Inverse Grading Research Articles

Investigation on the failure mechanism of the internal erosion in inverse grading sand based on DDA and Darcy’s law

Inverse grading structures are commonly found in landslide accumulations of high-speed and long-runout landslides. However, there is limited research on the internal erosion of inverse grading deposits. This study developed a program called discontinuous deformation analysis (DDA) and Darcy’s law coupling program (DDCP) using C language to investigate the failure mechanism associated with internal erosion in inverse grading sand. The results indicate that as the hydraulic gradient increases, the internal erosion phenomenon becomes more pronounced. The failure of the inverse grading sand due to internal erosion can be attributed to three processes: loss of fine particles in the lower layer, accumulation of the particles from the upper layer in the lower layer, and flow of fine particles from the upper layer through the lower layer and exit from the bottom. During the internal erosion process, both the hydraulic conductivity and porosity initially decrease and then increase. Similarly, the average velocity of fine particles exceeds that of coarse particles in each layer. The stability of the inverse grading sand during the internal erosion increases with a higher content of fine particles in the lower layer. These findings are significant in understanding the mechanism of internal erosion within inverse grading deposits.

Wave‐Driven Vertical Sorting of Density‐Varying Particles

AbstractIt has been recognized that the vertical sorting of polydispersed sand grains via the “Brazil Nut effect” can lead to inverse grading (upward coarsening) in the surface layer of a sand bed. However, the addition of nonnative particles not inherently observed on the beach, characterized by different densities and sizes, may complicate the vertical sorting processes and the fate of the nonnative particles. For example, olivine particles, which may be released on natural sandy beaches to facilitate carbon dioxide capture via weathering from wave action, have a density about 25% larger than the typical native sands. An Eulerian‐Lagrangian model, which couples Computational Fluid Dynamics for the fluid phase and Discrete Element Method for the particle phase was utilized to investigate how the so‐called “Reverse Brazil Nut effect” due to the larger density of nonnative (olivine) particles may be counteracted by the so‐called “Brazil Nut effect” for a range of nonnative particle sizes. Numerical simulations showed that the higher‐density nonnative particles tended to sink into the sand bed consistent with the “Reverse Brazil Nut effect”; however, the vertical sorting structure of native sand, driven by the “Brazil Nut effect,” also was a controlling factor determining the fate of nonnative particles. To maintain the presence of the nonnative (olivine) particles in the active layer of sediment transport, the higher‐density nonnative particle size must be larger than the mean native sand size found at the bottom of the active layer.

Grain-size variability in debris flows of different runout lengths, Wenchuan, China

Debris-flow grain-size distributions (GSDs) control runout length and mobility. Wide, bimodal GSDs and those containing a higher proportion of silt and clay have been shown experimentally to increase runout length. However, the relationship between grain size and mobility has not been well established in field conditions. Here, we compared the grain-size characteristics of two debris flows with considerably different runout lengths (1.5 km vs. 8 km) to understand the role of grain size in governing runout. The two debris flows were triggered in the same rainfall event from coseismic landslide debris generated in the 2008 Wenchuan earthquake in catchments with similar lithology and topography. We compared the deposited GSDs and their spatial patterns using our rare, three-dimensional GSD datasets. Surprisingly, the proportions of each size fraction deposited by the two flows were statistically indistinguishable. The spatial pattern of grain size differed between the two flows, with evidence of inverse grading only preserved in the smaller deposit. From these observations, we can infer that the GSDs of both flows were determined by the coseismic landslide source material, and that there was little difference in the GSDs of material entrained as the flows bulked. The contrasting spatial distributions of grains indicated that different internal processes were dominant within the two flows. These findings demonstrate that where GSDs are dominated by coarse grains and are governed by similar source conditions, grain size plays a lesser role relative to sediment supply and hydrology in controlling the runout length of large catastrophic post-earthquake debris flows.

