Backset Beds Research Articles

Depositional elements and evolution of gravity-flow deposits on Lingshan Island (Eastern China): An integrated outcrop-subsurface study

Understanding how subaqueous sediment gravity flows (SSGF) evolve in time and space, and how their deposits vary spatially, is a key research focus for gravity flow sedimentology. This study investigates depositional facies, depositional elements, and sediment transport processes of supercritical flows and hybrid event beds (HEBs) of the Lingshandao Formation in Lingshan Island, Eastern China. Three kinds of depositional elements were recognized: mass transport deposits (MTDs), channel-lobe transition zone (CLTZs), and lobe complexes. MTDs can be sub-divided into proximal and distal deposits. CLTZs are characterized by facies changes from massive coarse-grained pebbly sandstone to backset bedding sandstone in a down-slope direction, which are the deposits of supercritical turbidity currents. HEBs are common in both proximal and distal lobe settings. Tripartite structure in HEBs, which may be caused by up-slope substrate erosion, implies a relatively proximal origin compared with bipartite HEBs, perhaps caused by fluid fractionation in the research area. The succession is formed by a prograding lobe unit, followed by MTDs which are themselves overlain by deposits from a CLTZ. This vertical stacking pattern implies the potential longitudinal facies tract from CLTZ; to lobe complex deposits, is accompanied by emplacement of MTDs at the slope break. The flow types accompanied by sediment transport processes imply that erosion by supercritical turbidity currents associated with a hydraulic jump in the channel-lobe transition zone may be the main reason for mud-clasts and matrix addition to the flow. The large-scale addition of mud-clasts and suspended mud may dampen turbulence in proximal to medial lobe settings, and result in medium-to thick-bedded HEBs with common erosional features and tripartite structures. The remaining suspension flow (with abundant mud) may further transport down-dip and form medium-to thin-bedded HEBs with bipartite structures and rare mud-clasts. These findings may be applicable to other SSGF systems with supercritical flow deposits and hybrid event beds, emphasizing the downdip and lateral variation in depositional elements associated with gravity flow evolution.

The supercritical question for pyroclastic dune bedforms: An overview

AbstractSedimentary structures deposited from dilute pyroclastic currents are largely dominated by the products of metre‐scale dune bedforms containing backset lamination. Since the first seminal works in the 1970s, the vast majority of the literature called these structures antidunes and chutes‐and‐pools. This consensus led to a loose terminology, either as descriptive terms for structures containing backset beds or as an interpretation of hydrodynamic conditions related to Froude‐supercritical flows. Here, the main characteristics of pyroclastic dune bedforms are summarized and classified under four categories. The literature is examined and a discussion is provided regarding a possible interpretation in terms of supercritical‐flow bedforms, including antidunes, chutes‐and‐pools and cyclic steps. The interpretation of pyroclastic dune bedforms as related to supercritical conditions is a possibility among others, yet open questions remain, and under the present state of knowledge alternative interpretations are equally valid. There is consensus that an interpretation of pyroclastic dune bedforms as related to supercritical flows implies deposition from a basal underflow supporting an internal free‐surface formed at a density interface within the overall current structure. Antidunes are likely to be limited to low‐angle, incipient bedforms. Chutes‐and‐pools may occur in the form of series of steep truncations formed by an erosive supercritical ‘chute’, with a depositional signature occurring exclusively in the subcritical ‘pool’ of a Froude jump. Rare examples of cyclic steps may be found as large‐scale undulations with superimposed metre‐scale bedforms, or for short‐scale, fully‐aggrading structures in periodic trains. Additionally, some pyroclastic bedforms may be formed by granular flows passing granular Froude jumps or as frozen granular stationary waves. The dominance of chute‐and‐pool structures over bedforms characterized by relatively more stable morphodynamics (antidunes and cyclic steps) could be the likely result of pulsating flow conditions and abrupt high‐rate deposition, impeding stable flow conditions. In a majority of cases, the growth of pyroclastic bedforms appears triggered by the influence of an inherited bed topography and by bed‐flow feedback effects. Apart from a possible interpretation as supercritical bedforms, other specific dynamics of dilute pyroclastic currents may well explain the characteristics of pyroclastic dune bedform structures. In particular, the inherent turbulence fluctuations, internal organization and highly depositional dynamics may be key to the formation of pyroclastic dune bedforms. In this respect, rather than solely focusing on the Froude dimensionless number of the formative current, theoretical considerations accounting for the Reynolds, Richardson and Rouse dimensionless parameters may be also implemented for the understanding of pyroclastic bedforms.

