Slope Deposits Research Articles

Corrigendum: Experimental study on landslides of loose sediment slope induced by stream bed incision

In the published article, there was an error in Figure 1 as published. Due to negligence, the ‘Landslide site’ in Figure 1 was mistakenly written as ‘Landslide stie’. The corrected Figure 1 and its caption appear below.The authors apologize for this error and state that this does not change the scientific conclusions of the article in any way. The original article has been updated.

Development and Emplacement of Ana Slide, Eivissa Channel, Western Mediterranean Sea

AbstractSubmarine landslides can destroy seafloor infrastructures and generate devastating tsunamis. In spite of decades of research into the functioning of submarine landslides there are still numerous open questions, in particular how different phases of sliding influence each other. Here, we re‐analyze Ana Slide—a relatively small (<1 km3) landslide offshore the Balearic Islands, which is unique in the published literature because it is completely imaged by high‐resolution 3D reflection seismic data. Ana Slide comprises three domains: (a) a source area that is almost completely evacuated with evidence of headscarp retrogression, (b) an adjacent downslope translational domain representing a by‐pass zone for the material that was mobilized in the source area, and (c) the deposit formed by the mobilized material, which accumulated downslope in a sink area and deformed slope sediment. Isochron maps show deep chaotic seismic units underneath the thickest deposits. We infer that the rapid deposition of the landslide material deformed the underlying sediments. A thin stratified sediment unit between three lobes suggests that Ana Slide evolved in two failure stages separated by several tens of thousands of years. This illustrates the problem of over‐estimating the volume of mobilized material and under‐estimating the complexity even of relatively simple slope failures without high‐quality 3D reflection seismic data.

Cenozoic Depositional Evolution and Stratal Patterns in the Western Pearl River Mouth Basin, South China Sea: Implications for Hydrocarbon Exploration

Investigating the deposition evolution and stratal stacking patterns in continental rift basins is critical not only to better understand the mechanism of basin fills but also to reveal the enrichment regularity of hydrocarbon reservoirs. The Pearl River Mouth Basin (PRMB) is a petroliferous continental rift basin located in the northern continental shelf of the South China Sea. In this study, the depositional evolution process and stacking pattern of the Zhu III Depression, western PRMB were studied through the integration of 3D seismic data, core data, and well logs. Five types of depositional systems formed from the Eocene to the Miocene, including the fan delta, meandering river delta, tidal flat, lacustrine system, and neritic shelf system. The representative depositional systems changed from the proximal fan delta and lacustrine system in the Eocene–early Oligocene, to the tidal flat and fan delta in the late Oligocene, and then the neritic shelf system in the Miocene. The statal stacking pattern varied in time and space with a total of six types of slope break belts developed. The diversity of sequence architecture results from the comprehensive effect of tectonic activities, sediment supply, sea/lake level changes, and geomorphic conditions. In addition, our results suggest that the types of traps are closely associated with stratal stacking patterns. Structural traps were developed in the regions of tectonic slope breaks, whereas lithological traps occurred within sedimentary slope breaks. This study highlights the diversity and complexity of sequence architecture in the continental rift basin, and the proposed hydrocarbon distribution patterns are applicable to reservoir prediction in the PRMB and the other continental rift basins.

Eocene Terrane Accretion in Northern Cascadia Recorded by Brittle Left‐Lateral Slip on the San Juan Fault

AbstractThe San Juan fault (SJF), on southern Vancouver Island, Canada, juxtaposes the oceanic Wrangellia and Pacific Rim terranes in the northern Cascadia forearc, and has been suggested to play a role in multiple Mesozoic‐Cenozoic terrane accretion events. However, direct observations of the SJF's kinematics have not been documented and its exact role in accommodating strain arising from terrane accretion is unknown. To test if, how, and when the SJF accommodated accretion‐related strain, we use geologic mapping, kinematic inversion of fault‐plane slickenlines, and dating of marine sediments to constrain the timing and direction of brittle slip of the SJF. P‐ and T‐axes from kinematic inversions indicate predominantly left‐lateral slip. Left‐lateral brittle faulting cross‐cuts ∼51 Ma magmatic intrusions and foliation, providing a maximum age of brittle deformation. The fault zone is non‐conformably overlain by a >300 m‐thick sequence of clastic marine shelf and slope sediments that are not left‐laterally offset. A strontium isotope age of foraminifers helps constrain the depositional age of the sediments to late Eocene–early Oligocene, bracketing left‐lateral slip to the Eocene. Eocene left‐lateral slip is temporally and kinematically consistent with regional southwest‐northeast compression during accretion of the Siletzia ocean island plateau, suggesting brittle slip on the SJF accommodated strain resulting from the accretion of this terrane. This result does not support hypotheses that brittle slip along the SJF directly accommodated earlier accretion of the Pacific Rim terrane to Wrangellia, instead it offsets the older accretionary boundary between these two terranes.

