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

a miocene coarse volcaniclastic mass flow deposit in the shimane peninsula sw japan product of a deep submarine eruption a subaqueous volcaniclastic mass flow deposit in the miocene josoji formation shimane peninsula is 15 16 m thick and comprises mainly blocks and lapilli of rhyolite and andesite pumices and non to poorly vesiculated rhyolite it can be divided into four layers in ascending order layer 1 is an inversely to normally graded and poorly sorted lithic breccia 0 3 6 m thick layer 2 is an inversely to normally graded tuff breccia to lapilli tuff 6 11 m thick this layer bifurcates laterally into minor depositional units individually composed of a massive lithic rich lower part and a diffusely stratified pumice rich upper part with inverse to normal grading of both lithic and pumice clasts layer 3 is 2 5 3 m thick and consists of interbedded fines depleted pumice rich and pumice poor layers a few centimeters thick layer 4 is a well stratified and well sorted coarse ash bed 1 5 2 m thick the volcaniclastic deposit shows internal features of high density turbidites and contains no evidence for emplacement at a high temperature the mass flow deposit is extremely coarse grained dominated by traction structures and is interpreted as the product of a deep submarine explosive eruption of vesicular magma or explosive collapse of lava

dry bulk density and caco3 relationships in upper quaternary sediments of the eastern equatorial pacific abstract results from 1110 paired caco3 and bulk density measurements from cores raised from the eastern equatorial pacific permit formulation of well constrained caco3 dbd relationships for that region the cores lie along a n s transect at 110 w from 10 n to 3 s underneath the different currents of the equatorial current system and along an e w transect from 110 to 90 w at approximately 3 s two distinct crescent shaped dry bulk density caco3 relationships are observed for equal caco3 percentages sediments from those sites at 110 w which are situated in the high productivity zone have lower dry bulk density cores raised from closer to shore have relatively greater dbd

a marine origin for the late mesoproterozoic copper harbor and nonesuch formations of the midcontinent rift of laurentia abstract the c 1 1 ga copper harbor and nonesuch formations of the keweenawan supergroup exposed along the canadian united states shorelines of lake superior are part of the surface exposures of the laurentian midcontinent rift these units have long been considered non marine in origin and have figured prominently in ideas regarding the evolution of microbial life and the redox conditions of earth s ocean atmosphere system at the close of mesoproterozoic time however these rocks also host hydrothermal metal deposits the emplacement of which may have compromised primary geochemical signals that are used to underpin those ideas here we highlight new sedimentological observations to provide an independent framework for assessing the depositional setting and geochemistry of those strata we show that the totality of sedimentological features leads to the conclusion that parts of the upper copper harbor formation and the entirety of the nonesuch formation were deposited along tide and wave influenced shorelines and in shallow marine settings under evaporitic conditions evidence for this interpretation includes the abundance of flaser wavy linsen and pinstripe bedding ubiquity of reactivation surfaces and mud drapes associated with all ripple forms superposed sets of ripple cross lamination showing bimodal herring bone sediment transport directions desiccation cracks and metre scale hummocky cross stratification further evaporite fabrics and pseudomorphs after gypsum in the upper 200 m of the copper harbor formation and in numerous stratigraphic positions within the nonesuch formation indicate that the water body was saline not fresh the emerging palaeogeographic image is one of a large shallow marine embayment with fringing sabkha like shorelines ideas about late mesoproterozoic biospheric evolution and earth s surface redox and oxygenation that rely on the nonesuch formation and copper harbor stromatolites having been deposited within a lacustrine setting require reassessment

a consideration of the dune antidune transition in fine gravel hydraulic data defining the dune antidune transition in fine gravel are compared with potential flow theory and information is drawn from published experiments and field based studies attention is given to both transitional bedforms and the development of downstream migrating antidunes in the latter case most data pertain to sand beds and not to gravel empirical data provide some weak support for the theoretical notion that the transition occurs at progressively lower froude numbers at greater relative depths although a critical froude number of 0 84 may reasonably be applied for the beginning of the dune to antidune transformation lag effects and a possible depth limitation ensure that transitional bedforms may persist across a broad range of froude numbers from 0 5 to 1 8 this latter observation has great relevance for palaeohydraulic estimates derived from outcrop data whereas the application of theoretical bedform existence fields based upon potential flow theory to fine gravel was previously purely speculative the addition of experimental and field data to these plots provides a degree of confidence in applying stability theory to practical geological problems for the first time laboratory data pertaining to downstream migrating gravel antidunes are compared with theory these bedforms have been reported from certain experimental near critical flows above sand or gravel beds but have been observed infrequently in natural streams however there are no detailed studies from natural rivers and only a few contentious identifications from outcrops nevertheless the limited hydraulic data conform to theoretical expectations

a new conceptual model for the deposition process of homogenite application to a cretaceous megaturbidite of the western pyrenees basque region sw france abstract the north pyrenean megaturbidite is a 19 63 m exceptional thick bed deposited during the late turonian and extending over more than 90 km in the basque region country sw france its thickness varies from more than 63 m to about 19 m from east mauleon area to west basque coast it represents a total compacted volume of about 90 km3 of carbonates on the field rare sedimentary structures are visible in the turbidite bed they consist of laminar planar lamination and rare cross laminations in the eastern region and antidune like structures in the western region five sites have been sampled with a vertical step of 50 cm to 1 m thin sections have been quantitatively analyzed for counting the terrigeneous fraction the quartz grain size and the mineral orientation the deposits fine westward which suggests a source located in the east of the mauleon basin this is consistent with the quartz grain orientation in the lower part of the megaturbidite the deposits fine upward from medium sand to clayey silt this is consistent with the classical definition of homogenites and suggests that these kinds of deposits are turbidites these observations also suggest that the term megaturbidite is appropriated for these deposits the sedimentary analysis indicates that the deposit results from a single event the volume of sediment involved in the process as well as the quartz grain orientation indicating flow motion in the opposite direction of the initial flow and the antidune like structure suggest the formation of reflected flows and the formation of standing waves over the complete water column in the basque flysch sub basins corresponding to a seiche effect the origin of the megaturbidite is probably an earthquake generated collapse on the carbonate platform this example allows providing a new conceptual model for the process of homogenite deposition this model explains the deposition of thick fine grained crudely graded megaturbidites

reply to the discussion by roger higgs on hummocky cross stratification like structures in deep sea turbidites upper cretaceous basque basins western pyrenees france by mulder et al sedimentology 56 997 1015

small scale hummocky cross stratification in turbidites a form of antidune stratification small scale hummocky cross stratification occurs in upper cretaceous calciclastic turbidites exposed in the western basque pyrenees facies associations and microfossil assemblages indicate slope to base of slope bathyal depositional environments it is developed in the fine grained portion of beds and displays spacings mostly between 0 2 and 0 7 m the beds fine upward with no sharp grain size breaks or mud partings suggesting that deposition occurred during a single flow event hummocky intervals are 0 1 0 8 m thick and consistently grade laterally and vertically into flat planar laminations of bouma b divisions suggesting that deposition occurred under upper flow regime conditions they have wave like geometries with laminae continuous across crests and troughs and display a ratio of wavelength to estimated underflow thickness of 11 3 12 8 combining the above observations and inferences these examples of small scale hummocky cross stratification are interpreted as a form of antidune stratification generated by standing waves along the interface of a thinner denser underflow main body tail of the turbidity current and an overlying thicker low density layer this occurrence is further evidence that small scale hummocky cross stratification is multigenetic and therefore not indicative of a particular flow condition or depositional environment

