laboratory experiments on the formation of subaqueous depositional gullies by turbidity currents abstract gullies and furrows are shallow sub parallel features widespread on the seafloor and some lacustrine environments vast fields of linear channels of erosional origin called furrows with spacings of meters and up to hundred of meters are common in settings with relatively low seafloor slopes and have been previously ascribed to helical motion in benthic boundary layers submarine gullies are linear donwslope trending features of erosional depositional or mixed nature commonly observed in the upper continental slope submarine canyon walls and other relatively steep submarine environments while gully fields have been associated to the action of unconfined turbidity currents and other cascading dense flows their relatively large spacings ranging from several hundreds of meters to 1 2 km suggest that these features might be the result of other mechanisms different than boundary layer helical motion in this paper we propose an alternative mechanism for the initiation of gullies observed in relatively steep seabed gradients downslope of a seabed slope change such as those observed in continental margins self slope break or submarine canyon walls our working hypothesis is that gullies could be the result of a centrifugal instability that sets in interfaces of gravity currents as they flow through a bedslope transition since for these cases the centrifugal force excess is of the same order of magnitude of the driving force and therefore a potential source of flow instability we propose that gully spacing might be controlled by the magnitude of the centrifugal force scaling roughly with the radius of curvature of the transition slope break we also present experimental evidence on the formation of longitudinal features resembling slope gullies that were created in a laboratory tank after running successive supercritical depositional turbidity currents through a slope change similar to that observed in continental margins gullies in the tank developed through differential deposition only and showed spacing an order of magnitude larger that that expected from a boundary layer helical motion origin these experimental results along with the observation that gullies were not formed in the tank without a bed slope change low to high support the hypothesis of a centrifugal instability origin for longitudinal features in relatively steeper submarine settings our experimental results also support our first order approximate gully spacing rule which is in general agreement with field observations of gully spacing in upper continental slopes and submarine canyon walls

the initiation of sedimentary furrows by standing internal waves longitudinal helical vortices are generally considered to be the cause of longitudinal sedimentary bedforms in tidal oscillatory flows however it is not clear how a regular system of vortices will become fixed for long enough to establish the bedforms it is proposed that the presence of standing internal waves with axes parallel to the tidal current have provided the basic flow pattern on which longitudinal helical vortices develop in southampton water the observed furrows then developed in a pattern determined by the standing internal waves

the barremian aptian arid phase in western europe abstract evidence is presented from nine areas in western europe speeton yorkshire southern north sea channel and weald basins southern england jeanne d arc lusitanian and aquitanian basins of the north atlantic continental margins lower saxony basin north germany the paris basin and southern france for a phase of aridity which began during the hauterivian middle barremian and continued to dominate the climate of the region until the mid aptian although conditions during this period were not uniformly dry throughout criteria for recognising arid environments include the occurrence of evaporites abundances of carbonate rich sediments red beds firmgrounds carbonate nodules and pyrite concretions depletion of kaolinite spore assemblages of limited morphological diversity and the presence of numerous coniferalean and cheirolepidiacean pollen grains the change from generally humid to dry climates during the barremian is thought to have been linked to a lowering of sea level the arid phase clearly falls within the time span of the first cretaceous oceanic anoxic event

ecological distribution of cenomanian terrestrial plants based on 13c 12c ratios the cenomanian lagoonal member of the argiles du baugeois anjou france yielded a rich and exceptionally well preserved fossil leaf flora stable carbon isotope ratios of fossil plants were measured in order to investigate the presence of a palaeoenvironmental signal the small intraspecies variations of 13c values observed for most of the fossil leaves suggested that the isotopic signal had not been significantly altered hence an isotopic approach was used as an attempt to assist in reconstructing the ecological distribution of the flora the 13c enrichment of the fossil leaves suggested that the plants underwent water or salt stress the occurrence of the conifer frenelopsis suggested at least locally a saline environment such as a lagoon according to their decreasing 13c values all the plants collected could have been distributed along a decreasing salinity transect from the lagoon to a flood plain the predominant species eretmophyllum andegavense a fossil ginkgo exhibited a wide range of 13c values this could indicate that this taxon grew in a wide range of habitats with varying salinities the origin of the organic matter appeared to be terrestrial with a major contribution from water stressed plants such as frenelopsis as suggested by the similarity between 13c values of the conifer and the sedimentary organic matter 13c values of the sediment indicated stable palaeoenvironmental conditions during the deposition of the argiles du baugeois member hence while detailed palaeovegetation reconstructions are generally limited to the few deposits which present in situ palaeoflora the contribution of isotope geochemistry could allow reconstructions for a wider range of fossil plant deposits

