Conceptual model of fracture‐limited sea cliff erosion: Erosion of the seaward tilted flyschs of Socoa, Basque Country, France

Abstract

Sea cliff morphology and erosion rates are modulated by several factors, including rock control that reflects both lithology and rock structure. Erosion is anticipated to preferentially exploit 'fractures', broadly meant as any discontinuity in an otherwise continuous medium, where the rock mass is weakest. Unpicking the direct control of such fractures on the spatial and temporal pattern of erosion remains, however, challenging. To analyse how such fractures control erosion, we monitored the evolution of a 400 m-long stretch of highly structured sedimentary cliffs in Socoa, Basque Country, France. The rock is known as the Socoa flysch formation. This formation combines decimetre-thick turbidites composed of repeat triplets of medium to strong calcareous sandstone, laminated siltstones and argillaceous marls. The sequence plunges at 45 into the sea with a shore-parallel strike. The cliffs are cross-cut by two normal and reverse fault families, with 10-100 m alongshore spacing, with primary and secondary strata-bound fractures perpendicular to the bedding, which combined delimit the cliff rock mass into discrete blocks that are exploited by the erosion process. Erosion, and sometimes plucking, of such beds and blocks on the cliff face was monitored using ground-based structure-from-motion (SfM) photogrammetry, over the course of 5.7 years between 2011 and 2017. To compare with longer time change, cliff-top retreat rate was assessed using SfM-orthorectified archive aerial photographs spanning 1954-2008. We show that the 13,250 m 2 cliff face released 4500 blocks exceeding 1.45 Â 10 À3 m 3 , removing a total volume of 170 m 3 . This equates to an average cliff erosion rate of 3.4 mm/year, which is slightly

The interplay of bedrock fractures and glacial erosion in defining the present‐day land surface topography in mesoscopically isotropic crystalline rocks

“…Recent studies have shed light towards understanding coastal erosion processes, and the findings indicate that fracture‐controlled quarrying is the dominant erosion process (Buchanan et al, 2020). Major coastal erosion through quarrying typically requires rocks characterized by distinct layering, including mechanically weak layers that allow detachment of larger blocks (Prémaillon et al, 2021). Further, as the glacial striations are preserved on the oucrop surfaces, we conclude that the potential effect of coastal erosion in the Geta Island is limited to intersections of the fault damage zones and shorelines (with joint‐bound block size of less than 2 m, e.g., right side of Figure 3d), while the bulk of erosion relates to glacial processes.…”

Quantifying the topographical structure of rocky and coral seabeds

“…The platform is approximately 100 m long. The shore platform and the associated cliff display the so-called Flysch marno-calcaire de Socoa , corresponding to a marl and limestone Flysch formation [ 50 , 51 ].…”

Quantitative Characterization of Coastal Cliff Retreat and Landslide Processes at Portonovo–Trave Cliffs (Conero, Ancona, Italy) Using Multi-Source Remote Sensing Data

“…In particular, geological structures can play a key role in controlling the morphology and stability of high coastlines [17,18], changing asymmetrically the values of shear strength and uniaxial compressive strength along the cliff. Furthermore, wave action and sea-level rise can lead to the creation of coastal notches and progressively to the collapse of large parts of the cliffs [2,19].…”