Abstract

The Upper Cretaceous deposits of me Southern Pyrenees (Iberian margin) are localized in pull-apart basins controlled by NE-SW sinistral strike-slip faults. The Aren sandstone extends regionally over 100 km and represents the regressive infill of the Tremp basin. The Maestrichtian sequence (Aren Fm) starts with a basin-wide collapse, which was tectonically triggered. All through the sequence, local folding and faulting produced paleogeographical changes. The sea was open to the north and the shelves extended over a stable southern zone as well as around some unstable and active relief to the east, next to the main NE-SW faults. An unstable shelf developed around the synsedimentary anticline structure of S. Cornell and was subjected to continuous tilting at an angle to the strike of the slope. Major tilting events define depositional cycles characterized by: 1. a reworking phase after the initial tilting, with erosion in the inner area and distribution of sand by longshore drift over the outer shelf; and 2. a shale onlap due to further subsidence. The upbuilding of the Aren sequence results from repetition of such sequences. During the first stages, deposition of successive outer shelf sandstone bodies onlapped the anticlinal structure.

During the second stage, major tilting resulted in a basal erosive surface and coarse siliciclastic influxes, followed by sand distribution. The depositional system includes brackish shale, channel sandstone, aeolian and marine (storm and tidal) reworking and, locally, turbidites. These first two stages are related to the movement of the anticline. A third stage is characterized by stability and beach progradation, as the system was locally tilted by normal faults. The intricate superposition of sandstone bodies through the successive stages of evolution results in an apparent onlap-offlap geometry that would be better interpreted as a fold-controlled uplap.