facies distribution and textural variation in faro drift contourites velocity fluctuation and drift growth abstract detailed study of seismic profiles piston cores and bottom photographs from the faro drift on the southern margin of portugal has led to a better understanding of drift development and its relationship to bottom current circulation data on the contourite facies characteristics and the surface microphysiography have been published elsewhere here we concentrate on sediment distribution and geometry longitudinal trends in facies types mean grain size sedimentary structures and composition can be interpreted in terms of relative intensity of currents over different parts of the drift these are generally more intense in the marginal channels and at the upstream or eastern end of the drift three different scales of vertical variation of facies can be identified at the large scale 300 500 m of sediment has accumulated over 4 6 ma in a regular vertical succession due to the northward progradation of the drift at the medium scale the upper 20 30 m of sediment shows alternating phases of active lateral progradation and uniform vertical accumulation that may correlate with episodes of more and less current activity related to high and low sea level stands respectively over the past 0 3 ma at the small scale the topmost 2 3 m of sediment deposited in approximately 0 03 ma shows three zones of coarser grained sediments separated by finer grained contourites this sequence can also be interpreted in terms of long term fluctuation in bottom current activity although the signal is clearly complex this kind of analysis of sedimentary drifts can lead to more accurate reconstruction of paleocirculation patterns

the western grand banks of newfoundland transgressive holocene sedimentation under the combined influence of waves and currents abstract the surficial sediments on the western grand banks of newfoundland have been reworked from older deposits primarily late wisconsinan glacial and glaciomarine sediments during the post glacial sea level fall and holocene rise winnowed medium to coarse sands and lag gravels that are generally less than 0 5 m thick chracterize the areas above the lowstand elevation at 110 to 120 m the holocene sediments which have accumulated in the deeper interbank channels isolated basins and on the upper continental slope are thicker and consists of the fine sands and muds that were removed from the banks the deposits below the lowstand elevation coarsen upward because little deposition occured during the sea level rise in general the holocene succession becomes thicker and finer grained to the south active sediment movement occurs at all depths less than about 110 m wave ripples composed of sandy gravel form to depths in excess of 100 m in response to long period 15 s surface waves but the predominant bedforms consist of current generated sand ribbons megaripples and sandwaves which exhibit unidirectional migration azimuths toward the southwest even though the storm current directions are highly variable and the maximum sediment transport rates are directed toward the east or southeast the bedform orientation and resultant transport are instead parallel to the southerly and southwesterly storm currents which occur during the prolonged post storm spin down period and to the weak 1 labrador current and the orbital motion associated with the largestwaaves thus the interaction of the oceanic current and wave orbital motion with the directionally variable strom currents is responsible for the consistent orientation of the current generated bedforms and for the distribution of holocene deposits

model for turbidite to contourite continuum and multiple process transport in deep marine settings examples in the rock record abstract petrological analysis of geological sections in st croix in the caribbean the niesenflysch in switzerland and the annot sandstone in the french maritime alps sheds light on multiple process transport in deep marine settings a model depicting a turbidite to contourite continuum of stratal types is applied to these three rock units recognition of a diverse suite of bedforms coupled with analysis of paleocurrents helps to better interpret depositional origin and basin paleogeography the st croix strata record emplacement by gravity flows and subsequently by bottom currents flowing parallel to the base of slope these sediments accumulated on a lower slope apron a niesenflysch section in the swiss alps west of adelboden includes turbidites which were deposited at fairly regular intervals beyond the base of slope in a setting more distal than that of the st croix sequences most of these turbidites appear to have been partially reworked by bottom currents related to basin circulation or to density flows from the basin margins in the annot sandstone reworked turbidites termed transitional variants and packets of entirely rippled strata are observed in submarine fan and slope sequences in the peira cava area in contrast to those in st croix and the niesenflysch the current emplaced deposits of the annot sandstone are directly associated with fan valley deposits such rippled strata in channels are deposits of gravity flow origin which were subsequently reworked downslope by currents generated by successive gravity flows they also occur on levees by overbank flow consideration of multiple process transport is of special help to interpret sections which are poorly exposed or which can be examined in cores or which are located in sequences that have been highly deformed structurally

