Relationships between climatically influenced sedimentation and salt diapirism in the French Western Alps based on evidence from organic and inorganic geochemistry

Abstract

In the Sub-alpine Basin (French Western Alps), the Terres Noires and Marnes Bleues Formations (shales of Bathonian-Oxfordian and Aptian-Albian ages, respectively) record a rhythmic distribution of organic matter. In the Terres Noires Formation, this rhythmic distribution is related to coloured bands visible in outcrop over a limited area of the Sub-alpine Basin. At present in this area, diapirs crop out which were supposedly active as early as Jurassic times when the Sub-alpine Basin was subjected to overstretching. In contrast, the colour of the shales is uniform in areas where salt tectonics occurred after the Terres Noires deposition. The study of organic matter and mineral content permits determination of the role played by brines in the genesis of banded facies in a basin where sedimentation was climatically influenced. These brines could have been formed by dissolution of Triassic evaporites exposed along faults. In addition, as the basin entered a deformation stage, during the Marnes Bleues deposition, halokynetic manifestations seem to have been absent at the bottom of the basin. At present, phenomena of density stratification and anoxia induced by brines related to the dissolution of evaporites occur in the Eastern Mediterranean. This paper is an attempt to recognized such phenomena in the Mesozoic sediments of the Sub-alpine Basin. The use of these techniques also provides a means of determining the age of halokynesis in the Western Alps, which is indicated to be at least Callovian.