Turbidity currents are destructive flows that are hazardous to critical seafloor infrastructure on submarine slopes because run-up heights can be 10-100s of meters, as their relative density is 2-3 orders of magnitude lower than terrestrial flows. Currently, risk analysis is hindered by poor prediction of run-up heights that are mainly derived from confined 2D experiments, and/or numerical models, and are restricted to a specific configuration whereby the flow strikes topographic barriers orthogonally. Here, a new analytical model is presented, informed by and validated against physical experiments, which predicts run-up heights for flows encountering three-dimensional slopes as a function of any slope angle, and incidence angle, of the impinging turbidity current. This has important implications for reducing geohazards by informing routing and positioning of seafloor infrastructure, and for more accurately interpreting submarine landscapes and their deposits.

Gravity currents interacting with planar slopes have been thought to always 'reflect' a component of flow orthogonal to the slope irrespective of the flow incidence angle. Incoming flows are argued to undergo gravitational collapse, and generate internal waves, that propagate perpendicular to the bounding slope. These processes have been used to explain the widespread observation of palaeocurrents from sole marks at high angles to those in the associated ripple division. This paradigm for gravity current interactions with planar slopes has stood for more than three decades. Herein, these ideas are tested using three-dimensional low-density saline currents interacting with (but not overtopping) planar slopes of varying gradients, at a range of incidence angles. Fifteen new experiments show that the dominant flow process transitions from divergence-, through reflection- to deflection-dominated as the flow incidence angle decreases from 90 degrees to 15 degrees and the slope gradient increases from 20 degrees to 40 degrees. Multidirectional combined flows are documented above topographic slopes, varying as a function of location on a single slope, and the orientation and gradient of the slope. However, discrete internal waves are not observed, likely due to the spatial and temporal variability of flows on the slope. These findings challenge the paradigm of flow deflection and reflection in the existing model; there is not always a component of 'reflected' flow orthogonal to the planar slope. A new process model for flow-planar-slope interactions is presented. Flows broadly parallel to topographic slopes lead to up-slope and down-slope flow oscillations orthogonal to the bounding slope, providing new mechanics for the observation of high-angle variation between sole marks and ripple derived palaeocurrents in elongate basin-fills. Results also provide insights into the spatial distribution of distinctive combined-flow bedforms, which are most applicable to settings where flows interact with high-relief intrabasinal topography and/or basin margins.

Turbidity current behaviour is affected by interactions with seafloor topography. Changes in flow dynamics will depend on the physiographic configuration of the topography (orientation and gradient), and the character of the incoming flow (magnitude and rheology). A better understanding of how unconfined turbidity currents interact with topography will improve interpretations of the stratigraphic record; we address this using 3D flume tank experiments with unconfined saline density currents 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, super-elevation 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 super-elevation 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 topographies; a planar slope 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) and triggers for soft-sediment deformation processes to produce a spatial model of gravity-current interaction, and deposition, on slopes to support palaeogeographic reconstructions.

Nearshore incised valleys are important conduits for the transport of sediment, nutrients, pollutants and organic carbon from the continents to the sea. Therefore, it is essential to understand the autogenic evolution of deltas confined within incised valleys and how such evolution is affected by relative sea-level rise. To date, limited research has focused on how deltas constrained by incised valleys or other forms of antecedent topography respond to rising sea level. An existing theory of autostratigraphy envisages scenarios in which two-dimensional or unconfined three-dimensional fan deltas can experience three evolutionary stages under constant rates of relative sea-level rise and sediment supply: progradation, autoretreat and post-autobreak transgression. In this work, an integrated study of geometric numerical models and physical experiments is undertaken to investigate autostratigraphic delta evolution for a variety of incised-valley geometries, under conditions of constant rates of relative sea-level rise and sediment supply. Results indicate that interplays of antecedent topography (valley geometries) and sediment mass balance expressed in resultant deltaic geometries can result in autogenic changes in shoreline dynamics and river avulsion frequency on deltas. The following primary findings arise. (i) Compared to valleys with rectangular and trapezoidal cross-sectional profiles, valleys with triangular cross-sections tend to contain deltas that experience faster rates of progradation, autoretreat and post-autobreak transgression under rising sea level, and exhibit a more prominent convex-seaward shoreline trajectory. (ii) The shoreline trajectory is also related to delta topset geometry, becoming more convex-seaward under decreasing topset slopes. (iii) River avulsion frequency on deltas with rising sea level varies markedly across valleys with different geometries, even under the same rate of relative sea-level rise; this is attributed to the difference in temporal evolution of shoreline migration for different valley geometries and the resultant difference in the delta topset aggradation. This study highlights complexities in responses of sedimentary systems under the confinement of different topographic configurations that have hitherto largely been overlooked in sequence-stratigraphic models. The findings provide insight into future shoreline behaviour and river avulsion hazard on confined deltas, and for decoding the stratigraphic record.

