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ABSTRACT Turbidity current behaviour is affected by interactions with seafloor topography. Changes in flow dynamics will depend on the orientation and gradient of the topography, and the magnitude and rheology of the incoming flow. A better understanding of how unconfined turbidity currents interact with topography will improve interpretations of the stratigraphic record, and is addressed herein using three‐dimensional flume tank experiments with unconfined saline density currents that enter a horizontal basin before interacting with a ramp orientated perpendicular to flow direction. The incoming flow parameters remained constant, whilst the slope angle was independently varied. On a 20° slope, superelevation of the flow and flow stripping of the upper, dilute region of the flow occurred high on the slope surface. This resulted in a strongly divergent flow and the generation of complex multidirectional flows (i.e. combined flows). The superelevation and extent of flow stripping decreased as the slope angle increased. At 30° and 40°, flow reflection and deflection, respectively, are the dominant flow process at the base of slope, with the reflected or deflected flow interacting with the parental flow, and generating combined flows. Thus, complicated patterns of flow direction and behaviour are documented even on encountering simple, planar topographies orientated perpendicular to flow direction. Combined flows in deep‐water settings have been linked to the interaction of turbidity currents with topography and the formation of internal waves with a dominant oscillatory flow component. Here, combined flow occurs in the absence of an oscillatory component. A new process model for the formation and distribution of hummock‐like bedforms in deep‐marine systems is introduced. This bedform model is coupled to a new understanding of the mechanics of onlap styles (draping versus abrupt pinchout) to produce a spatial model of gravity‐current interaction, and deposition, on slopes to support palaeogeographical reconstructions. [ABSTRACT FROM AUTHOR]

Inherited rift topography controls the sediment routing, timing of sand supply, and sedimentary linkage of early post‐rift depocentres. Exhumed examples of early post‐rift turbidite systems are rare and previous studies have examined the evolution of individual depocentres; in contrast, the detailed evolution of early post‐rift turbidite systems across multiple depocentres has never been documented. Current fill‐and‐spill models do not detail the stratigraphic architecture and evolution of sedimentological characteristics of multiple intraslope fans developed across topography, including bed type distributions. Here, the evolution of three intraslope fans that developed across two early post‐rift depocentres is documented along an 18 km long transect in the southern Neuquén Basin, Argentina. The relative chronology of sand supply in depocentres is constrained with new U–Pb ages, and sediment source areas with provenance analysis. The early post‐rift intraslope fans record progradation of the system and progressive sedimentary linkage of post‐rift depocentres, transverse to local syn‐rift structures, with sediment routing subparallel to the cratonic basin margin. The large‐scale stratigraphic architecture of intraslope fans indicates an evolution as a fill‐and‐spill system, with initial confinement through flow stripping and overspill to spillover with erosion and bypass across a transverse topographic high separating the depocentres. Changes in early post‐rift intraslope fan characteristics, including thickness, sandstone content, lobe complex stacking patterns, stratal termination patterns and bed type distribution, record changing confinement through time within a depocentre, and spatially across depocentres. The strong spatial and vertical stratigraphic variability of transitional flow deposits and hybrid event beds reflects enhanced erosion, sediment bypass and flow transformation across transverse relief between the two depocentres during the spillover phase. These findings advance current understanding of early post‐rift turbidite systems and refine fill‐and‐spill models, which will help the prediction of spatial and vertical changes in rock quality and connectivity in subsurface hydrocarbon reservoirs and CO2 storage sites. [ABSTRACT FROM AUTHOR]

ABSTRACT Sole structures on the base of turbidites, and other bed types, are typically classified into scour marks and tool marks, such as flutes, grooves, skim marks and prod marks. Yet, there are a range of other common sole marks that are unrelated to scouring or tools, and whose origin is poorly understood. Prominent among these sole structures are longitudinal ridges and furrows, and ‘dinosaur leather’ structures associated with mud ripples. Herein, these features are described and it is argued that they are the product of deformation of the substrate during a sediment gravity flow event. In these flow‐induced interfacial deformation structures (FIDS), a soft cohesive substrate undergoes deformation in response to a buoyant force induced by the denser basal component of an overriding flow, and the flow interacts with this buoyant deformation through shear to remould the substrate. Variations in the relative strength of these buoyant and shear‐induced forces explain the wide range of FIDS that can form. This FIDS model reinterprets the formation of longitudinal ridges and furrows, which have previously been classified as scour marks, and explains their distinctive spatial patterns. Furthermore, the new model builds on the seminal work of Dżułyński and colleagues in the 1960s and 1970s, who identified that these structures contain key palaeocurrent information, and it is argued that such information is largely under‐utilized. Importantly, alongside their utility as palaeocurrent indicators, FIDS provide insights into the rheology of the substrate at the time of their formation, and thus the nature of basal flow conditions in the formative flows. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Fluvially‐dominated, fine‐grained, shallow‐water deltas are more variable than is generally recognized. Studies of the Mississippi River imply rather persistent river‐mouth jet dynamics, resulting in either progressive channel bifurcation around middle‐ground mouth bars on lobate deltas, or steady channel progradation to form elongate deltas akin to the modern Mississippi Delta. By contrast, satellite imagery and historical maps of less well‐known fluvially‐dominated deltas show diverse river‐mouth deposits, with plan‐form shapes ranging from river‐mouth fans, through splay, triangular, frond and tongue‐shaped mouth bars, to elongate channels with prominent subaqueous levées. Critically, such deposits may vary along individual channels and across or between individual lobes suggesting that jet variability can be normal, and that a spectrum of jets forms a suite of stable river‐mouth deposits. Channel elongation can also be common. In addition to high‐inertia jets and delta‐head or backwater‐mediated avulsion, two further mechanisms are recognized to form elongate channels. Following channel splitting, dominant splits can shoulder aside mouth bars, so that subordinate splits wither and dominant splits extend the parent channel. Alternatively, river‐mouth fans, subordinate distributary networks and lobes may be abandoned, so that a dominant parent distributary is rejuvenated by receiving an increase in discharge, allowing progradation to continue. Individual real‐world deltas are expected to be characterized by a range of river‐mouth deposits, both laterally and over time. However, such variable river‐mouth deposits, and different modes of channel elongation, are not generally considered in models of fluvially‐dominated deltas – omissions that may have significant impact for land remediation projects on modern deltas, and descriptions of the subsurface constructed to aid resource extraction. [ABSTRACT FROM AUTHOR]