a walk down the cap de creus canyon northwestern mediterranean sea recent processes inferred from morphology and sediment bedforms the cap de creus canyon northwestern mediterranean sea belongs to a complex network of submarine canyons cutting the western gulf of lion continental shelf and opening into the larger sete canyon swath bathymetry data mak 1m deep towed side scan sonar imagery and 5 khz high resolution seismic reflection profiles show striking morphologies in the cap de creus canyon floor and walls as a consequence of the canyon head and the upper reach severe incision the continental shelf dramatically narrows in front of the creus cape promontory the upper canyon has a flat bottomed thalweg incised in a mega scale sediment furrow field displaying hyperbolic seismic facies the tens of kilometres long linear furrows extend also over the middle canyon down to 1400 m of water depth the furrows on either side of the canyon are not parallel but oblique and display varying degrees of excavation mid channel sediment bars are locally present in the thalweg which is made of sandy lag deposits as revealed by its acoustic response and verified by sediment samples the middle canyon is linear and steep with an up to 700 m high southern wall contrasting with the sinuous smooth lower canyon which is controlled by flowage of the underlying messinian evaporites large sections of the canyon are affected by sediment instability processes the lower cap de creus canyon hangs up to 260 m over its distalmost reach and the sete canyon through a narrow less than 1 km wide gorge numerous scours up to 10 m deep suggesting bed load transport occupy the lower sete canyon immediately downstream of the cap de creus canyon mouth the data set provides the first complete very high resolution imaging of a submarine canyon from its upper part down to its distalmost reach the observations evidence a wide set of erosion transport and deposition processes along the cap de creus canyon including sediment entrapment at the canyon head furrow generating dense water cascading through the southern wall along channel currents strong enough to excavate specific sections of the channel floor and bed load sediment transport as demonstrated by the presence of mega ripples crescent scours and grooves in the lower canyon

sedimentary furrows and organized flow structure a study in lake superior investigations of the geology and physical limnology of a field of sedimentary furrows in lake superior have provided information on the characteristics of these bedforms and their relationship to the wind driven currents and associated turbulence structures in the bottom boundary layer the results generally support the existing conceptual model for furrow formation furrows in the study area 2 km2 at 100 m depth are spaced 20 l 00 m apart and exceed 800 m long the troughs being 0 5 m deep and 3 5 m wide sediment cores collected by submersible are poorly sorted cohesive terrigenous silts with a mean grain size of 20 pm relative to sediments in the interfurrow area those in the trough contain higher fractions of sand and coarse debris and accumulate at a rate 15 slower strong bottom currents speed 6 cm s l usually flow within 20 of the furrow direction and are most frequent between late october and early february when wind forced currents reach the bottom annual peak speeds approach 30 cm s l the relationship of the current and temperature records to the wind field is consistent with predictions of coastal jet theory profiling current meters deployed on either side of a furrow showed near bottom cross stream flow converging over the furrow with divergence higher in the boundary layer when the current speed exceeded 6 cm s l a preliminary numerical model exhibits a cross stream flow pattern consistent with the profiler observations

short distance variability in slope bed forms along the southwestern adriatic margin central mediterranean abstract the slope of sw adriatic margin sam is impinged today by two bottom water masses of thermohaline origin that interact constructively along a preferred path the levantine intermediate water liw generated in the eastern mediterranean through intense evaporation and flowing along the contour and the north adriatic dense water naddw formed on the shallow north adriatic and cascading episodically off the shelf these two southward flowing bottom water masses impact the upper slope which is characterized by an irregular physiographic setting and together create the bottom current deposits e g fine grained contourite drifts and sediment waves and related erosional features e g furrows scours comet marks this study is focussed on a small open slope area of the sam ca 35 25 km where high resolution seismic data allow observe elongated contour parallel scours widespread erosion surfaces accompanied by wide crescent shape scours and furrow fields and giant comet marks spatially associated and genetically linked along a common current path further downslope and away from this highly energetic current path a field of sediment waves migrates upcurrent and shows bifurcated crests in some cases the downcurrent limbs of the sediment waves display a well defined erosional character on seismic profiles and furrows on side scan sonar mosaics all bottom current features observed are in equilibrium with the present day oceanographic regime as documented by the available current metre data moreover sediment cores suggest that the sediment waves are actively migrating during the present interglacial possibly implying that bottom waters formation was enhanced after the shallow adriatic shelf was drowned during the late quaternary eustatic rise the sam slope represents therefore an ideal site to improve our understanding of the depositional impact of bottom currents along the mediterranean margins and may provide useful hints on the activity of bottom currents elsewhere on the mediterranean margins

comparing the smooth parabolic shapes of interfluves in continental slopes to predictions of diffusion transport models abstract computer models of stratigraphic development at continental margins often include elements in which sediment topography evolves according to a diffusion equation representing effects of down slope sediment transport however there has been little effort to verify the model for example by showing whether deposit geometries are as expected from it or by considering whether transport is consistent with the model s assumptions the issue is important for understanding transport erosion and deposition in continental slopes more generally and the extent to which such processes are recorded in seabed morphology examples of slope topography illustrating diffusive like characteristics are examined here in the uppermost slope of the usa central atlantic margin sandy areas between canyon heads are smoothly convex upwards and almost parabolic a morphology similar to subaerial soil mantled hillslopes known to evolve according to a diffusion equation other data of interfluves in muddy sediment from both atlantic and pacific usa margins also show this smooth convex upwards form in subaerial hill crests the upwards convex parabolic shape arises where steady erosion by streams along channels provide constant flux boundary conditions i e the parabola is the steady state solution to the diffusion equation although submarine processes will likely be different the similar morphology of submarine interfluves prompts the question as to whether erosion along channels and movements of sediment between channels can also sometimes occur to cause a diffusive like evolution of topography the discussion therefore focuses on submarine processes affecting erosion and deposition in particular to address whether the morphology results from true diffusion which requires that sediment moves down slope with a transport flux proportional to local bed gradient or is fortuitous sediment transport is instead unrelated to local bed gradient and the morphology arises from other effects on accumulation rates