hummocky cross stratification tropical hurricanes and intense winter storms most previous workers have inferred a storm origin for hummocky cross stratification which typically occurs in shallow marine deposits on the modern earth the only storms capable of profoundly affecting shallow marine depositional environments are severe tropical cyclones hurricanes and mid latitude winter wave cyclones intense winter storms this paper examines the palaeogeographic distribution including palaeolatitude and palaeogeographic setting of 107 occurrences of hummocky cross stratification ranging in age from the proterozoic to recent in each of these stratigraphic units both palaeolatitude and palaeogeography are consistent with a direct storm influence associated with the passage of hurricanes or winter storms directly over the site of deposition this palaeogeographic evidence lends support to the inferred storm origin for hummocky cross stratification further the distribution of the structure suggests that most occurrences 73 were generated by tropical hurricanes the remaining 27 being generated by intense mid latitude winter storms the preferential generation of hummocky cross stratification by hurricanes is consistent with 1 the known differences in the nature of the bottom flows generated by the two major storm types and 2 the inferred nature of the flows which form hummocky cross stratification hurricanes couple less effectively with the water column than do intense winter storms due to this ineffective coupling hurricane generated bottom flows tend to be oscillatory or multidirectional dominant with only minor unidirectional components of motion in contrast intense winter storms generally do couple effectively with the water column generating bottom flows which possess a dominant or significant unidirectional component most previous workers have suggested that hummocky cross stratification forms under oscillatory or multidirectional dominant flow thus it is conceptually reasonable that the vast majority of ancient occurrences of hummocky cross stratification were probably hurricane generated as suggested by the aforementioned palaeogeographic distribution the proterozoic palaeozoic neogene and quaternary were times when global climate was similar to that of today the distribution of hummocky cross stratification deposited during these times suggests that both hurricanes and intense winter storms occupied latitudinal belts during these times which were essentially identical to those occupied by their modern counterparts the mesozoic and palaeogene were non glacial times when global climate was much warmer than that of today the distribution of hummocky cross stratification deposited during this interval suggests that hurricanes occurred more frequently at higher latitudes during non glacial times than they do at present the possibility of a broadened hurricane belt during the mesozoic and palaeogene is consistent with climatic considerations a limited number of mesozoic and palaeogene rock units containing hummocky cross stratification were deposited in palaeogeographic settings that preclude a direct hurricane influence these examples were deposited in the middle latitudes suggesting that intense winter storms continued to form hummocky cross stratification in the middle latitudes during these much warmer times some previous workers have suggested that tsunamis may be capable of generating hummocky cross stratification the palaeogeographic distribution of the structure does not support an origin due to tsunamis lacustrine examples of hummocky cross stratification reported herein are the first known non marine occurrences they suggest that storm effects strongly influence the sedimentary record of some lakes

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

glacial to holocene fluctuations in hydrography and productivity along the southwestern continental margin of india oxygen isotope and sedimentological records of a sediment core from the southwestern continental margin of india are used to reconstruct the fluctuations in sea surface hydrography and productivity during the last deglaciation the delta o 18 records of globigerinoides ruber and globigerinoides sacculifer exhibit high amplitude oscillations during the early deglaciation and holocene significant variations in delta o 18 during the holocene and a high glacial interglacial amplitude in delta o 18 values delta delta o 18 similar to 2 1 o suggest large fluctuations in sea surface hydrography related to monsoonal precipitation on land our results suggest that although the last deglacial events are coherent in time with the regional and global climatic events fluctuations in sea surface salinity during holocene are unique to this region maximum monsoonal precipitation appears to have occurred after similar to9 8 ka bp unlike the records from the western arabian sea biological productivity was maximum between similar to 18 and 15 ka bp and substantially diminished between similar to 13 and 6 ka bp the increased strength in the winter monsoon during the glacial period could have enhanced convective mixing and nutrient injection leading to high primary production reduced surface productivity during the early holocene may be attributed to the complex dynamics of the circulation system and hydrological cycle along the southwest coast of india c 2001 elsevier science b v all rights reserved

swaley cross stratification produced by unidirectional flows bencliff grit upper jurassic dorset uk unusual bedforms occur in the bencliff grit at osmington mills dorset the geometry of cross strata in stacked predominantly fine grained sandstone units is dominated by low angle troughs with locally perfectly preserved antiforms which wedge out onto climbing low angle erosion surfaces many of the antiforms hummocks and troughs swales appear to have grown at similar rates of sediment fall out others show draped antiforms and thickening into swales in both cases laminae are concordant with the lower bounding surfaces of the cross sets demonstrating that vertical accretion was the dominant process this geometry together with the conspicuous absence of angle of repose cross strata give the bedforms a resemblance to amalgamated hummocky cross stratification or swaley cross stratification many workers consider amalgamated hummocky cross stratification and swaley cross stratification to be identified with storm sedimentation on the shelf to shoreface thereby bracketting the structures both in terms of process and environment however the structures in the bencliff grit suggest that such a precise interpretation of amalgamated hummocky cross stratification or swaley cross stratification is inadvisable the geometry of the cross stratification suggests deposition under a highly sediment charged but essentially unidirectional flow this study therefore supports recent views that hummocky and swaley styles of cross stratification are in reality a form of low angle trough cross stratification produced primarily by unidirectional flows clearly since undulatory forms of stratification are substrate and process dependent and not environmentally specific great care must be taken in the interpretation of palaeoenvironmental settings especially where exposures are limited as for instance in borehole cores it is unlikely that core data can be used to identify bedforms analogous to hummocky cross stratification or swaley cross stratification unless boreholes are highly deviated

the supply and accumulation of silica in the marine environment rivers and submarine hydrothermal emanations supply 6 1 1014g sio2 yr to the marine environment approximately two thirds of the silica supplied to the marine environment can be accounted for in continental margin and deep sea deposits siliceous deep sea sediments located beneath the antarctic polar front convergence account for over a fourth 1 6 1014g sio2 yr of the silica supplied to the oceans deep sea sediment accumulation rates beneath the polar front are highest in the south atlantic with values as large as 53cm kyr during the last 18 000 yr siliceous sediments in the bering sea sea of okhotsk and subarctic north pacific accumulate 0 6 1014g sio2 yr or 10 of the dissolved silica input to the oceans the accumulation of biogenic silica in estuarine deposits removes a maximum of 0 8 1014g sio2 yr although estuarine silica versus salinity plots indicate extensive removal of riverine silica from surface waters the removal rates must be considered as maximum values because of dissolution of siliceous material in estuarine sediments and bottom waters siliceous sediments from continental margin upwelling areas e g gulf of california walvis bay or peru chile coast have the highest biogenic silica accumulation rates in the marine environment 69 g sio2 cm2 kyr despite the rapid accumulation of biogenic silica upwelling areas account for less than 5 of the silica supplied to the marine environment because they are confined laterally to such small areas

sedimentary responses to the sea level and indian summer monsoon changes in the central bay of bengal since 40 ka abstract we present a sedimentary multi index comprehensive study on sediment core bob 56 collected from the central bay of bengal bob with an attempt to understand the sedimentary processes and their responses to sea level change and the indian summer monsoon ism since the last glacial period we revealed the temporal evolution of terrigenous input and provided a brief discussion of the implications of our sedimentary indexes for ocean productivity then we proposed a simple depositional model in the central bob since the last glacial period as indicated by the geochemistry mineralogy and physical indexes including linear sedimentary rate lsr ti ca rb sr and quartz calcite ratios the terrigenous input was higher during the last glacial period than during the holocene period and the sea level is suggested to be responsible for this significant distinction through the depositional center transition in the bob at the glacial interglacial scale similar terrigenous input patterns with solar radiation and precipitation before 18 ka indicated that the monsoon controlled these situations at precession related scales synchronous changes with records in the north atlantic ocean during the last deglaciation and early holocene suggested terrigenous input responses to the climate changes that also recorded by high latitude sedimentary system at millennial scales indicated by the element caco3 ba and sr authigenic accumulation rates the paleoproductivity in the central bob was at a roughly equivalent level during the last glacial period and the holocene period different terrestrial nutrient inputs and ism related ocean surface stratifications were suggested to be responsible for the level of paleoproductivity this study provides a brief understanding of the sedimentary response to the climate and emphasizes the different roles of the sea level and ism in the central bob