Submarine meandering channels are conduits that transport gravity-driven flows and sediments into the deep sea. Such channel systems form distributive networks across submarine fans, ultimately forming the largest sedimentary deposits on Earth. Despite this, our understanding of flow processes and the sedimentary evolution of sinuous submarine channel systems remains poor, primarily due to a lack of, thus far elusive, direct field observation and measurements of flows within submarine channel bends. In the absence of direct field measurements, our understanding of these systems has necessarily been speculative, relying until very recently on bend flow theory derived from research in subaerial river channel bends. Although recent measurements and results from scaled laboratory experiments and numerical modeling of submarine channel bends have advanced our understanding of some of the relations between flows, forms, and the implications for sedimentary deposits, key results from this recent research have been contradictory. Notably, several studies have indicated that the helical flow structures within submarine bend flows closely match those found ubiquitously in subaerial river channels, while conflicting research has reported a reversal of this helical flow structure, with associated far-reaching implications for deep-sea sedimentology. This paper presents the first direct three-dimensional measurements of the flow field in a natural submarine channel bend. The results, from a submarine channel bend on the Black Sea shelf, demonstrate for the first time that a reversed helical flow structure can occur in seafloor channel bends. Such findings have major implications for process sedimentology in these environments, because the direction and strength of helical flow fields are known to impart a significant influence on cross-stream sediment sorting in bends and thus the stratigraphy of the deposits produced by these channel systems. doi: 10.1130/G31121.1

The planform geometry of submarine channels commonly exhibits a spatio-temporal stability generally not observed in fluvial channels. As such, submarine channels tend to lack the meander loop cutoffs and frequent avulsion history typical of fluvial channels. Fluvial sinuosity develops through inner-bend deposition and outer-bend erosion. Inner-bend deposits have also been recognized in submarine channels, from subsurface and seafloor images and from ancient channel outcrops, and have been demonstrated within physical models. However, outer-bend sediment accumulations are a feature thought to be unique to submarine channels. We report on physical experiments on channelized, subaqueous, particle-driven turbidity currents that demonstrate that channel-fill architecture relates directly to the degree of flow bypass, in turn largely determined by the degree of confinement. In general, weakly bypassing flows deposit at the outer bend, whereas strongly bypassing flows deposit at the inner bend. Therefore flows within aggradational channel systems whose axes are bypass dominated may preferentially deposit at the inner bend, ultimately having the effect of increasing channel sinuosity through time; this is an evolution pattern commonly observed in seismic images. Once developed, the apparent spatio-temporal longevity of sinuosity within many systems may be explained by the passage of turbidity currents of varying magnitude (and consequently bypass potential) depositing preferentially at either the inner or outer bank of the channel, maintaining a quasi-stable morphological equilibrium. Fluvial channels do not have the ability to reduce or maintain their sinuosity in this way, which plausibly explains why they tend to develop cutoffs at higher rates than subaqueous channels. Keywords: turbidity currents, channel geometry, sedimentary processes, submarine fans, turbidite.

Meander cutoffs and oxbow lakes are very common features of fluvial landscapes that add complexity and diversity to floodplain alluvial architecture and riverine habitats. Following initial cutoff, sediment accumulates within the entrance and exit of the original bend forming plugs that eventually disconnect the abandoned bend from the main channel. While studies have examined the sedimentology of these plugs once they have fully disconnected from the abandoned bend, fewer studies have detailed the sedimentological processes occurring during the early stages following cutoff initiation. Furthermore, recent studies have highlighted the importance that planform geometry plays in the evolution of neck cutoffs. This study examines the spatial depositional patterns of two neck cutoffs on the White River in central Arkansas, USA, that remain hydrologically connected to the main channel, providing a unique opportunity to research sedimentological processes before disconnection of the abandoned bend. Sediment cores were collected in key locations of each cutoff, including the entrance and exit of the abandoned bends, abandoned bend apices, and newly developed cutoff bars in the downstream channel. The cores were logged and interpreted and grain‐size analyses were performed. In addition to sediment cores, repeat high‐resolution multibeam echo sounding surveys were conducted roughly four hours apart to estimate bedload transport rates and patterns of bedload routing through each cutoff. Results from this research are different from previous studies. Sediment core results show a pattern of deposition typically associated with lower diversion angle chute cutoffs instead of higher diversion angle neck cutoffs. Previous research has indicated that plugging of abandoned bends drives disconnection from the active channel; however, this research shows that disconnection is more associated with the prevention of sediment from being delivered to the abandoned bends due to flow being pulled away from the abandoned bends and the evolving channel morphology. [ABSTRACT FROM AUTHOR]