Effect of Including Sand Component in a Debris Flow on Concentration of Coarser Particles at the Flow Front

We conducted flume experiments for a debris flow consisting of coarser particles, finer particles and sand, focusing on the concentration of coarser particles at the flow front. Our experimental results revealed that the concentration of coarser particles at the flow front using sediment mixture with sand was less than that without sand. This may be because including of sand component in the material contributed to be a smaller averaged interstice between particles in the flow layer and a smaller averaged particle size. These may lead to reduce the falling volume of sand or finer particles and dispersion pressure for the rise of coarser particles, respectively, resulting in the inhibition of inverse grading formation. Our experimental results also suggested that the changing trend in the proportion of finer particles depended on the relationship between their particle size and the average particle size of the flow. These are consistent with our previous experimental results using material without sand. This consistency suggested that for the concentration of coarser particles at the flow front, the behavior of the sand component can be considered in the same manner as other coarser-sized components.

Entrainment and Transport of Well‐Sorted and Mixed Sediment Under Wave Motion

AbstractEntrainment and suspension of sediment particles with the size distribution similar to a range of natural sands were simulated with a focus on the vertical size sorting and transport dynamics in response to different wave conditions. The simulations were performed using a two‐phase Eulerian‐Lagrangian model by combining the LIGGGHTS discrete element method solver for sediment and SedFoam solver for the fluid phase. The model was first validated for a range of sand grain sizes from 0.21 to 0.97 mm having well‐sorted and mixed (bimodal) size distributions using laboratory oscillatory flow data. Three sediment bed configurations were studied under a wide range of velocity‐skewed waves with different wave intensity and skewness. It was found that the bimodal distribution having only 30% of coarse fraction and 70% of medium fraction responds similar to a well‐sorted coarse sand configuration. Sediment fluxes of the bimodal distribution were slightly higher than those of well‐sorted coarse sand because of the pronounced inverse grading in the bimodal distribution. Furthermore, for the bimodal distribution the medium fraction acted as a relatively smooth foundation underneath the coarse fraction which facilitated the mobilization of the coarser particles. Under high energy wave conditions, the smoothing feature was exacerbated and further caused the formation of plug flow where a thick layer of intense sediment flux was observed. Model results also showed that under high skewness waves, phase‐lag effect occurred in well‐sorted medium sand which caused lower net onshore sediment transport rates but the effect was significantly reduced for mixed sediments.

Experimental Study on the Accumulation Characteristics and Mechanism of Landslide Debris Dam

According to physical model tests, we analyzed the accumulation characteristics of landslide dams formed under three different slope characteristics, namely, uniform slope, parallel slope, and intersecting slope and investigated the accumulation mechanisms of the debris dams. The relationship between slope types and accumulation characteristics was also explored using the tracer particle analysis method. The damming process and accumulation mechanism of the landslide dam were changed with slope conditions, which lead to the difference in the accumulation characteristics of the dam, especially in transverse cross-sectional shape and grain size distribution. The transverse cross-sectional shape of the landslide dams formed by different slope conditions can be divided into three categories: the flat pattern, unidirectional pattern, and undulating pattern. The characteristics of the slope body are closely related to those of the landslide dam with respect to debris distribution. The debris distributions in the longitudinal and transverse directions of the slope body are consistent with those in the longitudinal and sliding regions of the dam. A general inverse grading characteristic of debris gains occurs in the vertical direction of landslide dams. For the uniform and parallel slopes, obvious inverse grading distribution is induced by overall-starting initiation of the slope body and strong vertical infiltration of the fine sands during the movement. However, inverse grading distribution is generated by the effects of pushing–climbing and lateral infiltration that existed among particles caused by a layered-starting mode for the intersecting slope body. This study provides a basis for the prediction of landslide dam formation and backtracks the initial structure of the slope.

