what controls submarine channel development and the morphology of deltas entering deep water fjords river deltas and associated turbidity current systems produce some of the largest and most rapid sediment accumulations on our planet these systems bury globally significant volumes of organic carbon and determine the runout distance of potentially hazardous sediment flows and the shape of their deposits here we seek to understand the main factors that determine the morphology of turbidity current systems linked to deltas in fjords and why some locations have well developed submarine channels whilst others do not deltas and associated turbidity current systems are analysed initially in five fjord systems from british columbia in canada and then more widely this provides the basis for a general classification of delta and turbidity current system types where rivers enter relatively deep 200 m water fjord delta area is found to be strongly bimodal avalanching of coarse grained bedload delivered by steep mountainous rivers produces small gilbert type fan deltas whose steep gradient 11 25 approaches the sediment s angle of repose bigger fjord head deltas are associated with much larger and finer grained rivers these deltas have much lower gradients 1 5 10 that decrease offshore in a near exponential fashion the lengths of turbidity current channels are highly variable even in settings fed by rivers with similar discharges this may be due to resetting of channel systems by delta top channel avulsions or major offshore landslides as well as the amount and rate of sediment supplied to the delta front by rivers

large scale sediment waves and scours on the modern seafloor and their implications for the prevalence of supercritical flows large scale 20 m to 7 km wavelength bedforms are common on the seafloor yet there is a lack of consensus on how they form and thus what to call them we conducted statistical analysis on a dataset of 82 seafloor bedforms that span a range of water depths and environments the data form three distinct groups 1 small scale 20 300 m wavelength sediment waves with mixed relief made of medium sand to cobble sized sediment that form in confined settings which we call small sediment waves 2 large scale 300 7000 m wavelength sediment waves with mixed relief made of fine grained sediment that form in relatively unconfined settings which we call large sediment waves and 3 large scale fully enclosed depressions in the seafloor which we call scours there is a statistically significant data gap in the size of bedforms between small sediment waves and large sediment waves that does not appear to be a sampling artefact this data gap probably results from the environments in which sediment waves form being either confined e g channel or canyon or unconfined e g open slope bedform migration direction is available for 36 of the data and includes small and large scale sediment waves of these examples all are shown to migrate up current up current migration is indicative of supercritical flow thus this data suggests that supercritical flows operate in a wide range of environments and can generate both small and large sediment waves therefore we suggest that small and large sediment waves form by similar processes despite the gap in bedform wavelength and sediment size the migration direction for scours remains unknown scours may form from similar processes to small and large sediment waves or alternatively they may be a completely separate bedform type that form when erosive flows exploit pre existing defects in the seafloor this novel statistical analysis of a global database shows that up current migrating bedforms associated with supercritical flow are unusually widespread and are recognised at two distinct scales

bedforms on the submarine flanks of insular volcanoes new insights gained from high resolution seafloor surveys a comparative analysis of bedform fields along the submarine flanks of insular volcanoes characterized by different morpho structural settings volcanic and meteo marine regimes vanuatu kermadec bismark madeira and aeolian archipelagos is presented here to provide insights on the size distribution morpho dynamic and genesis of such bedforms two main types of bedforms are recognized according to their size location and preconditioning triggering processes small scale bedforms have wavelengths of tens to hundreds of metres and wave heights of metres because of their small size they are typically not recognizable at water depths greater than 400 m from vessel mounted bathymetric surveys however few examples of small scale bedforms are reported from upper volcanic flanks where steep gradients commonly hinder their formation their recognition is mostly limited to the thalweg of shallow and flat bottomed channels that carve the insular shelf on slope gradients 15 small scale bedforms are mostly related to erosional depositional processes due to sedimentary gravity flows that are often the result of a cascading effect between volcanic and non volcanic processes for example flood discharges and retrogressive landslides large scale bedforms occur at all water depths having wavelengths of hundreds thousands of metres and wave heights up to few hundreds of metres the origin of large bedforms is more difficult to ascertain especially if only bathymetric data are available some diagnostic criteria are presented to distinguish between bedforms associated with landslide deposits and those associated with density currents in this latter case relevant sediment sources and slope gradients 8 are key factors for bedform development erosional depositional bedforms are typically related to eruption fed density flows formed during large caldera collapses or to large turbidity flows bedforms generated by turbidity flows are often observed in the lower volcanic flanks where an abrupt decrease of gradients is present often matching a change from confined to unconfined settings in summary this study provides insights to interpret bedforms in modern and ancient marine volcaniclastic settings elsewhere