glacial enrichments of authigenic cd and u in subantarctic sediments a climatic control on the elements oceanic budget we examine the possibility that glacial increase in the areal extent of reducing sediments might have changed the oceanic cd inventory thereby decoupling cd from po4 we suggest that the precipitation of cd sulfide in suboxic sediments is the single largest sink in the oceanic cd budget and that the accumulation of authigenic cd and u is tightly coupled to the organic carbon flux into the seafloor sediments from the subantarctic ocean and the cape basin south atlantic where oxic conditions currently prevail show high accumulation rates of authigenic cd and u during glacial intervals associated with increased accumulation of organic carbon these elemental enrichments attest to more reducing conditions in glacial sediments in response to an increased flux of organic carbon a third core overlain by circumpolar deep water cpdw as are the other two cores but located south of the antarctic polar front shows an approximately inverse pattern to the subantarctic record the contrasting patterns to the north and south of the antarctic polar front suggest that higher accumulation rates of cd and u in subantarctic sediments were driven primarily by increased productivity this proposal is consistent with the hypothesis of glacial stage northward migration of the antarctic polar front and its associated belt of high siliceous productivity however the increase in authigenic cd and u glacial accumulation rates is higher than expected simply from a northward shift of the polar fronts suggesting greater partitioning of organic carbon into the sediments during glacial intervals lower oxygen content of cpdw and higher organic carbon to biogenic silica rain rate ratio during glacial stages are possible causes higher glacial productivity in the cape basin record very likely reflects enhanced coastal up welling in response to increased wind speeds we suggest that higher productivity might have doubled the areal extent of suboxic sediments during the last glacial maximum however our calculations suggest low sensitivity of seawater cd concentrations to glacial doubling of the extent of reducing sediments the model suggests that during the last 250 kyr seawater cd concentrations fluctuated only slightly between high levels about 0 66 nmol kg 1 on glacial initiations and reaching lowest values about 0 57 nmol kg 1 during glacial maxima the estimated 5 lower cd content at the last glacial maximum relative to modern levels 0 60 nmol kg 1 cannot explain the discordance between cd and 13c such as observed in the southern ocean this low sensitivity is consistent with foraminiferal data suggesting minimal change in the glacial cd mean oceanic content

comparison and correlation of physical properties from the plain and slope sediments in the ulleung basin east sea sea of japan abstract high resolution profiles chirp sonar 2 7 khz acquired from the ulleung basin east sea sea of japan revealed several discrete echo types suggesting deposits formed mainly by mass flow processes the ulleung basin plain sediments are characterized by turbidite hemipelagic deposits while slide slump and debrite deposits are mainly found in the slope area rock fall deposits also appear near the ulleung island the core sediments show distinctive downslope variations in most physical properties studied over the plain and the slope mass physical properties water content porosity wet bulk density shear strength and velocity show considerably different values in spite of the nearly constant sediment texture however the relationships among the physical properties have relatively good correlations although the absolute values are different grain density in the plain is relatively lower than the slope probably because of a tephra and calcite compensation depth ccd therefore we suggest that the slope sediments may be affected by sedimentological changes resulting from diagenesis such as a certain degree of compaction and or consolidation

anatomy of a siliciclastic turbidite basin the gorgoglione flysch upper miocene southern italy physical stratigraphy sedimentology and sequence stratigraphic framework abstract the gorgoglione flysch is a tortonian siliciclastic turbidite formation well exposed in the lucania apennines southern italy with an average thickness of 1500 m it is one of the few examples of collisional basin succession for which field sedimentological investigations and visual correlations have permitted the reconstruction of the physical stratigraphy of proximal to distal turbidite deposits a northern and a southern turbidite complex have been identified they are divided by a structural high and both consist of three turbidite systems the middle and upper systems are composed of growth stages that show proximal to distal downcurrent facies variations the stacking pattern of each turbidite stage of growth consists of 5 10 cycles with thicknesses between 10 and 100 m mesocycles each of which is composed of cycles with a thickness of less than 10 m microcycles stratigraphical and sedimentological data are fitted to a sequence stratigraphy scheme represented by two nonconformity bounded depositional sequences internally composed of type i ii and iii stages of growth with aggradational to retrogradational and progradational stacking patterns the sequence stratigraphy scheme can be used as a model for turbidite collisional basins and is predictive of the stratigraphical features geometry and stacking patterns of depositional units formed when physiographic sedimentary basin evolution occurs under a regime of tectonic control this process consists of three phases i formation of the basin on top of a argille varicolori formation thrust system ii in sequence substratum thrust propagation towards the foreland basin confinement and elongation aggradational type i and retrogradational type ii stages of growth with two main sediment entry points iii deformational regime change with deactivation of in sequence thrusting decrease of deformation rate and decrease of lateral and vertical mobility the basin assumes a foreland ramp type configuration with development of a thick type iii pelitic stage of growth related to a predominant eustatic control

record of continental to marine transition from the mesoproterozoic ampani basin central india an exercise of process based sedimentology in a structurally deformed basin abstract the mesoproterozoic ampani group of rocks a structurally deformed sedimentary package hosted within the bastar craton in central india was studied for process based facies and paleoenvironmental analyses outcrop mapping on 1 1500 scale deconvolution of deformation pattern and process based facies analyses have led to the identification of fifteen facies types clubbed under four facies associations a range of paleoenvironmental settings varying from continental fluvial to distal marine shelf is inferred deductive paleohydrology revealed poorly efficient dirty river character for the ampani river system with low water discharge however at times of catastrophic sheet floods release of sediments trapped at the river mouth in form of hyperpycnal underflows triggered formation of river mouth delta reworking of delta front sediment in wave dominated coastline resulted development of beach foreshore and shoreface proximal to distal variation in the relative proportion of bar and interbar products within the shoreface successions exposed at different studied sections is interpreted as signature of relative bathymetric variation the pro deltaic ampani shelf was storm infested tectonic perturbance in the basin hinterland in course of ampani sedimentation is inferred from occurrence of a disparately thick lobate high density flow deposit towards the top of shoreface succession and increase in feldspar content upward within the shoreface succession addition of detritus from a 1600 ma mesoproterozoic provenance in upper part of shoreface also strengthen the contention deconvolution of deformation pattern and delineation of environmental products ranging between continental and deep marine allowed us to infer the ampani sediment package as fining upward in character evolved in a transgressive mode

chemical accumulation variations under the peru current during the past 130 000 years the titanium to aluminum ratio in core v19 29 is correlated with aluminosilicate accumulation rates this correlation may be due to pleistocene eolian transport fluctuations which alter the mean grain size of sedimented eolian material the relation between aluminum accumulation rate and ti al established from accumulation rates integrated over 11 000 50 000 year intervals can be inverted to compute a high resolution record of aluminosilicate and calcium carbonate accumulation rates over the past 130 000 years carbonate accumulation rates are closely related to the oxygen isotope record in the core with a phase lag and damping constant that is compatible with the response time shown to be only 6000 years of calcium carbonate in the ocean carbonate sedimentation at this site responds to several processes independently correlated with climatic change the relative importance of these processes for carbonate sedimentation at this site can be constrained by the record in this core and other lines of evidence 15 of the increased carbonate deposition at this site during glacial periods may be due to diminished nadw north atlantic deep water formation 10 is due to carbonate productivity decreases in the north atlantic 25 may be due to a diminished shallow sea carbonate sink and the residual 50 must be due to a local productivity increase these assignments are consistent with observations on carbonate paleoceanography in the north atlantic aluminosilicate accumulation rate variations correlate with the record of eolian quartz deposition near northwest africa and in a general way with the climatic record but in detail the record differs substantially from the oxygen isotope record and may provide independent evidence on the nature of climate dynamics

tsunami induced sediment transport in the abyssal mediterranean sea an unusual stratigraphic unit nicknamed homogenite fills topographic lows in the complex ridge and trough bathymetry at two survey sites on the western mediterranean ridge and the calabrian ridge on near bottom 4 khz seismic reflection profiles this unit us an acoustically transparent near surface flat lying layer whereas in cores it is a homogeneous gray marl as much as 7 m thick grain size decreases upcore within the unit implying that it was deposited in a single event controlled by gravitational settling the stratigraphic position of the homogenite relative to a firmly dated sapropel bed suggests emplacement between 4400 and 3100 yr b p the source of the homogenite is inferred to be the nearby basin walls farther east two other sites with similar rugged topography lack homogenite entirely a triggering mechanism is required which is capable of initiating massive sediment transport simultaneously in many separate basins at the western two survey sites but which is not effective at the eastern sites a large archeologically recorded earthquake of the correct age is considered and rejected because its epicenter is closer to the nonhomogenite bearing sites than to the sites where this sediment type was observed and because several other earthquakes of comparable magnitude have since been recorded in the area whereas the homogenite is unique the 3 500 yr b p collapse of the caldera of the volcano of santorini caused a huge tsunami which is recorded archeologically and geologically around the eastern mediterranean because of refraction of the tsunami by the bathymetry and because the caldera collapsed in its southwest corner a disproportionate amount of tsunami energy was directed toward the western area where homogenite is observed in contrast the homogenite free sites were relatively sheltered an order of magnitude calculation shows that the near bottom oscillating currents accompanying the santorini tsunami were at or above the threshold erosion velocity at the homogenite bearing sites in addition the near bottom pressure pulse under the tsunami at the homogenite bearing sites was sufficient to cause liquefaction of sediments neither mechanism was adequate to cause sediment transport or slope failure at the homogenite free sites

