Geological and geomorphological controls on rocky shore platforms: insights from the basque coast, France

Rocky shore platforms are critical coastal landforms whose morphology reflects the interplay between geological inheritance, sea-level history, and erosional processes. While Sunamura’s (1992) classification distinguishes three main platform types (A, B, C), the detailed morphology of these features—especially their submerged components—remains poorly understood due to limited access. Here, we investigate the geological and geomorphological controls on shore platform development along the Basque Coast (France), using high-resolution bathymetric data acquired during the SPLASHALIOT-1 cruise ( R/V Haliotis, 2014 ). Our study focuses on two sites near Saint-Jean-de-Luz: Socoa (a long-term monitoring site of the Dynalit national observatory) and Kokotia, both underlain by Cretaceous–Paleogene turbidites deformed during the Pyrenean orogeny. By integrating bathymetric mapping, and geological observations, we reveal that both sites exhibit the characteristics of a Type A platform (sensu Sunamura, 1992 ): a planar, gently sloping surface (1–2°) extending 100–500 m offshore. The platform morphology is strongly influenced by lithological layering, with resistant turbidite beds shaping roughness and structural features. Notably, we observe no seaward cliff, challenging the hypothesis of ubiquity of low-tide cliffs in models of rocky coast evolution. Geological mapping extends onshore stratigraphy offshore, highlighting thrusts and normal faults (e.g., the Bidart–Pointe Sainte Anne Fault) that compartmentalize the platform. Our results suggest that platform morphology primarily records Holocene sea-level stabilization, with limited evidence of deeper terraces from the Last Interglacial (MIS5). The absence of prominent scarps or stepped features implies that cliff retreat rates (4–20 cm/a) have been insufficient to preserve older morphologies. We further discuss how structural roughness and lithological contrasts modulate wave energy dissipation, with implications for future cliff retreat under sea-level rise. This study underscores the need to account for geological heterogeneity in predictive models of rocky coast evolution.