Flood-generated hyperpycnal delta front sands of the Brora Arenaceous Formation (upper Callovian–middle Oxfordian) of the Inner Moray Firth, Scotland, record the onset of rifting

Sandstones of the Middle–Upper Jurassic Brora Arenaceous Formation of the Inner Moray Firth, NE Scotland have hitherto been interpreted as representing coastal, tidally-influenced bars. The formation is exposed close to the northern basin-bounding Helmsdale Fault, and the middle member of the formation, the Clynelish Quarry Sandstone, consists of thick, mainly structureless sandstone beds with wavy, commonly amalgamated boundaries. It also includes sandstone bodies with sigmoidal clinothems, erosional surfaces and backset beds. Rich marine faunas dominated by bivalves and ammonites occur at a few levels, whereas trace fossils are rare or absent. The Clynelish Quarry Sandstone is here reinterpreted as reflecting deposition by hyperpycnal sandy density flows in flood-generated marine, subaqueous, delta-scale clinoforms and lobes in front of local mountain streams. The reinterpretation of these sandstones implies the presence of a tectonically controlled, relatively steep basin margin along the line of the Helmsdale Fault. The Brora Arenaceous Formation thus dates the onset of Jurassic rifting in the Inner Moray Firth to the latest Callovian rather than the late Oxfordian as previously interpreted from seismic data.

Pyroclastic dune bedforms: macroscale structures and lateral variations. Examples from the 2006 pyroclastic currents at Tungurahua (Ecuador)

AbstractPyroclastic currents are catastrophic flows of gas and particles triggered by explosive volcanic eruptions. For much of their dynamics, they behave as particulate density currents and share similarities with turbidity currents. Pyroclastic currents occasionally deposit dune bedforms with peculiar lamination patterns, from what is thought to represent the dilute low concentration and fluid‐turbulence supported end member of the pyroclastic currents. This article presents a high resolution dataset of sediment plates (lacquer peels) with several closely spaced lateral profiles representing sections through single pyroclastic bedforms from the August 2006 eruption of Tungurahua (Ecuador). Most of the sedimentary features contain backset bedding and preferential stoss‐face deposition. From the ripple scale (a few centimetres) to the largest dune bedform scale (several metres in length), similar patterns of erosive‐based backset beds are evidenced. Recurrent trains of sub‐vertical truncations on the stoss side of structures reshape and steepen the bedforms. In contrast, sporadic coarse‐grained lenses and lensoidal layers flatten bedforms by filling troughs. The coarsest (clasts up to 10 cm), least sorted and massive structures still exhibit lineation patterns that follow the general backset bedding trend. The stratal architecture exhibits strong lateral variations within tens of centimetres, with very local truncations both in flow‐perpendicular and flow‐parallel directions. This study infers that the sedimentary patterns of bedforms result from four formation mechanisms: (i) differential draping; (ii) slope‐influenced saltation; (iii) truncative bursts; and (iv) granular‐based events. Whereas most of the literature makes a straightforward link between backset bedding and Froude‐supercritical flows, this interpretation is reconsidered here. Indeed, features that would be diagnostic of subcritical dunes, antidunes and ‘chute and pools’ can be found on the same horizon and in a single bedform, only laterally separated by short distances (tens of centimetres). These data stress the influence of the pulsating and highly turbulent nature of the currents and the possible role of coherent flow structures such as Görtler vortices. Backset bedding is interpreted here as a consequence of a very high sedimentation environment of weak and waning currents that interact with the pre‐existing morphology. Quantification of near‐bed flow velocities is made via comparison with wind tunnel experiments. It is estimated that shear velocities of ca 0·30 m.s−1 (equivalent to pure wind velocity of 6 to 8 m.s−1 at 10 cm above the bed) could emplace the constructive bedsets, whereas the truncative phases would result from bursts with impacting wind velocities of at least 30 to 40 m.s−1.