Imaging of karst cave using ground penetrating radar and electrical resistivity tomography: a case study in Shandong, China

A denudation-dissolution hilly area about 20km southeast of Jinan city, Shandong Province, China is characterized by karst features. The upper part of the area is covered by quaternary Holocene flood and slope deposits, and the lower bedrock is Ordovician limestone, which is a moderately weathered massive structure with developed joints and fissured fractures, and solution grooves can be seen locally. The local government to better serve the surrounding residents and the surrounding facilities, here is planned for the substation, for later security construction, electrical resistivity tomography (ERT) method and ground penetrating radar (GPR) method were used to investigate the underground karst caves. The aim of the joint use of the two methods is to better describe the characteristics of karst landform, a 160 MHz shielding antenna was employed in the GPR survey, while the frequency limits the scope of the depth of the electromagnetic waves can reach. However, it provides a lot of shallow stratigraphic information, which well represents the boundary between overburden and bedrock and the development of karst. The use of ERT method further makes up for the depth defects of the GPR method, and two-dimensional imaging of underground resistivity distribution was carried out, which also helps to further identify the characteristics of underground karst. The combination of the two geophysical methods can accurately delineate the development of underground karst, which also proves that the combination of the GPR method and ERT method has good application value in karst landform detection, and can also be applied to a similar geological survey.

The mid-Cretaceous Peninsular Ranges orogeny: a new slant on Cordilleran tectonics? III: the orogenic foredeep

The Cretaceous Western Interior Basin reflects the interplay between the North American craton and allochthonous terranes to the west. We divide the basinal stratigraphy into three successions, Aptian–Albian, Cenomanian–Turonian, and Santonian–Maastrichtian, each related to periods of deformation in the adjacent fold-thrust belt. Here we focus on the Cenomanian–Turonian succession, where progressive west to east uplift and fluvial incision of older Aptian–Albian sedimentary rocks (Cedar Mtn–San Pitch–Thermopolis–Skull Creek–Mannville) are interpreted as a migrating forebulge. Uplift was underway at 103 Ma in the west (Paddy–Blackleaf–Muddy sandstones) and propagated eastward throughout the trough by 99.5 Ma (Viking–Bow Island–Newcastle sandstones). The incised fluvial valleys were subsequently filled by swampy and shallow marine facies, then overlain by dark, marine Neogastroplites-bearing shale and associated bentonites of the 100–97.5 Ma Shell Creek–Mowry–Slater River–Goodrich–Shaftesbury–Westgate shales. The shales are characterized by a distinctive condensed horizon with abundant fish scales, teeth, and bones. They are interpreted as outer-trench slope deposits, with the overlying anoxic horizon representing a starved isochronous unit formed atop the slope deposits. The starved horizon is overlain by prodeltaic muddy clinoforms of easterly migrating clastic wedges (Trevor–Dunvegan–Frontier–Cintura–Mexcala) that can be traced 800 km atop the fish-scale hash and contain hinterland-derived 99–90 Ma detrital zircons. Although the Western Interior Basin has long been considered a retro-arc trough, the overall succession instead suggests that the Cretaceous–Turonian part represents a collisional foredeep created during the ∼100 Ma collision between the arc-bearing Peninsular Ranges composite terrane and North America. The accretion brought tyrannosaurids, pachycephalosaurs, snakes, and marsupials to North America.