For improved prediction of reservoir distribution and quality in the infill of rift basins, it is paramount to characterize the extent and relative dominance of simultaneously active depositional systems fed by axial and transverse drainages, and to understand how these vary in response to geological controls. This study demonstrates how integration of seismic, wireline-log, core, petrographic and heavy-mineral datasets, combined with existing palaeotopographic reconstructions, can be utilized to characterize the spatiotemporal distribution and reservoir potential of syn-rift depositional systems associated with axial and transverse sediment routes to a lacustrine basin. In the Eocene of the Dongpu Depocentre (Bohai Bay Basin, China), three distinct delta types are identified: footwall-derived coarse-grained fan deltas and sand-rich deltas, and axial deltas that are dominated by siltstone and fine sandstone. The spatiotemporal distribution of these systems in the basin fill arose from the interaction of tectonic, lake-level, and climatic controls. Sedimentological and provenance analyses indicate the predominance (>85%) of axial systems over transverse systems in the infill of this elongate, asymmetrical graben. Locally, the interaction and amalgamation of transverse and axial systems was controlled by the presence of intra-basinal highs, through their influence on sediment-delivery pathways, and by high rates of sediment supply that caused overfill of isolated depocentres. Texturally mature, well-sorted fine sandstones of axial deltaic systems, which were sourced from comparatively large catchments, are attractive reservoir targets. By contrast, conglomerates of transverse fan deltas are poorly sorted and have more limited reservoir potential. However, footwall-derived fans may serve as viable reservoir targets locally, where fed by major hinterland drainages associated with long sediment-transport distances. Identification of the position of relay ramps in seismic data and recognition of bedrock-incised drainage outlets on the footwall basement of the basin margin may allow prediction of the presence and location of reservoir units associated with oversized deltas.

Namurian (Carboniferous) eustatic fluctuations drove the incision and backfill of shelf‐crossing valley systems located in humid subequatorial regions, which are now preserved in successions of the United Kingdom and Ireland. The infills of these valleys archive the record of palaeoriver systems whose environmental, hydrological and palaeogeographic characteristics remain unclear. A synthesis of sedimentological data from fluvial strata of 18 Namurian incised‐valley fills in the United Kingdom and Ireland is undertaken to elucidate the nature of their formative river systems and to refine regional palaeogeographic reconstructions. Quantitative analyses are performed of facies proportions, of geometries of incised‐valley fills and related architectural elements, and of the thickness of dune‐scale sets of cross‐strata. Reconstruction of the size of the drainage areas that fed these valleys is attempted based on two integrative approaches: flow‐depth estimations from dune‐scale cross‐set thickness statistics and scaling relationships of incised‐valley fill dimensions derived from late‐Quaternary examples. The facies organization of these incised‐valley fills suggests that their formative palaeorivers were perennial and experienced generally low discharge variability, consistent with their climatic context; however, observations of characteristically low variability in cross‐set thickness might reflect rapid flood recession, perhaps in relation to sub‐catchments experiencing seasonal rainfall. Variations in facies characteristics, including inferences of flow regime and cross‐set thickness distributions, might reflect the control of catchment size on river hydrology, importance of which is considered in light of data from modern rivers. Palaeohydrological reconstructions indicate that depth estimations from cross‐set thickness contrast with observations of barform and channel‐fill thickness, and projected thalweg depths exceed the depth of some valley fills. Limitations in data and interpretations and high bedform preservation are recognized as possible causes. With consideration of uncertainties in the inference of catchment size, the palaeogeography of the valley systems has been tentatively reconstructed by integrating existing provenance and sedimentological data. The approaches illustrated in this work can be replicated to the study of palaeohydrological characteristics and palaeogeographic reconstructions of incised‐valley fills globally and through geological time.

