The Barrême Basin and the Gévaudan diapir - an example of the interplay between compressional tectonics and salt diapirism Rod Graham¹, Lajos Adam Csicsek¹ ¹Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, London, SW7 2AZ, UK email: [email protected], [email protected] 1) Abstract, Regional setting 2) Stratigraphy System/ Series/ Code/ Stage/ Regional System/ Series/ Code/ Stage/ Regional period epoch index age tectonics period epoch index age tectonics Mio-Pliocene units AR N Our understanding of the role of salt diapirism in determining the finite Ba m-pl n7 Albian Pliocene Neogene Undiff. Mio- geometry of fold and thrust belts has grown apace in the last few years, g3V AT b g3G Oligocene units Aptian g2S Chattian a n6 g2R g1S g1B e7g1,G Colliosion but the interplay between the two remains a significant problem for n4-5 Barremian Oligocene Rupélian e7 Priabonian e6-7C structural interpretation. The Gevaudan diapir in the fold and thrust belt Lower Eocene units n3 Hauterivian e Bartonian Subduction of the Cretaceous Paleogene eC Eocene Lutetian? European margin of the sub-Alpine chains of Haute Provence is well known and has been Foreland basin succession Cretaceous n2c Subduction of the Upper Cretaceous units Pe n2b documented by numerous eminent alpine structural geologists. n2 Valanginian Paleocene Valais Ocean DT Ar n2a (Rifting of the Valais Ocean) Maastrichtian of the PLO Graciansky, Dardot, Mascle, Gidon and Lickorish and Ford have all n1 Berriasian Subduction Campanian Lower Cretaceous units A D j9 Tithonian j6-9 c5 Santonian described and illustrated the geometry and evolution of the structure, BA j7-8 Kimmeridgian j5-6 j5 and Lickorish and Ford’s interpretation is figured as an example of Upper Jurassic units c3b4 Coniacian G Upper c3 Turonian diapirism in a compressional setting by Jackson and Hudec in their text M Cretaceous c2 Upper Jurassic Cénomanian S Ocean c1 on salt tectonics. We review these various interpretations and present Middle Jurassic units j2-4 Oxfordian Postrift succession another. Terres noires Cretaceous Lower Jurassic units n7 Albian Ocean The differences between the various interpretations say much about the Postrift succession j3 Callovian CA Spreading of the Piemont-Liguria Lower V j2-3 j2-3 Bathonian Jurassic complex interplay of salt diapirism and thin-skinned thrusting and have 200 km Cretaceous Triassic units Spreading of the Valais j2-3 j1S profound implications for the way we interpret the tectonic and Bajocien Middle j1 j1 Jurassic sedimentary evolution of the Barreme basin which lies adjacent to the Permian units j1D Sandstone l9 Aalenian diapir. l7 l6 Toarcien Conglomerates 10 km l5 Pliensbachian Ocean (PLO) The Barreme basin is a thrust-top fragment of the Provencal foreland Crystalline basement Synrift succession Lower l3-4 Sinemurian basin and has been described in detail from both sedimentological (e.g. Jurassic lD Chalky limestone and marl Rifting of the Piemont-Liguria Figure 1. The fold and thrust belt of the Southern Subalpine Chains comprises the Digne and Authon thrust l1-2 Hettangian Evans and Elliott, 1999) and structural (e.g. Antoni and Meckel, 1998) t10 Rhaetian Upper t7-9 sheets. The thrust system has polyphase deformational history (Graham et al. 2012.) Thrusting started before the Triassic Keuper tG,tK Evaporites Marl points of view. Here we make the case that it is also a salt related Triassic M.Triassic t2 Muschelkalk minibasin - a secondary minibasin developed on a now welded Eocene. During the Eocene and Oligocene a series of thrust-sheet-top basins evolved one of which is the Barréme L.Triassic t1 Buntsandstein Interbedded marl basin. During the Miocene and Pliocene the mountain front reached the Valensole basin. Permian allochthonous Middle Cretaceous salt canopy. We believe that within and limestone The study area is indicated by the black rectangle. The location of the regional cross-section (Fig. 3,4) is indicated Units autochtonous Permian units the basin it is possible to interpret successive depocentres which may Autochtonous Undifferentiated Autochtonous Cherty limestone crystalline basement by the black line. The white line indicates the edge of the Provencal platform during the Early and Middle Jurassic. Carboniferous record progressive salt withdrawal. We argue that though thrust loading AR: Argentera; D: Dôme de Barrôt; A: Annot basin; Ar: Argens syncline: Pe: Peyresq syncline; CA: Castellane Arc; 500 m must be the fundamental driving mechanism responsible for salt 400 Limestone Bioclastic limestone DT: Digne thrust sheet; AT: Authon thrust sheet; G: Gevaudan diapir BA: Barrême basin; Ba: Barles; M: Majestres 300 movement late in the tectonic history of the region, thrusting has not syncline; S: St Jurs imbricates; V: Valensole basin 200 Sandstone Evaporites done much more than modify existing salt related geometry. 