Upper-flow Regime Research Articles

Towards a new generation of fluvial facies models for the interpretation of ancient deposits, based on inter‐annual peak discharge variance of modern rivers

ABSTRACTFluvial facies models based on river planform are flawed and not fully fit for purpose. An alternative approach to classifying fluvial deposits is based on river discharge characteristics. A recent paper showed that the co‐efficient of variance of annual peak discharge (CVQp) correlates well with the characteristics of preserved alluvium for a suite of modern rivers. Here, the database of rivers is expanded and augmented, and the utility of this statistic confirmed. For modern rivers, plotting the running‐value of CVQp for a set interval (for example, 30 years) over a longer discharge record shows that the statistic varies both spatially and temporally. The CVQp can rise abruptly at a given location in response to a single flow event if that event is significantly larger than is common in that river. Such abrupt changes in CVQp are superimposed on longer‐term patterns of more gradual change stimulated by environmental factors or anthropogenic effects such as dam construction. If the period of calculation for CVQp is relatively short, ‘anomalous’ events appear more frequently. Sequential studies of modern rivers indicate that such anomalous events have a high probability of being preserved in the alluvial record of the river, and suggest that, over the timeframes of channel filling and abandonment, they may constitute the dominant (characteristic) portion of the record in high CVQp systems. Since flow events with strongly peaked hydrographs (flashy discharges) are more common in high CVQp systems and leave a record dominated by upper flow regime stratification styles, it follows that the alluvial record of high CVQp rivers will be dominated by upper flow regime stratification. In contrast, in lower CVQp systems the occurrence of an event that changes the running CVQp value significantly may form a distinct event deposit, and the temporarily changed CVQp is unrepresentative for the duration of the channel's life. The dominance of upper flow regime stratification, together with decreased preservation of identifiable macroform structure, increased lateral lithological heterogeneity, and in many cases increased preservation of in situ tree fossils in channel deposits of higher CVQp rivers, forms the basis for a series of new facies models for low, moderate, high and very high/ultra‐high CVQp alluvial systems and deposits. The new models can be applied to ancient successions either in combination with traditional, planform‐based models, or not. These models provide information about ancient palaeoenvironmental conditions, and changes in such conditions over time at various scales via stratigraphic investigations.

Downslope evolution of supercritical bedforms in a confined deep-sea fan lobe, Amantea Fan, Paola Basin (Southeastern Tyrrhenian Sea)

The sedimentology of upper flow regime bedforms represents an important research topic at the present. Deposits interpreted as those of supercritical flows are widely recognized in modern fan systems, but their recovery is challenging. Most of the sedimentological information has come from channel thalwegs but supercritical bedforms are also frequently downslope from the channel mouths. Such an environment has been identified in the Paola basin, where erosive and depositional cyclic steps have been imaged and identified in a sandy submarine lobe of the Amantea Fan. High-resolution sub-bottom profiles provide insight into the bedform internal architecture and their relationships with a frontally-confining ridge. For the first time, supercritical bedforms in a submarine lobe have been interpreted in two distinct positions: in the scour of an erosional cyclic step and in the stoss side of a depositional cyclic step. Coarse to medium-grained massive sand with flame structures, indicating rapid sediment fall-out and frequently associated with the occurrence of hydraulic jumps, has been identified in the scour and at the toe of the ridge. The latter represents an example of topographically induced hydraulic jumps driven by a frontal confinement. Top-cut-out medium to fine sands with tractive structures have been interpreted as the deposits related to the stoss side of a cyclic step or small-scale antidune superimposed on the cyclic step surface. The presented data broaden the understanding of the range of processes that are driven by the interaction between turbidity currents and seafloor topography and the dip of the slope. The recognition that topography influences the density structure and the degree of criticality of the flow and, consequently, the morphodynamics and facies of the relative deposits may help to explain sediment distribution and improve depositional models of fan lobes in confined settings.

