Geometry,distribution and fill of erosional scours in a heterolithic,distal lower shoreface sandstone reservoir analogue: Grassy Member,Blackhawk Formation,Book Cliffs,Utah, USA

Many shoreface sandstone reservoirs host significant hydrocarbon volumes within distal intervals of interbedded sandstones and mudstones. Hydrocarbon production from these reservoir intervals depends on the abundance and proportion of sandstone beds that are connected by erosional scours, and on the lateral extent and continuity of interbedded mudstones. Cliff‐face exposures of the Campanian ‘G2’ parasequence, Grassy Member, Blackhawk Formation in the Book Cliffs of east‐central Utah, USA , allow detailed characterization of 128 erosional scours within such interbedded sandstones and mudstones in a volume of 148 m length, 94 m width and 15 m height. The erosional scours have depths of up to 1·1 m, apparent widths of up to 15·1 m and steep sides (up to 35°) that strike approximately perpendicular (N099 ± 36°) to the local north–south palaeoshoreline trend. The scours have limited lateral continuity along strike and down dip, and a relatively narrow range of apparent aspect ratio (apparent width/depth), implying that their three‐dimensional geometry is similar to non‐channelized pot casts. There is no systematic variation in scour dimensions, but ‘scour density’ is greater in amalgamated (conjoined) sandstone beds over 0·5 m thick, and increases upward within vertical successions of upward‐thickening conjoined sandstone beds. There is no apparent organization of the overall lateral distribution of scours, although localized clustering implies that some scours were re‐occupied during multiple erosional events. Scour occurrence is also associated with locally increased amplitude and laminaset thickness of hummocky cross‐stratification in sandstone beds. The geometry, distribution and infill character of the scours imply that they were formed by storm‐generated currents coincident with riverine sediment influx (‘storm floods’). The erosional scours increase the vertical and lateral connectivity of conjoined sandstone beds in the upper part of upward‐thickening sandstone bed successions, resulting in increased effective vertical and horizontal permeability of such intervals.     

Composition and origin of stromatactis‐bearing mud‐mounds (Upper Devonian,Frasnian), southern Rocky Mountains,western Canada

Stromatactis‐bearing mud‐mounds remain an enigmatic reef type despite being common in Palaeozoic ramp settings. Two well preserved Upper Devonian (Frasnian) mud‐mounds in the Mount Hawk Formation crop out side by side in the southern Rocky Mountains of west‐central Alberta and provide an opportunity to develop a new case study that can be compared with the other coeval examples, such as those well‐known ones in southern Belgium, as well as evaluate competing hypotheses for mud‐mound formation. The southern mud‐mound is 46·2 m thick and 38·6 m wide at the base, whilst the northern one is 53·3 m thick and 72·2 m wide at the base, and they exhibit three or four growth stages indicated by interfingering and onlapping geometries with flanking strata. The biota is diverse, but fossils only occupy 10·7% by volume, among which sponge spicules, echinoderms, ostracods, brachiopods and calcimicrobes belonging to Girvanella and Rothpletzella are the most common. Five microfacies are discriminated in the mud‐mounds: biomicrite, clotted micrite, spiculite, stromatolite and laminite, with clotted micrite comprising the largest proportion. There is no internal vertical or lateral palaeoecological zonation, and the presence of calcimicrobes and calcareous algae throughout indicates accretion entirely within the photic zone, in a deeper ramp setting seaward of a large carbonate platform to the east. Stromatactis is abundant and the cavities were mostly due to excavation by currents rather than physical collapse of spiculate siliceous sponges. Formation of lime mud involved a combination of multiple organisms, mechanisms and processes. Cyanobacteria were integral to mud‐mound frame‐building and accretion because they stabilized the surface, often permineralized to form Girvanella and provided organic matter that was decomposed by bacteria. This induced precipitation of micrite, forming early indurated rigid masses, evidenced by the presence of intraclasts, stromatactis cavities, isopachous marine cements, absence of bioturbation and rare synsedimentary brittle deformation. The same microbial components, invertebrate biota and clotted micrite occur in underlying strata, suggesting that there was a protracted period of potential mud‐mound initiation before the exact conditions arose to trigger it. The ramp setting, antecedent sea floor topography and relative sea‐level likely contributed together to control this. This study indicates that mud‐mound formation was controlled by a combination of processes, but they are essentially a microbial buildup.     

