patterns of deposition from experimental turbidity currents with reversing buoyancy turbidity currents are turbulent sediment laden gravity currents which can be generated in relatively shallow shelf settings and travel downslope before spreading out across deep water abyssal plains because of the natural stratification of the oceans and or fresh water river inputs to the source area the interstitial fluid within which the particles are suspended will often be less dense than the deep water ambient fluid consequently a turbidity current may initially be denser than the ambient sea water and propagate as a ground hugging flow but later reverse in buoyancy as its bulk density decreases through sedimentation to become lower than that of the ambient sea water when this occurs all or part of the turbidity current lofts to form a buoyant sediment laden cloud from which further deposition occurs deposition from such lofting turbidity currents containing a mixture of fine and coarse sediment suspended in light interstitial fluid is explored through analogue laboratory experiments complemented by theoretical analysis using a box and cloud model particular attention is paid to the overall deposit geometry and to the distributions of fine and coarse material within the deposit a range of beds can be deposited by bimodal lofting turbidity currents lofting may encourage the formation of tabular beds with a rapid pinch out rather than the gradually tapering beds more typical of waning turbidity currents lofting may also decouple the fates of the finer and coarser sediment depending on the initial flow composition the coarse fraction can be deposited prior to or during buoyancy reversal while the fine fraction can be swept upwards and away by the lofting cloud an important feature of the results is the non uniqueness of the deposit architecture different initial current compositions can generate deposits with very similar bed profiles and grading characteristics highlighting the difficulty of reconstructing the nature of the parent flow from field data it is proposed that deposit emplacement by lofting turbidity currents is common in the geological record and may explain a range of features observed in deep water massive sands thinly bedded turbidite sequences and linked debrites depending on the parent flow and its subsequent development for example a lofting flow may lead to a well sorted largely ungraded or weakly graded bed if the fines are transported away by the cloud however a poorly sorted largely ungraded region may form if during buoyancy reversal high local concentrations and associated hindered settling effects develop at the base of the cloud

experiments on non channelized turbidity currents and their deposits abstract large scale experiments on non channelized turbidity currents show that a wide flow opening angle forms and a rapid dilution of the current with distance takes place the thickness of the deposit decreases radially away from the source resulting in tongue shaped isopachs the mean grain diameter also decreases with distance while the sorting improves with increasing distance the following succession of bedforms was observed non deposition parallel lamination ripples parallel lamination this corresponds to the b c and d division of the bouma sequence for turbidites the experiments are in good agreement with the models presented by bouma mutti and ricci lucchi and walker for classical turbidites in the depositional lobes of submarine fans

conditions for formation of massive turbiditic sandstones by primary depositional processes it can be shown from bedform stability diagrams that each turbidity current regardless of whether its density is low or high or whether its history is simple e g surging flow or complex e g sustained flow should pass through a final waning phase in which flow conditions are prone to bedform development yet many thick deep marine sandstones of supposed turbidity current origin are totally devoid of tractional structures such as plane parallel lamination and ripple cross lamination tdure a mathematical model for the calculation of duration of sediment deposition from high density turbidity currents hdtcs is used to show that the absence of bouma tb and tc divisions cannot be explained by rapid temporal flow deceleration only the model calculations imply that discernible tractional structures are formed even at residence times within the upper stage plane bed and ripple stability regimes of several seconds and several tens of seconds respectively therefore other explanations have to be sought for the absence of lamination in turbiditic massive sandstones these include incompatible grain size post depositional removal or obstruction of lamination by bioturbation and liquefaction and erosion of laminated divisions followed by rapid burial

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

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 setting of the 2 1 ga francevillian macrobiota gabon rapid mud settling in a shallow basin swept by high density sand flows the depositional setting of the 2 1 ga fill to the franceville basin of gabon is important for understanding the habitat energy and availability of light and oxygen and taphonomy of recently discovered early macro organisms buried in black shales in unit fb the available data bearing on the stratigraphy and sedimentology of unit fb provide new insight into processes acting on the palaeo seafloor the shales are interpreted to have formed as fluid mud deposits interstratified with structureless sands the latter poubara sandstones were emplaced during a forced regression during the terminal infill of fault bounded sub basins following a stage characterised by a ferruginous to anoxic water column the structureless sandstones were deposited from high density gravity currents along with a locally strong bottom oscillation of the water column tuft structures preserved in cyanobacterial mats together with the position of the macro organisms at the top of the sandstone beds within associated black shales point to a water depth of less than 80 m the relative sea level fall that drove deposition of the poubara sandstones controlled the rise of a phototrophic ecosystem and also possibly favoured the supply of oxygen and nutrients via density flows this article is protected by copyright all rights reserved