Middle Ordovician bottom current deposits in the western margin of the North China craton: Evidence from sedimentary and magnetic fabrics

AbstractIn many modern deep‐water slope environments, deposits formed by bottom currents (contourites) are rather common. However, reports on ancient contourites in deep‐water environments are scarce and most of them remain contentious. Based on the study of high‐resolution sedimentological, sedimentary geological and magnetic fabric characteristics, this study presents the key evidence of bottom current activity in the Middle Ordovician Kelimoli Formation from the western margin of the North China craton, and reconstructs the deep‐sea circulation pattern and the direction of deep‐water bottom currents in the middle and low latitudes during the Middle Ordovician. Six typical carbonate microfacies are identified in the Kelimoli Formation, which can be grouped into three sedimentary facies types related to the main interpreted depositional processes (bottom current, undifferentiated bottom current and hemipelagic settling, and hemipelagic settling). The evidence of bottom current deposits are five‐fold. Firstly, it is possible to observe a variety of traction current structures, which mainly represent bed‐load‐dominated transport of sediments. Secondly, there are multi‐scale inverse grading and bi‐grading, as well as inversely‐graded and normally‐graded sedimentary sequences, which reflect the periodic change of bottom current activity. Thirdly, gradual or sharp contact boundaries and small internal erosional surfaces indicate omission surfaces caused by the oscillating variation of bottom current velocity. Fourthly, the continuous rhythmic change of sedimentary fabrics on the microscopic scale reflects sustained, waxing and waning current intensities. Fifthly, the anisotropy of magnetic susceptibility reveals the microscopic fabric characteristics of imbricated magnetic grains, which represent the continuous action of directional current, and the restored palaeocurrent direction indicates that the current is mainly parallel to the slope strike. After the palaeo‐north correction of the North China craton clockwise rotation of 120° to 150° since the Ordovician, anisotropy of magnetic susceptibility shows that the direction of bottom current in the study area is towards the north‐east/east. Therefore, it is speculated that there was a clockwise deep‐water circulation in the middle and low latitudes of the southern hemisphere during the Ordovician. This study not only provides sedimentological evidence for the Ordovician bottom current activity, but also contributes to a better understanding of the Ordovician palaeoceanographical environment, especially the Middle Ordovician deep‐sea circulation pattern.

Effect of artificial structures on the formation process of the 2011 Tohoku-oki tsunami deposits

The interaction between tsunami hydrodynamics and sediment behavior results in a complex depositional process for onshore tsunami deposits. Furthermore, the formation of modern tsunami deposits can be affected by artificial structures, which can make the depositional processes difficult to understand. In the present study, we focus on artificial structures to reveal the relationship between sedimentary structures and hydrodynamics of the 2011 Tohoku-oki tsunami. Field observations indicated that a coastal dike and three shore-parallel roadways significantly affected the source, thickness, and sedimentary structures of the tsunami deposits. The sandy tsunami deposits were mainly sourced from the backshore, which was severely eroded at the landward side of the dike by tsunami overflow. The thickness of the tsunami deposits increased seaward of roadways, while the deposit became thinner landward of roadways. A one-dimensional numerical simulation indicated a local decrease in the flow velocity at the seaward side of roadways, which elucidated the increase in the deposit thickness. Most tsunami deposits are characterized by normal graing, inverse grading, and ungraded units. Inverse grading was observed 0.05 km from the shoreline and at the seaward side of the roadways, except at the most landward roadway. Changes in the grain-size distribution of the deposits and variations in the simulated flow velocity across the roadways suggested that the inverse grading was formed because of waxing and depletive flow conditions with high concentrations of suspended sediments. Our study elucidated the key role that artificial structures play in the formation of tsunami deposits and the resultant sedimentary structures, in developed coastal areas. Artificial structures, which have a somewhat extreme topography compared to the natural geomorphology, also contributed to demonstrate the concept of the acceleration matrix. This is because of the remarkable change in the steadiness and uniformity of the tsunami flow and the resultant sedimentary structures.