The role of shelf morphology on storm‐bed variability and stratigraphic architecture,Lower Cretaceous,Svalbard

The dominance of isotropic hummocky cross‐stratification, recording deposition solely by oscillatory flows, in many ancient storm‐dominated shoreface–shelf successions is enigmatic. Based on conventional sedimentological investigations, this study shows that storm deposits in three different and stratigraphically separated siliciclastic sediment wedges within the Lower Cretaceous succession in Svalbard record various depositional processes and principally contrasting sequence stratigraphic architectures. The lower wedge is characterized by low, but comparatively steeper, depositional dips than the middle and upper wedges, and records a change from storm‐dominated offshore transition – lower shoreface to storm‐dominated prodelta – distal delta front deposits. The occurrence of anisotropic hummocky cross‐stratification sandstone beds, scour‐and‐fill features of possible hyperpycnal‐flow origin, and wave‐modified turbidites within this part of the wedge suggests that the proximity to a fluvio‐deltaic system influenced the observed storm‐bed variability. The mudstone‐dominated part of the lower wedge records offshore shelf deposition below storm‐wave base. In the middle wedge, scours, gutter casts and anisotropic hummocky cross‐stratified storm beds occur in inferred distal settings in association with bathymetric steps situated across the platform break of retrogradationally stacked parasequences. These steps gave rise to localized, steeper‐gradient depositional dips which promoted the generation of basinward‐directed flows that occasionally scoured into the underlying seafloor. Storm‐wave and tidal current interaction promoted the development and migration of large‐scale, compound bedforms and smaller‐scale hummocky bedforms preserved as anisotropic hummocky cross‐stratification. The upper wedge consists of thick, seaward‐stepping successions of isotropic hummocky cross‐stratification‐bearing sandstone beds attributed to progradation across a shallow, gently dipping ramp‐type shelf. The associated distal facies are characterized by abundant lenticular, wave ripple cross‐laminated sandstone, suggesting that the basin floor was predominantly positioned above, but near, storm‐wave base. Consequently, shelf morphology and physiography, and the nature of the feeder system (for example, proximity to deltaic systems) are inferred to exert some control on storm‐bed variability and the resulting stratigraphic architecture.     

Stratigraphic organization and predictability of mixed coarse‐grained and fine‐grained successions in an upper slope Pleistocene turbidite system of the Peri‐Adriatic basin

The early Pleistocene clastic succession of the Peri‐Adriatic basin, eastern central Italy, records the filling of a series of piggyback sub‐basins that formed in response to the development of the eastward‐verging Apennine fold‐thrust belt. During the Gelasian (2·588 to 1·806 Ma), large volumes of Apennine‐derived sediments were routed to these basins through a number of slope turbidite systems. Using a comprehensive outcrop‐based dataset, the current study documents the depositional processes, stratigraphic organization, foraminiferal age and palaeodepth, and stratigraphic evolution of one of these systems exposed in the surroundings of the Castignano village. Analysis of foraminiferal assemblages consistently indicates Gelasian deposition in upper bathyal water depths. Sediments exposed in the study area can be broken into seven main lithofacies, reflecting specific gravity‐induced depositional elements and slope background deposition: (i) clast‐supported conglomerates (conglomerate channel‐fill); (ii) amalgamated sandstones (late stage sandstone channel‐fill); (iii) medium to thick‐bedded tabular sandstones (frontal splay sandstones); (iv) thin to thick‐bedded channelized sandstones (sandy channel‐fill); (v) medium to very thin‐bedded sandstones and mudstones (levée‐overbank deposits); (vi) pebbly mudstones and chaotic beds (mudstone‐rich mass‐transport deposits); and (vii) massive mudstones (hemipelagic deposits). Individual lithofacies combine vertically and laterally to form decametre‐scale, disconformably bounded, fining‐upward lithofacies successions that, in turn, stack to form slope valley fills bounded by deeply incised erosion surfaces. A hierarchical approach to the physical stratigraphy of the slope system indicates that it has evolved through multiple cycles of waxing then waning flow energy at multiple scales and that its packaging can be described in terms of a six‐fold hierarchy of architectural elements and bounding surfaces. In this scheme, the whole system (sixth‐order element) is comprised of three distinct fifth‐order stratigraphic cycles (valley fills), which define sixth‐order initiation, growth and retreat phases of slope deposition, respectively; they are separated by discrete periods of entrenchment that generated erosional valleys interpreted to record fifth‐order initiation phases. Backfilling of individual valleys progressed through deposition of two vertically stacked lithofacies successions (fourth‐order elements), which record fifth‐order growth and retreat phases. Fourth‐order initiation phases are represented by erosional surfaces bounding lithofacies successions. The component lithofacies (third‐order element) record fourth‐order growth and retreat phases. Map trends of erosional valleys and palaeocurrent indicators converge to indicate that the sea floor bathymetric expression of a developing thrust‐related anticline markedly influenced the downslope transport direction of gravity currents and was sufficient to cause a major diversion of the turbidite system around the growing structure. This field‐based study permits the development of a sedimentological model that predicts the evolutionary style of mixed coarse‐grained and fine‐grained turbidite slope systems, the internal distribution of reservoir and non‐reservoir lithofacies within them, and has the potential to serve as an analogue for seismic or outcrop‐based studies of slope valley fills developed in actively deforming structural settings and under severe icehouse regimes.     