proterozoic tectonism of the arabian shield abstract new field and analytical work together with a new aeromagnetic map and a geologic structural geochemical and geochronologic synthesis and reappraisal offer a new view of the anatomy and geologic history of the arabian shield although early proterozoic rocks have been found in the eastern part of the shield the main geologic evolution of the shield is limited to a period ranging from 900 to 530 ma that led to the formation amalgamation and final cratonization of several tectonostratigraphic terranes the pre panafrican structures 690 ma which are difficult to decipher due to younger deformation were essentially the result of the formation amalgamation and accretion of these terranes the panafrican tectonism 690 590 ma was marked by the formation of the nabitah belt and peripheral ranges punctuated by gneiss domes various sedimentary formations contemporaneous with this tectonism represent foreland or intracontinental molasse basins after the panafrican tectonism widespread extension 590 530 ma brought about crustal thinning that generated bimodal magmatism and significant dike swarms associated volcanics form the shammar group a marine transgression associated with passive margin type structures with tilted blocks marked the end of the thinning the platform facies produced by this transgression correspond to part of the jibalah formation other basins formed as deep continental pull apart basins along transform faults this updated view of the arabian proterozoic geodynamic evolution provides a framework for reviewing the associated mineralizing events and places them in a new chronology and structural history

pyroclastic dune bedforms macroscale structures and lateral variations examples from the 2006 pyroclastic currents at tungurahua ecuador pyroclastic currents are catastrophic flows of gas and particles triggered by explosive volcanic eruptions for much of their dynamics they behave as particulate density currents and share similarities with turbidity currents they occasionally deposit dune bedforms with peculiar lamination patterns from what is thought to represent the dilute low concentration and fluid turbulence supported end member of the pyroclastic currents here we present a high resolution dataset of sediment plates lacquer peels with several closely spaced lateral profiles representing sections through single pyroclastic bedforms from the august 2006 eruption of tungurahua ecuador most of the sedimentary features contain backset bedding and preferential stoss face deposition from the ripple scale few cm to the largest dune bedform scale several m length similar patterns of erosive based backset beds are evidenced recurrent trains of sub vertical truncations on the stoss side of structures reshape and steepen the bedforms in contrast sporadic coarse grained lenses and lensoidal layers flatten bedforms by filling troughs the coarsest clasts up to 10 cm least sorted and massive structures still exhibit lineation patterns that follow the general backset bedding trend the stratal architecture exhibits strong lateral variations within tens of centimeters with very local truncations both in flow perpendicular and flow parallel direction we infer that the bedforms sedimentary patterns result from four formation mechanisms differential draping slope influenced saltation truncative bursts and granular based events whereas most of the literature makes a straightforward link between backset bedding and froude supercritical flows we reconsider this interpretation indeed features that would be diagnostic of subcritical dunes antidunes and chute and pools can be found on the same horizon and in a single bedform only laterally separated by short distances 10s of cm our data stress the influence of the pulsating and highly turbulent nature of the currents and the possible role of coherent flow structures such as gortler vortices backset bedding is interpreted here as a consequence of a very high sedimentation environment of weak and waning currents that interact with the pre existing morphology quantification of near bed flow velocities are made via comparison with wind tunnel experiments we estimate that shear velocities of ca 0 30 m s 1 equivalent to pure wind velocity of 6 to 8 m s 1 at 10 cm above the bed could emplace the constructive bedsets whereas the truncative phases would result from bursts with impacting wind velocities of at least 30 40 m s 1

