Abstract

It is possible to define a particular wave dominated delta pattern in consideration of morphologic, sedimentologic and stratigraphic features of the four greatest tyrrhenian deltas. This pattern developed, after the sea level still stand (last 6 kyr), on gently sloping shelf of a passive continental margin. The delta body shows a composite geometry with a tabular inner part, few meters thick, and a lenticular outer part about 70 m thick. The delta body lies on a maximum flooding surface of the last glacial depositional sequence (Bellotti et alii, 1994) Morphologic features. The inner delta plain is very flat and there were, as far as 150 years B.P., ponds and marshes today reclaimed. The outer delta plain shows beach ridges and coastal dunes which locally include small and ephymeral brackish basins. The river present a single channel without levees which is somewhat steady, it is winding in the inner delta plain and more rectilinear in outer delta plain. The delta front shows a gently sloping flat bottom between 0 and -20 m; littoral bars are present only close to the beach. The pro-delta shows steep bottom (as far as 1.3 degrees ) with convex profile and sometimes synsedimentary creep (Bellotti et alii, 1989). Sediments distribution. Mud, muddy clay and organic clay are widely present in inner delta plain. Sand and muddy sand are present close to the river channel. In the outer delta plain, medium to Line sands constitute beach ridges and coastal dunes, mud and organic mud are present only into brackish ephemeral basins. Evolution. At high sea level still-stand, the river flowed directly into a coastal lagoon closed seaward by a barrier beach which represents the last beach of the transgressive system tract. In the first time (6-3 kyr B.P.), the lagoon was the depocenter of the system and lagoon infilling constituted a tabular body correspondents to actual inner delta plain. In the last 3 kyr the depocenter moved into the sea and a wave dominated delta developed. The fluvial load/wave power ratio, drives the delta progradation. In the first time the progradation occurs with development of parallel beach ridges. Successively due to an increased of the fluvial load/wave power ratio, delta progradation developed particularly in river mouth area and cuspate delta was generated. Due to a reduction of the fluvial load/wave power ratio, the apical part was eroded and progradation took place along the delta wings. Variations in the fluvial load/wave power ratio depends by secular climatic modifications and anthropic activity (Bartolini, 1983).

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