Integrating transitional-flow signatures into hybrid event beds: Implications for hybrid flow evolution on a submarine lobe fringe

Alongside turbidites and debrites, hybrid event beds are now recognized as a common occurrence in deep-marine environments. Yet, many variations in the standard H1–H5 facies model of Haughton et al. (2009, Marine and Petroleum Geology, v. 26, p. 1900–1918) have been described since its introduction, with the role of transient-turbulent flows, i.e., flows that are transitional between fully turbulent turbidity currents and fully laminar debris flows, being particularly enigmatic. Based on a comprehensive dataset collected from the lobe fringe and distal fringe of a submarine fan (Silurian Aberystwyth Grits Group and Borth Mudstone Formation, West Wales, United Kingdom), transitional-flow signatures were integrated into the standard hybrid-event-bed model. These signatures include muddy sandstones and sandy mudstones with large ripples (formed by turbulence-enhanced transitional flows), low-amplitude bed waves and heterolithic lamination (formed by turbulence-attenuated transitional flows), and banding (formed by turbulence-enhanced to turbulence-attenuated transitional flows). The field data reveal that: (a) H1 divisions are generated by turbulent flows that form not only massive, structureless facies but also plane-parallel-laminated and ripple-cross-laminated facies; (b) H2 divisions are formed by transitional flows that form banded facies, but also facies with large ripples and low-amplitude bed waves, as well as heterolithic facies; (c) H3 divisions are formed by laminar debris flows of varied rheology; (d) H4 divisions can form from both tractional turbulent and transitional flows; and (e) H5 divisions can be hemipelagic, deposited from the dilute tail of the flow or originate from cohesive freezing of a late-stage muddy debris flow. Based on embedded Markov-chain analysis, the vertical stacking of facies in the five principal hybrid-event-bed divisions suggests a transformation from turbidity current via transitional flow to debris flow (H1 to H3), followed by a repetition of this transformation in the H4 and H5 divisions, but in overall finer-grained sediment. In addition to this complete extended facies model for hybrid event beds, three incomplete bed types could be defined: turbulent-flow-prone, transitional-flow-prone with an H3 division, and transitional-flow-prone without an H3 division. The sedimentary successions in the study area reveal a basinward change from predominantly turbidites and turbulent-flow-prone hybrid event beds via a mixture of turbulent-flow and transitional-flow signatures in hybrid events beds to H3 missing hybrid event beds with transitional-flow and muddy-debrite signatures. Hence, sediment gravity flows became increasingly muddy and cohesive from lobe fringe to lobe distal fringe

Mixed sand–mud bedforms produced by transient turbulent flows in the fringe of submarine fans: Indicators of flow transformation

The fringe of fine‐grained deep‐marine 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 current‐induced sedimentary structures found within these facies. This study provides the first comprehensive description and interpretation of mixed sandstone–mudstone bedforms observed in the fringe of the mud‐rich 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: ‘classic’ sandy current ripples, large current ripples and low‐amplitude bed‐waves. 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 sandstone–mudstone and possess greater dimensions than the sandy current ripples, with an average height of 19 mm and an average length of 274 mm. The low‐amplitude bed‐waves are long thin bedforms composed commonly of mixed sandstone–mudstone, with an average height and length of 10 mm and 354 mm, respectively. The large current ripples and low‐amplitude bed‐waves are strikingly similar to experimental bedforms produced under decelerating mixed sand–mud flows and are interpreted to form beneath transitional flows with enhanced and attenuated near‐bed 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 low‐amplitude bed‐waves, suggesting that the flows changed from turbulent to increasingly turbulence‐modulated. 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 shear‐thinning nature of clay‐rich suspensions. Large current ripples and low‐amplitude bed‐waves are likely to be common in the fringe of other submarine fans. The presence and spatial trends in mixed sand–mud bedform types may be an important tool in interpreting fan fringe environments

On the origin of chevron marks and striated grooves, and their use in predicting mud bed rheology

Understanding of the formative conditions of many sole structures is limited, with chevron marks and striated groove marks being particularly enigmatic. These sedimentary structures are examined here through laboratory modelling. An idealized tool, resembling an armoured mud clast, was dragged through substrates of kaolinite–seawater mixtures of different yield strengths while submerged in seawater. The experiments suggest that armoured mud clasts are the likely tools producing fine striae in striated grooves and, given the common occurrence of striated groove marks in outcrops, that these clasts are more prevalent in deep‐marine settings than previously thought. Chevron marks were observed to form over a narrow range of substrate yield stresses, likely explaining their relative rarity. Furthermore, their form is shown to be a function of substrate rheology, with chevron angle relative to the movement direction of the tool being less in weaker substrates. Moreover, the size of cut chevron marks, characterized by a narrow central cut, bears no relationship to the size of the incising tool, but rather reflects a substrate with a low yield stress that is sufficiently mobile to close behind the tool. In contrast, interrupted chevron marks, characterized by a distinct central groove, reflect greater substrate strength. Striated grooves without chevrons formed at the highest yield stresses simulated in the experiments. The relationship between tool mark type and yield stress, in combination with changes in chevron angle, enables these sole structures to be utilized as indicators of palaeosubstrate rheology. The conditions required to preserve such features include a prolonged period of bed consolidation, flow bypass and lack of bioturbation. Given changes in seafloor communities and bioturbation over time and their impact on substrate rheology, particularly during the early Palaeozoic, the present work supports the idea that the frequency of these sole structures likely changed over geological time

Discontinuity in Equilibrium Wave–Current Ripple Size and Shape and Deep cleaning associated with Cohesive Sand–Clay Beds

Mixtures of cohesive clay and noncohesive sand are widespread in many aquatic environments. Ripple dynamics in sand-clay mixtures have been studied under current-alone and wave-alone conditions but not combined wave-current conditions, despite their prevalence in estuaries and the coastal zone. The present flume experiments examine the effect of initial clay content, C0, on ripples by considering a single wave-current condition and, for the first time, quantify how changing clay content of substrate impacts ripple dimensions during development. The results show inverse relationships between C0 and ripple growth rates and clay winnowing transport rates out of the bed, which reduce as the ripples develop toward equilibrium. For C0 ≤ 10.6%, higher winnowing rates lead to clay loss, and thus the presence of clean sand, far below the base of equilibrium ripples. This hitherto unquantified “deep-cleaning” of clay does not occur for C0 > 10.6%, where clay-loss rates are much lower. The clay-loss behavior is associated with two distinct types of equilibrium combined flow ripples: (a) Large asymmetric ripples with dimensions and plan geometries comparable to their clean-sand counterparts for C0 ≤ 10.6% and (b) small, flat ripples for C0 > 10.6%. The 10.6% threshold, which may be specific to the experimental conditions, corresponds to a more general 8% threshold found beneath the ripple base, suggesting that clay content here must be <8% for clean-sand-like ripples to develop in sand-clay beds. This ripple-type discontinuity comprises a threefold reduction in ripple height, with notable implications for bed roughness

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 sand–gravel 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