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Abstract Deltas are deposits directly accumulated by land‐generated gravity flows in a standing body of water. The paradigm of deltaic sedimentation has dramatically changed during recent years, from the popular very simplified ternary models of marine littoral deltas towards more realistic and comprehensive models, considering the importance of sediment‐laden river discharges. Ternary delta models were designed for clean rivers, where a stream flow drags the sediments. Depending on the basin dynamics, these littoral deposits can be modified, forming tidal‐dominated, wave‐dominated or fluvial‐dominated littoral deltas. In recent years, a new classification of delta systems was proposed, based on contrasting the salinity of the receiving water body with the bulk density of the incoming fluvial discharge. Rivers are highly dynamic systems, and their discharges can be very variable in terms of flow duration and sediment concentration. Additionally, the salinity of the receiving water body can exhibit significant variability, especially in closed lakes and epicontinental seas, ranging from freshwater to brines. This scenario allows the distinction of three major delta categories (hypopycnal, homopycnal and hyperpycnal deltas) which can be in turn subdivided, defining seven delta types. Hypopycnal deltas form when the bulk density of the incoming flow is lower than the density of the water in the basin, allowing the definition of three delta types, corresponding to hypersaline littoral deltas, marine littoral deltas and brackish littoral deltas. Homopycnal deltas form when the bulk density of the incoming flow is similar to the density of the water in the basin, defining a delta type termed homopycnal littoral deltas. Hyperpycnal deltas form when the bulk density of the incoming flow is higher than the density of the water in the basin, allowing the definition of three categories termed hyperpycnal littoral deltas, hyperpycnal subaqueous deltas and hyperpycnal fan deltas.

Deltas are deposits directly accumulated by land‐generated gravity flows in a standing body of water. The paradigm of deltaic sedimentation has dramatically changed during recent years, from the popular very simplified ternary models of marine littoral deltas towards more realistic and comprehensive models, considering the importance of sediment‐laden river discharges. Ternary delta models were designed for clean rivers, where a stream flow drags the sediments. Depending on the basin dynamics, these littoral deposits can be modified, forming tidal‐dominated, wave‐dominated or fluvial‐dominated littoral deltas. In recent years, a new classification of delta systems was proposed, based on contrasting the salinity of the receiving water body with the bulk density of the incoming fluvial discharge. Rivers are highly dynamic systems, and their discharges can be very variable in terms of flow duration and sediment concentration. Additionally, the salinity of the receiving water body can exhibit significant variability, especially in closed lakes and epicontinental seas, ranging from freshwater to brines. This scenario allows the distinction of three major delta categories (hypopycnal, homopycnal and hyperpycnal deltas) which can be in turn subdivided, defining seven delta types. Hypopycnal deltas form when the bulk density of the incoming flow is lower than the density of the water in the basin, allowing the definition of three delta types, corresponding to hypersaline littoral deltas, marine littoral deltas and brackish littoral deltas. Homopycnal deltas form when the bulk density of the incoming flow is similar to the density of the water in the basin, defining a delta type termed homopycnal littoral deltas. Hyperpycnal deltas form when the bulk density of the incoming flow is higher than the density of the water in the basin, allowing the definition of three categories termed hyperpycnal littoral deltas, hyperpycnal subaqueous deltas and hyperpycnal fan deltas. [ABSTRACT FROM AUTHOR]

Turbidity currents are a form of gravity-driven flow that occurs in subaqueous environments. These currents contain a low volumetric percentage of particles but are very important in transporting them from coastal to deep-marine environments. In the Var sediment routing system, southeastern France, there is a sedimentary record of turbidity currents formed by submarine landslides and flooding of the Var River. In 2006, there was continuous monitoring of the marine section that recorded four turbidity currents. To explore the connectivity between the terrestrial source and marine basin we linked two reduced complexity models of sediment transport and landscape change. The aim of the paper is to combine two models, one dedicated to the prediction of river water and sediment run-off, and the second dedicated to the transport, erosion, and sedimentation by a turbidity current. The landscape evolution model CAESAR-Lisflood was used to model discharge and sediment yield. The behaviour of turbidity currents was modelled with the cellular automata model CATS. From calibration of the two models against observations of discharge and suspended sediment in the Var River system, we find that in 2006, two rainfall events would have led to hyperpycnal turbidity currents. These two events match well with two of the four-recorded events. Focusing on the largest event we find that the source pulse of sediment from the terrestrial environment is short-lived, less than half a day, but contains a significant quantity of fine particles. The event transferred on the order of 105 m3 of suspended sediment from source to sink. This study demonstrates that hyperpycnal turbidity currents can be generated with concentrations as low as 2-6 kg/m3 at the Var River mouth far below the theoretical 40 kg/m3 threshold, suggesting active convective sedimentation in the surface plume and sea-water dilution at the flooding river mouth. A substantial amount of sediment (35% of the input volume) is directly transferred towards the deepest part of the system during a short-living hyperpycnal turbidity current. However, a considerable (65%) part remains in the surface as a hypopycnal plume feeding the hemipelagic sedimentation.

Deltas are at the transition between fluvial and marine sedimentary environments where sediment density flows are often triggered during high river discharge events, forming submarine channels and sediment waves. On wave‐influenced deltas, longshore currents are particularly efficient at transporting sediment alongshore, reducing the likelihood of sediment density flows from occurring at river mouths. This study describes four deltaic sedimentary systems at different stages of their evolution on a formerly glaciated continental inner shelf of eastern Canada in order to better understand the distribution of sediment density flows on wave‐influenced deltas. Three types of settings are recognized as being prone to sediment density flows: (i) in the early stages of wave‐influence and on large deltas, converging longshore currents can lead to offshelf sediment transport; (ii) on wave‐influenced to wave‐dominated deltas, a sandy spit can re‐route the river mouth and sediment density flows form where the spit intersects the delta lip; (iii) in advanced stages of wave‐dominated deltas and during their demise, rocky headlands are exposed and can intersect the slope, where off‐shelf sediment transport occurs. These types of sediment density flows were all characterized by debris flows or surge‐type turbidity currents which have limited offshore run‐out. More rarely, hyperpycnal flows form at the river mouths, especially where the river incises glaciomarine clays prone to landsliding in the river, which increases fine‐grained fluvial suspended sediment concentration. Overall, these results highlight the predominance of fluvial‐dominated deltas during a phase of relative sea‐level fall combined with high sediment supply. However, as soon as sediment supply diminishes, wave action remobilizes sediment alongshore modifying the distribution and types of sediment density flows occurring on wave‐influenced deltas. [ABSTRACT FROM AUTHOR]

Oceanic anoxia is regarded as the immediate cause of the Late Devonian Frasnian–Famennian (F–F) bio-crisis and Annulata bio-event. The oceanic anoxia has been linked to continental weathering and input. However, some previous findings indicate oxic marine environments during the F–F bio-crisis. Moreover, the relative impact of terrestrial input on the Late Devonian oceanic redox conditions, when comparing a bio-crisis and a bio-event, remains unknown. Here, we present a decoupled oceanic redox model suggesting the redox conditions were different between shallow and deep water columns during the F–F bio-crisis, i.e., intensified euxinia in the photic zone and a more oxygenated condition at the seafloor. Geochemical (biomarker and trace metal) and sedimentologic evidence from the Late Devonian Seaway, eastern U.S., indicate that the shallow-deep decoupled oceanic redox conditions were regulated by terrestrial weathering and input. During the F–F bio-crisis, land nutrient input intensified photic zone euxinia through eutrophication, while hyperpycnal flows oxygenated seafloors. After the F–F bio-crisis, the seafloor redox conditions shifted from oxygenation to silled euxinia, and the watermass became hydrographically restricted. During this post-F–F interval, the euxinia at the seafloor intensified due to land nutrient runoff during the Annulata bio-event. Importantly, the land weathering and input during the F–F bio-crisis were more prominent than during the Annulata bio-event. This resulted in widespread and intense photic zone euxinia during the F–F bio-crisis, relative to the seafloor euxinia that developed in restricted areas during the Annulata bio-event. Our decoupled oceanic redox model may explain the preferential decimation of shallow-water organisms during the F–F bio-crisis. The seafloor euxinia during the Annulata bio-event may provide a niche for the turnover of benthic faunas adapting to low oxygen levels. This study suggests that continental weathering and influxes played an important role in regulating oceanic oxygen evolution and impacting life evolution and bio-diversity. • PZE intensified but seafloor became more oxygenated in the Late Devonian Seaway during the F–F bio-crisis. • Seafloor oxygenation may be caused by hyperpycnal flows. • Land input caused redox changes during the F–F bio-crisis and Annulata bio-event. • Seafloor shifted from oxygenation to silled euxinia after the F–F bio-crisis. • The redox model may explain the preferential decimation of shallow-water organisms in the Late Devonian Seaway during the F–F bio-crisis. [ABSTRACT FROM AUTHOR]