on the architecture of intra formational mass transport deposits insights from the carbonate slopes of great bahama bank and the apulian carbonate platform abstract large sub tropical carbonate platforms shed huge volumes of sediment onto their adjacent slopes resulting carbonate successions are comprised of gravity flow beds turbidites debrites which are occasionally reworked into mass transport deposits mtds by the failure of inconsistently lithified portions of slope strata despite their significance in the depositional record the internal architecture of mtds along relatively tectonically stable and sediment producing carbonate platforms is poorly understood in comparison with counterparts in active compressional tectonic settings this study compares carbonate slope derived mtds imaged in seismic reflection data with those observed in outcrop the internal architecture of mtds from two case studies is presented the seismically imaged mtds in miocene recent carbonate slope deposits from the bahamas and mtds off the cretaceous apulian carbonate margin exposed in outcrops of the ionian basin albania understanding the large scale kilometers to tens of kilometers internal architecture of mtds in seismic is crucial for identification of these structures in outcrop in turn only outcrop studies allow the detailed analysis of the reworking style within these massive sediment bodies at a smaller scale meters to kilometers the internal architecture of the studied carbonate mtds testifies to the existence of an along slope continuum of deformation structures characterized by vertical partitioning into distinct mass flows during single mass transport events mass wasting processes are inferred from strain partitioning structures recording along slope and slope parallel architectural heterogeneities three deformation domains are recognized extensional headwall domain translational body domain and contractional toe domain confined mtds are limited by steep and deep lateral margins and frontal ramps that mark the boundaries between failed deposits and undisturbed strata vertical strain partitioning is pronounced in the toe domain upward intensification of deformation within mtds is common and corresponds to a top down reorganization of failed strata during downslope transport intra formational mtds along carbonate slopes that are lacking exotics such as the ones studied here differ from extra formational mtds occurring in compressional tectonic settings that form sedimentary melanges ductile layer deformation structures are predominant in carbonate mtds as opposed to block in matrix fabrics in sedimentary melanges the development of shear surfaces rather than ductile shear zones in the body and toe domains presents predictive in modern environments and diagnostic criteria in fossil examples of mass wasting events along carbonate slopes

how turbidity current frequency and character varies down a fjord delta system combining direct monitoring deposits and seismic data submarine turbidity currents are one of the most important processes for moving sediment across our planet they are hazardous to offshore infrastructure deposit petroleum reservoirs worldwide and may record tsunamigenic landslides however there are few studies that have monitored these submarine flows in action and even fewer studies that have combined direct monitoring with longer term records from core and seismic data of deposits this article provides one of the most complete studies yet of a turbidity current system the aim here is to understand what controls changes in flow frequency and character along the turbidite system the study area is a 12 km long delta fed fjord howe sound in british columbia canada over 100 often powerful up to 2 to 3 m sec 1 events occur each year in the highly active proximal channels which extend for 1 to 2 km from the delta lip about half of these events reach the lobes at the channel mouths however flow frequency decreases rapidly once these initially sand rich flows become unconfined and only one to five flows run out across the mid slope each year many of these sand rich channelized delta sourced flows therefore dissipated over a few hundred metres once unconfined rather than eroding and igniting upflow migrating bedforms indicate that supercritical flow dominated in the proximal channels and lobes and also across the unconfined mid slope these supercritical flows deposited thick sand beds in proximal channels and lobes but thinner and finer beds on the unconfined mid slope the distal flat basin records far larger volume and more hazardous events that have a recurrence interval of ca 100 years this study shows how sand rich delta fed flows dissipate rapidly once they become unconfined that supercritical flows dominate in both confined and unconfined settings and how a second type of more hazardous and much less frequent event is linked to a different scale of margin failure