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

late cenozoic changes in atmospheric circulation deduced from north pacific eolian sediments abstract isolation and analysis of the eolian component of late cenozoic pelagic sediments from the north pacific provides direct information concerning changes in atmospheric circulation a 50 increase in intensity of both the prevailing westerlies and the tradewinds coincides with increasing pole to equator temperature gradients resulting from the onset of northern hemisphere glaciation at the same time the mass flux of dust from continents to the north pacific increased by a factor of 4 5 apparently reflecting significantly increased continental aridity associated with the late cenozoic glacial ages

organic carbon preservation in southeastern arabian sea sediments since mid holocene implications to south asian summer monsoon variability the earlier studies show a contrasting long term trend of the south asian summer monsoon sasm after attaining the precessional forcing induced mid holocene maximum the increasing total organic carbon toc concentration of marine sediments in the southeastern arabian sea seas has been interpreted to imply strengthening sasm since mid holocene by a few studies however toc concentration is also influenced by redox conditions sedimentation rate and an influx of terrigenous matter depending on the regional settings so it needs to be ascertained whether the toc concentration of the sediments in the seas is a signal of productivity related to the sasm strength or preservation therefore we studied multiple proxies toc total nitrogen atomic c n 13corg caco3 and major and trace elements concentration for determining the productivity redox conditions detrital supply and provenance in a sediment core from the upper continental slope of the seas spanning the past 4700 years at centennial scale resolution the present study shows that the observed toc increase is not a result of enhanced productivity but is because of better preservation due to the increased sedimentation rate along with increasingly reducing conditions since mid holocene we further show that the sasm has been declining since mid holocene after attaining a precession forced maximum which corroborates the earlier model ensemble studies

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

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

detailed anatomy of a deep water carbonate breccia lobe upper jurassic french subalpine basin abstract detailed correlations across tithonian carbonate breccia deposits in the drome river area northern part of the so called vocontian through suggest the depositional system was that of an elongated deep water lobe up to 70 km long and 20 to 30 km wide for a thickness reaching 200 m the drome lobe as it is now called is mainly made of slope to basinal mudstones breccias with minor platform components interpreted as debris flow and mud flow deposits associated with slump deposits it is basically a base of slope system whose elongated depositional area implies it was a point sourced gravity system thus perhaps connected to a small canyon cut onto the western slope of the basin but the mostly mudstone material of the breccias also suggests that the walls of this inferred canyon were the main supplier of the lobe not the carbonate platform proper the updip part of the lobe has a complex internal geometry as the deposition of breccia bed packages is interrupted by scourings locally 50 m deep indicating maybe a canyon mouth environment the middle part of the lobe is dominated by pure vertical aggradation of breccia beds with minor intervening erosion in the downdip part of the system a morphological compensation mechanism occurs as breccia beds tend to spread laterally a huge slump carrying large mudstone olistoliths ends the breccia deposition at the beginning of the berriasian this megaslump deposit was mostly emplaced on the right side of the breccia lobe supporting the idea of a depositional relief our observations thus show that previous interpretations as a submarine canyon infilling or as shallow water breccias formed in situ by cyclic loading under attenuating hurricane waves approaching the platform are not consistent with our observations the internal geometry of the system studied brings new data about a poorly studied kind of turbidite systems that of deep water carbonate breccias

sedimentary indicators of atmospheric activity in the northern hemisphere during the cenozoic abstract the concentrations and accumulation rates of eolian phases and the mineralogy of sediments from a north pacific core ll44 gpc3 have been used to evaluate changes in eolian sedimentation and atmospheric activity during the cenozoic the quartz content and clay mineralogy of the sediments reflect changing source areas for eolian material whereas the accumulation rates of eolian phases reflect climatic variation and wind intensity eolian sedimentation decreased from 70 to 50 m y ago at the site as it moved north out of the equatorial pacific where it was supplied with eolian debris from north america and africa low rates of eolian sediment accumulation 50 to 25 m y ago reflect the temperate humid environment of the early tertiary and the lack of vigorous atmospheric circulation at that time from 25 to 7 m y ago eolian accumulation increased as the site moved into the influence of the westerlies from 7 to 3 m y ago there was a more rapid increase in accumulation of eolian material at the site which was not accompanied by changes in mineralogy or quartz content and was therefore related to climatic change and more vigorous atmospheric circulation about 2 5 m y ago a dramatic increase in eolian sedimentation accompanied the onset of dry northern hemisphere glaciation

did the indo asian summer monsoon decrease during the holocene following insolation a few studies from the western arabian sea indicate that the indian summer or southwest monsoon ism after attaining its maximum intensity at ca 9 ka declined during the holocene as did insolation in contrast earlier and later observations from both the eastern and the western arabian sea do not support this inference analysis of multiple proxies of productivity in a new sediment core from the western arabian sea fails to confirm the earlier single proxy e g abundance of globigerina bulloides based inference of the holocene weakening of ism following insolation the reason for the observed decreasing trend in foraminiferal abundance the basis for the earlier inference could be the favouring of silicate rather than carbonate productivity by the increased ism wind strength although ism exhibits several multi millennial scale fluctuations there is no evidence from several multi proxy data to conclude that it declined during the holocene this is consistent with the phase lag analysis of longer time series of monsoon proxies thus on sub milankovitch timescales ism did not follow insolation highlighting the importance of internal feedbacks a comparison with east asian summer monsoon easm records suggests that both ism and easm varied in unison implying common forcing factors on such longer timescales copyright 2010 john wiley sons ltd

depositional environments and ichnology of the deep marine succession of the amiran formation upper maastrichtian paleocene lurestan province zagros fold thrust belt iran abstract the paleoenvironmental significance of deep marine trace fossil assemblages in the upper maastrichtian paleocene amiran formation has been done for the first time in lurestan province nw zagros from the zagros fold thrust belt a semi quantitative study of trace fossil abundance bioturbation indices in the amiran formation shows that trace fossil assemblages can be very powerful discriminators of deep marine clastic fan and related environments the amiran formation is a sand rich thick bedded and coarse grained turbidite succession deep marine paleoenvironments from basin slope to basin floor settings are preserved these strata contain a diverse and abundant pre and post depositional ichnofauna proximal and axial parts of the sandy systems show low diversity and low density trace fossil assemblages dominated by post depositional trace fossils but distal environments show a general increase in trace fossil diversity abundance and number of pre depositional trace fossils the post depositional association essentially consists of dwelling feeding and grazing traces the pre depositional assemblage is rich in graphoglyptids and grazing trails and feeding structures also occur the ichnodiversity composition ethology and morphologic complexity of the pre depositional association of the heterolithic successions of thin to thick bedded turbidite sandstone and inter turbidite mudstone are characteristic of three ichnosubfacies of the nereites ichnofacies these ichnosubfacies are identified in amiran formation deposits the typical succession of ichnosubfacies can express a bathymetric trend from shallower to deeper parts and from higher to lower hydrodynamic conditions they are 1 the ophiomorpha rudis ichnosubfacies the proximal and axial parts with thick bedded channel and lobe related environments 2 the paleodictyon ichnosubfacies channel lobe transition in the distal lobe facies and 3 the nereites ichnosubfacies distal part of off axis lobe or fan fringe basin floor transition and overbank settings there is also mixed o rudis to paleodictyon ichnosubfacies channel margin lobe fringe in the proximal lobe facies the dominance of zoophycos and chondrites in poor oxygen basin floor facies may represent zoophycos ichnofacies the number of graphoglyptids increases from distal basin floor to proximal fan fringe basin floor transition fan fringe lobe complex fringe lobe complex off axis settings they are absent in the most proximal axial lobe environments due to increased volume frequency erosive power of turbidity current events and sedimentation rate grain size decreased preservation potential of shallow tier and pre turbidite trace fossils the ichnosubfacies method used here as utilized by other authors has the potential to improve paleoenvironmental analysis of other deep marine depositional settings and in outcrop investigation of turbidite systems

