The Oroz-Betelu massif is a large anticline composed of Paleozoic and Triassic rocks, surrounded by Cretaceous and Tertiary strata. Study of the biofacies and lithofacies shows that this structure was an emerged zone during the Cenomanian. It was invaded by the Santonian Lacazines sea and oscillations brought about a partial emergence at the end of the Santonian or at the beginning of the Campanian. This was followed by the deposition of a flysch facies at the end of the Campanian. A marine regime continued throughout the remainder of the upper Cretaceous.