The tectonosedimentary study of the southern and northwestern margins of the BortzirieCinco Villas Palaeozoic Massif, which constitute the eastern margin of the BasqueeCantabrian basin, has yielded a detailed synrift stratigraphic architecture and led to the proposal of formal lithostratigraphic units, as well as to an interpretation of the evolution of different marine depositional systems during Aptian to Cenomanian times. During this time interval, carbonate platform to deeper siliciclastic depositional systems were deposited transgressively onto the margin of the BortzirieCinco Villas Palaeozoic Massif. The onset and evolution of sedimentary depositional systems were controlled by strong synsedimentary tectonic activity. The analysis and interpretation of the sedimentary systems has permitted the characterization of 5 tectonosedimentary phases separated by 5 main discontinuities (D1 to D5): Phase I, early Aptianeearly Albian; Phase II, middle Albian; Phase III, late Albianelatest Albian; Phase IV, latest Albianeearly Cenomanian; Phase V, early CenomanianeCampanian. The observed tectonic and sedimentary features indicate the presence of regional transtensional strain during all the tectonosedimentary phases, except for Phase IV when compressive features are recognized related to the main basement faults. All of the above-mentioned tectonosedimentary phases correlate well with major Pyrenean tectonosedimentary events related to the opening of the Bay of Biscay and the Central Atlantic during the Cretaceous.