The sub-Alpine chains of Haute Provence and Alpes Maritimes are a thin-skinned fold and thrust belt detaching on Triassic evaporites. Basinal rocks are thrust south-westward over an ‘L’- shaped carbonate platform to form a great arc - the Arc de Castellane. The structure and stratigraphy are well known and described, as is the existence of diapiric salt, though recent insights from seismic data and analogue modelling have helped us to add much to the ‘salt story’.
In 2012, three of us from Hess (along with Martin Jackson of AGL) described evidence for an allochthonous salt sheet overlying the inverted strata of the Barre de Chine north of Digne, (the main town in Haute Provence) and proposed that a submarine glacier must have covered some 25sq Km of the Oxfordian seabed. More recently and a little further north, Célini et al. (2021) have recognised a much bigger 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 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 Middle Cretaceous to cover at least 10 sq km. More extensive 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 Palaeocene Poudingues d’Argens conglomerate, and most probably much of the classic 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, but the most spectacular example of Tertiary allochthonous salt 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.
Speaker
Dr Rod Graham - William Smith Medallist Winner 2022
Rod graduated from the University College of Wales, Aberystwyth in 1966 and gained a PhD from Imperial College in 1969 with a thesis on the structure and metamorphism of Lewisian rocks in the Outer Hebrides. He became a lecturer at University College, Swansea in 1971 and switched his research interest to the structural evolution of the external Alpine chains of Provence. 1984 he joined BP as a structural geologist at a time when there was great interest in the prediction of geological structures at depth using the principles of ‘thin skinning’ and section balancing. At BP he developed his Alpine interest by running training courses for the company on the evolution of the Tethyan continental margin along with David Roberts, Marcel Lemoine and Pierre-Charles de Graciansky. After BP he joined Monument as chief Geologist, then LASMO as a global adviser, then Emerald as Exploration Manager. The emphasis was mostly on global new ventures, with a particular focus on Colombia. Finally, he worked for Hess in London as a global consultant until retirement in 2011. After retirement he became an independent consultant and Visiting Professor at Imperial College and became closely involved with ION Geophysical in the interpretation of their deep seismic data sets of the world’s continental margins. Together with Jim Pindell he ran numerous courses for the oil industry on the evolution of rifts and passive margins using ION data, with the Alps as a field analogue. More recently his research focus has been on a re-appraisal of the evolution of the French sub-Alpine chains by applying recently developed concepts of salt tectonics.
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