Article

Hyperpycnal (over density) flows and deposits

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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.

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... Un flujo hiperpícnico episódico usualmente dura unas pocas horas y desarrolla un fan-delta con gradientes pronunciados y pequeñas áreas de captación. Mientras que los flujos hiperpícnicos sostenidos se producen por descargas de grandes ríos que pueden durar semanas o incluso meses dependiendo del clima, el tamaño y la forma del área de drenaje fluvial asociada (Zavala y Pan, 2018;Zavala 2020). ...
... Interpretación: se interpreta que esta facies se habría acumulado a partir de altas tasas de decantación en flujos fluidos de fango (Otharán et al. 2018;2020). La existencia de cambios cíclicos en el tamaño de grano sugiere un flujo de fondo en movimiento, rasgos diagnósticos que los diferencian de la decantación pura en aguas calmas (Zavala et al., 2014). ...
Thesis
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... Possibly, the sand beds represent short episodes of extreme run-off, which caused incoming high-density river water to plunge beneath the seawater, flowing over the delta slope as turbidity currents (e.g. Mulder and Syvitski, 1995;Zavala, 2020). Such situations may be related to flash floods in the upstream reach of the river and may have been amplified by ebb currents in the river mouth. ...
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... The abundance of platy micas and amorphous organic matters could be linked to a terrigenous origin (Saller et al., 2006). The mudclast type thus could be less likely to be incorporated from initial slope failures, but it may be largely attributed to erosion of underlying flood-related, plume-generated turbidites and even organic-rich debrites (Zavala and Arcuri., 2016;Hage et al., 2019;Zavala, 2020;Hussain et al., 2021) via successive turbidity currents (Fig. 19a), which occurred prior to the occurrence of flow transformation. The comparatively lower cohesive mud content suggests that the mudclast type contributed less to the formation of HEBs through mud-driven flow transformation. ...
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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.
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Ancient fluvio-deltaic systems deposited in tectonically active basins are essentially built up by inter-gradational fan-delta and river-delta systems dominated by catastrophic flooding. These systems and their component depositional elements cannot therefore be described and interpreted in terms of current sedimentological models based on "normal" fluvial and deltaic processes, facies and geomorphic settings derived from the study of modern environments. Direct and indirect sedimentological and stratigraphic evidence indicates that "normal" sedimentation occurred also in these systems, but its preservation potential appears to be very small with the exception of tidal diffusion in estuarine settings. Facies and facies associations of ancient flood-dominated depositional systems include a very broad spectrum of essentially poorly described and understood sediments that vary from thick-bedded and disorganized conglomerates to thin-bedded graded mudstones via a great variety of pebbly-sandstone and sandstone facies. Despite this variability, all these sediments are characteristically composed of graded flood units in both alluvial and marine environments. The greatest preservation potential of individual flood units is found in the final marine depositional zones of each system considered. Ancient flood-dominated fluvio-marine systems comprise huge accumulations of conglomerates, sandstone and mudstone facies whose origin and stratigraphic importance have been essentially overlooked in previous literature. These depositional systems can be understood only in terms of tectonically-controlled physiographic settings characterized by small and medium-sized fluvial systems with high-elevation drainage basins and high-gradient transfer zones located close to marine basins. In settings of this type, sediment flux to the sea can dramatically increase when climatic conditions provide sufficient amounts of water to produce catastrophic floods. These floods generate mixtures of water and sediment that can enter sea waters with sufficient velocity and sediment concentration to produce hyperpycnal flows and related, self-sustained turbidity currents. The resulting depositional settings are thus dominated by flood-related facies that can develop in shelfal or deeper marine regions. Thick and laterally extensive successions of shelfal sandstone lobes with flood-generated HCS are the fundamental depositional element of both fan-delta and river-delta systems considered in this study. These lobes are essentially similar to deeper-water turbidite sandstone lobes in terms of geometry, facies tracts, and high-frequency cyclic stacking patterns. Shelfal sandstone lobes probably represent the only possible expression of fluvial-dominated delta-front sandstone facies, since, in the absence of flood-generated hyperpycnal flows, river-borne sands can only be redistributed in marine environments by waves and tides. As indicated by their overall stacking patterns, the evolution of ancient flood-dominated fluvio-deltaic systems with time is apparently controlled by the initial uplift of the drainage basin, the rate of denudation, the gradient of each system, and the volume and sediment concentration of individual floods, the latter being a function of the amount of water and sediment made available to the system considered. A flood-dominated system of this type comes to an end when the sediment flux to the sea is progressively reduced to "normal" conditions. This occurs when relief and elevation of drainage basins and related sediment availability, as well as the gradient of transfer zones, have been substantially reduced through progressive denudation and sediment exportation to marine depositional zones. The occurrence of cyclic stacking patterns developed at different hierarchical orders is one of the most striking aspects of flood-dominated fluvio-deltaic systems. The most complete record of this cyclicity is preserved in the final depositional zone of each system. These stacking patterns are apparently very similar to those which are thought to be characteristic of sequence-stratigraphic models. Despite this apparent similarity, we suggest that the overall vertical evolution of flood-dominated systems is primarily controlled by Davisian-type cycles produced by alternating periods of orogenic uplift and denudation. In their most complete development, these cycles are ideally recorded by an overall forestepping-backstepping succession recorded by a basal turbidite system (basin floor fan of sequence stratigraphy) overlain by a flood-dominated fluvio-deltaic system which passes upward and landward into a "normal" fluvial or fluvio-deltaic system with time. Higher-frequency stacking patterns developed within each of the above stages are essentially produced by forestepping-backstepping episodes of sand deposition which are essentially controlled by cyclic climatic variations. The relationships between Davisian-type and higher-frequency climatic cycles and eustasy-driven cycles of relative sealevel variations remain to be explored through careful stratigraphic, sedimentological and structural studies carried out without preconceived ideas. It is likely, however, that the eustatic control on flood-dominated sedimentation patterns of high-gradient, tectonically active settings cannot generally compare with the importance of tectonism and related cycles of uplift and denudation.