High Carbon Mineralization Rates in Subseafloor Hadal Sediments—Result of Frequent Mass Wasting

AbstractIn the past 20 years, the exploration of deep ocean trenches has led to spectacular new insights. Even in the deepest canyons, an unusual variety of life and unexpectedly high benthic oxygen consumption rates have been detected while microbial processes below the surface of the hadal seafloor remains largely unknown. The information that exist comes from geophysical measurements, especially related to seismic research, and specific component analyses to estimate the carbon export. In contrast, no information is available on metabolic activities in deeper buried sediments of hadal environment. Here we present the first pore water profiles from 15 up to 11 m long sediment cores recovered during three expeditions to two hadal zones, the Japan Trench and the Atacama Trench. Despite low levels of organic debris, our data reveal that rates of microbial carbon turnover along the trench axes can be similar to those encountered in much shallower and more productive oceanic regions. The extreme sedimentation dynamics, characterized by frequent mass wasting of slope sediments into the trenches, result in effective burial of reactive, microbially available, organic material. Our results document the fueling of the deep hadal biosphere with bioavailable material and thus provide important understanding on the function of deep‐sea trenches and the hadal carbon cycle.

Sedimentary geochemistry of deepwater slope deposits in southern Lake Tanganyika (East Africa): Effects of upwelling and minor lake level oscillations

AbstractLake Tanganyika ranks among the most valuable modern analogs for understanding depositional processes of carbonaceous sediments in ancient tropical rifts. Prior research on Lake Tanganyika has emphasized the importance of bottom-water anoxia, depositional processes (hemipelagic settling versus gravity flows), and large-scale (100s of meters) lake level change on the quality of sedimentary organic matter content. Here, facies analysis and numerous organic geochemical tools (elemental, carbon isotope, and programmed pyrolysis) were applied to a radiocarbon-dated core from southern Lake Tanganyika to investigate the accumulation of carbonaceous sediments in a deepwater slope environment influenced by high-frequency climatic fluctuations accompanied by only minor (10s of meters) lake level changes. Considerable variability in lithofacies and geochemistry characterizes the ∼ 1030-year-long core record, chiefly driven by climate-mediated changes to the lake's upwelling system. Laminated diatom oozes and sapropels with mean total organic carbon (TOC) concentrations and hydrogen indices of 6.9 wt.% and 385 mg hydrocarbon/g TOC, respectively, characterize sediments deposited during periods of strong upwelling and variable water levels. Silty sediments deposited via gravity-flow processes were likewise rich in organic matter, likely due to preservation-enhancing bottom-water anoxia. Dilution by reworked tephra was the chief constraint on organic enrichment at the study site. Data from this study reveal that oscillations in atmospheric and limnological processes in the absence of major shoreline movements can result in geochemically diverse deepwater slope sediments, which have implications for improving depositional models of petroliferous continental rift basins.

Estimating sediment transport diffusion coefficients from reconstructed rifted margin architecture: measurements in the Ogooué and Zambezi deltas

AbstractDiffusion‐based stratigraphic models are widely used to simulate sedimentary systems and margin deltas. Diffusion‐based models assume that the topographic evolution primarily depends from its slope. Limited attention has however been given to the calibration of the transport coefficients. Here, we evaluate transport coefficient values from natural examples, the Ogooué and Zambezi rifted margin deltas over the last 5 to 12 Ma respectively. We developed a method to estimate transport coefficients based on high resolution seismic stratigraphy analysis of the stratigraphic architecture of these deltas. For each stratigraphic sequence, we calibrated the sand/shale ratios of the deposits, we restored their depositional slopes, we estimated their uncompacted accumulated volumes and we calculated the transport coefficient (Kd) from the sediment flux/slope ratio. Estimated values of Kd fall within one order of magnitude (×0.1 km2/ka), a much narrower range than previously published values (×0.0001 to ×100 km2/ka). We show that the diffusion approximation is optimal at 10–100 km scale and 0.5–1 Ma time resolution, independently of the stratigraphic context. We show that the diffusion assumption is appropriate for the formation of the clinoforms (mainly gravity driven). It is however not optimal for the shelf and distal domains where additional processes (e.g., wave, flood, hemipelagic, turbidites, oceanic current), not accounted for it the diffusion assumption, significantly impact sediment transport. We documented a significant increase of Kd values after 0.9 Ma, coeval of an increase in the amplitude of eustatic variations at this time indicating that the calibration of Kd from present day sedimentary systems might not be optimal for simulations of sedimentary systems before the last million years.