erosion and deposition by supercritical density flows during channel avulsion and backfilling field examples from coarse grained deepwater channel levee complexes sandino forearc basin southern central america abstract erosion and deposition by supercritical density flows can strongly impact the facies distribution and architecture of submarine fans field examples from coarse grained channel levee complexes from the sandino forearc basin southern central america show that cyclic step and antidune deposits represent common sedimentary facies of these depositional systems and relate to the different stages of avulsion bypass levee construction and channel backfilling during channel avulsion large scale scour fill complexes 18 to 29 m deep 18 to 25 m wide 60 to 120 m long were incised by supercritical density flows the multi storey infill of the large scale scour fill complexes comprises amalgamated massive normally coarse tail graded or widely spaced subhorizontally stratified conglomerates and pebbly sandstones interpreted as deposits of the hydraulic jump zone of cyclic steps the large scale scour fill complexes can be distinguished from small scale channel fills based on the preservation of a steep upper margin and a coarse grained infill comprising mainly amalgamated hydraulic jump zone deposits channel fills include repeated successions deposited by cyclic steps with superimposed antidunes the deposits of the hydraulic jump zone of cyclic steps comprise regularly spaced scours 0 2 to 2 6 m deep 0 8 to 23 m long infilled by intraclast rich conglomerates or pebbly sandstones displaying normal coarse tail grading or backsets these deposits are laterally and vertically associated with subhorizontally stratified low angle cross stratified or sinusoidally stratified sandstones and pebbly sandstones which were deposited by antidunes on the stoss side of the cyclic steps during flow re acceleration the field examples indicate that so called spaced stratified deposits may commonly represent antidune deposits with varying stratification styles controlled by the aggradation rate grain size distribution and amalgamation the deposits of small scale cyclic steps with superimposed antidunes form fining upwards successions with decreasing antidune wavelengths indicating waning flows such cyclic step antidune successions form the characteristic basal infill of mid fan channels and are inferred to be related to successive supercritical high density turbidity flows triggered by retrogressive slope failures

supercritical flow structures on a late carboniferous delta front sedimentologic and paleoclimatic significance comment ventra et al 2015 propose a model of hyperpycnal supercritical flow regime deposition related to a megamonsoonal climate for a series of interpreted cyclic steps on a carboniferous delta front the new model has major implications for paleoenvironmental reconstructions of this well studied area in terms of climate and geographical setting we contend that fundamental sedimentological observations do not support their proposed model ventra et al interpret that each cross bedded set is the deposit of a single mega monsoonal flood event we propose that each set represents multiple events e g their set 3 is at least 30 discrete events event bed sandstones are represented by variable to normal grading crude cross stratification and plane parallel lamination and a finer grained top which is generally ripple cross laminated fig 1 these are capped by a very thin silty to fine grained sand layer with abundant organic material representing interflood deposition hampson 1997 this repeated motif is only interrupted where bed tops are eroded abundant loading dewatering and flame structures between beds and rip up clast horizons support the interpretation of discrete depositional events fig 1 a mega monsoonal origin cannot be invoked for 20 cm thick beds ventra et al state that there is a lack of ripple cross lamination or stratification aside from some upflow dipping crude laminae most bed tops are laminated to crosslaminated fig 1 these are interpreted as current ripples rather than antidunes or climbing ripple backsets as they have clearly developed tangential foresets these ripples suggest paleoflow broadly toward the east in contrast to the overall southwesterly paleoflow eastward paleocurrents typically occur on the updip side of the undulatory beds of mccabe 1977 and hampson 1997 suggesting that the exposed section is oblique to depositional dip rather than parallel as proposed we infer that there is a significant lateral as well as downstream accretion component to these three dimensional bedforms the lack of bioturbation between beds cannot be used as evidence for the short duration of events as there is in general a lack of bioturbational structures in the sandy marine sediments of the british carboniferous basins however the depositional environment as proposed by ventra marine delta front is at odds with the existing fluvial models which are based on detailed mapping and correlations mccabe 1977 hampson 1997 the large scale architecture shows a progressive decrease in depositional dip that we interpret to reflect healing of scour related topography the depth of the scour minimum of 13 m corresponds to that observed in major fluvial confluences and is aligned with hampson s 1997 interpretation the overlying section which ventra et al do not discuss is represented by many smaller scale trough cross beds interpreted to represent local shallowing in a fluvial setting associated with filling of the scour in this stratigraphic unit features of a similar geometry but at a smaller scale are convincingly shown to represent supercritical flow conditions bijkerk 2014 in summary we concur with the observations and interpretations of hampson 1997 that these beds represent the fill of a major scour potentially associated with a fluvial confluence which filled in a stepwise manner by gravity flow in the deepest parts with periods of minor erosion of the downslope sides of the bedforms e g ullah et al 2015 figure 1 a short sedimentological log from the base of ventra s 2015 set 2 showing the general character of event beds b character of event beds and finer interbeds i and amalgamation surfaces a c small dune with tangential foresets f d ripple cross lamination r with tangential foresets e loading l between beds

