Joint interpretation of beach-ridge architecture and coastal topography show the validity of sea-level markers observed in ground-penetrating radar data

Abstract

Geological studies of past and present sea level rely on valid and robust features marking sea level in geological sections. Present sea-level markers around the upper shoreface to beachface transition are detected in a beach-ridge plain formed in a microtidal regime. These sea-level markers identify specific relative sea levels at the time of formation. Ground-penetrating radar (GPR) data collected across the youngest part of the beach-ridge system of Feddet, Denmark are compared with independent coastal morphological and sedimentological data of the active strand plain and interpreted in relation to sea-level data. The data show consistency between dip values of the present beachface and upper shoreface compared with dip values of interpreted beachface and upper shoreface GPR reflections. A clear change in dip value is observed between beachface and upper shoreface deposits in both data sets. Within few centimetres, this break point coincides with actual sea level and is interpreted to correspond to downlap points observed in the GPR reflection data. Furthermore, our observations may indicate that downlap points of deposits, formed under both relatively high and low water levels, are preserved and may be identified in GPR reflection data. Downlap points identified in GPR data across microtidal beach-ridge systems from other localities can also constitute markers of palaeo-sea level at the time of deposition. Records of these sea-level markers can be used to reconstruct the local relative sea-level history during the Holocene.