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A long held geologic paradigm is that mudrocks and shales are basically the product of ‘hemipelagic rain’ of silt- and/or clay-sized, detrital, biogenic and particulate organic particles onto the ocean floor over long intervals of time. However, recently published experimental and field-based studies have revealed a plethora of micro-sedimentary features that indicate these common fine-grained rocks also could have been transported and/or reworked by unidirectional currents. In this paper, we add to this growing body of knowledge by describing such features from the Paleozoic Barnett Shale in the Fort Worth Basin, Texas, U.S.A. which suggests transport and deposition was from hyperpycnal, turbidity, storm and/or contour currents, in addition to hemipelagic rain. On the basis of a variety of sedimentary textures and structures, six main sedimentary facies have been defined from four 0.3 meter intervals in a 68m (223 ft) long Barnett Shale core: massive mudstone, rhythmic mudstone, ripple and low-angle laminated mudstone, graded mudstone, clay-rich facies, and spicule-rich facies. Current-induced features of these facies include mm- to cmscale cross- and parallel-laminations, scour surfaces, clastic/biogenic particle alignment, and normal- and inverse-size grading. A spectrum of vertical facies transitions and bed types indicate deposition from waxing-waning flows rather than from steady ‘rain’ of particles to the sea floor. Detrital sponge spicule-rich facies suggests transport to the marine environment as hypopycnal or hyperpycnal flows and reversal in buoyancy by transformation from concentrated to dilute flows; alternatively the spicules could have originated by submarine slumping in front of contemporaneous shallow marine sponge reefs, and then transported basinward as turbidity current flows. The occurrence of dispersed biogenic/organic remains and inversely size graded mudstones also support a hyperpycnal and/or turbidity flow origin for a significant part of the strata. These processes and facies reported in this paper are probably present in other organic-rich shales.
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Nearly instantaneous melting of snow and ice by the March 19, 1982, eruption of Mount St. Helens released a 4 × 106 m3 flood of water from the crater that was converted to a lahar (volcanic debris flow) through erosion and incorporation of sediment by the time it reached the base of the volcano. Over the next 81 km that it traveled down the Toutle River, the flood wave was progressively diluted through several mechanisms. A transformation from debris flow to hyperconcentrated streamflow began to occur about 27 km downstream from the crater, when the total sediment concentration had decreased to about 78% by weight (57% by volume). The hyperconcentrated lahar-runout flood wave, transporting immense quantities of sand in suspension, continued to experience progressive downstream dilution. Although turbulence was significantly dampened by the extremely high suspended load, very large standing waves and antidune waves were observed. The hyperconcentrated lahar-runout flow deposited an unusual, faintly stratified, coarse sand which locally contained small, isolated gravel lenses. Very similar deposits in the Quaternary stratigraphy of Mount St. Helens and other Cascades volcanoes suggest that lahars may be more frequent than previously recognized.
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Classifications of flowing sediment-water mixtures have, in the past, been based primarily on relative, qualitative differences in the style and rate of movement as well as on morphology and sedimentology of deposits. A more quantitative and physically relevant classification is presented here, based on thresholds in rhéologie behavior. The classification is constructed on a two-dimensional matrix in which flows are located according to deformation rate (mean velocity) and sediment concentration, with composition of the mixture constant. Three major rhéologie boundaries are crossed as sediment concentration increases from 0 (clear water) to 100 percent (dry sediment): (1) the acquisition of a yield strength-the transition from liquid "normal streamflow" to plastic "hyperconcentrated streamflow"; (2) an abrupt increase in yield strength coinciding with the onset of liquefaction behavior-the transition to "slurry flow"; and (3) the loss of the ability to liquefy-the transition of "granular flow." These three rhéologie boundaries shift according to particle-size distribution and composition of the mixture. Processes controlling flow behavior depend on deformation rate (velocity). Rateindependent frictional and viscous forces dominate at lower velocities and in finer grained mixtures; rate-dependent inertial forces dominate at higher velocities and in coarser grained mixtures. As velocity increases, grain-support mechanisms change from low-energy varieties (buoyancy, cohesion, structural support) to progressively higher energy mechanisms (turbulence, dispersive stress, fluidization). Existing nomenclatures of geologic flow phenomena can fit within this rhéologie classification. The morphology and sedimentology of flow deposits commonly can be used to deduce rhéologie behavior, but caution needs to be exercised in inferring processes from deposits.