Unusual intraclast conglomerates in a stormy, hot‐house lake: The Early Triassic North China Basin

AbstractEarly Triassic temperatures were some of the hottest of the Phanerozoic, sea‐surface temperatures approached 40°C, with profound consequences for both the sedimentology and faunal distributions in the oceans. However, the impact of these temperatures in terrestrial settings is unclear. This study examines shallow lacustrine sediments from the Lower Triassic succession of North China. These consist of diverse fluvial to shallow lacustrine sandstones and also spectacular, coarse conglomerates composed of diverse, intraformational clasts reworked from the interbedded sediments. The conglomerate beds can show inverse grading and high angle, flat‐pebble imbrication in their lower part and vertically orientated flat pebbles in their upper part. The cobbles include cemented and reworked conglomerate intraclasts and sandstone concentrically‐laminated concretions that record multi‐step histories of growth and reworking, pointing to rapid cementation of the sandy lake bed (likely facilitated by high temperatures). The conglomerates record frequent, high‐energy events that were capable of brecciating a lithified lake bed and transporting cobbles in wave‐influenced sediment‐gravity flows. Initially, powerful oscillatory flows brecciated and deflated the lake bed and subsequently helped to sustain turbulence during short‐distance lateral flow. It is possible that hurricanes, originating from the adjacent hyper‐warm, Palaeo‐Tethyan Ocean travelled into the major lakes of the North China continent during the Early Triassic.

Acceleration of Debris Flow Due to Granular Effect

Pore water pressure has been recognized as an important factor to enhance the mobility of debris flow moving in channel of very gentle slope. The creation and dissipation of pore water pressure are associated with interaction between grains. This study proposes a physical model for the pressure on mobility of flows with different granular configurations: the flow with overlying coarse-grained layer (i.e., inverse grading) and the flow with fully-mixed grains. The flow velocity is derived by the effective stress principle and the relationship between acceleration and pore water pressure is analyzed under different conditions. The results show that a high excess pore water pressure leads to high velocity of flow, and the pressure increases during the movement; and acceleration increases with time and flow depth under given pore water pressure. Moreover, compared with the flow with mixed grains, the flow with overlying coarse-grained layer is more effective to promote the excess pore water pressure and the liquefaction slip surface. Therefore, the internal drag reduction due to pore water pressure produces an acceleration effect on the flow.

Experimental investigation of mobility and deposition characteristics of dry granular flow

This paper presents an experimental investigation of mobility and deposition characteristics of dry granular flow, by a number of flume tests on silica sand no. 3 and silica sand no. 7, to interpret the effects of angle of slope, granular volume, cushion, granular structure, and granular size on the mobility and deposition characteristics of granular flow. Along a given slope, an increase of the amount of sand impaired its mobility. However, for a given amount of sand along a slope, an increase of the angle of slope resulted in a V-shaped change of the angle of mass center movement, implying the existence of a characteristic combination of the angle of slope and the amount of sand to yield the maximum mobility of granular flow. The angle of mass center movement increased while increasing the thickness of cushion, showing that the cushion impaired the mobility of granular flow. Granular structure using the mixed structures of silica sand no. 3 and silica sand no. 7 affected greatly the mobility of granular flow, by showing an inverted structure in the near runout area with the deposition of the materials in the upper half of original grading structure in the far runout area for the inverse grading structure and normal grading structure. The mobility of granular flow increased with the change in turn of the inverse grading structure, the uniform structure, and the normal grading structure. The increase of the grain sizes of granular material enhanced its mobility. In addition, the angle of mass center movement showed a more reliable assessment for the mobility of granular flow in comparison with the angle of maximum mass movement.

