Single and superimposed bedforms: a synthesis of San Francisco Bay and flume observations

Rubin, David M.; McCulloch, David S.

doi:10.1016/0037-0738(80)90012-3

Cited by 224 publications

(140 citation statements)

References 20 publications

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“…The structures of inclined sets that can be traced into the topsets in location 2 suggest that the proportion of a set that remains preserved increases significantly where dunes overtake a bar lee slope. This matches observations of decreasing heights, and hence locally increased sediment flux to the bed, of superimposed dunes migrating down the lee slope of larger dunes (Pretious and Blench 1951;Rubin and McCulloch 1980;Rubin and Hunter 1982;Amsler and Garcia 1997;Parsons et al 2005). The decrease in heights of the down-climbing bedforms indicates a systematic flux of sediment to the host slope and hence a systematic increase in set thickness.…”

Section: Foresets: Inclined Co-setssupporting

confidence: 84%

Evidence of Bedform Superimposition and Flow Unsteadiness In Unit-Bar Deposits, South Saskatchewan River, Canada

Reesink¹,

Bridge²

2011

Journal of Sedimentary Research

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Unit-bar deposits are ubiquitous components of river-channel deposits and strongly influence their hydrological properties, yet they are not easy to interpret. This paper concerns details of the internal structures of six unit bars from the South Saskatchewan River, Canada, that were investigated using trenches, epoxy resin peels, and 900 MHz ground-penetrating radar (GPR) profiles. The composition of unit bars depends on flow unsteadiness and superimposed bedforms. Flow unsteadiness causes changes in the mean grain size of the sediment in transport, but is expressed primarily as a change in the type and direction of migration of smaller ripple-and dune-scale bedforms superimposed on unit bars. Superimposed bedforms with heights that exceed 25% of the host bedform height reduce their host's slope and generate inclined sets. Host bedforms with smaller superimposed bedforms form angle-of-repose cross strata with a visible pre-sorting pattern. The relationship between the formative host-and superimposed bedforms and such pre-sorted cross strata can be used to interpret numerous aspects of the three-dimensional geometries of the bedforms. Such detailed interpretations rely on: (i) regularly spaced finegrained drapes deposited during the passage of troughs of superimposed bedforms, (ii) grain-size sorting patterns generated by sorting within the superimposed bedforms before deposition, (iii) grain-size sorting during deposition by grainfall and intermittent grainflows down the bar lee-slope, and (iv) the presence and nature of low-angle reactivation surfaces.The detailed interpretations revealed that the deposits of dam-related floods were significantly smaller than the deposits of individual unit bars in this study. The unsteady flow and sediment transport conditions are reflected by changes in the structural composition of the unit bars. Reactivation surfaces associated with flow unsteadiness, as opposed to those formed by large superimposed bedforms, were characterized by changes in mean grain size, the buildup of sediment at the base of the bar lee slope, and changes in structures formed by superimposed bedforms. Reactivation surfaces generated by large superimposed bedforms indicate that bedform preservation is likely increased in areas of flow deceleration. The trough deposits of exposed unit bars contained bubble sand, planar laminae due to upper-stage plane beds, and low-angle inclined laminae due to antidunes. These structures indicate that unit-bar troughs can act as ephemeral channels. The composition of the investigated unit bars thus emphasizes the interplay between flow unsteadiness and bed morphology in the formation of sedimentary structures in river channels.Sediment surfaces generate visible GPR reflections where the thickness and contrast in electrical properties of the sediment layers are sufficiently large. Most cross-stratified sets were represented only by reflections from the base of the set, and not by high-angle inclined reflections from the cross strata. The local absence of refle...

