Evolution of the Baztán Basin (navarra, N Spain) During the Early Stages of the Alpine Cycle: A Key Area at the Junction Between the Western Pyrenees and the Cantabrian Range

The Baztán Basin (western Pyrenees, N Spain) represents a unique geological archive located at the junction between the Western Pyrenees and the Cantabrian Range, two major orogenic systems whose tectono-stratigraphic connections remain poorly understood. Despite significant advances in the study of neighboring Permo-Triassic basins since the 1970s, this area has received limited attention. Here we present the results of a multidisciplinary investigation integrating detailed stratigraphic logging, sedimentological analysis, clay mineralogy (XRD), scanning electron microscopy (SEM), structural fieldwork, and high-resolution Digital Outcrop Models (DOMs) acquired via UAV photogrammetry for geometry interpretation. Our study of eight stratigraphic sections across the basin allowed us to define three Permian tectono-sedimentary units (P1, P2, P3) and three Triassic units (T1, T2, T3), separated by erosional unconformities that record distinct phases of tectonic activity, paleogeographic reorganization, and climatic change. The Permian succession, which age is poorly defined, documents the Variscan extensional collapse, with continental deposition in narrow fault-controlled basins evolving from lacustrine and debris-flow environments to braided fluvial and playa-lake systems under increasingly arid conditions. The Middle Triassic (Anisian) succession, records the onset of the Alpine extensional stage, displaying Germanic-type facies (Buntsandstein, Muschelkalk, Keuper) linked to the early rifting associated with the opening of the Bay of Biscay. The stratigraphic, sedimentological, and magmatic record of the Baztán Basin shows strong affinity with the central and eastern Pyrenean domain, rather than with the Cantabrian Range, redefining its geodynamic significance. The basin thus emerges not merely as a transitional zone, but as the westernmost expression of the Pyrenean Permo-Triassic system, providing a link for understanding the lateral continuity and structural evolution of both the Pyrenean and Cantabrian orogenic belts.