CONTINENTAL ORIGIN OF THE PALEOGENE EVAPO RITES FROM THE VALENCE TROUGH (DROME, FRANCE) - ISOTOPIC, MINERALOGICAL AND PALEOECOLOGICAL ARGUMENTS

The find of fossil fishes in the borehole DP 108 (Montchenu-I) was the starting point of investigations relative to the depositional conditi ons of the Palaeogene evaporitic series of the Valence trough (Drome, France). The isotopic study shows that the isotopic content of the sul phates (O-18 and S-34) is very similar to that observed in the Upper A lsace Palaeogene potash basin. This demonstrates that they result from a secondary precipitation of redissolved salts of Permo-Triassic orig in. As well, O-18 and C-13 contents of the carbonates are indicative o f precipitation in continental waters. Palaeoecological information pr ovided by the malacofauna indicates variations in water salt content w hich fully agree with the isotopic interpretation. More precisely, the occurrence of fishes belonging to the genus Dapalis GISTEL suggests t hat the deposition of the ''formation marneuse intermediaire'' took pl ace in oligo- to mesohaline waters.