paralava produced by combustion of dead gorse near colac bay southland new zealand abstract heat from combustion of dead gorse ulex europaeus fabaceae stacked on volcanogenic graywacke on a ridge crest near colac bay southland new zealand has induced surface melting of the graywacke at temperatures reaching 1330 c driblets of vesicular paralava trickled down vertical surfaces of rock where heat has been funneled between torlike blocks up to 5 m high where partly devitrified and less abundantly where clear and fresh it contains new formed swallow tailed microlites of labradorite an56 62 and slender prisms of al rich diopside chemical data are given for parent rock clear glass of trachytic composition partly devitrified darkened glass enriched in fe mg and ca and al rich diopside

upstream migrating knickpoints and related sedimentary processes in a submarine canyon from a rare 20 year morphobathymetric time lapse capbreton submarine canyon bay of biscay france abstract the capbreton submarine canyon is a striking feature of the south east of the bay of biscay this canyon forms a deep incision through the continental shelf and slope and displays remarkable structures linked to its present day hydrosedimentary activity its head has been disconnected from the adour river since 1310 ce but remains close enough to the coast to be supplied with sediment by longshore drift gravity processes in the canyon body are abundantly described and documented but activity in the head and the fan of the canyon is poorly constrained furthermore many questions remain regarding the details of processes affecting the head the body and the fan of the capbreton canyon in this work we address the paucity of documentation concerning 1 the temporal evolution of sediment transfer between the head and the deep reaches of the canyon and 2 the interaction between gravity processes and the morphology of the canyon floor including both shaping and feedback mechanisms this study is based on the analysis and comparison of eight multibeam bathymetric surveys acquired in the upper part of the capbreton canyon between 1998 and 2018 in depths ranging 10 320 m below sea level this rare time series exposes the morphological evolution of this outstanding dynamic area over the last 20 years our work shows that much of the changes are located in the canyon floor and head following a period characterized by a unique flat floor thalweg the canyon was affected by an incision with low lateral terraces which resulted in a narrow axial thalweg the deepening of the narrow thalweg was induced by retrogressive erosion according to the presence of upstream migrating knickpoints while low elevation residual terraces formed as the canyon reached a local equilibrium profile the flat thalweg observed in 1998 is likely a result of a partial filling of the canyon thalweg by a substantial emptying of the canyon head and significant mass transfer to the proximal part of the canyon a flat floor thalweg was not observed again in the remaining of our time series since 2010 suggesting a possible quieter working mode of the canyon we also propose the first accurate volume quantification of sediment displacement on the canyon floor our findings underline the alternation of filling and erosive periods in the canyon axis and an unexpected continuous sediment deposition in the canyon head during the last 20 years

reconstruction of bedform dynamics controlled by supercritical flow in the channel lobe transition zone of a deep water delta sant llorenc del munt north east spain eocene

upslope migrating bedforms in a proglacial sandur delta cyclic steps from river derived underflows abstract backstepping cross strata on steep foresets of a gilbert type proglacial fluviodeltaic system are ascribed to cyclic steps and other associated supercritical bedforms they provide insight into how sandur river flows transition into the marine realm these sedimentary structures are located on steep foresets up to 17 with corresponding top lying flat based topsets in an upper pleistocene delta on the north shore of the gulf of st lawrence quebec canada packages of backstepping cross strata of sand and gravel lying in the lower part of the delta front outcrop are organized in 10 20 m spaced pseudo foresets with a mean slope of 11 12 seawards backstepping strata include frequent internal erosion surfaces that onlap upslope on pseudo foresets and are interpreted as cyclic steps narrow deep and asymmetrical scours and upslope climbing cross beds are interpreted as chutes and pools and antidunes respectively very shallow 15 m depositional paleo bathymetry is inferred from the preservation of the delta brink the well organized stratal pattern in cyclic step to antidune deposits indicates relatively steady and uniform flow patterns there is insufficient distance for a headscarp large enough to transform to the volume of observed accreted sands or for a flow transformation from a gravitational collapse to net depositional cyclic steps these deposits are sandier than the topsets beds and are thus not derived from them but rather correspond to topset erosional surfaces the development of cyclic steps from hyperpycnal flows was likely enhanced by tidal drawdown processes the resulting sediment laden supercritical flows plunged inertially and evolved into an underflow that generated the cyclic steps on the upper foresets the cyclic steps have a high aspect ratio and represent an end member of coarse grained sediment deposited on steep slopes in contrast to low gradient low aspect ratio muddy deposits