shallow plio pleistocene contourites on the hebrides slope northwest u k continental margin abstract british geological survey borehole 88 7 was drilled on the continental margin of the northeast rockall trough in a water depth of 560 m with the intention of coring an unbroken plio pleistocene succession the borehole penetrated 102 m of pleistocene and tertiary sediments with 60 recovery the majority of the plio pleistocene succession is composed of bioturbated muds and sandy muds with rare dropstones in most of the core bioturbation has obscured any primary sedimentary structures although in places the muds are laminated and graded on a centimetric scale subtly different trace fossil assemblages in the homogeneous and laminated core indicate that the preservation of laminae is not simply the result of a decrease in burrowing activity x radiographs of the apparently homogeneous sections of core indicate that both coarsening and fining upward sequences between 5 and 50 cm in thickness are present particle size analysis has identified larger variations in grain size over several metres sands in the plio pleistocene are not common and comprise under 8 of the total succession most of which directly overlie older tertiary sediments the mineralogy of the plio pleistocene sands implies reworking of the lower beds and their base is marked by a conglomeratic lag deposit the change between homogeneous and laminated muds appears to be the result of variations in the style of sediment deposition at times deposition rate was relatively rapid and formed a laminated sediment with a trace fossil assemblage containing bedding parallel forms this more rapid sedimentation rate may be the result of greater down slope transport of material to the north or south of 88 7 giving rise to the lateral redistribution of fine sediment which was laid down in laminated beds similar in form to distal turbidites when this lateral source was not present the background sedimentation of ice rafted and ice margin derived material gave rise to a poorly sorted deposit which was reworked into both coarsening and fining upward units which have similarities with muddy contourites all the plio pleistocene muds have been influenced by along slope currents during deposition

contourite drift molded by deep mediterranean outflow since its inception in early pliocene time the deep mediterranean outflow has eroded and sculptured the southern iberian margin the 50 km long 300 m thick faro drift constructed by this powerful bottom current has been the subject of detailed study sandy silty and muddy contourite facies can be clearly characterized and a distinctive vertical arrangement or sequence of these facies can be related to hydrodynamic fluctuation these results are important for the identification of ancient contourites the reconstruction of paleocirculation patterns and documentation of the effects of quaternary climates on the mediterranean outflow

identification of ancient sandy contourites proposed criteria for the identification of ancient sandy contourites include occurrence thin beds reworked sandy turbidites coarse lag structure bioturbated no regular vertical sequence orientation of cross lamination perpendicular to regional downslope paleocurrents grain size silt or sand little mud low or negative skewness fabric grain orientation and composition downslope and along slope provenance concentrations of heavy minerals a suggested three stage approach to recognition of sandy contourites in the field involves work on a progressively increasing scale with particular emphasis on paleocurrent data the various deep water settings in which disparate current directions may occur are summarized and those in which sandy contourites may be formed and recognizably preserved are indicated a broad definition of the term contourite is now possible because of the widespread documentation of contourites even in freshwater environments along slope flow that is persistent in time and space is a requirement

marine geology of the sodwana bay shelf southeast africa abstract the geostrophic current controlled northern zululand shelf displays a unique assemblage of interesting physical sedimentological and biological phenomena the shelf in this area is extremely narrow 3 km and is characterised by submarine canyons coral reefs and steep gradients on the continental slope three submarine canyons occur in the study area and are classified as mature or youthful phase canyons depending on the degree to which they breach the shelf these canyons originated as mass wasting features which were exploited by palaeo drainage during sea level regressions shelf lithology is dominated by a series of coast parallel patch coral reefs which have colonised beachrock and aeolianite sequences that extend semi continuously from 5 to 95 m and delineate late pleistocene palaeocoastline events the unconsolidated sediment on the shelf is either shelf sand mainly terrigenous quartz grains or bioclastic sediment large scale subaqueous dunes commonly form in the unconsolidated sediment on the outer shelf due to the agulhas current flow these dunes occur as two distinct fields at depths of 35 to 70 m the major sediment transport direction is towards the south but occasional bedload parting zones exist where the bedform migration direction changes from south to north