sedimentology of a glaciofluvial landsystem lough ree area central ireland implications for ice margin characteristics during devensian deglaciation eskers in a glaciofluvial landsystem in the lough ree area central ireland exhibit two orientations the most southerly esker in the area the athlone esker forms part of a dendritic esker system with paleocurrent directions indicating eastward drainage while the rooskagh esker and associated eskers and kames immediately to the north were formed during southward drainage sediments indicate that sharp crested steep sided sections of both ridges were formed within subglacial tunnels while fan shaped and flat topped areas were formed as subaqueous outwash fans and deltas in standing water immediately in front of an ice margin the deposition of ice marginal deposits indicating southward paleoflows against the side of the eastward flowing athlone esker indicates a 90 shift in the direction of ice surface slope and a similar shift in the likely orientation of the ice margin the absence of any evidence of intermediate ice or water flow directions indicates that the shift reflects a recession of ice followed by a readvance from the north

retrospective dry bulk density estimates from southeast indian ocean sediments comparison of water loss and chloride ion methods abstract knowledge of absolute accumulation rates permits quantitative comparison between and within sediment components from cores of differing age and lithologic character dry bulk density is an essential parameter in the calculation of absolute accumulation rates dry bulk density measurements require a wet volume term and are best made when a core is first recovered such measurements were not routinely made on eltanin cores from the southeast indian ocean in an effort to recover this information we have estimated the dry bulk density of sixty samples from five eltanin cores using a water loss method and a chloride ion method which is independent of the state of desiccation of the cores based on the consistency of the results using these two independent methods we feel confident in our estimation of dry bulk density comparison of dry bulk density and caco 3 allowed us to develop the following non linear regression equation for estimating the dry bulk density based on caco 3 a more easily and accurately measured property 3 104x10 3 caco 3 2 2 176x10 3 caco 3 0 430 this equation can be used to estimate dry bulk density to within 0 097 g cm 3 thus providing the information needed to reliably calculate absolute accumulation rates our indian ocean dry bulk density equations are different from those developed by similar techniques for the pacific and atlantic oceans indicating that different lithologies necessitate the development of regional equations

impact of productivity events on the benthic foraminiferal fauna in the arabian sea over the last 150 000 years we have studied the sediment geochemistry and benthic foraminiferal fauna in a piston core from the northwest arabian sea covering the last 150 000 years and compared the results with the stable isotopic record from both benthic uvigerina peregrina and planktonic globigerinoides sacculifer foraminifera the 18o record of the planktonic foraminiferal species shows excursions towards heavier 18o values when compared to the benthic record and the specmap stack particularly in stage 3 between approximately 30 and 45 50 kyr we call this the stage 3 event species diversity and relative abundance data on benthic foraminifera reveal that some species e g cibicidoides wuellerstorfi oridorsalis umbonatus pullenia bulloides pyrgo spp and the uvigerinids increased in relative abundance during glacial stages 2 and 4 and during the stage 3 event whereas other e g astrononion novozealandicum cibicidoides bradyi eggerella bradyi gyroidina altiformis hoeglundina elegans pullenia subcarinata and sigmoilina edwardsii decreased using correspondence analysis we determined that approximately 50 of the total sample variation may be accounted for by three first factors which all are interpreted as primarily related to surface water productivity and the quality of the organic matter that reaches the seafloor the relative abundance of some species appears to vary on a precessional cycle perhaps responding to monsoon driven upwelling and associated increased food supply whilst the abundances of other species are related to the 100 kyr ice volume cycle and may be responding to changes in intermediate and deep water chemistry geochemical paleoproductivity tracers such as the ba content of the sediment have a very similar distribution to specific foraminiferal factors in both time and frequency domains certain planktonic foraminifera e g globigerina bulloides also display obvious maxima in the stage 3 event the stage 3 event occurs at a time of reduced rate of amplitude change in both solar radiation and monsoon pressure index we conclude that there was a period 15 to 20 kyr duration during stage 3 with cooler than normal surface waters enhanced productivity and rapid settling of biogenic matter

reverse flow in turbidity currents the role of internal solitons pickering hiscott 1985 have demonstrated amply the presence of reverse flow units within the thick bedded calcareous wacke tcw beds of the turbiditic cloridorme formation middle ordovician gaspe peninsula quebec canada these reverse flow units are underlain and overlain by units which reveal flow in the primary obverse direction in this paper a model is proposed for this reverse flow based on the probable nature of the primary turbidity flow it appears that the initial flow was highly elongated thickness h length l with h 500 m velocity u 2 m s 1 and sediment concentration c 1 25 o the rate of momentum loss of the flow is estimated by means of a useful parameter which we call the drag distance symbol dd defined by where h and l are the thickness and length of the flow respectively ccd is a combined drag coefficient representing friction on the bottom and at the upper interface and fcd is a form drag coefficient related to the shape and size of the head dd is the distance travelled by a current of constant h and l flowing over a horizontal bottom and obeying a quadratic friction law for an e fold reduction in velocity simple considerations confirmed by our own experiments described in this paper show that such an elongated turbidity current cannot be reflected as a whole from an adverse slope when the nose of the current reaches the slope it forms a hump which surges backwards and sooner or later breaks up into a series of internal solitons the latter probably numbering 4 7 will cause reverse flow at a given point as they pass by provided that the residual velocity in the tail is not too great flow in the original obverse direction will be re established after the passage of the solitons quiescent periods in front of between and behind the solitons when soliton associated currents cancelled out the residual obverse flow would allow the deposition of thin mud drapes additional flow reversals observed in a few of the tcw beds cannot be explained readily by the re passage of solitons since wave breaking at the ends of the basin would cause massive energy loss internal seiches are the preferred explanation for these later reversals

shallowing upward cycles in pelagic troughs upper jurassic subbetic southern spain abstract abundant calcareous tempestites with chert and hummocky cross stratification hcs occur in upper jurassic successions milanos formation of the subbetic betic cordillera southern spain these calcareous tempestite beds are calcisiltites and very fine calcarenites with peloids and bioclasts and have thicknesses between 25 and 75 cm beds show symmetrical wave ripple lamination near the top and are usually capped with wave ripples according to palaeogeographic reconstructions these strata were deposited in a subsiding trough in the pelagic domain of the southern iberian continental margin which was the northwestern most border of the tethys calcisiltite and calcarenite beds deposited in this pelagic trough form small scale to large scale shallowing upward cycles shallowing upward cycles with calcareous tempestites in the lower part and oolitic limestone beds in the upper part have also been recognized in some sectors of the subbetic pelagic basin the large scale cycle occurs in the upper part of the jarropa radiolarite formation which directly underlies upper oxfordian kimmeridgian and tithonian strata of the milanos formation two types of large scale shallowing upward cycles have been recognized according to the presence or absence of thick units of oolitic limestone with large scale cross stratification the water depth during deposition ranged from below but near storm wave base at the base of the large scale cycles to very shallow carbonate platform at the top

