Sandstone tempestite beds in the Starshot Formation, cen- tral Transantarctic Mountains, were deposited in a range of shoreline to shelf environments. Detailed sedimentological analysis indicates that these beds were largely deposited by wave-modified turbidity currents. These currents are types of combined flows in which storm-generated waves overprint flows driven by excess-weight forces. The interpreta- tion of the tempestites of the Starshot Formation as wave-dominated turbidites rests on multiple criteria. First, the beds are generally well graded and contain Bouma-like sequences. Like many turbidites, the soles display abundant well-developed flutes. They also contain thick divisions of climbing-ripple lamination. The lamination, however, is dominated by convex-up and sigmoidal foresets, which are geometries identical to those produced experimentally in current-dominated com- bined flows in clear water. Finally, paleocurrent data support a tur- bidity-current component of flow. Asymmetric folds in abundant con- volute bedding reflect liquefaction and gravity-driven movement and hence their orientations indicate the downslope direction at the time of deposition. The vergence direction of these folds parallels paleocur- rent readings of flute marks, combined-flow ripples, and a number of other current-generated features in the Starshot event beds, indicating that the flows were driven down slope by gravity. The wave component of flow in these beds is indicated by the presence of small- to large- scale hummocky cross-stratification and rare small two-dimensional ripples. Wave-modified turbidity currents differ from deep-sea turbidity cur- rents in that they may not be autosuspending and some proportion of the turbulence that maintains these flows comes from storm waves. Such currents are formed in modern shoreline environments by a com- bination of storm waves and downwelling sediment-laden currents. They may also be formed as a result of oceanic floods, events in which intense sediment-laden fluvial discharge creates a hyperpycnal flow. Event beds in the Starshot Formation may have formed from such a mechanism. Oceanic floods are formed in rivers of small to medium size in areas of high relief, commonly on active margins. The Starshot Formation and the coeval Douglas Conglomerate are clastic units that formed in response to uplift associated with active tectonism. Sedi- mentological and stratigraphic data suggest that coarse alluvial fans formed directly adjacent to a marine basin. The geomorphic conditions were therefore likely conducive to rapid fluvial discharge events asso- ciated with storms. The abundance of current-dominated combined- flow ripples at the tops of many Starshot beds indicates that excess- weight forces were dominant throughout deposition of many of these beds.

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Wave-Modified Turbidites: Combined-Flow Shoreline and Shelf Deposits, Cambrian, Antarctica

The Oak Ridges Moraine in southern Ontario is a poly- genetic moraine constructed of a number of coalesced deposits of gla- cifluvial and glacilacustrine origin. A detailed study of the facies ar- chitecture has been completed on a series of pit sections extending ; 300 m subparallel to the paleoflow direction. Eight major lithofacies and five facies associations have been described. These data have been interpreted to be upper-flow-regime hyperconcentrated-flood-flow de- posits emplaced under a regime of rapid flow expansion and loss of transport capacity within a plane-wall jet with an associated hydraulic jump. Deposition from the plane-wall jet with jump occurred in three zones of flow transformation: zone of flow establishment, transition zone, and zone of established flow. Massive gravels with unconsolidated sand intraclasts and open-work gravel / gravel-sand couplets were de- posited in the zone of flow establishment by hyperconcentrated and supercritical flows, respectively. Immediately downflow low-angle cross-stratified sand incised by steep-walled scours infilled by diffusely graded sand define the transition zone, the zone of maximum vortex erosion, and the distal limit of deposits emplaced under upper-flow- regime conditions. These strata record rapid bed aggradation from sediment-laden supercritical flows that episodically were scoured by large vortices generated within migrating hydraulic jumps. Strati- graphically upward and downflow strata consist only of lower-flow- regime sedimentary structures. Medium-scale, planar cross-strata and small-scale cross-lamination related to migrating 2-D dunes and cur- rent ripples, respectively, characterize the zone of established flow. The facies and sediment architecture suggest that this fan was deposited during a relatively short period of time (days, weeks) by energetic sed- iment-laden floods.

