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

sediment transport processes in an ancient mud dominated succession a comparison of processes operating in marine offshore settings and anoxic basinal environments abstract few studies describe and compare the transport mechanisms operating to disperse mud in different parts of basins instead the physical processes operating to disperse mud in offshore environments where storm and tidal processes are interpreted to dominate are generally considered in isolation from those occurring in basinal settings where changes in bottom water anoxia and suspension settling from buoyant plumes are mostly interpreted to dominate using microtextural mineralogical and geochemical data derived from the analyses of 151 thin sections obtained from the lower jurassic mudstone dominated succession exposed on the coast of ne england we investigate how varying sediment dispersal mechanisms bioturbation and early diagenesis operated to produce the lithofacies variability observed in particular we consider the processes of sediment delivery while bottom waters were interpreted to be euxinic analyses of these samples reveal that the succession is highly variable at millimetre to centimetre scales six main lithofacies were observed 1 sand and clay bearing silt rich mudstones 2 silt bearing clay rich mudstones 3 clay rich mudstones 4 clay calcareous nannoplankton and organic carbon bearing mudstones 5 fine grained muddy sandstones 6 cement rich mudstones these units are organized typically into stacked successions of sharp based normally graded thin

facies and beddding sequences in shelf storm deposited carbonates fayetteville shale and pitkin limestone mississippian arkansas abstract the fayetteville shale miss is a black marine shale that was deposited on a southward deepening ramp in northern arkansas it is conformably overlain by skeletal oolitic grainstones of the pitkin limestone and together they form a shoaling upward succession of 1 deep muddy shelf 2 storm dominated muddy shelf and 3 ooid skeletal shoal and shoreface facies rhythmically bedded limestones of the upper fayetteville shale and some lower pitkin ooid grainstones are interpreted to be shelf storm deposits because of their stratigraphic position and bedding sequences upper fayetteville limestone beds are transitionally positioned between cephalopod bearing black shales deep muddy shell underneath and cross bedded ooid grainstones shoal and shoreface above fayetteville and pitkin beds are similar to other reported shelf storm deposits in that they contain hummocky cross stratification hcs and unidirectional climbing ripple cross stratification thought to have been laid down by combined flows six principal types of bedding sequences were noted in order of increasing water depth and or distance from the inferred shore 1 cross bedded and planar laminated beds with some keystone vugs beach 2 graded units of coarse skeletal ooid grainstone shoreface 3 planar laminated and hummocky cross stratified limestones lower shoreface 4 hummocky cross stratified limestones and thin shales storm dominated muddy shelf 5 fine grainstones with parallel laminae in lower half followed above by climbing ripple cross stratification storm dominated muddy shelf and 6 structureless lime mudstones and black shale deep muddy shelf reconstruction of the ancient shelf suggests that it sloped southward at 0 08 to 0 14 and the water depth at 30 km estimated width of storm dominated shell was 40 to 70 m the paleolatitudinal position of northern arkansas 5 15 s indicates that strong tropical storms or hurricanes rather than intense winter storms were responsible for eroding carbonate sediment from lower shoreface environments and depositing it as much as 30 km offshore in the muddy shelf

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

the bouma division a and the possible duration of turbidity currents abstract experimental data arnott and hand 1989 combined with theoretical arguments suggest that the transition in a turbidite from division a to division b occurs at a deposition rate of unsupported grains of about 6 7 10 4 m s 1 in the case of fine medium quartz sand a second rate may be calculated from properties measured at the transition from division b to division c allen 1971b these rates together permit an estimate to be made of the order of magnitude of the duration of deposition from a turbidity current the calculated order of a few tens of minutes for a mio pliocene turbidite described and interpreted hydraulically by komar 1985 is in broad agreement with his independent estimate based on the sequenc of sedimentary structures

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

numerical simulations of tidal and wind driven circulation in the cretaceous interior seaway of north america twenty two numerical experiments using a multi layer numerical model of turbulent flow in shallow seas better define fluid circulation and sediment transport paths in the cretaceous interior seaway of north america each experiment consists of a different combination of paleogeography paleobathymetry coriolis acceleration boundary tidal amplitudes wind stresses and bed friction the paleogeographies and paleobathymetries represent three seaway configurations a seaway of intermediate size and depth 200 m during a transgressive peak t5 in late albian time a seaway of maximum size and depth 400 m during peak transgression t6 in early turonian time and a seaway of minimum size and depth 100 m during a peak regression r8 in the campanian values for the other key model parameters are m 2 co oscillating boundary tides of 0 1 to 0 2 m range at the arctic ocean boundary and 0 5 to 1 m range at the proto gulf of mexico boundary coriolis accelerations corresponding to 30 n 45 n and 60 n latitude chezy friction factors ranging from 31 to 70 m s 1 and average winter winds and two winter storms computed by the community climate model at the national center for atmospheric research for paleogeographic conditions during the late albian results of the numerical experiments and comparison of these results with geologic observations allow the following conclusions 1 circulation in the seaway was generally storm dominated 2 typical winter storms crossing the seaway from west to east generated 0 3 m s 1 shore parallel geostrophic currents on the shelves north of arizona at first flowing weakly to the north but later during the storm flowing strongly to the south 3 extreme storms could have produced 0 8 m s 1 currents over the shelves and 0 3 m s 1 currents in 200 m of water 4 co oscillating tides propagated into the seaway as progressive kelvin waves and therefore tidal ranges in the seaway were mesotidal to macrofdal along the southeastern margin but microtidal everywhere else 5 significant deep water wave heights and periods of the fully developed wave field are predicted to have been 5 to 6 m high and 10 s respectively the northwestern shore of the seaway would have experienced these storm waves approaching from the north and thus net sediment drift should have been to the south limited available field data support these conclusions

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

ancient lacustrine hyperpycnites a depositional model from a case study in the rayoso formation cretaceous of west central argentina abstract hyperpycnal flows originate when sediment laden fluvial discharges enter standing lower density water because of their excess density the flows plunge near the river mouth and continue to travel basinward as a quasi steady and fully turbulent underflow the related deposits are hyperpycnites and constitute a particular type of turbidite with poorly known facies and facies tracts although hyperpycnal flows seem to be quite common in present times their occurrence in fossil strata is poorly documented this paper addresses the characteristics and depositional processes of shallow lacustrine sandy hyperpycnites on the basis of the field analysis of well exposed lower cretaceous strata rayoso formation in the neuquen basin of west central argentina the rayoso formation is composed of clastic and minor evaporitic red beds up to 1200 m thick deposited in a shallow perennial lake of variable salinity affected by long lived hyperpycnal flows main clastic facies are composed of fine grained sandstones with climbing ripples and plane beds other common sandstone facies include massive beds and low angle cross stratification most sandstone facies are related to traction plus fallout processes and often show a vertical fluctuation between sedimentary facies originated under different traction plus fallout conditions within single beds these fluctuations are interpreted to be evidence of deposition from flow fluctuations in sustained hyperpycnal flows most beds internally show the existence of three depositional phases acceleration ap erosion plus bypass ep and deceleration dp which record the complete evolution of a single long lived hyperpycnal flow at a fixed point additionally the depositional evolution of a single long lived hyperpycnal flow with distance records initially the progressive basinward migration of the ap and ep phases and finally an overall deposition under the dp phase both in proximal and distal areas this evolution provides an adequate explanation for the basinward extension of channelized features and for the occurrence of fine grained sandstones with climbing ripples both in proximal and distal positions within the same hyperpycnal system consequently facies analysis derived from application of the bouma sequence is not valid for deposits of quasi steady hyperpycnal flows