Shelf‐margin clinothem progradation,degradation and readjustment: Tanqua depocentre,Karoo Basin (South Africa)

Degradation of basin‐margin clinothems around the shelf‐edge rollover zone may lead to the generation of conduits through which gravity flows transport sediment downslope. Many studies from seismic‐reflection data sets show these features, but they lack small‐scale (centimetre to metre) sedimentary and stratigraphic observations on process interactions. Exhumed basin‐margin clinothems in the Tanqua depocentre (Karoo Basin) provide seismic‐reflection‐scale geometries and internal details of architecture with depositional dip and strike control. At the Geelhoek locality, clinothem parasequences comprise siltstone‐rich offshore deposits overlain by heterolithic prodelta facies and sandstone‐dominated deformed mouth bars. Three of these parasequences are truncated by a steep (6 to 22°), 100 m deep and 1·5 km wide asymmetrical composite erosion surface that delineates a shelf‐incised canyon. The fill, from base to top comprises: (i) thick‐bedded sandstone with intrabasinal clasts and multiple erosion surfaces; (ii) scour‐based interbedded sandstone and siltstone with tractional structures; and (iii) inverse‐graded to normal‐graded siltstone beds. An overlying 55 m thick coarsening‐upward parasequence fills the upper section of the canyon and extends across its interfluves. Younger parasequences display progressively shallower gradients during progradation and healing of the local accommodation. The incision surface resulted from initial oversteepening and high sediment supply triggering deformation and collapse at the shelf edge, enhanced by a relative sea‐level fall that did not result in subaerial exposure of the shelf edge. Previous work identified an underlying highly incised, sandstone‐rich shelf‐edge rollover zone across‐margin strike, suggesting that there was migration in the zone of shelf edge to upper‐slope incision over time. This study provides an unusual example of clinothem degradation and readjustment with three‐dimensional control in an exhumed basin‐margin succession. The work demonstrates that large‐scale erosion surfaces can develop and migrate due to a combination of factors at the shelf‐edge rollover zone and proposes additional criteria to predict clinothem incision and differential sediment bypass in consistently progradational systems.     