depositional characteristics and reservoir potential of paleogene sediment gravity flow deposits on a faulted slope of the zhanhua sag bohai bay basin china abstract deep water sandstones related to sediment gravity flows are becoming an increasingly important exploration domain in continental rift basins in china however ambiguities remain regarding the initiation evolution and deposition of sediment gravity flows on faulted slopes where step faults are commonly developed to address the uncertainties related to these processes grain size analysis lithofacies analysis logging interpretation and seismic interpretation were employed to study paleogene sediment gravity flow deposits from well cores on a faulted slope in the zhanhua sag of the bohai bay basin twelve lithofacies related to sediment gravity flows were recognized and attributed to intrabasinal sediment gravity flows isgfs induced by slope failures and extrabasinal sediment gravity flows esgfs generated by floods active faults were important triggers for isgfs on the faulted slope in the zhanhua sag the fault activity rate had a prominent role in determining the volume of re transported sediments which further influenced the evolution of the resulting flows travelling across multiple faults downslope isgfs generally underwent considerable velocity losses with the result that almost all sediments were deposited in shallow water moreover unconsolidated isgf deposits could be transported again to generate sediment gravity flows obscuring the depositional characteristics of isgfs esgfs originated from rivers during floods and involved the transformation of cohesive into turbulent hyperpycnal flows in humid and warm climates active sag border faults favoured the generation of long lived esgfs these flows usually travelled along intra sag faults and therefore considerable volume of sediments could be transferred into deep water the resulting thick bedded hyperpycnites contributed substantially to the reservoirs of bonan oilfield meanwhile short lived esgfs usually generated in the regions where sag border faults were extinct they generally travelled down the faulted slope and deposited sediments in shallow water these features suggest that the initiation evolution and deposition of sediment gravity flows on the faulted slope are mainly controlled by faults and climate hyperpycnites can serve as excellent reservoirs for hydrocarbon accumulation and they are probably common in other sags of the bohai bay basin

a preliminary experimental study of turbidite fan deposits we present preliminary laboratory experimental results that demonstrate lobe switching in a turbidite fan after a significant number of individual event beds more than eight in both experimental fans made in this study the flow began to focus itself into a lobe at the center of which appeared a subtle channel form initial lobe switching occurred after nearly twenty event beds subsequent switching appeared to scale with lobe size and sedimentation rate the densest coarsest runs tended to initiate the switching of the depocenter instabilities within the turbidity current itself appear to be responsible for initial flow confinement to a lobe and ultimate switching events currently untreated scale effects and measurement difficulties limit quantitative extension of the results to natural settings though some general relationships are proposed

basin floor fans in the north sea sequence stratigraphic models vs sedimentary facies discussion original article shanmugan g r b bloch s m mitchell g w j beamish r j hodgkinson j e damuth t straume s e syvertsen and k e shields 1995 aapg bulletin v 79 p 477 512

reappraisal of the cambrian glauconite bearing anse maranda formation quebec appalachians from deep sea turbidites to clastic shelf deposits the cambrian anse maranda formation is a glauconite bearing clastic succession within the quebec reentrant it is part of the appalachians humber zone particularly of the shale feldspathic sandstone assemblage which represents the rift to drift deposits of the laurentian margin the massive sandstones of the anse maranda were originally interpreted as the proximal turbidites of a deep sea fan the occurrence of glauconite and the high degree of bioturbation which give the sandstones a massive aspect suggest a different depositional environment detailed ichnology and lithofacies descriptions indicate that the anse maranda predominantly represents shelf sediments deposited in storm influenced environment and that intense bioturbation destroyed most of the sedimentary structures the paleo geographic reconstruction suggests that the anse maranda was deposited seaward of a headland identified as the montmorency promontory along a narrow shelf with an irregular topography some coeval turbidites of sim

