Petrography and temporal changes in petrofacies of deep-marine Ainsa–Jaca basin sandstone systems, Early and Middle Eocene, Spanish Pyrenees

A petrographic study of 157 samples from the Early to Middle Eocene deep-marine sandy systems, Ainsa–Jaca basin, Spanish Pyrenees, shows that each system has a characteristic petrofacies. Three main petrofacies are recognized. Petrofacies 1 sandstones comprise mainly siliciclastic grains (≥80%), subordinate terrigenous carbonate grains and negligible intrabasinal grains. Petrofacies 2 hybrid arenites are characterized by significant amounts (≥10%) of intrabasinal carbonate grains. Petrofacies 3 calcilithites contain relatively abundant (≥10%) extrabasinal carbonate grains. On the basis of these petrofacies, a revised correlation of the sandy systems is proposed between the more proximal Ainsa basin, and the more distal Jaca basin sediments, now separated by the Boltaña anticline, across which it is impossible to actually trace out individual beds or sandstone packages between both basins. The arenite composition in the Ainsa and Jaca basins is interpreted as being controlled mainly by synsedimentary tectonic processes that led to changes in sediment sources during basin evolution.