Vertically persistent sedimentary facies boundaries along growth anticlines and climate-controlled sedimentation in the thrust-sheet-top South Pyrenean Tremp-Graus Foreland Basin

Compression of the thrust sheet underlying the central South Pyrenean Tremp-Graus Foreland Basin led to weak folding of the overlying basin fill during deposition of Eocene sediments. From the distribution of sedimentary facies and the presence of these folds, it is interpreted that thrusting of deep-seated competent units was accommodated by weak synsedimentary folding with a shorter wavelength at shallower levels. This led to differential subsidence at the surface but no unconformities are observed. The slower subsidence along the active anticlines locally influenced the distribution of sedimentary facies: lateral boundaries between different sedimentary facies are found to extend (sub)vertically up to more than 60 metres within narrow zones a few 100 m to 1.5 km wide. The growth folds thus led intermittently to the fixation of the position of facies boundaries, including a fixation of the coastline, over long periods (104 to 105 years). Sediment transport paths were also influenced by the slight folding of the surface. The orientation of the weak ‘en echelon’ anticlines and of related facies boundaries agrees with the inferred compressional pattern during the Eocene. Explanations for the regular occurrence of 50–60 m thick sedimentary cycles in terms of tectonic pulses or orbitally driven climatic changes and resulting pulses in sediment yield are discussed. For cycles of shorter length (10–15 m), with durations of the order of tens of thousands of years, it is inferred that these are due to regular climatic changes, probably related to orbital forcing, and resulting cyclic alternations of arid and wet periods. Such periodic changes of climate would have caused the intermittent waxing and waning of coarse-grained sediment