Article
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A marine hyperpycnal plume is a particular kind of turbidity current occurring at a river mouth when the concentration of suspended sediment is so large that the density of the river water is greater than the density of sea water. The plume can then plunge and possibly erode the seafloor to become self-maintained for a particular period of time (hours to weeks). Frequency of hyperpycnal plumes emanating from river discharge can be predicted with knowledge of rating curve characteristics, particularly during flood conditions. Examples of these curves are shown for middle-sized North American rivers. Semi-empirical relationships among average discharge, average sediment concentration, and the discharge during flood are proposed and applied to 150 world rivers. Results show the importance of small and medium sized rivers in their ability to trigger underflow at their mouth. There are at least nine ''dirty'' rivers that may trigger underflows during one or more periods of the year. Most other rivers are cleaner and have hyperpycnal plumes only during floods. Large rivers do not generate underflows at their mouth because sediment retention within their expansive coastal flood plains effectively reduces the upper limit of the suspended concentration. Underflow transport may be an important process in marine-delta construction and should be considered in sedimentary basin-fill modeling. Proposed equations and nomograms may assist engineers in infrastructure design seaward of a river mouth.
Conference Paper
A hyperpycnal flow forms when a land derived dense flow enters a marine or lacustrine water reservoir. As a consequence of its excess in density, the 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 type) and basin salinity the resulting deposits (hyperpycnites) can be very variable. According to flow duration, hyperpycnal flows can be classified into short lived (SLHF) or long lived (LLHF) hyperpycnal flows. SLHF lasts for minutes or hours, and are mostly related to small mountainous river discharges, alluvial fans, collapse of natural dams, landslides, volcanic eruptions, jökulhlaups, etc. LLHF last for days, weeks or even months, and are mostly associated to medium to large size river discharges. Concerning the characteristics 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 (bedload, sandy or muddy dominated fully 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 to evolve into a turbidity current and travel farter basinward. Their resulting deposits are difficult to differentiate from those related to intrabasinal turbidites. On the contrary, Newtonian subcritical hyperpycnal flows (NSHF) are capable of transfer huge volumes of sediment, freshwater and organic matter far from the coast with gentle or flat slopes. In marine settings, the buoyant effect of interstitial freshwater in bedload and sandy hyperpycnal flows can result in lofting due to 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 basins. NSHF can also erode the basin bottom during its travel basinward, allowing the incorporation and transfer of intrabasinal organic matter and sediments. Long lived NSHF deposits exhibit typical characteristics that allow a clear differentiation respect to those related to intrabasinal turbidites. Main features include (1) complex beds with gradual and recurrent changes in sediment grain size and sedimentary structures, (2) mixture of extrabasinal & intrabasinal components, (3) internal and discontinuous erosional surfaces and (4) lofting rhythmites in marine settings.
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Sedimentary, isotopic and bulk geochemical proxies measured in sediment samples of five gravity cores collected in the distal part of the Ogooue turbidite system (around 4000 m-depth) were used to develop a conceptual model to describe the accumulation of terrigenous organic matter (OM) during the last 200,000 yrs BP in the eastern part of the Gulf of Guinea. This model takes into account the influence of the different depositional processes (turbiditic vs hemipelagic sedimentation), geomorphological features and sea-level variations.
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The Congo River, the second largest river in the world, is a major source of organic matter for the deep Atlantic Ocean because of the connection of its estuary to the deep offshore area by a submarine canyon which feeds a vast deep-sea fan. The lobe zone of this deep-sea fan is the final receptacle of the sedimentary inputs presently channelled by the canyon and covers an area of ~2500 km². The quantity and the source of organic matter preserved in recent turbiditic sediments from the distal lobe of the Congo deep-sea fan were assessed using Rock-Eval pyrolysis analyses. Six sites, located at approximately 5000 m water-depth, were investigated. The mud-rich sediments of the distal lobe contain high amounts of organic matter (~3.5 to 4% Corg), the origin of which is a mixture of terrestrial higher-plant debris, soil organic matter and deeply oxidized phytoplanktonic material. Although the respective contribution of terrestrial and marine sources of organic matter cannot be precisely quantified using Rock-Eval analyses, the terrestrial fraction is dominant according to similar hydrogen and oxygen indices of both suspended and bedload sediments from the Congo River and that deposited in the lobe complex. The Rock-Eval signature supports the 70% to 80% of the terrestrial fraction previously estimated using C/N and δ¹³Corg data. In the background sediment, the organic matter distribution is homogeneous at different scales, from a single turbiditic event to the entire lobe, and changes in accumulation rates only have a limited effect on the quantity and quality of the preserved organic matter. Peculiar areas with chemosynthetic bivalves and/or bacterial mats, explored using ROV Victor 6000, show a Rock-Eval signature more or less similar to background sediment. This high organic carbon content associated to high sedimentation rates (> 2 to 20 mm.yr⁻¹) in the Congo distal lobe complex implies a high burial rate for organic carbon. Consequently, the Congo deep-sea fan represents an enormous sink of terrestrial organic matter when compared to other turbiditic systems over the world.