Quantitative facies analysis of a fluvio-aeolian system: Lower Triassic Buntsandstein Group, eastern France

Understanding the factors that control the temporal and spatial evolution of the Lower Triassic Buntsandstein Group, eastern France, is important not just for palaeoenvironmental reconstruction but also because it is an important reservoir for the lithium-rich geothermal brines in the Upper Rhine Graben region. The interval of interest in this study, the Lower Grès Vosgien Formation (LGV), is made up of c. 200 m of clastic deposits of mixed fluvial and aeolian origin. Given the proximity between the outcrops and the areas of mineral and heat exploration, this region offers a unique opportunity for quantitative analysis and correlation between outcrops and reservoirs. The LGV is the thickest Formation in the Buntsandstein Group. However, hitherto, no detailed architectural analysis has been published to reveal the controlling factors of its depositional elements. In this study, high-resolution facies analysis was applied to thirteen outcrops, and a core, to quantify fluvial and aeolian depositional architectures, and to comprehend palaeoenvironmental conditions during their deposition. Two facies associations were identified. The fluvial channel facies association, composed mainly of sandstones deposited in poorly confined, occasionally ephemeral channels, corresponds to 93 % of the total thickness. The analysis of >800 sets of strata characterised by lower-, transitional-, and upper-flow regime sedimentary structures, distributed within different architectural elements, enabled the distinction of temporal and spatial variabilities, and an estimate of fluvial palaeohydraulic characteristics. The wind- and water-laid facies association, which accounts for 7 % of the total thickness, records an architecture controlled by a fluctuating water-table level, and ephemeral floods. Results revealed that aeolian dunes and sand sheets accumulated in distinct temporal moments. The vertical trends, recorded in the core, indicate an upward increase in the frequency of intercalations between the two facies associations, and a decrease in the occurrences of thicker cross-bedded sandstone sets of fluvial origin, which, associated with the overall retrogradational stacking pattern of the LGV, is interpreted as the preserved record of a large distributive fluvial system (DFS). The occurrence of laterally extensive aeolian deposits, overlying fluvial deposits, is often associated by authors with periods of relatively more arid conditions at a regional scale. However, evidence indicates channel belt avulsion as an alternative interpretation for the establishment of the two facies associations. The findings enhance the knowledge about the depositional controlling factors in braided fluvial and aeolian systems, and improve predictive models that account for reservoir heterogeneity in accumulated successions of this type.

Prediction of Bedform Dimensions on Alluvial Bed in Unidirectional Flow

In this study, the bedform dimensions of an alluvial bed in a unidirectional flow were experimentally investigated. A series of flume experiments was conducted; 700 sets of flume and field data were used in developing formulae for predicting the bedform dimensions on alluvial beds in unidirectional flow. Bedform dimensions include the length and height of bedforms generated by the lower and upper flow regimes; the resistance coefficient for the flow in different flow regimes is introduced into the proposed formulae. The momentum boundary-layer thickness was introduced as an independent variable instead of the flow depth. Based on a large amount of flume and field data, the coefficients of each parameter were determined; four typical formulae were used to compare the accuracy of the proposed formulae. The experimental results show that the momentum boundary-layer thickness, hydraulic radius, and resistance coefficient for the flow in different regimes correlate well with the bedform dimensions. The calculation results show that the dimensionless particle size should not be ignored in the calculation of bedform dimensions. The bedform dimensions have an obvious trend of rapid increase with an increase in the ratio of flow depth to sand size (H/d). The bedform dimensions obtained using the van Rijn method and the Engelund and Hansen method did not represent the variation trend of the bedform length in the upper flow regime with an increase in H/d when H/d was greater than 103. The calculations using the proposed formulae are more accurate and reasonable than those in previous studies predicting the bedform height and length on an alluvial bed in a unidirectional flow.

The Kuching Formation: A deep marine equivalent of the Sadong Formation, and its implications for the Early Mesozoic tectonic evolution of western and southern Borneo

The pre-Cretaceous history of Borneo remains relatively poorly studied. Limited exposures of Palaeozoic and lower Mesozoic rocks are located in NW Kalimantan and in West Sarawak, an area interpreted as the West Borneo basement. Lower Mesozoic sedimentary rocks in West Sarawak were analysed to study their depositional environments and implications for the tectonic evolution. Upper Triassic turbidites in West Sarawak, exposed in the northern part of Kuching city, informally named the Kuching Formation, are the deep marine equivalent to the more widespread, shallow marine Sadong Formation. The Kuching Formation comprises thinly-bedded stacked turbidites, consisting of incomplete Bouma sequences, with multiple, erosive channel sandstone bodies deposited under upper flow regime waning flows. Thin debrites with abundant coaly-material are interbedded with the channel sandstones. The Kuching and Sadong formations both contain volcaniclastic detritus that was derived from the westward-subducting Palaeo-Pacific plate, forming a Triassic Andean-type arc which extended from West Borneo in the south to southern China, Taiwan and Japan in the north. Palaeoproterozoic to Archean detrital zircons in the Kuching and Sadong formations reveal a Cathaysian basement source, providing insights into the nature of the West Borneo basement. Quartz-mica schists (Kerait Schist, Tuang Formation) in fault-contact with the two sedimentary successions may have formed during accretion.