Gravity flows: Types, definitions, origins, identification markers, and problems

Abstract This review covers 135 years of research on gravity flows since the first reporting of density plumes in the Lake Geneva, Switzerland, by Forel (1885). Six basic types of gravity flows have been identified in subaerial and suaqueous environments. They are: (1) hyperpycnal flows, (2) turbidity currents, (3) debris flows, (4) liquefied/fluidized flows, (5) grain flows, and (6) thermohaline contour currents. The first five types are flows in which the density is caused by sediment in the flow, whereas in the sixth type, the density is caused by variations in temperature and salinity. Although all six types originate initially as downslope gravity flows, only the first five types are truly downslope processes, whereas the sixth type eventually becomes an alongslope process. (1) Hyperpycnal flows are triggered by river floods in which density of incoming river water is greater than the basin water. These flows are confined to proximity of the shoreline. They transport mud, and they do not transport sand into the deep sea. There are no sedimentological criteria yet to identify hyperpycnites in the ancient sedimentary record. (2) A turbidity current is a sediment-gravity flow with Newtonian rheology and turbulent state in which sediment is supported by flow turbulence and from which deposition occurs through suspension settling. Typical turbidity currents can function as truly turbulent suspensions only when their sediment concentration by volume is below 9% or C < 9%. This requirement firmly excludes the existence of 'high-density turbidity currents'. Turbidites are recognized by their distinct normal grading in deep-water deposits. (3) A debris flow (C: 25-100%) is a sediment-gravity flow with plastic rheology and laminar state from which deposition occurs through freezing en masse. The terms debris flow and mass flow are used interchangeably. General characteristics of muddy and sandy debrites are floating clasts, planar clast fabric, inverse grading, etc. Most sandy deep-water deposits are sandy debrites and they comprise important petroleum reservoirs worldwide. (4) A liquefied/fluidized low (>25%) is a sediment-gravity flow in which sediment is supported by upward-moving intergranular fluid. They are commonly triggered by seismicity. Water-escape structures, dish and pillar structures, and SSDS are common. (5) A grain flow (C: 50-100%) is a sediment-gravity flow in which grains are supported by dispersive pressure caused by grain collision. These flows are common on the slip face of aeolian dunes. Massive sand and inverse grading are potential identification markers. (6) Thermohaline contour currents originate in the Antarctic region due to shelf freezing and the related increase in the density of cold saline (i.e., thermohaline) water. Although they begin their journey as downslope gravity flows, they eventually flow alongslope as contour currents. Hybridites are deposits that result from intersection of downslope gravity flows and alongslope contour currents. Hybridites mimic the "Bouma Sequence" with traction structures (Tb and Tc). Facies models of hyperpycnites, turbidites, and contourites are obsolete. Of the six types of density flows, hyperpycnal flows and their deposits are the least understood.

Armoring and exposure effects on the wave-driven sediment transport

Coastal sediment transport of graded spherical particles with the material properties of sand was simulated and compared to transport of the uniform-sized spherical particles using a two-phase Eulerian-Lagrangian model. The fluid phase solver was based on SedFoam developed in OpenFOAM and the open-source discrete element method solver LIGGGHTS was used for the particle phase. We validated the model for sheet flow of well-sorted medium sand (d50 = 0.28 mm) and mixed sand with bimodal size distribution in velocity-skewed oscillatory flows as well as well-sorted coarse sand (d50 = 0.51 mm) in velocity-skewed and acceleration-skewed oscillatory flows. Simulation results of graded particles showed the formation of inverse grading (upward coarsening) in sediment bed under oscillatory flows, suggesting that the effects of armoring and exposure were important in the resulting transport rate. Examining different particle size distributions under onshore velocity-skewed flows, it was found that the largest increase of the net onshore sediment transport rate due to size gradation corresponded to the moderately sorted particle size distribution (d90/d10 = 3.41), where the coarse fraction (d > d50) had the maximum contribution to the transport. By analyzing intra-wave sediment transport quantities, the response of size gradation to the flow skewness and asymmetry and velocity intensity was investigated. Model results revealed that the armoring effect (reduction of sediment flux due to inverse grading) was dominant when flow velocity magnitude was lower (wave trough) or the fluid acceleration was higher. On the other hand, when flow velocity magnitude was larger (wave crest), the armoring effect was reduced or the exposure effect (enhancement of sediment flux due to inverse grading) may become more pronounced. Overall, we found that onshore net sediment transport was enhanced up to 30% due to particle size gradation under onshore velocity-skewed oscillatory flows. Conversely, the size gradation reduced the net onshore transport rate up to 35% under onshore acceleration-skewed oscillatory flows. Model results also suggested that the thickness of the active layer (surface layer affected by vertical sorting) in sheet flows can be quantified by the peak erosion depth. The simulation results presented here provide insights into the role of sediment size gradation (armoring and exposure effects) in wave-driven onshore transport which is important for predicting morphological evolution.