Coarse‐sand beach ridges at Cowley Beach,north‐eastern Australia: Their formative processes and potential as records of tropical cyclone history

Storm surges generated by tropical cyclones have been considered a primary process for building coarse‐sand beach ridges along the north‐eastern Queensland coast, Australia. This interpretation has led to the development of palaeotempestology based on the beach ridges. To better identify the sedimentary processes responsible for these ridges, a high‐resolution chronostratigraphic analysis of a series of ridges was carried out at Cowley Beach, Queensland, a meso‐tidal beach system with a >3 m tide range. Optically stimulated luminescence ages indicate that 10 ridges accreted seaward over the last 2500 to 2700 years. The ridge crests sit +3·5 to 5·1 m above Australian Height Datum (ca mean sea‐level). A ground‐penetrating radar profile shows two distinct radar facies, both of which are dissected by truncation surfaces. Hummocky structures in the upper facies indicate that the nucleus of the beach ridge forms as a berm at +2·5 m Australian Height Datum, equivalent to the fair‐weather swash limit during high tide. The lower facies comprises a sequence of seaward‐dipping reflections. Beach progradation thus occurs via fair‐weather‐wave accretion of sand, with erosion by storm waves resulting in a sporadic sedimentary record. The ridge deposits above the fair‐weather swash limit are primarily composed of coarse and medium sands with pumice gravels and are largely emplaced during surge events. Inundation of the ridges is more likely to occur in relation to a cyclone passing during high tide. The ridges may also include an aeolian component as cyclonic winds can transport beach sand inland, especially during low tide, and some layers above +2·5 m Australian Height Datum are finer than aeolian ripples found on the backshore. Coarse‐sand ridges at Cowley Beach are thus products of fair‐weather swash and cyclone inundation modulated by tides. Knowledge of this composite depositional process can better inform the development of robust palaeoenvironmental reconstructions from the ridges.     

Stratigraphic record in the transition from basin floor to continental slope sedimentation in the ancient passive-margin Windermere turbidite system

Well-exposed, vertically dipping, glacially polished outcrops of the Neoproterozoic Windermere Supergroup in the southern Canadian Cordillera include basin-floor deposits of the Upper Kaza Group overlain by slope channel complexes of the Isaac Formation. Within the 2·5 km thick Kaza and Isaac succession is an up to 360 m thick interval composed of diverse deep-water stratal elements including scour and interscour deposits, distributary channels, fine-grained turbidites, terminal splays, mass-transport deposits, erosional and levéed channels and avulsion splays, which collectively were formed during the development of an ancient passive-margin channel-lobe system. The proportion and vertical and lateral arrangement of stratal elements reveal three distinct complexes. The lower complex, consisting mostly of distributary channels and small and large scours, is interpreted to represent the detachment of lobes from an upflow levéed channel, wherein a well-developed channel-lobe transition zone was formed by efficient, siliciclastic flows during a period of sustained transport bypass and limited deposition coincident with the onset of falling relative sea level. The middle, comparatively thicker and more sandstone-rich complex, comprises distributary channel fills, fine-grained turbidites and lesser terminal splays that are interspersed with small scours, capped by a slope levéed channel filled with coarser-grained siliciclastic sediment. The abundance of basin-floor elements suggests negligible separation between the levéed channel and lobe, and therefore a poorly-developed channel-lobe transition zone, resulting from inefficient, siliciclastic-rich depositional flows that became dominant during lowstand and/or ensuing transgression. The stratal makeup of the upper complex resembles the lower detached complex, suggesting a return to efficient flows, and an abrupt change to mixed carbonate–siliciclastic sediments associated with highstand conditions. Accordingly, the stratigraphic architecture and stacking pattern of the Kaza–Isaac interval, which relate to the formation of multiple channel-lobe transition zones, were controlled by temporal changes in sediment supply and flow characteristics during the long-term progradation of the Laurentian continental margin.     

Re-evaluation of coquinoid sandstone depositional model, Upper Jurassic of central Wyoming and south-central Montana

The most extensive Jurassic marine transgression in North America reached its maximum limits during the Oxfordian Age. At this time, siliciclastic sediments were being brought into the North American seaway from an uplifted zone to the west. Within this setting, complexes of sand ridges and coquinoid sands layers were deposited. Coquinoid sandstones appear to fill erosional scours and were interpreted as channel fills. Re-evaluation of these features in the light of recently discovered attributes of modern shelf sediments and processes has produced a revised model of coquinoid sand deposition in this setting. Coquinoid sandstones which fill ‘channel-like’ scours in the Oxfordian (Upper Jurassic) rocks of central Wyoming and south-central Montana, appear to have formed through the migration of sand waves across the crests of inner shelf sand ridges during periods of storm and tidal flow. Erosion in the zone of flow reattachment in the troughs between sand waves resulted in the development of shell lags. Migration of these scour zones as the sand waves advanced resulted in the deposition of sheet-like coquinoid sandstone bodies. Sand waves crossing the ridge crest tended to migrate more slowly and to be overstepped by later sand waves. Sand wave troughs thus buried have channel-like geometries with apparent epsilon bedding.     