Abstract A hyperpycnal flow forms when a relatively dense land-derived gravity flow enters into a marine or lacustrine water reservoir. As a consequence of its excess of density, the incoming flow plunges in coastal areas, generating a highly dynamic and often long-lived dense underflow. Depending on the characteristics of the parent flow (flow duration and flow rheology) and basin salinity, the resulting deposits (hyperpycnites) can be very variable. According to flow duration, land-derived gravity flows can be classified into short-lived or long-lived flows. Short-lived gravity flows last for minutes or hours, and are mostly related to small mountainous river discharges, alluvial fans, collapse of natural dams, landslides, volcanic eruptions, jökulhlaups, etc. Long-lived gravity flows last for days, weeks or even months, and are mostly associated with medium- to large-size river discharges. Concerning the rheology of the incoming flow, hyperpycnal flows can be initiated by non-Newtonian (cohesive debris flows), Newtonian supercritical (lahars, hyperconcentrated flows, and concentrated flows) or Newtonian subcritical flows (pebbly, sandy or muddy sediment-laden turbulent flows). Once plunged, non-Newtonian and Newtonian supercritical flows require steep slopes to accelerate, allow the incorporation of ambient water and develop flow transformations in order to evolve into a turbidity current and travel further basinward. Their resulting deposits are difficult to differentiate from those related to intrabasinal turbidites. On the contrary, long-lived Newtonian subcritical flows are capable of transferring huge volumes of sediment, freshwater and organic matter far from the coast even along gentle or flat slopes. In marine settings, the buoyant effect of interstitial freshwater in pebbly and sandy hyperpycnal flows can result in lofting due to flow density reversal. Since the excess of density in muddy hyperpycnal flows is provided by silt-clay sediments in turbulent suspension, lofting is not possible even in marine/saline basins. Muddy hyperpycnal flows can also erode the basin bottom during their travel basinward, allowing the incorporation and transfer of intrabasinal sediments and organic matter. Long-lived hyperpycnal flow deposits exhibit typical characteristics that allow a clear differentiation respect to those related to intrabasinal turbidites. Main features include (1) composite beds with gradual and recurrent changes in sediment grain-size and sedimentary structures, (2) mixture of extrabasinal and intrabasinal components, (3) internal and discontinuous erosional surfaces, and (4) lofting rhythmites in marine/saline basins.

This paper presents a new depositional model for the Amasiri Sandstone of the southern Benue Trough, Nigeria. Variants of facies and facies relationships suggesting fluvial, storm, and sediment gravity processes gave rise to controversies in the interpretation of environments of deposition for the formation. However, newly exposed sections in the quarries allowed further studies, to understand the intrinsic depositional facies. Field description and lithofacies analysis were used in understanding the facies relationships, depositional processes, and environments. Results of the sedimentary facies analysis revealed twelve facies grouped into foreshore facies association (FA 1), proximal delta front facies association (FA 2), distal delta front facies association (FA 3), and prodeltaic facies association (FA 4). FA 1 consists of cross-bedded sandstone, interpreted as subtidal sand bars in a littoral setting and occurred at the top of each deltaic cycle in the Amasiri Sandstone. FA 2 comprises asymptotic cross stratified sandstone, hummocky cross stratified sandstone, massive bedded sandstone, conglomeratic sandstone with limestone rip-up clasts, and parallel laminated sandstone. It was interpreted as deposits of proximal delta front (proximal mouth bar/ dune front and delta front lobes), deposited by bedload process, and suspension influenced by sustained hyperpycnal flows. FA 3 consists of wavy/ripple laminated sandstone, bioturbated sandstone, coquinoid limestone, bioturbated mudstone with rip-up clasts, shell hash, and plant debris, interpreted to be deposits of distal delta front transported as suspension load by sustained hyperpycnal flows. FA 4 comprises graded cycles and couplets of thinly interbedded very fine sandstones, mudstones, siltstones, and shales inferred as prodeltaic facies deposited by fallout and buoyancy reversal (lofting process). A new depositional model for the Amasiri Sandstone shows that FA 1 was deposited by the fluvial/tidal process. In contrast, FA 2 to FA 4 were deposited by hyperpycnal flows in flood-dominated deltaic environments. [ABSTRACT FROM AUTHOR]

Abstract In this reply, I respond to 18 issues associated with comments made by Zavala (e.g., inverse- to normally-graded sequence, origin of massive sands, experimental sandy debris flows, tidal rhythmites, facies models, etc.), and 10 issues associated with comments made by Van Loon et al. (e.g., 16 types of hyperpycnal flows, anthropogenic hyperpycnal flow, etc.).

Abstract I basically agree with the viewpoints of Shanmugam (Journal of Palaeogeography 7(3):197-238, 2018) and Zavala (Journal of Palaeogeography 8(3), 2019) who cited, refined and interpreted the definitions of hypopycnal flow, homopycnal flow and hyperpycnal flow. I appreciate two typical case studies of hyperpycnal flows induced by the Yellow River and Yangtze River, and the Gaoping River. The former is a normal type while the latter is catastrophic. They make up a complete knowledge about hyperpycnal flows and hyperpycnites. According to the interpretation of the word “hyperpycnal” from Greek to English, the “hypopycnal flow” should be “less density flow” or “lower density flow” (“低密度流”), the “homopycnal flow” should be “equal density flow” (“等密度流”), and the “hyperpycnal flow” should be “higher density flow” or “over density flow” (“高密度流” or “超密度流”). Some geologists called the “hypopycnal flow” as “异轻流” (“abnormally light flow”) and called the “hyperpycnal flow” as “异重流” (“abnormally heavy flow”). There are at least more than 10 names or terms about the “density flows” and the “deposits of density flows”. It is a problem indeed. In addition, the density could be changed by salinity, temperature and pressure of water. Therefore, the term “density flow” may be problematic either. Another problem is that reliable and irrefutable identification markers of ancient heperpycnites are lacking. We should observe the policy of “A hundred flowers blossom and a hundred schools of thought contend” to discuss these problems and to promote progress and development of hyperpycnal flows and hyperpycnites.

Deltas constitute complex depositional systems formed when a land-derived gravity-flow (carrying water and sediments) discharges into a marine or lacustrine standing body of water. However, the complexity of deltaic sedimentary environments has been oversimplified by geoscientists over the years, considering just littoral deltas as the unique possible type of delta in natural systems. Nevertheless, a rational analysis suggests that deltas can be much more complex. In fact, the characteristics of deltaic deposits will depend on a complex interplay between the bulk density of the incoming flow and the salinity of the receiving water body. This paper explores the natural conditions of deltaic sedimentation according to different density contrasts. The rational analysis of deltaic systems allows to recognize three main fields for deltaic sedimentation, corresponding to (1) hypopycnal (2) homopycnal and (3) hyperpycnal delta settings. The hypopycnal delta field represents the situation when the bulk density of the incoming flow is lower than the density of the water in the basin. According to the salinity of the receiving water body, three different types of hypopycnal littoral deltas are recognized: hypersaline littoral deltas (HSLD), marine littoral deltas (MLD), and brackish littoral deltas (BLD). The basin salinity will determine the capacity of the delta for producing effective buoyant plumes, and consequently the characteristics and extension of prodelta deposits. Homopycnal littoral deltas (HOLD) form when the density of the incoming flow is roughly similar to the density of the water in the receiving basin. This situation is typical of clean bedload-dominated rivers entering freshwater lakes. Delta front deposits are dominated by sediment avalanches. Typical fallout prodelta deposits are absent or poorly developed since no buoyant plumes are generated. Hyperpycnal deltas form when the bulk density of the incoming flow is higher than the density of the water in the receiving basin. The interaction between flow type, flow density (due to the concentration of suspended sediments) and basin salinity defines three types of deltas, corresponding to hyperpycnal littoral deltas (HLD), hyperpycnal subaqueous deltas (HSD), and hyperpycnal fan deltas (HFD). Hyperpycnal littoral deltas are low-gradient shallow-water deltas formed when dirty rivers enter into brackish or normal-salinity marine basins, typically in wave or tide-dominated epicontinental seas or brackish lakes. Hyperpycnal subaqueous deltas represent the most common type of hyperpycnal delta, with channels and lobes generated in marine and lacustrine settings during long-lasting sediment-laden river-flood discharges. Finally, hyperpycnal fan deltas are subaqueous delta systems generated on high-gradient lacustrine or marine settings by episodic high-density fluvial discharges. [ABSTRACT FROM AUTHOR]