Fine‐grained gravity flow deposits and their depositional processes: A case study from the Cretaceous Nenjiang Formation, Songliao Basin, NE China

The Songliao Basin in NE China is a large rift basin filled with Cretaceous terrestrial sediments. Lacustrine mudstones of the Nenjiang Formation form an important source rock in the Cretaceous Songliao Basin. These shales are commonly thought to have been deposited in deep, quiet, and anoxic environments. Samples obtained from the core of the SK‐2 scientific borehole provide critical insights to understand the hydrodynamic and hydroclimatic environments, which are, however, different from the traditional views regarding the deposition of these rocks. By following a mudstone description guide, five different mudstone lithofacies (LF) transported and deposited by muddy hyperpycnal flows and muddy debris flows were recognized. They are laminated fine mudstone (LF1), laminated medium mudstone (LF2), and laminated coarse mudstone (LF3) showing pairs of inverse grading (Ha) and normal grading (Hb) under the microscope, graded coarse mudstone (LF4) and massive coarse mudstone (LF5). We found that mudstones of the First Member of the Nenjiang Formation are dominated by siliciclastic detritus and argillaceous components and show frequent variations in grain size. Because large‐scale sub‐lacustrine channels travelling long distance (>80 km) were widely distributed in the Songliao palaeolake during the deposition of the Nenjiang Formation, fluctuations in mudstone grain size might have been caused by velocity fluctuations in flows. Sedimentary structures and textures preserved in mudstones of the First Member of the Nenjiang Formation indicate that the majority of these lithofacies were accumulated by muddy hyperpycnal flows and muddy debris flows. Therefore, a depositional model dominantly influenced by muddy hyperpycnal flows and debris flows is proposed. This work not only provides a new view for the depositional process of mudstones of the Songliao Basin, NE China, but also give insights to understand lacustrine palaeoenvironment and terrestrial palaeoclimate.

Bioturbation, sedimentation rates, and preservation of flood events in deltas

We examine the nature and number of event beds preserved in heterolithic prodelta and delta-front strata in five proximal-accumulation-dominated (PAD) North American delta systems including the Quaternary Gulf of Mexico and the Late Cretaceous Western Interior Seaway. We also explore the continuum between PAD, more distal deposits of mud belts, and thicker subaqueous-delta-clinoforms (SDC). Sediment accumulation rates in modern PAD and SDC systems are commonly on the order of several to severaltens of centimeters per year. They also show that several event beds may be created annually, with preservation depending on supply and reworking. The PAD heterolithics commonly comprise interbedded sand, silt and clay. These show normal and inverse grading and Bouma sequences, indicating deposition from turbidity currents and hyperpycnal flows that are interpreted as the products of river plumes generated during flood episodes. Re-equilibrated Rosselia socialis in stacked delta front turbidite bedsets in the Cretaceous examples show that floods may occur as a cluster of events over several months. Shelf areas adjacent to river mouths may experience high rates of sediment delivery from rivers, but oceanographic processes are instrumental in moving much of the silt and clay farther along the shelf. Direct, hyperpycnal delivery from a river mouth on a low gradient shelf (i.e., typically <0.05°) is rare, owing to the difficulty of maintaining a high suspended sediment concentration, but sedimentation rates are nevertheless high. Shelf facies show evidence for extensive and frequent remobilization during storms and other meteorological events. Storm-wave-enhanced sediment-gravity flows (WESGFS) may form under favorable conditions, such as strong waves, high suspended sediments, and convergent currents, and can be significant in building SDC and other shelf deposits. The proximal portions of PAD and SDC systems typically show low intensity and diversity of bioturbation (BI 0–1). The bioturbation styles encountered in these deposits record: 1) high sedimentation rates, which preclude wholesale biogenic reworking; 2) high-frequency river floods and storm reworking, expressed by reduced colonization windows; 3) soupy substrates, likely reflecting formation of a permanent fluid-mud lutocline; and 4) elevated turbidity near the sediment-water interface, which may cause further suppression of trace-making.