Systematically variable geometry and architecture of incised-valley fills controlled by orbital forcing: A conceptual model from the Pennsylvanian Breathitt Group (Kentucky,USA)

Incised-valley fills preserved within ancient coastal to shallow-marine successions represent important archives of environmental and sea-level change. Most current knowledge about the origin of incised valleys stems from Quaternary case studies; however, research on pre-Quaternary examples can shed light on valley formation and evolution across longer timespans. This article describes different types of incised-valley fills from Lower to Middle Pennsylvanian fluvio-deltaic successions of the Breathitt Group (eastern Kentucky), accumulated in the Central Appalachian Foreland under prevalent glacioeustatic forcing driven by Gondwanan glaciations. Based on well-established criteria for their recognition, numerous incised-valley fills were identified from outcrop and subsurface data through more than 300 m of clastic successions consisting of fourth-order stratigraphic sequences stacked into third-order composite sequences. Incised-valley fills were categorized into three archetypes based on lateral extent and aspect ratio (relatively wide versus narrow valley fills), nature of infill (fully continental versus mixed marine and continental facies associations) and relationships to underlying coal zones (truncating versus non-truncating). The systematic occurrence of each incised-valley fill type at specific stratigraphic positions within every third-order sequence suggests control by a periodic allogenic factor. Valley-fill archetypes are interpreted in terms of variable accommodation-supply ratios driven by variable duration of formative base-level cycles. For example, relatively wide incised-valley fills with alluvial infill evolved during long-lived cycles whose prolonged base-level drawdown maintained low accommodation/supply ratios. Deeper valleys with low aspect ratios and mixed marine-continental infills were generated by short-lived base-level drawdown that forced higher accommodation/supply ratios. Available chronological data for the studied successions consent to estimate base-level cycles spanning 10^4–5 years that were likely modulated by interference patterns of orbital parameters (obliquity and eccentricity) via global climate and glacioeustatic fluctuations. This conceptual model, relating incised-valley fill morphometry and internal architecture to orbital forcing patterns, provides a possible approach to predicting and interpreting incised-valley fill variability through successions accumulated during icehouse conditions.     

Channel formation by flow stripping: large-scale scour features along the Monterey East Channel and their relation to sediment waves

The Monterey East system is formed by large‐scale sediment waves deposited as a result of flows stripped from the deeply incised Monterey fan valley (Monterey Channel) at the apex of the Shepard Meander. The system is dissected by a linear series of steps that take the form of scour‐shaped depressions ranging from 3·5 to 4·5 km in width, 3 to 6 km in length and from 80 to 200 m in depth. These giant scours are aligned downstream from a breech in the levee on the southern side of the Shepard Meander. The floor of the breech is only 150 m above the floor of the Monterey fan valley but more than 100 m below the levee crests resulting in significant flow stripping. Numerical modeling suggests that the steps in the Monterey East system were created by Froude‐supercritical turbidity currents stripped from the main flow in the Monterey channel itself. Froude‐supercritical flow over an erodible bed can be subject to an instability that gives rise to the formation of cyclic steps, i.e. trains of upstream‐migrating steps bounded upstream and downstream by hydraulic jumps in the flow above them. The flow that creates these steps may be net‐erosional or net‐depositional. In the former case it gives rise to trains of scours such as those in the Monterey East system, and in the latter case it gives rise to the familiar trains of upstream‐migrating sediment waves commonly seen on submarine levees. The Monterey East system provides a unique opportunity to introduce the concept of cyclic steps in the submarine environment to study processes that might result in channel initiation on modern submarine fans.     

