Internal structure of mixed-sand-and-gravel beach deposits revealed using ground-penetrating radar



ABSTRACT Mixed-sand-and-gravel beaches are a distinctive type of coarse-clastic beach. Ground-penetrating radar (GPR) and photographic records of previous excavations are used to investigate the stratigraphy and internal sedimentary structure of mixed-beach deposits at Aldeburgh in Suffolk, south-east England. The principles of radar stratigraphy are used to describe and interpret migrated radar reflection profiles obtained from the study site. The application of radar stratigraphy allows the delineation of both bounding surfaces (radar surfaces) and the intervening beds or bed sets (radar facies). The deposits of the main backshore berm ridge consist of seaward-dipping bounding surfaces that are gently onlapped by seaward-dipping bed sets. Good correspondence is observed between a sequence of beach profiles, which record development of the berm ridge on the backshore, and the berm ridge's internal structure. The beach-profile data also indicate that backshore berm ridges at Aldeburgh owe their origin to discrete depositional episodes related to storm-wave activity. Beach-ridge plain deposits at the study site consist of a complex, progradational sequence of foreshore, berm-ridge, overtop and overwash deposits. Relict berm-ridge deposits, separated by seaward-dipping bounding surfaces, form the main depositional element beneath the beach-ridge plain. However, the beach ridges themselves are formed predominantly of vertically stacked overtop/overwash units, which lie above the berm-ridge deposits. Consequently, beach-ridge development in this progradational, mixed-beach setting must have occurred when conditions favoured overtopping and overwashing of the upper beachface. Interannual to decadal variations in wave climate, antecedent beach morphology, shoreline progradation rate and sea level are identified as the likely controlling factors in the development of such suitable conditions.