furrowed outcrops of eocene chalk on the lower continental slope offshore new jersey a sea bottom of middle eocene calcareous claystone cut by downslope trending furrows was observed during an alvin dive to the mouth of berkeley canyon on the continental slope off new jersey the furrows are 10 to 50 m apart 4 to 13 m deep linear and nearly parallel in water depths of 2 000 m they have steep walls and flat floors 3 to 5 m wide of fine grained sediment mid range sidescan sonar images show that similarly furrowed surfaces are found on nearby areas of the lower continental slope not associated with canyons the furrows are overlain in places by pleistocene sediments although they show evidence of erosional origin they do not appear to be related to observed structures and their straight parallel pattern is not well understood a general cover of flocky unconsolidated sediments implies that bottom current erosion is not active now

morphological changes and sedimentary processes induced by the december 2003 flood event at the present mouth of the grand rhone river southern france the study which is based on repetitive bathymetric surveys assesses changes and effects of one of the most important floods recorded in the rhone delta area bottom morphology and sediment distribution in the rhone outlet after the december 2003 flood are discussed by comparison between digital terrain maps dtms of november 2003 and january 2004 the post flood morphology shows that the whole of the system is active mainly in the east the channel of the rhone has been hollowed out on the left bank by more than 5 m the eastern coast of the mouth has retreated 400 m the mouth bar has prograded 200 m and the slope of the delta front has increased by 0 19 the overlay of pre and post flood dtms makes it possible to estimate the total volume deposited in the 0 to 20 m zone as 7 8 10 the acquisition of bathymetric measurements immediately before and after a major flood allows more than a simple morphological description in addition an analysis of the solid load transfer processes towards the prodelta is achievable three mechanisms are highlighted most of the material eroded in the channel and supplied to the delta front corresponds to fine sediment that drape homogeneously over the pre flood morphology of the delta front the transit of the coarsest sediment primarily sand is slowed down in the channel of the rhone this sediment builds up in relatively small areas leading to the formation of gullies on the prodelta slope the gullies have fixed positions and disappear gradually towards the west following the progressive migration of the active band towards the east these old features are not reactivated by floods occurring after their formation during the flood only a small volume of coarse sediment bypasses through the gullies and the fine deposits are remobilised rapidly or compacted flood input concerns essentially the delta front which traps 90 of the fluvial solid discharge between 0 and 20 m depth the coarse prodelta supply is then due mainly to mass movements of unconsolidated material deposited beforehand on the top of the delta front consequently the main nourishment of the prodelta does not occur directly during and or immediately after a flood event but is delayed by sediment being temporarily trapped on the delta front

evolution of continental slope gullies on the northern california margin abstract a series of subparallel downslope trending gullies on the northern california continental slope is revealed on high resolution seismic reflection profiles imaging the uppermost 50 m of sediment the gullies are typically 0 m wide and have 1 to 3 m of relief they extend for 10 to 15 km down the slope and merge into larger channels that feed the trinity canyon in the lower half of the 50 m stratigraphic section the gullies increase in both relief and number up section to maxima at a surface 5 to 10 m below the last glacial maximum lowstand surface gully relief increased as interfluves aggraded more rapidly than thalwegs erosion is not evident in the gully bottoms therefore gully growth was probably due to reduced sediment deposition within the gullies relative to that on interfluves as the gullies increased in relief their heads extended upslope toward the shelfbreak at all times a minimum of 10 km of non gullied upper slope and shelf stretched between the heads of the gullies and the paleo shoreline the gullies did not connect with a subaerial drainage network at any time gully growth occurred when the gully heads were in relatively shallow water 200 m paleo water depth and were closest to potential sediment sources we suggest that prior to the last glacial maximum the mad river then within 10 km of the gully heads supplied sediment to the upper slope which fed downslope eroding sediment flows these flows removed sediment from nearly parallel gullies at a rate slightly slower than sediment accumulation from the eel river 40 km to the south the process or processes responsible for gully growth and maintenance prior to the last glacial maximum effectively ceased following the lowstand when sea level rose and gully heads lay in deeper water 300 m water depth farther from potential sediment sources during sea level highstand the mad river is separated from the gully heads by a shelf 30 km wide and no longer feeds sediment flows down the gullies which fill with sediment from the distal eel river approximately one half of the subsurface gullies have no expression on the seafloor because they have completely filled with sediment following the last glacial maximum lowstand of sea level

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

side scan targets in lake superior evidence for bedforms and sediment transport a high resolution side scan sonar survey of the lake bed off the keweenaw peninsula lake superior demonstrates that bottom currents are affecting lake bed morphology at depths up to 240 m numerous lineations which run parallel to the shore appear to be sand ribbons a field of sedimentary furrows which occurs in one area demonstrates the long term directional stability of the near bottom flow large 100 300 m in diameter 2 5 m deep unusual ring like or arcuate depressions are common throughout the western half of lake superior these rings themselves do not appear to have been formed by bottom currents but may have developed as water was released by the rapid compaction of glacial sediments which underlie the lake bed off the keweenaw peninsula the forms of the rings have been modified by bottom currents the bottom currents which have modified the lake bed are probably generated when storms cross the lake at times when it is poorly stratified