depositional environments of campanian maastrichtian successions in the kirthar fold belt southwest pakistan tectonic influences on late cretaceous sedimentation across the indian passive margin abstract the creation of a complex submarine morphology on the passive margin of the indo pakistan continental plate during later stages of its northward drift just prior to collision with eurasia is clearly demonstrated by the nature architecture and regional distribution of sandstone bodies within the campanian maastrichtian successions mughal kot and pab formations exposed within the north south trending kirthar fold belt of southwest pakistan these successions range in thickness from 7 m to more than 500 m and are dominated by fine to coarse sometimes pebbly thin to thick bedded sandstones with subordinate mudstones and marls all of which were deposited on the north facing passive margin of the indo pakistan plate twelve facies have been identified in these sequences and these have been grouped into nine facies associations which were formed within two broadly coeval depositional systems that occupied the contiguous southern kirthar and central kirthar sub basins it is argued here that these sub basins and their fill sequences developed mainly in response to major tectonically induced changes in both the morphology of this sector of the palaeo margin and in the sediment supply systems the more proximal sequences in the east and southeast sectors of the southern kirthar sub basin are dominated by fluviodeltaic deposits while more distal areas to the northwest are characterized by turbidite sandstone bodies formed in channel levee and lobe complexes within slope and basin floor settings of a relatively confined deep marine trough in this southern kirthar sub basin the mughal kot formation comprises basin floor lobes submarine channel fill sand bodies and base of slope mud rich lobes whereas the succeeding pab formation reflects overall progradation of the system being dominated by submarine slope fan lobes channels and levee deposits the central kirthar sub basin displays a broadly east to west transition from shoreface inner shelf through shelfal delta lobes middle shelf and deeper shelf ramp outer shelf facies associations all formed on a gently inclined storm and flood dominated clastic ramp synsedimentary normal faults observed in this study were controlled the depositional architecture and creation of closed deep marine southern kirthar sub basin during late cretaceous

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

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

early holocene coastal environment change inferred from deposits at okinoshima archeological site boso peninsula central japan abstract a series of deposits recording the early holocene drastic changes in coastal environment was investigated in terms of sedimentology and paleontology at the okinoshima archeological site on the southwest coast of boso peninsula central japan the deposits ca 3 m thick exposed in trenches consist of structureless mud and overlying stratified sand divided into three units i ii and iii in ascending order unit i is poorly sorted sandy mud with abundant opal phytoliths unit ii gradually overlying unit i is poorly sorted sandy mud containing more organic material and pieces of wood and archeological remains unit iii overlies unit ii with a sharp erosional contact and is composed of well sorted medium grained sand intercalated by mud layers with abundant plant fossils unit i suggests a dry surface on a coastal plain near uplands unit ii was transported from the upland onto the coastal plain by a debris flow unit iii deposits resulted from high energy river and storm surge flows onto the coastal plain the succession records that the sedimentary environments of the study site changed successively from a slope near the uplands to a fluvial plain to a coastal plain in response to a sea level rise in the early holocene ams ages of the units are approximately 10 000 cal bp and deposition occurred within 1000 years

ichnology of a subaqueously prograding clastic wedge late pliocene morne l enfer formation fullarton trinidad implications for recognition of autogenic erosional surfaces and delineation of stress factors on irregular echinoids abstract the morne l enfer formation outcrops near fullarton trinidad expose an erosional surface between the shelf deposits below and a prograding clastic wedge of the late pliocene paleo orinoco system above the surface exhibits similarities as well as dissimilarities with examples of regressive surfaces of marine erosion rsmes in wave influenced shelf settings and also with incised valleys inherited from fluvial incision of shelf during a forced regression episode however detailed ichnological studies reveal that the surface is a subaqueous autogenically controlled erosional surface on a shelf dominated by the hyperpycnal and hypopycnal discharges with transient wave influence integration of ichnological and sedimentological characteristics also suggests complicated interrelationships among different stress factors affecting the infauna in different subenvironments especially the irregular echinoids the delta prograded subaqueously through autogenically establishing its lobe on the open shelf without stratigraphically significant erosional removal of sediments in the subaqueously prograding clastic wedge the fluvial influence pertains high stress conditions not only for the adult infauna but also for their larvae within the water column especially in and near the subaqueous distributary channels

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

numerical simulation of a midlatitude squall line in two dimensions abstract a two dimensional anelastic cloud model was used in attempts to numerically replicate the observed structure of a midlatitude squall line initial conditions were adapted from observations of the 22 may 1976 oklahoma line model simulations were made with and without considering the ice phase of water these model storms have within the constraints of the model s geometry replicated the basic multicellular character and general line normal airflow typical of these lines in addition the structure and intensity of the subcloud cold air pool and the propagation speeds developed by the storms appear to be reasonable further it was found that the initial conditions chosen resulted in model storms which were not only long lasting but also essentially repetitive indicating that a state of quasi equilibrium had been attained the storms did not decay because the environmental conditions ahead of the storms were favorable and essentially unchanged during the course of the simulations the inclusio

quasi planar laminated sandstone beds of the lower cretaceous bootlegger member north central montana evidence of combined flow sedimentation abstract lower shoreface to shallow shelf strata of the bootlegger member of the lower cretaceous blackleaf formation are characterized by interbedded sandstone and mudstone sandstone beds are characterized by a peculiar planar lamination showing a subtle although perceptible undulation spacing to height ratios of the undulation are generally 100 or more typically the undulation shows no evidence of lateral accretion but only vertical aggradation and as a result most beds consist of a single laminaset aspects of quasi planar laminated beds indicate single event storm sedimentation and paleocurrent data indicate offshore sediment transport by its sedimentary characteristics and its similarity with a bed configuration generated in un experimental wave duct quasi planar lamination is produced by high energy combined flows this style of stratification should be common in the shallow marine stratigraphic record and its recognition should aid in interpreting high energy combined flow depositional events

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

sedimentology stratigraphy and reservoir properties of an unconventional shallow frozen petroleum reservoir in the cretaceous nanushuk formation at umiat field north slope alaska numerous oil and gas accumulations exist in the brooks range foothills of the national petroleum reserve in alaska npra we use cores and well logs from 12 abandoned legacy wells at umiat field near the southeastern boundary of the npra to characterize the sedimentology and stratigraphy of unconventional shallow frozen reservoirs in sandstones of the cretaceous albian cenomanian nanushuk formation the nanushuk formation at umiat has five facies associations offshore and prodelta lower shoreface upper shoreface delta front and delta plain three stratigraphically distinct regionally extensive nanushuk formation depositional systems at umiat contain several potential petroleum reservoirs the lower nanushuk formation including a reservoir interval known informally as the lower grandstand primarily consists of marine mudstone and shoreface sandstones the middle nanushuk formation is dominantly deltaic and contains a second major reservoir interval in the informal upper grandstand sandstone both the upper grandstand and lower grandstand are regressive the transgressive upper nanushuk formation contains an additional potential reservoir interval in shoreface sandstones of the informal ninuluk interval the primary reservoir intervals at umiat field are upper shoreface and delta front sandstones in the upper grandstand and lower grandstand where increased sorting and decreased bioturbation in high energy depositional environments affect overall permeability and permeability anisotropy