The Cambro-Ordovician Gondwana alluvial megafan in Central Africa: Insights from the Paleozoic sandstones of the Inkisi group, Congo Republic and Democratic Republic of the Congo.

The Paleozoic arkoses of the Inkisi Group in both the Republic and the Democratic Republic of the Congo has been investigated in order to examine whether they can represent an alluvial fan originating from the Gondwana supermountains in Northern and East Africa.This study focuses on outcrops’ description, lithofacies, bounding surfaces, architectural and sequential organization of sedimentary bodies and paleocurrents. The aim is to determine the nature of the Inkisi sandstones, the provenance of its sediments and the depositional setting, in order to test whether the East African orogen was also the sediments source for the formation of sedimentary fan in the western part of Central Africa.The study shows that the Inkisi Group consists of various sandstone lithofacies, amalgamated by conglomerates. The sediments are immature composed of quartz and feldspars associate with lithic fragments and micas. The sediments originated from the erosion of the West-Congolian belt and the Congo Craton, and were transported by braided fluvial system under mainly upper flow regime and secondarily lower flow regime. Sediments were deposited in the Inkisi basin, which was extensional and affected by burial.The Inkisi sediments setting up in a proximal to medial distributary zone of an alluvial megafan.The flows were directed southward, as opposed to paleofans originating from the Gondwana supermountain in Northern and East Africa. Thus, the Inkisi sandstone constitutes a new alluvial megafan distinguished from the Gondwana superfan derived from the Gondwana supermountain. It originated from the Pan-African belt of Mayombe and the Congo Craton. We name this megafan “the Cambro-Ordovician alluvial megafan of Central Africa”.

Solids dispersion in high density circulating fluidized beds

Transient local solid tracer concentration distributions were measured by a novel, simple and accurate measurement system with fast response to obtain axial solids dispersion in the riser of a circulating fluidized bed operated at solids fluxes up to 450 kg/m2s and gas velocities up to 8 m/s. Phosphorescent-coated FCC particles were used as the tracer which were activated by high intensity UV light and injected into the return leg of high-density circulating fluidized bed unit. A radially non-uniform axial dispersion model was utilized to determine axial solids dispersion coefficients, and the results were interpreted with the help of hydrodynamic data obtained in the same column. Comparison of the axial dispersion coefficients at different operating conditions revealed that dispersion decreases when solids circulation flux increases to 450 kg/m2s together with the increase in gas velocity to 8 m/s. Low solids dispersion coefficients (Daz < 0.1) at high solids circulation fluxes and high velocities resulted from the disappearance of the net downward flow of particles at the riser walls, signifying the existence of the dense suspension upflow regime.

Progradational‐to‐retrogradational styles of Palaeogene fluvial fan successions in the San Juan Basin, New Mexico

AbstractBasin‐scale outcrop analyses of fluvial architecture in the Palaeogene San Juan Basin, New Mexico, document lateral and vertical trends in channel, floodplain and palaeosol characteristics. Herein, the uppermost part of the Palaeocene Nacimiento Formation and lower Eocene Cuba Mesa and Regina Members of the San Jose Formation are identified as deposits of large fluvial fans based on trends observed across the basin. Stratigraphic trends suggest two packages originated by fluvial fan progradation. Progradation of the lower fan system provides a new explanation for the transitional nature of a disconformity at the Nacimiento–San Jose Formation contact, previously thought to be a low‐angle unconformity. The two fan systems are separated by a retrogradational interval that culminates in a depositional hiatus at the contact between the Cuba Mesa and Regina Members. This, combined with poor age constraints, indicates that the duration of the disconformity at the base of the Cuba Mesa Member may have been overestimated. Furthermore, the succession is interpreted as deposits of variable‐discharge rivers, based on the combined abundance of upper flow regime and high deposition rate sedimentary structures indicative of intense flooding events, preservation of in‐channel bioturbation and paedogenic modification indicating periods of prolonged dryness, lack of identifiable bar strata and alternations of poorly drained and well‐drained floodplain deposits with pedofacies indicating alternating wet–dry cycles. This dataset adds to a growing body of evidence linking the formation of large fluvial fans to discharge variability and thus to hydroclimates with significant inter‐ and intra‐annual precipitation variability and intense rainfall. A long‐term stratigraphic shift from poorly drained to well‐drained floodplain deposits across two progradational fan successions indicates that a predictive model suggesting downstream decreases in soil drainage conditions is not encompassing of all large fan systems, and instead suggests a transition to a more arid climate across the Palaeocene–Eocene boundary.

