Coupling of X-ray fluorescence and strontium isotopes to track the influence of brine in continental deposits: study of Oligocene sediments in the Digne area (SE basin, France)

Abstract

It is often difficult to decipher past salt tectonics events in orogenic domains. The southwestern Alpine foreland in France presents a long and extensive salt tectonics history in the Mesozoic, which was later inverted during the Cenozoic Alpine Orogeny. Synorogenic Cenozoic salt-related deformations are difficult to identify due to the contemporaneous shortening experienced by the foreland. This study is based on sedimentological and geochemical (Sr concentrations and 87 Sr/ 86 Sr ratios) analyses of the Oligocene non-marine succession of St-Geniez (Digne region, France). Oligocene salt influences are highlighted by (1) high Sr concentrations (>1000 ppm) in most of the series, (2) the occurrence of halophytic gastropods and (3) the deposition of two gypsum beds. The 87 Sr/ 86 Sr ratios measured in the limestones and the gypsum beds confirm a Triassic origin for the Sr, originating from the Sorine diapir located to the SW, for the first part of the series and from the Authon thrust, located to the north, for the upper part of the series. The geochemistry also suggests the occurrence of two Triassic evaporite levels: one already known and attributed to the Carnian–Norian (Late Triassic) and one attributed to the Olenekian–Anisian (Early to Mid-Triassic). These results show that the combined use of field geology and geochemistry can provide information about previously erased salt tectonics events.