Seismic stratigraphy of the Deglacial deposits of the Rhône prodelta and of the adjacent shelf

Abstract

In order to achieve a synthesis of the stratigraphic organization of the Deglacial deposits of the inner/middle shelf in front of the Rhône delta plain, we merged high resolution (Mini-sparker and Sparker), and very high resolution (chirp and mud-penetrator) seismic data into a single seismic database. Thus, the merged seismic database improves the lateral correlation between eastern and western parts, separated by the Rhône Incised Valley deposits. As a result the interpretation of seismic units in relation to local and global environmental changes was refined. The Deglacial deposits rest on a basal erosional discontinuity capping a complex of Pleistocene prograding wedges. The identified units make up Transgressive and Highstand Systems Tracts, and are bounded by flooding surfaces. The main flooding surfaces are (1) the transgressive surface (D200) which forms the lower boundary of the Deglacial deposits and (2) the maximum flooding surface which forms the boundary between the Transgressive and Highstand Systems Tracts. Regarding the data set the study area is divided into three parts depending on the stacking pattern and main control factors. In the western area the units present an aggradational stacking pattern and the rate of sediment supply and dynamic conditions seems coupled with glacio-eustacy role on sedimentary units building and evolution. In the central area the units present an overall backstepping pattern mainly controlled by glacio-eustacy. In the eastern area the units present an aggradational stacking pattern and both the glacio-eustacy and rate of sediment supply have an important role. The lower parasequence of the Transgressive Systems Tract (U200) is due to the reworking of Würmian terraces. Above, the parasequence U300 is a transgressive body formed during a rapid sea-level rise. The two upper parasequences are interpreted as ancestral coastal systems with a backstepping pattern. The first coastal system (U400) is due to a decrease in the rate of sea-level rise that occurred during the Younger Dryas. The second coastal system (U500) may be due to a similar event or to an increased rate of sediment supply during a constant sea-level rise. Finally the Highstand Systems Tract is composed of three units: (i) unit U601, located to the West and interpreted as a subaqueous delta, (ii, iii) units U600 and U610, located to the East and interpreted as prograding deltaic lobes.