/pdf/morphologic-and-facies-trends-through-the-fluvial-marine-28tnn15m7c.pdf

Morphologic and facies trends through the fluvial–marine transition in tide-dominated depositional systems: A schematic framework for environmental and sequence-stratigraphic interpretation

Supercritical-flow phenomena are fairly common in modern sedimentary environments, yet their recognition and analysis remain difficult in the stratigraphic record. This fact is commonly ascribed to the poor preservation potential of deposits from high-energy supercritical flows. However, the number of flume data sets on supercritical-flow dynamics and sedimentary structures is very limited in comparison with available data for subcritical flows, which hampers the recognition and interpretation of such deposits. The results of systematic flume experiments spanning a broad range of supercritical-flow bedforms (antidunes, chutes-and-pools and cyclic steps) developed in mobile sand beds of variable grain sizes are presented. Flow character and related bedform patterns are constrained through time-series measurements of bed configurations, flow depths, flow velocities and Froude numbers. The results allow the refinement and extension of some widely used bedform stability diagrams in the supercritical-flow domain, clarifying in particular the morphodynamic relations between antidunes and cyclic steps. The onset of antidunes is controlled by flows exceeding a threshold Froude number. The transition from antidunes to cyclic steps in fine to medium-grained sand occurs at a threshold mobility parameter. Sedimentary structures associated with supercritical bedforms developed under variable aggradation rates are revealed by means of combining flume results and synthetic stratigraphy. The sedimentary structures are compared with examples from field and other flume studies. Aggradation rate is seen to exert an important control on the geometry of supercritical-flow structures and should be considered when identifying supercritical bedforms in the sedimentary record.

/pdf/morphodynamics-and-sedimentary-structures-of-bedforms-under-2rymedppki.pdf

Morphodynamics and sedimentary structures of bedforms under supercritical-flow conditions: New insights from flume experiments

A comparative study of sediment waves and cyclic steps based on geometries, internal structures and numerical modeling

As defined in this chapter, an estuary forms during a shoreline transgression and then fills during a progradational phase that is transitional to a delta. The spatial distribution of processes, grain sizes and facies within tide-dominated estuaries is predictable in general terms. Tidal currents dominate sedimentation along the axis, with wave-dominated sedimentation occurring along the flanks of the estuary in its outer part. Tidal energy increases into the estuary but then decreases toward the tidal limit, with a gradual transition to river-dominated sedimentation at its head. The interaction of the tidal wave with the morphology of the estuary, and with river currents, causes the outer estuary to be flood-dominant, with a net landward movement of sand. By contrast, the inner estuary is ebb-dominant, creating a bedload convergence within the estuary. The axial sandy deposits are typically finest at this location. In transgressive-phase estuaries, the main channel shows a low—high—low pattern of sinuosity, with the tightest bends (sinuosity ≥ 2.5) occurring at the bedload convergence. These bends experience neck cutoff in the transition to the progradational phase of estuary filling. The estuary-mouth region is characterized by cross-bedded sands deposited on elongate sand bars, although wave-generated structures can be important in some cases. Estuaries that are down-drift of major rivers have anomalously muddy outer estuarine deposits. Further landward, upper-flow-regime parallel lamination can be widespread. The margins of the inner estuary are flanked by muddy salt-marsh and tidal-flat deposits that can contain well-developed tidal rhythmites and evidence of seasonal variations in river discharge.