biogenic opal in southern ocean sediments over the last 450 000 years implications for surface water chemistry and circulation we present records of biogenic opal percentage and burial rate in 12 piston cores from the atlantic and indian sectors of the southern ocean these records provide a detailed quantitative description of changing patterns of opal deposition over the last 450 kyr the striking regional coherence of these records suggests that dissolution in the deep sea and sediment pore waters does not obscure the surface productivity signal and therefore these opal time series can be used in combination with other surface water tracers to make inferences about the chemistry and circulation of the southern ocean under different global climate conditions three broad depositional patterns can be distinguished northernmost records 39 42 s latitude are characterized by enhanced opal burial during glacial periods and strong 41 kyr periodicity records from cores just north of the present antarctic polar front 46 49 s show even larger increases in opal burial rate during glacial intervals but have variance concentrated in the 100 and 23 kyr bands southernmost records 51 55 s are completely out of phase with those to the north with greatly reduced opal burial rates during glacial periods taken as a whole the opal records show no evidence for the increased total antarctic productivity predicted by recent geochemical models of atmospheric co2 variability the areal expansion of southern ocean sea ice over the present zone of high siliceous productivity provides one plausible explanation for the glacial interglacial opal patterns the excess silica not taken up in this zone during glacial periods would contribute to greater nutrient availability and thus higher productivity in the subantarctic region however local circulation changes may act to modify this basic signal possibly accounting for the observed differences in the opal variance spectra

architecture and morphodynamics of subcritical sediment waves in an ancient channel lobe transition zone in modern systems submarine channel lobe transition zones show a well documented assemblage of depositional and erosional bedforms in contrast the stratigraphic record of channel lobe transition zones is poorly constrained because preservation potential is low and criteria have not been established to identify depositional bedforms in these settings several locations from an exhumed fine grained base of slope system unit b laingsburg depocentre karoo basin south africa show exceptional preservation of sandstone beds with distinctive morphologies and internal facies distributions the regional stratigraphic context lack of a basal confining surface wave like morphology in dip section size and facies characteristics support an interpretation of subcritical sediment waves within a channel lobe transition zone setting some sediment waves show steep 10 to 25 unevenly spaced 10 to 100 m internal truncation surfaces that are dominantly upstream facing which suggests significant spatio temporal fluctuations in flow character their architecture indicates that individual sediment wave beds accrete upstream in which each swell initiates individually lateral switching of the flow core is invoked to explain the sporadic upstream facing truncation surfaces and complex facies distributions vertically within each sediment wave variations in bedform character are related to the axial to marginal positions within a channel lobe transition zone the depositional processes documented do not correspond with known bedform development under supercritical conditions the proposed process model departs from established mechanisms of sediment wave formation by emphasising the evidence for subcritical rather than supercritical conditions and highlights the significance of lateral and temporal variability in flow dynamics and resulting depositional architecture

volcaniclastic sedimentation in a shallow water marginal basin the early miocene koura formation sw japan abstract the early miocene koura formation sw japan is composed mainly of reworked volcaniclastics and andesite to rhyolite pyroclastics and constitutes part of the volcanic zone of sw japan of that time reworked volcaniclastic sequences are thick and mostly coarse grained and based on conventional facies concepts are interpreted as shoreface to beach deposits turbidites and debris flow deposits thick acid tuffs intercalated in the volcaniclastic sequences individually show internal features characteristic of either subaerial or subaqueous ash flow emplacement or in some cases both andesite pyroclastic facies include surge and fall deposits facies relationships together with the fossil record indicate deposition in a shallow lacustrine environment the deposition rate of the koura formation was probably high enough to rapidly fill the shallow lake but sedimentation was balanced by rapid subsidence of the lake similar to lakes present within the modern volcanic zones of taupo new zealand and hohi japan the koura formation grades upward into marine deposits and may represent the initial volcanism and volcaniclastic sedimentation during the early stages of back arc or intra arc rifting

lateral and vertical facies relationships of bedforms deposited by aggrading supercritical flows from cyclic steps to humpback dunes abstract the preservation of bedforms related to supercritical flows and hydraulic jumps is commonly considered to be rare in the geologic record although these bedforms are known from a variety of depositional environments this field based study presents a detailed analysis of the sedimentary facies and stacking pattern of deposits of cyclic steps chutes and pools antidunes and humpback dunes from three dimensional outcrops the well exposed middle pleistocene successions from northern germany comprise glacilacustrine ice contact subaqueous fan and glacial lake outburst flood deposits the studied successions give new insights into the depositional architecture of bedforms related to supercritical flows and may serve as an analogue for other high energy depositional environments such as fluvial settings coarse grained deltas or turbidite systems deposits of cyclic steps occur within the glacial lake outburst flood succession and are characterised by lenticular scours infilled by gently to steeply dipping backsets cyclic steps formed due to acceleration and flow thinning when the glacial lake outburst flood spilled over a push moraine ridge these bedforms are commonly laterally and vertically truncated and alternate with deposits of chutes and pools and antidunes the subaqueous fan successions are dominated by laterally extensive sinusoidal waveforms which are interpreted as deposits of aggrading stationary antidunes which require quasi steady flows at the lower limit of the supercritical flow stage and high rates of sedimentation humpback dunes are characterised by downflow divergent cross stratification displaying differentiation into topsets foresets and bottomsets and are interpreted as deposited at the transition from subcritical to supercritical flow conditions or vice versa gradual lateral and vertical transitions between humpback dunes and antidune deposits are very common the absence of planar parallel stratification in all studied successions suggests that the formation of these bedforms is suppressed in flows characterised by hydraulic jumps under highly aggradational conditions the large scale lateral and vertical successions of bedforms are interpreted as representing the temporal and spatial evolution of the initial supercritical flows which was strongly affected by the occurrence of hydraulic jumps small scale facies changes and the formation of individual bedforms are interpreted as controlled by fluctuating discharge bed topography and pulsating unstable flows

placer formation in gravel bedded rivers a review the literature on placer formation processes within fluvial systems is widespread and ranges between detailed laboratory studies of the hydrodynamic segregation processes through to the intuitive interpretation of the distribution of minerals within geological sections however there are few if any comprehensive reviews of the literature surprisingly given the economic importance of placers the theoretical framework relating to the hydrodynamics of physical grain sorting is not well developed and there are relatively few detailed laboratory hydraulic investigations to inform theory in this wide ranging review the history of the development of principles of placer formation is explored as far as possible in a non technical fashion a consideration is given to the hydrodynamics of physical grain sorting above lower stage and upper stage bedforms and the typical internal sedimentary structures associated with placer concentrations are detailed finally examples of the depositional environment of diamond tin and gold placers are considered

the occurrence of paleodictyon in shallow marine deposits of the upper cretaceous mikasa formation hokkaido island northern japan implications for spatiotemporal variation of the nereites ichnofacies abstract the trace fossil paleodictyon is characterized by a regular hexagonal network and has been widely considered to be a characteristic of the nereites ichnofacies indicative of a deep marine environment recently however some researchers have reported paleodictyon occurrences in relatively shallow settings leading to suggestions that the habitat of these burrowers was not restricted to deep marine environments a new occurrence of this trace fossil is presented in this study from a shallow marine succession of upper cretaceous deposits in northern japan paleobathymetric and temporal variations in the habitat range of shallow marine examples of this ichnotaxon are also discussed based on a compilation of previous works the new paleodictyon specimens discussed here are from the lowermost part of the mikasa formation within the yezo group interpreted as a deposit representing the lower shoreface through to the inner shelf transitional zone this paper presents the first record of a shallow marine paleodictyon from the pacific region combined with data from previous studies records suggest that shallow marine paleodictyon are observable from paleozoic to mesozoic shelf deposits although no reliable records of their occurrences have so far been reported from the cenozoic increasing bioturbation intensity and predation pressure during the mesozoic and cenozoic may have been significant factors preventing the preservation of this ichnogenus in post mesozoic shallow marine settings the paleodictyon bearing interval of the mikasa formation also preserves evidence for low intensity bioturbation which may account for preservation of the traces in this case