In the deepest portions of many lakes, zones of high-conductivity bottom water (HCBW) depleted in dissolved oxygen (DO) are present. HCBW and DO are important for determining benthic diversity and abundance, nutrients, and contaminant cycling and understanding the long-term evolution of lakes. We investigate the persistence and removal of HCBW and DO replenishment in a High Arctic lake using physical properties and flow velocity data along with hydrometric and suspended sediment inflow data over a 4 year monitoring period (2007–2010). HCBW was removed in 2007 and 2008 but largely remained in 2009 and 2010. Catchment disturbances in 2007 increased suspended sediment concentrations (SSC) in the inflowing river in 2007 and 2008. In the later two years of monitoring (2009 and 2010), fluvial sediment availability relaxed to pre-disturbance levels. High SSC in 2007 and 2008 caused by landscape disturbances formed sustained river-generated hyperpycnal flows during the snowmelt period that are linked to HCBW removal. In 2009 and 2010, inflowing river water was periodically denser than lake water; however, HCBW was not removed in these years. Hyperpycnal flows were likely either of insufficient strength or duration, deposited on the delta front, or followed paths that led away from the deepest portion of the lake. Results suggest that hyperpycnal flow frequency will increase and HCBW persistence will decrease with projected climate change due to an increase in fluvial SSC inflow. Water density changes resulting from increased electrical conductivity and water temperature are not likely to have a similarly large effect on hyperpycnal flow frequency and HCBW.

A hyperpycnal flow forms when a relatively dense land-derived gravity flow enters into a marine or lacustrine water reservoir. As a consequence of its excess of density, the incoming flow plunges in coastal areas, generating a highly dynamic and often long-lived dense underflow. Depending on the characteristics of the parent flow (flow duration and flow rheology) and basin salinity, the resulting deposits (hyperpycnites) can be very variable. According to flow duration, land-derived gravity flows can be classified into short-lived or long-lived flows. Short-lived gravity flows last for minutes or hours, and are mostly related to small mountainous river discharges, alluvial fans, collapse of natural dams, landslides, volcanic eruptions, jökulhlaups, etc. Long-lived gravity flows last for days, weeks or even months, and are mostly associated with medium- to large-size river discharges. Concerning the rheology of the incoming flow, hyperpycnal flows can be initiated by non-Newtonian (cohesive debris flows), Newtonian supercritical (lahars, hyperconcentrated flows, and concentrated flows) or Newtonian subcritical flows (pebbly, sandy or muddy sediment-laden turbulent flows). Once plunged, non-Newtonian and Newtonian supercritical flows require steep slopes to accelerate, allow the incorporation of ambient water and develop flow transformations in order to evolve into a turbidity current and travel further basinward. Their resulting deposits are difficult to differentiate from those related to intrabasinal turbidites. On the contrary, long-lived Newtonian subcritical flows are capable of transferring huge volumes of sediment, freshwater and organic matter far from the coast even along gentle or flat slopes. In marine settings, the buoyant effect of interstitial freshwater in pebbly and sandy hyperpycnal flows can result in lofting due to flow density reversal. Since the excess of density in muddy hyperpycnal flows is provided by silt-clay sediments in turbulent suspension, lofting is not possible even in marine/saline basins. Muddy hyperpycnal flows can also erode the basin bottom during their travel basinward, allowing the incorporation and transfer of intrabasinal sediments and organic matter. Long-lived hyperpycnal flow deposits exhibit typical characteristics that allow a clear differentiation respect to those related to intrabasinal turbidites. Main features include (1) composite beds with gradual and recurrent changes in sediment grain-size and sedimentary structures, (2) mixture of extrabasinal and intrabasinal components, (3) internal and discontinuous erosional surfaces, and (4) lofting rhythmites in marine/saline basins. [ABSTRACT FROM AUTHOR]

The Upper Cretaceous Prairie Canyon Member of the Mancos Shale, Book Cliffs, Utah, contain outstanding examples of prodeltaic turbidity and hyperpycnal flow deposits. Sandstone‐rich, heterolithic and mudstone‐rich channel fills occur near the north‐west entrance to Tusher Canyon, Gunnison Butte and Bootlegger Wash. Mudstone‐rich and heterolithic‐rich hyperpycnal channel deposits are mostly unbioturbated, locally displaying a few specimens of Phycosiphon incertum, Protovirgularia dichotoma, Rosselia socialis, Schaubcylindrichnus coronus and Palaeophycus tubularis. Sandstone‐rich channel deposits consist of wave‐reworked turbidites and hyperpycnites, containing Helminthoidichnites tenuis, Lockeia siliquaria, Phycodes isp., Phycosiphon incertum, Protovirgularia dichotoma, Rosselia socialis, Skolithos linearis and Fugichnia. Scolicia isp. and Chondrites isp. occur locally. Strata along the south‐west entrance of Tusher Canyon record deposition in a prodelta turbidite lobe, but far from its axis. With the exception of a few specimens of Ophiomorpha isp., bioturbation is restricted to the top of the succession, where Curvolithus simplex, Gyrochorte comosa, Lockeia siliquaria, Palaeophycus tubularis and Ptychoplasma excelsum occur. Strata at Hatch Mesa record deposition in a hyperpycnal lobe, near to its axis. Sandstone beds include Curvolithus simplex, Gyrochorte comosa, Ophiomorpha nodosa, Palaeophycus tubularis, Phycosiphon incertum, Protovirgularia dichotoma, Ptychoplasma excelsum, Schaubcylindrichnus freyi, Skolithos linearis, large specimens of Rosselia socialis and indeterminate crustacean burrows. Chondrites isp. is present in the mudstone. High rates of both episodic and sustained sedimentation, degree of substrate consolidation, freshwater discharge and water turbidity are the most important stress factors in both channels and lobes. Taxonomic composition, uneven distribution of bioturbation through the successions, and overall low ichnodiversity help to distinguish these prodeltaic deposits from bathymetrically equivalent offshore strata in the same basin. Hyperpycnal flow deposits are formed in a wide variety of environmental settings, therefore displaying high ichnological variability. Such variability is summarized by characterising ichnofaunas from four different depositional settings: (i) lakes; (ii) shelf deltas; (iii) shelf‐edge deltas; and (iv) deep‐marine systems. [ABSTRACT FROM AUTHOR]

