Development of cusp-related, gravel size and shape facies at Malin Head, Ireland



Samples of beach gravels were obtained from foreshore cusp deposits at Portmore Beach, near Malin Head, Republic of Ireland. Axial dimensions (A, B and C) were measured, and the maximum projection sphericity was derived, for each of 4360 clasts. Each sample was also categorized according to depositional subenvironment. Principal components analysis and cluster analysis were used to identify potential facies zonation. Results indicate that reworking of the foreshore by incident and edge waves resulted in clast size and shape segregation both parallel and perpendicular to the beach. Patterns of zonation were distinctly different from those described in earlier facies descriptions, showing a complex set of foreshore facies. The distribution of these facies was controlled primarily by location within beach cusp systems. Primary and secondary facies demonstrate alongshore variability of a magnitude comparable to across shore differences. The results have important implications for palaeoenvironmental reconstruction, morphosedimentary description and research sampling design, and provide the basis for a conceptual model of gravel beach cusp facies.