a review of recent research in fluvial sedimentology this volume has been compiled to represent the current status of research into fluvial sedimentology as part of the proceedings for the 5th international conference on fluvial sedimentology 5icfs brisbane july 1993 the volume contains papers on a wide variety of topics representing the diversity of interests covered by the icfs this review serves to introduce these papers within the context of recent research post1988 in fluvial sedimentology icfs papers may be somewhat arbitrarily divided into those which report fundamental studies of sediment transport studies of modern alluvial and coastal systems studies of environmental management of fluvial systems facies analyses of ancient fluvial sequences studies of controls on alluvial stratigraphy and basin fill and applied sedimentological studies of fluvialhosted mineral resources it is interesting to compare the balance of papers in this volume with that of previous icfs volumes miall 1978 collinson and lewin 1983 ethridge et al 1987 marzo and puigdefabregas 1993 as a guide to changing emphasis in fluvial sedimentology figure 1 illustrates the distribution of papers over the five icfs meetings categorised as above among various trends two are worthy of comment the first being the marked peak in interest in controls on fluvial sequence and basin fill at the 4icfs and the second the influx of papers on environmental management in this volume although there has been a steady level of activity in fundamental aspects of sediment transport and bedform generation the rela

the base of slope carbonate breccia system of ceuse tithonian s e france occurrence of progradational stratification in the head plug of coarse granular flow deposits abstract once interpreted as a deep water canyon infilling and later as some kind of tempestite deposit the tithonian breccias of ceuse are here reinterpreted as a small lobe deposit emplaced at the mouth of a gully cutting into a slope between two deep water terraces in the eastern part of the subalpine basin the breccias belong to two basic categories mostly grain supported and occasionally mud supported spectacular large scale oblique geometries apparent lateral accretions are observed in the grain supported breccias both within the infilling of the lower feeder channel and in the updip part of the lobe detailed analyses of these geometries show that they correspond to progradational features both laterally and in a more distal position on the lobe these progradational bodies evolve into a pile of unsorted massive breccias sometimes showing a crude even stratification we thus propose a depositional model for the grain supported breccias linking the two facies as follows the deposition of a gravity flow begins at the toe of slope mouth of the tributary gully by a progradational head which evolves downslope and laterally to a splay of massive or crudely stratified breccia the internal oblique stratification flattens distally over a distance of a few kilometres elementary beds pile up in morphological compensation to build the 100 m thick breccia body of ceuse because the breccias are derived from the gravity reworking of deep water nodular carbonates there is no sand sized material available in the depositional system or only a little probably produced through an erosive clast grain diminution process during transport this explains why the thick breccia beds pass quickly eastward to thin bedded mud turbidites

submarine origin for the neoproterozoic wonoka canyons south australia abstract an examination of the deeply incised ediacaran wonoka canyons in the adelaide geosyncline most recently interpreted as subaerial valleys demonstrates their submarine origin and confirms them as some of the best examples of ancient outcropping submarine canyons in the world the entire canyon fill succession is interpreted to be of deep water below wave base origin consisting of calcareous shale and siltstone together with a variety of mass flow deposits including turbidites grain flows and debris flows the canyon fill lacks definitive shallow water structures e g mud cracks fenestral fabrics or wave ripples at all stratigraphic levels canyon lining carbonate crusts that have previously been interpreted as non marine calcretes or tufas and used to suggest a non marine origin for the canyons are argued to be of deep water marine microbial origin extremely negative carbon isotope values from the canyon fill and canyon lining crusts have a primary marine origin previously interpreted deepening upward trends in the canyon fill used as evidence of a subaerial erosion episode followed by drowning are suggested to be fining upward trends caused by the transition from canyon cutting to canyon filling with the majority of the fill being of deep water slope origin the basal conglomeratic canyon fill sediments represent the last vestiges of the high energy deep water canyon erosion environment in which the incisions formed a deep water origin for the canyons is consistent with all previous stratigraphic observations of the wonoka canyons including the conspicuous lack of regional unconformities in the lower wonoka formation and their emanation from the deep water facies of the wonoka formation a submarine canyon origin also removes the need for extreme 1 km relative sea level fluctuation and associated problems i e an enclosed basin with messinian style evaporative drawdown or thermal uplift above a migrating mantle plume required by the subaerial valley hypotheses

formative conditions and sedimentary structures of sandy 3d antidunes an application of the gravel step pool model to fine grained sand in an experimental flume antidunes and their sedimentary structures can be useful in reconstructing paleo hydraulic conditions especially for large discharge events however three dimensional 3d antidunes in sand sized sediments have not yet been studied extensively as compared to either two dimensional 2d antidunes or antidunes in gravel sized sediments in this study we estimated formative conditions of gravel step pool morphologies and applied them to the formation of 3d antidunes over a sand bed formative conditions are expressed in terms of a relationship between the water discharge per unit width and the bed slope flume experiments demonstrated that 3d mound like antidune configurations and their associated internal sedimentary structures could be preserved internal sedimentary structures were characterized by shallow lens like structures whose bases were erosional although gently dipping concave upward lamination was dominant convex upward lamination was occasionally observed the dimensions of lenticular lamina sets can be used to estimate antidune geometry thus if 3d antidunes can be interpreted in the stratigraphic record it is possible to estimate the paleo hydraulic parameters such as water discharge and bed slope more precisely than previously copyright 2010 john wiley sons ltd

late quaternary caco3 production and preservation in the southern ocean implications for oceanic and atmospheric carbon cycling recent geochemical models invoke ocean alkalinity changes particularly in the surface southern ocean to explain glacial age pco2 reduction in such models alkalinity increases in glacial periods are driven by reductions in north atlantic deep water nadw supply which lead to increases in deep water nutrients and dissolution of carbonate sediments and to increased alkalinity of circumpolar deep water upwelling in the surface southern ocean we use cores from the southeast indian ridge and from the deep cape basin in the south atlantic to show that carbonate dissolution was enhanced during glacial stages in areas now bathed by circumpolar deep water this suggests that deep southern ocean carbonate ion concentrations were lower in glacial stages than in interglacials rather than higher as suggested by the polar alkalinity model broecker and peng 1989 our observations show that changes in southern ocean caco3 preservation are coherent with changes in the relative flux of nadw suggesting that southern ocean carbonate chemistry is closely linked to changes in deepwater circulation the pattern of enhanced dissolution in glacials is consistent with a reduction in the supply of nutrient depleted water nadw to the southern ocean and with an increase of nutrients in deep water masses carbonate mass accumulation rates on the southeast indian ridge 3200 3800 m and in relatively shallow cores 3000 m from the kerguelen plateau and the south pacific were significantly reduced during glacial stages by about 50 the reduced carbonate mass accumulation rates and enhanced dissolution during glacials may be partly due to decreases in caco3 corg flux ratios acting as another mechanism which would raise the alkalinity of southern ocean surface waters the polar alkalinity model assumes that the ratio of organic carbon to carbonate production on surface alkalinity is constant even if overall productivity in the southern ocean were held constant a decrease in the caco3 corg ratio would result in increased alkalinity and reduced pco2 in southern ocean surface waters during glacials this ecologically driven surface alkalinity change may enhance deepwater mediated changes in alkalinity and amplify rapid changes in pco2

seismic characteristics of central brazil crust and upper mantle a deep seismic refraction study 1 a two dimensional model of the brazilian central crust and upper mantle was obtained from the traveltime interpretation of deep seismic refraction data from the porangatu and cavalcante lines each approximately 300 km long when the lines were deployed they overlapped by 50 km forming an e w transect approximately 530 km long across the tocantins province and western sao francisco craton the tocantins province formed during the neoproterozoic when the sao francisco the paranapanema and the amazon cratons collided following the subduction of the former goias ocean basin average crustal vp and vp vs ratios moho topography and lateral discontinuities within crustal layers suggest that the crust beneath central brazil can be associated with major geological domains recognized at the surface the moho is an irregular interface between 36 and 44 km deep that shows evidences of first order tectonic structures the 8 05 and 8 23 km s 1 p wave velocities identify the upper mantle beneath the porangatu and cavalcante lines respectively the observed seismic features allow for the identification of 1 the crust has largely felsic composition in the studied region 2 the absence of the mafic ultramafic root beneath the goias magmatic arc and 3 block tectonics in the foreland fold and thrust belt of the northern brasilia belt during the neoproterozoic seismic data also suggested that the bouguer gravimetric discontinuities are mainly compensated by differences in mass distribution within the lithospheric mantle finally the goias tocantins seismic belt can be interpreted as a natural seismic alignment related to the neoproterozoic mantle domain