mediterranean undercurrent sandy contourites gulf of cadiz spain abstract the pliocene quaternary pattern of contourite deposits on the eastern gulf of cadiz continental slope results from an interaction between linear diapiric ridges that are perpendicular to slope contours and the mediterranean undercurrent that has flowed northwestward parallel to the slope contours and down valleys between the ridges since the late miocene opening of the strait of gibraltar coincident with the northwestward decrease in undercurrent speeds from the strait there is the following northwestward gradation of sediment facies associations 1 upper slope facies 2 sand dune facies on the upstream mid slope terrace 3 large mud wave facies on the lower slope 4 sediment drift facies banked against the diapiric ridges and 5 valley facies between the ridges the southeastern sediment drift facies closest to gibraltar contains medium fine sand beds interbedded with mud the adjacent valley floor facies is composed of gravelly shelly coarse to medium sand lags and large sand dunes on the valley margins compared to this the northwestern drift contains coarse silt interbeds and the adjacent valley floors exhibit small to medium sand dunes of fine sand further northwestward sediment drift grades to biogenous silt near the faro drift at the portuguese border because of the complex pattern of contour parallel and valley perpendicular flow paths of the mediterranean undercurrent the larger scale bedforms and coarser grained sediment of valley facies trend perpendicular to the smaller scale bedforms and finer grained contourite deposits of adjacent sediment drift facies the bottom current deposits of valleys and the contourites of the cadiz slope intervalley areas are distinct from turbidite systems the valley sequences are not aggradational like turbidite channel levee complexes but typically exhibit bedrock walls against ridges extensive scour and fill into adjacent contourites transverse bedform fields and bioclastic lag deposits both valley and contourite deposits exhibit reverse graded bedding and sharp upper bed contacts in coarse grained layers low deposition rates and a regional pattern of bedform zones textural variation and compositional gradation the surface sandy contourite layer of 0 2 1 2 m thickness that covers the gulf of cadiz slope has formed during the present holocene high sea level because high sea level results in maximum water depth over the gibraltar sill and full development of the mediterranean undercurrent the late pleistocene age of the mud underlying the surface sand sheet correlates with the age of the last sea level lowstand and apparent weak mediterranean undercurrent development thus the cyclic deposition of sand or mud layers and contourite or drape sequences appear to be related to late pliocene and quaternary sea level changes and mediterranean water circulation patterns since its pliocene origin the contourite sequence has had low deposition rates of

late cenozoic sediment drift complex northeast rockall trough north atlantic seismic reflection profiles and two shallow cores have revealed a sediment drift complex in the northeastern rockall trough the drift complex consists of an elongate drift with associated sediment waves a broad sheeted drift with a larger field of sediment waves and smaller moat related isolated drifts the internal reflection configuration of these features indicates consistent upslope migration throughout the late cenozoic the larger field of sediment waves has been described previously with a southerly migration a reevalution of these waves suggests a migration direction to the east conforming with the other drift complex features the most vigorous current activity probably occurred during the miocene with a more reduced current flow prevailing during the pliocene to holocene interval core evidence from the elongate drift and the moat has revealed upper pleistocene and lower holocene glaciomarine sediments reworked to produce muddy silty and sandy contourites sedimentation rates for the early holocene pre 7 5 ka are up to 4 times greater on the drift compared to the moat it is suggested that the sediment drift complex has formed through the interaction of a northward flowing slope current of north atlantic deep water origin with an area of complex bathymetry at the northeastern end of the rockall trough where the wyville thomson ridge intersects the hebridean margin

fossil contourites a critical review abstract despite three decades of study there is still great controversy over the recognition and interpretation of fossil contourites exposed in ancient series on land in order to best examine this problem we briefly review the evidence from modern systems including the many examples of cenozoic contourites that have been recovered from dsdp odp drilling on major drifts in the present day oceans the range of contourite facies described from both deep water 2000 m and mid water 300 2000 m drifts are mostly fine grained bioturbated and homogeneous often with a distinct bedding cyclicity and with some coarser grained sandy contourites developed under higher energy bottom currents there are also a number of current controlled sediment bodies that have formed in outer shelf upper slope settings 50 300 m under the influence of counter currents underflows and major surface currents these are not considered contourites sensu stricto but may be mistaken as such in ancient examples the most commonly described fossil contourites in the literature have been interpreted by the authors concerned as bottom current reworked turbidites however a critical review suggests that these are the facies most subject to misinterpretation and many of the sediments claimed as fossil contourites are almost certainly fine grained turbidites whereas others were more likely formed under outer shelf upper slope current systems there remain very few ancient examples that are more closely comparable to modern contourites these include the cretaceous talme yafe formation in israel the ordovician jiuxi drift in china and parts of the paleogene lefkara formation cyprus and the neogene misaki formation in japan we present a set of possible criteria for the recognition of fossil contourites and bottom current reworked turbidites