how coriolis forces can limit the spatial extent of sediment deposition of a large scale turbidity current abstract the rotation rate of the earth controls the deposition scale of turbidity currents when their flow transit time is comparable to one day using laboratory experiments it is shown that the maximum length scale l of deposition is set by the scale where the rossby number of the current is equal to one the dimensionless rossby number is defined as ro u fl where u is a depth averaged velocity of the turbidity current and f is the coriolis parameter in these new laboratory experiments the coriolis parameter is varied by changing the platform rotation rate and there is good agreement between experimental observations and the prediction that the extent of the deposition scales as l 4 1 4 g v f 1 4 where g is the reduced gravity and v the initial volume of the sediment laden material this scaling implies that the depositional patterns will vary with latitude and that at higher latitudes one would expect smaller thicker deposits using this scaling implies that the 200 400 km spatial extent of the turbidite arising during the 1929 grand banks earthquake had ro 1 and so coriolis forces controlled the length scale of the turbidite deposition

spatio temporal evolution of velocity structure concentration and grain size stratification within experimental particulate gravity currents we describe a flume study of the spatio temporal evolution of particulate gravity currents time series of the vertical structure of flow in terms of the forward component of velocity flow concentration and grain size of suspended sediment were co measured at a different position along the flume for each of a series of nominally identical flows the data are combined to show the spatial evolution of a single idealised flow such a flow fed into the flume from an overhead reservoir first propagates as a quasi steady jet the subsequent evolution of coherent spatial and temporal trends in velocity grain size and concentration are developed by the internal action of the flow itself rather than being inherited from the flow generation mechanism quasi steady input currents evolved down flume into surge type flows that wax very rapidly upon arrival then progressively wane thus the velocity history of currents at source may differ from that experienced downstream indicating that flow steadiness measured or interpreted in downstream positions may not necessarily be indicative of the flow generation mechanism the fore most parts of the flow travelled more rapidly than the hind most parts and thus gradually drew further away as a consequence the duration of the experimental currents systematically increased along the length of the flume the flows stretched out natural scale flows may therefore wane more rapidly in proximal regions than distal ones this may impact upon sedimentation style with higher sedimentation rates potentially more prevalent in proximal than in distal regions grain size maxima are recorded in the head of the current convincing evidence of coarse tail lag behind the head is only patchily developed within the flow body a consistent pattern of upward fining then coarsening is observed this can be related to an upward flux of coarse particles from the head which subsequently settle downwards into the body at the natural scale such a phenomenon may have implications for the development of coarse tail grading q 2003 elsevier ltd all rights reserved

duration of deposition from decelerating high density turbidity currents using recent advances in the stability analysis of current ripples a new model for the calculation of duration of sediment deposition from decelerating high density turbidity currents is proposed the model named tdure refines a duration model proposed by allen 1991 j sediment petrol 61 291 295 by calculating the accumulation time of the rippled tc division separately from the massive and plane parallel laminated tab divisions in bouma type turbidites tdure consists of three modules in the first module the accumulation time for theta and tb divisions is approximated by assuming a linear decrease in sedimentation rate with height in the turbidite as in the original model a straight line is fitted through inferred sedimentation rates at the tab and tbc boundaries in the second module angle of climb of ripples thickness of the tc division and grain size distribution are used in empirical relationships between ripple migration rate and a grain related mobility parameter to estimate the accumulation time of the tc division in the third module the expected development of ripple size across the tc division is calculated using empirical relationships between rate of development of ripple height and grain related mobility parameter and subsequently compared with the observed development of ripple size in the turbidite in this way the accuracy of the accumulation time calculated in module 2 can be verified independently tdure was tested using bouma type turbidites from the doheny channel capistrano fm ca usa and the flysch di motta calabria italy accumulation times of 12 and 10 75 min for the tc division and 19 and 16 min for the tabc sequence were calculated for the doheny channel and the flysch di motta turbidites respectively although module 3 underestimates the rate of development of current ripples near the tbcboundary in both beds ripple size at the tcd boundary is calculated accurately the underestimation of development rate may be caused by differences between flow conditions in experimental flumes on which the model calculations are based and turbiditycurrent dynamics q 2000 elsevier science b v all rights reserved

