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Structure and Salt Tectonics in the Offshore Parentis Basin, Eastern Biscay Bay

Ferrer, Oriol 1; Roca, Eduard 1; Jackson, Martin 2; Ellouz, Nadine 3; Muñoz, Josep Anton 1
1 GEOMODELS, Departament de Geodinàmica i Geofísica, Universitat de Barcelona, Barcelona, Spain.
2 Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin, Austin, TX.
3 Geology and Geochemistry Division, Institute Français du Pétrole, Rueil-Malmaison, France.

The Parentis Basin is one of the main oil provinces of France and has been widely explored since the 1960s. However, little has been published on the salt tectonics here. This basin lies on thinned continental crust in the eastern Bay of Biscay and is bounded by the Landes structural high to the south.

The Parentis Basin is an E-W-trending, intracontinental basin 65 to 120 km wide by 150 km long. The basin fill comprises a thick Jurassic-Lower Cretaceous synrift sequence above lowermost Jurassic to Upper Triassic evaporites and Lower Triassic-Permian siliciclastic rocks. The southwestern part of the basin appears bounded by a north-dipping master fault (Landes Fault), which shifts northward in the eastern part of the basin (Ibis Fault). The Landes High remained high until the Late Cretaceous. Uppermost Cretaceous to Lower Miocene synorogenic deposits of the North-Pyrenean Foreland Basin cap the entire region.

Detailed interpretation of old industrial seismic surveys recently reprocessed from the offshore Parentis Basin elucidates the role of salt tectonics in the basin’s evolution. Salt anticlines and diapirs are concentrated and more structurally evolved mainly along the edges of the basin, where Jurassic-Lower Cretaceous infill sequences are thinner especially in the east of the basin. Salt walls and diapirs grew south of the Ibis fault, whereas salt anticliclines cluster above the Ibis fault and the northern edge of the basin. In the west, diapirs are small and restricted to the Landes Fault along the southern edge of the basin. In contrast, salt ridges are larger than in the east.

Salt structures formed at Albian-late Cretaceous times, during the Bay of Biscay opening. Subsequently, Pyrenean compression rejuvenated these salt structures. Anticlines cored by Triassic evaporites and stocks absorbed most regional shortening during the Pyrenean orogeny, forming squeezed diapirs, salt glaciers and salt welds, some of which were later reactivated as reverse faults. No new diapirs formed during orogeny, and salt tectonics ended with the close of the Pyrenean orogeny in the Middle Miocene. The lack of significant inversion structures in the Parentis Basin denotes that the Landes High acted as a partial buffer for northward propagation of Pyrenean shortening. However, orogenic compression in the basin was enough to squeeze the relatively weak salt structures, which responded as sensitive strain markers, while the country rocks resisted inversion.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009