Supercritical-flow structures (backset-bedded sets and sediment waves) on high-gradient clinoform systems influenced by shallow-marine hydrodynamics

Inferred supercritical structures and bedforms, including sediment waves and backset-bedded sets, are identified as components of coarse-grained siliciclastic and bioclastic, high-gradient clinoform wedges (Plio-Pleistocene of southern Italy) and canyon head infills (Tortonian of Venetian pre-Alps), showing evidence of having been built out in a setting influenced by shallow-marine hydrodynamics. The facies identified are dominated by a range of traction carpets, formed after segregation of coarser particles in the lower part of bipartite density underflows. The generation of backset-bedded sets is thought to imply scouring due to impact of a submerged hydraulic jump on the bed, and upstream migration of the jump, concomitant with the deposition of backset beds on the stoss side of the developing bedform. Submerged hydraulic jumps apparently formed spontaneously and in any position on the foreset and toeset, without requiring any precursor bed defect. The mostly solitary, non-cyclical character of the bedforms prevents their attribution to cyclic steps. The sets of backset beds are locally underlain by chaotic infills of deep, steep-sided scours attributed to vigorous erosion at the hydraulic jump, accompanied by instantaneous loss in transport capacity which results in rapid plugging of the scour (hydraulic jump facies of Postma et al., 2014). Gravel waves have a distinct internal stratigraphy, and their length to amplitude ratios show lower mean values and higher variability when compared to sediment waves consisting of sand. The presence of supercritical bedforms on steep foreset slopes of the studied clinoform systems, even in proximity to the topset-foreset rollover, is believed to reflect high inefficiency of mud-poor and short run-out bipartite underflows episodically transporting relatively small volumes of coarse-grained sediment. This may also account for common solitary, non-cyclical bedforms. It is proposed that during intense oceanographic events, such as coastal storms, seaward sediment entrainment, assisted by gravity, was very effective on the gently sloping subaqueous topset, and that, beyond the topset-foreset rollover, the flows evolved to high-concentration turbidity underflows with supercritical Froude numbers. The flows are inferred to have been sustained, probably lasting for the duration of the meteorological events, and to have commonly been unsteady in discharge, fluctuating in concentration and size of transported sediments, and subject to peaks in velocity. The characteristics of the structures are regarded as typical of the systems fed by oceanographic processes, and may fall into the class of coarse-grained “small sediment waves with mixed relief” of Symons et al. (2016), formed from a combination of erosion and deposition, and by the action of stratified flows depositing from denser basal layers, and typically restricted to small-scale shallow-marine slope systems.

Hydraulic and sediment transport properties of autogenic avulsion cycles on submarine fans with supercritical distributaries