Hydraulic-Jump and Hyperconcentrated-Flow Deposits of a Glacigenic Subaqueous Fan: Oak Ridges Moraine, Southern Ontario, Canada

https://hal-insu.archives-ouvertes.fr/insu-00178600/document

Late Quaternary co-seismic sedimentation in the Sea of Marmara's deep basins

Early Miocene lacustrine deposits and sequence stratigraphy of the Ermenek Basin, Central Taurides, Turkey

Turbidity currents generated nearshore have been suggested to be the source of some sandy marine event beds, but in most cases the evidence is circumstantial. Such flows must commonly travel through a field of oscillatory flow caused by wind-generated waves; little is known, however, about the interactions between waves and turbidity currents. We explore these interactions through detailed process-oriented sedimentological analysis of sandstone event beds from the Pennsylvanian Minturn Formation in north-central Colorado, U.S.A. The Minturn Formation exhibits a complex stratigraphic architecture of fan-delta deposits that developed in association with high topographic relief in a tectonically active setting. An ~20–35-m-thick, unconformity-bounded unit of prodelta deposits consists of dark green shale and turbidite-like sandstone beds with tool marks produced by abundant plant debris. Some of the sandstone event beds, most abundant at distal localities, contain reverse-to-normal grading and sequences of sedimentary structures that indicate deposition from waxing to waning flows. In contrast, proximal deposits, in some cases less than a kilometer away, contain abundant beds with evidence for deposition by wave-dominated combined flows, including large-scale hummocky cross-stratification. We interpret the majority of these event beds as a record of deposition from hyperpycnal flows, i.e., turbidity currents generated directly from highly concentrated river plumes, which accelerated and decelerated in response to a rising and falling flood discharge. Additional support for this interpretation includes the following: (1) a variety of sole marks including flute and gutter casts, as well as tool marks made by relatively large (up to tens of centimeters across) woody debris (i.e., groove, prod, and chevron marks); (2) consistent unimodal orientation of sole marks and abundant ripple cross-stratification, which indicate strong downslope-directed flow; (3) a well documented sedimentological framework for the formation of fan-delta deposits adjacent to nearby highlands; and (4) plant fossils typical of middle- to high-elevation habitats that are abundant in the turbidite beds but absent in underlying and overlying shoreline and marginal marine deposits, which have a separate floral assemblage. Differences in grain sizes, vertical stratification sequences, and bed thicknesses between outcrops are interpreted to result from the spatial distribution of wave effects, the time history of hyperpycnal flows, and the interaction of these processes. The latter varied both spatially and temporally and produced a wide range of bed types, which are incorporated into a new conceptual model for storm-influenced hyperpycnal flows.

/pdf/deposits-from-wave-influenced-turbidity-currents-krgqmkqht0.pdf

Deposits from Wave-Influenced Turbidity Currents: Pennsylvanian Minturn Formation, Colorado, U.S.A.

We studied sand particle movement on migrating combined-flow ripples in an experimental flume. The water-surface waves propagated against the flow in the flume. The resultant combined-flow ripples have characteristic rounded profiles. Motion pictures reveal that vortices, generated only close to the lee sides of ripples, are responsible for the rounded profile. These vortices excavate rounded troughs and lift sand particles. The lee sides are nourished by the sand, contributing a convex-upward profile. The flow that separates at the crests is dragged into the vortices at the troughs, so that there is no clear reattachment point with focused erosion, as is the case for current ripples. Sand that fulls to the bed from a floating cloud over the stoss side nourishes the convex upward roun ed stoss side. The relative strength of the oscillatory flows controls the positions at which the vortices lift sand particles to deposit, and determine whether the ripple profiles are rounded or sharp. A lower relative oscillatory flow velocity yields smaller vortices, which cause rounded ripple profiles. Conversely, a higher relative oscillatory flow velocity results in larger vortices and sharper crests. The movement of sand particles is unlike that on ripples beneath either purely oscillatory flows or unidirectional flows.

Sand Particle Movement on Migrating Combined-Flow Ripples

Grain fabric of experimental gravity flow deposits

Theoretical Study on Breaking of Waves on Antidunes

Grain fabric and the depositional volume of modern foreshore deposits from the Pacific coast of southern Ibaraki Prefecture, Japan, are influenced by tides. Reversals of imbrication dip were found that may result from predominance of uprush transport during the flood stage and backwash transport during the ebb stage. The thickness of sediment deposited during both rising and falling water stages varies directly with tidal range for that cycle, so that diurnal inequality leads to alternations of thick (3-4 cm) and thin (1-2 cm) layers on the foreshore. Pleistocene foreshore deposits exposed near the Pacific coast are also characterized by reversals of grain imbrication and may have been controlled by tidally-influenced uprush-backwash processes. The Pleistocene example also possesses alternating thin and thick layers, suggesting a diurnal variability in tidal pattern similar to the modern tidal pattern along the Pacific coast.

Miwa Yokokawa

Papers

Sand Particle Movement on Migrating Combined-Flow Ripples

Grain fabric of experimental gravity flow deposits

Theoretical Study on Breaking of Waves on Antidunes

Tidal Influence on Foreshore Deposits, Pacific Coast of Japan

Grain-size distributions of sediment carried by single transportation modes in an experimental microdelta system