Sedimentary environment and petrological features of organic-rich fine sediments in shallow water overlapping deposits: A case study of Cambrian Yuertus Formation in northwestern Tarim Basin, NW China

Abstract Taking the Cambrian Yuertus Formation outcrop profiles in the Aksu-Keping-Wushi areas of northwestern Tarim Basin as examples, the depositional environments of organic rich fine sediment were analyzed by examining the outcrop profiles macroscopically and microscopically. The study reveals that: (1) The lower part of the Yuertus Formation consists of organic-rich fine sediment or thin rhythmic interbeds of organic-rich fine sediment and siliceous sediment, the formation transforms to terrigenous diamictic grain shoal and inverse grading carbonate rocks upward. (2) The thin limestone interbedded with dark shale rhythmically has inverse grading. (3) The thin-bedded siliceous rock has metasomatic residual granular texture, stromatolithic structure and cementation fabric in vugs. (4) There are iron crust layers at the top of the shallowing diamictic grain shoal, beneath which exposed karst signs, such as karrens, dissolved fissures, sack-like vugs, near surface karst (plastic) breccia, breccia inside the karst system and terrigenous clastic fillings, can be seen. (5) Both the outcrops and seismic profiles show that organic-rich fine sediments above the unconformities or exposed surfaces are characterized by overlapping. The organic-rich fine sediment of the Cambrian Yuertus Formation was deposited in the anoxic-suboxidized restricted gulf lagoon environment, and its formation was controlled by high paleoproductivity and poor oxygen exchange jointly, then a shallow-water overlapping sedimentary model has been established. The results will help enrich and improve the sedimentary theory of organic-rich fine sediments.

Source- vs topographic-forcing in pyroclastic currents: the case of the Orvieto-Bagnoregio Ignimbrite, Vulsini, central Italy

The main pyroclastic flow unit of the Orvieto-Bagnoregio Ignimbrite (Vulsini, central Italy) provides a striking example of increasing thickness with distance from the vent, accompanied by opposite size-distance trends for lithic and juvenile clasts. Lithic clasts show normal lateral grading, while dense (1120-1400 kg/m 3 ) scoria clasts show inverse lateral grading. The latter trends are attributed to the opposite density contrast with respect to the flow medium of gas and fine particles and put a constraint to the minimum density of the transporting flow. Thus, the study example approaches the high-concentration, non-turbulent end-member of pyroclastic currents. By applying the topological aspect ratio approach, we infer a forced behavior of the parent flow in proximal to intermediate settings, due to sustained feeding with high mass discharge rate at source. In distal settings, the source-forced regime was enhanced by topographic forcing due to channeling along radial topographic lows, thus resulting in increasing bulk density and runout of the current. By analogy with the mobility of dry debris flows, the sliding component of transport prevailed in proximal to intermediate settings, accounting for the prevailing tendency of the pyroclastic current to transport than to deposit, thus forming a relatively thin deposit with normal lateral grading of lithics. The spreading component dominated downcurrent, resulting in increasing pyroclast accumulation (up to tens of meters of thickness) and delayed deposition of coarsest scoria clasts as far as the final runout.

Structural Study of Nano-Sized Gahnite (ZnAl2O4): From the Average to the Local Scale.

