Sedimentary Processes in Mediterranean Deltas and Shelves

Abstract

Recent studies enable us to compare the geographic and physiographic settings, general structural configuration, emerged delta evolution, hydrodynamic regime, suspended transport processes, and geometry of recent shelf deposits seaward of the four major Mediterranean deltas: the Rhone, Ebro, Po, and Nile. This synthesis indicates that some features of the deltas and adjacent continental shelves appear to have remained constant during the last high sea-level stand (Holocene) around the Mediterranean. These include prismatic geometry of Holocene deposits; nearshore smectite-rich prodelta muds; multilayer system of suspended sediments on the continental shelf, with a bottom nepheloid layer; and longitudinal bottom and surface currents. Sedimentation depends on the balance between climatic-eustatic changes, river discharge, and current dispersal. Terrigeneous material is essentially supplied by a bottom nepheloid layer, which is initiated at the fresh-water-salt-water contact at the river mouth and is not only the result of mud resuspension as previously supposed. Gravitational depositional processes from this bottom layer result in a seaward decrease in silt content and sediment size. This results in the geometry of the basic Holocene unit, a sedimentary prism. Electrochemical flocculation processes also take place in the prodelta environment, thus explaining extensive muddy deposits. These mechanisms take place not only in front of the major Mediterr anean deltas but also off secondary rivers such as those on the Gulf of Lion shelf and many of the other Mediterranean continental shelf sectors. Observed variations of some importance are mainly the result of man-made processes and, to a lesser degree, the effects of the regional tectonic evolution. Comparable processes have been active during each of the Quaternary high sea-level stages. Emerged deltas were built out and immediately eroded during each subsequent low sea-level stage, as indicated by the truncation of successive transgressive series on the continental shelf. This deltaic destructive phase supplies terrigeneous material that contributes to slope progradation and the growth of the associated deep-sea fan. The partial preservation of deposits is the result of subsidence of the margin during Quaternary time. The shelves appear to alternate as areas of entrapment, as presently observed, and of by-passing during erosional phases.