Upper-flow regime bedforms in a subglacial triangular-shaped landform (murtoo), Late Pleistocene, SW Finland: Implications for flow dynamics and sediment transport in (semi-)distributed subglacial meltwater drainage systems

We know less about subglacial meltwater flow properties in distributed inefficient and semi-efficient systems in comparison to those of ice marginal eskers and proglacial environments. While previous studies have indicated the overall common presence of upper-flow-regime (UFR) bedforms in glacigenic settings, facies expressions of subglacial meltwater flows remain poorly documented. Three ca. 3 m deep and up to 70 m long trenches excavated across a triangle-shaped subglacial landform called a murtoo in a Lateglacial to Holocene meltwater route in SW Finland provide a detailed window into sedimentary structures presumably formed ca. 40–50 km away from the coeval subaqueous margin of the Fennoscandian Ice Sheet (FIS). The aim of this paper is to document small-scale bedforms, which formed subglacially by meltwater flow and to characterize the proximal and central parts of the studied murtoo during its early evolutionary phase. We defined seven main facies types that characterize the depositional processes of the unit. Overall, the studied deposits reflect increasing meltwater delivery through time and are characterized by abrupt lateral changes in sedimentary structures and grain size. While the initial deposits are dominated by massive and horizontally laminated silt with sand lenses interpreted as lower-flow-regime deposits, the latter sediments are characterized by sinusoidal stratification, sigmoidal cross-stratification and scours with backsets or chaotic fill interpreted as deposits of antidunes, humpback dunes, chutes-and-pools and cyclic steps of the upper-flow regime. The upper-flow-regime bedforms developed on a 1 m high and 15 m long bed slope and are associated with the formation of a short-lived enlarged water-filled cavity or pond, where supercritical density flows allowed for the deposition of upper-flow regime bedforms. The final coarse-grained murtoo head-bar development, characterized by planar-cross stratified gravel and pebbly sand, indicates avalanche processes that were controlled by grain size.Our results confirm that the core of the murtoo is depositional and meltwater processes played a key role in its deposition. Despite the subglacial setting with a subaqueous ice-sheet margin, the meltwater flow was not permanently characterized by pipe-flow conditions. Overall, the findings contribute to the understanding of semi-distributed subglacial meltwater systems during the retreat of a continental ice sheet (FIS) in a rapidly warming climate.

Solids flux profiles of Geldart Group A particles in high‐velocity circulating fluidized bed risers

AbstractCirculating fluidized bed (CFB) risers using Group A particles have traditionally, mostly, been considered to operate in the fast fluidization regime, which consists of a core‐annulus flow profile with solids refluxing in the annulus layer. High gas and solids flow riser studies, however, suggest the existence of additional types of flow behaviours. Therefore, more studies are still needed to help clear uncertainties of local solids flow patterns in CFB risers of Group A particles, especially at gas and solids flow rates at or near those in commercial units. In this study, riser density and local solids flux profiles were measured in 0.3 m diameter risers of three CFB units at gas velocities of 9–16 m/s and solids fluxes of up to 700 kg/s · m2. A variety of radial solids flux profiles were obtained, including a parabolic profile with a peak flux at about the radial centre, a nearly flat profile across the riser cross‐section and an inverted parabola with peak upwards flux near the wall. At high gas velocity and solids flux, risers have no solids downflow at the wall. Multiple fluidization regimes were found to exist in the riser. The bottom dense part of the riser was in the dense suspension upflow regime, and the dilute upper part was in the dilute pneumatic transport regime. With commercial fluid catalytic cracking (FCC) risers operating at nearly similar conditions as tested here, it is likely that they also have one or both fluidization regimes and not the traditional fast fluidization regime. The data in this study fitted well on the Kim et al. fluidization regime map.

