coarse grained sediment waves in hyperpycnal clinoform systems miocene of the austral foreland basin argentina in this paper we present an outcrop example of coarse grained sediment waves generated by hyperpycnal discharges at the toe of miocene clinoform systems exposed in the northeast atlantic coast of tierra del fuego argentina an individual set of these sediment waves is formed during the waxing and waning phases of the hyperpycnal discharge during the waxing phase accumulation of coarse grained sediments with generation and migration of dune bedforms occurs at the base of the clinoforms these deposits show transitions of tractive sedimentary structures with evidence of bedload transport and a continuous reworking by the overpassing hyperpycnal flow the continued erosion and reworking during the waxing phase affect mainly the top and the stoss side of the dunes resulting in isolated lenses having upcurrent aggradation structures during the waning phase these isolated lenses are covered by fine grained heterolithic strata that are thicker on the stoss side than on the lee side constituting a differential draping the pulsating and sustained character of the hyperpyc nal flows and the steep depositional slope clinoforms controlled the stacking pattern of the coarse grained sediment waves during successive hyperpycnal events in this way small scale coarse grained sediment waves with wavelengths of 10 40 m and up to 5 m amplitude and a stacking pattern showing an up current growth are generated by high density hyperpycnal flows on slopes having steep gradients

modelling facies belt distribution in fan deltas coupling sequence stratigraphy and geostatistics the eocene sant llorenc del munt example ebro foreland basin ne spain abstract the eocene sant llorenc del munt fan delta complex developed in the ebro foreland basin the stratigraphy of this succession about 1000 m thick has been described as made up of several transgressive regressive composite sequences each composed of a stack of fundamental sequences in turn made up of a transgressive systems tract and an overlying regressive systems tract systems tracts are composed of several facies belts proximal alluvial fan distal alluvial fan fan delta front carbonate platform and fan delta slope and prodelta a 3d facies model of a 130 m thick stratigraphic succession was built using outcrop data from one of the composite sequences the modelling followed a hierarchical approach a deterministic method was used to reconstruct the surfaces bounding fundamental sequences and their constituent systems tracts data input was outcrop traces digitized on photorealistic terrain models the stacking trend and interfingering of adjacent facies belts within each systems tract was added by means of a geostatistical algorithm and was conditioned to measured stratigraphic logs the analysis of the resultant 3d facies distributions allows revising the architectural interpretation of the studied interval leading to the conclusion that the observed arrangements are accommodation not supply driven furthermore the 3d models were flow simulated in two injector producer scenarios up depositional and down depositional dip waterflooding flow simulation results show that the depositional architecture of the studied succession controls potential recovery efficiencies in that a considerable proportion of moveable oil is trapped at the dead ends in depositional pinch outs of the main potential reservoir fan delta front deposits also that up dip waterflooding predicts better recovery efficiencies than down dip the dataset methods and results herein make the sant llorenc del munt complex a ready for application analogue for marine transitional reservoirs

anatomy of regressive tracts in a regressive sequence set vilomara unit sant llorenc del munt ebro basin ne spain abstract the sant llorenc del munt fan delta succession forms a clastic wedge ca 3 my duration that built out into the ne ebro basin in response to transpression and uplift along the paleogene catalan coastal range this fan delta succession is arranged in a series of transgressive regressive sequences of different scales a large scale composite megasequence the milany megasequence includes the whole fan delta succession this megasequence is subdivided in seven composite sequences that in turn can be subdivided into fundamental sequences the vilomara composite sequence the vilomara unit is located in the regressive part of the milany megasequence the regressive part of the vilomara composite sequence consisting of a seaward stepping set of fundamental sequences the vilomara unit has been studied to understand the facies relations geometries and the characteristics of key surfaces in progradational settings the vilomara regressive sequence set is formed by stacked alluvial fan fan delta plain proximal and distal fan delta front fan delta slope within the regressive part of fundamental sequences and carbonate platform usually within the transgressive part of fundamental sequences deposits the vilomara regressive unit shows a sigmoidal geometry in contrast to the underlying el marcet transgressive unit which thickens landwards this sigmoidal geometry for the regressive unit and landward thickening for the transgressive unit is common for all scales of sequence composite megasequences composite sequences and fundamental sequences there are evident facies arrangement differences between transgressive and regressive deposits within a fundamental sequence key surfaces have been recognised from vertical trend changes from transgressive to regressive and vice versa the facies successions immediately above and below the surfaces bounding fundamental sequences may indicate at different stratigraphic levels a basinward or a landward shift of facies belts