metallogeny of the lietinggang leqingla fe cu mo pb zn polymetallic deposit evidence from geochronology petrogenesis and magmatic oxidation state lhasa terrane abstract lietinggang leqingla deposit consists of lietinggang fe cu mo ore block and leqingla pb zn fe cu ore block locating in the southern margin of the central lhasa subterrane the ore occurs as laminated or lenticular units in skarns distributed in the contact zone between the jubuzhari complex intrusions and carbonate strata of the mengla formation intrusions associated with the two ore blocks are granodiorite granodiorite porphyry and granite porphyry of 62 4 1 2 ma 62 6 0 6 ma and 63 7 1 0 ma by zircon u pb age dating molybdenite re os analyses yield an isochron age of 59 0 1 7 ma with a mswd of 1 6 comparable isotopic ages between the intrusions and mineralization indicate that they are genetically related and occurred during or shortly before the main stage of the indian asian continental collision granodiorites have sio2 content of 62 0 69 1 wt al2o3 content of 15 1 15 3 wt total alkali concentration k2o na2o of 6 63 7 46 wt and vary in terms of the differentiation index di 67 78 rocks from the granodiorite porphyry and granite porphyry are characterized by high sio2 contents 72 3 74 0 wt 74 8 75 8 wt and di 89 90 93 94 contents of cao al2o3 mgo fe2o3 p2o5 tio2 of granodiorite granodiorite porphyry and granite porphyry show negative correlations with sio2 samples from the granite porphyry have the most negative eu anomalies eu 0 35 0 36 which are lower than the granodiorite porphyry and granodiorite eu 0 46 0 55 and 0 46 0 66 all the rocks are enriched in large ion lithophile elements rb ba pb k and th and depleted in high field strength elements ta ti and nb the granodiorites have zircon hf t values from 2 5 to 8 4 while hf t values of the granodiorite porphyries and granite porphyries range from 6 3 to 6 8 and 1 8 to 6 9 respectively these data suggest that the three types of intrusions are genetically related and the less evolved granodiorites are i type granites that were derived from a mantle crust mixed source the granodiorite porphyries and granite porphyries were strongly fractionated and most likely derived from the melts represented by granodiorites through fractional crystallization of plagioclase hornblende biotite ilmenite titanite apatite and zircon zircon trace elements average fmq values of granodiorites granodiorite porphyries granite porphyries are 0 15 0 07 and 0 25 respectively indicate that the lietinggang leqingla magmas were weakly oxidized to moderately reduced the involvement of mantle materials and medium oxygen fugacity controlled the fe cu mineralization while the contribution of ancient crustal end member and strongly fractional crystallization caused the pb zn mineralization of the lietinggang leqingla deposit combined with previously published data we suggest that the fe cu pb zn fertilities of magmas in the central lhasa subterrane and their metal series were not only influenced by magma sources but also by the evolution degree fractional crystallization and oxidation state of the magmas

