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ABSTRACT: Holocene Prodeltaic Depositional History in the Gulf of Rosas, Northwestern Mediterranean Sea

Jose I. Diaz, Gemma Ercilla

Prodeltaic sediment in the Gulf of Rosas, northwestern Mediterranean Sea, is supplied mainly by the seasonally controlled Fluvia and Muga rivers and, until the seventeenth century, by a distributary of the southern located Ter River. These rivers, which had several distributaries in the past, have developed a 120-km2 delta plain in a wave-dominated coast, prograding offshore by beach ridge development.

High-resolution seismic profiles (3.5 kHz) and gravity cores were examined to evaluate the transgressive and highstand progradational history of prodelta deposits in the Gulf of Rosas. They cover an extension of about 365 km2 between the 20-m and 120-m isobaths. They extend 23 km offshore and, toward the south, lie with the prodeltaic body of Ter River. Over a major flat erosional unconformity, related to postglacial Versilian transgression, the following pattern of seismic facies is seen progressing offshore: (1) stratified facies (5-15 m thick) with closely spaced southeastward-prograding reflectors, (2) chaotic facies (4-21 m thick) in which lenses with reflectors of low continuity and high acoustic amplitude predominate, and (3) thin transparent facies (2-7 m thick). An area of gas-charged sediment (90 km2) is located between the 20 m and 80 m isobaths, mainly affecting the stratified facies, from 2 to 6 m below the sediment/water interface. Sediment stratigraphy is well correlated, with the pattern of seismic facies showing a complex pattern of coarsening- and fining-upward sequences.

Since the last Holocene transgressive stage and the following establishment of the present sea level highstand, a regressive depositional wedge-shaped sequence has been prograding on the Gulf of Rosas continental shelf. Storm-related bottom flows and mass movement processes in proximal areas chiefly controlled the prodeltaic depositional history in an area where relatively rapid deposition occurs from a multidistributary river supply and where a cyclonic eddy of the main southwestward geostrophic Mediterranean flow develops.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990