Ground-penetrating radar study of Pleistocene ice scours on a glaciolacustrine sequence boundary

Ice scouring of lake and sea-floor substrates by the keels of drifting ice masses is a common geological process in modern northern lakes and continental shelves, and was widespread during the Pleistocene. Nonetheless, the importance of scouring as a geological process is not yet matched by many sedimentological studies of scour structures exposed in outcrop. This article presents an integrated study combining outcrop sedimentology and subsurface ground-penetrating radar (GPR) data from a relict late Pleistocene ice-scoured glacial lake floor now preserved below beach sediments in Ontario, Canada. Scours occur along a regressive sequence boundary where deep-water muddy facies are abruptly overlain by shallow-water sands resulting from an abrupt drop in water levels. This has allowed the keels of drifting ice masses to scour into muds. Three-dimensional data gained from the GPR survey show that scours are as much as 2.5 m deep and 7 m wide; they have berms of displaced sediment and are oriented parallel to the former shoreline. Scoured shoreface sediments that fill scours show abundant liquefaction structures, indicating substrate dewatering during repeated scouring events similar to that recently reported in the modern Beaufort Sea in Canada's far north. Marked changes in water depths are typical of glacially influenced lakes and seas, creating opportunities for drifting ice to scour into offshore muddy cohesive facies and be preserved. The data presented here may aid identification in ancient successions elsewhere.     

Coastal boulder deposit as evidence of an ocean-wide prehistoric tsunami originated on the Atacama Desert coast (northern Chile)

A Late Holocene cliff-top deposit of large boulders well above the limits of modern storm waves is described from the southern coast of the Atacama Desert (northern Chile). The largest moved boulder weighs >40 t and field data point to a flood height >18·5 m above high tide level and an inland penetration greater than 284 m from the cliff edge. The minimum flow velocity needed for particle entrainment was estimated as 10·1 ms⁻¹ and the most likely processes of sediment deposition for different boulders were deduced. The boulder distribution, sorting and orientation of imbricated debris, together with the significant wave height of extreme storms reported and the occurrence of interplate earthquakes in the study area indicate that the deposit records a single event, interpreted here as a tsunami wave train rather than exceptional storm waves. The boulder field was dated to between the 13th and the 16th Centuries ce and possibly correlates with the 1420 Oei orphan tsunami, that affected the eastern coast of Japan. A magnitude of 8·8 to 9·4 has been estimated for the earthquake, which may be one of the larger events of a super-cycle of earthquakes in the southern Atacama Desert. These cycle-ending earthquakes involve large rupture areas (lengths in excess of 600 km) and highly destructive ocean-wide tsunamigenic events.     

Grain‐size variability within a mega‐scale point‐bar system,False River,Louisiana

Point bars formed by meandering river systems are an important class of sedimentary deposit and are of significant economic interest as hydrocarbon reservoirs. Standard point‐bar models of how the internal sedimentology varies are based on the structure of small‐scale systems with little information about the largest complexes and how these might differ. Here a very large point bar (>25·0 m thick and 7·5 × 13·0 km across) on the Mississippi River (USA) was examined. The lithology and grain‐size characteristics at different parts of the point bar were determined by using a combination of coring and electrical conductivity logging. The data confirm that there is a general fining up‐section along most parts of the point bar, with a well‐defined transition from massive medium‐grained sands below about 9 to 11 m depth up into interbedded silts and fine–medium sand sediment (inclined heterolithic strata). There is also a poorly defined increase in sorting quality at the transition level. Massive medium sands are especially common in the region of the channel bend apex and regions upstream of that point. Downstream of the meander apex, there is much less evidence for fining up‐section. Finer sediment accumulated more readily after the establishment of a compound bar in the later stages of construction, at the terminal apex and in the bar tail. This work implies that the best reservoir sands are likely to be located in the centre of the point bar, deposited in a simple bar system. Reservoir quality decreases towards the bar edge. The early‐stage channel plug is largely composed of coarsening‐upward cycles of silt to clay and is dominated by clay and clayey silt material with poor reservoir characteristics.     