how to recognize crescentic bedforms formed by supercritical turbidity currents in the geologic record insights from active submarine channels submarine channels have been important throughout geologic time for feeding globally significant volumes of sediment from land to the deep sea modern observations show that submarine channels can be sculpted by supercritical turbidity currents seafloor sediment flows that can generate upstream migrating bedforms with a crescentic planform in order to accurately interpret supercritical flows and depositional environments in the geologic record it is important to be able to recognize the depositional signature of crescentic bedforms field geologists commonly link scour fills containing massive sands to crescentic bedforms whereas models of turbidity currents produce deposits dominated by back stepping beds here we reconcile this apparent contradiction by presenting the most detailed study yet that combines direct flow observations time lapse seabed mapping and sediment cores thus providing the link from flow process to depositional product these data were collected within the proximal part of a submarine channel on the squamish delta canada we demonstrate that bedform migration initially produces back stepping beds of sand however these back stepping beds are partially eroded by further bedform migration during subsequent flows resulting in scour fills containing massive sand as a result our observations better match the depositional architecture of upstream migrating bedforms produced by fluvial models despite the fact that they formed beneath turbidity currents

cyclic steps review and aggradation based classification abstract cyclic steps are a type of upper flow regime bedform consisting of trains of upstream and upslope migrating bed undulations the overriding flow is characterised by a series of hydraulic jumps occurring in the troughs of the undulations cyclic steps form in open channel flows such as rivers and comprise a common bedform in subaqueous density flows in oceans lakes and reservoirs cyclic steps are associated with alternating froude subcritical and froude supercritical flow on respectively the stoss side and lee side of individual bedforms the transition between these flow states is embodied by the hydraulic jump in the trough of the bedform leading to the permanent or quasi permanent morphology of cyclic steps over the past decade numerous studies affirmed the dominant role of cyclic steps in generating bed undulations in modern and ancient glacial outwash fluvial delta and turbidite environments as reviewed here cyclic steps were previously discriminated as net depositional climbing transportational and net erosional falling in different parts of sedimentary systems the following cyclic step descriptors with distinct depositional signatures are proposed fully depositional partially depositional transportational partially erosional and fully erosional partially depositional cyclic steps are most common they are associated with backset bedded sets high aggradation rate and nested scours filled with massive to backset bedded deposits low aggradation rate the new classification can be used as a predictive tool in the reconstruction of modern and ancient sedimentary successions using repeat bathymetry seismic reflection or outcrop data it is applied in three turbidite case studies classical deep sea system small scale delta slope and line sourced carbonate slope

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

modern sedimentary processes at the heads of martinez channel and steffen fjord chilean patagonia abstract chilean fjord sediments constitute high resolution archives of climate and environmental change in the southern andes to interpret such records accurately it is crucial to understand how sediment is transported and deposited within these basins this issue is of particular importance in glaciofluvial martinez channel and steffen fjord 48 s due to the increasing occurrence of glacial lake outburst floods glofs originating from outlet glaciers of the northern patagonian icefield hence the bathymetry of the head of martinez channel and steffen fjord was mapped at high resolution and the grain size and organic carbon content of grab sediment samples were examined results show that the subaquatic deltas of baker and huemules rivers at the head of martinez channel and steffen fjord respectively are deeply incised up to 36 m by sinuous channels the presence of sediment waves and coarser sediments within these channels imply recent activity and sediment transport by turbidity currents although several triggering mechanisms are possible we argue that elevated river discharge and the associated relatively high suspended sediment loads is the main cause of the occurrence of turbidity currents at the head of these fjords and are of key importance in shaping the fjord s subaquatic morphology this study shows that the heads of martinez channel and steffen fjord are dynamic sedimentary environments with rapidly migrating channels it highlights the importance of site selection and multi coring in any future project aimed at reconstructing environmental change using the sediments of either fjord