effects of sedimentation tectonics and glacio eustasy on depositional sequences pennsylvanian minturn formation north central colorado the pennsylvanian minturn formation was deposited adjacent to the ancestral front range on the eastern margin of the actively subsiding central colorado basin during a time of high frequency eustatic sea level changes attributed to glaciations in the southern hemisphere in the 100 km2 study area near mccoy colorado up to nine depositional cycles composed of marine and nonmarine deposits are recognized three tectonic blocks were subsiding at different rates in the study area detailed correlation of measured sections along well exposed outcrop belts has shown that seven of these cycles extend continuously across the field area which includes two delta complexes two cycles are absent on the central block the study area is well suited for examining the role of local tectonic vs eustatic processes in controlling the distribution of sedimentary deposits in the study area cycles are bounded by erosional surfaces with as much as 43 m of relief these surfaces are inferred to be sequence boundaries and are overlain by braided fluvial conglomerates inferred to be incised valley fill deposits marine flooding surfaces overlie the conglomerates the copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received january 17 1996 revised manuscript received october 18 1996 final acceptance march 20 1997 2department of geological sciences university of colorado at boulder campus box 250 boulder colorado 80309 0250 present address department of geology university of colorado at denver campus box 172 p o box 173364 denver colorado 80217 3364 i thank the members of my doctoral dissertation committee at the university of colorado ted walker martin lockley alan scott david budd and john pitlick for their advice and guidance during the course of my research financial support was provided by the rocky mountain section of sepm the colorado mountain club union pacific resources and the department of geological sciences at colorado university boulder charles ross provided fusulinid identifications and biostratigraphic zonation i thank paul weimer colorado university boulder for advice and encouragement and keith miller kansas state university for carefully reviewing a preliminary draft of this paper aapg reviewers william devlin romeo flores denise stone and elected editor kevin biddle provided many suggestions that greatly improved the final manuscript i also thank the residents of mccoy bond and copper spur colorado for granting access to their property and for all their kindness while i was working in the field

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

experiments on bed configurations in fine sands under bidirectional purely oscillatory flow and the origin of hummocky cross stratification abstract we made experiments on bed configurations in bidirectional purely oscillatory flows at a wide range of oscillation periods t and maximum orbital velocities um in two closed flow ducts 0 1 m and 0 4 m wide the motivation was that understanding of bed configurations in oscillatory flows with long periods and high velocities during storm induced deposition in the shallow ocean is needed for interpreting the stratification in such deposits the ripples produced were classified geometrically as two dimensional 2d or three dimensional 3d in plan view geometry and dynamically as oscillatory current ripples dependent upon the patterns of flow and sediment transport specific to the existence of the oscillation itself and reversing current ripples dependent on he existence of the current in one direction or the other during the oscillation runs in the small duct with effectively 0 19 mm and 0 30 mm sand showed the familiar small 2d oscillatory current ripples at short t and low um passing into much larger ripples with spacing up to 2 m at periods of 6 20 s and velocities up to 1 m s with no apparent limit on size with increasing t and um even in such a narrow duct there was some evidence of 3d geometry of the large ripples runs in the large duct with effectively 0 11 mm sand at a single period of about 9 5 s showed a progression from small 2d oscillatory current ripples through small 3d reversing current ripples to large round crested 3d oscillatory current ripples with prominent superimposed 3d to 2d reversing current ripples the large 3d ripples which increased in spacing to 1 3 m at the h ghest attainable um of 1 m s shifted in position and changed in size with time seemingly at random causing substantial local and temporary erosion and deposition at a given point on the bed there was no net aggradation during the runs but a deposit was synthesized by taking a time series of sidewall bed profiles during a run in which large 3d ripples developed from an initially plane bed and changed irregularly in size and position as they grew toward equilibrium and then plotting the profiles one above the previous with a constant upward increment the resulting stratification is strikingly similar to much hummocky cross stratification in ancient sandstones this suggests that some hummocky cross stratification is generated during sediment fallout from strong purely oscillatory flows at moderate to long oscillation periods as large 3d oscillatory current bed forms develop from a planar bed during strong but waning flow

thin fine grained storm layers in a muddy shelf sequence an example from the lower jurassic in the stenlille 1 well denmark thin storm sand layers are a characteristic feature of many sandy shelf sequences while little evidence of storm layers is preserved in muddy shelf deposits where bioturbation is often total the present study shows that storm generated depositional processes governed sedimentation in a muddy shelf environment the studied sequence is dominated by blackish mudshale and silt streaked mudshale with thinner intervals of pale fine grained heterolithic sandstone it represents the lower jurassic fjerritslev formation in the eastern part of the danish basin and has been studied in four cores from the stenlille 1 well within the muddy outer shelf association four sedimentary facies and 14 types of laminae 0 5 10 mm thick may be distinguished most of the laminae share the characteristics of graded bedding erosive bases and occasional fading ripples with storm layers of sand grade the structures indicate that the mudshale laminae formed by settling of the suspension load carried by storm generated currents this kind of episodic deposition of fine grained storm layers may be common though rarely preserved in outer shelf environments

exploratory flow duct experiments on combined flow bed configurations and some implications for interpreting storm event stratification abstract experiments were conducted to define equilibrium combined flow bed configurations developed under a wide range of oscillatory uo and unidirectional uu velocity components with a constant oscillation period of 8 5 sec and median grain size of 0 09 mm in the range of flows studied uo 0 0 80 m sec uu 0 0 25 m sec the bed phases are no movement small 2d ripples small 3d ripples large 3d ripples and plane bed small 2d ripples are stable at low uo and uu by increasing either velocity component the plan view bed configuration becomes progressively less regular as small 3d ripples develop small 3d ripples are stable at low to moderate uo and a wide r nge of uu under purely oscillatory flow as uo increases beyond about 0 40 m sec large scale hummocky bed forms become stable when a unidirectional component is applied these bed forms quickly develop a downstream asymmetry and generally move in the direction of uu the dip angles of the lee flanks increase with increasing uu although they remain less than the angle of repose at high oscillatory velocities uo 0 70 0 80 m sec plane bed is the stable bed phase when uu 0 05 m sec much hummocky cross stratification shows a preferred dip direction of the coset laminae our experiments support the idea that such cross stratification is produced by combined flow the large 3d ripples produced at high oscillatory velocities and small to moderate unidirectional velocities would presumably generate what might be called anisotropic hummocky cross stratification if we had been able to aggrade the bed during the runs this would be the case even in dominantly oscillatory flow the experiments show that even a weak unidirectional current of several centimeters per second superimposed upon a strong oscillatory flow results in bed forms that move predominantly in a single direction and that leave sets of laminae that dip predominantly in that direction in runs at large uo the purely oscillatory large scale bed forms were quickly planed off as uu was increased from zero but a series of almost stationary gentle undulations on the bed surface persisted for indefinitely long times where the original topographic highs were situated even at the highest unidirectional flow investigated 0 25 m sec this suggests a possible origin for the gentle undulation observed in cretaceous parallel laminated shallow marine sandstones described by arnott 1987 1988 with spacings up to a few meters and heights not much more than a centimeter

lake cyclicity as response to thermal subsidence a post camp scenario in the parnaiba basin ne brazil abstract the jurassic cretaceous transition was marked by the fragmentation of the west gondwana supercontinent and consequent opening of the atlantic ocean this event resulted in extensive lava flows in the central portion of west gondwana composing the central atlantic magmatic province in the parnaiba basin ne brazil a large lacustrine system succeeded this event nevertheless the post camp scenario and the influence of the decreasing isotherms in the mesozoic sedimentation are poorly understood in this sense cyclostratigraphy and outcrop based facies analysis were carried out in the lacustrine stratigraphic record allowing the recognition of four paleoenvironments central lake sheet like delta front lakeshore and ephemeral fluvial channels mainly organized in shallowing upward cycles the upsection transition of thin black shales and limestones to thick oxidized mudstones and stratified sandstones reflects the evolution of underfilled to overfilled lake settings the lacustrine succession is organized in four major depositional cycles characterized by millimeter to centimeter scale cycles bounded by unconformities or flooding surfaces the cycles define a retrogradational progradational retrogradational stacking pattern with random and frequent changes in lithology and cycle thickness cyclostratigraphic data suggest that the upward increasing accommodation space was controlled by the post camp thermal subsidence as well as shifts in sedimentary supply and water inflow outflow the time span of this subsidence phase lasted from even before the camp magmatism 200 ma affecting the whole deposition of the lacustrine succession and was interrupted by the cretaceous magmatic event 126 ma subsidence was also enhanced by crustal loading associated with thick subsurface jurassic intrusions and probably influenced the whole basin in consequence of flexural effects