classification and sedimentary characteristics of lacustrine hyperpycnal channels triassic outcrops in the south ordos basin central china abstract subaquatic channels are known as active conduits for the delivery of terrigenous sediments into related marine and lacustrine basins as well as important targets for hydrocarbon exploration compared to submarine channels lacustrine subaqueous channels created by hyperpycnal flows are understudied using well exposed outcrops collected from three different locations in the southern ordos basin central china morphologies and architecture of a channelized hyperpycnal system were studied and classified six facies associations represent sedimentary processes from strong erosion by bedload dominated hyperpycnal flows to transitional deposition jointly controlled by bedload and suspended load dominated hyperpycnal flows finally to deposition from suspended load dominated hyperpycnal flows on the basis of channel morphologies infilling sediments and sedimentary processes the documented channels can be classified into four main categories which are erosional bedload dominated suspended load dominated and depositional channels in very proximal and very distal locations erosional channels and depositional channels serve as two end members while in middle areas bedload dominated channels and suspended load dominated channels are transitional types erosional channels as a response to strong erosion from bedload dominated hyperpycnal flows on upper slope were mainly filled by mud interbedded with thin sand beds as flow energy decreases bedload dominated channels develop on middle slopes which are characterized mainly by under to balanced sediment infillings with cross bedded sandstones and or minor massive sandstones compared to bedload dominated channels suspended load dominated channels mainly develop in deeper water and were filled mainly by massive or planar laminated sandstones depositional channels as a response to suspended load dominated hyperpycnal flows in deep water areas are characterized by thin medium bed classical turbidites with bouma sequences and thin to thick massive sandstones such evolution patterns of hyperpycnal channel systems are ascribed to the progressive decrease in flow capacity of hyperpycnal flows and provide an adequate explanation for the basinward channelization behavior of hyperpycnal systems

directional petrological characterisation of deep marine sandstones using grain fabric and permeability anisotropy methodologies theory application and suggestions for integration abstract different methods for quantifying preferred grain orientation or grain shape fabric in sandstones are reviewed and their value for the reconstruction of palaeocurrent direction and preferred direction of pore fluid flow is reassessed critically based on existing knowledge from literature and new grain fabric data it is shown that petrographic analysis has distinct merits because grain fabric can be determined with low equipment costs and population statistics are based on the orientation of individual grains several recommendations are given how to standardise procedures for acquisition of grain fabric through microscopy these recommendations include 1 the careful selection of cross section orientations to acquire apparent long grain axes that are representative of true long grain axes 2 the preferred measurement of the coarsest grains in cross section and 3 a new statistically founded method to select a subset of grains that is representative of the total population of grains in a sandstone sample magnetic techniques that use anisotropy of magnetic susceptibility ams as a proxy for grain orientation require specialised equipment and are more restricted in statistical evaluation but these methods benefit greatly from being able to measure a large number of grains in a three dimensional space in a short amount of time and with a lower sensitivity to user bias of the two magnetic techniques discussed in this paper the new enhanced ams method in which rock magnetic properties are enhanced by introducing magnetite into the pores and as precipitates on grain surfaces is regarded to be an improvement over the traditional natural ams method in which only the natural assemblage of magnetite particles present in the rock is analysed enhanced ams measures the preferred orientation of essentially all grains in a rock sample thus providing the shape fabric of a hydrodynamically more accurate representation of framework grains than natural ams through a small technical modification the enhanced ams method can be used to obtain a magnetic cast of the pore network of a rock sample so that the resulting ams is controlled by the shape anisotropy of the individual pores it is commonly assumed that long axes of sand grains in sedimentary deposits parallel the direction of the flows that formed the deposits this concept is challenged by a new systematic study of grain fabric in deep marine turbidites it indeed appears that grains in ripple cross laminated divisions of turbidites dominantly attain a preferred orientation parallel to the palaeoflow direction but grains in plane parallel laminated divisions have an additional flow perpendicular mode which suggests a contribution of rolling grain transport three different fabric modes occur in massive structureless sandstones and massive divisions in turbidites apart from flow aligned and flow perpendicular modes there is a statistically significant but hydrodynamically enigmatic flow oblique mode with an average azimuthal deviation of 40 this large variation of fabric types suggests that a similar variation exists in preferred paths of pore fluid flow within turbidites controlled to at least some degree by depositional process this is confirmed by independent laboratory measurements of permeability anisotropy on samples for which grain fabric data are available and thus defines grain fabric analysis as a viable alternative to traditional permeametry techniques

