71 research outputs found
Effect of biological polymers on mobility and run-out distance of cohesive and non-cohesive sediment gravity flows
Mixed sandâmud bedforms produced by transient turbulent flows in the fringe of submarine fans: Indicators of flow transformation
The fringe of fineâgrained deepâmarine systems often exhibits complex sedimentary facies and facies associations, because the presence of clay promotes the development of transient turbulent flows with complex depositional properties. Relatively little is known about the variation of currentâinduced sedimentary structures found within these facies. This study provides the first comprehensive description and interpretation of mixed sandstoneâmudstone bedforms observed in the fringe of the mudârich submarine fan that makes up the Aberystwyth Grits Group and Borth Mudstone Formation (Wales, United Kingdom). Using textural and structural descriptions, 158 bedforms in sediment gravity flow deposits were characterized into three main types: âclassicâ sandy current ripples, large current ripples and lowâamplitude bedâwaves. The sandy current ripples comprise clean sandstone, with average heights and lengths of 11 mm and 141 mm, respectively. The large current ripples are composed of mixed sandstoneâmudstone and possess greater dimensions than the sandy current ripples, with an average height of 19 mm and an average length of 274 mm. The lowâamplitude bedâwaves are long thin bedforms composed commonly of mixed sandstoneâmudstone, with an average height and length of 10 mm and 354 mm, respectively. The large current ripples and lowâamplitude bedâwaves are strikingly similar to experimental bedforms produced under decelerating mixed sandâmud flows and are interpreted to form beneath transitional flows with enhanced and attenuated nearâbed turbulence, respectively. From the fringe to the distal fringe of the fan, the dominant bedform type changed from sandy current ripples, via large current ripples, to lowâamplitude bedâwaves, suggesting that the flows changed from turbulent to increasingly turbulenceâmodulated. It is proposed that the flow Reynolds number reduced, reflecting this flow transformation, from a combination of constant or decreasing flow height, flow deceleration from sediment deposition, and increasing flow viscosity due to the shearâthinning nature of clayârich suspensions. Large current ripples and lowâamplitude bedâwaves are likely to be common in the fringe of other submarine fans. The presence and spatial trends in mixed sandâmud bedform types may be an important tool in interpreting fan fringe environments
Sole marks reveal deep-marine depositional process and environment: implications for flow transformation and hybrid event bed models
Discontinuity in Equilibrium WaveâCurrent Ripple Size and Shape and Deep cleaning associated with Cohesive SandâClay Beds
Streamwise turbulence modulation in non-uniform open-channel clay suspension flows
Cohesive sediment particles are ubiquitous in environmental flows. The cohesive properties of clay promote the formation of clay flocs and gels and relatively small suspended clay concentrations can enhance or suppress turbulence in a flow. Furthermore, flows are naturally non-uniform, varying in space and time, yet the dynamics of non-uniform open-channel clay suspension flows is poorly understood. For the first time, the adaptation time and length scales of non-uniform clay suspension flows were quantified using novel experiments with spatially varying but temporally uniform flow. Different levels of turbulence enhancement and attenuation were identified as the flow decelerates or accelerates. Results highlight that decelerating clay suspension flows crucially have a longer adaptation time than accelerating clay suspension flows. This is explained by the longer timescale required for the formation of bonds between cohesive particles in turbulence attenuated flows after deceleration than the rapid breakdown of bonds in turbulent flows after acceleration of clay suspension flows. This hysteresis is more pronounced for higher concentration decelerating flows that pass through a larger variety of clay flow types of turbulence enhancement and attenuation. These different adaptation time scales and associated clay flow type transitions are likely to affect clay flow dynamics in a variety of fluvial and submarine settings
Dynamics and Deposition of Sediment-Bearing Multi- Pulsed Flows and Geological Implication
