Abstract

Detailed analysis of shallow penetration single-channel seismic data, integrated with piston core data, reveals that the stratigraphic architecture on the Rhône shelf of the western Mediterranean Sea is characterized by a complex stratigraphy comprising both the regressive and transgressive parts of late Pleistocene depositional sequences. Several cycles of deposition are observed and are interpreted to be associated with fourth-or possibly fifth-order cycles of relative change of sea level. The regressive parts of the sequence are inferred to have been deposited either during late highstand or during periods of relative fall of sea level. These deposits are characterized, in some instances, by discrete downstepping wedges and internal downward shift surfaces; however, in other instances this evidence is not present. The transgressive parts of the depositional sequences are characterized by backstepping wedges and isolated sand bars. In general, these types of deposits are more common on the western part of the Rhône shelf and are largely absent on the extreme eastern part of the shelf. The transgressive deposits seem to be preferentially preserved at both inner and outer shelf locations and less so in the middle shelf. The exception to this is the area near the Rhône Delta depocenter, where transgressive deposits are observed across the entire shelf. Key surfaces separating stratigraphic units include ravinement surfaces, downlap surfaces, and subaerially formed erosional surfaces. These stratal discontinuity surfaces constitute the basis for analysis of the stratigraphic architecture.

This area is dominated by seismic reflection geometries suggesting a high-energy depositional environment. Relatively steeply dipping seismic reflections bounded by horizontal to irregular erosional surfaces characterize most of the upper Pleistocene section in this area. The high-energy seismic facies correspond to three types of deposits: (1) thick regressive sands of lobate delta-front origin, (2) retrogradational beach barriers overlying the regressive wedge on the outer to middle shelf, and (3) late transgressive beach sands and ridges observed at the inner shelf. The stratigraphic complexity illustrated here is likely analogous to similar, although commonly undetected, complexity that characterizes petroleum fields in shallow-shelf settings.

A variety of geomorphic elements have been interpreted here. These include transgressive sand bars, wave-dominated distributary mouth bars, recurved spits, isolated shelf edge shoreface/beaches, and distributary channels. These elements are consistent with an interpretation of a depositional environment characterized by wave-dominated delta deposition. The morphology of the delta in the eastern part of the shelf (i.e., near the depocenter) seems to be consistent from the outer to inner shelf location, suggesting that the position of the shoreline relative to the shelf edge and the presence of submerged shelf outboard of the shoreline had only minor impact on deltaic deposition; moreover, climatic change and changes of fluvial discharge that likely characterized this area during the late Pleistocene similarly had only minor effect on delta morphology.

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