The U. S. National Aeronautics and Space Administration (NASA) has archived thousands of satellite images of density plumes in its online publishing outlet called ‘Earth Observatory’ since 1999. Although these images are in the public domain, there has not been any systematic compilation of configurations of density plumes associated with various sedimentary environments and processes. This article, based on 45 case studies covering 21 major rivers (e.g., Amazon, Betsiboka, Congo [Zaire], Copper, Hugli [Ganges], Mackenzie, Mississippi, Niger, Nile, Rhone, Rio de la Plata, Yellow, Yangtze, Zambezi, etc.) and six different depositional environments (i.e., marine, lacustrine, estuarine, lagoon, bay, and reef), is the first attempt in illustrating natural variability of configurations of density plumes in modern environments. There are, at least, 24 configurations of density plumes. An important finding of this study is that density plumes are controlled by a plethora of 18 oceanographic, meteorological, and other external factors. Examples are: 1) Yellow River in China by tidal shear front and by a change in river course; 2) Yangtze River in China by shelf currents and vertical mixing by tides in winter months; 3) Rio de la Plata Estuary in Argentina and Uruguay by Ocean currents; 4) San Francisco Bay in California by tidal currents; 5) Gulf of Manner in the Indian Ocean by monsoonal currents; 6) Egypt in Red Sea by Eolian dust; 7) U.S. Atlantic margin by cyclones; 8) Sri Lanka by tsunamis; 9) Copper River in Alaska by high-gradient braid delta; 10) Lake Erie by seiche; 11) continental margin off Namibia by upwelling; 12) Bering Sea by phytoplankton; 13) the Great Bahama Bank in the Atlantic Ocean by fish activity; 14) Indonesia by volcanic activity; 15) Greenland by glacial melt; 16) South Pacific Ocean by coral reef; 17) Carolina continental Rise by pockmarks; and 18) Otsuchi Bay in Japan by internal bore. The prevailing trend in promoting a single type of river-flood triggered hyperpycnal flow is flawed because there are 16 types of hyperpycnal flows. River-flood derived hyperpycnal flows are muddy in texture and they occur close to the shoreline in inner shelf environments. Hyperpycnal flows are not viable transport mechanisms of sand and gravel across the shelf into the deep sea. The available field observations suggest that they do not form meter-thick sand layers in deep water settings. For the above reasons, river-flood triggered hyperpycnites are considered unsuitable for serving as petroleum reservoirs in deep-water environments until proven otherwise. [ABSTRACT FROM AUTHOR]

This paper aims at documenting the spatial and genetic relationships between fluvial and subaqueous gravity-flow deposits in an ancient fluvio-deltaic system exposed in the eastern border of the Paraná Basin, southern Brazil. The examined interval comprises the Pedra Alta sandstone and associated deposits, which are included in the glacially-influenced, Late Paleozoic Itararé Group. Five facies associations were identified and comprise the following coeval depositional settings from the more proximal to the more distal: A) braided fluvial/delta-plain, B) channel-mouth zone, C) delta-slope, D) distal marine glacially-influenced and E) non-cohesive density-flow deposits. These facies associations form a conformable, shallowing-upward succession about 200 m thick, whose stacking pattern is interpreted as a postglacial progradation. The stratigraphic architecture suggests a genetic correlation between high-energy fluvial discharges (possibly related to meltwater release) and coarse-grained deposits (non-cohesive density-flow deposits) accumulated on the delta foreset and bottomset. Mass-transport deposits were more likely triggered by sediment load increase on the delta slope due to high sediment supply, rather than by relative sea-level fall. The stratigraphic evolution can be reconstructed considering an initial stage characterized by a grounded ice margin and the deposition of mudstones with ice-rafted debris (glacial stage). This stage was succeeded by a second phase in which a fluvial system formed as a result of ice-margin retreat farther to the SE (postglacial stage). This glacial-postglacial stratigraphic succession is different from that recognized by previous workers in the Itararé Group, a conclusion that impacts greatly in reservoir prediction and modeling in the Paraná Basin. [ABSTRACT FROM AUTHOR]

Tropical cyclones directly trigger turbidity currents in submarine canyons as a consequence of storm surges, high waves, onshore blowing winds and extreme currents. The resultant supply of sediment at the heads of the canyons plays a crucial role in the genesis of turbidity currents and thus is key in understanding frequency and duration of their flows. Here we present a single numerical framework capable of modelling turbidity currents driven by cyclone-induced winds and waves through resolved, quasi-3D hydrostatic equations. Our simulations predict the occurrence of a canyon-confined turbid underflow induced by a tropical cyclone that caused a seafloor pipeline to shift. Turbidity current occurrence is shown to be related not just to tropical cyclone intensities but also to their tracks with respect to the canyon head. The modelled underflow is well approximated by similarity profiles from laboratory and field observations which demonstrates the reliability of the model in capturing the structure of turbidity currents. The proposed triggering of turbidity currents off the centre of coastal embayments is likely to occur when the abrupt rotation of incoming winds induced by a passing cyclone remains always coastal-bound all across the cyclone's waxing and waning stages. Our results show that these conditions can give rise to simultaneous, opposite alongshore currents and eventually result in offshore-bound rip currents. Conversely, it is unlikely that turbidity currents will be triggered by cyclone-induced rip currents when the cyclonic rotation results in peak offshore winds (coming from the land), as no fetch is available for generating large breaking waves to induce simultaneous, opposite alongshore currents. Nevertheless, the sole presence of strong alongshore currents deflected at the headlands of a coastal embayment (or delta) is likely to trigger sediment-gravity flows and eventually result in turbidity currents offshore the edge of the embayment without the aid of rip currents. • A quasi-3D model is used to hindcast turbidity current triggering by tropical cyclones • Direct triggering off embayment centre occurs if cyclonic wind rotation is coastal-bound • Indirect triggering depends on cyclonic storm tail and river flood runoff interaction [ABSTRACT FROM AUTHOR]

High-resolution swath bathymetry data collected in fjord-lakes Pentecôte, Walker and Pasteur (eastern Québec, Canada) allowed imaging in great detail the deltas of four rivers in order to understand the factors controlling the formation and downslope evolution of bedforms present on their slopes. The morphometry and morphology of 199 bedforms reflect the behaviour of sediment density flows. The shape of the bedforms, mostly crescentic, and the relationships between their morphological properties indicate that they were formed by supercritical density flows and that they are cyclic steps. The crescentic shape suggests an upslope migration while the aspect ratios and increasing wavelengths with distance from the shore (and decreasing slopes) are compatible with a cyclic step origin. At the rollover point, the acceleration of the density flows on steep slopes produces tightly spaced hydraulic jumps and favours short wavelength and symmetrical bedforms. Further downslope, decreasing slopes and increasing specific discharge increase the wavelength and asymmetry of the bedforms. The wavelength and asymmetry are increased because density flows require longer distances to become supercritical again on lower slopes after each successive hydraulic jump. Bedform morphometry and morphology are used to reconstruct density flow behaviour downslope. Froude numbers are high near the rollover point and gradually decrease downslope as the slope becomes gentler. Conversely, the specific discharge and flow depth are low near the rollover point and gradually increase downslope as the flow either erodes sediments or becomes more dilute due to sediment deposition and water entrainment. The supercritical density flows are believed to be triggered mainly by hyperpycnal flows but some evidence of delta-front slope failures is also observed. The differences in delta morphology and bedform development between the four deltas are linked to basin morphology and watershed hydrology, but also mainly to the fjord heritage of the lakes that allowed the focusing of sediment at the delta front. [ABSTRACT FROM AUTHOR]

The discovery of turbidites represents perhaps the major genuine advance of sedimentology during the twentieth century. Turbidites are the deposits of turbidity currents and were originally related to the gravitational instability and re-sedimentation of previously accumulated shallow water sediments into deep waters. As these flows originate and entirely evolve within a marine or lacustrine basin, their associated deposits are here termed intrabasinal turbidites. Controversially, increasing evidences support that turbidity currents can also be originated by the direct discharge of sediment–water mixtures by rivers in flood (hyperpycnal flows). Since these flows are originated in the continent, their associated deposits are here termed extrabasinal turbidites. Deposits related to these two different turbidity currents are often confused in the literature although they display diagnostic features that allow a clear differentiation between them. Intrabasinal turbidites are mostly related to surge-like (unsteady) flows that initiate from a cohesive debris flow that accelerates along the slope and evolves into a granular and finally a turbulent flow. Its flow behavior results on the accumulation of normally graded beds and bedsets that lacks terrestrial phytodetritus and lofting rhythmites. Extrabasinal turbidites, on the contrary, are deposits related to fully turbulent flows having interstitial freshwater and sustained by a relatively dense and long-lived river discharge. According to the grain size of suspended materials, hyperpycnal flows can be muddy or sandy. Sandy hyperpycnal flows (with or without associated bedload) often accumulate sandy to gravelly composite beds in prodelta to inner basin areas. Their typical deposits show sharp to gradual internal facies changes and recurrence, with abundant plant remains. In marine waters, the density reversal induced by freshwater results in the accumulation of lofting rhythmites at flow margin areas. Muddy hyperpycnal flows are loaded by a turbulent suspension dominantly composed of a mixture of silt and clay-sized particles (< 62.5 μm) of varying compositions. Since the suspended sediment concentration does not substantially decrease in waning flows, muddy hyperpycnal flows will be not affected by lofting, and the flow will remain attached to the sea bottom until its final accumulation. Typical deposits compose cm to dm-thick graded shale beds disposed over an erosive base with displaced marine microfossils and dispersed plant remains. [ABSTRACT FROM AUTHOR]

