Upper Miocene Salt Layer in the Western Mediterranean Basin

THE problem of salt deposits in a present deep-sea basin has been discussed recently in the case of the Atlantic Oceanl. In this article we present new data on the salt layer in the Western Mediterranean Basin. In the Balearic Basin and the Ligurian Sea, anticline structures and dornes disturbing the upper part of the sedimentary cover have been reported by Alinat and Cousteauz, Menard et aL3, Hersey4, Alinat et al.', Glangeaud6, Leenhardt7, Mauffretn, and Watson and Johnson9. Various hypotheses have been put forward toexplain their origin, and the most widely accepted is that they are salt structures; the presence of sulphur, chalcopyrite and an increase of salinity in interstitial waters have been notedl0.l1. There are various interpretations of the age and areal extent of the salt layer'2.13. Because of their limited penetrating capabilities, the conventional profiling techniques do not penetrate beyond the layer disturbed by diapirism. In the Northern Balearic Basin, however, one of us14 has interpreted a strong, diffracting, deep reflector (two-way travel time of more than 1 s) as the top of a salt layer. This interpretation has received strong support recently from records obtained with the "Flexotir" technique and new data have been acquired on the age and thickness of the salt layer15. From records obtained during the Polymede cruise of the R/V Jean Charcot (May-July 1970) we have traced the salt horizon over a large portion of the Western Mediterranean Basin. Moreover, we are able to distinguish a sequence of deep, flat or slightly folded reflectors (1.5 to 2.5 s two-way travel time) in contrast with the upper layer. strongly disturbed by diapirism. The top of the lower sequence is marked by a strong reflector, assurned to be the bottom of the evaporite layer. Five sections, representative of the margins and central zone of the Western Mediterranean Basin, are presented here (Fig. 1).