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Geochemical data (total organic carbon-TOC content, δ¹³Corg, C:N, Rock-Eval analyses) were obtained on 150 core tops from the Angola basin, with a special focus on the Congo deep sea fan. Combined with the previously published data, the resulting dataset (322 stations) shows a good spatial and bathymetric representativeness. TOC content and δ¹³Corg maps of the Angola basin were generated using this enhanced dataset. The main difference in our map with previously published ones is the high terrestrial organic matter content observed downslope along the active turbidite channel of the Congo deep sea fan till the distal lobe complex near 5,000 m of water-depth. Interpretation of downslope trends in TOC content and organic matter composition indicates that lateral particle transport by turbidity currents is the primary mechanism controlling supply and burial of organic matter in the bathypelagic depths.
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The discovery of turbidites represents perhaps themajor 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 (b62.5 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.
Article
Primary sedimentary structures from so-called “turbidites” (deposits made by turbidity currents) may be divided into two groups: (1) those that were formed by deposition from turbulent suspensions, and (2) those that were not formed by deposition from turbulent suspensions. Structures in Group 1 include: (a) scour marks made on cohesive mud bottoms; (b) syndepositional deformation structures in fine-grained sediment; (c) structureless fine-grained beds; and (d) traction-plus-fallout structures (plane parallel laminae, “ripple drift with deposition from above,” and convoluted laminae). Structures in Group 2 include: (a) heterogeneous structureless beds; (b) cross-strata; (c) certain syndepositional structures in sands; (d) bedding surfaces showing linear tool marks without scour marks; (e) inversely graded beds; (f) coarse-grained graded beds; and (g) beds of coarse-grained sediment that are more delicately adjusted to bottom microrelief than interbedded layers of fine-grained sediment. Because turbidity currents are defined as density currents caused by turbulently suspended sediment, it is here argued that only deposits of Group 1 should be classified as turbidites. Previous usage has assigned structures from both groups to “turbidites,” but a revised nomenclature is presented that assigns structures of both groups to the process of resedimentation. Turbidity currents comprise the “suspended-load” part of resedimentation, whereas other processes, including slumps, flowing-grain layers, and moving viscous suspensions, comprise the “bed-load” part of resedimentation. Structures of Group 2 are inferred to originate by “bed-load” resedimentation. Three implications of the present interpretation are discussed: (1) The so-called fixed sequence of bedding types in “turbidites” should be modified to include more varieties, based on different proportions of “bed-load” parts and “suspended-load” parts of resedimented deposits as well as on differential distance of travel from the origin of resedimented deposits with equal proportions of these two parts. (2) The conditions of drag at the base of a turbidity current vary widely and range from low drag where the bottom is cohesive, to large drag where the bottom consists of cohesionless grains, and also include zero or “negative” drag where a more rapidly moving flowing-grain layer is in motion at the base of the turbidity current. (3) The distinctive features of resedimented deposits are the primary sedimentary structures of Group 2, which are here excluded from turbidites and assigned to the “bed-load” part of resedimentation.
Article
The rheological character of past flow events can generally be distinguished by an in-depth study of the geomorphic and sedimentologic features that they produce. Clearly, where the consequences of misdesign or poor prediction are grave, a geomorphic-sedimentologic analysis of past flow events is an essential component of flood studies for small, mountainous drainage basins. -from Editors
Article
Flume experiments were conducted to determine how a pervasive rain of sand might affect the development of bedforms, internal structures, and grain fabric in a bed aggrading from this fallout. Depositional fabrics, deduced from anisotropy of magnetic susceptibility, confirmed that under upper-plane-bed conditions (representative of Bouma divisions A and B) grain orientation is current-parallel and equally well developed for all sedimentation rates. -from Authors
Article
More than 100 offshore mass-movement deposits have been studied in Holocene and Pleistocene sediments. The processes can be divided into three main types: slides/slumps, plastic flows, and turbidity currents, of which 13 main varieties have been recognized. The three types are differentiated mainly by motion, architecture, and shape of failure surface. For slides, the morphology of deposits can usually be linked to a process, but for plastic flows and turbidity currents, information about the motion is mainly provided by the sedimentary record. A static classification based on these features is given, and is related to a dynamic classification system to try to underline the morphological transformation of an offshore event from initiation to deposition.
