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Chutes‐and‐pools are hybrid bedforms that occur in upper‐flow‐regime conditions, populating the stability field between antidunes and cyclic steps. Chutes‐and‐pools consist of a superimposition of a train of antidunes on a longer‐wavelength cyclic‐step instability. The presence of a hydraulic jump is more persistent in chutes‐and‐pools than the bores occasionally seen on breaking‐wave antidunes, yet not permanent as over cyclic‐step bedforms. The nature of chute‐and‐pool structures preserved in the depositional record is controlled largely by aggradation rate. This paper documents chute‐and‐pool structures generated beneath highly aggradational sediment density‐flows at the subaqueous toe‐of‐slope of a Pleistocene carbonate wedge. Quarry exposures enable the morphodynamic reconstruction of the sediment‐bed configuration that produced the identified structures. Wave‐breaking on growing antidunes occurred without the destruction of the antidune, which deviates from observations in low‐aggradation subaerial open‐channel flows. The antidune build‐up was followed by hydraulic jump controlled deposition in the antidune trough. Two‐dimensional depth‐resolved numerical simulations have been used to validate this process interpretation. The term aggradational chutes‐and‐pools is proposed for these bedforms, associated with the formation of build‐and‐fill structures consisting of interstratified convex lenses (in‐phase wave regime) and concave lenses (hydraulic jump regime). Such structures contrast with the scour‐and‐fill structures produced by low‐aggradation chutes‐and‐pools frequently described from subaerial open‐channel‐flow deposits. This study demonstrates a potential difference in flow behaviour between open‐channel flows and density flows in the supercritical regime. [ABSTRACT FROM AUTHOR]

Cyclic‐step bedforms typifying a Froude‐supercritical flow regime are a recurrent component of depositional/erosional turbiditic systems. Over modern delta slopes, cyclic steps have been inferred from observations of upslope‐migrating crescent‐shaped bedforms. However, the recognition in the sedimentary record of the resulting stratal pattern and depositional facies remains challenging. In this study, the depositional facies observed in exposed late Quaternary glaciofluvial upper delta‐slope sands (Portneuf‐Forestville, Québec) are compared to those cored from a modern analogue consisting of sediment waves interpreted as cyclic steps (Southwind Fjord, Baffin Island). The fossil and modern delta slopes share similar context, morphology and stratigraphic record. The clinoform foresets dip 2 to 6° and consist of prevailing sand‐sized material including subhorizontal to upslope‐dipping top‐cut‐out turbidites. Individually, the latter are 5 to 20 cm thick and massive to planar laminated (prevailing TA and TB subdivisions). In the fossil delta slope, related successions form relatively thick, well‐bedded suites, which are truncated downcurrent by, and onlap upstreamward on, inclined composite erosion surfaces here referred to as pseudo‐foresets. Pseudo‐foresets are regularly spaced (10 to 30 m) and have dips greater than the clinoform foresets (ca 20°). Large composite scours form pseudo‐channel structures filled in by structureless pebbly sand deformed by sheared flame structures, in association with coarse sand showing undulating lamination and rip‐up clasts. Similar depositional facies are observed on the modern delta slope. The stratal pattern is best compatible with upslope‐migrating bedforms and structureless sand indicates hydraulic jump deposits typical of cyclic steps. Cyclic‐step flow events, encompassing a succession of genetically linked erosional cyclic steps, depositional cyclic steps and subsequent waning‐flow conditions, were associated with the dense basal layer of high‐density (stratified) turbidity currents. They are specifically associated with pseudo‐channel incision and infill. The deposition of well‐bedded suites on the stoss side, occasionally also on the lee side, is interpreted to result from a different type of turbidity current, which decelerated over the bedform stoss and accelerated over the lee sides of pre‐existing cyclic‐step morphologies. Antidunal flow conditions in expanded (non‐stratified) turbidity currents are tentatively suggested, deposition being linked, in this case, to interactions between inherited bedform morphologies and a near‐bed tractive layer. In the resulting dual flow model, the bulk of delta‐slope sands was mainly deposited from turbidity currents not developing a cyclic‐step instability, yet cyclic steps were instrumental in shaping and/or re‐organizing the delta‐slope morphology, bedforms and resulting stratal patterns. In some delta systems, the upslope migration of supercritical crescentic bedforms may not only be due to repetitive cyclic steps but could also result from antidunal turbidity current conditions remobilizing an inherited cyclically‐stepped morphology. [ABSTRACT FROM AUTHOR]

