Sand-mud transition zone

The sand-mud transition zone may be described in terms of the net flux of sand over the accreting deposits of marine mud. Under excitation by the currents sand moves westwardly from the eastern Sound by ad-vection and turbulent diffusion until it is incorporated into the accreting... 

It was found that the sand-mud transition zone could be adequately represented by allowing the sedimentary processes to proceed at constant rates. When this was done, the calculated sand flux agrees well with the sand flux that has been measured over the sand-wave field in the eastern Sound. Since the tides and the estuarine circulation control the sand fluxes, the resulting distribution of sand approaches a steady state very quickly. It is likely that the sand-mud transition was established soon after the Sound became an arm of the sea and has persisted unchanged to the present day. As a corollary to this hypothesis, it would seem that Mattituck Sill has been accreting over the lifetime of the Sound. Small variations in the sand content preserved in short cores are probably due to the perturbation of the sand distribution by a series of winter storms rather than individual storm events. 

Fig. 13. Structure of the bottom of Long Island Sound revealed by acoustic reflection profiles made with 7-kHz acoustic pulses. (Upper echo is produced by a 2(X)-kHz echo sounder.) (a) Section of end moraine capped by boulders and almost buried by marine mud. (b) Thick deposit of marine mud in central Long Island Sound on top of outwash sand with reflector above thought to be surface of lacustrine deposits, (c) Sand-to-mud transition zone in central Long Island Sound. In all records each division on the vertical scale is 600 mm.

Sound. For these reasons the transition zone was described using a onedimensional form of the mass-balance equation. The muddy floor of the Sound is blanketed with a layer of readily resuspendable agglomerates. This layer is about 1 cm thick. Any sand in this layer was assumed to be regularly set in motion by the currents, while sediment grains below this layer remained immobile. The formation of a permanent mud deposit occurs at the base of this layer at the rate of about 1 mm/year. This value was taken as the speed with which sand grains are removed from the zone of mobile sediment. 

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