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The North Pyrenean basin in southern France was initiated in middle Cretaceous time along the North Pyrenean fault zone. An understanding of the middle Cretaceous is critical because it corresponds in time to opening in the Bay of Biscay, to postulated strike-slip along the North Pyrenean fault, to a controversially dated episode of metamorphism, and to emplacement of lherzolite. A field study of the Albian and Cenomanian fill of the North Pyrenean basin has been undertaken in order to provide constraints on postulated regional relationships during this time.
Two steps are required for the study. The first is to reconstruct the structure of the basin that was deformed during the Late Cretaceous to Oligocene Pyrenean orogeny. The orogeny is here summarized as a north-south shortening that reactivated the North Pyrenean fault zone as a north-vergent reverse fault. Dominantly south-directed thrusting followed, and detachment along incompetent Triassic shale and evaporite layers was important in both phases. The second step is a sedimentologic analysis. The basin fill is dominantly marine, clastic mud, up to 4 km thick. Abrupt lateral thickness and facies variations demonstrate that the basin was bounded by active faults. Excepting the absence of high organic productivity, reconstruction suggests that the North Pyrenean basin is a partial anal g in terms of geometry, facies arrangement, and regional setting to Cenozoic marine basins in California, particularly to those in which Monterey Formation lithologies were deposited.
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