orbital forcing of bottom current enhanced sedimentation on feni drift ne atlantic during the mid pleistocene feni ridge is a plastered sediment drift deposited along the northwestern flank of rockall trough ne atlantic under the influence of deep geostrophic currents formed by intermittent overflows of arctic intermediate water from the norwegian sea across the iceland scotland ridge nsow lower deep water ldw a derivative of antarctic bottom water from the south atlantic mixed with old north atlantic deep water nadw also bathes the lower flanks of the drift below 2500 m deep sea drilling project site 610 was drilled in a sediment wave field on the surface of feni drift 2417 m though the sediments recovered proved to be typically pelagic preserving a high resolution 5 cm kyr 1 quaternary record of orbitally modulated ice rafting and biogenic productivity cycles but showing little evidence of current affected deposition from particle size and compositional analyses of the biogenic and lithogenic 10 m silt which we then compared with records of the percentage and flux of ice rafted detrital ird sand foraminifera and coccoliths for the same interval cross spectral comparisons were then made between these records and variations in a model of northern hemisphere ice volume ice sheet growth and decay in response to orbitally forced insolation changes at 65 n variations in the relative proportions and fluxes of ird sand foraminifera and coccoliths all covary closely with orbitally forced ice volume fluctuations but with differing amplitude coherency and phase relationships at each milankovitch frequency however there is a significant component of variance in pelagic and ice rafted sedimentation which is controlled by bottom currents and this shows excellent phase and coherency agreement with the ice volume model at all orbital frequencies the relative enrichment coarseness and flux of the contourite silt fraction of feni drift sediments all increase during glacial or cold stages of the mid pleistocene relative to warm stages suggesting that currents were stronger during these periods possible explanations for such changes in current activity include 1 an increased flux of nsow via the wyville thompson ridge into the rockall trough during glacials but not due to increased production of nsow 2 ldw presently lying below 2500 m rises during glacials by at least 50 60 m thereby flooding over the surface of feni ridge at site 610 3 the path of a major recirculation gyre in the lower nadw ldw system presently situated marginally south of site 610 encroaches upon the southern tip of feni ridge during glacials and 4 the rockall trough itself is a site of deep water formation during glacials analogous to labrador sea deep water formation today

i n atei palaeolake documents past environmental changes in central sahara at the time of the green sahara charcoal carbon isotope and diatom records during the green sahara event water bodies developed throughout the sahara and sahel reflecting the enhanced influence of the atlantic monsoon rainfall major lakes then dried out between 6 5 and 3 5 ka this study investigates land cover change and lacustrine environment during the holocene at i n atei southern algeria a desert region lying in the hyperarid core of the sahara this site is remarkable by its extent up to 80 km 2 and by the exceptional preservation and thickness of the lacustrine deposits 7 2 m i n atei was a lake from 11 to 7 4 ka then it dried out and left place to a swampy environment charcoal concentrations show that the surroundings of the lake were vegetated throughout the wet period with two short phases of possible vegetation deterioration associated with a lowering of the lake level at 9 3 and 8 2 ka coeval with well known dry events in the tropics the stable carbon isotope record reflects the penetration of c4 herbaceous populations in replacement of the original c3 typical of the regional vegetation at the time of the maximum lake expansion the 13 c of charcoals increase non linearly with the 14 c based ages from 24 5 to 13 0 v pdb assuming that these extreme values sample both c3 and c4 plant end members mass balance calculations suggest that c3 were replaced by c4 plants according to an exponential decay law with a half life t 1 2 of 850 110 years the replacement of c3 by c4 plants occurred in two main steps a mixed c3 c4 vegetation of wooded grassland type was present from 10 ka to 8 4 ka while a c4 exclusive vegetation developed after 8 4 ka after the end of the lacustrine phase a catastrophic event flooding provoked the lifting of most of the lacustrine deposits and their re deposition above the lacustrine sequence

a fossil carbonate contourite drift on the lower ordovician palaeocontinental margin of the middle yangtze terrane jiuxi northern hunan southern china abstract the early ordovician tremadocian in part of jiuxi northern hunan province comprises a succession of deep water carbonate sediments deposited on a palaeocontinental margin at the southern edge of the yangtze platform a distinctive elongate mound like form some 350 450 m thick can be recognized lying between shallow water platform carbonates and deeper water mudstones this elongate body here called the jiuxi drift is made up dominantly of sediments interpreted as contourites on the basis of their mid to base of slope location alongslope current indicators features of traction flow processes coupled with intense bioturbation and distinctive contourite sequences typically 30 80 cm thick five contourite facies are recognized calcilutites calcisiltites calcarenites calcirudites possibly contourite lag deposits and bioclastic contourites hemipelagites and turbidites make up only a small proportion of the drift which accumulated at an average sedimentation rate of 38 m m y large scale cross stratified units found in parts of the calcilutitic contourite section are believed to result from seafloor development of mudwaves and or erosional furrows under the influence of a semipermanent bottom current regime