Effects of material migration on the spatial distribution of topsoil moisture at the slope scale

Soil moisture is strongly spatially variable and scale dependent, but there is a lack of research focusing on factors that link soil actual evaporation to the spatial distribution of topsoil moisture at the slope scale. External factors (e.g., meteorological) have similar effects across entire slopes; hence, this paper instead explores the internal factors controlling differential soil actual evaporation. We consider the effects of historical rainfall on soil erosion, and adopt a combined approach encompassing slope investigations, a slope rainfall simulation experiment, slope erosion analyzed by 3D laser terrain scanning, and evaporation experiments. Along the runoff direction, we show that the relative topsoil clay content increases, but slope surface deformation and failure lead to a decrease in the relative content of clay in deposits located on the lower of slope. The original thickness of slope surface decreases in the upper part of the slope, due to particle migration, while the slope surface relatively far away from the upper part of the slope tends to be thicker, due to the accumulation of slope deposits. The actual evaporation of the topsoil relatively far from the upper part of the slope is significantly weaker (63%–72% probability), which may be related to the high clay content, dense soil structure, and high content of clay minerals. The topsoil evaporation also tends to weaken as overburden thickness increases. Here, we suggest that marked differences in the original thickness of the slope surface and in the properties of the topsoil caused by the migration and deposition of soil particles under successive years of rainfall erosion, are important factors controlling differences in topsoil actual evaporation in different parts of the slope. This understanding lays the foundation for the analysis of hydrological processes in different parts of a slope, which is crucial for the study of shallow slope mechanical behavior.

Failure process of a lateral slope deposit and its effect on debris flood formation

Failure process of a lateral slope deposit and its effect on debris flood formation

The starting mechanism study on rainfall-induced sloop loose source in strong earthquake area

The 5.12 Wenchuan earthquake in 2008 led to a large number of loose deposits on the slope on both sides of the gully in the formation area. Under the triggering action of heavy rainfall, the previously unrecognized slope loose source in formation areas easily failed and started earlier than the overall start of the gully debris flow, which has practical significance for realizing the early warning of the gully debris flow starting. The special loose source conditions and postearthquake geological environment change the hydraulic mechanism of traditional debris flow start-up, which brings great difficulties to the monitoring and early warning of postearthquake debris flows. Therefore, based on hydraulics, on the premise of building a hydraulic model of groundwater level change of loose slope accumulation body, the characteristics and rules of groundwater level change are analyzed with the aid of hydraulic seepage theories, and the action characteristics of hydrodynamic pressure and hydrostatic pressure on the accumulation body are quantitatively studied. Based on field exploration data and average slope gradient, we divided the accumulation body in a reasonable manner; completed the sliding force, anti-sliding force and residual-sliding force mechanical expression and establishment; and performed a slope stability analysis. The research results show that the slope starting mode is divided into the thrust-type landslide and the retrogressive landslide mode of the subsection disintegration. The hydraulic deposit mechanism analytical results can also reverse the critical rainfall condition of slope failure. Finally, taking the Yindongzi gully in Dujiangyan as an example, the hydraulic mechanism of typical slope deposits in the formation area was analyzed and combined with the historical debris flow events in the study area to verify the physical simulation test, which was consistent with the field investigation results after the disaster.

Lithofacies variability of the Yarakta horizon as a complex result of geodynamic processes of tectonogenesis and sedimentogenesis

The purpose of the research is to analyze the lithological and facies conditions of formation and the causes of lithological composition variability of the Yarakta horizon deposits as well as formation and distribution features of reservoir rocks in the horizon. The paper uses the data on drilling and geophysical work results and research materials from the public sources. The Yarakta horizon is commercially productive in the deposits of the southeastern slope and the central part of the Nepa-Botuoba anteclise: Dulisminskoe, Yaraktinskoe, Ayanskoe, Danilovskoe deposits, etc. The deposits of the Yarakta horizon were formed during the initial transgressive stage of the Vendian sedimentation cycle and are characterized by extreme lithological irregularity of rock composition and thickness. Investigations of a number of scientists testify that in the early formation of the Yarakta horizon its territory in geomorphological terms was predominantly presented by a coastal plain, which was occasionally flooded by the sea. The rocks in the area are represented by the facies of temporary proluvial-deluvial flows of the coastal plain replaced by coastal shallow sediments. The change in the lithological composition of rocks and thicknesses of the Yarakta horizon is a result of changes in the nature of oscillatory movements, while its structural and textural features are determined mainly by the dynamics of water flows of the sedimentation medium and the paleorelief nature of the underlying surface. The conducted research allows to conclude that lithological heterogeneity of the Yarakta horizon in sections (vertically) and along the strike (laterally) is determined by the frequent alternation of various lithotypes, the formation of which is determined by the complex nature of tectonic movements and irregularity features of sedimentary layer facies.