a fluid dynamic model of volcaniclastic turbidity currents based on the similarity with the lower part of dilute pyroclastic density currents evaluation of the ash dispersal from ash turbidites subaqueous sediment density currents have variable flow structures depending on particle size and volumetric concentration they form both in ocean basins and in volcanic districts where subaqueous density currents are generated by collapse of slope and primary pyroclastic material entering the sea respectively a particular type of such flows is represented by volcaniclastic turbidity currents which form by the subaqueous remobilization of primary pyroclastic deposits in this work a fluid dynamic model of volcaniclastic turbidity currents is presented and applied to the currents that generated the craco southern italy volcaniclastic deposits the deposits consist of thick laminated to massive beds which formed from the remobilization of thin fine ash pyroclastic fall deposits in an area far away from the original volcanic source our model is based on the sedimentological features of the deposits and particle physical characteristics it allows the calculation of velocity thickness and reynolds number of the currents furthermore a relationship between the current thickness laminae thickness and paleo slope angle is obtained which allows to evaluate the thickness of the primary pyroclastic fallout deposits our model can be useful for obtaining an estimation of the primary distal ash dispersal by the characteristics of the associated subaqueous volcaniclastic deposits

re sedimented deposits in the rift section of the campos basin abstract an integrated petrologic sedimentologic stratigraphic seismic study of the lagoa feia group rift section of the campos basin has shown that rift sedimentation was dominantly intrabasinal carbonate and stevensitic deposits with siliciclastic deposits restricted to the proximity of graben border faults the bivalve bioclastic rudstones coquinas that constitute the rift reservoirs show commonly limited abrasion of the bioclasts and were deposited both on basement highs and lows throughout the rift section although in situ deposits of stevensite ooids and peloids occur dominantly at the base of the succession these particles are ubiquitous to the entire rift section mixed in variable proportion with siliciclastic and carbonate sediments the environmental conditions required for the formation of stevensite and the growth of bivalves are mutually exclusive as stevensite forms only at ph greater than 10 while bivalves cannot tolerate ph greater than 9 the common mixture of well rounded basaltic rock fragments with angular granitic gneissic rock fragments and feldspars in the sandstones and conglomerates indicates recycling of epiclastic deposits from the early rift section combined with first cycle contribution from the plutonic basement the studied cores show no evidence of subaerial exposure and there is a lack of bioturbation suggesting harsh environmental conditions the rift deposits are dominantly massive or faintly laminated with diffuse facies boundaries structures indicative of unidirectional or oscillatory flow are subordinate integration of seismic sedimentologic and petrographic evidence indicates that the campos basin rift section is formed mostly by re sedimented gravitational deposits the onset of the rift sedimentation occurred in synformal depressions where bivalve banks or stevensite ooids were formed in shallow lacustrine environments under variable alkalinity conditions with the development of half grabens and concentration of the tectonic activity along the border faults recurrent tectonic events promoted the mixing and gravitational re deposition of stevensitic clastic and bioclastic sediments in deeper fault bounded troughs large scale units hundreds of meters thick were generated by major tectonic events whereas compositional variations in the scale of meters were possibly a product of lake level climatic fluctuations thus due to syn rift and mainly to post rift erosion the neo aptian unconformity the preserved rift section of the lagoa feia group comprises mostly sediments deposited in the central troughs of the half graben structures marginal sediments were extensively eroded and re sedimented as gravity driven mixed deposits this new model constructed form the integration of seismic stratigraphic sedimentologic and petrologic data diverges substantially from the presently accepted model for the sedimentation of the rift section opening new possibilities for the exploration of campos basin as well as of similar settings as in the adjacent santos basin

heterozoan carbonate deposition on a steep basement escarpment late miocene almeria south east spain heterozoan temperate water carbonates mixed with varying amounts of terrigenous grains and muddy matrix azagador limestone accumulated on and at the toe of an inherited escarpment during the late tortonian early messinian late miocene at the western margin of the almeria nijar basin in south east spain the escarpment was the eastern end of an uplifting antiform created by compressive folding of triassic rocks of the betic basement channelized coralline algal bryozoan rudstone to coarse grained packstone together with matrix supported conglomerate are the dominant lithofacies in the higher outcrops comprising the deposits on the slope these sediments mainly fill small canyon shaped half graben depressions formed by normal faults active before during and after carbonate sedimentation roughly bedded and roughly laminated coralline algal bryozoan rudstone to coarse grained packstone are the main lithofacies forming an apron of four small kilometre scale lobes at the toe of the south eastern side of the escarpment almeria area channelized and roughly bedded coralline algal bryozoan rudstone to coarse grained packstone conglomerates packstone and sandy silt accumulated in a small channel lobe system at the toe of the north eastern side of the escarpment las balsas area carbonate particles and terrigenous grains were sourced from shallow water settings and displaced downslope by sediment density flows that preferentially followed the canyon shaped depressions roughly laminated rudstone to packstone formed by grain flows on the initially very steep slope whereas the rest of the carbonate lithofacies were deposited by high density turbidite currents the steep escarpment and related break in slope at the toe favoured hydraulic jumps and the subsequent deposition of coarse grained low transport efficiency skeletal dominated sediment in the apron lobes accelerated uplift of the basement caused a relative sea level fall resulting in the formation of outer ramp carbonates on the apron lobes which were in turn overlain by lower messinian coral reefs this is the first known example of a base of slope apron within temperate water carbonate systems this article is protected by copyright all rights reserved

