Detailed Survey of a Giant Scar at the Distal Part of the Var Valley (off Nice, Southern France)

L. Droz, G. Auffret, B. Savoye, P. Cochonot, W. B. F. Ryan, A. Malinverno

A Sea-Marc survey of the Var Canyon and Ridge was performed in April 1984 aboard the R/V Conrad under an IFREMER/Lamont Doherty Geological Observatory collaboration. At the distal end of the ridge (2,400 m water depth) the sonar vehicle crossed a spoon-shaped scar (1.5 km in diameter, 70 m in relief). This structure was selected as a target for a CYANA dive during the SAME cruise aboard the RV Suroit in November 1986. Later, a detailed seismic survey of the area was conducted by the Laboratoire de Geodynamique Sous-Marine of Villefranche, and additional piston cores were sampled.

The seismic profiles show a homogeneous sequence of continuous acoustic reflections truncated by the scar. The dive revealed that the scarp consists of a step-like succession of 1 to 3 m-thick lithologic units with debris accumulations at their foot. Detailed observations exhibit thinning-upward sequences of relatively indurated horizontal sandy and muddy beds. These sequences are, in some cases, underlain by fining-upward sequences grading from lenses of gravels (channel infillings?) to obliquely stratified sandy layers to horizontally laminated muddy beds.

We interpret these observed sequences as gravity deposits (turbidites and debris flow). The giant scar itself could result from either retrogressive erosional processes or catastrophic massive strippings of the floor. Both processes could be controlled by high-energy gravity flows and/or salt tectonics.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.