re evaluating the paleogeography of the river dominated and wave influenced ferron notom delta southern central utah an integration of detailed facies architecture and paleocurrent analysis abstract this study re evaluates the paleogeography of a river dominated and wave influenced deltaic system in a single parasequence 6a of the cretaceous ferron sandstone south central utah which was previously interpreted as a typical asymmetric delta the conceptual asymmetric delta model of bhattacharya and giosan 2003 has been widely applied to interpret ancient deltaic systems however reconstructed paleogeographies relying on this conceptual model are commonly not scaled to the architectural elements which could possibly lead to inaccurate interpretations especially when data are relatively sparse the extensively exposed ferron sandstone outcrops provide an opportunity to demonstrate how detailed facies architecture work can be integrated with regional facies relationships to elucidate a properly scaled paleogeography seven facies associations fa are identified in parasequence 6a and are arranged into two distinctively different successions the river dominated succession consists of prodelta distal delta front proximal delta front and terminal distributary channel fa while the wave and storm dominated succession comprises offshore shelf lower shoreface and upper shoreface fa the river dominated succession is muddier and less bioturbated than the wave and storm dominated succession facies architecture analysis shows that the river dominated proximal delta front facies consists of mouth bar and terminal distributary channel elements that are not seen in the wave and storm dominated facies the scales of mouth bars and terminal distributary channels suggest that they are rather small features compared with the river dominated delta lobe which is incompatible with the simple bar elongation asymmetric delta model presented by li et al 2011 instead parasequence 6a is interpreted to be a valley fed river dominated symmetric delta protected from waves by an updrift barrier system which is analogous to the modern mongky river delta on the west coast of madagascar the progressive change in orientation of west directed mouth bars to north directed and east directed lobes around the periphery of the river dominated delta indicates a highly radial lafourche type delta moving away from the protection of the barrier system there is a gradual change from highly river dominated facies to increasingly wave and storm influenced facies toward the southeast due to the entrenched nature of the feeder river associated with an incised valley system the parasequence 6a delta was bifurcation dominated rather than avulsion dominated this study shows that it is critical to integrate detailed facies architecture analysis in order to reconstruct properly scaled paleogeographies and hypothesizes that stable bifurcation dominated deltas should be more common than avulsion dominated deltas in a lowstand delta system due to the entrenched nature of the feeding trunk river which inhibits avulsion

hummocky cross stratification like structures and combined flow ripples in the punta negra formation lower middle devonian argentine precordillera a turbiditic deep water or storm dominated prodelta inner shelf system abstract turbidity current and storm induced deposits may exhibit similarities in particularly when the latter is laid down by a combination of oscillatory and unidirectional flows recent progress in facies analysis helps to discriminate the sedimentary effects of oscillatory from unidirectional components of the flow on the basis of detailed analysis of sedimentary facies strata geometry and palaeocurrent data the present study reinterprets the punta negra formation pnf lower middle devonian argentine precordillera previously considered as a depositional system of deep water as a storm dominated prodeltaic shelf depositional system in the sandstone beds of the pnf planar low angle and undulating laminations with weakly asymmetric hummocky and swaley bedforms combined flow ripples accretionary hummocky cross stratification like hcs like and anisotropic hcs like suggest the action of oscillatory currents combined with unidirectional currents in forming the deposits different hypotheses on the origin of the oscillatory currents have been examined the most convincing interpretation is that the oscillatory component of the velocity is attributed to storm induced waves the palaeocurrent data indicate offshore current directions suggesting that the unidirectional flow was a gravity induced bottom current inverse grading at the base and overlying normally graded divisions of the sandstone beds testify to waxing waning behaviour of the depositional flows interbedding of sedimentary structures undulating laminations low angle and parallel laminations and combined flow ripples in the lower and intermediate divisions of the beds indicate fluctuations of flow velocity this organisation of the sedimentary structures permits association of the unidirectional component with hyperpycnal bottom currents the terrestrial origin of the hyperpycnal flows is suggested by the abundance of terrestrial plant remains the mineralogical and textural immaturity of the sandstone composition and the relative scarcity of bioturbation which was likely controlled by fresh water input and a high rate of sedimentation storm influenced hyperpycnal flows generated subaqueous channelised forms at the mouth of the river deltas which later filled with sand at the distal end of the channelised forms lobe shaped sandstone beds were deposited evolving distally into thin sandstone beds alternating with sandy mudstone

dynamical influence of microphysics in tropical squall lines a numerical study abstract based on environmental conditions of the 22 and 23 june squall lines during the convection profonde tropicale in 1981 copt81 experiment in west africa a series of numerical simulations are performed with a two dimensional nonhydrostatic cloud model to examine the dynamical effect of microphysics in tropical squall lines the role of ice phase microphysics strongly depends on ambient conditions for the environment of strong convective instability the ice phase is important regarding the system scale structure but is not important to the convective scale dynamics on the other hand the ice influence is crucial to the squall line convective system if the environment has a weak convective instability and is almost saturated at low levels

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

storm and tide dominated shorelines in cretaceous moosebar lower gates interval outcrop equivalents of deep basin gas trap in western canada the moosebar and gates formations crop out in the deformed foothills belt south of fort st john british columbia they permit examination of the depositional environments of sandstones and conglomerates that are gas bearing in the equivalent wilrich falher interval of the deep basin of alberta and british columbia the moosebar gates interval consists of several upward coarsening and marine to nonmarine sequences moosebar bioturbated shales pass upward into turbidites and offshore storm deposits flow directions indicate a north dipping paleoslope a second coarsening upward sequence at the top of the moosebar and base of the gates terminates in a lenticular fluvial conglomerate up to 30 m thick which is overlain by coals carbonaceous mudstones and sandstones above the carbonaceous zone a major lateral facies change is observed fluvial conglomerates in the south grade into beach conglomerates northward the upper part of the beach conglomerate consists of exposed three dimensional storm berms at least 500 m long up to 1 5 m high with wavelengths of 12 to 24 m the berms trend east west seaward of the conglomerate beach are herringbone cross bedded tidal sandstones with a marine trace fauna graded sharp based conglomerates also are present probably washed out to sea by storms farther north at the same stratigraphic level is a series of partly overlapping conglomerate bars up to 5 m thick and 40 to 50 m across the tops of these bars are covered with storm formed symmetric conglomeratic dunes details of the orientation and geometry of these conglomerate bodies along with their internal structure and pebble fabrics suggest that four major environments fluvial beach offshore conglomerate bars and offshore graded storm conglomerates should be identifiable and predictable in the subsurface

