Quaternary geomorphological evolution of a granitic shore platform constrained by in situ 10Be concentrations, Penmarc'h, SW Brittany, France

The Penmarc'h granitic coastal domain, SW Brittany (Western France), offers the opportunity to address the little investigated issue of erosional processes in a granitic coastal context. From land to sea, the granitic coast is composed of 1) an aerial marine terrace (< 4 km), 2) a large shore platform (< 800 m), 3) an extensive submarine rocky platform (< 7 km), corresponding to the inner shelf, and 4) an outer shelf. Our geomorphological study, further completed by cosmogenic nuclide (Be-10) analyses, supplies new constraints on the weathering and denudation processes recorded by the Penmarc'h platform in Quaternary times. The role of at least two denudation events is emphasized: (i) a > 1.8 m-thick level of granitic material is assumed to have been removed by erosion during the MISS sea-level highstand, both in the offshore domain and in the low part of the marine terrace; (ii) an additional 0.6 m-thick level of the granitic substratum was later removed during the last Holocene transgression, mainly in the marine domain. The lack of any morphological step between the subaerial marine terrace and the shore platform is here attributed to the combined effect of multiple factors, including (i) a Quaternary tectonic quiescence, (ii) a major eustatic transgression (MISS) shaping both the shore platform and the low part of the marine terrace, (iii) the mechanically strong behaviour of the granitic basement with regard to erosion, and (iv) the moderate influence of the Holocene sea-level stillstand (since 6 ka) on the shore platform. Thanks to Be-10 content analysis, a range of vertical erosion rates of ca. 3-6 m.Ma(-1) is obtained for the onshore domain, showing no discernable difference between the intertidal and aerial erosion rates. From that result, it is thus determined that the long-term development of the Penmarc'h granitic shore platform was mainly driven by marine erosive processes during the MISS and Holocene marine transgressions, whereas subaerial erosive processes prevailed after the MISS event.