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

tectonic and climatic controls on the development of foreland fan deltas montserrat and sant llorenc del munt systems middle eocene ebro basin ne spain abstract along an early cenozoic foreland hinterland boundary in northeastern spain we synthesize the depositional environment climatic conditions structural evolution age catchment geometry and altitudinal characteristics from sedimentologic palynologic stratigraphic structural and paleomagnetic data as the transpressional catalan coastal ranges rose during the paleogene two large fan deltas prograded into the ebro foreland basin adjacent to the northeastern part of the range the apices of the fans likely were localized by lateral ramps or tear faults along which rivers from hinterland catchments debouched into the foreland beginning in the late lutetian proximal debris flow sheetflood and distal fluvial deposits maintained the fan surface at or above sea level despite rapid basin subsidence during the succeeding 4 4 my palynological data suggest that a warm humid climate prevailed throughout this interval the mapped extent of the two fans permits an estimation of their volumes whereas the spatial distribution of distinctive lithologies within the ancestral catalan coastal ranges serves to delimit the approximate catchment areas for each of the fans we estimate mean hinterland denudation rates to range from 100 to 180 m my and mean catchment elevation to range from 700 to 1250 m the steep gradients between these catchments and the low lying fan deltas is attributed to the tectonic style of the ancestral catalan coastal ranges which are characterized by an uplifted basement block along a steep frontal thrust accompanied by folding of cover rocks the considerable topographic relief of these catchments is inferred to have combined with co seismic shaking to produce landslides and rockfalls which were reworked as debris and fluid gravity deposits on the fan surfaces

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

supercritical flows and their control on the architecture and facies of small radius sand rich fan lobes abstract new insights into flow characteristics of supercritical high density turbidity currents initiated renewed interest in a sand rich lobe complex near the hamlet of mizala in the sorbas basin tortonian se spain the field study was done using drone made images taken along bed strike in combination with physical tracing of bounding surfaces and section logging the studied lobe systems show a consistent built up of lobe elements of 1 5 2 0 m thick which form the building blocks of the lobe system the stacking of lobe elements shows lateral shift and compensational relief infill the new model outlined in this paper highlights three stages of fan lobe development i an early aggradational stage with lobe elements characterized by antidune and traction carpet bedforms and burrowed mud intervals here called distal fan deposits ii a progradational stage where the distal fan deposits are truncated by lobe elements of amalgamated sandy to gravelly units characterized by cyclic step bedform facies designated as supra fan deposits the supra fan is much more channelized and scoured and of higher flow energy than the distal fan aggradation of the supra fan is terminated by a pappy pebbly sandstone and by substrate liquefaction pappy referring to a typical porridge like texture indicating rapid deposition under conditions of little to no shear the facies bounded termination of the supra fan is here related to its maximum elevation causing the lobe feeding supercritical flow to choke and to expand upwards by a strong hydraulic jump at the channel outlet iii a backfilling stage characterized by backfilling of the remaining relief with progressively thinning and fining of turbidite beds and eventually with mud the three stage development for fan lobe building is deducted from reoccurring architectural and facies characteristics in three successive fan lobes the validity of using experimental supercritical flow fan studies for understanding the intrinsic mechanisms in sand rich fan lobe development is discussed

on traction carpet sedimentation traction carpets are highly concentrated bedload layers that are developed beneath and driven by turbulent overlying flows they have a convex up velocity profile and comprise a lower frictional and an upper collisional region the frictional region having a particle concentration more than 80 of the packed bed is characterized by nearly continuous grain contacts low strain rate and hampered grain segregation on the other hand the collisional region has a particle concentration between 15 and 80 of the packed bed and is characterized by active grain collisions and higher strain rate deposition from the traction carpets occurs via progressive aggradation of the bed rather than via en masse freezing while the downward grain flux from the overlying flow maintains them the thickness of a traction carpet stratum is therefore determined by the cumulative amount of sediment settled during the lifespan of a traction carpet and can be much larger than the thickness of a moving traction carpet inverse grading can be produced in the collisional region by the vertical gradient of dispersive pressure which is related to nonuniform distribution of particles when a thick frictional region develops however the inverse size distribution in the collisional region is poorly recorded in the deposits depositional features of traction carpet strata are therefore determined by the duration of a traction carpet and the thickness ratio of collisional to frictional regions the thickness ratio is further controlled by the applied shear stress sediment fallout rate and grain size generally a collisional region is better developed beneath a highly competent and coarse grained gravelly flow whereas a thick frictional region is developed beneath a heavily sediment laden fine grained sandy flow this explains the more common occurrence of inverse grading in coarse grained 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

