Allochthonous salt sheets in the external SW Alps

                Allochthonous salt sheets in the external SW Alps

Rod Graham₁, Sam Brooke-Barnett₁, Naïm Célini₂, Jean Paul Callot₂, Adam Csicsek₁, Lidia Lonergan₁, Jean Claude Ringenbach₃.

₁ Imperial College London ₂Université de Pau et des Pays del’Adour ₃Total Energies SE

Understanding of the importance of salt tectonics in the Alpine fold and thrust belts of Europe has evolved considerably in the last few years. Our particular area of focus has been in the sub-Alpine chains of Haute Provence, an area where initial ground-breaking work on diapirism was published many years ago by Graciansky, Dardeau, Mascle and others.

 Our own work demonstrates that salt related phenomena like minibasins, megaflaps, welds and anomalous changes in stratigraphic thickness and facies are widespread over the region, but perhaps the most startling discovery is that, at several times in the past, salt was squeezed out over topographic surfaces to form allochthonous sheets of considerable extent. The salt breakouts were mostly submarine, comparable with the allochthonous salt in the Gulf of Mexico and coincided with times of shale deposition during Oxfordian, Albo- Cenomanian and , although breakout  onto land surfaces occurred in the Tertiary.

Graham et al (2012) described an allochthonous salt sheet overlying the inverted strata of the Barre de Chine north of Digne which covered some 25sq Km of the Oxfordian seabed,. North of this structure Célini et al. (2021) describe a major allochthonous sheet rooting in the Astoin diapir. 10 sq km of this remains as gypsum and cargneule, but the original extent of the salt glacier on the Oxfordian sea floor must have been much greater since stratigraphically out of place remnants of Liassic resting on Oxfordian black shales are found 10km north of the remnant allochthonous gypsum.

Further east on the flanks of the Dome de Barrot inlier near Daluis, there is evidence of Oxfordian breakout, renewed more extensively in the Apto-Albian with an extent of at least 10 sq km. More extensive Apto-Albian breakout is associated with the Gevaudan diapir near Barreme on which secondary minibasins developed from the Late Cretaceous to the Miocene. These include the hugely rotated minibasin containing the Poudingues d’Argens conglomerate, and, we suggest, much of the Barreme basin itself. The minimum extent of this sheet was about 80sq km, though it may have been larger.  

20km to the east, Eocene breakout is indicated by an enormously expanded growth in Nummulitic  shales - a possible Roho system, with salt expelled into the active strike slip system of the Rouaine-Daluis transcurrent fault zone. The most spectacular example of Tertiary allochthonous salt, however,  is  that into which the secondary minibasin of the famous  Esclangon Velodrome sank, providing a major depocentre for  Burdigalian to Pliocene sediments. Célini et al. (in press) have documented the evolution of the Velodrome and convincingly compared it with the Tuzlagözü minibasin of the Sivas basin in Turkey. The salt glacier into which the Velodrome sank must have flowed out onto an Oligocene land surface and may be analogous with the Salt Range of Pakistan.