spatial variability in sedimentary processes on the eel continental slope the northern california continental margin receives episodically large quantities of sediment from the eel river which seasonally discharges to the shelf regional circulation broadcasts this sediment widely not only affecting sedimentation on the shelf but on the slope as well a suite of 60 cores has been collected to examine the processes of sediment accumulation and sediment delivery to the eel continental slope in waters 800 m deep spatial surveys of accumulation rate 0 2 1 3 g cm 2 year 1 and surficial sediment grain size 4 8 5 demonstrate that sediment is accumulating throughout the eel margin reflecting a rapid and widespread redistribution of the fluvial discharge downcore grain size profiles of heterogeneous sediments suggest that hemipelagic and other episodic sediment delivery processes may be important for the slope in areas proximal to the river in more distal areas downcore profiles reveal relatively homogeneous sediments delivered dominantly by hemipelagic processes a budget for fine grained sediment shows that this upper portion of the slope contains at a maximum 20 of the river s annual discharge a combined budget for the shelf and upper slope suggests that at least 60 of the annual sediment load is not accounted for in these areas the eel continental slope because of the large terrigenous sediment input the narrow width of the adjacent shelf and its energetic environment is a good analogue for slopes during transitional stands of sea level and for modern tectonically active settings processes typically associated with these geological settings i e supply of sediment to the slope and downslope mass movement are actively occurring 1999 elsevier science b v all rights reserved

shelf slope sedimentation during the late quaternary on the southwestern kuril forearc margin northern japan abstract we studied an active forearc margin off eastern hokkaido northern japan to identify the main influences on stratigraphic development from the last glacial to the present highstand this paper presents new data on the environment texture and sedimentation rates of forearc shelf slope sediments based on more than 300 samples of seafloor sediments and densely gridded sub bottom profiling records lowstand sedimentary wedges developed upon the shelf margins in areas with a large sediment supply and without incising canyons the transgressive and highstand deposits formed on the shelf in extensive low gradient and topographically low areas the narrow shelf is covered by sandy sediments where winnowed fines are likely to have escaped to the slope via gravity driven across shelf transport or ocean current induced along shelf transport the slope has a mid slope mud belt at water depths of 700 1600 m the sedimentation rates on the slope subsequent to 15 ka the late transgressive to highstand stage were just 10 70 of the rates prior to this period these changes in sedimentation rates are ascribed to spatially variable topography high sedimentation rates were maintained at topographically low and gently sloping areas even during highstand periods due to concentrations of nepheloid layers or deposition via sediment gravity flows on the other hand low sedimentation rates were recognized on topographic highs of interfluves on the upper slope or on axes of anticlines where main flows or overspills of turbidity currents decreased as sealevel rose these results suggest that sedimentologic and stratigraphic variations are tied to variations in the physical configuration of the shelf slope system being influenced by the local topography in addition to the climatic and oceanographic processes

long lived magmatic phases at los azufres volcanic center mexico abstract the los azufres volcanic complex has had a long eruptive history of 1 5 myr or more less than 3 myr subsequent to the building of a large miocene and pliocene andesitic complex since about 1 5 ma its development consists of two main periods of volcanic activity the first period from 1 5 to 0 8 ma has two magmatic cycles about 200 000 years in duration characterized by acidic followed basaltic volcanism such a magmatic evolution suggests successive emptyings of a shallow reservoir in which differentiation previously occurred about 0 6 ma major deep magma supplies were probably responsible for the resurgent doming which led to uplift of the southern part of the caldera from 0 6 ma to present the second period exposes volcanic products grading from basalts to rhyolites considering the recent age of the last ignimbrites 26 000 to 29 000 years one can assume that the volcanic activity related to a voluminous differentiated magma body at shallow depth has not yet ended especially in the southern area in and around the resurgent zone occupied by the geothermal field

experimental investigation on self channelized erosive gravity currents submarine canyons are the main conduits for sediment transfer from continental shelves to deep abyssal plains a large number of bathymetric and seismic surveys provide detailed information on their morphology and structure shedding light on the mechanisms involved in their formation however because of the difficulty in deploying instruments in deep marine environments little is known about the processes by which turbidity currents erode and incise the canyon floors in this study we use a micro scale tank experiment to investigate the incision of a mobile sediment bed by a gravity current we inject a brine at the top of a ramp covered with low density plastic sediments conditions for channel formation are investigated through systematic exploration of the slope vs discharge phase space measurements of the sediment bed topography and the gravity current velocity profile allow us to characterize the channel incision dynamics a channel forms when the bed shear stress exceeds the sediment critical shear stress in this case a channel starts forming near the upstream end of the incline and propagates down slope while slowly deepening after a transient state of about 1 2 hours the channel reaches steady state experimental results suggest that the slope controls the rate of erosion and the speed of channel incision while the brine discharge controls the channel geometry the erosional morphologies created by our experimental gravity currents show strong analogies with the axial incision of canyons the primary mechanism by which canyons connected to continental shelves or large rivers evolve

plant taphonomy and palaeoecology in the lacustrine una delta late barremian iberian ranges spain abstract plant cuticle compressions and sporomorphs were studied in coaly clays belonging to the upper barremian la huerguina formation in una south western iberian ranges cuenca spain cuticle assemblages are extremely monotonous and formed by the genera frenelopsis schenk emend watson classostrobus alvin et al and glenrosa watson and fisher sporomorphs are dominated by classopollis pflug emend srivastava the genus glenrosa is described for the first time in europe sedimentological and taphonomical analyses show that these assemblages originated by fragmentation and size selection during residence in the leaf litter and by fluvial transport final deposition occurred in crevasse splay deltaic front and pro deltaic environments our results indicate that the una delta was fed almost exclusively by remains of cheirolepidiaceous conifers living in the upper deltaic plain of a lacustrine delta a comparison with the flora found in the open lacustrine facies of las hoyas which is laterally equivalent of the beds studied shows that land plant assemblages of las hoyas are more diverse the parautochthonous matoniaceous tree fern weichselia reticulata dominated the lakeshore vegetation of las hoyas in contrast to the vegetation of the upper delta plain of una which was dominated by frenelopsis the habitats of frenelopsis and glenrosa from the upper barremian oligohaline basin of una las hoyas contrast with the habitats commonly hypothesized for both taxa in coastal marine environments or on saline edaphic profiles in conclusion frenelopsis appears to have a wider tolerance to salinities than previously thought and their adaptations to saline edaphic conditions should be considered species specific rather than genus specific