Article
Many volcano-hydrologic events produced high-discharge, sediment-laden flows originating on or near volcanoes, during or following eruptions. Water for some flows is derived from normal precipitation, commonly with enhanced runoff because of reduced infiltration into slopes mantled by pyroclastic debris and on which most vegetation has been destroyed. Flows triggered by eruptions may contain large volumes of water mobilized by eruption-induced snowmelt, discharge of crater lakes, or liquefaction of saturated debris-avalanche material. Sediment is available as unconsolidated pyroclastic or autoclastic material, principally of sand to fine-gravel size, that is thickest on the steep slopes of the volcano itself. Resulting flows generally possess the characteristics of debris flows or hyperconcentrated flows. -from Authors
Article
A new model for predicting the long-term flux of sediment from river basins to the coastal ocean is applied to a global data set of 340 river basins. The model is based on relief, basin area (or, averaged discharge), and basin-averaged temperature. Basin-averaged temperature is determined from basin location (latitude, longitude) and the lapse rate across the basin relief (hypsometric approximation). The sediment flux model incorporates climate through basin temperature and hydrologic runoff. Solutions are provided for each of the major hemispheric climate regions (polar, temperate and tropic). The model successfully predicts the pre-anthropogenic flux of sediment to within the uncertainties associated with the global observations (within a factor of two for 75% of rivers that range across five orders of magnitude in basin area and discharge). Most of the ''problem'' rivers are associated with low observational loads (often smaller rivers where anthropogenic impacts are often magnified, and temporal variability is high). Model predictions provide a baseline for researchers: (1) to question the quality of observational data where available and disagreement is greatest, (2) to examine a river basin for unusually large anthropogenic influences (i.e. causes of erosion or causes of hinterland sediment retention), and (3) to uncover secondary factors not addressed by our model (lithology, lakes). The model provides a powerful tool to address the impact of paleo-climate fluctuations (warmer/colder; wetter/drier) on the impact of sediment flux to the coastal ocean.
Article
Kitimat, British Columbia has a recent history of slope failure on the delta front at the head of the fjord, culminating in a major failure in 1975. The resulting sea-floor morphology and sediments have many characteristics similar to terrestrial debris flows. Deformational mechanisms include delta-front sliding, downslope loading and mixing, translational shearing and remolding, and distal-lobe block gliding over weak, high-water-content fjord bottom sediments.-after Authors
Article
Dispersions of solid spherical grains of diameter D = 0\cdot 13 cm were sheared in Newtonian fluids of varying viscosity (water and a glycerine-water-alcohol mixture) in the annular space between two concentric drums. The density sigma of the grains was balanced against the density rho of the fluid, giving a condition of no differential forces due to radial acceleration. The volume concentration C of the grains was varied between 62 and 13%. A substantial radial dispersive pressure was found to be exerted between the grains. This was measured as an increase of static pressure in the inner stationary drum which had a deformable periphery. The torque on the inner drum was also measured. The dispersive pressure P was found to be proportional to a shear stress T attributable to the presence of the grains. The linear grain concentration lambda is defined as the ratio grain diameter/mean free dispersion distance and is related to C by lambda =1/(C0/C)1/3-1, where C0 is the maximum possible static volume concentration. Both the stresses T and P, as dimensionless groups Tsigma D2/lambda eta 2 and Psigma D2/lambda eta 2, were found to bear single-valued empirical relations to a dimensionless shear strain group lambda 1/2sigma D2(dU/dy)/eta for all the values of lambda < 12 (C = 57% approx.) where dU/dy is the rate of shearing of the grains over one another, and eta the fluid viscosity. This relation gives T propto \ sigma (lambda D)2 (dU/dy)2 and T propto \ lambda 3/2eta dU/dy, according as dU/dy is large or small, i.e. according to whether grain inertia or fluid viscosity dominate. An alternative semi-empirical relation T = (1 + lambda ) (1 + 1/2lambda ) eta dU/dy was found for the viscous case, when T is the whole shear stress. The ratio T/P was constant at 0\cdot 3 approx. in the inertia region, and at 0\cdot 75 approx. in the viscous region. The results are applied to a few hitherto unexplained natural phenomena.
Article
Turbidity currents were formed by releasing suspensions of plastic beads (density 1.52, median diameter 0.18 mm) from a lock into a horizontal water-filled flume. Graded beds were formed; the mechanism of deposition was studied by motion photography and the size grading by 150 size analyses.Deposition of sediment took place behind the head even at a time when there was no deceleration of the head: the greater part of the thickness of the bed was deposited during a period of rapid decline in velocity of flow within the body of the current. The mechanism of deposition and the type of grading differed for beds deposited from suspensions with concentrations less than and greater than about 30% by volume. Low concentration suspensions formed 'distribution grading' in which all percentiles showed vertical grading and at least the coarser half of the distribution showed lateral size decrease away from the gate. High concentration suspensions formed 'coarse-tail grading' in which there was almost no lateral size ...
Article
Lenticular lamination is a fabric that is known from shales of all ages, but its origin and paleoenvironmental significance is poorly understood. We have successfully reproduced this fabric in flume experiments. Beds of water-rich mud were eroded in a flume and yielded sub-millimeter to centimeter-size fragments that can be transported in bedload for distances of ten kilometers or more. Upon redeposition and compaction, these deposits have the same textural qualities as lenticular laminated shales from the rock record. Although accumulation of fecal pellets or abundant burrow tubes in a shale may produce comparable fabrics upon compaction, these can be distinguished from erosion-produced lenticular lamination via petrographic criteria. Lenticular lamination in shales that is due to deposition of water-rich mud fragments most likely records intermittent erosion and transport of surficial muds by currents.