Storm‐dominated shelf‐edge delta successions in a high accommodation setting: The palaeo‐Orinoco Delta (Mayaro Formation), Columbus Basin,South‐East Trinidad

Pliocene age deposits of the palaeo‐Orinoco Delta are evaluated in the Mayaro Formation, which crops out along the western margin of the Columbus Basin in south‐east Trinidad. This sandstone‐dominated interval records the diachronous, basinwards migration of the shelf edge of the palaeo‐Orinoco Delta, as it prograded eastwards during the Pliocene–Pleistocene (ca 3·5 Ma). The basin setting was characterized by exceptionally high rates of growth‐fault controlled sediment supply and accommodation space creation resulting in a gross basin‐fill of around 12 km, with some of the highest subsidence rates in the world (ca 5 to 10 m ka^?1). This analysis demonstrates that the Mayaro Formation was deposited within large and mainly wave‐influenced shelf‐edge deltas. These are manifested as multiple stacks of coarsening upward parasequences at scales ranging from tens to hundreds of metres in thickness, which are dominated by storm‐influenced and wave‐influenced proximal delta‐front sandstones with extensive, amalgamated swaley and hummocky cross‐stratification. These proximal delta‐front successions pass gradationally downwards into 10s to 100 m thick distal delta front to mud‐dominated upper slope deposits characterized by a wide variety of sedimentary processes, including distal river flood and storm‐related currents, slumps and other gravity flows. Isolated and subordinate sandstone bodies occur as gully fills, while extensive soft sediment deformation attests to the high sedimentation rates along a slope within a tectonically active basin. The vertical stratigraphic organization of the facies associations, together with the often cryptic nature of parasequence stacking patterns and sequence stratigraphic surfaces, are the combined product of the rapid rates of accommodation space creation, high rates of sediment supply and glacio‐eustasy in the 40 to 100 Ka Milankovitch frequency range. The stratigraphic framework described herein contrasts strikingly with that described from passive continental margins, but compares favourably to other tectonically active, deltaic settings (for example, the Baram Delta Province of north‐west Borneo).     

Transition from storm wave‐dominated outer shelf to gullied upper slope: The mid‐Pliocene Orinoco shelf margin,South Trinidad

Shelf‐edge deltas are a key depositional environment for accreting sediment onto shelf‐margin clinoforms. The Moruga Formation, part of the palaeo‐Orinoco shelf‐margin sedimentary prism of south‐east Trinidad, provides new insight into the incremental growth of a Pliocene, storm wave‐dominated shelf margin. Relatively little is known about the mechanisms of sand bypass from the shelf‐break area of margins, and in particular from storm wave‐dominated margins which are generally characterized by drifting of sand along strike until meeting a canyon or channel. The studied St. Hilaire Siltstone and Trinity Hill Sandstone succession is 260 m thick and demonstrates a continuous transition from gullied (with turbidites) uppermost slope upward to storm wave‐dominated delta front on the outermost shelf. The basal upper‐slope deposits are dominantly mass‐transport deposited blocks, as well as associated turbidites and debrites with common soft‐sediment‐deformed strata. The overlying uppermost slope succession exhibits a spectacular set of gullies, which are separated by abundant slump‐scar unconformities (tops of rotational slides), then filled with debris‐flow conglomerates and sandy turbidite beds with interbedded mudstones. The top of the study succession, on the outer‐shelf area, contains repeated upward‐coarsening, sandstone‐rich parasequences (2 to 15 m thick) with abundant hummocky and swaley cross‐stratification, clear evidence of storm‐swell and storm wave‐dominated conditions. The observations suggest reconstruction of the unstable shelf margin as follows: (i) the aggradational storm wave‐dominated, shelf‐edge delta front became unstable and collapsed down the slope; (ii) the excavated scars of the shelf margin became gullied, but gradually healed (aggraded) by repeated infilling by debris flows and turbidites, and then new gullying and further infilling; and (iii) a renewed storm wave‐dominated delta‐front prograded out across the healed outer shelf, re‐establishing the newly stabilized shelf margin. The Moruga Formation study, along with only a few others in the literature, confirms the sediment bypass ability of storm wave‐dominated reaches of shelf edges, despite river‐dominated deltas being, by far, the most efficient shelf‐edge regime for sediment bypass at the shelf break.     

