The analysis of calcareous micro fauna (foraminifers, ostracods) of the Arceniega Formation allows to decipher different phases on the palaeoenvironmental evolution of this part of the Navarro-Cantabrian Basin during the Cenomanian. The generalised marine ingression of the beginning of the Upper Cretaceous occurred in the region during the deposition of the Arceniega unit, in the early Cenomanian. After an interval with regressive trend, a sustained transgressive phase was developed during the late Cenomanian, which represents the deepest sediments of shelf settlements of the Mesozoic. Some biotic proxies, particularly that of the platycopic ostracod signal, evidence intervals with important influx of hypoxic waters