are commonly oriented normal to paleoshoreline and the trend of paleobathymetric contours in the basin asymmetrical tool marks typically indicate flows directed offshore several workers have attributed their formation to storm generated shallow marine turbidity currents this interpretation conflicts with observations from modern shelves where storm driven circulation generally is geostrophically balanced and time averaged bottom currents approximately parallel bathymetric contours and the local shoreline the resolution of these apparently conflicting observations may lie in the realization that tool marks and many other small paleoflow indicators form almost instantly as the result of instantaneous flow conditions very near the bed beneath storm generated flows in the shallow ocean instantaneous and time averaged characteristics of the bottom boundary layer generally exhibit little similarity storm generated tool marks are formed by the movement of large tools within the thin less than 1 m inner boundary layer resulting from the superimposition of waves and currents the orientation of the peak instantaneous shear stress moving large tools under such combined flows mainly reflects wave orbital motions which typically are normal to shore the magnitude of stress is greatly increased in the offshore direction and decreased in the onshore direction by superimposition of a steady current with an offshore component of flow but the direction of stress is only slightly affected in ancient storm influenced sequences therefore shore normal tool marks generally were not formed by turbidity currents rather their orientation is best attributed to shoaling waves approaching the coast at a very high angle asymmetrical tool marks are directed offshore due to enhanced shear stress on the offshore stroke of waves superimposed on a geostrophic current with an offshore flow component tool marks do not reflect the time averaged bottom flow direction in fact they provide almost no information concerning steady bottom currents in contrast high angle cross beds formed in coarser sediment by the migration of dunes and sandwaves although relatively rare in offshore storm deposits generally reveal approximately shore parallel flows in ancient systems cross beds closely reflect the time averaged flow direction in the outer boundary layer of a geostrophic current for three reasons 1 the net transport direction for sand moving as bed load beneath a combined flow lies between the directions of peak instantaneous shear stress and time averaged shear stress 2 large ripples disrupt the thin inner boundary layer and 3 long time intervals relative to wave induced velocity oscillations are required to form large ripples