distribution morphology and origin of sedimentary furrows in cohesive sediments southampton water patches of sedimentary furrows are developed at several locations in the cohesive estuarine sediments of southampton water water depth 1 12 m these furrows apparently result from short periods of erosion followed by long periods of deposition although all the furrows are similar regularly spaced parallel troughs 0 5 15 m wide aligned with the dominant current furrows in different patches have different characteristics in some areas furrow width is 1 5 1 15 of furrow spacing termed narrow whereas in other areas furrow width is about 1 2 of the spacing termed wide narrow furrows have developed where sediment accumulation rates are greater than 3 6 cm yr 1 wide furrows where accumulation rates are lower cockle shells and other coarse sediments concentrated on the furrow floors and on floors of smaller 2 10 cm wide minifurrows play an important role in furrow formation and evolution as they act to widen the furrows when mobilized during current episodes uniform sedimentation across the profile during slack periods tends to narrow the furrow some of the larger furrows have remained in the same position for 12 years while mini furrows have duration scales of a few months or less well developed furrows are also found in a recently dredged channel bedforms similar to those described here may be preserved in the sedimentary record while no analogues to the larger furrows are presently known minifurrows may be morphologically similar to the gutter casts described from ancient rocks

effect of sea level variation on upper slope depositional processes offshore of tiber delta tyrrhenian sea italy abstract the upper slope and outer shelf in front of the tiber river mouth east central tyrrhenian sea exhibits a series of about 15 gullies that are not clearly related to present day erosional or depositional processes an extensive high resolution seismic reflection profiling survey of a 450 km 2 area in front of the tiber delta shows several generations of similar features within the older depositional sequences underlying the outer continental shelf the gully relief appears dominantly depositional probably developed during lowstand periods when the tiber river mouth was relatively near the shelf break the position of the gullied intervals including those on the modern slope relict from the last lowstand shows a successive northward shift with time this northward shift indicates continued tilting of this part of the eastern tyrrhenian continental margin probably resulting from a continued subsidence to the north

mesozoic extensional tectonics stratigraphy and crustal evolution during the alpine cycle of the eastern iberian basin abstract sequence stratigraphy subsidence analysis and the integration of the basin fill data allow to the identification of four successive evolutionary stages in the basins of the eastern iberian margin during mesozoic extension 1 triassic rift late permian hettangian 2 early and middle jurassic postrift sinemurian oxfordian 3 late jurassic and early cretaceous rift kimmeridgian middle albian and 4 late cretaceous postrift late albian maastrichtian the present day crustal structure of the eastern iberian range and evidence of its evolution are deduced from the analysis of a new gravity map and other geophysical data a regional gravity low along the iberian range is interpreted in terms of crustal thickening beneath the orogen as a result of the collision between the ebro block and the iberian plate during the paleogene the boundary between these blocks coincides with a magnetic lineament that is interpreted as an inherited signature of the thinning that occurred during mesozoic extension a geodynamic scenario for the crustal evolution of the eastern part of the iberian peninsula based on the evaluation of mesozoic extensional tectonics and an analysis of the available geophysical data includes three main successive evolutionary stages 1 1 mesozoic crustal thinning 2 2 paleogene crustal thickening 3 3 neogene crustal thinning based on new geological and geophysical arguments this study offers an alternative and a more complete geodynamic history of the eastern iberian basins related to the evolution of the iberian plate and the central north atlantic

classification of sedimentary furrows and a model for furrow initiation and evolution sedimentary furrows are longitudinal bed forms which form in fine grained cohesive sediments of the deep sea floor as well as in similar sediments in estuaries and in large lakes deep sea furrows have been observed where bottom currents often flow in one direction at 5 to 20 cm sec currents are much stronger greater than 50 cm sec in furrowed estuarine environments but sediment accumulation rates are higher a review of furrows studied in a variety of locations and environments suggests that these bed forms fall into two broad morphological classes those developed in muddy cohesive sediments tend to have steep walls and flat floors while those developed in carbonate rich cohesive sediments are broad and have gently sloping walls at abyssal depths furrows can often be detected by their distinctive echo patterns on 3 5 khz echo sounders a series of similar evenly spaced hyperbolic echoes which often have their apices tangent to the sea floor however different furrow cross sections may generate distinctive hyperbolic echo patterns a model for furrow initiation and development in muddy cohesive sediments suggests that furrows develop in depositional environments swept by recurring directionally stable and episodically strong currents furrows are initiated when secondary circulations in the boundary layer align coarse generally light debris in sand ribbon like bed forms as the coarse light sediments move along the sea floor they abrade small longitudinal troughs into the muds the grooves may fill in during intervals between strong current events if sedimentation rates are too high once a furrow is established on the sea floor further development may be controlled by the balance between sediment deposition and erosion where deposition occurs continuously and erosion occurs only episodically during brief periods of increased current strength narrow furrows width 1 5 to 1 15 of spacing appear to develop where deposition exceeds erosion whereas wide furrows width 1 2 of spacing develop where erosion exceeds or equals deposition dissolution may play an important role in the formation and evolution of furrows in carbonate rich cohesive sediments secondary circulation patterns over established furrows may be strongly controlled by bed form topography