AbstractSubmarine fans, like other distributive systems, are built by repeated avulsion cycles. However, relative to deltas and alluvial fans, much less is known about avulsions in subaqueous settings. In this study, we ran a set of subaqueous fan experiments to investigate the mechanics associated with autogenic avulsion cycles of self‐formed channels and lobe deposits on steep slopes. The experiments used saline density currents with crushed plastic to emulate sustained turbidity currents and bed load transport. We collected detailed hydraulic and bathymetric measurements and made use of a 1‐D laterally expanding density current model to better understand different aspects of the avulsion cycle. Our results reveal three major components of the avulsion cycles: (1) distributary channel incision, extension, and stagnation; (2) mouth bar aggradation and hydraulic jump initiation; and (3) hydraulic jump sedimentation and upstream retreat. Interestingly, in all but one experiment, the avulsion cycles led to fans that remained perched above the basin slope break. Experimental data and hydraulic theory were used to unravel actual mechanics associated with cycles. We found that channels stopped extending into the basin due to a decay in sediment transport capacity relative to sediment supply and that the reduction in capacity was primarily an outcome of expansion‐driven velocity reduction; dilution played a secondary role. Once channel extension ceased, mouth bar deposits aggraded to a thickness approximately equal to the critical step height needed to create a choked flow condition. The choke then initiated a hydraulic jump on the upstream side of the bar. Once formed, the jump detained a majority of the incoming sediment and forced the channel‐to‐lobe transition upstream, filling the channel with steep backset bedding and capping the entire channel with a mounded lobate deposit. These intrinsic processes repeated through multiple avulsion cycles to build the fan.

Cyclic steps along the South Taiwan Shoal and West Penghu submarine canyons on the northeastern continental slope of the South China Sea

Large-scale step-like features within the South Taiwan Shoal and West Penghu submarine canyons on the northeastern continental slope of the South China Sea are investigated by integrating high-resolution multibeam bathymetric data and multichannel seismic profiles. These step-like features, ranging from 1.2 to 10.0 km in wavelength and 5.4–80.9 m in wave height, are mostly interpreted as cyclic steps formed by turbidity currents flowing through the canyons, based on their characteristic step-like morphology, in-train alignment, large wavelengths and aspect ratios (ratio of wavelength to wave height), and typical upstream-sloping backset bedding, among others. A train of 19 continuous steps delineated along the thalweg of the South Taiwan Shoal canyon measures up to 100 km and may be the longest ever reported. Nine short trains of scours identified on a terrace of the South Taiwan Shoal canyon are oriented parallel to the distributaries draining over the terrace and roughly perpendicular to the main canyon thalweg, indicating a complicated flow pattern within the canyon valley. Two trains of scours separated by an intracanyon high in the steeper middle reach of the West Penghu canyon are interpreted as transitional bed forms between antidunes and cyclic steps, which develop downstream into a train of five net-depositional cyclic steps with typical backset bedding in the gentler-sloping lower reach of the canyon. Average slope gradients for the canyon reaches with cyclic steps range from 0.26° to 1.24°. Along each thalweg step train, a slope break is identified to separate the net-erosional cyclic steps in the steeper upstream segment from the net-depositional ones in the gentler downstream segment. Rough estimations indicate that the paleoflows are 100 to 300 m thick with maximum velocities of up to 10 m s–1. The estimated flow depths match well with those inferred from geomorphologic analysis. Estimated paleodischarges of ?7–23 × 105 m3 s–1 are equivalent to ten times the discharge of the modern Amazon River.

Distinctive erosional and depositional structures formed at a canyon mouth: A lower Pleistocene deep‐water succession in the Kazusa forearc basin on the Boso Peninsula, Japan

AbstractThe canyon mouth is an important component of submarine‐fan systems and is thought to play a significant role in the transformation of turbidity currents. However, the depositional and erosional structures that characterize canyon mouths have received less attention than other components of submarine‐fan systems. This study investigates the facies organization and geometry of turbidites that are interpreted to have developed at a canyon mouth in the early Pleistocene Kazusa forearc basin on the Boso Peninsula, Japan. The canyon‐mouth deposits have the following distinctive features: (i) The turbidite succession is thinner than both the canyon‐fill and submarine‐fan successions and is represented by amalgamation of sandstones and pebbly sandstones as a result of bypassing of turbidity currents. (ii) Sandstone beds and bedsets show an overall lenticular geometry and are commonly overlain by mud drapes, which are massive and contain fewer bioturbation structures than do the hemipelagic muddy deposits. (iii) The mud drapes have a microstructure characterized by aggregates of clay particles, which show features similar to those of fluid‐mud deposits, and are interpreted to represent deposition from fluid mud developed from turbidity current clouds. (iv) Large‐scale erosional surfaces are infilled with thick‐bedded to very thick‐bedded turbidites, which show lithofacies quite similar to those of the surrounding deposits, and are considered to be equivalent to scours. (v) Concave‐up erosional surfaces, some of which face in the upslope direction, are overlain by backset bedding, which is associated with many mud clasts. (vi) Tractional structures, some of which are equivalent to coarse‐grained sediment waves, were also developed, and were overlain locally by mud drapes, in association with mud drape‐filled scours, cut and fill structures and backset bedding. The combination of these outcrop‐scale erosional and depositional structures, together with the microstructure of the mud drapes, can be used to identify canyon‐mouth deposits in ancient deep‐water successions.