Processes, Morphodynamics, and Facies of Tide-Dominated Estuaries

Facies analysis of the Upper Cretaceous (Turonian) Ferron Sandstone in the western Henry Mountains of south-central Utah, U.S.A., indicates sediment accumulation in a series of flood-dominated, marine current- and wave-influenced, deltas. Twelve lithofacies are recognized: 1. erosionally based, thick cross-bedded sandstone bodies (Distributary Channels), 2. similar though thinner bodies containing common bioturbation (Marine-Influenced Distributary Channels), 3. root-penetrated, plant fossil bearing siltstone with minor sandstone (Coastal Floodplain and Floodbasin), 4. coal and carbonaceous shale (Coastal Mire), 5. thin-bedded, carbonaceous and bioturbated sandstone–siltstone (Coastal Lagoon), 6. erosionally based sandstone with large- and small-scale cross-bedding and bioturbation (Mouth-Bar Complex), 7. sharply based sandstone bodies internally dominated by hummocky cross-stratification, soft-sediment deformation, or lacking structure (Proximal Delta Front), 8. sharply bounded, calcareous, fossiliferous and bioturbated sandstone sheets (Abandoned Delta Lobe), 9. thickly interbedded sandstone, coarse- and fine-grained siltstone (Medial Delta Front), 10. thinly interbedded sandstone, coarse- and fine-grained siltstone (Distal Delta Front), 11. mainly siltstone with minor thin-bedded sandstone (Prodelta) and 12. fine-grained siltstone with bentonite beds (Offshore). Deltas prograded into shallow water, forming sharp-based mouth-bar sand bodies. The upper delta front was evidently fluidal and prone to failure, leading to the development of rotational slope failures, debris–flow filled gullies, and, in places, growth faults. Paleocurrent data indicate that the regional sediment dispersal direction was eastward. Data from delta-front facies, however, suggest that outflow plumes and associated bottom currents were deflected towards the southeast, giving rise to an asymmetric delta planform. The Holocene and modern Burdekin River Delta of NE Australia is considered a close planform, process, and facies analog for the Ferron Notom deltas. The Burdekin Delta facies assemblage is vertically and laterally heterogeneous, despite being the product of a consistent array of environmental controls. Adopting a model that incorporates such a degree of heterogeneity negates the need for multiple depositional models for complex stratigraphic intervals such as the Ferron Sandstone. The facies model also suggests that asymmetric deltas may be produced by directional growth of delta lobes, rather than by deflection of beach ridges. Stratigraphic stacking patterns strongly suggest that sediment accumulation in the Ferron Sandstone of the western Henry Mountains was forced by a regime of progressively more limited accommodation through time.

Planform and Facies Variability in Asymmetric Deltas: Facies Analysis and Depositional Architecture of the Turonian Ferron Sandstone in the Western Henry Mountains, South-Central Utah, U.S.A.

n mesotidal settings the transition of a coastal plain estuary to the river is marked by the change of a multiple ebb and flood channel configurationto a single channel system. At high river discharge fluvial processes operate, whereas in periods of low discharge the flow is complicated by a tidalcomponent and a landward intrusion of the salt wedge. These hydraulic and morphological characteristics make the transitional zone differentfrom the ‘pure’ fluvial and estuarine environment. Inspection of published and unpublished data from a number of outcrops of Recent andTertiary deposits of the Rhine reveals that also in a sedimentary sense a transitional zone can be recognized. In order to separate this zone fromthe upstream fluvial and downstream estuarine environment a sedimentological definition of the fluvial-tidal zone is proposed being the part ofriver that lies between the landward limit of observable effects of tidal-induced flow deceleration on fluvial cross-bedding at low river dischargeand the most seaward occurrence of a textural or structural fluvial signature related to the high river stage.Many structures of the fluvial-tidal zone suggest conditions of rapid deposition. This may reflect the high mobility of channels that characterisethe fluvial-tidal zone, or deposition in a bay-head delta setting. The diagnostic features of the fluvial-tidal transition zone, as manifested in thestudied outcrops, are described and illustrated in detail. They primarily refer to large-scale dune cross bedding and decimetre- scale successions(cycles) in vertical sense:Dune cross bedding: Foresets tend to be more or less regularly spaced or bundled. These bundles are often delineated by slightly erosive discontinuities,arranged in a daily variance mode due to remote tidal effects. The discontinuities are draped by silt and peat detritus rather than byclays, as is the case in the estuarine environment. In the pebble size grade, peat also predominates over clay. The dune foresets often passdownward into thick bottomsets, produced by suspension load on the gently inclined toe of the dune, leading to a knicked (‘shovel’) appearanceof this assemblage. In contrast with the fluvial and tidal counterparts, bottomsets in the transitional zone show a disorder in direction of crosslaminations.Foresets are frequently used by ripples moving up high on them (‘setclimbers’). These ripples usually are not of the backflow typeas found in fluvial deposits. Directions of ripple movement parallel to foreset-strike are also found. Incidentally, setclimbing is developed to sucha degree that entire crossbedded sets are made up of multi directional small scale cross lamination.Decimetre-scale successions: In the case of tidal deposits decimeter-scale rhythmic successions are generated either by seasonal changes or springneaptidal cyclicity. Fluvial cyclic successions can be found in fills of abandoned channels and represent high and low discharge alternations. Incontrast to this, diagnostic decimeter-scale successions of the transitional environment consist of alternating units reflecting fluvial dominanceat high river discharge versus dominance of tidal currents at low discharge.Although none of the sedimentary features of the fluvial-tidal zone is unique for this environment, they allow a rather safe signature of thisenvironment when several are found together.

Diagnostic sedimentary structures of the fluvial-tidal transition zone – Evidence from deposits of the Rhine and Meuse

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Diagnostic sedimentary structures of the fluvial-tidal transition zone – Evidence from deposits of the Rhine and Meuse