Fine Grained Braided River Sedimentation in Brahmaputra River, Majuli, Assam, India

Abstract The Brahmaputra River in Assam exhibits a typical braided pattern and is characterized by a sediment load comprising of sand and mud. The river contains channel-bars of varied types and dimensions and embraces the world’s largest river island – Majuli, which at present is under the effect of severe bank-erosion along its southern margin. The objective of this study is to examine the characteristics of fluvial sedimentation prevailing along the southern margin of the Majuli landmass. Both channel as well as bank sediments were analyzed to understand the mechanism of transportation, depositional process and hydrodynamic behavior. Both channel and bank sediments consist of very fine to medium sand with subordinate amount of silt and mud. Majority of the sediments exhibit well to very well sorting and positive skewness. The Probability curves of grain size distribution indicate transportation of sediments dominantly by saltation mode with minor amount by suspension and surface-creep mode. CM plot suggest transportation by graded suspension (saltation) with subordinate rolling and uniform suspension process by means of tractive current. The internal stratifications are dominated by planar cross stratifications followed by horizontal stratifications, trough cross stratifications, ripple laminated layers and massive sand. The bank sediments in few locations exhibit a general fining upward sequence which ends up with the deposition of a mud layer.The lithofacies sequences of the study area show prevalence of both lower as well as upper flow regime energy conditions indicating fluctuations of energy conditions during their respective sedimentation.

Alluvial ichnofacies from Upper Triassic red beds in India: Implications for palaeoenvironment and palaeoclimate

Trace fossils are highly sensitive to the hydrodynamic conditions of depositional systems, substrate, and climate and hence yield information about spatio-temporal changes in palaeoenvironment and palaeoclimate. In this paper, we report alluvial trace fossil assemblages from the Upper Triassic Tiki Formation of the Rewa Basin, central India. Based on detailed facies analysis and architecture, two depositional environments are identified in this formation: (1) perennial braided channel belts and (2) extensive and complex floodplains containing ephemeral small fluvial channels, lakes, and crevasse splay deposits. Upper flow regime primary structures in laterally extensive sandstones encased in the floodplain deposits indicate short-lived high magnitude flood discharge, suggestive of seasonal climates. Trace fossils are sporadically distributed through these facies, generally in suites of low abundance, although in places the bioturbation index is higher. Three major ichnofacies, namely Skolithos, Scoyenia and Mermia are recognized, based on the association of different trace fossils preserved in the channel, crevasse splay of proximal floodplain and pond/lake of distal floodplain deposits, respectively. Vertical Beaconites in palaeosols and an abrupt change from Skolithos to Scoyenia ichnofacies indicate a highly variable water table, controlled by seasonal precipitation. The transitions between the Skolithos, Scoyenia and Mermia ichnofacies reveal that trace fossil preservation was variously controlled by channel abandonment, floodplain dewatering, substrate desiccation or periodic waterlogging of overbank areas. Specifically, increased water discharge over the floodplain during high precipitation, followed by post-flood recovery of hot, dry, low-water table conditions may have been a key controlling factor in shaping the ichnological characteristics of these Upper Triassic red beds.

Permafrost as the main factor controlling the fluvial sedimentation style on glaciomarginal fans

This study describes the sedimentary successions of two glaciomarginal fans (=end moraines) deposited during MIS 6 and MIS 2 in the eastern and north-western part of Poland. Sedimentological analyses of the successions indicate they were both deposited close to the ice sheet margin and are dominated by extensive gravelly and sandy sheet beds derived from the upper flow regime with single ice-wedge structures. Qualitative and quantitative analysis of the pancake-like gravelly and sandy sediments was performed using Markov chain analysis. The presence of these, admittedly common, sedimentary successions raises two key points: why sheetfloods are so common on glaciomarginal fans instead of in-channel flows, and whether sedimentary rhythms deposited by sheetfloods represent the full depositional records, or only partial ones due to accompanying erosion. While both points remain unclear, the answers to these questions may be connected to the presence of permafrost, which occupied the foreland of the ice sheet where the glaciomarginal fans were deposited: it can control the type of proglacial river flows on glaciomarginal fans, the depth of erosion and influence the record of sedimentary rhythms.

