Selective Entrainment of Sediment Graded by Size and Density Under Waves

Tomkins, Matthew R.; Nielsen, Peter; Hughes, Michael G.

doi:10.1306/031703730906

Cited by 16 publications

(7 citation statements)

References 19 publications

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“…A similar differential style of compositional segregation of the mixed silici-bioclastic sand can be expected in a wave-dominated littoral environment (see Kraus, 1985; and experimental study by Tomkins et al, 2003), although the assemblage of shoreface stratification types will probably be quite different (De Raaf et al, 1977;Driese & Dott, 1984) and the swash-backwash oscillation series will probably have sub-harmonic frequencies (Huntley & Bowen, 1975;Miles & Russel, 2004). Instead of strata-scale segregation, the longer-term fluctuations in wave energy may result in metre-scale intervals of bioclast-rich deposits alternating with similar intervals of siliciclast-dominated deposits (see the 'temporal variability' of Budd & Harris, 1990).…”

Section: Compositional Dichotomy Of Sand Stratasupporting

confidence: 55%

Neogene to Quaternary tidal mixed deposits of southern Italy: implications for palaeogeographic reconstructions

Longhitano¹

2011

Rendiconti Online Della Società Geologica Italiana

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The Pliocene-Pleistocene peripheral marine basins of the Mediterranean Sea in southern Italy, from Basilicata and western Calabria to northern and eastern Sicily, represent tectonically formed coastal embayments and narrow straits.Here, units of cross-stratified, mixed silici-bioclastic sand, 25 to 80 m thick, record strong tidal currents. The Central Mediterranean Sea has had a microtidal range of ca 35 cm, and the local amplification of the tidal wave is attributed to tides enhanced in some of the bays and to the out-of-phase reversal of the tidal prism in narrow straits linking the Tyrrhenian and Ionian basins. The siliciclastic sediment was generated by local bedrock erosion, whereas the bioclastic sediment was derived from the contemporaneous, foramol-type cool-water carbonate factories. The cross-strata sets represent small to medium-sized (10 to 60 cm thick) two-dimensional dunes with mainly unidirectional foreset dip directions. These tidalites differ from the classical tidal rhythmites deposited in mud-bearing siliciclastic environments. Firstly, the foreset strata lack mud drapes and, instead, show segregation of siliciclastic and bioclastic sand into alternating strata. Secondly, the thickness variation of the successive silici-bioclastic strata couplets, measured over accretion intervals of 2 to 3 m and analysed statistically, reveal only the shortest-term, diurnal and semi-diurnal tidal cycles. Thirdly, the record of diurnal and semi-diurnal tidal cycles is included within the pattern of neapspring cycles. Differences between these sediments and classical tidal rhythmites are attributed to the specific palaeogeographic setting of a microtidal sea, with the tidal currents locally enhanced in peripheral basins. It is suggested that this particular facies of mud-free, silici-bioclastic arenite rhythmites in the stratigraphic record might indicate a specific type of depositional sub-tidal environment of straits and embayments and the shortest-term tidal cycles.

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Section: Compositional Dichotomy Of Sand Stratasupporting

confidence: 55%

Neogene to Quaternary tidal mixed deposits of southern Italy: implications for palaeogeographic reconstructions

Longhitano¹

2011

Rendiconti Online Della Società Geologica Italiana

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“…A similar differential style of compositional segregation of the mixed silici–bioclastic sand can be expected in a wave‐dominated littoral environment (see Kraus, 1985; and experimental study by Tomkins et al. , 2003), although the assemblage of shoreface stratification types will probably be quite different (De Raaf et al.…”

Section: Discussionmentioning

confidence: 64%

The record of tidal cycles in mixed silici–bioclastic deposits: examples from small Plio–Pleistocene peripheral basins of the microtidal Central Mediterranean Sea