Incised valleys that develop due to relative sea-level change are common features of continental shelves and coastal plains. Assessment of the factors that control the geometry of incised-valley fills has hitherto largely relied on conceptual, experimental or numerical models, else has been grounded on case studies of individual depositional systems. Here, a database-driven statistical analysis of 151 late-Quaternary incised-valley fills has been performed, the aim being to investigate the geological controls on their geometry. Results of this analysis have been interpreted with consideration of the role of different processes in determining the geometry of incised-valley fills through their effect on the degree and rate of river incision, and on river size and mobility. The studied incised-valley fills developed along active margins are thicker and wider, on average, than those along passive margins, suggesting that tectonic setting exerts a control on the geometry of incised-valley fills, probably through effects on relative sea-level change and river behaviour, and in relation to distinct characteristics of basin physiography, water discharge and modes of sediment delivery. Valley-fill geometry is positively correlated with the associated drainage-basin size, confirming the dominant role of water discharge. Climate is also inferred to exert a potential control on valley-fill dimensions, possibly through modulations of temperature, peak precipitation, vegetation and permafrost, which would in turn affect water discharge, rates of sediment supply and valley-margin stability. Shelves with slope breaks that are currently deeper than 120 m contain incised-valley fills that are thicker and wider, on average, than those hosted on shelves with breaks shallower than 120 m. No correlation exists between valley-fill thickness and present-day coastal-prism convexity, which is measured as the difference in gradient between lower coastal plains and inner shelves. These findings challenge some concepts embedded in sequence stratigraphic thinking, and have significant implications for analysis and improved understanding of 'source to sink' sediment route-ways, and for attempting predictions of the occurrence and characteristics of hydrocarbon reservoirs.

Facies models of the internal fills of incised valleys developed in shelf and coastal settings during cycles of relative sea-level change are largely conceptual, descriptive and qualitative in form; moreover, they are commonly bespoke to individual examples. Here, a database-driven quantitative statistical analysis of 87 late-uaternary incised-valley fills (IVFs) has been undertaken to assess the general validity and predictive value of classical facies models for IVFs, and to investigate the relative importance of possible controls on their strati graphic organization. Based on datasets from the published literature stored in a sedimentological database, the geometry and proportion of systems tracts, and of architectural elements of different hierarchies within IVFs are quantified. These variables were analysed to assess how they vary in relation to parameters that represent potential controlling factors: relative sea-level stage, continental-margin type, drainage-basin area, valley geometry, basin physiography and shoreline hydrodynamics. The stratigraphic organization of the studied coastal-plain IVFs is generally consistent with that represented in fades models, the primary control being the rate and magnitude of relative sea-level change. However, results from this study demonstrate significant variability in the stratigraphic architectures of IVFs, which is not accounted for by existing models. Variations in the fades architecture of coastal-plain and cross-shelf valley fills can be attributed to controls other than sea level, and expressed in relationships with continental-margin type, basin physiography, catchment area, river-system size and shoreline hydrodynamics. The following primary findings arise from this research. (i) Compared to their counterparts on passive margins, coastal-plain IVFs hosted on active margins contain, on average, a higher proportion of fluvial deposits and a lower proportion of central-basin estuarine deposits; estuarine deposits tend however to be thicker. This suggests a control on IVF stratigraphic architecture exerted by distinct characteristics of the tectonic setting of the host continental margins, notably basin physiography, rates and mode of sediment supply, and nature of sediment load. (ii) The thickness and proportion of lowstand systems tract are positively correlated with coastal-plain IVF dimensions, likely reflecting the role of drainage-basin area in dictating the scale of the fluvial systems that carved and infilled the valleys. (iii) Positive correlations are observed between the thickness of fluvial deposits, bayhead-delta deposits and central-basin estuarine deposits, versus coastal-plain IVF dimensions and valley catchment area. This suggests a control exerted by the river-system scale on sediment-supply rates and on the accommodation determined by valley size. (iv) Positive correlations between the thickness and proportion of barriercomplex deposits within cross-shelf IVFs versus mean shelf gradient indicate that the geometry of the shelf might control the establishment and preservation of barrier-island environments in incised valleys located on the shelf. (v) Correlations between the width of coastal-plain IVFs and present-day mean tidal range at the shoreline indicate that tidal dynamics may contribute to the widening of the incised valleys. Positive correlation is observed between the proportion of tide-dominated elements in highstand IVF deposits and IVF width, suggesting possible effects of interplays between hydrodynamic conditions and the geometry of incised valleys on their infills. This study highlights the complexity of the internal fills of incised valleys, which must be considered when attempting the application of fades models of IVFs to rock-record interpretations or as predictive tools in subsurface studies.