Copyright © 2019, SEPM (Society for Sedimentary Geology) Previous studies on dilute, multi-pulsed, subaqueous saline flows have demonstrated that pulses will inevitably advect forwards to merge with the flow front. On the assumption that pulse merging occurs in natural-scale turbidity currents, it was suggested that multi-pulsed turbidites that display vertical cycles of coarsening and fining would transition laterally to single-pulsed, normally graded turbidites beyond the point of pulse merging. In this study, experiments of dilute, single- and multi-pulsed sediment-bearing flows (turbidity currents) are conducted to test the linkages between downstream flow evolution and associated deposit structure. Experimental data confirm that pulse merging occurs in laboratory-scale turbidity currents. However, only a weak correspondence was seen between longitudinal variations in the internal flow dynamics and the vertical structure of deposits; multi-pulsed deposits were documented, but transitioned to single-pulsed deposits before the pulse merging point. This early transition is attributed to rapid sedimentation-related depletion of the coarser-grained suspended fraction in the laboratory setting, whose absence may have prevented the distal development of multi-pulsed deposits; this factor complicates estimation of the transition point in natural-scale turbidite systems
The hiding-exposure effect revisited:A method to calculate the mobility of bimodal sediment mixtures
Predicting seabed mobility is hampered by the limited accuracy of sediment transport models when the bed is composed of mixed sediments. The hiding-exposure (HE) effect modifies the threshold of motion of individual grain classes in sediment mixtures and its strength is dependent on the grain size distribution. However, an appropriate method of predicting this effect for bimodal sediment mixtures remains to be developed. The prototypical example of a bimodal mixture is that consisting of a well-sorted sand and gravel for the fine and coarse fractions respectively. Through a comprehensive series of laboratory experiments, the HE effect has been quantified for a full range of sand-gravel mixtures from pure sand to pure gravel, the choice of which has been underpinned by an integrated study of offshore geophysical and sedimentological data found in coastal and shelf seas. In the sandâgravel mixtures used in the present study the critical shear stress needed to mobilise the sand and gravel fractions increased by up to 75% and decreased by up to 64%, respectively, compared to that needed to mobilise well-sorted sediment of similar size. The HE effect was found to be dependent on the percentage of gravel (coarse mode) present in the bimodal mixture, whereby the effect for the mixture is the weighted sum of the HE effect for the fine and coarse modes
Dynamics and deposition of sediment-bearing multi-pulsed flows and geological implication
Previous studies on dilute, multi-pulsed, subaqueous saline flows have demonstrated that pulses will inevitably advect forwards to merge with the flow front. On the assumption that pulse merging occurs in natural-scale turbidity currents, it was suggested that multi-pulsed turbidites that display vertical cycles of coarsening and fining would transition laterally to single-pulsed, normally graded turbidites beyond the point of pulse merging. In this study, experiments of dilute, single- and multi-pulsed sediment-bearing flows (turbidity currents) are conducted to test the linkages between downstream flow evolution and associated deposit structure. Experimental data confirm that pulse merging occurs in laboratory-scale turbidity currents. However, only a weak correspondence was seen between longitudinal variations in the internal flow dynamics and the vertical structure of deposits; multi-pulsed deposits were documented, but transitioned to single-pulsed deposits before the pulse merging point. This early transition is attributed to rapid sedimentation-related depletion of the coarser-grained suspended fraction in the laboratory setting, whose absence may have prevented the distal development of multi-pulsed deposits; this factor complicates estimation of the transition point in natural-scale turbidite systems
Repetitive Late Pleistocene softâsediment deformation by seismicityâinduced liquefaction in northâwestern Lithuania