the study of barrier fracture in relation to earthquake and rockburst prediction one of the simplest models of earthquakes and rockbursts was studied in laboratory conditions in which the fracture was initiated in a barrier between two preexisting faults the treated models were built of concrete during the construction of the models the stress concentrators were inserted inside the models this arrangement enabled the shear displacement to occur during uniaxial loading of the model the tests were made on the series of models the sizes of which were mutually varying more than one order in the process of the barrier fracture propagation the following were investigated the time and space changes of local deformations as well as the acoustic emission velocities and amplitudes of elastic waves electrical conductivity and proper electro induction the process of barrier fracturing can be understood in three fundamental stages in the course of the first stage the density of small tensile cracks increases considerably while the volume in which they exist is gradually enlarged in the second stage as the result of these cracks coupling longer shear cracks are created during the coupling due to the development of numerous regions of unstable deformation the zones of nonequal rigidity arise in the treated medium distributed within it in a mosaic like pattern in the third stage the main fracture is prepared and formed with simultaneous unloading of the surrounding medium the process of crack forming is gradually localized into a more narrow zone

a flume study on the development and equilibrium morphology of current ripples in very fine sand an empirical model is constructed for the development and equilibrium dimensions of small scale unidirectional bedforms in sand with a median grain size of 0 095 mm based on a series of steady flow experiments in a flume current ripples always attain a linguoid plan morphology with constant average height 13 1 mm and wavelength 115 7 mm provided that sufficient time is allowed for their formation the development pattern of these ripples on a flat bed is independent of flow velocity and involves four stages 1 incipient ripples 2 straight and sinuous ripples 3 non equilibrium linguoid ripples and 4 equilibrium linguoid ripples straight and sinuous ripples are non equilibrium bedforms at all flow velocities the time needed to reach equilibrium dimensions is related to the inverse power of flow velocity and ranges from several minutes to more than hundreds of hours at flow velocities where washed ripples are stable the equilibrium wavelength is similar to that of equilibrium linguoid ripples but the equilibrium height rapidly decreases from 13 1 mm to zero towards upper stage plane bed conditions the results of the flume experiments correspond reasonably well with those of previous studies provided that various complicating factors such as different experimental methods different sediment characteristics shallow flow depths and non equilibrium runs are accounted for

experimental quasi steady density currents abstract hyperpycnal flows may be important mechanisms for transporting particles from continents to oceans given their frequency of generation at many modern river mouths their deposits are probably very common in the rock record although they are not frequently recorded in the literature these currents may last longer and be steadier than surge like turbidity flows and consequently their deposits are likely to be significantly different in this paper simple laboratory experiments are used to investigate how flows and deposits may vary the thickness of the experimental flows near the flow front varies with distance from the source depending on slope effluent discharge grain size and sorting runout distance varies with the proportion of fine grained sediment rather than with mean grain size flow front velocity varies with effluent discharge and sediment concentration and in a complex manner with mean grain size and sorting the variation of deposit mass with runout distance depends on proximal slope angle and the maximum mass per unit area approximating to bed thickness is inversely dependent on slope angle variation in initial suspended particle concentration results in deposits of similar shape but different volume although the total mass of the deposit depends on the discharge the maximum mass per unit area approximating to maximum bed thickness does not the distance over which particles are deposited increases and the deposit flattens as particles are deposited progressively further downstream at higher discharges perhaps relating to more efficient turbulent suspension at higher re more complete energy cascade in addition to longer horizontal component of settling trajectory at higher velocity the amplitude of mass variation depends on the grain size as with laboratory surges gladstone et al sedimentology 1998 833 844 the finer the sediment the more efficient is the sediment transport