Geological and Geomorphological Controls on Rocky Shore Platforms: Insights from the Basque Coast, France
Abstract
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.
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BibTeX
@article{regard2026geological,
title = {{Geological and Geomorphological Controls on Rocky Shore Platforms: Insights from the Basque Coast, France}},
author = {Vincent Regard and Grégoire M Maillet and Philippe Razin and Adrien Dublanchet and Mélody Prémaillon and Thomas J B Dewez and Anne Briais and Anne Duperret and Clara Lévy and Kevin Pedoja and Sebastien Baratchart and César Dyson and Nicole Guerrero and Yannick Thiery},
journal = {Bulletin de la Société Géologique de France},
year = {2026},
month = jul,
doi = {10.1051/bsgf/2026010},
url = {https://normandie-univ.hal.science/hal-05751684v2},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Geological and Geomorphological Controls on Rocky Shore Platforms: Insights from the Basque Coast, France
AU - Vincent Regard
AU - Grégoire M Maillet
AU - Philippe Razin
AU - Adrien Dublanchet
AU - Mélody Prémaillon
AU - Thomas J B Dewez
AU - Anne Briais
AU - Anne Duperret
AU - Clara Lévy
AU - Kevin Pedoja
AU - Sebastien Baratchart
AU - César Dyson
AU - Nicole Guerrero
AU - Yannick Thiery
PY - 2026
DA - 2026/07/28
JO - Bulletin de la Société Géologique de France
DO - 10.1051/bsgf/2026010
UR - https://normandie-univ.hal.science/hal-05751684v2
AB - 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.
SN - 0037-9409
SN - 1777-5817
ER -