The sedimentology of river confluences

Channel confluences are key nodes within large river networks, and yet surprisingly little is known about their spatial and temporal evolution. Moreover, because confluences are associated with vertical scour that typically extends to several times the mean channel depth, the deposits associated with such scours should have a high preservation potential within the rock record. Paradoxically, such scours are rarely observed, and their preservation and sedimentological interpretation are poorly understood. The present study details results from a physically‐based morphodynamic model that is applied to simulate the evolution and alluvial architecture of large river junctions. Boundary conditions within the model were defined to approximate the junction of the Ganges and Jamuna rivers, Bangladesh, with the model output being supplemented by geophysical datasets collected at this junction. The numerical simulations reveal several distinct styles of sedimentary fill that are related to the morphodynamic behaviour of bars, confluence scour downstream of braid bars, bend scour and major junction scour. Comparison with existing, largely qualitative, conceptual models reveals that none of these can be applied simply, although elements of each are evident in the deposits generated by the numerical simulation and observed in the geophysical data. The characteristics of the simulated scour deposits are found to vary according to the degree of reworking caused by channel migration, a factor not considered adequately in current conceptual models of confluence sedimentology. The alluvial architecture of major junction scours is thus characterized by the prevalence of erosion surfaces in conjunction with the thickest depositional sets. Confluence scour downstream of braid bar and bend scour sites may preserve some large individual sets, but these locations are typically characterized by lower average set thickness compared to major junction scour and by a lack of large‐scale erosional surfaces. Areas of deposition not related to any of the specific scour types highlighted above record the thinnest depositional sets. This variety in the alluvial architecture of scours may go some way towards explaining the paradox of ancient junction scours, that while abundant large scours are likely in the rock record, they have been reported rarely. The present results outline the likely range of confluence sedimentology and will serve as a new tool for recognizing and interpreting these deposits in the ancient fluvial record.     

Sorted bedforms off Western Long Island,New York,USA: Asymmetrical morphology and twelve‐year migration record

Sorted bedforms are widely present in sediment‐starved littoral and inner shelf settings; they are indicators for hydrodynamic conditions and a primary contributor for the subsurface structure. This study investigated the morphology and migration of sorted bedforms on the inner shelf of Long Beach Barrier Island, New York, USA , by repeat geophysical and geological surveys in 2001, 2005 and 2013 (following superstorm Sandy) involving swath bathymetry, backscatter, chirp seismic reflection data and grab sampling. Swath data revealed that the western sector, comprising the western 75% of the survey region, is dominated by NNE –SSW ‐oriented, 0·5 to 1·0 km wide sorted bedforms with highly asymmetrical cross‐sections, with steeper slopes and coarser sands on the eastern (stoss) flanks. Many secondary bedforms were also observed (north–south to north‐east/south‐west oriented lineation structures) at the western edges of coarse sand zones. The eastern sector displays an unusual sorted bedform pattern that is dominated by coarse‐grained substrate, with isolated patches of fine‐grained sands oriented north‐east/south‐west which are 0·15 to 1·0 km in length and ca 30 to 200 m in width, similar in scale and orientation to the secondary bedforms in the western sector. Comparison analysis of the swath data sets indicates that the primary transverse sorted bedform morphology within the western sector was largely stable over this time frame, although the swales were deepened following the storms. The coarse/fine sand boundaries did migrate, however, moving ca 1 to 5 m eastward between 2001 and 2005, and ca 5 to 20 m westward between 2005 and 2013; the higher migration rates (up to 2·5 m year⁻¹) in the latter time period may be attributable to large storm forcing (for example, hurricanes Irene and Sandy). Significant north‐westward migration of the secondary bedforms and coarse sand patches in the western sector, as well as fine sand patches in the eastern sector were also observed; these features are far more mobile than the primary sorted bedforms, possibly because they are fine sand drifts that do not erode into the coarse substrate. Seismic reflection data revealed a transgressive ravinement beneath sorted bedforms, merging with the sea floor at the bottom of swales. The authors hypothesize that long‐term topographic migration of transverse sorted bedforms contributes to the formation and evolution of the ravinement.