Longhitano

2010

Sedimentology

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The Pliocene-Pleistocene peripheral marine basins of the Mediterranean Sea in southern Italy, from Basilicata and western Calabria to northern and eastern Sicily, represent tectonically formed coastal embayments and narrow straits. Here, units of cross-stratified, mixed silici-bioclastic sand, 25 to 80 m thick, record strong tidal currents. The Central Mediterranean Sea has had a microtidal range of ca 35 cm, and the local amplification of the tidal wave is attributed to tides enhanced in some of the bays and to the out-of-phase reversal of the tidal prism in narrow straits linking the Tyrrhenian and Ionian basins. The siliciclastic sediment was generated by local bedrock erosion, whereas the bioclastic sediment was derived from the contemporaneous, foramol-type cool-water carbonate factories. The cross-strata sets represent small to medium-sized (10 to 60 cm thick) two-dimensional dunes with mainly unidirectional foreset dip directions. These tidalites differ from the classical tidal rhythmites deposited in mud-bearing siliciclastic environments. Firstly, the foreset strata lack mud drapes and, instead, show segregation of siliciclastic and bioclastic sand into alternating strata. Secondly, the thickness variation of the successive silici-bioclastic strata couplets, measured over accretion intervals of 2 to 3 m and analysed statistically, reveal only the shortest-term, diurnal and semi-diurnal tidal cycles. Thirdly, the record of diurnal and semi-diurnal tidal cycles is included within the pattern of neapspring cycles. Differences between these sediments and classical tidal rhythmites are attributed to the specific palaeogeographic setting of a microtidal sea, with the tidal currents locally enhanced in peripheral basins. It is suggested that this particular facies of mud-free, silici-bioclastic arenite rhythmites in the stratigraphic record might indicate a specific type of depositional sub-tidal environment of straits and embayments and the shortest-term tidal cycles.

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“…Tomkins et al (2003) observed this effect for regular waves above a rippled bed of mixed quartz and heavy mineral sand. The vertical gradients of the time-averaged suspended Cprofiles were found to be similar for the light and heavy minerals, despite their settling velocities differing by a factor of about 1.5, implying a convective rather than diffusive distribution mechanism, at least for the heavy particles.…”

Section: Convection and Diffusion In The Wave Boundary Layermentioning

confidence: 84%

On the suspension of graded sediment by waves above ripples: Inferences of convective and diffusive processes

Davies

Thorne

2016

Continental Shelf Research

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The relationship between the grain size distribution of the sediment on the bed and that found in suspension due to wave action above ripples is assessed here using detailed, pumped sample, measurements obtained at full-scale and also at laboratory scale. The waves were regular and weakly asymmetrical in most tests, and irregular in a minority of tests. The beds comprised fine and medium sand and were rippled in all tests. The cycle-mean sediment concentrations (C) from the pumped samples were split into multiple grain size fractions and then represented by exponential Cprofile shapes. The analysis of these profiles was carried out in two stages to determine: i) the relationship between the size distribution of the sediment on the bed and that found in the reference concentration, and ii) the behaviour of the exponential decay scale of the C-profiles. From this analysis inferences are made about the relative roles of diffusion and convection in the upward sediment flux linked to the process of vortex shedding from the ripple crests. The Transfer Function (Tr) defined to relate the bed sediment size distribution to that of the reference concentration indicates that, while finer fractions are relatively easily entrained, the suspension of some coarser fractions is caused by an additional convective effect that supplements diffusion. The evidence for this becomes pronounced above steep ripples, and the Transfer function suggests further that irregular waves increase the occurrence of coarser fractions in suspension. A functional form for Tr is suggested incorporating these principles. The exponential decay scale LS arising from the fractional C-profiles is also examined to assess the mechanisms responsible for the upward transfer of grains and a parameterisation of LS related to ripple size is suggested. The separate findings for Tr and LS present supporting evidence of diffusion affecting the finer fractions in suspension and combined diffusion + convection affecting the coarser fractions. The methodology developed allows the vertical profile of suspended median grain size to be predicted given knowledge of both the bed grain size distribution and also the flow conditions.

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Selective Entrainment of Sediment Graded by Size and Density Under Waves

Cited by 16 publications

References 19 publications

Neogene to Quaternary tidal mixed deposits of southern Italy: implications for palaeogeographic reconstructions

Neogene to Quaternary tidal mixed deposits of southern Italy: implications for palaeogeographic reconstructions

The record of tidal cycles in mixed silici–bioclastic deposits: examples from small Plio–Pleistocene peripheral basins of the microtidal Central Mediterranean Sea

On the suspension of graded sediment by waves above ripples: Inferences of convective and diffusive processes

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