tectonic implications of new shrimp and tims u pb geochronology of rocks from the sask craton peter lake domain and hearne margin trans hudson orogen saskatchewan this study reports new sensitive high resolution ion microprobe shrimp and thermal ionization mass spectrometry tims u pb geochronological data for thirteen rocks from the archean to paleoproterozic sask craton the peter lake domain and hearne margin saskatchewan seven samples from the exposed sask craton in both the glennie domain and pelican window record zircon growth between 2425 and 2525 ma and at 1800 ma older precursor material has been identified most notably a refined crystallization age of 3117 ma for a sample of quartzofeldspatic gneiss q gneiss in the pelican window which also experienced ca 2450 ma zircon growth three samples from the peter lake domain and a sample from the linn island inlier of the wollaston domain yield ages of 2575 ma one sample from the peter lake domain is distinctly older yielding an age of 2640 ma but also displays zircon growth at 2575 ma the simplest interpretation of these data is that the peter lake domain is likely a part of the hearne margin

structureless coarse tail graded bouma ta formed by internal hydraulic jump of the turbidity current abstract turbidity currents are commonly super critical if they travel on slopes steeper than 0 001 hence upon deceleration such flows must pass a hydraulic jump it is therefore surprising that sedimentary features related to the hydraulic jump are so poorly known and not clearly related to the bouma sequence to study this missing link a stability diagram of bed forms generated by decelerating initially super critical turbidity currents is presented here to sketch the position of hydraulic jump deposits relative to super and sub critical bed forms observed in turbidites the axes of the diagram are formed by the densiometric froude number the grain size and the particle fall out rate experimental evidence shows that the hydraulic jump is hardly manifested in the deposit if turbidity currents pass the jump as a low concentration single phase suspension flow only those turbidity currents that develop two phase suspension flow with a super critical flowing traction carpet cause vigorous erosion and liquefaction of the substrate at the jump locality to produce rip up of the substrate rip up clasts and flame structures which are trapped in structureless coarse tail graded deposits characteristic of the classic bouma ta unit the location of the jump is sensitive to density velocity variations of the traction carpet and can shift rapidly up and down the slope this would explain why structureless coarse tail graded deposits such as bouma ta develop sheet like

anatomy of high sediment supply transgressive tracts in the vilomara composite sequence sant llorenc del munt ebro basin ne spain abstract the sant llorenc del munt fan delta succession forms a clastic wedge ca 3 m y duration that built out into the ne ebro basin in response to transpression and uplift along the catalan coastal range the wedge contains a series of composite transgressive regressive sequences the transgressive part of one of these sequences the vilomara composite sequence has been examined to highlight the character of transgressive successions in a highly aggradational setting the analysis focuses on the nature of the component high frequency fundamental sequences there is a marked difference in how alluvial and marine sediment volumes are partitioned within the transgressive and regressive tracts of the three fundamental sequences within this overall landward stepping succession in transgressive tracts the marine shoreline clastics are well developed but only in narrow 250 m wide strike parallel zones which are closely associated with ravinement clusters the time equivalent alluvial strata are thick and extensive in regressive tracts the marine clastics are extensively developed along both dip and strike as prograding shoreline sheets whereas the interfingering alluvial deposits are only thinly developed or eroded out completely by their overlying sequence boundary in regressive intervals there was significant sediment bypass in the alluvial reaches with feeding to the shorezone and shelf in transgressive intervals in contrast great volumes of sediment were stored in the coastal plains this partitioning of sediment volumes commonly led to wedge shaped seaward thickening regressive geometries and wedge shaped landward thickening transgressive geometries the unusual thickness of transgressive strata the aggradationally stacked nature of successive ravinement surfaces and the thick development of nonmarine strata during times of transgression are symptomatic of an unusually high supply of sediment to the system during high rates of accommodation creation and a near continuous rise of relative sea level an unusual type of parasequence in the transgressive tracts showing well developed transgressive and regressive components is probably also symptomatic of the high sediment supply setting