The Cretaceous Beolkeum Member was formed in a lacustrine environment affected by explosive volcanic eruptions, and hyperpycnal flow deposits are well developed following the eruptions. Beds of hyperpycnal flow deposit are generally less than 25 cm thick and consist of an inversely graded and planar laminated lower part, a poorly-sorted and massive middle part, and a normally graded, planar laminated upper part. An internal erosional surface is common between the lower and middle parts. After explosive volcanic eruptions, subaerial drainage systems would be highly disturbed by deposition of fine-grained and loose pyroclasts. The pyroclasts can easily be remobilized by surface runoff, and the surface runoff evolved into sedimentladen floods with excess density to plunge into the lake, providing favorable conditions for the occurrence of the hyperpycnal flows. Compared with classic models of hyperpycnal flow deposits, predominant planar laminations in the lower and upper parts suggest high fallout rates of suspended sediments from the hyperpycnal flows during initial and late stages of deposition. This implies that the hyperpycnal flows were driven from relatively highly concentrated subaerial floods owing to erodible subaerial conditions following the eruptions. Relatively thinly bedded hyperpycnal flow deposits (<25 cm thick) in comparison with the classic models (1 to 4 m thick) can be attributed to short-lived hyperpycnal flows, arising from the disturbed subaerial conditions following eruptions together with relatively small-scale drainage basins around the lake. [ABSTRACT FROM AUTHOR]

Global sea-level fall during the late Eocene-Oligocene icehouse, in conjunction with the Alpine orogenesis, resulted in the epicontinental Paratethys Sea formed at the northern margin of the Tethys Ocean. Among the Paratethys sub-basins, the foreland Dukla Basin hosted dark dysoxic Menilite Shales. Within these the lenticular body of deep-marine Cergowa Beds was deposited. Due to its marginal position and orogenic activity, the Cergowa depocenter was sensitive to a broad range of influences reflected by: separation from and re-connection with the Tethys Ocean; reactions to global sea-level change; variations in calcareous nannoplankton palaeoecology and fluctuations in water chemistry; changing sediment gravity flow mechanisms; facies progradation and basin tectonics; palaeotransport direction; and changes in the calcite compensation depth. The proximal north-western part of the Cergowa depocenter consisted of two troughs evolving towards the south-east into an unconfined distal area. This is shown by palaeocurrent patterns changing down current from low-dispersion to fanning out distributions. Sedimentation during the succeeding NP23 nannoplankton zone was controlled by glacio-eustatic shallowing, enhanced by tectonic uplift, both resulting in isolation from the Tethys Ocean, forming a brackish environment affected by fluvial supply. Aggrading sand-rich proximal turbidite facies common at this stage include hyperpycnally-generated sustained turbidites that were sourced from shelf-edge delta, as well as hybrid event beds that reflect erosion of morphologically unbalanced slopes modified by fold-thrust tectonics. Nannoplankton of the succeeding NP24 zone documents normal salinity of an open sea that was reconnected with the Tethys Ocean, which is contrary to the global cooling and related eustatic sea-level lowstand during this time. At this stage, deposits of low-density turbidity currents predominate and form a succession prograding towards the south-east. Therefore, the oceanic reconnection was likely controlled by local subsidence, the rate of which exceeded the rate of deposition, especially in the south-east. The reconnection also influenced the calcite compensation depth level and resulted in an increase in coccolithophore productivity, recorded by laminated pelagic limestone interbeds within the youngest fining-upwards part of the Cergowa Beds succession. This fining-upwards sequence of low-density turbidites reflects retrogradation of the arenaceous material within the distributary system and a transition to mudstone-dominated disoxic facies of the Menilite Beds, being the most productive hydrocarbon and helium source rock unit in the Outer Carpathians. These processes and relationships show that due to the marginal position of the synorogenic basin, the Cergowa Beds record environmental sensitivity to extrabasinal and intrabasinal factors, and a broad range of global, regional, and local influences, thus providing insights applicable to research into the evolution of other basins placed in a similar context. [ABSTRACT FROM AUTHOR]

Thin-bedded delta-front and prodelta facies of the Upper Cretaceous Ferron Notom Delta Complex near Hanksville in southern Utah, USA, show significant along-strike facies variability. Primary initiation processes that form these thin beds include surge-type turbidity currents, hyperpycnal flows and storm surges. The relative proportion of sedimentary structures generated by each of these depositional processes/events has been calculated from a series of measured sedimentological sections within a single parasequence (PS6-1) which is exposed continuously along depositional strike. For each measured section, sedimentological data including grain size, lithology, bedding thickness, sedimentary structures and ichnological suites have been documented. Parasequence 6-1 shows a strong along-strike variation with a wave-dominated environment in the north, passing abruptly into a fluvial-dominated area, then to an environment with varying degrees of fluvial and wave influence southward, and back to a wave-dominated environment further to the south-east. The lateral facies variations integrated with palaeocurrent data indicate that parasequence 6-1 is deposited as a storm-dominated symmetrical delta with a large river-dominated bayhead system linked to an updip fluvial feeder valley. This article indicates that it is practical to quantify the relative importance of depositional processes and determine the along-strike variation within an ancient delta system using thin-bedded facies analysis. The wide range of vertical stratification and grading sequences present in these event beds also allows construction of conceptual models of deposition from turbidity currents (i.e. surge-type turbidity currents and hyperpycnal flows) and storm surges, and shows that there are significant interactions and linkages of these often paired processes. [ABSTRACT FROM AUTHOR]

The easternmost Gulf of Finland is the recipient of the Neva River, the largest river draining to the Baltic Sea. The depositional history and endozoobenthic activity in the area have not been studied before based long sediment cores, although such studies are desirable to provide a paleoenvironmental context for the recent environmental changes and the ecosystem projections for the future Baltic Sea. Two replicate long sediment cores from the easternmost Gulf of Finland record brackish-water mud accumulation beginning at 5900±60 calendar years before present (cal. BP). The brackish-water muds are characterized by thin beds with erosional bases, fining-upward grain sizes and increasing-upward organic contents. A storm-induced flow origin is inferred for the thin mud beds, contrasting with previous interpretations that these muds accumulate by quiescent suspension settling. The bed primary microstructures are overprinted by biodeformation and burrow mottling by incipient small Planolites and Arenicolites/Polykladichnus ichnofossils of the continental Skolithos Ichnofacies, left behind by endobenthic oligochaetes and chironomid larvae. The depositional succession is cut by a strong erosional hyperpycnal flow event at 3320±40cal. BP, caused by the birth of the Neva River, which is now documented offshore for the first time. Mineral magnetic properties show pseudosingle-domain magnetite as the main magnetic mineral present. Increased magnetic grain sizes and reduced LOI values indicate a strong lithic influx immediately after the Neva River birth. The dominance of riverborne material in the accumulating sediments ever since highlights the strong influence of reworking in the shallow sea area. Increased diameters and the deeper penetration of biogenic sedimentary structures demonstrate improved seafloor oxygenation after the river birth. During the Medieval Warm Period (ca. 1000–700cal. BP) and the last century, increased sedimentary lithic contents and magnetic grain sizes indicate an elevated riverine influx due to increased precipitation, and/or the increased reworking of sediments by storms. The results provide a paleoenvironmental context for the study area, and deepen the understanding of storm-induced sediment dispersal and the formation of sedimentary fabric in shallow, wave-dominated sea areas. [ABSTRACT FROM AUTHOR]

