The Effects of Late Quaternary Sea-level Changes on the Rhone Slope Sedimentation (Northwestern Mediterranean), as Indicated by Seismic Stratigraphy

Abstract

ABSTRACT High-resolution, single-channel seismic profiles have been collected on the shelf and slope off the present Rhone Delta (western Mediterranean) using a 750 J Sparker. These profiles have been correlated with earlier high-resolution seismic work on the shelf that identified several superimposed wedge-shaped seismic units, interpreted to be Late Quaternary shelf-perched lowstand wedges. These wedge-shaped seismic units are traced onto the slope in an intercanyon area between the Grand-Rhone and Petit-Rhone submarine canyons. Detailed analysis of the seismic facies shows that Upper Quaternary slope deposits also define sedimentary cycles. Each basic cycle is composed of a couplet of two different subunits. The lowest one is mostly transparent, has a gentle seaward-convergent geometry, and correlates with one complete perched lowstand wedge on the shelf. The upper subunit is well stratified, has a gentle seaward-divergent geometry, and correlates with an erosional unconformity on the shelf. 14C dating of the youngest slope cycle indicates that most of the deposition on the slope occurred during low sea-level periods and that the couplet of transparent and layered subunits are the result of the interaction between eustatic sea level, sediment flux, and the geometry of the continental shelf and slope. Stratified elongated wedges on the slope are interpreted as deposition when the depocenter moves out from the shelf to the upper slope and deep basin. This model is compared to the Late Quaternary eustatic sea-level curve. Some cycles in this curve seem to be absent on the slope or completely eroded on the shelf, indicating that caution is necessary in deducing relative sea-level cycles from continental-margin stratigraphy.