development of sediment drifts approaching an active plate margin under the sw pacific deep western boundary current the abyssal pacific ocean is fed by a 1000 km wide deep western boundary current dwbc that flows northward along the continental margin east of new zealand between the passive margin of chatham rise and the subduction zone of kermadec trench a distance of 1200 km the dwbc has formed a suite of sediment drifts over a depth range of 2200 5700 m airgun and 3 5 khz profiles record a variety of drift types that reflect regional variations in bathymetry sediment supply and the tectonic volcanic framework on chatham rise the dwbc has deposited a sinuous linear body along the south flank 3000 m an extensive apronlike drift on the north flank 2200 4500 m and a ridgelike drift about the rise base 4500 5200 m the flow has also deposited a body of sediment over 400 km long within a moat at the base of the nearby louisville seamount chain further downcurrent the 250 km long rekohu drift 3600 4190 m has developed northward to 39 s south of this latitude drifts comprise mainly reworked pelagic hemipelagic material and sediment transported from distant southerly sources in contrast drifts north of 39 s have received a major injection of terrigenous sediment from hikurangi channel which runs 1400 km from new zealand eastward across the hikurangi plateau to disgorge on to the abyssal floor at the plateau edge en route turbidity current overspill from the channel has moved north under the influence of the shallow dwbc to contribute to a series of small ridge and patch drifts among the numerous seamounts on the plateau at 3500 4200 m off hikurangi channel mouth a large fan has accumulated the dwbc has extended the fan into a drift running over 250 km along the base of hikurangi plateau 5150 5770 m toward kermadec trench here drift sediment becomes increasingly disrupted by mass wasting associated with the active subduction in this area the seismic stratigraphy reveals the drifts to rest mainly on a widespread erosional surface that is interpreted to mark the inception of the dwbc in the region with the late oligocene opening of the australian antarctic seaway drift construction commenced during the miocene but was punctuated in the late miocene by another period of erosion that coincided with increased bottom water production in antarctica deposition resumed in plio pleistocene times when large quantities of sediment from the rapidly rising landmass of new zealand were injected into the boundary current the modern flow continues to affect drift deposition as manifest by an active boundary channel along the foot of hikurangi plateau and widespread scour zones and sediment wave fields

contourite drift types and their distribution in the north and south atlantic ocean basins abstract patterns of sediment accumulation under the control of deep bottom currents depend on interacting factors intensity of the regional deep circulation morphology of the sea floor coriolis effects abundance of terrigenous and biogenic supplies subsequent turbidity of deep nepheloid layers and depth of the ccd based on many examples of bottom current deposits from the atlantic and from other oceans we recognise three major types of contourite accumulation 1 giant elongate drifts 2 contourite sheets and 3 channel related drifts they generally occur in distinct topographic settings and show different geometries facies and rates of deposition in the north atlantic strong bottom circulation an open basin system and high sediment supply have led to the construction of large elongate contourite drifts which have striking relief 200 1000 m above the surrounding sea floor and are composed of muddy contourites rich in calcareous pelagic material in the south atlantic strong bottom currents are often trapped in gyre like circulation patterns around topographically closed basins or are accelerated through deep narrow connecting passages the resulting deposits are mainly extensive contourite sheets covered by fields of large sediment waves and channel related drifts including contourite fans constructed at the downstream exit of deep passages

sedimentary characteristics of late pleistocene bottom current deposits barents sea slope off northern norway abstract the barents sea slope off northern norway received large amounts of terrigenous sediments from an extensively glaciated norwegian continental margin during the late pleistocene a sedimentological study of seven gravity cores from the barents sea slope suggests that some terrigenous sediments were transported by both downslope currents and contour following bottom currents the bottom current deposits comprise four sedimentary facies 1 indistinctly laminated mud 2 layered mud 3 indistinctly layered mud and 4 bioturbated mud the indistinctly laminated mud on the mid to lower slope is characterized by a relatively low degree of bioturbation and consists of monotonous diffuse and discontinuous silt rich and silt depleted laminae 1 mm scale and or discontinuous trains of coarse grains these characteristics are suggestive of sedimentation from contour currents driven by water exchange with the north atlantic ocean these bottom current deposits can be distinguished from fine grained turbidites silt clay couplets and intensively bioturbated hemipelagites on the barents sea slope