the bude formation lower westphalian sw england siliciclastic shelf sedimentation in a large equatorial lake the 1300 m thick turbiditic bude formation was deposited in a lake lake bude but disagreement persists over whether the environment was a deltaic or deep water fan the tectonic setting of the lake was the northern flank of a northerly advancing variscan foreland basin close to the westphalian palaeo equator palaeocurrents indicate sediment sourcing from all quadrants except the south there is a dm m scale cyclicity whereby sandstone bodies comprising amalgamated event beds alternate with mudstone intervals containing non amalgamated event beds the ideal cycle is a symmetrical coarsening up fining up cycle consisting of three facies i 2 and 3 arranged in 12321 order facies 3 in the middle of the cycle is an amalgamated sandstone body up to 10 m thick which interfingers laterally with thin cm mudstone layers the sandstone body comprises amalgamated beds of very fine sandstone which are largely massive and up to 0 4 m thick channels are absent except for scours up to 0 2 m deep which truncate the interfingering mudstone layers sandstone bodies are inferred to be tongue shaped in three dimensions facies 1 and 2 completing the 12321 cycle are respectively dark grey fine and light grey coarse varved mudstone containing thin 0 4 m sandstone event beds fossils and burrows indicate that facies 1 and 2 were deposited respectively in brackish rarely marine and fresh water hence the ideal cycle 12321 reflects an upward decrease then increase in salinity brackish fresh brackish this is attributed to the lake sill being periodically overtopped by the sea due to glacio euitatic sea level oscillations the resulting oscillations in lake depth produced the coarsening up fining up regressivetransgressive cyclicity the central sandstone body representing the regressive maximum event beds are interpreted as river fed turbidites deposited during catastrophic storm floods combinedflow ripples and other wave influenced structures occur in event beds throughout the ideal cycle suggesting deposition of the entire bude formation above storm wave base the proposed environment is a shelf of continental shelf dimensions but lacustrine instead of marine sandstone bodies are interpreted to be river connected tongues or lobes the absence of cycles containing nearshore or emergent facies is attributed to i the lake sill preventing the water level from falling below sill level thereby insulating the lake floor from eustatically forced emergence and ii relatively distal deposition beyond the reach of shoreline progradations the lack of palaeoflow from the south is attributed to a now eroded deep water trough lying to the south in front of the northerly advancing orogen some facies 2 laminated mudstone beds grade laterally into massive and or contorted beds interpreted as in situ seismites facies 4 consistent with an active foreland basin setting development of seismites was possibly facilitated by gas bubbles and or weak cohesion in the fresh water bottom mud the late quaternary black sea with its broad northwestern shelf is probably a good physiographical analogue of lake bude and was likewise fresh at times

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

evidence for plunging river plume deposits in the pahrump hills member of the murray formation gale crater mars recent robotic missions to mars have offered new insights into the extent diversity and habitability of the martian sedimentary rock record since the curiosity rover landed in gale crater in august 2012 the mars science laboratory science team has explored the origins and habitability of ancient fluvial deltaic lacustrine and aeolian deposits preserved within the crater this study describes the sedimentology of a ca 13 m thick succession named the pahrump hills member of the murray formation the first thick fine grained deposit discovered in situ on mars this work evaluates the depositional processes responsible for its formation and reconstructs its palaeoenvironmental setting the pahrump hills succession can be sub divided into four distinct sedimentary facies i thinly laminated mudstone ii low angle cross stratified mudstone iii cross stratified sandstone and iv thickly laminated mudstone sandstone the very fine grain size of the mudstone facies and abundant millimetre scale and sub millimetre scale laminations exhibiting quasi uniform thickness throughout the pahrump hills succession are most consistent with lacustrine deposition low angle geometric discordances in the mudstone facies are interpreted as scour and drape structures and suggest the action of currents such as those associated with hyperpycnal river generated plumes plunging into a lake observation of an overall upward coarsening in grain size and thickening of laminae throughout the pahrump hills succession is consistent with deposition from basinward progradation of a fluvial deltaic system derived from the northern crater rim into the gale crater lake palaeohydraulic modelling constrains the salinity of the ancient lake in gale crater assuming river sediment concentrations typical of floods on earth plunging river plumes and sedimentary structures like those observed at pahrump hills would have required lake densities near freshwater to form the depositional model for the pahrump hills member presented here implies the presence of an ancient sustained habitable freshwater lake in gale crater for at least ca 10 3 to 10 7 earth years

hummocky cross stratification like structures in deep sea turbidites upper cretaceous basque basins western pyrenees france by mulder et al 2009 sedimentology 56 997 1015 discussion

non uniform across shelf variations in thickness grain size and frequency of turbidites in a transgressive outer shelf the middle pleistocene kakinokidai formation boso peninsula japan abstract across shelf variations in thickness grain size and frequency of sandstone beds in a transgressive outer shelf succession were investigated from the middle pleistocene ca 0 7 ma kakinokidai formation on the boso peninsula japan the transgressive deposits are generally muddy and contain slumps and slump scars the intercalated sandstone beds are interpreted to have been formed from turbidity currents as a response to erosion and resuspension of sandridge complex deposits in the southwestern upslope area during storm events mapping of volcanic ash beds and a transgressive surface in the base of the formation permits detailed bed by bed correlation of the outer shelf sandstone beds although overall thickness grain size and frequency of sandstone beds decrease in the downslope direction some sandstone beds locally thin out and coarsen in association with slump scars in the surrounding muddy deposits these sandstone beds subsequently thicken and fine and finally thin out in the farther downslope area in addition to the local thinning of sandstone beds the frequency of sandstone beds first decreases and then increases in the farther offshore direction from this evidence we concluded that these non uniform patterns of across outer shelf variations in thickness grain size and frequency of sandstone beds were caused by the local increases in flow speeds and subsequent expansion and reduced speeds of turbidity currents along with a local increase in the seafloor gradient that was induced by the development of slump scars in the transgressive outer shelf floor these physiographic features in the outer shelf are interpreted not to have permitted monotonous downslope thinning and fining of sandstone beds compared with the bed shape models of depletive turbidity currents and with the proximality trend of shelf sandstones from modern and ancient highstand stage shelf systems

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

combined flow model for vertical stratification sequences in shallow marine storm deposited beds abstract we develop a model to help explain the wide variety of vertical stratification sequences in shallow marine sandstones this model embodied in a set of vertical sequence cartoons predicts or accounts for vertical stratification sequences in beds of free sandstone deposited from a full range of decelerating combined flows as well as from purely unidirectional and oscillatory flows according to the model vertical stratification sequences in storm deposited sandstone beds are the outcome of the time history of the bed configuration as deposition proceeds the idealized sequences are based on the initial velocities and subsequent deceleration histories of the unidirectional and oscillatory flow components twenty qualitatively different sequences starting with plane bed are generated and an additional 26 sequences result from omitting one or more of the lowermost intervals in succession the model thus accounts for 46 distinct kinds of sequences five major simplifying assumptions are built into the model bidirectional flows linear deceleration of both unidirectional and oscillatory flow components time independent deposition rate time independent sediment size and equilibrium bed configurations owing to these assumptions the model is likely to be applicable to only a small subset of vertical stratification sequences observed in shallow marine sandstones as more information on combined flow bed configurations becomes available these assumptions could be relaxed to provide a more refined and complete representation of vertical stratification sequences since the model generates vertical sequences formed from any kind of decelerating flow the bouma sequence for deposits from purely unidirectional turbidity currents and also sequences resulting from decelerating purely oscillatory flows similar in appearance to many proposed generalized tempestite sequences are end member cases in the model the model is also extended slightly to explain the geometry of the uppermost bedding surfaces of storm generated sandstones by consideration of disequilibrium effects

evaluating along strike variation using thin bedded facies analysis upper cretaceous ferron notom delta utah thin bedded delta front and prodelta facies of the upper cretaceous ferron notom delta complex near hanksville in southern utah usa show significant along strike facies variability primary initiation processes that form these thin beds include surge type turbidity currents hyperpycnal flows and storm surges the relative proportion of sedimentary structures generated by each of these depositional processes events has been calculated from a series of measured sedimentological sections within a single parasequence ps6 1 which is exposed continuously along depositional strike for each measured section sedimentological data including grain size lithology bedding thickness sedimentary structures and ichnological suites have been documented parasequence 6 1 shows a strong along strike variation with a wave dominated environment in the north passing abruptly into a fluvial dominated area then to an environment with varying degrees of fluvial and wave influence southward and back to a wave dominated environment further to the south east the lateral facies variations integrated with palaeocurrent data indicate that parasequence 6 1 is deposited as a storm dominated symmetrical delta with a large river dominated bayhead system linked to an updip fluvial feeder valley this article indicates that it is practical to quantify the relative importance of depositional processes and determine the along strike variation within an ancient delta system using thin bedded facies analysis the wide range of vertical stratification and grading sequences present in these event beds also allows construction of conceptual models of deposition from turbidity currents i e surge type turbidity currents and hyperpycnal flows and storm surges and shows that there are significant interactions and linkages of these often paired processes