Spinel gahnite (ZnAl2O4) has been obtained through a hydrothermal synthesis method with a grain size of about 2 nm. The sample was calcined for a few hours at two different temperatures (800 and 900 °C) in order to obtain larger grain sizes to be analyzed by means of powder diffraction with the Rietveld method, and by means of total scattering with the Pair Distribution Function (PDF) method. The idea is to compare the average to the local structure, as a function of increasing grain size. The total scattering data were collected at the European Synchrotron Radiation Facility (ESRF), Grenoble. The samples have been also characterised by means of high resolution Transmission Electron Microscopy (TEM), showing an increasing grain size up to about 9 nm. The average structure presented variations in the inversion degree and an increase in grain size. TEM observations demonstrated that the small crystals are well crystallised: the high resolution images neatly showed the atomic planes, even in the smallest particles. However, the average structure did not properly fit the PDF data in the local region, owing to a slightly different coordination among the octahedra. A new structural model is proposed for the local region of the PDF, that helped our understanding of the differences between a real nanostructured sample and that of a microcrystalline one. The oxygen disorder, due to the inversion grade of the spinel, is demonstrates to be at the basis of the local deviation. No signals of interstitial Zn atoms were detected.

Emplacement mechanisms of the Tagarma rock avalanche on the Pamir-western Himalayan syntaxis of the Tibetan Plateau, China

To determine the emplacement mechanisms of rock avalanches, the surficial and internal sedimentological characteristics of avalanche deposits are attracting increased scientific interest. In this study, deposits of the massive paleoseismic Tagarma rock avalanche in the Tarim Basin of the Pamir-western Himalayan syntaxis, China, are investigated to enrich the knowledge on rock avalanche dynamics. Based on a combination of remote sensing data and in situ surveys, a series of typical surficial landforms, including a Toreva block, arc-shaped transverse ridges, aligned megablocks, and longitudinal lineament, are studied in detail. Clusters of superimposed gneiss megablocks preserving the bedrock structures of the source scar are widely distributed along the travel path. Moreover, inverse grading in the deposit and a substrate characterized by clast-supported pebbles with imbricate structures are also observed. Cracks and fissures are common in most pebbles, indicating intense interactions between the avalanche mass and the substrate. Through analysis of these surficial and internal characteristics, we propose that the avalanche mass should have propagated rapidly as a single unit with slight internal mixing and high internal pressure fluctuations. These internal pressure fluctuations obviously exceeded the overburden pressure and contributed to the intense shattering of the main deposit, especially the basal facies. The basal facies was mainly emplaced by a frictional, simple shear process and controlled the avalanche hypermobility. From the proximal to the distal parts, the transport of the avalanche mass can be classified as an extension-dominated sliding process in the transition zone and a compression-dominated sliding process in the accumulation zone.

Seismic cycle recorded in cockade-bearing faults (Col de Teghime, Alpine Corsica)

Few fault rocks are known to be associated undoubtedly with seismic faulting. Here, we investigated the formation mechanism of cockade breccias found in transtensional faults cutting marbles and quartzites from the Col de Teghime area (Alpine Corsica, France). Field surveys coupled with detailed microanalytical investigations indicated that: (i) the core clasts of the cockades are composed of host rock fragments >310 μm in size that are suspended in the slipping zones and arranged in inverse grading; (ii) the concentric rims of the cockades show a cyclic zoning made of saddle dolomite + Mg-calcite + goethite + anatase; (iii) the cockade-bearing veins are associated with minor fault veins filled with fine fragments (<300 μm in size) cemented by the same minerals of the cockade rims.We propose that the cockade-bearing faults formed at shallow crustal depths (<2 km) and recorded the main phases of the seismic cycle: (1) co-seismic fragmentation of the wall rocks in presence of fluids; (2) co-seismic fluidization of the rock fragments resulting in elutriation of the finer particles and formation of residual porous and well-sorted slipping zones, where cockades will nucleate. Inverse grading resulted from co-seismic shaking and shearing; (3) post-seismic to inter-seismic cementation by deposition of carbonate-rich rims due to slow mineral pressure growth, resulting in the suspension of the clasts within the slipping zones. The formation mechanism of cockade breccias proposed here provides an alternative view of earthquake-related processes in fluid-rich environments at shallow crustal depths.