Multiple Types of Porosity – P‐Wave Velocity Relationships for the Nankai Trough

The empirical relationship between the porosity and P-wave velocity is a useful tool for probing large-scale underground physical properties and stress states based on P-wave velocity structures acquired by seismic surveys. In this study, the porosity-velocity curves were examined using local core sample and logging datasets acquired along the Kumano transect in the Nankai Trough, Southwest Japan. We tested Hashimoto et al.’s (2010) hypothesis that slope apron and accreted sediments have different relationships. Our advantage is using a large amount of logging and core data obtained at multiple sites in the Nankai Trough under various geological conditions, from the incoming oceanic plate to the inner wedge. We identified multiple types of porosity – P-wave velocity relationships: 1) The first type agrees with the low-velocity model in the global empirical relation of Erickson and Jarrard 1998), which is observed at lithostatic stress state condition (incoming upper Shikoku basin and slope sediments), 2) The second type consistent with high-velocity models of the global empirical relation observed at compressive stress state condition (accreted sediments), which have a higher velocity and larger dependence on the porosity than the first type with the same porosity, and 3) The third type for incoming sediments with high smectite content has lower velocity than the first type. Based on our results, the transition between the first and second porosity – velocity relationships occurs in the prism toe, implying that compressive stress due to subduction controls acoustic properties and the lithification process of these sediments.

Stratal characteristic and depositional origin of two-part (Mud-poor overlain by mud-rich) and associated deep-water strata: Components in a lateral depositional continuum related to particle settling in negligibly sheared mud-rich suspensions

AbstractDeep-marine strata consisting of a sandy basal part overlain sharply by a muddier and mud-clast-rich upper part are increasingly being recognized in the deep-marine sedimentary record, and have been termed linked debrites, cogenetic debrite–turbidite beds, hybrid event beds, transitional-flow deposits, and bipartite beds. These composite strata are generally reported to develop downflow of clean (less muddy) sand, but the variable distance (hundreds of meters to tens of kilometers) over which this lithological change is reported to take place has resulted in a variety of physical depositional models. As such, the details about the origin and spatial evolution of these admixed sand–mud strata still remain poorly understood. Part of this uncertainty is that many earlier studies, with a few exceptions, have been based on macroscopic observations in drill core or discontinuous outcrops, and therefore interpreted bed continuity is based on similarity of textural or other sedimentological attributes. However, in slope to proximal basin-floor deposits of the Neoproterozoic Windermere Supergroup, and distal basin-floor deposits of the Ordovician Cloridorme Formation, exceptional outcrop exposure permitted individual two-part (bipartite) facies and their associated strata to be continuously traced and lithological changes to be documented along the strike of the outcrop.In both the Windermere and the Cloridorme, the bipartite facies consists of a lower sandy (25–60% matrix) part overlain sharply by a muddier (40–80% matrix), commonly mud-clast-rich upper part, and occurs downflow of muddy sandstone and upflow of sandy mudstone in a hundreds-of-meters long proximal to distal depositional continuum. These strata are then overlain everywhere by a thin- to very thin-bedded, traction-structured sandstone and/or silty mudstone cap. Notably, the interface separating the lower sand-rich part from the upper mud-rich part in the bipartite part of the depositional transect is always planar in the Windermere, but often alternates between planar and irregular in the Cloridorme. The continuum is interpreted to represent deposition downflow of an avulsion node that resulted in erosion of the local mud-rich seafloor and charged the flow with fine-grained sediment, resulting in increased effective fluid viscosity and significant changes in the characteristics, intensity, and transfer of fluid turbulence needed to maintain the particle suspension. Collapse of the suspension and development of overcapacity conditions resulted in rapid particle settling in the now rapidly depleting and negligibly sheared mud-enriched suspension that formed along the margins of the avulsion-wall jet in proximal-basin-floor and slope deposits (Windermere), or at the downflow terminus of the flow in more distal basin-floor deposits (Cloridorme). Additionally, when normalized to the total length of the continuum, the transition from muddy sandstone to bipartite facies and bipartite facies to sandy mudstone are equally proportioned in all examples, suggesting that, once formed, the depleting suspension evolved in a spatially systematic and dimensionally proportionate manner—a consistent depositional evolution difficult to reconcile with two mechanistically different parts to the flow. Moreover, differences in the morphology of the bipartite interface (i.e., planar vs. irregular) is not a primary depositional feature, but instead is interpreted to be a consequence of local postdepositional deformation of a previously continuous planar surface.