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Section: Foresets: Inclined Co-setssupporting

confidence: 84%

Evidence of Bedform Superimposition and Flow Unsteadiness In Unit-Bar Deposits, South Saskatchewan River, Canada

Reesink¹,

Bridge²

2011

Journal of Sedimentary Research

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“…According to the pathway of the winter storms and hurricanes and the distribution of the emerged areas, the offshore migration of the dunes was due to these episodic return currents. The progressive southward (seaward) reduction of the dunes, indicates the coeval offshore decrease of the energy of the return currents (Rubin and McCulloch, 1979).…”

Section: Description Of the Modelsmentioning

confidence: 98%

Proximal–distal facies relationships and sedimentary processes in a storm dominated carbonate ramp (Kimmeridgian, northwest of the Iberian Ranges, Spain)

Bádenas

Aurell

2001

Sedimentary Geology

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Facies analysis of the upper Kimmeridgian rocks in the outcrops located near Ricla (Zaragoza province, northeast Spain) and the integration of the resultant data in a broader context (the northern part of the Iberian Basin), has produced two general models showing the facies distribution and the processes that controlled the sedimentation in the Kimmeridgian carbonate ramp. Using these two models the transition from shallow to relatively deep environments of the carbonate ramp is examined in detail. Model 1 corresponds to the development of a mixed carbonate-siliciclastic ramp during a slow rise and stillstand of sea level (Sequence 1-HST), whereas Model 2 represents the growth of a pure carbonate ramp during a rapid rise of sea level (Sequence 2-TST).Carbonate production was higher in the shallow ramp domains (coral reefs and oolitic shoals in Model 1 and reefs in Model 2) than in deeper domains, where there is no indication of signi®cant pelagic or benthic production. The activity of unidirectional return¯ows induced by winter storms and hurricanes, played an important role in the redistribution of the sediment across the ramp, generating different coarse-grained deposits. In the inner and mid-ramp settings dunes, lower scale bedforms and tempestites occur in Model 1, and storm lobes, bars and tempestites in Model 2. Moreover, a signi®cant bulk of the carbonate mud produced in shallow areas would eventually be resedimented in the outer ramp as suspended load in the density currents. Stillstand of sea level in Model 1 involved a rapid progradation of the inner and proximal mid-ramp carbonate and siliciclastic facies. The rapid relative sea level rise of Model 2 is determined by the dominance of the carbonate facies and by the presence of aggradational geometries in the transitional area between shallow and deep-ramp domains. The presence of relatively thick sections in the outer-ramp settings (instead of condensed sections, as observed in Model 1) during times of sea level rise (Model 2) can mainly be explained by the increase of the shallow production in the reef dominated areas. q

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“…These information are useful in estimating U. Specific bedwaves (for example, ripples, dunes) are stable within specific ranges of grain size, flow depth and flow velocity 26,32 (Supplementary Fig. S1).…”

Section: Methodsmentioning

confidence: 99%

River-margin habitat of Ardipithecus ramidus at Aramis, Ethiopia 4.4 million years ago

Gani

2011

Nat Commun

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The nature and type of landscape that hominins (early humans) frequented has been of considerable interest. The recent works on Ardipithecus ramidus, a 4.4 million years old hominin found at middle Awash, Ethiopia, provided critical information about the early part of human evolution. However, habitat characterization of this basal hominin has been highly contested. Here we present new sedimentological and stable isotopic (carbon and oxygen) data from Aramis, where the in situ, partial skeleton of Ar. ramidus (nicknamed 'Ardi') was excavated. These data are interpreted to indicate the presence of major rivers and associated mixed vegetations (grasses and trees) in adjacent floodplains. our finding suggests that, in contrast to a woodland habitat far from a river, Ar. ramidus lived in a river-margin forest in an otherwise savanna (wooded grassland) landscape at Aramis, Ethiopia. Correct interpretation of habitat of Ar. ramidus is crucial for proper assessment of causes and mechanisms of early hominin evolution, including the development of bipedalism.

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Single and superimposed bedforms: a synthesis of San Francisco Bay and flume observations

Cited by 224 publications

References 20 publications

Evidence of Bedform Superimposition and Flow Unsteadiness In Unit-Bar Deposits, South Saskatchewan River, Canada

Evidence of Bedform Superimposition and Flow Unsteadiness In Unit-Bar Deposits, South Saskatchewan River, Canada

Proximal–distal facies relationships and sedimentary processes in a storm dominated carbonate ramp (Kimmeridgian, northwest of the Iberian Ranges, Spain)

River-margin habitat of Ardipithecus ramidus at Aramis, Ethiopia 4.4 million years ago

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