Abstract: 

A buried system was observed using seismic data along the northern side of the Roussillon coastal plain and inner shelf (Western Gulf of Lion, Mediterranean Sea) far from the nearest Agly and Têt rivers. It was interpreted as an example of a “compound” type, which is subject to controversy.

A core 60 meters long was obtained on the sandy beach barrier between sea and lagoon approximately on the axis of the basal surface of erosion mapped from seismic data and long core logs obtained previously on the coastal plain.

In the lower part of the core (−64 m to −26 m b.p.s.l.) the main estuarine muddy-silt facies and thin levels of fluvial gravels intercalated in the mud suggest that successive cycles of base-level and sea-level falls and rises were recorded. This interpretation is confirmed by pollen diagrams showing several successive warming periods during climatic changes (differentiated as “interglacial” phases) recorded by the estuarine muds. Owing to these correlated data we can attest that this incised-valley system is of a “compound” type.

Every vertical succession of fluvial gravels and estuarine muddy silts represents a fundamental depositional sequence typical of the ”simple” model of wave-dominated incised-valley filling, truncated at the top by a subsequent base-level fall. Several depositional sequence remnants are stacked upon each other.

The chronostratigraphic benchmark is based on microforaminiferal biomarkers which indicate that the time period covered by the successive phases of base-level and sea-level cycles extends from MIS 16 (about 600,000 years B.P.) to MIS1 (present day). The exact correlation of base level and sea level cycles with identified climate cycles remains partly approximate, but the cycles which contain well-known plant associations are quite reliable.

The internal geometry of the incised-valley system, based upon high-resolution seismic data, shows that the lateral migration of the successive phases of incision and filling explains both the preservation of several cycles and the incomplete preservation of the typical facies due to the reworking of the upper part of all individual depositional sequences.

Eustasy is the dominant controlling factor, and differential subsidence, an important factor on the mid and outer shelf, has a reduced impact at this location. The preservation of the “compound” system depends upon the variable maximum depth of erosion reached at each maximum sea-level lowstand. We propose that different interglacial conditions took place in the drainage basins, rather than differences in the lowstand-sea-level values. The transverse shape of the incisions, with lateral terraces and deeper channels, combined with a continuous lateral shift of the successive incisions, also contributed to preservation. The lateral shifting is partly due to a normal fault and substratum tilting, and partly to the oceanic regime.

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