constraining the efficiency of turbidity current generation from submarine debris flows and slides using laboratory experiments abstract results from a small set of laboratory experiments are presented here that help further constrain the processes governing the production of turbidity currents from impulsive failures of continental shelf and slope deposits three mechanisms by which sediment can be transferred from a parent debris flow to a less dense turbidity current were observed and quantified these mechanisms are grain by grain erosion of sediment from the leading edge of the parent flow detachment of thin layers of shearing material from the head of the parent flow and turbulent mixing at the head of the parent flow which transfer process dominates an experimental run depends on whether the large dynamic stresses focused on the head of the debris flow are sufficient to overcome a effective yield strength for the parent sediment water mixture and on whether the dynamic stresses are sufficient to induce the turbulent flow of the parent mixture analysis of data from marr et al geol soc am bull 113 2001 1377 and mohrig et al geol soc am bull 110 1998 387 support the use of a shear strength to dynamic stress ratio in constraining necessary critical values for occurrence of the different production mechanisms direct sampling of turbidity currents using racks of vertically stacked siphons was used to measure both the quantity of sediment eroded from the heads of non mixing parent flows and the distribution of particle sizes transported by the developing turbidity currents acoustic backscatter imaging was used to better resolve the internal boundary separating any turbulent mixing zone near the front of a flow from unmodified parent material

experiments on oscillatory flow and combined flow bed forms implications for interpreting parts of the shallow marine sedimentary record abstract analog laboratory experiments were conducted to investigate subaqueous bed forms generated under storm like oscillatory and combined flow experiments were carried out in a large wave tunnel and used a range of sand sizes fine and very fine wave periods 10 5 and 8 s oscillatory velocities 0 to 125 cm s and unidirectional velocities 0 to 25 cm s at low unidirectional velocities 10 cm s addition of an increasing collinear oscillatory flow caused the bed to evolve from small scale wavelength 100 cm symmetric orbital ripples to plane bed at higher unidirectional velocities 10 cm s a similar trend was noted but ripples were more asymmetric three phase diagrams are presented to summarize the observed relationships of bed configurations to flow conditions these diagrams are intended to assist in the interpretation of shallow marine sedimentary environments where oscillatory and combined flow are thought to be omnipresent distinctive features of small scale asymmetric combined flow ripples generated are a 3d planform round crest and convex up sigmoidal profile with local pronounced scour at the toe of the stoss side giving the ripple profile a boxy appearance similarly large scale asymmetric combined flow ripples had broad and round crests convex up stoss sides and compressed profiles due to scouring at the toe of the stoss side hummocky bed forms were generated under moderate to high oscillatory velocities and low unidirectional velocities hummocks were not observed as a distinct bed state but rather appeared to mark transitions in bed form scale and symmetry hummocks were more prevalent at longer oscillatory periods and in finer grained sediment stratification produced by synthetically aggrading hummocky bed profiles closely resembles hummocky cross stratification with the introduction of only a small unidirectional flow component hummocks evolve into downstream migrating large scale asymmetric ripples and the resultant cross stratification becomes similar to that produced by unidirectional flow dunes accordingly these experiments suggest that much of the hummocky cross stratification observed in the stratigraphic record is produced by storm generated long period oscillatory dominant combined flows inasmuch as long period high energy waves require deep wide basins to form hummocky cross stratification may therefore serve as a useful indicator of deposition in unrestricted open water conditions

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

desert flash floods form hyperpycnal flows in the coral rich gulf of aqaba red sea abstract running rivers are very scarce in globally distributed hyperarid and arid coastlines 28 000 km worldwide and it is ephemeral rivers that carry most terrestrial sediment into the sea in these regions however there is very little information regarding the contribution of terrestrial sediment and dynamics of transport of sediment that enter marine basins from these rivers one hyperarid region the gulf of aqaba received an exceptional number of flashflood events during the winter of 2012 2013 the results illustrate for the first time how the high volume of flashflood sediment influences the distribution of coral reefs dwarfs the contribution of airborne dust elevates floodwater densities to produce hyperpycnal flows despite highest ocean salinities and is subsequently transported to the deep basin where it may be preserved as a climate archive

hyperpycnal plume derived fans in the santa barbara channel california 1 hyperpycnal gravity currents rapidly transport sediment across shore from rivers to the continental shelf and deep sea although these geophysical processes are important sediment dispersal mechanisms few distinct geomorphic features on the continental shelf can be attributed to hyperpycnal flows here we provide evidence of large depositional features derived from hyperpycnal plumes on the continental shelf of the northern santa barbara channel california from the combination of new sonar lidar and seismic reflection data these data reveal lobate fans directly offshore of the mouths of several watersheds known to produce hyperpycnal concentrations of suspended sediment the fans occur on an upwardly concave section of the shelf where slopes decrease from 0 04 to 0 01 and the location of these fans is consistent with wave and auto suspending sediment gravity current theories thus we provide the first documentation that the morphology of sediment deposits on the continental shelf can be dictated by river generated hyperpycnal flows

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

hyperpycnites deposited 700 km away from river mouths in the central japan sea abstract relatively little is known about the characteristics of deposits and potential runout distances of hyperpycnal currents this study describes and discusses evidence of sediment deposition from sustained long duration quasi steady turbidity currents in the distal part of the toyama deep sea channel which extends ca 700 km from river mouths in the central japan sea the study is based on gravity cores and airgun seismic reflection profiles obtained from the channel distal reaches the silty turbidite beds of the channel s terminal fan show rhythmic layering that indicates sustained turbidity currents with distinct flow strength fluctuations some of the rhythmite beds show a fining upward internal trend net flow strength waning whereas others show an upward coarsening net flow strength waxing followed by fining seismic reflection profiles from the channel levees show large bedforms of climbing dune type attributed to the spillover of thick sustained turbidity currents the deposition of the terminal fan rhythmites and accretionary levee bedforms is attributed to turbidity currents generated by hyperpycnal river effluent a quantitative assessment of sediment concentrations in the coastal rivers indicates that their effluents could become hyperpycnal nearly every year or during every major seasonal flood the density of a river generated underflow would increase by the entrainment of saline water and seafloor sediment on the steep slope of the toyama bay resulting in robust long runout turbidity currents the estimated flow velocities of these currents were around 0 3 m s and their recurrence period for the last 1000 years was of the order of 70 years the estimated duration of hyperpycnal flows required for the deposition of rhythmite beds 700 km away from the river mouth is of the order of several days to 3 4 weeks the study provides new insights into the recognition and classification of hyperpycnites in distal zones of deep marine turbiditic systems

ichnology of the laisong formation late eocene early oligocene of the naga hills indo burma range ibr paleoenvironmental significance abstract the late eocene early oligocene laisong formation basal barail group from the naga hills indo burma range has yielded a low diversity of trace fossils i e chondrites gyrolithes ophiomorpha palaeophycus planolites and thalassinoides three ichnofabrics namely ophiomorpha palaeophycus thalassinoides and chondrites planolites have been identified the ophiomorpha palaeophycus ichnofabric grades upsection into chondrites planolites ichnofabric in a fine to medium grained sandstone unit through a succession of alternating thinly bedded sandstone and mudstone containing thalassinoides ichnofabric the ichnofabric distribution is primarily controlled by the change in depositional energy levels from low to high a low diversity trace fossils and lack of graphoglyptids in the laisong formation of naga hills supports the presence of a proximal part of the hyperpycnal lobes of delta fed marine coarse grained turbidite system