Significance of Granulometric Composition and Physicochemical Properties for Interpreting the Lake Grand Sedimentation

Abstract —The paper is concerned with a study of the slope deposits being weathering products of rhyolites and their tuffs in the vicinity of Lake Grand (northern Okhotsk area). The samples were divided into grain size fractions of 2500, 250, 140, 100, 63, 40, 20, and 1 μm. Analysis for major and trace elements and mineralogical and petrophysical studies were performed for each fraction. It is shown that a decrease in the fraction size is accompanied by the enrichment of the sediments with Al2O3, Fe2O3, TiO2, MgO, Y, Rb, Ni, and paramagnetic minerals and by an increase in LOI. The fine fractions are characterized by low contents of SiO2 and Na2O and high values of CIA, PIA, and Rb/Sr. The maximum magnetic susceptibility, saturation magnetization, and contents of CaO, Sr, and Zr are established in the fractions of 40 μm. The values of magnetic susceptibility, Js, and Jrs decrease in passing from the fractions of 40 μm to the fractions of 1 μm, whereas the values of Bc and Bcr increase. A specific feature of these fractions is high paramagnetic susceptibility and the presence of lepidocrocite. The qualitative composition of ferrimagnetic minerals is the same for the slope deposits and lacustrine sediments. In passing from coarse fractions to fine ones, the contents of quartz and K-feldspar decrease, and the contents of muscovite, biotite, chlorite, and kaolinite increase. The distribution of petromagnetic and geochemical characteristics in the Lake Grand sediments of varying grain size is similar to that in the slope deposits. The complex characteristics of the lacustrine sediments that formed in cold and warm periods are consistent with the distribution of these parameters in various fractions of deluvium and colluvium. Moreover, the fine-grained material acquired specific geochemical, mineralogical, and petrophysical properties when being on the slopes. Comparison of the SiO2/TiO2 and Fe2O3/TiO2 ratios of the slope deposits and lacustrine sediments makes it possible to identify the intervals of biogenic and chemogenic sedimentation in water basins.

Coal and shale distributions on half-graben slopes: A case study of the Eocene Pinghu Formation in Xihu Sag, East China Sea

Steep and gentle slopes in a half-grabben basin often form oil source rocks of coal and shale. However, which source rock is dominant and what the relation is with the slope type and sedimentary facies are not clear. The Pinghu Formation in Xihu Sag is a half-graben basin with coal seam as the main source rock. Five condensate reservoirs have been found in its west slope, with several wells and 3D seismic data, which makes it possible to study the relationship between slope types and source rocks. We have used logging data, lithologic identification statistics, and seismic inversion to reconstruct the sedimentary facies and paleoslopes in the western Xihu Sag and have analyzed the variations in the proportions of coal and shale in the study area and their relationships with the facies, slope types, and subsag. The results indicate that the upper slope contained a higher proportion of coal than the lower slope. In contrast, the lower slope contained more shale than the upper slope. Coal is dominant in the delta plain and front facies, whereas shales are dominant in the prodelta facies. Although the proportions of coal and shale vary among the southern hinged margin, the central escarpment margin, and the northern hinged margin, the thicknesses of the coal and shale increases northward, independent of the rift margin type. These variations may be related to the filling of the delta from north to south during the deposition of the Eocene Pinghu Formation. These results also suggest that there were no substantial differences in the depositional slopes and water depths of the hinged and escarpment margins. The abundance of shales in the lower slope and at the center of the depression suggest that, in addition to coal, shales should have contributed to petroleum generation in Xihu Sag.

Asymmetric abundances of submarine sediment waves around the Azores volcanic islands