Plastic pollution affects all oceans and sequestration of plastics in sediments is considered its ultimate sink. We report evidence of macroplastic burial retrieved within a sediment core collected at 38 m depth at the mouth of the Mazzarrà River, a torrential river able to carry a large amount of sediment during seasonal flash-floods. Two macroplastic items were found at 68 and 255 cm below the core top (corresponding to the seafloor). Their association with terrestrial vegetal debris and their inclusion in decimetre-thick sandy/silty intervals showing coarsening- and fining-upward trends, suggest that they were deposited by hyperpycnal flows possibly triggered by flood events. These findings testify the potential of sedimentary flows in burying macroplastic at depth below the seafloor, especially in nearshore prodelta environments. Furthermore they raise the quest on the magnitude of macroplastic storage in the subsurface and on the lack of specific devices and strategies for their reckoning. • Macroplastic items retrieved within a sediment core collected in a prodelta deposit. • Two items were found at 68 and 255 cm below the core top (i.e. the seafloor). • Plastics were enclosed in sandy sediments and associated with vegetal debris. • Plastics were transported and buried by river flood-generated hyperpycnal flows. [ABSTRACT FROM AUTHOR]

The lower Eocene Roda Sandstone (Figols Group, south-central Pyrenees) mainly consists of mouth bars and delta-front sandstone lobes deposited in a flood-dominated river-delta system. The deposition of these bodies was strongly controlled by an interaction between flood-dominated gravity flows entering seawater, topographic confinement and tidal currents. The Roda Sandstone is made up of six depositional sequences of different hierarchical order each of which is characterized by a basal deltaic sandstone wedge (R1 to R6) that passes upward into a siltstone and mudstone interval. Each basal deltaic sandstone wedge is composed of three types of facies association and respective facies tract (sensu Mutti 1992) that, from proximal to distal zones, are indicated as T1, T2 and T3. These three facies tracts are created by the downcurrent evolution of different types of sediment-laden stream flows entering seawater and related hyperpycnal flows. Their deposits are constituted by three different types of coarse-grained mouth bars and corresponding fine-grained delta-front sandstone lobes. The tidal influence is present in facies tract T3 in the R5 and R6 sandstone units, where the passage between flood-dominated mouth bars and the delta-front sandstone lobes occurs through intermediate facies characterized by different types of sigmoidal-cross stratification whose meaning will be discussed. The basal deltaic sandstone wedges of Roda sandstone are characterized by a progressive forestepping culminating in the R6 unit that erodes the underlying R5 unit and by an overlying backstepping unit indicated as R7. The erosive surface at the base of R6 unit is interpreted as a sequence boundary that divides the Roda Sandstone into two parts: 1) an underlying highstand system tract (HST) and falling stage system tract (FSST) (units R1 to R5) and 2) an overlying low-stand delta (the R6 unit) that passes upward into highstand mudstone through a transgressive system tract represented by the R7 unit. Pdf

Abstract: Two meter-scale pyroclastic levels are interbedded within the early Miocene succession of the Estancia 25 de Mayo (Patagoniense transgression) and Santa Cruz formations in the foreland Austral (or Magallanes) Basin, Argentina. The Lower Pyroclastic Level (LPL) is a tabular body interbedded within offshore marine deposits, laterally continuous for 30km and varying in thickness from few centimeters to around 4m. Grain-size grades from coarse to extremely fine ash with upward-fining along with a northeastern-fining trends. Structureless fine to very fine tuffs dominate and rare parallel laminations are the only tractive sedimentary structures. The Upper Pyroclastic Level (UPL) lies within low energy fluvial deposits and is laterally discontinuous, and it is composed by lenticular bodies reaching a maximum of 15m thick and 100m wide, with a concave-up base and a plane top. Grain-size range is similar to the LPL but it coarsens upward. The lower portion of the UPL shows parallel lamination, current ripple lamination and mud drapes with large pumice lapilli and plant debris, whereas the upper portion shows parallel lamination and trough cross-stratification. Both pyroclastic levels are composed mainly of pumice grains and glass shards with minor proportions of quartz and plagioclase crystals and lithic fragments. The LPL shows no mixing with epiclastic material whereas the UPL shows an upward increase in epiclastic material, and an upward increment in the scale of cross-bedding. The large thickness in relation to the possible emission center and the content of plant debris of the LPL does not suggest a direct, submarine, ash-fallout origin. The LPL is interpreted as a deposit of hyperpycnal-flows generated at the coastal zone when tephra-laden rivers plunged into the ocean. Large amounts of well preserved plant debris support the hypothesis of a terrestrial source of the sediments. The UPL is entirely composed of tractive deposits, so an ash fallout origin is disregarded. This, together with the lenticular shape and the alluvial plain origin of the encasing sediments, suggests accumulation within fluvial channels. Cycles of upper-flow-regime parallel lamination, current-ripple lamination and mud drapes at the lower portion, suggest short-lived turbulent flows that initially filled semi-abandoned channels. They were followed by sheet floods and channel reactivation, expressed by large-scale cross-bedding. The low degree of particle mixing observed in both levels is explained by the inability of streams to erode the substrate as they are suddenly over-saturated with pyroclastic sediments during and after the eruption. The grain-size distribution of the LPL and geochemical data indicate a contemporaneous volcanic source located to the west/southwest in the Andean ranges, where the South Patagonian Batholith is presently located. [Copyright &y& Elsevier]

Abstract: Numerical experiments in an idealized river mouth are conducted using a three-dimensional hydrodynamics model (EFDC model) to examine the impacts of suspended sediment concentration (SSC), settling velocity of sediment and tidal mixing on the formation and maintenance of estuarine hyperpycnal flows. The standard experiment presents an illustrative view of hyperpycnal flows that carry high-concentrated sediment and low-salinity water in the bottom layer (>1.0 m in thickness) along the subaqueous slope. The structure and intra-tidal variation of the simulated hyperpycnal flows are quite similar to those previously observed off the Huanghe (Yellow River) mouth. Results from the three control experiments show that SSC of river effluents is the most important parameter to the formation of hyperpycnal flows. High SSC will increase the bulk density of river effluents and thus offset the density difference between freshwater and seawater. Low SSC of river effluents will produce a surface river plume, as commonly observed in most large estuaries. Both the settling velocity of sediment particles and the tidal mixing play an important role in maintaining the hyperpycnal flows. Increasing settling velocity enhances the deposition of sediment from the hyperpycnal layer and thus accelerates the attenuation of hyperpycnal flows, whereas increasing tidal mixing destroys the stratification of water column and therefore makes the hyperpycnal flows less evident. Our results from numerical experiments are of importance to understand the initiation and maintenance of hyperpycnal flows in estuaries and provide a reference to the rapidly decaying hyperpycnal flows off the Huanghe river mouth due to climatic and anthropogenic forcing over the past several decades. [Copyright &y& Elsevier]

Most Mediterranean prodeltas show undulated sediment features on the foresets of their Holocene wedges. These features have been described all along the Mediterranean for the last 30 years and interpreted as either soft sediment deformation and incipient landsliding, and more recently, as sediment transport structures. We perform a review and detailed analysis of these undulated sediment features using ultrahigh-resolution seismic and bathymetric data as well as geotechnical information and hydrodynamic time series and hydrographic transects. In this study we show that the characteristics of the sediment undulations (configuration of the reflections down section and between adjacent undulations and overall morphologic characteristics) are incompatible with a genesis by sediment deformation alone and do not show evidence of sediment deformation in most cases. Various processes in the benthic boundary layer can be invoked to explain the variety of features observed in the numerous areas displaying sediment undulations. [ABSTRACT FROM AUTHOR]

Summary: Over the past 60years, climate change and human activities in the Huanghe (Yellow River) basin have changed the river system. In particular, construction of large reservoirs and soil-conservation practices within the river basin have reduced sediment flux to the sea by ∼90% and increased the grain size of suspended sediment delivered to the sea (30μm now versus 18μm before in median grain size). Scouring of the channel in the lower reaches has added a new sediment source to those derived from the loess region of the middle reaches. Before construction of the Xiaolangdi Reservoir at the downstream end of the middle reaches, suspended sediment concentrations at station Lijin, the last hydrographic station before the river enters the sea, were high enough (>35kg/m3) in the flood season to offset the density contrast between freshwater and ambient seawater. Thus, hyperpycnal flows developed along the subaqueous slope at the river mouth during the flood season. Observations from two cruises near the Huanghe river mouth, one before and one after the operations of the Xiaolangdi Reservoir, suggest that buoyant hypopycnal plumes, rather than hyperpycnal plumes, have occurred at the river mouth since the dramatic changes in concentration and grain size of suspended sediment discharged to the sea. The rapid decrease of sediment discharge to the sea and the increase of grain size of suspended sediments not only changed the sediment dispersal pattern at the river mouth, but also modified the shoreline and subaqueous slope. The effects of climate change and human activities in the river basin have been transferred along the hydrological pathway to the estuary and delta, and have changed estuarine sediment dynamics and delta morphology. The Huanghe presents an illustrative example of river basin–coastal zone interactions in the context of climate change and human activities. [ABSTRACT FROM AUTHOR]

The article discusses the hyperpycnal flows off the Huanghe River in China, and the relationship of the hyperpycnal flows to tidal cycles. It mentions that high concentrations of sediment input from river and tide modulations had initiated the hyperpycnal flows at the mouth of the river. It states that the tidal behaviors of hyperpycnal flows are associated with the suspended sediment concentration (SSC), salinity and stratification of water columns.