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

a theory for strong long lived squall lines revisited based on the analysis of idealized two and three dimensional cloud model simulations rotunno et al hereafter rkw and weisman et al hereafter wkr put forth a theory that squall line strength and longevity was most sensitive to the strength of the component of low level 0 3 km agl ambient vertical wind shear perpendicular to squall line orientation an optimal state was proposed by rkw based on the relative strength of the circulation associated with the storm generated cold pool and the circulation associated with the ambient shear whereby the deepest leading edge lifting and most effective convective retriggering occurred when these circulations were in near balance since this work subsequent studies have brought into question the basic validity of the proposed optimal state based on concerns as to the appropriate distribution of shear relative to the cold pool for optimal lifting as well as the relevance of such concepts to fully complex squall lines especially considering the potential role of deeper layer shears in promoting system strength and longevity in the following the basic interpretations of the rkw theory are reconfirmed and clarified through both the analysis of a simplified two dimensional vorticity streamfunction model that allows for a more direct interpretation of the role of the shear in controlling the circulation around the cold pool and through an analysis of an extensive set of 3d squall line simulations run at higher resolution and covering a larger range of environmental shear conditions than presented by wkr

effects of settling and preferential deposition of sediment on ripple roundness under shoaling waves abstract a comprehensive series of wave flume experiments were performed to examine the effects of settling of entrained sediment on ripple roundness using three sizes of well sorted quartz sand d50 0 32 0 49 0 73 mm the observed ripple profiles were closely examined with reference to ripple roundness and ripple asymmetry in order to quantify the effect of sediment settling settling time factor s w0t is ripple height w0 is settling velocity and t is wave period is newly introduced it is shown that ripples developed under shoaling waves manifest the following geometrical features pertaining to ripples 1 the measured performance of ripple roundness correlates systematically with s irrespective of grain size 2 the preferential deposition of entrained sediment on the ripple crest and the onshore side slope of each ripple is accentuated under conditions of smaller s such deposition takes place under shoaling waves when the oscillatory flow direction is reversed from onshore to offshore thereby allowing the entrained sediment to stall to some extent prior to being subjected to the subsequent peak offshore going flow thus round crested ripples ensue 3 the way in which preferential deposition occurs under shoaling waves is similar to the deposition process that occurs under combined flows on rippled beds yokokawa et al 1995 4 this reasoning led to a comparison of the ripple roundness and ripple symmetry in both flow regimes ripples formed under shoaling waves are in cases significantly round crested with marked asymmetry in contrast equally round crested ripples under combined flows yokokawa 1995 are commonly more symmetrical in profile thus combined flow ripples can be identified from the geological record using their degree of roundness and symmetry of their profile

shallow water marl limestone alternations in the late jurassic of western france cycles storm event deposits or both abstract the contribution of event deposits to various basin fills can be very significant higher than 90 in some cases events may lead to the formation of marl limestone alternations which can also result from cyclic changes in sea level or climate for example the marl limestone alternations of the late jurassic of western france contain abundant coarse grained accumulations that resemble storm deposits described in other western european successions the detailed analysis of facies evolution and hierarchical high frequency stacking pattern of depositional sequences of the phare de chassiron section ile d oleron western france allows the controls on marl limestone formation to be defined this section contains nearshore and shallow marine mud deposits that were exposed to high energy events elementary small and medium scale depositional sequences are defined the stacking pattern and the duration of these sequences suggest an orbital control on sedimentation precession 20 ka cycles notably controlled the formation of elementary sequences that correspond to marl limestone alternations the deposition of marly or carbonate mud occurred in this storm dominated system because of muddy sea beds the gentle slope of the shelf and the great amount of particles in suspension which reduced water energy resulting from storms sediment supply was also sufficient to limit bioturbation and favour the preservation of numerous storm deposits the production of carbonate mud was localised on positive structures and partly controlled by milankovitch scale sea level cycles transport by storms of carbonate mud to the adjacent marly depressions during high carbonate production periods led to the formation of calcareous beds marl limestone alternations in the late jurassic of western france therefore result from the combined effects of cyclic changes in carbonate production and high energy episodic events

genesis and evolution of hierarchical cloud clusters in a two dimensional cumulus resolving model abstract a two dimensional cloud ensemble model is integrated over a basin scale domain with prescribed sea surface temperature sst to study the formation and evolution of cloud clusters over a large scale warm pool neither a basic zonal flow is prescribed nor is a single perturbation initially given the results show that deep convective clouds appear in hierarchical clustered patterns and are limited to the area of warm sst above 28 c the most fundamental cloud cluster in the model has a horizontal scale of a few hundred kilometers in which new cumulus clouds are generated at the leading edge of a propagating surface cold air pool the gust front it may last for days and propagate for a long distance if the background flow is broad and persistent as is the case in the low level convergence zone of the sst induced background flow the largest hierarchical propagating cloud systems in the model have horizontal scales up to 3000 km and consist of up to four cloud clusters that are generally of gust

relationship of diagenetic chlorite rims to depositional facies in lower cretaceous reservoir sandstones of the scotian basin the relationship between diagenetic chlorite rims and depositional facies in deltaic strata of the lower cretaceous missisauga formation was investigated using a combination of electron microprobe bulk geochemistry and x ray diffraction data the succession studied comprises several stacked parasequences the delta progradational facies association includes i fluvial or distributary channel sandstones some with tidal influence ii thick bedded delta front graded beds of sandstone interpreted as resulting from fluvial hyperpycnal flow during floods and storms and iii more distal muddier delta front and prodeltaic facies the transgressive facies association includes lag conglomerate siderite cemented muddy sandstone and mudstone and bioclastic sandy limestone chlorite rims are absent in the fluvial facies and best developed in thick sandstones lacking mudstone baffles good quality chlorite rims are well correlated with ti in bulk geochemistry ti is a proxy for fe availability principally from the breakdown of abundant detrital ilmenite fetio3 under conditions of sea floor diagenesis the abrupt decrease in sedimentation rate at transgressive surfaces caused progressive shallowing of the sulphate depletion level and of the overlying eh controlled diagenetic zones resulting in conditions suitable for diagenetic formation of berthierine to migrate upwards through the packet of reservoir sandstones this early diagenetic berthierine suppressed silica cementation and later recrystallized to chlorite thick euhedral outer chlorite rims were precipitated from formation water in sandstone lacking muddy baffles on this chlorite substrate and inhibited late carbonate cementation this study thus shows that the preservation of porosity by chlorite rims is a two stage process rapidly deposited delta front turbidite facies create early diagenetic conditions that eventually lead to the formation of chlorite rims but the best quality chlorite rims are restricted to sandstones with high permeability during burial diagenesis