Sediment waves are common on the submarine flanks of volcanic islands, but the processes that form them are difficult to decipher from geophysical data alone. Here, we identify downslope-trending trains of sediment waves in multibeam sonar data from five islands of the Azores. Train abundances are derived by counting the train intersections with depth contours around each island, revealing that they are typically twice as common on the north slopes of the islands compared with their south slopes. To provide context, we also characterise the Holocene mobility of sediments on the shelves, ocean density structure (for evaluating internal wave critical gradients) and sediment lithologies in four cores from beneath the slopes, which contain shelf carbonates in some volcaniclastic turbidites. Holocene mobility is assessed using sidescan sonar data, which reveal shelf-crossing sand streams. Using results of a high-resolution wave model, wave-imposed shear stresses are found to have exceeded the stresses at threshold of motion for most of the modern shelf sediments during recent extreme wave conditions.Sediment waves could potentially arise from sedimentary flows, gravity mass movements (shallow slumping) or ocean internal waves. Sedimentary flow origins are favoured by streamwise changes of wavelengths and upslope bedform migrations suggested by seismic reflections. The asymmetry of abundances north and south of each island is also an important clue. For example, it is unlikely that systematically different sediment geotechnical properties occur on the two sides of the islands, hence no reason to expect greater mass movements on northern slopes. Internal waves originate mainly from the south and hence also do not explain the asymmetry. Primary volcaniclastic flows are likely to be equally important on both sides. Sedimentary flows originating from shelf sediment agitated during storms or from failed slope sediment appear to explain best the asymmetry. In that case, prevailing wind and waves approaching the islands from the northwest lead to more frequent sediment gravity flows because they lead to greater flux of sediment from coastal erosion towards northerly outer shelves and upper slopes, where it is prone to slope failure. Offshore-directed near-bed currents originating from northwesterly wind-driven surface currents also transport shelf sediment efficiently to the north, further aided by greater wave agitation on that side. The high abundances of wave trains on the north sides of the islands therefore favours origins involving sedimentary gravity flows associated with wind and wave effects.

Anti-Erosion Influences of Surface Roughness on Sloping Agricultural Land in the Loess Plateau, Northwest China

The roughness of surface soil and the benefits produced by tillage for slope runoff and sediment reduction have attracted considerable interest; however, there are inconsistencies in existing research results. In this study, we have studied the anti-erosion influences of several typical tillage practices on both runoff and sediment generation in areas of sloping farmland in the Loess Plateau of northwest China. Rough surfaces were prepared manually, according to the surface microtopography of the plateau’s sloping farmland, using four tillage practices; a smooth surface was used as a control. Rainfall simulation experiments were performed using three rainfall intensities and five slope gradients. A path analysis was used to analyze the interactive effects of the slope gradient, rainfall intensity, and the surface roughness on the sediment yield and runoff volume. According to our findings, the gradient of a slope and the intensity of the rainfall both had a positive effect, while the surface roughness had a negative effect; the rate of 40.8% and 21.0% was lower than the values under CK on sediment yield and runoff volume. The interaction between the rainfall intensity and surface roughness always had a runoff reduction effect. Conversely, there was a critical slope gradient between 5° and 10° for sediment yield. The interaction between the slope gradient and surface roughness also had a runoff reduction effect, which was diminished by increasing the rainfall intensity. However, their interactive influence on sediment yield was inconsistent, with a critical slope gradient between 10° and 15°. Based on the comprehensive interactive effects among all three factors, we concluded that rainfall intensity, slope gradient, and surface roughness collectively played a crucial role in promoting runoff and sediment generation under tillage. The results support soil and water conservation by tillage on the sloping farmlands of the Loess Plateau.

Quantification of the effect of loess admixture on soil hydrological properties in sandy slope deposits

The admixture of loess in soils formed in sandy parent materials has considerable impact on pedogenesis and soil ecological functions. This study aimed to evaluate these effects on soil hydrology by quantifying the relationships between loess content and soil hydrological properties in a sandstone landscape covered by Pleistocene periglacial slope deposits in central Europe. Studied properties were saturated soil water capacity, field capacity, permanent wilting point, available water capacity, macro-porosity, matrix porosity and saturated hydraulic conductivity. The effect of loess addition on soil hydrological properties differed between topsoil (pedogenic A horizons) and subsoil (pedogenic B and C horizons). In the subsoil, the studied soil hydrological properties were mainly controlled by loess content. By contrast, in the topsoil the effects of loess content on soil hydrological properties were largely masked by the effects of soil organic matter (reflected by soil organic carbon, SOC). The enhancing effect of SOC on soil hydrological properties was most prominent for saturated soil water capacity and was also significant for all other parameters except permanent wilting point. After the effects of SOC were accounted for, the residual effects of loess on soil hydrological properties were the same in both topsoils and subsoils, with one exception: Saturated hydraulic conductivity decreased with increasing loess content for subsoils, but not for topsoils. This study highlighted the ecohydrological significance of loess admixture in coarse-textured soils, especially for subsoils with low SOC contents. However, for coarse-textured topsoils, SOC content plays the dominant role in affecting soil hydrological properties.