first isotopic and multidisciplinary evidence for nonmarine coelacanths and pycnodontiform fishes palaeoenvironmental implications abstract the recent coelacanth latimeria chalumnae one of the best known living fossils dwells in deep marine water fossil coelacanths have been globally related to marine environments specially after the triassic an association that has led to a general belief that they have always been marine previous reports of fossil coelacanths in continental deposits have been largely neglected prior to this report uncontested cretaceous freshwater coelacanths were unknown in turn the pycnodontiform fishes have always been considered exclusively marine apart from a few controversial exceptions here we present the first multidisciplinary evidence for nonmarine coelacanths and pycnodonts our conclusions are based upon palaeogeographic sedimentologic taphonomic and palaeoecologic criteria strongly supported by strontium 87 sr 86 sr and stable carbon and oxygen isotopic studies the coelacanth provisionally attributed to the genus holophagus and the pycnodontiforms eomesodon sp and macromesodon aff bernissartensis were unearthed at the early cretaceous locality of las hoyas cuenca spain where they grew in and inhabited a freshwater environment without marine influence fossil coelacanths and pycnodonts cannot therefore be used as unambiguous indicators of a marine environment caution is needed when using a single or a few taxa as palaeoenvironmental indicators especially fish in this sense communities are much more reliable arguments based on actualism or taxonomic uniformitarianism morphologic convergence and functional morphology are truly significant only within the framework of a sound multidisciplinary approach to the study of the palaeoenvironment

post lgm deglaciation in pine island bay west antarctica abstract to date understanding of ice sheet retreat within pine island bay pib following the last glacial maximum lgm was based on seven radiocarbon dates and only fragmentary seafloor geomorphic evidence during the austral summer 2009 2010 restricted sea ice cover allowed for the collection of 27 sediment cores from the outer pib trough region combining these cores with data from prior cruises over 133 cores have been used to conduct a detailed sedimentological facies analysis these results augmented by 23 new radiocarbon dates are used to reconstruct the post lgm deglacial history of pib our results record a clear retreat stratigraphy in pib composed of from top to base terrigenous sandy silt distal glacimarine pebbly sandy mud ice proximal glacimarine and till initial retreat from the outer continental shelf began shortly after the lgm and before 16 4 k cal yr bp as a likely response to rising sea level bedforms in outer pib document episodic retreat in the form of back stepping grounding zone wedges and are associated with proximal glacimarine sediments a sub ice shelf facies is observed in central pib and spans 12 3 10 6 k cal yr bp it is possible that widespread impingement of warm water onto the continental shelf caused an abrupt and widespread change from sub ice shelf sedimentation to distal glacimarine sedimentation dominated by widespread dispersal of terrigenous silt between 7 8 and 7 0 k cal yr bp the final phase of retreat ended before 1 3 k cal yr bp when the grounding line migrated to a location near the current ice margin

submarine erosional troughs in the northern south china sea evidence for early miocene deepwater circulation and paleoceanographic change abstract the paleoceanography of the south china sea scs is poorly understood although recent cruises and an odp expedition leg 184 have provided a useful general framework here we report previously unrecognized multi km scale sedimentary structures consisting of coupled ridges and troughs within the early miocene strata using high resolution 3d seismic data from the northern scs margin the troughs are interpreted as linear and or arcuate erosional features which truncate the underlying strata they range in orientation from e w in the southern part of the study area to nearly ne sw in the northwestern part of the study area the spacing of erosional troughs varies from less than 0 1 km in the southern part to more than 3 km in the northern part and they are about 50 1500 m in width with a relief of 10 90 m the erosional troughs are parallel to the regional strike of the early miocene slope of the northern scs from their internal geometry context regional distribution and most importantly their orientation relative to the paleo slope gradient at the time of their formation it is argued that they formed due to the action of bottom currents linked to the broader paleo circulation regime in the basin we infer that the paleo bottom currents intruding from the pacific flowed along the northern scs slope resulting in the occurrence of localized erosional troughs as deep water masses impinged on and interacted with the sea floor topography this study provides new insights into the early miocene paleoceanography of the scs and demonstrates the utility of 3d seismic data to enhance our understanding of paleo bottom current circulation regimes

sediment drifts and cold water coral reefs in the patagonian upper and middle continental slope abstract the north flowing falkland malvinas current has generated sediment drifts at a depth of 1200 1600 m in the patagonian middle continental slope out of early holocene hemipelagics late pleistocene ice rafted clastics and neogene fluvial sediments possibly there may be two generations of drifts pleistocene on the outer middle slope and holocene on the inner shelf the ice rafted debris originated in antarctica at a distance of 2000 4000 km south of patagonia scattered over the upper and middle slopes at depths ranging from 300 to 1400 m are cold water coral reefs of less than a meter to about ten of meters in relief it is inferred that most of cold water coral structures flourish as a consequence of the falkland malvinas current that concentrates the food supply at the reef sites growth of cold water coral reefs documented by digital submarine photographs on the upper slope at a depth of 300 400 m may be promoted by upwelling of nutrient rich waters and associated plankton blooms created by the intrusion of falkland malvinas current onto the outer shelf

