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

Abstract

Deciphering past salt tectonics events is often difficult in orogenic domains. The southwestern Alpine foreland in France presents a long Mesozoic extensive salt tectonics history, inverted during the Cenozoic Alpine orogeny. Syn-orogenic Cenozoic salt-related deformations are rather difficult to identify because of the contemporaneous shortening experienced by the foreland. This study is based on sedimentologic and geochemical approaches (Sr concentration and 87 Sr/ 86 Sr ratio) of the Oligocene non-marine succession of St Geniez (Digne region, France. Oligocene salt influences are highlighted by (1) high strontium concentrations (>1000 ppm) in most of the series, (2) the occurrence of Halophyticgastropods and (3) the deposit of two gypsum beds. 87 Sr/ 86 Sr ratio measured in the limestones and the gypsum beds confirm a Triassic origin for the strontium, originating from the diapir of Sorine, located to the south-west for the first part of the series, and from the Authon Thrust, located to the north, for the upper part. Geochemical values also suggest 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 (Ealy-Middle Triassic). These results support that combined use of field geology and geochemistry can provide information on erased past salt-tectonics.