progradational patterns at the head of single units of base of slope submarine granular flow deposits conglomerats des gâs coniacian se france upper cretaceous conglomerates sandstones and calcarenites outcropping in the glandage syncline les gas gorge belong to two superimposed turbidite systems the first being the shortest about two hundred metres thick five kilometres wide and a mere ten kilometres long both onlap a sw ne oriented palaeoslope created by faulting in the latest turonian on the southern edge of the vercors carbonate platform the first system is coniacian in age the second probably campanian this study deals only with the first system which comprises in stratigraphic order classical sandstone turbidites clast supported conglomerates and coarse grained cross bedded sandy calcarenites the transition between the last two lithologies is progressive allowing the mechanisms responsible for apparent lateral accretions found pervasively in both the conglomerates and the calcarenites to be understood here cross bedding is not the result of changes in the direction of accretion in lateral bars or in oblique infillings of sinuous channels as is often suggested in other similar deep water carbonate systems each event bed comprises a prograding conglomeratic head plug blanketed in continuous sedimentation by forward spreading bioclastic calcarenites the superimposition of such mixed beds gives rise to a low angle cross stratification an unusual feature for a system interpreted as a base of slope apron conglomerates were probably deposited through a granular flow mechanism at a strong hydraulic jump at the base of the slope associated calcarenites show supercritical flow features the undulating transverse architecture of its large head onlapping the paleoslope suggests it is a multi apex apron the sandstone conglomerate calcarenite succession of the depositional sequence is interpreted as overall transgressive after the late turonian tectonic phase that led to the exposure of the vercors carbonate platform updip

depositional environments organic carbon accumulation and solar forcing cyclicity in smackover formation lime mudstones northern gulf coast upper jurassic smackover lime mudstones a major hydrocarbon source rock in the u s gulf coast show three distinct lithofacies the basal laminated lithofacies consists of organic and siliciclastic rich lamina sets that alternate with organic poor clean carbonate lamina sets both lamina sets are composed of lamina couplets interpreted to be varves this lithofacies lacks current induced structures allochems and rip up clasts benthic and planktonic fossils are absent one organic rich lamina set and one organic poor lamina set comprise one cycle 0 5 3 cm thick the middle thin bedded lithofacies is composed of organic and siliciclastic rich carbonate beds that alternate with organic poor clean carbonate beds both bed types are internally laminated one organic rich bed and one organic poor bed comprise one cycle 2 60 cm thick the upper burrowed lithofacies is extensively bioturbated massive shows no evidence of cyclicity and contains little organic carbon sedimentologic characteristics suggest that the basal laminated lithofacies was deposited under anoxic conditions below storm wave base in a basinal environment the middle thin bedded lithofacies is copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received march 5 1996 revised manuscript received august 2 1996 final acceptance december 10 1996 2basin research institute louisiana state university baton rouge louisiana 70803 3lss international 1440 lake front circle suite 180 the woodlands texas 77380 formerly with department of geology and geophysics louisiana state university baton rouge louisiana 70803 4department of geology and geophysics louisiana state university baton rouge louisiana 70803 the study was supported by grants from the center for energy studies and the basin research institute of louisiana state university we thank h d klemme and g t moore for providing unpublished data p meyers and l m pratt reviewed an earlier version of the manuscript and offered valuable comments j wrenn kindly provided characteristics of organic matter we thank j kropog w s leblanc k lyle p o neill and b simpson for technical assistance we appreciate the helpful comments of r l folk b kirkland and c h moore critical comments by k t biddle n b harris b a prather and p d wagner significantly improved the quality of concepts presented

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

chemical transport to the seafloor of the equatorial pacific ocean across a latitudinal transect at 135 w tracking sedimentary major trace and rare earth element fluxes at the equator and the intertropical convergence zone we have analyzed the major trace and rare earth element composition of surface sediments collected from a transect across the equator at 135 w longitude in the pacific ocean comparing the behavior of this suite of elements to the caco3 opal and corg fluxes which record sharp maxima at the equator previously documented at the same sampling stations enables us to assess the relative significance of the various pathways by which trace elements are transported to the equatorial pacific seafloor the 1 1 high biogenic source at the equator associated with equatorial divergence of surface water and upwelling of nutrient rich water and 2 2 high aluminosilicate flux at 4 n associated with increased terrigenous input from elevated rainfall at the intertropical convergence zone itcz of the tradewinds are the two most important fluxes with which elemental transport is affiliated the biogenic flux at the equator transports ca and sr structurally bound to carbonate tests and mn primarily as an adsorbed component trace elements such as cr as pb and the rees are also influenced by the biogenic flux at the equator although this affiliation is not regionally dominant normative calculations suggest that extremely large fluxes of ba and p at the equator are carried by only small proportions of barite and apatite phases the high terrigenous flux at the itcz has a profound effect on chemical transport to the seafloor with elemental fluxes increasing tremendously and in parallel with ti normative calculations however indicate that these fluxes are far in excess of what can be supplied by lattice bound terrigenous phases the accumulation of ba is greater than is affiliated with biogenic transport at the equator while the p flux at the itcz is only 10 less than at the equator this challenges the common view that ba and p are essentially exclusively associated with biogenic fluxes many other elements including mn pb as and rees also record greater accumulation beneath the itcz than at the equator thus adsorptive scavenging by terrigenous paniculate matter or phases intimately associated with them appears to be an extremely important process regulating elemental transport to the equatorial pacific seafloor these findings emphasize the role of vertical transport to the sediment and provide additional constraints on the paleochemical use of trace elements to track biogenic and terrigenous fluxes

contained reflected turbidity currents from the middle ordovician cloridorme formation quebec canada an alternative to the antidune hypothesis the parautochthonous cloridorme formation is a syn orogenic flysch succession that was deposited in an elongate foredeep basin as mainly lower middle fan outer fan and basin floor deposits the basin floor deposits about 1 5 km thick are confined to members 1 2 and 1 and are characterized by graded thick 1 10 m mud rich calcareous greywacke beds previously interpreted as deposits of concentrated muddy unidirectional turbidity currents that locally generated backset antidune lamination in internally stratified flows the dominant flow directions were from east to west but west to east transport also occurred as seen in the orientation of ripples climbing ripples flutes consistently overturned flames and grain imbrication we believe that the flows that deposited these thick calcareous greywacke beds reversed by roughly 180 one or more times during deposition of the lower sandy part of the beds flow reversals are consistent with the sharp grain size breaks and mud partings within sandy divisions measurement of grain fabric relative to stratification in the most celebrated antidune bedforms indicates flow from west to east thus the bedforms were produced by west to east migration of megaripples not by the upcurrent migration of antidunes the thick muddy beds were deposited by large volume muddy flows that were deflected and reflected from the side slopes and internal topographic highs of a confined basin floor much like the contessa and similar beds of the italian apennines large quantities of suspended mud were ponded above the irregular basin floor and settled to produce the thick silty mudstone caps seen on each bed because of their mode of emplacement we propose that these beds be called contained turbidites

ordovician proglacial sediments in algeria insights into the controls on hydrocarbon reservoirs in the in amenas field illizi basin abstract ordovician proglacial deposits form gas reservoirs in the in amenas field illizi basin algeria depositional models were developed to understand the context and disposition of the main reservoirs through an evaluation of core and analogous outcrops from the tassili n9ajjer tunnel valleys initially accumulated sandstones with tractional structures subsequent failures of subaqueous grounding line sediment deposited proglacial debrites comprising poorly sorted argillaceous sandstone with granules these were interbedded with high density turbidity sandstones their fine grain size indicates they were dynamically disconnected from the lithologically varied debrites a lobate geometry has been defined for one subsurface composite turbidite periodic catastrophic outflows possibly evacuating subglacial lakes incised the network of subglacial tunnels and in the process delivered sand to the turbidite outwashes bedforms indicate high energy transcritical to supercritical outflows that were stable for extended periods during ice retreat a period of ice margin stability may have occurred due to grounding over the in amenas granitic palaeohighs outwash fan apices were located along this grounding line with feeder channels developing where the substrate was more easily eroded such as between the palaeohighs following further ice retreat deposition evolved to variably sinuous channels and thence to pelagic fines with dropstones