100 and siltstone 0 Crystalline rocks Limestone and dolomite Figure 2. Lithostratigraphic column of the study area and the major tectonic events in the Western Alps. We refer the lithostratigraphic units as they are shown in this 3) Various interpretations of the Gévaudan diapir and the surrounding basins lithostratigraphic column. Figure 5. A “non-salt attempt” interpretation of R. Graham around Gévaudan Figure 3. Modified after Graham in Elliott et al. 1985, a pure fold and thrust belt interpretation Figure 4. Csicsek 2019, one of the first attempt of a regional section during a PhD project. Note the thickness changes in the Mesozoic succession and a lot of possible, welded salt related contacts. Figure 6. Artoni & Meckel 1998, an another structural interpretation. Note the thick salt layer at the top of the basement, thickness changes in the Mesozoic succession , the steep thrust faults bounding the different units and a possible young-on-older contact between the Gévaudan-Reichard unit and the Douroulles syncline. Middle Eocene (Bartonian) 40 Ma~ a 6.35°E 6.40°E 6.45°E West East PROVENÇ AL FORELAND Onset of flexural subsidence synchronous BARRÊME BASIN with Calcaires Nummulitiques transgression SEA LEVEL + + + + + MESOZOIC COVER + + ? Paleo-normal faults + THINNED CONTINENTAL CRUST + + + + + THRUST + + WEDGE sites of development + + + + + + + + + + + + + + of later thrust faults + + + + + + + + + + + + + + + + +++ + + + + + + + + + + St. Jacques + Late Eocene (Priabonian) 36 Ma~ 43.96°N Initial thrust front established west of Barrême BARRÊME GRÈS D’ANNOT SUB-BASINS SEA LEVEL N Barrême + + + Turbidites onlap structured basin floor + + DIGNE + + + + + + THRUST + + + + + + + Section line + + + + + 43.95°N + diapir + + + + + + + + + +? + THRUST + + + WEDGE + + + + + + + Gévaudan + + + + + + + + + + + + + + + + + + + + + mbl = blue marls, apto-albian; cN = limestones with Nummulites; mGlob = Globigerine marls from the Priabonian; + + gS = Sénez sandstone; mR = red molasse; cgC = calcareous conglomerates (top of the red molasse) + + + 0 1 km + + + + Figure 7.Gidon 1997. Cross-setion of the southern part of the Barrême basin. Late Eocene (Priabonian) 36 Ma~ Diapir overthrusts SHALLOW MARINE BASIN ST. LIONS ANTICLINE mid-Jurassic SEA LEVEL b Back-stepping shoreface of Former shale ? Calcaires Nummulitiques (CN) Allochthonous sheet C Southwest reactive Northeast AP CN PER RETA TIA UP CE buried by N N -A O EOC L PTIAN - A US diapir? DI OM BIAN A LB mid-Cretaceous Digne G IAN SHALES IA Barrême N SH EO N thrust E ALES N CO shale T AND LIMES ONES MI lateral tip HR T AN syncline U JU ST RA JURASSIC Reactivation SS IC of paleo-normal 1 TRIASSIC EVA PORITES faults as thrust faults + + + + + + + + + + + Gévaudan + + + + + + + + + + + + + + + + + 0 diapir Early Oligocene (Rupelian) 30 Ma~ -1 CONTINENTA L BASIN ER CRETACE PP OU A U S -2 PT CN MARINE BASIN TIAN - AL Elevation (km) B N IA FILL AP IA EO N - N CO AL 0 5 km M BI NEOCO IA AN MIA -3 DI N SH N G SH ALES NE AL JU V.E. × 1 T ES RA JU H AND SS R RU L IC A ST IMESTONES SS IC Recent alluvium Tithonian limestone JURASSIC + + + + + + + + TRI ASSIC EVA PORITES + + + + + Oligocene Middle Jurassic shale Barrême thrust-sheet-top Figure 5. Sketch illustrating the regional tectonic setting of the Barrême thrust-sheet-top Eocene Liassic limestone basin between middle Eocene (ca. 40 Ma) and early Oligocene (ca. 30 Ma) time. This ~10 m.y. Upper Cretaceous limestone Triassic gypsum time interval spans the initiation of the marine thrust-sheet-top basin to the onset of nonmarine sedimentation at Barrême, the so-called flysch to molasse transition of Allen et al. (1991). Note Middle Cretaceous shale Basement that the thicknesses of stratigraphic units are not drawn to scale. Lower Cretaceous limestone Figure 8. Schematic map of the structures of the Southern Figure 9. Cartoons shoving the evolution of the Gévaudan diapir Figure 10. Lickroish and Ford 1998. Series of cross sections, form N to Figure 11. Lickorish & Ford 1998 cross section in Jackson and Hudec 2017. Their interpretation Figure 12. Sketch showing the evolution of the Barrême Subalpine Chains, Gidon 1997. Note the strike-slip faults and the sourroundign basins. S. Their interpretation includes the eastward migration of the includes an allochthonous salt sheet buried by mid-Cretaceous shale (n7c1). The Eocene thrust-sheet-top basin. Note the generally constant thickness around the Gévaudan area. Note the relatively short period of structural evolution. Gidon 2000. depocenter in the Barrême basin and the intrusion of the Gévaudan and Oligocene units are thickening towards to east.