Abstract: A series of piston cores from Flathead Lake, Montana, USA and a new radiocarbon date from the sedimentary record provide the basis for describing sedimentary processes related to deglaciation of the Flathead Lobe of the Cordilleran Ice Sheet and for interpreting the retreat history of the lobe. The oldest part of Flathead Lake sediment core records is Late Pleistocene in age and consists of cm-scale rhythmites of silt and clay, interpreted here as annual varves. Each varve contains a light-colored coarser-grained portion, inferred to represent deposition during peak annual runoff, and a darker-colored finer-grained portion interpreted to represent sediment accumulation during seasonal low-flow conditions. The coarser-grained portions, especially in the stratigraphically older sections of each core, contain sedimentary structures that reflect traction transport. Based on these sedimentary structures, their facies characteristics, and their spatial distribution within the lake, we interpret the thicker, light-colored portion of each varve to be deposited by hyperpycnal flows caused by seasonal melt events rather than more classic turbidity currents. Immediately overlying Late Pleistocene rhythmites in all Flathead Lake cores is a unique, significantly coarser-grained dm-scale silt bed with a median grain size up to 50µm. This silt bed has a sharp, locally erosional base and fines upward but does not contain any other sedimentary structures. In contrast to the rhythmites, we interpret this silt bed to represent a single, short-lived catastrophic sedimentation event generated by a large glacial outburst flood. Overlying this distinct bed are several other cm-scale beds of mainly silt that exhibit a basal upward-coarsening followed by an upward-fining median grain size profile. We interpret these beds and their grain size trends as reflecting the rising and falling hydrograph limbs of outburst floods derived from more distal sources located in the upstream parts of the upper Flathead watershed. The sediment record from Flathead Lake, together with results from geologic and geomorphologic 1:24,000 scale mapping around the lake margins, provide a series of constraints regarding the paleogeographic evolution of the area during deglaciation. Overall upward-thinning and upward-fining of the varved portion of the sediment core records reflects northward retreat of the southern Flathead Lobe ice margin starting at latest 14,475±150cal yr BP, the depositional age of the oldest varve in any of our cores. The depositional age of silt beds overlying the varved records is constrained as between 14,150±150cal yr BP and 13,180±120cal yr BP. Within the available chronostratigraphic constraints, the outburst floods interpreted to have delivered this silt to the Flathead Lake basin also downcut a bedrock nick point below the Flathead Lake outlet, oriented a series of large boulders downstream, and deposited a series of large flood bars on the lower Flathead River floodplain. [Copyright &y& Elsevier]

The concept of turbidite has evolved so much since its original definition by Kuenen and Migliorini in 1950 – i.e. the deposit of turbidity currents exemplified by the sandy flysch successions of the Northern Apennines – that it is now used to define a variety of deposits, some of which have little in common with sandy flysch formations in terms of facies, geometry and geological significance. The extension of the concept to other geodynamic settings and deposits of non-siliciclastic composition is considered only briefly in the concluding sections. With the diffusion of the concept of turbidity current, in the 1950s and early 1960s, an entirely new branch of sedimentology came into being, concerned with the inventory of sedimentary structures, palaeocurrent measurements and bedding patterns. The most representative expression of this branch came from the ‘Dutch school’ of Philip H. Kuenen and his students. Between the late 1960s and the mid-1970s, there was a new development: facies analysis, in terms of modern environments and depositional systems. This development led to the introduction and discussion of ‘fan models’ that became an increasingly thorny issue with the accumulation of data from modern deep-marine settings. In particular, most researchers emphasized the importance of channel and lobe elements and their mutual relationships in space and time... [ABSTRACT FROM AUTHOR]

Abstract: This paper focuses on the delivery of water and sediment to the northern Adriatic to better understand the short-term evolution of continental margin sedimentation under natural and human impact. For that reason, the Po and six Apennine rivers (Metauro, Musone, Potenza, Tronto, Chienti and Pescara) are investigated. The climate-driven hydrological model HydroTrend is used to simulate discharge and sediment loads where observational data are limited. The northern Apennine hinterland has a significant impact on the sediment flux leaving the Po River, contributing 56% of the sediment it delivers to the Adriatic Sea. The Po River experienced a strong decrease in its sediment load (17.2–6.4Mt/yr) across 1933–1987, in contrast to a small increase in its water discharge. The rivers draining the southern Apennine hinterland contribute more than 50% of the sediment load entering the Adriatic Sea, and this is in spite of human modification of their discharge through numerous small reservoirs that invariably reduce a river''s sediment load. As a result, hyperpycnal flows, which historically carried 20–40% of the sediment flux from these Apennine rivers, become rare. Sediment load reduction is also reflected by retreat of the Apennine coastline. Based on the ART model (used in HydroTrend), the total sediment load to the northern Adriatic is 43MT/yr where the northern Alpine rivers contribute 8MT/yr, the Po River 13MT/yr and the Apennine rivers contribute 22MT/yr. [Copyright &y& Elsevier]

Abstract: Meteoric-water flux and formation of kaolinite owing to the dissolution of detrital silicates are common features of continental and paralic sandstones. In deep-water marine sandstones, meteoric-water flux is commonly considered unlikely to occur. However, the study of deep-water, marine sandstones of the Shetland–Faroes Basin on the British continental shelf revealed widespread and extensive dissolution and kaolinitization of mica and feldspar grains, which are attributed to meteoric-water flux during a sea-level lowstand. We suggest that this apparently enigmatic meteoric-water flux mechanism is likely to have occurred by hyperpycnal flow. Hyperpycnal flow occurs when river effluent directly transfers into sediment gravity flow, and enters seawater as a mixture of sediment and fresh water. The likelihood for hyperpycnal flows increases at times when rivers and distributary channels reach the shelf edge, and their flows are delivered directly onto the deepwater slope. [Copyright &y& Elsevier]

Abstract: Northern Gulf of Mexico is a key archival site of the Laurentide Ice Sheet melting history and the only margins containing evidence of meltwater outflows contemporaneous with global meltwater pulse mwp-1A. Inconsistencies between these meltwater floods and the absence of strong climate responses to large freshwater fluxes predicted by climate models raised questions concerning the validity of planktonic foraminiferal δ 18O record in the Gulf of Mexico as a reliable index of freshwater fluxes. Isotope records of depth-stratified foraminifera document unusual trends compatible with entrainment of 18O-depleted freshwaters in hyperpycnal flows reaching the seafloor and rising to the surface as buoyant plumes. These exceptionally large outbursts of floodwaters, coeval with a prominent erosion intensity peak in continental paleohydrology, were catastrophically released in a rapid succession over a period of ca. 500 yr (14.7–14.2 kyr). Correspondence in time and magnitude between the torrential floods and the mwp-1A event supports the view that LIS was likely a substantial freshwater source. Drainage of meltwaters through torrential hyperpycnal flows that underwent substantial mixing with seawater may resolve the problem with ocean modeling results that predict a much stronger climate response to large freshwater hypopycnal fluxes into the ocean. [Copyright &y& Elsevier]