Article
Four principal mechanisms of deposition are effective in the formation of sediment gravity flow deposits. Grains deposited by traction sedimentation and suspension sedimentation respond invidually and accumulate directly from bed and suspended loads, respectively. Those deposited by frictional freezing and cohesive freezing interact through either frictional contact or cohesive forces, respectively, and are deposited collectively, usually by plug formation. Sediment deposition from individual sediment flows commonly involves more than one of these mechanisms acting either serially as the flow evolves or simultaneously on different grain populations. -from Author
Article
Relatively little is known about the characteristics of deposits and potential runout distances of hyperpycnal currents. This study describes and discusses evidence of sediment deposition from sustained (long-duration), quasi-steady turbidity currents in the distal part of the Toyama deep-sea channel, which extends ca. 700 km from river mouths in the central Japan Sea. The study is based on gravity cores and airgun seismic reflection profiles obtained from the channel distal reaches. The silty turbidite beds of the channel's terminal fan show rhythmic layering that indicates sustained turbidity currents with distinct flow-strength fluctuations. Some of the rhythmite beds show a fining-upward internal trend (net flow-strength waning), whereas others show an upward coarsening (net flow-strength waxing) followed by fining. Seismic reflection profiles from the channel levees show large bedforms of climbing-dune type, attributed to the spillover of thick sustained turbidity currents. The deposition of the terminal fan rhythmites and accretionary levee bedforms is attributed to turbidity currents generated by hyperpycnal river effluent. A quantitative assessment of sediment concentrations in the coastal rivers indicates that their effluents could become hyperpyenal nearly every year or during every major seasonal flood. The density of a river-generated underflow would increase by the entrainment of saline water and seafloor sediment on the steep slope of the Toyama Bay, resulting in robust, long-runout turbidity currents. The estimated flow velocities of these currents were around 0.3 m/s, and their recurrence period for the last 1000 years was of the order of 70 years. The estimated duration of hyperpycnal flows required for the deposition of rhythmite beds 700 km away from the river mouth is of the order of several days to 3-4 weeks. The study provides new insights into the recognition and classification of hyperpycnites in distal zones of deep-marine turbiditic systems.
Article
We report laboratory experiments that demonstrate that the fronts of subaqueous debris flows can hydroplane on thin layers of water. The hyhdroplaning dramatically reduces the bed drag, thus increasing head velocity. These high velocities promote sediment suspension and turbidity-current formation. Hydroplaning causes the fronts of debris flows to accelerate away from their bodies to the point of completely detaching from the bodies, producing surging. Instigation of hydroplaning is controlled by the balance of gravity and inertia forces at the debris front and is suitably characterized by the densimetric Froude number. The laboratory flows constrain hydroplaning to cases where the calculated densimetric Froude number is greater than 0.4. The presence of a basal lubricating layer of water underneath hydroplaning debris flows and slides offers a possible explanation for the long run-out distances of many subaqueous flows and slides on very low slopes.
Article
Modern and ancient volcaniclastic sedimentary sequences contain depositional units whose depositional processes demonstrate a sediment/water ratio intermediate between two end-members, debris flow and stream flow processes. The term 'hyperconcentrated flood flow' is proposed for describing this intermediate condition. The resulting deposits are distinguished from the other two by: 1) lack of matrix support or reverse grading and lack of cross-stratification in sand facies; 2) normal grading and horizontal stratification; 3) very poor sorting and poor imbrication; and 4) numerous clasts with long axes oriented parallel to flow directions. Hyperconcentrated flood flow deposits are not unique to volcanic settings but are most common and much thicker and extensive in such areas; they also occur in arid alluvial-fan sequences.-L.C.H.
Article
Some types of graded bedding, especially minor or isolated occurrences and varved "clays," can be readily accounted for by normal processes of sedimentation. Volcanic eruptions, dust storms, annual and longer climatic rhythms, rejuvenation of relief at the source or filling in of the sedimentary basin, churning up of sediment by storm waves, are among the more obvious potential causes of graded deposits. But the majority of occurrences cannot be explained by any of these processes. The authors believe turbidity currents of high density may be invoked to have supplied the sediment and deposited it in graded beds. The absence or insignificance of current bedding and ripple marking in graded deposits, the deposition of coarse material on the unconsolidated fine-grained top of the preceding bed, the enormous extent of each individual member without apparent change in thickness or character, and the frequent inclusion of angular fragments (even composed of clay) and of redeposited fossils are among the chief characteristics opposing transport and deposition by normal agents. All these and several other features, such as the often observed combination of grading with slumping, are explained by turbidity currents of high density. The graded graywacke series of some geosynclines, coarse graded conglomerates and breccias deposited in deep water, and some deep-sea sands can be accounted for in this manner. Experimental investigations on the nature of turbidity currents and the production of graded bedding are described, and the formation of certain graded rocks from the Apennines are treated in detail. Finally, an attempt is made to give a general elucidation of the formation of graded sediments by the action of turbidity currents of high density.
