Observations during field experiments on tidal flats in northeastern South America revealed that subaqueous mass-movement processes occur in soft, fluid muds on slopes of only 0.03° to 0.08°. Systems of linear failure chutes bounded by well-formed shear zones become visible during low tide and carry muds seaward at average rates of 1 to 10 cm/min. In situ measurements of pressure within the mud suggest that excess pore-water pressures which develop as tide waters recede may initiate this offshore movement of mud, described herein as “flowsliding.” Flowslide movement is significant as a transport mechanism for returning tidal flat muds to the wave-dominated environment from which they were derived.

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