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Large, northwestwardly migrating mud shoals, parts of which are composed of fluid mud (bulk density 1.05 to 1.25 g/cc), front the shoreline every 30 to 60 km along the 1,600-km-long Guiana coast. The rather spectacular density variations that occur in these soft, gel-like
silts and clays have provided considerable insight into the dynamics of sedimentary and mass-movement processes. Time-series measurements taken along the coast of Surinam using pressure-sensitive instruments indicate that periodic density variations may range in frequency from that of waves (~10 sec) to that of the tide (~12.4 hour). The density fluctuations are the result of sediment suspension, loss and gain of pore waters, and subaerial and subaqueous mass-movement processes.
In muds where density is less than 1.20 g/cc and water depth is less than 5 m, clouds of sediment are suspended as shallow-water waves propagate shoreward. Although suspended-sediment concentrations may reach 50,000 ppm under wave crests, rapid settling takes place before the next wave arrives. Wave-by-wave suspension is superimposed on a lower frequency process whereby accumulations of fluid mud up to 80 cm thick are suspended and redeposited during a tidal cycle.
In muds where density exceeds 1.20 g/cc, less than 1 cm of the bottom is suspended by incoming waves or by tidal currents. Measurements indicate that cyclic density variations result from pore-water loss during a falling tide followed by pore-water gains during a rising tide.
Spectral analysis shows the presence of a third frequency of density perturbation, typically 1 to 5 minutes, which we hypothesize to be the result of mass movement of fluid mud offshore. Observations on mud flats at low tide reveal well-formed shear planes bounding linear mudflow failure chutes. Sediment-flux determinations indicate that most of the estimated 2 × 108 m3/year of sediment moved onshore by waves can be roughly balanced by a slow, periodic mass-movement offshore. The result may be a sawtoothed pattern of sediment movement to the northwest.
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