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 17th century, by a distributary of the Ter River, located more to the south. These rivers, which had several distributaries, have developed a 120-km² delta plain on 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 the prodelta deposits in the Gulf of Rosas. The deposits cover an extended area of about 365 km² between the 20-m and 120-m isobaths. The profiles extend 23 km offshore and, toward the south, include the prodeltaic body of Ter River. Across a major flat erosional unconformity related to the postglacial Versilian transgression, the following patterns of seismic facies are seen offshore: (1) stratified facies (5 to 16 m thick) with closely spaced southeastwardly prograding reflectors, (2) chaotic facies (4 to 21 m thick), in which lenses with predominantly low-continuity, high-acoustic amplitude reflectors, and (3) a thin transparent facies (2 to 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 2 to 6 m below the sediment/water interface. Sediment stratigraphy is well correlated with the seismic facies showing a complex pattern of coarsening- and fining-upward sequences.

Since the last Holocene transgressive stage and its succeeding 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, and were active in an area where relatively rapid deposition occurred from a multidistributary river supply and where a cyclonic eddy of the main southwestward geostrophic Mediterranean flow developed.

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