tectonics sedimentation and erosion in northern california submarine geomorphology and sediment preservation potential as a result of three competing processes abstract on the continental margin of northern california north of the mendocino triple junction the geomorphic grain varies dramatically between the coast and the toe of the accretionary wedge this grain represents the composite signature of three competing processes tectonics sedimentation and erosion all three are active across the region but their rates and orientations vary spatially and temporally affecting the resultant grain and preservation potential in this rapid sedimentation environment at the toe of the accretionary wedge the relative rate of tectonic deformation exceeds the rate of sediment accumulation and erosion is largely limited to areally restricted but at times large mass wasting features in this region isobaths representing the geomorphic grain are parallel to structural highs and lows as in most forearcs and sediment accumulation is restricted to small basins located between anticlinal ridges the slope between the shelf break and the accretionary complex shows the competing effects of sediment accumulation erosion and tectonism slope perpendicular gullies are the prominent erosional features in this area especially in the region surrounding the trinity submarine canyon anticlinal ridges crop out on the seafloor although the ridge crests decrease in bathymetric expression both up and down slope on the shelf the rate of sediment accumulation 6 12 mm yr exceeds the relative rate of tectonic deformation with 1 3 mm yr of active uplift on land in this region the geomorphic grain is controlled by sediment accumulation with compressed but continuous stratigraphic sections above anticlinal highs and expanded sections in actively subsiding regions

abyssal bedforms as indicators of changing bottom current flow examples from the u s east coast continental rise direct indications of circulation patterns in the deep sea especially along continental rises and sediment drifts can be obtained through the analysis of abyssal bedforms such as furrows and mud waves such direct evidence of current flow will supplement indirect indicators of circulation provide by paleochemical indicators preserved in ocean sediments water that sinks in the northern north atlantic forms part of the western boundary undercurrent along the continental rise of eastern north america and the understanding of temporal changes in the speed and depth of flow along this continental rise can provide important information on how the speed at which the global conveyor system is operating changes with time studies of furrows and mud waves along the blake bahama outer ridges show that they are active sedimentary features today and that bottom flow speeds near the time of the younger dryas were increased over later intervals a more detailed analysis of bedform structure and sediments is needed to more clearly show the timing and reason for this change in sedimentation pattern

the spiral troughs of mars as cyclic steps we combine observations of stratigraphy morphology and atmospheric processes to relate the spiral troughs on mars polar layered deposits to a class of features known as cyclic steps cyclic steps are quasi stable repeating and upstream migrating bed forms that have been studied in terrestrial and submarine environments the repeating pattern is bounded by hydraulic jumps which act to stabilize the form we use radar stratigraphy from the shallow radar instrument on mars reconnaissance orbiter to examine trough evolution and constrain lateral transport we examine visible images from the thermal emission imaging system and observe low altitude clouds that we interpret to be the result of katabatic jumps i e the aeolian counterpart of hydraulic jumps in open channel flow we then devise a theoretical framework for understanding the origin of the spiral troughs that agree with 10 criteria that should be explained for any scenario to satisfactorily model the spiral troughs finally we use froude and geometrical analysis to estimate the rate of upstream migration caused by katabatic winds for the spiral troughs

storm induced sediment gravity flows at the head of the eel submarine canyon northern california margin 1 as part of the strataform program a bottom boundary layer bbl tripod was deployed at 120 m depth in the northern thalweg of the eel canyon during winter 2000 increases of the near bottom suspended sediment concentrations ssc recorded at the canyon head were not directly related to the eel river discharge but were clearly linked to the occurrence of storms bbl measurements revealed that during intensifications of the wave orbital velocity sediment transport at the head of the canyon occurred as sediment gravity flows directed down canyon observational evidence for near bed sediment gravity flow transport included an increase toward the bed of the down canyon component of wave averaged velocity and high estimated ssc at higher sampling frequencies 1 hz the current components during these events fluctuated at the same periodicity as the pressure reflecting a clear influence of the surface wave activity on the generation and maintenance of the sediment gravity flows the origin of such flows is not related to the formation of fluid muds on the shelf or to intense wave current sediment resuspension around the canyon head region rather liquefaction of sediment deposited at the head of the canyon induced by wave load excess pore water pressures during storms combined with elevated slopes around the canyon head appear to be the mechanisms initiating sediment transport the resulting fluidized sediment layer can easily be eroded entrained into the water column and transported down canyon as a sediment gravity flow results from this study reveal that storm induced sediment gravity flows occur periodically in the eel canyon head and suggest that this kind of sediment transport process can occur in other submarine canyons more frequently than previously expected