The sedimentary architecture of outburst flood eskers: A comparison of ground-penetrating radar data from Bering Glacier, Alaska and Skeithararjokull, Iceland

We present ground-penetrating radar (GPR) profiles that reveal the sedimentary architecture of an esker deposited during a surge-associated outburst flood at the Bering Glacier, Alaska. The wide, up-flow end of the esker contains a transition from large backset beds to large foreset beds interpreted to reflect composite macroform development in an enlarged part of the conduit. By contrast, the narrow, down-flow portion of the esker is dominated by plane beds interpreted to have been deposited where the conduit was constricted and the flow was faster. A previously studied outburst esker at Skeidararjokull, Iceland, has a similar morphology and stratigraphic architecture. This suggests that outburst floods generate distinct depositional signatures in eskers, both in terms of morphology and sedimentary architecture. Identification of these distinct signatures in ancient eskers will help assess the paleohydraulic conditions under which ancient eskers formed and, by extension, the nature of meltwater drainage systems beneath the Laurentide and Eurasian ice sheets.

Sedimentary architecture of large-scale, jökulhlaup-generated, ice-block obstacle marks: Examples from Skeiðarársandur, SE Iceland

During glacial outburst floods (jokulhlaups) large ice-blocks are often released from the glacier margin. Ice-blocks ground at points of flow deceleration, generating distinctive patterns of scour and deposition scaled to the obstacle size. The large-scale ice-block obstacle marks produced within the proglacial zone are diagnostic of jokulhlaups. We use ground-penetrating radar (GPR) to investigate the sedimentary architecture of ice-block obstacle marks generated by the November 1996 jokulhlaup at Skeiðararjokull, Iceland. The GPR data reveal a varied sedimentary architecture, interpreted to suggest that ice-block obstacle mark sedimentology is controlled by flow conditions, sediment supply, and whether ice blocks are deposited in isolation or as a cluster. Following ice-block grounding, deflection of flow down and away from the obstacle develops horseshoe vortices that maintain a non-depositional hollow around the obstacle, whilst continued outwash deposition away from the obstacle raises the level of the surrounding sandur. Where ice-blocks ground early during the jokulhlaup, hollows are infilled with foreset beds at the up-flow end and backset beds (antidune) distally. However, where ice-blocks ground on the waning stage when sediment flux is reduced, antidunes are not preserved and the hollow is infilled with low-angle foreset beds. In both cases the lee-side shadow ridge is composed of pseudo-anticlinal beds proximal to the obstacle (where shed vortices are strong) and plane beds distally (where flows converge and the shed vortices weaken). Where flow parallel ice-block clusters develop, the grounding of smaller ice-blocks in the lee of the initial obstacle results in only limited shadow ridge aggradation. These data provide improved insight into the controls on ice-block obstacle mark sedimentary architecture and allow identification of high-magnitude jokulhlaups in the sedimentary record.