Liquefaction can cause deformation of unconsolidated sediment, but specific processes involved and the trigger mechanisms often remain obscured. This study describes multiple deformed sediment layers in a succession of lacustrine sand, silt and clay deposited during the Marine Isotope Stage 5d in north-western Lithuania. The deformation structures (load casts, pseudonodules, ball-and-pillow structures, broken-up laminae and injections) are embedded in ten separate layers of fine-grained, laterally continuous sediments. Detailed mesoscale sedimentological analyses suggest that each deformation event consisted of numerous successive stages of sediment advection facilitated by liquefaction. Low-permeability fine-grained laminae contributed to localized pore-water pressure build-up and lowering of sediment strength. Erosional top surfaces that truncate layers with soft-sediment deformation structures suggest that at least seven deformation events were separated by successive periods of initial erosion and then uninterrupted deposition in the lake. The most likely trigger of the deformation was recurrent palaeoseismic activity possibly linked to a late glacial isostatic adjustment following the Scandinavian Ice Sheet melting after the Saalian glaciation. This study emphasizes the potential role of seismic processes in shaping the sedimentary record of the intraplate region of north-eastern Europe and contributes to constraining the depth of liquefaction, regardless of the actual trigger mechanism
Current- and wave- generated bedforms on mixed sand-clay intertidal flats : a new bedform phase diagram and implications for bed roughness and preservation potential
Funding: This work was supported by the United Kingdomâs Natural Environment Research Council (NERC) under Grant NE/I027223/1 (COHBED). JM, Julie Hope, and Daniel Parsons were partially funded by a Horizon 2020 European Research Council Consolidator Award (GEOSTICK, Grant 725955). The GEOSTICK project also kindly contributed the article processing fees. Andrew Manningâs contribution toward this research was partly supported by the National Science Foundation grants OCE-1924532 and OCE-1736668, TKI-MUSA project 11204950-000-ZKS-0002, and the HR Wallingford company research project FineScale (Grant Nos. ACK3013_62). DP received funding from the MASTS pooling initiative (The Marine Alliance for Science and Technology for Scotland) and their support is gratefully acknowledged. MASTS is funded by the Scottish Funding Council (Grant reference HR09011) and contributing institutions.The effect of bedforms on frictional roughness felt by the overlying flow is crucial to the regional modelling of estuaries and coastal seas. Bedforms are also a key marker of palaeoenvironments. Experiments have shown that even modest biotic and abiotic cohesion in sand inhibits bedform formation, modifies bedform size, and slows bedform development, but this has rarely been tested in nature. The present study used a comprehensive dataset recorded over a complete springâneap cycle on an intertidal flat to investigate bedform dynamics controlled by a wide range of wave and current conditions, including the effects of waveâcurrent angle and bed cohesion. A detailed picture of different bedform types and their relationship to the flow, be they equilibrium, non-equilibrium, or relict, was produced, and captured in a phase diagram that integrates wave-dominated, current-dominated, and combined waveâcurrent bedforms. This bedform phase diagram incorporates a substantially wider range of flow conditions than previous phase diagrams, including bedforms related to near-orthogonal waveâcurrent angles, such as ladderback ripples. Comparison with laboratory-derived bedform phase diagrams indicates that washed-out ripples, lunate interference ripples and upper-stage plane beds replace the subaqueous dune field; such bedform distributions may be a key characteristic of intertidal flats. The field data also provide a means of predicting the dimensions of these bedforms, which can be transferred to other areas and grain sizes. We show that an equation for the prediction of equilibrium bedform size is sufficient to predict the roughness, even though the bedforms are highly variable in character and only in equilibrium with the flow for approximately half the time. Whilst the effect of cohesive clay is limited under more active spring conditions, clay does play a role in reducing the bedform dimensions under more quiescent neap conditions. We also investigated which combinations of waves, currents, and bed clay contents in the intertidal zone have the highest potential for bedform preservation in the geological record. This shows that combined waveâcurrent bedforms have the lowest preservation potential and equilibrium current ripples have the highest preservation potential, even in the presence of moderate and storm waves. Hence, the absence of wave ripples and combined-flow bedforms and their primary stratification in sedimentary successions cannot be taken as evidence that waves were absent at the time of deposition.Publisher PDFPeer reviewe
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