Facies architecture of fluviatile deposits of the Jurassic-Cretaceous Bertangga Formation, Peninsular Malaysia

Bertangga Formation is a part of the Jurassic-Cretaceous non-marine sequences in Thailand and Malaysia. However, its facies analysis and depositional model have not been investigated in detail. Eleven lithofacies have been described in the Bertangga Formation and combined five facies associations including channel, point bar, floodplain, crevasse splay and swamp facies associations. Channel deposits are stacked bodies of fining upward sequences with prevalent erosional bases, formed by vertical aggradation and avulsion of channels. Point bar sands comprise cross bedded sandstone bodies formed in upper flow regime and possible lateral accretion surfaces. Crevasse splay deposits form sheets of fine-to-medium-grained sandstone. Floodplain sediments are composed of motteled grey mudstone. Swamp depositional environment is characterized by an association of coal, carbonaceous shale and siltstone. Facies analysis allows reconstruction of the depositional environment of the Bertangga Formation as a meandering fluvial system. Facies association also shows the increasingly distal and fine-grained trend from west to east of the studied area, which suggests possible eastward paleo-flow direction of the river. The existence of kaolinite in all samples indicates weathering of felsic rocks under acidic conditions. In the same time, the presence of smectite in the eastern part of the study area may suggest a contribution of mafic and/or volcanic rocks to the source of sediments in this area.

The sedimentology of an ephemeral fluvial–aeolian succession

ABSTRACTEphemeral fluvial systems are commonly associated with arid to semi‐arid climates. Although their complex sedimentology and depositional settings have been described in much detail, depositional models depicting detailed lateral and vertical relationships, and interactions with coeval depositional environments, are lacking compared to well‐recognized meandering and braided fluvial systems. This study critically evaluates the applicability of current models for ephemeral fluvial systems to an ancient arid fluvial example of the Lower Jurassic Kayenta Formation of the Colorado Plateau, USA. The study employs detailed sedimentary logging, palaeocurrent analysis and photogrammetric panels across the regional extent of the Kayenta. A generic model that accounts for the detailed sedimentology of a sandy arid ephemeral fluvial system (drawing upon both ancient and geomorphological studies) is developed, along with analysis of the spatial and temporal interactions with the aeolian setting. Results show that the ephemeral system is dominated by laterally and vertically amalgamated, poorly channelized to sheet‐like elements, with abundant upper flow regime flat beds and high sediment load structures formed between periods of lower flow regime conditions. Through interaction with a coeval aeolian system, most of the fluvial deposits are dominated by sand‐grade sediment, unlike many modern ephemeral fluvial systems that contain a high proportion of conglomeratic and/or finer grained mudstone and siltstone deposits. During dominantly fluvial deposition, high width to thickness ratios are observed for channelized and sheet‐like elements. However, with increasing aridity, the aeolian environment becomes dominant and fluvial deposition is restricted to interdune corridors, resulting in lower width to thickness ratio channels dominated by flash‐flood and debris‐flow facies. The data presented here, coupled with modern examples of ephemeral systems and flood regimes, suggest that ephemeral flow produces and preserves distinctive sedimentological traits that can not only be recognized in outcrops, but also within core.

Multistage damming of the Rhine River by tephra fallout during the 12,900 BP Plinian Laacher See Eruption (Germany). Syn-eruptive Rhine damming I