Article
We employ a combined interpretation of Hydrosweep swath bathymetry and high resolution multi-channel seismic reflection data to investigate the development of Cap Timiris Canyon, a newly discovered submarine canyon offshore Mauritania. The dominantly V-shaped and deeply entrenched canyon exhibits many fluvial features including dendritic and meander patterns, cut-off loops and terraces, and is presently incising. Distal meander patterns, confined within a narrow fault-controlled corridor, show several stages of evolution, the latest of which is dominated by a down-system meander-loop migration. Terraces exhibit a variety of internal structures suggesting they originated through different processes including sliding/slumping, uplift-induced incision and lateral accretion. We ascribe canyon origin to an ancient river system in the adjacent presently arid Sahara Desert that breached the shelf during a Plio/Pleistocene sea level lowstand and delivered sediment directly into the slope area. Our data suggest that the initial invading unchannelised sheet of sand-rich turbidity flows initiated canyon formation by gradually mobilising along linear seafloor depressions and fault-controlled zones of weakness. We propose that the development of canyon morphology and structure was influenced by the stages of active flow of the coupling river system, and hence could act as a proxy for understanding the paleo-climatic evolution of a ‘green’ Sahara since Plio/Pleistocene times.
Article
Relatively strong bottom currents in the deep-sea can be inferred from sedimentary structures observed in bottom photographs and cores. The majority of photographs of high and steep topographic prominences, such as seamounts, escarpments and the crests of major ridges, show ripples, scour and rock outcrops. Although photographic current evidence is uncommon in the deeper waters of the ocean basin floor, striking and significant examples do occur. Current scour and ripples are observed beneath the Antarctic bottom current in the western South Atlantic, below the Gulf Stream in the Florida Straits and on the Blake Plateau, beneath the outflowing Mediterranean water west of Gibraltar and beneath the deep currents in the Drake Passage.The three types of deep-sea sands and silts are turbidite, accretionary, and residual. The latter two types are always associated with currents and in some areas turbidites are reworked by bottom currents.Traction velocities necessary for the transport of deep-sea sediment particles probably range from 4–60 cm/sec, velocities similar to those found through dynamic computations of geostrophic currents and observed by recent deep-sea direct current measurements.
Article
It has been apparent for some time past that fine sediment material carried in suspension by a turbulent water stream flowing by gravity is apt to behave inconsistently with conventional theory. This demands that the concentration of suspended solids, which being heavier than the fluid tend to fall through it, must always increase downwards towards the bed. In fact, however, the concentrations of fine sediment grades present in rivers are often found to increase upwards instead of downwards. Further, while the discharge or transport rate of the coarser grades is found to be a function of the stream flow, that of the finer grades appears to be unlimited. The critical grain size below which these anomalies occur is usually put at about 50 µ for sediments of natural mineral density. But the factors on which the critical size depends have not previously been looked into. The reason for these anomalies has remained mysterious. Equally mysterious have been the conditions which enable a submarine turbidity current to maintain itself and to transport sand and silt in turbulent suspension for distances of many hundreds of miles over a very gently sloping ocean bed.
Article
Debris flows claim hundreds of lives and cause millions of dollars of property damage throughout the world each year. In Japan alone, an average 90 lives are lost annually from debris flows (Takahashi 1981). In 1970 a debris avalanche (a rapidly moving form of debris flow) triggered by an earthquake, completely destroyed the city of Yungay, Peru, killing an estimated 17,000 people and burying the whole city under 5 m of mud and debris (Plafker and Erickson 1978). Some countries with chronic losses from debris flows include Japan (Okuda et al. 1980); United States (Committee on Methodologies for Predicting Mudflow Areas, 1982; Scott 1972; Cummans 1981; Scott 1971; Flaccus 1958; Williams and Guy 1973; Woolley 1946; Morton and Campbell 1974); Indonesia (Scrivenor 1929); Tanzania (Temple and Rapp 1972); Scandinavia (Rapp and Stromquist 1976); Costa Rica (Waldron 1967); China (Li and Luo 1981; Chinese Society of Hydraulic Engineering 1980); Brazil (Jones 1973); Ireland (Prior et al. 1968); Romania (Balteanu 1976); India (Starkel 1972); Bangladesh (Wasson 1978); New Zealand (Selby 1967; Pierson 1980a, b); and the Soviet Union (Gol’din and Lyubashevskiy 1966; Niyazov and Degovets 1975; Gagoshidze 1969).
Article
Turbidity currents are notoriously difficult to monitor directly, therefore interpretation of their deposits forms the basis for much of our understanding of these flows. The deceleration rate of a flow is a potentially important yet poorly understood control on depositional processes. A series of experiments were conducted in an annular flume, in which fast (up to 3.5 m/s) and highly turbulent flows of sand (up to 250 mu m) and water were decelerated at different rates and processes of deposition and deposit character analyzed. Previously poorly documented depositional processes were observed in the experiments. This is because the flows were initially unusually fast and of prolonged duration, with sustained periods of sediment fallout as the flow slowed down. The conditions in these flows are thus likely to be closer to those at the base of a waning turbidity current than is achieved in other relatively slow experimental flows. The collapse of high-concentration, moving, thin (< 5 mm) near-bed layers (laminar sheared layers) were an important mechanism by which the bed aggraded beneath these unsteady flows. At bed aggradation rates in excess of 0.44 mm/s the sequential collapse of laminar sheared layers produced a structureless, poorly graded and poorly sorted deposit (Bouma T-a). When bed aggradation rates fell below 0.44 mm/s the collapsing laminar sheared layers were reworked by turbulence to form planar laminae (Bouma T-b). These laminae are formed in a very different manner than the planar laminae attributed to bedwaves in previous open-channel flow experiments. Collapse of laminar sheared layers is therefore an alternative process for generating the Bourna T-b division. Inverse grading developed at the base of the deposits of slowly decelerated flows. This inverse grading was probably a result of grain sorting in a high-concentration layer that persisted at the base of the flow for many minutes prior to the onset of deposition.