Stable supercritical‐flow bedform phases under two‐dimensional steady flow are geometrically simple and include long‐wavelength cyclic steps at high Froude numbers and antidunes characterized by in‐phase flow that is near critical. Less well understood are the transitional bedform phases at the boundaries of the stable bedform fields and bedforms developing in complex flow geometries like the channel–lobe transition zone. This complexity is exacerbated by the fact that natural flows are never steady. Stable antidune bedforms may be reworked by temporally increasing discharge into chute and pool, and cyclic step and chute and pool fields will be reworked into antidunes if discharge is sufficiently decreasing. In addition, the channel–lobe transition zone is continuously evolving in space and time due to the influence of solitary hydraulic jumps at the channel mouth on channel extension and back stepping. This detailed outcrop study of a deep‐water delta slope belonging to the Eocene Sant Llorenç del Munt clastic wedge exposed near El Pont de Vilomara (north‐east Spain), tackles the complex bedform architecture problems by applying a method previously developed for fluvial deposits. Analysis of surfaces traced on high‐definition, drone‐derived in‐strike images combines architectural studies with facies analysis. Set boundaries of the bedforms were thus established, revealing the upslope migration of hydraulic jump zones and the intricate stacking of antidunes and solitary, mouthbar related chute and pool like structures. Further analysis of the stacking of bedforms and bounding surfaces provide evidence that deposition occurred in a relatively short (few hundreds of metres) channel–lobe transition zone at the base of the delta slope. The usefulness of the bounding surface hierarchy approach for turbidite deposits lies in the careful evaluation of the spatial extent of bounding surfaces, which are easily overlooked in complex architectures such as those created in the channel–lobe transition zone. [ABSTRACT FROM AUTHOR]

Cyclic steps are a type of upper flow-regime bedform consisting of trains of bed undulations with hydraulic jumps in the troughs. They form in free-surface flows and comprise a common, if not the dominant, bedform in subaqueous density flows in oceans, lakes and reservoirs. Cyclic steps originate from alternating Froude-subcritical and Froude-supercritical flow on respectively the stoss side and lee side of individual bedforms. The transition between these flow states is embodied by the hydraulic jump in the trough of the bedform, leading to the permanent or quasi-permanent morphology of cyclic steps. Over the past decade, numerous studies affirmed the dominant role of cyclic steps in generating bed undulations in modern and ancient glacial outwash, deltaic and turbidite environments. Cyclic steps were previously discriminated as net-depositional, transportational and net-erosional in different parts of such systems.These bedforms are here classified as climbing, transportational and falling cyclic steps, respectively. In analogy to the classification of ripples and dunes, the adjectives 'supercritically' and 'subcritically' can be added to indicate the preservation or not of the lee side, respectively. To avoid confusion between the direction of crest-migration (indicated by the terms subcritically and supercritically climbing or falling) and the flow regime (Froude-subcritical and Froude-supercritical flow), it is in most cases preferable to use the following cyclic step descriptors: (1) fully depositional (supercritically climbing), (2) partially depositional (subcritically climbing), (3) partially erosional (subcritically falling) and (4) fully erosional (supercritically falling). The new classification defines distinct depositional signatures for each type of cyclic steps. The most common type of cyclic steps is partially depositional, which is associated with nested scours filled with massive to backset-bedded deposits.The new classification can potentially be used as a predictive tool in the reconstruction of modern and ancient turbidite environments using repeat bathymetry, seismic reflection or outcrop data. To demonstrate this, the classification is applied to the deep-marine West Penghu Channel in the South China Sea, the Howe Sound fjord-delta in British Columbia, Canada and the Pleistocene carbonate slope of Favignana Island, Italy. [ABSTRACT FROM AUTHOR]