The Rhine - the largest river in Western Europe – was dammed during the Plinian Laacher See Eruption (LSE; 12,900 BP). Damming during the climactic Plinian episode of LSE occurred both upstream and downstream of the broad tectonic Lower Neuwied Basin (LNB) that interrupts the narrow Rhine canyon. We here document details of the upstream damming at the bottleneck entrance to the LNB near the present city of Koblenz. Our reconstruction is based on a high-resolution analysis and correlation of the complex intercalation of primary fallout tephra relics with fluvially reworked Laacher See Tephra in the LNB. Tephra units representing complete eruptive cycles repeatedly fell on drained ground in between one minor and 4major flooding events - even at the base of side channels that had been active prior to the LSE and that had been flooded by a preceding flooding event. This demonstrates that flooding occurred generally during breaks and not during fallout events. The repeated formation and breach of a dam at the upstream entrance of the LNB (Koblenz Dam) consisting of fallout components and driftwood washed together convincingly explains the multiple repetition of the drainage of the channels in the LNB followed by large-magnitude flooding in rapid succession. The strongly pulsating nature of the LSE reflected in multiple interruptions of eruptive activity fundamentally controlled the damming and flooding dynamics. The Rhine became completely blocked during distinct fallout phases due to overloading with pumice that had fallen into the river and its major tributaries. The temporary dam collapsed during eruptive breaks. This is the first recorded example of tephra fallout damming a major watercourse. The extremely low gradient of the Rhine River allowed the repeated accumulation of large volumes of water in a long, multi-phase dammed-up lake (Lake Koblenz) that extended along the upstream course of the river for up to c. 30km despite the low height (<10m) of the dams. Each breach of Koblenz Dam caused extensive and wide-spread erosion and reworking of freshly deposited tephra throughout the entire LNB up to 7km perpendicular to the major axial Rhine channel. The floods deposited striking, large-scale upper flow regime structures interpreted as in-phase wave draping, antidunes and chute-and-pool structures consisting largely of gravel-sized tephra components. Primary tephra sheets - several meters thick - became detached by undercurrents above impermeable boundaries and floated potentially along the full length of both active and abandoned channels. Large tephra bodies with the dimensions of a garage were lifted off by the flood waves and transported downstream for at least tens of meters.Damming of a major river while a large Plinian eruption is in full progress represents an extraordinary challenge for hazard mitigation. This is especially pertinent for an area close to, and downwind from, the vent and therefore simultaneously affected by massive Plinian fall such as in the LNB.

Three-dimensional multiphase full-loop simulation of directional separation of binary particle mixtures in high-flux coal-direct chemical-looping combustion system

Coal-direct chemical-looping combustion (CDCLC) is a promising coal combustion technique that provides CO2 capture with a low energy penalty. In this study, we developed a three-dimensional Eulerian–Eulerian multiphase full-loop model for simulating the circulation and separation of binary particle mixtures in a novel high-flux CDCLC system. This model comprised a high-flux circulating fluidized bed as the fuel reactor (FR), a counter-flow moving bed as the air reactor (AR), a high-flux carbon stripper, two downcomers, and two J-valves. This model predicted the main features of complex gas–solid flow behaviors in the system. The simulation results showed that quasi-stable solid circulation in the whole system could be achieved, and the FR, AR, and J-valves operated in a dense suspension upflow regime, a near-plug-flow regime, and a bubbling fluidization regime, respectively. The multiphase flow model of binary particle mixtures was used to predict the mechanisms of directional separation of binary particle mixtures of an oxygen carrier (OC) and coal throughout the system. A decrease in the baffle aspect ratio of the inertial separator improved the coal selective separation efficiency but resulted in a slight decline in the OC selective separation; this is believed to be the result of weakening of particle collisions with the baffle. A higher FR gas velocity had a slightly negative effect on the OC selective separation efficiency, but improved the coal selective separation efficiency; this can be attributed to an increase in the particle-carrying capacity of the gas stream. A decrease in the coal particle size led to better entrainment of the coal particles by the gas stream and this increased the coal selective separation efficiency. In real CDCLC applications, the operating variables for separation of binary particle mixtures should be comprehensively assessed to determine their positive and negative effects on the carbon capture efficiency, OC regeneration efficiency, gas leakage restraint, energy consumption, and fuel conversion.

Water on Mars: Do submarine cyclic steps exist on the red planet?

Submarine cyclic steps have a significant impact on the evolution of the ocean floor on Earth. These upper-flow regime bedforms should be even more common in Martian reduced-gravity environments. We postulate that, if early Mars had an ocean, cyclic steps would have formed on Martian deltas spreading along the putative ancient shorelines in the northern plains. The main challenge to identifying submarine cyclic steps on Mars is geologic activity, in particular impacts, volcanism, and aeolian erosion, which have erased or modify much of the geomorphic evidence. Even though many Martian deltaic deposits do not bear much resemblance to the original depositional surfaces, we hypothesize that cementation and consolidation of marine sediments had facilitated the preservation of at least some Martian cyclic steps on deltaic deposits. We combine data from several orbital missions at Mars with a literature review and numerical analysis to inspect bedforms near the foreset-bottomset transition of 17 documented deltaic deposits. The most promising piece of evidence for Martian submarine cyclic steps is a linear series of rhythmic long-wavelength bedforms within a channel incised into the upper delta front of the Aeolis Mensae Delta. These bedforms are remarkably similar to their counterparts observed in channels on deltas and deep-water fans on Earth. Moreover, we speculate that two fields of cyclic steps, likely emanating from a crater upstream of the Aeolis Mensae Delta, are a potential example of contemporary cyclic steps and attest to the sporadic presence of flowing water on present-day Mars.