Article
Documents the ability of the flows to move substantial distances as subaqueous debris flows, to convert to turbidity currents by means of subaqueous hydraulic jumps, and to then move along the reservoir floor as turbidity currents. The findings illustrate the ability of the hydraulic jumps to control the location of sediment deposition. -from Author
Article
Despite the historical assumption that the bulk of marine ג€œshelfג€ mud is deposited by gradual fallout from suspension in quiet water, recent studies of modern muddy shelves and their associated rivers show that they are dominated by hyperpycnal fluid mud. This has not been widely applied to the interpretation of ancient sedimentary fluvio-deltaic systems, such as dominate the mud-rich Cretaceous Western Interior Seaway of North America. We analyze two such systems, the Turonian Ferron Sandstone Member of the Mancos Shale Formation, in Utah, and the Cenomanian Dunvegan Formation in Alberta. Paleodischarge estimates of trunk rivers show that they fall within the predicted limits of rivers that are capable of generating hyperpycnal plumes.The associated prodeltaic mudstones match modern hyperpycnite facies models, and suggest a correspondingly hyperpycnal character. Physical sedimentary structures include diffusely stratified beds that show both normal and inverse grading, indicating sustained flows that waxed and waned. They also display low intensities of bioturbation, which reflect the high physical and chemical stresses of hyperpycnal environments. Distinct ג€œmantle and swirlג€ biogenic structures indicate soupground conditions, typical of the fluid muds that represent the earliest stages of deposition in a hyperpycnal plume. Hyperpycnal conditions are ameliorated by the fact that these rivers were relatively small, dirty systems that drained an active orogenic belt during humid temperate (Dunvegan Formation) to subtropical (Ferron Sandstone Member) ג€œgreenhouseג€ conditions. During sustained periods of flooding, such as during monsoons, the initial river flood may lower salinities within the inshore area, effectively ג€œpreppingג€ the area and allowing subsequent floods to become hyperpycnal much more easily. Although shelf slopes were too low to allow long-run-out hyperpycnal flows, the storm-dominated nature of the seaway likely allowed fluid mud to be transported for significant distances across and along the paleo-shelf. Rapidly deposited prodeltaic hyperpycnites are thus considered to form a significant component of the muddy shelf successions that comprise the thick shale formations of the Cretaceous Western Interior Seaway.
Article
Sandy hyperpycnal flows and their deposits, hyperpycnites, have been documented in modern environments and, more recently, in Cretaceous and Tertiary strata; they may be more common in the rock record, and within petroleum reservoirs, than has been previously thought. Muddy hyperpycnites also occur within the rock record, but these are more difficult to document because of their finer-grained nature and lack of common sedimentary structures. This paper documents the presence of submarine slope mudstone and siltstone hyperpycnites (and muddy turbidites) in the delta-fed, Upper Cretaceous Lewis Shale of Wyoming; based on field measurements, analyses of rock slabs and thin sections, and laser grain-size distributions. Four lithofacies comprise laminated and thin-bedded mudstones that are associated with levéed channel sandstones: (L1) grey, laminated, graded mudstone with thin siltstone and sandstone interbeds; (L2) dark grey to tan, laminated mudstone with very thin siltstone and sandstone stringers; (L3) light grey, laminated siltstones; and (L4) laminated mudstones and siltstones with thin sandstone interbeds. Two styles of mudstone grain-size grading have been documented. The first type is an upward-fining interval that typically ranges in thickness from 2·5 to 5 cm. The second type is a couplet of a lower, upward-coarsening interval and an upper, upward-fining interval (sometimes separated by a micro-erosion surface) which, combined, are about 3·8 cm thick. Both individual laminae and groups of laminae spaced millimetres apart exhibit these two grain-size trends. Although sedimentary structures indicative of traction-plus-fallout sedimentary processes associated with sandier hyperpycnites are generally absent in these muddy sediments, the size grading patterns are similar to those postulated in the literature for sandy hyperpycnites. Thus, the combined upward-coarsening, then upward-fining couplets are interpreted to be the result of a progressive increase in river discharge during waxing and peak flood stage (upward increase in grain-size), followed by waning flow after the flood begins to abate (upward decrease in grain-size). The micro-erosion surface that sometimes divides the two parts of the size-graded couplet resulted from waxing flows of sufficiently high velocity to erode the sediment previously deposited by the same flow. Individual laminae sets which only exhibit upward-fining trends could be either the result of waning flow deposition from either dilute turbidity currents or from hyperpycnal flows. The occurrence of these sets with the size-graded couplets suggests that they are associated with hyperpycnal processes.