Abstract: Channels are relatively common on river-mouth deltas, but the process by which they arise from river sediment discharge is unclear because they can potentially be explained either by negatively buoyant (hyperpycnal) flows produced directly from the river outflow or by flows generated by repeated failure and mobilisation of sediment rapidly deposited at the delta front. Channels eroded through a dump site of dredge spoils are described here from multibeam and older sonar data collected in Commencement Bay, at the mouth of the Puyallup River. Shallow channels on the seaward upper surface of the dump site, away from any flows that could have been produced by delta front failures, suggest that at least some hyperpycnal flows were produced directly from the positively buoyant river outflow up to 200 m from the edge of the river mouth platform. The form of channel bed erosion is revealed by the longitudinal shape of the main eroded channel compared with the adjacent dump site profile. It suggests that the channel evolved by its steep front retreating, rather than by simple vertical entrenchment or diffusive-like evolution of the profile, a geometry interpreted as evidence that repeated failure of the bed occurred in response to shear stress imposed by bottom-travelling flows. Model calculations based on shear strengths back-calculated from the geometry of channel wall failures suggest that, if the main channel were eroded solely by hyperpycnal flows, their generation was remarkably efficient in order to create flows vigorous enough to cause channel bed failure. Besides the sediment concentration and discharge characteristics that have been considered to dictate the ability of rivers to produce hyperpycnal flows, it is suggested that the timing of floods with respect to the tidal cycle should also be important because extreme low tides may be needed to ensure that coarse sediment is transferred vigorously to the edge of river mouth platforms. [Copyright &y& Elsevier]

Abstract: The Turonian lower Ferron Sandstone and Juana Lopez Members of the Mancos Shale in the Western Interior foreland basin form a series of regressive shoreface to shelf depositional complexes sourced from the Sevier fold-thrust belt to the west. Continuous, dip-parallel outcrop exposures around the northern and eastern edges of the Tertiary San Rafael Anticline (SRA) in central Utah provide an insight into the origins of anomalous, marine mudstone-encased sandstones. Lower Ferron strata consist of both coarsening-and-thickening upward and sharp-based shoreface successions, interpreted to have been deposited as sequential highstand and falling stage systems tracts. Overlying these shallow marine deposits is a surface that marks sediment bypass that is correlated down-dip into coarse-grained, cross-bedded sandstones, interpreted as the deposits of channelised turbidity flows. These turbidites are considered to be the products of sustained hyperpycnal flows exiting rejuvenated lowstand rivers based on sustained flow indicators such as dune-scale cross-bedding, absence of genetically related slumps and delta front deposits in up-dip areas, surfaces of sediment bypass, coarser grain-sizes than the underlying shoreface deposits, and regional palaeogeography. The flows are interpreted to have been ignitive in proximal areas down a tectonically-induced slope in the uplifted area of the Farnham Dome. The change in depositional style from falling-stage to lowstand systems tracts is attributed to increased rates of sediment supply to the shoreline, promoting hyperpycnal flows. A flooding surface caps the interval and separates the lower Ferron Sandstone from the overlying Juana Lopez Member. This interval in contrast, consists only of heterolithic, parallel and ripple-laminated graded sandstone–mudstone couplets interpreted to have been deposited by turbulent, storm-induced geostrophic flows. The couplets are arranged into bundles and are interpreted as parasequences, which in turn are arranged into a progradational parasequence set deposited during a period of sea-level highstand. Divergent sole mark and bedform palaeocurrents from both members suggest slight eastward deflection of the turbulent flows off the proto-SRA. Further evidence for this topography is inferred from an absence of turbidites along the western side of the structure. The inferred influence of local basin floor structures on sedimentation of these units has important implications for structural models for this basin, for generalized sequence stratigraphic models applied to ramp-type foreland basin sequences and for elucidating the origin of anomalous marine mudstone-encased sandstone bodies. [Copyright &y& Elsevier]

Hyperpycnal flows form in the marine environment when river discharge enters the ocean with suspended concentrations in excess of 36 kg m−3 due to buoyancy considerations, or as little as 1–5 kg m−3 when convective instability is considered. They form at a river mouth during floods of small to medium size rivers including extreme events such as jo¨kulhaups, dam breaking and draining, and lahars. Associated with high-suspended concentration, they can transport considerable volume of sediment to ocean basins. The typical deposit or hyperpycnite sequence is a compound of a basal coarsening-up unit, deposited during the waxing period of discharge, and a top fining-up unit deposited during the waning period of discharge. Hyperpycnites differ from other turbidites because of their well-developed inversely graded facies and intrasequence erosional contacts. These observations lead to a complete redefinition and interpretation of fine-grained turbidites. Hyperpycnite stacking can locally generate high-sedimentation rates, in the range of 1–2 m per 100 year. Because hyperpycnites are related to climate through flood frequency and magnitude, their record should vary with sea level and climate change. They can also be associated with proximal ice-melting settings. Hyperpycnal flows could also be involved in the formation of meandering canyons and channels. [Copyright &y& Elsevier]

Hyperpycnal flows may be important mechanisms for transporting particles from continents to oceans given their frequency of generation at many modern river mouths. Their deposits are probably very common in the rock record, although they are not frequently recorded in the literature. These currents may last longer and be steadier than surge-like turbidity flows and consequently their deposits are likely to be significantly different. In this paper simple laboratory experiments are used to investigate how flows and deposits may vary. The thickness of the experimental flows near the flow front varies with distance from the source depending on slope, effluent discharge, grain size and sorting. Runout distance varies with the proportion of fine-grained sediment rather than with mean grain size. Flow front velocity varies with effluent discharge and sediment concentration and in a complex manner with mean grain size and sorting. The variation of deposit mass with runout distance depends on proximal slope angle and the maximum mass per unit area (approximating to bed thickness) is inversely dependent on slope angle. Variation in initial suspended particle concentration results in deposits of similar shape but different volume. Although the total mass of the deposit depends on the discharge the maximum mass per unit area (approximating to maximum bed thickness) does not. The distance over which particles are deposited increases and the deposit flattens as particles are deposited progressively further downstream at higher discharges, perhaps relating to more efficient turbulent suspension at higher Re (more complete energy cascade), in addition to longer horizontal component of settling trajectory at higher velocity. The amplitude of mass variation depends on the grain size. As with laboratory surges [Gladstone et al., Sedimentology (1998) 833–844], the finer the sediment the more efficient is the sediment transport. [Copyright &y& Elsevier]

The Holocene morpho-stratigraphic evolution of a compound submarine deltaic system linked with the Antas and Almanzora Rivers on the narrow (< 5 km) shelf along the Palomares margin (southeastern Iberia) has been reconstructed from the integrated analysis of geomorphology, seismo-stratigraphy and sedimentology. The shelf morphology is characterized by a 3–4 km wide deltaic body off the Almanzora River, where seafloor undulations and gullies are present. Larger and more incised gullies, crescent-shaped bedforms and pockmarks are recognizable at or close to the head of shelf-indenting canyons associated with the Almanzora-Garrucha canyon system. The seismo-stratigraphy displays a strong and irregular basal reflector overlain by a sedimentary wedge, showing variable thicknesses (1–25 m) and seismic characteristics along and across the shelf. The main depocenter of this wedge is not located off the Almanzora River mouth (i.e., the main sedimentary input in the study area) but along the southern shelf encompassing the Almanzora and Antas Rivers. Grain-size and sand compositional analyses show that silty sand with high terrigenous content (mostly quartz and mica) is mainly found on the submarine deltaic body. In contrast, sandy silt with variable ratios between terrigenous and biogenic contents is retrieved from the canyon head and associated gullies as well as from the surrounding mid-outer shelf. The evolution of the Antas-Almanzora compound delta has been divided into three evolutionary stages, mainly driven by the interplay between paleo-topography and riverine and marine processes and modulated by sea level changes during the Holocene. The origin of the present-day morphological features is also discussed, again highlighting a complex interaction between sedimentary gravity flows (mainly flash flood-generated hyperpycnal flows), storm-waves and shelf currents, fluid seepage and retrogressive slope failures. [ABSTRACT FROM AUTHOR]