supercritical and subcritical turbidity currents and their deposits a synthesis common facies models of turbidite deposits are based on idealized sequences of turbidite units which are assumed to reflect the depositional processes of a decelerating turbidity current we show how suites of turbidite units i e distinct turbidite facies associations that are easily described from core and outcrop may characterize the entire range of large scale dynamics of turbidity currents enabling estimates of their densimetric froude number fr subcritical versus supercritical and suspension fall out rate stratified versus nonstratified flows the linking of facies associations with large scale flow dynamics resolves process facies links that were hitherto unresolved by the common turbidite facies models

retreat pattern of glaciers controls the occurrence of turbidity currents on high latitude fjord deltas eastern baffin island glacier and ice sheet mass loss as a result of climate change is driving important coastal changes in arctic fjords yet limited information exists for arctic coasts regarding the influence of glacial erosion and ice mass loss on the occurrence and character of turbidity currents in fjords which themselves affect delta dynamics here we show how glacial erosion and the production of meltwaters and sediments associated with the melting of retreating glaciers control the generation of turbidity currents in fjords of eastern baffin island canada the subaqueous parts of 31 river mouths along eastern baffin island were mapped by high resolution swath bathymetry in order to assess the presence or absence of sediment waves formed by turbidity currents on delta fronts by extracting glaciological and hydrological watershed characteristics of these river mouths we demonstrate that the presence and areal extent of glaciers is a key control for generating turbidity currents in fjords however lakes formed upstream during glacial retreat significantly alter the course of sediment routing to the deltas by forming temporary sinks leading to the cessation of turbidity currents in the fjords due to the different deglaciation stages of watersheds in eastern baffin island we put these results into a temporal framework of watershed deglaciation to demonstrate how the retreat pattern of glaciers through the formation and filling of proglacial lakes affects the turbidity current activity of deltas

supercritical flow structures on a late carboniferous delta front sedimentologic and paleoclimatic significance deposits of fluvial systems in highly seasonal tropical climates possess unique architectural and facies characters owing to a flood prone regime alternating with lengthy periods of ineffective discharge distally linked deltaic successions should also feature distinctive attributes with great potential to preserve the stratigraphic evidence of exceptional discharge events we describe late carboniferous delta front valley confined sandstones from the pennine basin uk originally deposited at paleoequatorial latitudes during final assembly of the pangean megacontinent and characterized by giant sedimentary structures with repetitively sigmoidal geometry facies traits indicate geologically instantaneous deposition of a large sediment volume from a density current at sustained supercritical flow conditions leading to aggradation of cyclic steps recently identified bedforms developing in high energy flows and of which this is the first complete outcrop example the lack of unconformable erosional surfaces and absence of different associated facies point to a single aggradational event during which the structures attained dimensions comparable to those indicated by seismic data sets from which they are remotely detected on modern seafloors cyclic step formation in a deltaic setting suggests that pangean megamonsoons could have triggered hydrologic events capable of imprinting sedimentologic signatures on shallow marine deposits

preconditioning and triggering of offshore slope failures and turbidity currents revealed by most detailed monitoring yet at a fjord head delta rivers and turbidity currents are the two most important sediment transport processes by volume on earth various hypotheses have been proposed for triggering of turbidity currents offshore from river mouths including direct plunging of river discharge delta mouth bar flushing or slope failure caused by low tides and gas expansion earthquakes and rapid sedimentation during 2011 106 turbidity currents were monitored at squamish delta british columbia this enables statistical analysis of timing frequency and triggers the largest peaks in river discharge did not create hyperpycnal flows instead delayed delta lip failures occurred 8 11 h after flood peaks due to cumulative delta top sedimentation and tidally induced pore pressure changes elevated river discharge is thus a significant control on the timing and rate of turbidity currents but not directly due to plunging river water elevated river discharge and focusing of river discharge at low tides cause increased sediment transport across the delta lip which is the most significant of all controls on flow timing in this setting