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

box 8 3 fennoscandian shield iron oxide copper gold deposits tjarrojakka northern sweden lat 67 40 n long 19 10 e fennoscandian shield iron oxide copper gold deposits tjarrojakka northern sweden lat 67 40 n long 19 10 e

flow patterns sedimentation and deposit architecture under a hydraulic jump on a non eroding bed defining hydraulic jump unit bars this paper presents results from two flume runs of an ongoing series examining flow structure sediment transport and deposition in hydraulic jumps it concludes in the presentation of a model for the development of sedimentary architecture considered characteristic of a hydraulic jump over a non eroding bed in run 1 a hydraulic jump was formed in sediment free water over the solid plane sloping flume floor ultrasonic doppler velocity profilers recorded the flow structure within the hydraulic jump in fine detail run 2 had identical initial flow conditions and a near steady addition of sand which formed beds with two distinct characteristics a laterally extensive basal wedge shaped massive sand bed overlain by cross laminated sand beds each cross laminated bed recorded the initiation and growth of a single surface feature here defined as a hydraulic jump unit bar a small massive sand mound formed on the flume floor and grew upstream and downstream without migrating to form a unit bar in the upstream portion of the unit bar sand finer than the bulk load formed a set of laminae dipping upstream this set passed downstream through the small volume of massive sand into a foreset which was initially relatively coarse grained and became finer grained downstream this downstream fining coincided with cessation of the growth of the upstream dipping cross set at intervals a new bed feature developed above and upstream of the preceding hydraulic jump unit bar and grew in the same way with the foreset climbing the older unit bar the composite architecture of the superimposed unit bars formed a fanning climbing coset above the massive wedge defined as one unit a hydraulic jump bar complex

initiation of turbidity currents outcrop evidence for eocene hyperpycnal flow turbidites abstract the most widely used although not as widely documented explanation for initiation of turbidites is the conversion of catastrophic sediment failures into turbidites such instantaneous sediment failures generate short lived surge type flows that deposit turbidite beds described as bouma sequences in this communication we present data from the eocene central basin of spitsbergen that shows turbidite systems that are volumetrically dominated by thick ungraded and laminated sandy turbidite beds deposited by gradual aggradation from sustained flows sustained flows are long lived and more or less continuous they can be generated by a variety of mechanisms e g 1 instability during volcanic eruptions and the consequent remobilisation of unconsolidated material 2 seismically triggered subaerial sliding within the drainage 3 storm surges 4 retrogressive slope failure 5 breaching and 6 hyperpycnal flows our database shows that in the eocene central basin the sustained flows were generated by hyperpycnal flows i e by direct river effluent the hyperpycnal flow turbidites have been recognized on the basis of 1 physical connection between fluvial and turbidite channels at the shelf edge 2 abundance of thick turbidite sandstone beds 3 sand prone nature of the turbidite systems 4 downslope changes of individual thick sandy turbidite beds and their collapsed pinch out segments 5 great abundance of continental material leaves coal fragments in turbidite beds 6 low abundance of associated slumped or debris flow beds and 7 the occurrence of turbidites in systematically accreted shelf margins these features strongly suggest that hyperpycnal flow generated by direct river effluent deposited significant number of turbidite beds in the documented slope and basin floor turbidite successions we also argue that the hyperpycnal flows are preferentially fed into the deepwater slopes beyond the shelf edge during the falling stage and lowstand of relative sea level the turbidite beds were studied in the context of seismic scale 1 15 km dip oriented outcrops by measuring detailed vertical sections following individual beds and packages of beds downslope for distances over 5 km lateral mapping and helicopter taken photomosaics

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

parallel lamination formed by high density turbidity currents experiments were conducted in order to investigate processes of sediment transport and deposition from sustained turbidity currents with initial volume fraction sediment concentration of 0 20 0 25 and 0 35 parallel lamination was formed at bed aggradation rates up to 4 mm s 1 and bedload layer sediment concentration up to 0 36 this observation contradicts the generally held assumption that parallel lamination forms only from low density flows and at low bed aggradation rates this misconception illustrates a poor understanding of threshold values of grain support mechanisms in the bedload layer the present results bring new insight into understanding sediment transport in the deep sea and interpreting the sedimentary record

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