significance of the fine drainage pattern for submarine canyon evolution the foix canyon system northwestern mediterranean sea abstract submarine gullies are relatively small valleys that occur in a variety of submarine slopes they are very common in continental slopes and in submarine canyon heads and flanks where they often form tributary networks gullies are smaller than submarine canyons though there is no standardised size criterion to distinguish between them gullies and gully networks have been often viewed as initial steps in the development of larger submarine canyons and more mature drainage networks the shelf incising submarine foix canyon system fcs is located in the northwestern mediterranean sea numerous well developed and morphologically diverse gullies carve its two heads and flanks the objective of this study is to analyse the drainage network of the fcs and decipher the role of gullies in its evolution submarine gully and canyon networks were extracted from swath bathymetry data of 50 m grid size using geographic information systems gis a systematic morphometric analysis was carried out on drainage network of the fcs by using the horton strahler method our results show that the fcs is formed by 1660 streams 1000 km in total length which can be classified to six stream orders to detect relevant morphological changes along valley sections the drainage density the stream frequency and the drainage area relief parameters were applied furthermore a branching index i b was developed to characterise the geometry of the submarine drainage network in the canyon heads i b values are 1 7 which correspond to a dendritic network whereas i b in the canyon branches displays values of 2 2 corresponding to a pinnate one at a finer scale we have identified two types of canyon flank gullies namely rim gullies and toe gullies 1 rim gullies form large dendritic networks that extend from the canyon thalweg up to the canyon rim and 2 toe gullies form smaller pinnate networks restricted to the lower part of the canyon flanks the formation and development of rim gullies are interpreted as the main process responsible for canyon head growth and across flank transport of material from the continental shelf toe gullies on the other hand are the morphological expression of the rejuvenation of rim gullies and the canyon itself axial incision and toe gully formation are interpreted as the result of bottom to top erosive processes acting at different scales in response to regional and local changes of the factors controlling the system respectively this study helps to identify a variety of gullies and the processes they result from and provides a morphometric methodology to characterise submarine drainage networks at fine resolution scales

hydraulic jumps in sediment driven bottom currents laboratory experiments were conducted to observe the behavior of turbidity currents in the vicinity of a slope transition both sediment laden and saline hydraulic jumps were produced the vertical structure of the currents was found to depend on flow regime the saline and turbid hydraulic jumps showed similar characteristics the amount of water entrained by the flows through a jump was small the change in flow regime caused a marked reduction of the bed shear stress downstream of the jump in nature a turbidity current experiencing a hydraulic jump will drop most of its bedload immediately downstream from the jump while the suspended load will respond more gradually to the change in flow regime and will deposit sediment over a distance far exceeding 1 000 times the jump height

organic matter composition and distribution through the aptian albian lacustrine sequences of the araripe basin northeastern brazil abstract the main characteristics and distribution of organic matter have been studied through several aptian albian lacustrine units rio da batateira crato and ipubi formations araripe basin ne brazil six main facies have been distinguished on the basis of their lithological sedimentological characteristics and the type and amount of organic matter 1 marginal lacustrine deltaic massive to coarsely laminated mudstones and sandstones found as lenticular sheet and channel infills 2 oxic marginal lacustrine massive to poorly laminated mudstones 3 oxic suboxic inner lacustrine mudstones and laminated bioclastic mudstones 4 anoxic inner lacustrine dark mudstones 5 anoxic inner lacustrine dark clay carbonate rhythmites and 6 anoxic inner lacustrine finely laminated dark micritic limestones these facies make up decimeter to several meter thick parasequences which stack up in several tens of meters thick successions water level changes and variations of the detritic sediment contribution in the basin controlled the evolution of the depositional system palynomorph associations showed that the lacustrine system evolved in an arid semiarid tropical area where the most characteristic flora consisted of conifers gnetales bennettitales and early angiosperms in facies 1 to 3 organic matter was scarce poorly preserved and mainly derived from higher land plants facies 4 is characterised by containing abundant algal bacterial derived well preserved organic matter facies 5 and 6 contained mainly branchlets and pollen debris blown by the wind or fed by lacustrine underflows the pattern of organic matter accumulation and preservation corresponds well with cyclical palaeoenvironmental changes from anoxic to oxic bottom conditions and from a fresh to saline upper water layer which affected the shallow lacustrine system the deposition of thin organic matter rich mudstones facies 4 probably occurred during higher water levels which resulted in relatively low salinity waters and low detrital contribution to the inner lacustrine zones less significant organic matter accumulation is recorded in the clay carbonate rhythmites and in the limestones facies 5 and 6 due to scarce autochthonous aquatic organic matter deposition and pervasive intense bacterial degradation caused by persistent meromictic conditions large differences in the degree and path of evolution of the humic material derived from higher land plants have been detected depending on the depositional conditions the humic material in the oxic facies underwent humification and gelification and had the highest huminite reflectances whereas it showed suppressed reflectance values in the oil shales facies 4 and in the carbonate dominated facies in the latter extensive bacterial reworking of humic material is also observed these differences are considered significant and applicable to other case studies in terms of evaluating the maturity of organic matter in ancient lacustrine systems

inception of channelization and drainage basin formation upstream driven theory the ubiquitous presence of river drainage basins in the terrestrial environment suggests that distributed overland flow generated by rainfall tends to spontaneously organize itself into dendritic systems of discrete channels several recent numerical models describe the evolution of complete drainage basins from the initial condition of rainfall on a flat tilted plateau the surface of which has been provided with random elevation perturbations these analyses model overland flow via the assumption of a perfect balance between gravitational and frictional terms i e in terms of normal flow linear stability analysis applied to the normal flow model has been shown however to fail to select a wavelength corresponding to a finite distance of separation between incipient basins this suggests that the normal flow model may not be a sufficient basis for studying drainage basin development especially at the finest scales of morphologic significance here the concept of a threshold condition for bed erosion is combined with an analysis of the full equations of shallow overland flow in order to study wavelength selection classical linear stability analysis is shown to be inadequate to analyse the problem at the level of inception an alternative linear analysis of bed perturbations based on the threshold condition is developed and shown to lead to the selection of finite wavelength of the correct order of magnitude the analysis here is driven from the upstream direction in that bed erosion is first caused only when sufficient flow has gathered from upstream due to rainfall a downstream driven theory of incipient channelization that is not necessarily dependent upon rainfall is presented in izumi 1993 and is presently in preparation for publication