Ichnology and sedimentology of the trace fossil-bearing fluvial red beds from the lowermost member of the Paleocene Funing Formation in the Jinhu Depression, Subei Basin, East China

Abstract The lowermost member of the Paleocene Funing Formation, which is composed of bioturbated fluvial red beds, is extensively distributed throughout the Jinhu Depression, Subei Basin, East China. Despite its economic value as one of the most important oil-bearing reservoirs in the Subei Basin, the sedimentology, ichnology and paleoenvironment of this formation in the Jinhu Depression remain poorly understood. In this work, we investigated the ichnofossils and sedimentary facies, analyzed the ichnofacies and potential tracemakers and reconstructed the paleoenvironment of the Jinhu Depression based on cores from the lowermost member of the Paleocene Funing Formation. This formation contains a moderately diverse ichnofauna (9 ichnotaxa of the Scoyenia ichnofacies) that includes meniscate burrows (Taenidium barretti, Taenidium satanassi and Beaconites coronus), simple horizontal cylindrical burrows (Palaeophycus tubularis, Palaeophycus heberti and Planolites isp.), simple vertical or subvertical burrows (Skolithos linearis and Skolithos isp.) and plant root traces. The other complex branched and chambered burrows are left in open nomenclature. These ichnofossils are widespread throughout the three facies associations identified in this study: (1) ephemeral fluvial channel, (2) crevasse splay and (3) distal floodplain facies associations. Multiple lines of sedimentological and ichnological evidence, namely, the predominance of massive and upper flow regime structures, the occurrence of invertebrate burrows (e.g., Skolithos and Taenidium) in the lower and middle parts of the channel deposits and the dramatic changes in the color and mottling in channel and overbank sediments, indicate that the river discharge was short lived and that the water table was highly variable. The variations in the discharge and water level are attributed to the influence of seasonal rainfall. The cyclic nature of intense rainfall events and dry periods suggests that the Subei Basin was likely located in a semiarid region with a seasonal rainfall pattern during the early Paleocene. This study also reveals that the ephemeral fluvial channel is the facies association with the highest potential to yield oil-bearing reservoirs in the Paleocene red beds of the Jinhu Depression. Additionally, the influence of bioturbation on the reservoir quality in this region should be considered in future research.

Clast imbrication in coarse-grained mountain streams and stratigraphic archives as indicator of deposition in upper flow regime conditions

Abstract. Clast imbrication is one of the most conspicuous sedimentary structures in coarse-grained clastic deposits of modern rivers but also in the stratigraphic record. In this paper, we test whether the formation of this fabric can be related to the occurrence of upper flow regime conditions in streams. To this end, we calculated the Froude number at the incipient motion of coarse-grained bedload for various values of relative bed roughness and stream gradient as these are the first-order variables that can practically be extracted from preserved deposits. We found that a steeper energy gradient, or slope, and a larger bed roughness tend to favor the occurrence of supercritical flows. We also found that, at the onset of grain motion, the ratio ϕ between the critical shear stress for the entrainment of a sediment particle and its inertial force critically controls whether flows tend to be super- or subcritical during entrainment. We then mapped the occurrence of clast imbrication in Swiss streams and compared these data with the hydrologic calculations. Results indicate that imbrication may record supercritical flows provided that (i) ϕ values are larger than ca. 0.05, which is appropriate for streams in the Swiss Alps; (ii) average stream gradients exceed ca. 0.5 ± 0.1°; and (iii) relative bed roughness values, i.e., the ratio between water depth d and bed sediment D84, are larger than ∼ 0.06 ± 0.01. We cannot rule out that imbrication may be formed during subcritical flows with ϕ values as low as 0.03, as demonstrated in a large number of flume experiments. However, our results from Alpine streams suggest that clast imbrication likely reflects upper flow regime conditions where clasts form well-sorted and densely packed clusters. We consider that these differences may be rooted in a misfit between the observational and experimental scales.