radiolarian contourites record eocene aabw circulation in the equatorial east atlantic abstract 50 m of middle eocene pure radiolarian ooze were drilled at odp site 660 in the equatorial east atlantic 80 km northeast of the kane gap the oozes comprise a 10 m high and 2 km broad mound of seismic reverberations covered by manganese rich sediment and contain trace amounts of sponge spicules and diatoms negligible organic carbon 0 15 clay and variable amounts of pyrite the yellow to pale brown silty sediments are relatively coarse grained 30 45 coarser than 6 m little bioturbated and commonly massive or laminated on a cm scale the unlithified radiolarian ooze may indicate an interval of high oceanic productivity probably linked to a palaeoposition of site 660 close to the equatorial upwelling belt during middle eocene time the absence of organic matter however and both the laminated bedding and the mound like structure of the deposit on the lower slope of a continental rise indicate deposition by relatively intense contour currents of oxygen rich deep water which passed through the kane gap winnowed the fine clay fraction and prevented the preservation of organic carbon the ooze may be either a contourite lag deposit or a contourite accumulation of displaced radiolarians originating south of the kane gap and being deposited in its northern lee thus documenting the passage of a strong cross equatorial bottom water current formed near antarctica these eocene contourites may be an analogue for ancient radiolarites in the tethyan ocean

ordovician contourites in the lachlan fold belt eastern australia abstract the ordovician sequence in the northeastern lachlan fold belt consists of three lithologic units representing an overall fining upward succession from turbidites to black shale the lowest unit dominated by turbidite beds represents a small segment of an extensive ordovician submarine fan sequence that covered much of southeastern australia the overlying unit consists predominantly of ripple bedded sandstone with beds 3 5 cm thick these beds are texturally mature containing heavy minerals mica and silt concentrated on the cross laminae the ripple bedded unit has a mean palaeocurrent trend to 321 which is at 60 to sole marks and almost normal to the trend of ripple cross bedding in the underlying and associated turbidite beds this unit is here interpreted as a sandy contourite sequence that can be attributed to contour currents moving northwards along the eastern margin of gondwana from areas of subpolar downwelling off southern africa and america the two major fining upward successions in the ordovician sequence in southeastern australia may reflect eustatic sea level rises during the middle and late ordovician and thus provide evidence of interglacial periods in the darrawilian and late gisbornian to bolindian the contourites would have accumulated in the latter stages of the glacial periods after sea level had risen sufficiently to cut off major sediment supply to the submarine fan but before the temperature had risen to the extent of preventing deep thermohaline circulation

turbidite to current reworked sand continuum in upper cretaceous rocks u s virgin islands abstract a purely turbidite or gravity emplaced interpretation does not adequately explain the origin of marine volcaniclastic strata in upper cretaceous units in st croix u s virgin islands most sandy layers herein termed intermediate variants record emplacement in two stages first by turbidity currents flowing toward the south and then a subsequent reworking of these layers by bottom currents flowing toward the west the sands accumulating on lower slope aprons were then displaced along bathymetric contours before burial a natural continuum of deposits results between downslope directed turbidity current and bottom current tractive end member deposits the lower part of a variant layer typically preserves the original graded basal a turbidite division while the texturally cleaner middle and upper parts of such a stratum usually show structures more typically associated with tractive transport an inventory of the diverse suite of reworked sandy lithofacies in st croix may be comparable to other cases in the rock record and modern oceans that involve interaction between gravity driven flows and bottom currents

sedimentation on the feni drift and late glacial bottom water production in the northern rockall trough abstract between the last glacial maximum 18 ka and the present day a variety of contrasting oceanographic conditions including changes in surface and bottom water circulation patterns have existed in the north atlantic evidence for the presence of bottom currents in winnowed silt horizons in the feni drift suggests that deep water may have been produced in this region during glacial deglacial intervals current activity and thus perhaps deep water production was apparently intermittent and may have been controlled by a process analogous to the formation of labrador sea water this involves cooling and deep ocean convection during winter months physical factors controlling this production would be temperature and salinity of upper waters which would govern the density and wind stress necessary for vertical overturn coupled with absence of ice cover evidence of bottom current activity suggests that deep water production occurred in the rockall trough throughout termination 1a which closely corresponds to fairbanks 1989 meltwater pulse 1a mwp 1a production of deep water in this region apparently ceased shortly after the decline in influence of the meltwater influx