Gravelly Gilbert-type fan delta on the Conway Coast, New Zealand: Foreset depositional processes and clast imbrications

Quaternary Gilbert-style fan deltas of the Conway Coast, New Zealand, are formed and exposed by the continuing uplift and erosion of the Hawkswood Range due to transpression associated with the modern New Zealand plate boundary. The fan deltas are exposed in 50–70 m high cliffs and crop out for over 5 km along the coast and up to 2 km inland. Depositional processes on the foresets were dominated by sediment gravity flows originating from hyperpycnal river flow and gravity induced slumps. Slumps may originate at any depth on the fan delta foreset and are common in the deposits. Foreset beds are interpreted as the deposits of sediment gravity flows with preserved foreset beds that are preserving normally graded, inversely graded or massive, and change abruptly laterally and down slope. Gravity induced slumps are the dominant mode of deposition on the foresets along the Conway Coast and presumably all Gilbert-type fan deltas. This study concludes that the sediment gravity flows developed basal traction carpets as evidenced by the regular appearance of ∼ 25° imbricated clasts in nearly all foreset beds. Behind slump deposits, there is a strong zone of ‘backstacking’ clast fabrics that prograde upslope into backset beds. These clast fabrics provide information about downslope slumps. Excellent exposures of distal foreset and nearshore marine deposits show the dynamic interface within a fan delta complex.

Backset Bedding in Turbidites: A Further Example from the Cloridorme Formation (Middle Ordovician), Gaspe, Quebec

ABSTRACT Backset bedding was found in a thick (144 cm) turbidite bed in the Cloridorme Formation, Gaspe Peninsula, Quebec. The backsets occur in coarse-medium sand below a train of three trochoidal profiles (wavelength 30 cm, amplitude 4 cm) and above a basal division (9-13 cm thick) of parallel lamination. Flute casts on the sole of the bed, and ripple-drift cross-lamination--forming the preservation blanket covering the trochoidal profiles and backsets--allow paleocurrent direction measurements. The backset bedding is interpreted to result from the upstream migration of antidunes. A suitable analysis for the formation of antidunes by density currents uses an equation appropriate for internal waves at a density interface between two superimposed fluids which are moving parallel and in the same direction.

Backset bedding developed in shooting flow in laboratory experiments

ABSTRACT Backset bedding was produced in a laboratory flume by feeding sand into a shooting (supercritical) flow that terminated downstream in a hydraulic jump. Sediment accumulated as a mound at the site of the jump, increasing the tailwater depth and causing the jump to move upstream. Concurrent with the upstream movement of the jump, there was a growth of the sediment mound in both the upstream and downstream directions. The downstream growth of the mound formed typical foreset bedding, whereas the upstream growth of the mound resulted in the deposition of backset bedding inclined in the up-current direction. Backset bedding was also produced in experiments where antidune and chute and pool bed configurations were formed in a recirculating flume. These experiments support Davis' theory of the origin of backset bedding in the fluvioglacial outwash deposits of New England and offer a plausable explanation for the origin of this type of cross-bedding in other sedimentary formations.

Backset Beds in the Coso Formation, Inyo County, California

ABSTRACT Some cross-beds in coarse-grained sandstone and pebble conglomerate of the Coso Formation (lower Pleistocene) are inclined against the direction of flow of depositing currents. Textural gradation, mineralogical changes, and structural considerations show that the depositing currents flowed westward. About one half the cross-beds are inclined to the east, and the others are inclined to the west. The diametrically opposed directions do not fit the pattern expected of meandering streams. The eastward-inclined cross-beds are called backset beds. Backset beds are the planar (inclined) type and make an angle of 25° or less with the normal bedding planes. Cross-laminations are long tapered wedges with the wide part of the wedge at the base. Material within the wedge is coarse near the bottom and grades to finer material at the top. Normal foreset laminations are either the inclined or festoon (trough) type, and material within each lamination is well sorted and not graded. The backset beds are believed to have formed during the antidune phase of sediment transport described by G. K. Gilbert. The factors contributing to the antidune phase of sediment transport are high load, high velocity, high resistance, and shallow depth of water. Inasmuch as the deposits were probably deposited in a manner analogous to the heavily overloaded streams of the Basin and Range province today, all these factors could well have been present.