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

minturn and sangre de cristo formations of southern colorado prograding fan delta and alluvial fan sequence shed from ancestral rocky mountains abstract the middle pennsylvanian minturn formation and the pennsylvanian and permian sangre de cristo formation of the northern sangre de cristo range form a 4 000 m 13 000 ft thick progradational sequence of fan delta and alluvial fan deposits this sequence was deposited along the western margin of the central colorado trough during faulting and uplift of the late paleozoic uncompahgre highland of the ancestral rocky mountains the minturn formation is composed mostly of sandstone and shale deposited by fan deltas that prograded into the central colorado trough the minturn of the northern sangre de cristo range is divisible into 1 a turbidite bearing facies 2 a limestone bearing facies and 3 a red bed facies the turbidite bearing facies is interpreted as deposits of fan deltas that prograded onto the sea bottom below wave base the limestone bearing facies is interpreted as deposits of fan deltas that prograded onto a shallow sea bottom above wave base turbidites and shallow marine limestones although they make up only a minor part of the minturn formation are mutually exclusive deposits that serve to distinguish the two facies in the limestone bearing facies sandstones containing deltaic fore ets overlie thin shallow marine limestones and are considered diagnostic of that facies both facies contain thick intervals of sandstone and shale interpreted as deltaic and alluvial deposits where it onlaps the uncompahgre highland the lower part of the minturn formation contains quartzose and arkosic red beds of probable alluvial origin the continental sangre de cristo formation conformably overlies the minturn formation basinward but it unconformably overlies minturn formation and precambrian basement near the uncompahgre highland the sangre de cristo formation contains 1 a sandstone facies deposited on the distal surfaces of alluvial fans and 2 a conglomerate facies crestone conglomerate member deposited on the proximal surfaces of alluvial fans the sandstone facies consists of fining upward cycles of red conglomeratic sandstone and siltstone interpreted as braided stream deposits the conglomerate facies consists of poorly sorted conglomerates interpreted as debris flow and mudflow deposits and sorted conglomerate and sandstone interpreted as streamflow and sheetflow deposits end of article last page 854

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

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

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

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

lateral variation in sandstone lithofacies from conventional core scotian basin implications for reservoir quality and connectivity1 2 1this article is one of a series of papers published in this cjes special issue on the theme of mesozoic cenozoic geology of the scotian basin 2geological survey of canada contribution 20120021 facies analysis in outcrops on land is strongly dependent on the lateral variability of lithofacies interpretation of conventional core in wells relies principally on the vertical succession of li

magnetic susceptibility of the quaternary red clay in subtropical china and its paleoenvironmental implications abstract magnetic susceptibility lf and other magnetic parameters of 11 typical profiles of quaternary red clay qrc in subtropical china were measured lf curves of the qrc profiles show similar vertical variations lf of the uppermost yellow brown earth ybe and uniform red clay urc of the profiles is high mostly ranging from 50 to 150 10 8 m 3 kg 1 that of the reticulate red clay rrc sharply decreases downwards especially in the lower rrc lrrc with dense reticulate pattern lf is often less than 10 10 8 m 3 kg 1 grain size characteristics and geochemical properties of the qrc suggest that the sharp decrease of lf in the rrc is not related to sudden change of parent materials but to the formation of the reticulate pattern further study indicates that pedogenic fine ferrimagnetic minerals fm superparamagnetic particles sp and or stable single domain ssd are the main magnetic carriers of the qrc the ybe and urc with higher lf show low coercivity and magnetic soft behavior the rrc especially the lrrc with weak magnetism shows high coercivity and magnetic hard behavior the temperature dependent susceptibility tds of the qrc suggests that the dominant type of fm in the ybe and urc is pedogenic maghemite the rrc especially the lrrc however contains little maghemite but a trace amount of lithogenic multi domain md magnetite inherited from the parent rock during post depositional hydromorphic processes pedogenic maghemite was more easily dissolved than hematite which caused the sharp decrease of lf in the rrc as lf of the qrc was mainly controlled by the intensity of hydromorphic processes rather than the weathering degree lf of the qrc in subtropical china cannot accurately indicate the paleoclimate during the quaternary period when the red clay was initially formed

hummocky cross stratification like structures in deep sea turbidites upper cretaceous basque basins western pyrenees france hummocky cross stratification is a sedimentary structure which is widely interpreted as the sedimentary record of an oscillatory current generated by energetic storm waves remobilizing surface sediment on the continental shelf sedimentary structures named hummocky cross stratification like structures similar to true hummocky cross stratification have been observed in the turonian senonian basque flysch basin south west france the bathymetry 1000 to 1500 m suggests that the observed sedimentary structures do not result from a hydrodynamic process similar to those acting on a continental shelf the morphology of these three dimensional structures shares similarities with the morphology of hummocky cross stratification despite a smaller size the lateral extent of these structures ranges from a few decimetres to many decimetres they consist of convex up domes hummock and concave up swales with a non erosive base four types of hummocky cross stratification like geometries are described they occur in association with structures such as climbing current ripple lamination and synsedimentary deformations in the basque flysch hummocky cross stratification like structures are only found in the tc interval of the bouma sequence hummocky cross stratification like structures are sporadic in the stratigraphic series and observed only in few turbidite beds or bed packages this observation suggests that hummocky cross stratification like structures are linked genetically to the turbidity current but form under a very restricted range of parameters these structures sometimes show an up current upslope migration trend antidunes in the described examples they could result from standing waves forming at the upper flow interface because of kelvin helmholtz instability

sand particle movement on migrating combined flow ripples abstract we studied sand particle movement on migrating combined flow ripples in an experimental flume the water surface waves propagated against the flow in the flume the resultant combined flow ripples have characteristic rounded profiles motion pictures reveal that vortices generated only close to the lee sides of ripples are responsible for the rounded profile these vortices excavate rounded troughs and lift sand particles the lee sides are nourished by the sand contributing a convex upward profile the flow that separates at the crests is dragged into the vortices at the troughs so that there is no clear reattachment point with focused erosion as is the case for current ripples sand that fulls to the bed from a floating cloud over the stoss side nourishes the convex upward roun ed stoss side the relative strength of the oscillatory flows controls the positions at which the vortices lift sand particles to deposit and determine whether the ripple profiles are rounded or sharp a lower relative oscillatory flow velocity yields smaller vortices which cause rounded ripple profiles conversely a higher relative oscillatory flow velocity results in larger vortices and sharper crests the movement of sand particles is unlike that on ripples beneath either purely oscillatory flows or unidirectional flows

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

bottomset and foreset sedimentary processes in the mixed carbonate siliciclastic upper jurassic lower cretaceous vaca muerta formation picun leufu area argentina abstract the upper jurassic lower cretaceous vaca muerta formation at the picun leufu area constitutes bottomset and foreset marine deposits comprising open bay siliciclastic basin and mixed carbonate siliciclastic shelf facies detailed sedimentological ichnological and sequence stratigraphic analysis of six stratigraphic sections allows establishing two depositional sequences depositional sequence 1 ds1 begins with lowstand eolian deposits included in the quebrada del sapo formation and continues with thin retrograding transgressive open bay facies of the vaca muerta formation encompassing coastal sand sheet and dune marginal marine bay margin and distal bay facies above the transgressive deposits the regressive hemicycle of ds1 consists of bottomset deposits representing a siliciclastic basin facies association lobe and lobe fringe hyperpycnal flow sedimentation alternated with hemipelagic basinal deposition in an oxygen deficient environment a relative sea level fall generated a sequence boundary which coincides with an angular unconformity that marks the base of depositional sequence 2 ds2 this sea level fall triggered the formation of an extensive lowstand channel fill and lobe complex at the base of the slope followed by retrogradational lobe facies during the subsequent transgression the regressive hemicycle of ds2 represents foreset mixed carbonate siliciclastic shelf facies forming a slope mud belt and slope sand bodies near the top sedimentation in a foreset topset transition resulted in the accumulation of bioturbated mixed slope and sandy shoal deposits sand bars and lagoonal facies occur on top of all sections and are truncated by a sequence boundary the combined analysis provides insights into the sedimentary processes affecting bottomset and foreset deposition underscoring the role of wave influenced hyperpycnal flows and the effect on organic matter dilution in one of the most important unconventional reservoirs from argentina