models for the deposition of mesozoic cenozoic fine grained organic carbon rich sediment in the deep sea the widespread occurrence of organic carbon rich strata black shales in certain portions of jurassic cretaceous and cenozoic sequences has been well documented from deep sea drilling project sites in the atlantic and pacific oceans and from sequences now exposed on land originally deposited in the tethyan ocean these ancient black shales usually have been explained by analogy with examples of modern deep sea sediments in which organic matter locally is preserved by 1 increasing the supply of organic matter 2 increasing the rate of sedimentation and or 3 decreasing the oxygen content of the bottom water however detailed examination of many black shales reveals characteristics that cannot be explained by simple local models including their approximate coincidence in time globally their occurrence in a variety of different environments including open oxygenated oceans restricted basins deep and shallow water their interbedding with organic carbonpoor strata which often dominate a so called black shale sequence their deposition by pelagic hemipelagic turbiditic and other processes and the variations in type and amount of organic matter that occur even within the same sequence a more complex model for the origin of black shales therefore appears most appropriate in which the cyclic preservation of organic matter depends on the interplay of the three main variables namely supply of organic matter sedimentation rate and deep water oxygenation each of which varies independently to some extent the variation and relative importance of these parameters in individual basins and widespread black shale deposition in general are linked globally and temporally by changes in global sea level climate and related changes in oceanic circulation an important and often overlooked factor for the supply of organic matter to deep basin sediments is the frequency and magnitude of redepositional processes the interplay of these variables is discussed in relation to the middle cretaceous and cenozoic organic carbon rich strata in particular which show marked differences in the relative importance of the different variables

sand waves echinocardium traces and their bathyal depositional setting monte torre palaeostrait plio pleistocene southern italy stacked cross sets up to 2 5 m thick produced by sand wave migration and meniscate trace fossils produced by echinocardium cordatum both considered in the literature as typical of shallow water marine depositional settings commonly occur in the bathyal plio pleistocene deposits of monte torre calabria southern italy the plio pleistocene sediments form two coarsening upward depositional sequences separated by an unconformity and by a palaeobathymetric gap of at least 300 m the lower sequence passes upwards from hemipelagic marls and thin bedded turbidites to thick bedded sandy turbidites then to sand wave deposits alternated with sandy turbidites and finally to base of slope megabreccias facies characteristics and relationships and the occurrence of deep sea faunal associations indicate deposition in the bathyal zone the facies of the upper sequence reflect a fan delta environment no deeper than a few tens of metres the depositional setting of the lower sequence where the sand wave deposits and meniscate trace fossils occur appears to have been a tectonically controlled seaway connecting the tyrrhenian and ionian seas this seaway became progressively narrower with time evolving into a strait the overall coarsening upward trend reflects the upward transition from a low to a high energy environment possibly caused by the tectonic narrowing of the seaway deposition and erosion from high concentration turbidity currents and from tidal bottom currents were important processes periods of tectonic activity producing first the uplift of the seaway margins and culminating with the uplift of the strait sequence itself are marked by scattered rockfall deposits the strait setting causing the development of powerful oxygenated bottom currents produced optimal conditions in the bathyal zone for the colonization of sandy bottoms by a single infaunal r selected species echinocardium sp

bottom current controlled sedimentation a synthesis of the contourite problem abstract an overview of the main items concerning deep bottom current controlled deposits is presented these include the definition of contourites their processes of deposition and facies characteristics contourite drift types and seismic patterns the interplay of turbidity and bottom current processes and the correlation between bottom current processes and global changes in climate sea level from this discussion several of the more significant problems are highlighted which we believe should be priority targets for future research 1 identification of contourites especially in ancient series where they are interbedded with turbidites and other facies 2 refinement of the numerous parameters used as tracers of palaeocirculation patterns 3 understanding the interaction between the different types of bottom currents and also between bottom current and other deep water processes 4 distinction in the sedimentary record between the effects of short time scale bottom current variations due to regional causes like benthic storms and geological scale current fluctuations related to climatic or astronomic control 5 understanding the control of glacial interglacial climatic cycles on the nature and rate of contourite deposition