Textural and compositional variations in beach sands along south Alagoas coast, Brazil

Abstract

Coastal and shallow marine sediments are usually of heterogeneous nature, composed of variable mixtures of siliciclastic and bioclastic particles. The comprehension of sedimentation dynamics in these environments is rendered problematic by their variable compositional and physical properties. Surface samples collected in the beach face from eighteen stations in southern Alagoas were analyzed. Grain size parameters were determined through the SYSGRAN package and 300 grains were identified for each grain size class coarser than 0.125 mm. Grain size distribution ranged from very fine sand to medium sand with dominance of the former, mostly classified as moderately sorted. Beach sediments are essentially siliciclastic and quartz is the major component. Pontal do Peba beach concentrates the highest content of bioclastic grains and northwards from it bioclastic grains content gradually diminishes. The main bioclastic grains are mollusc shells, red algae, and Halimeda fragments. Echinoderm spines and fragments, bryozoans and foraminifera tests also contribute significantly to beach deposit. Grain size and sorting patterns shows a correspondence with coastal plain width and the proximity of carbonate grains source. The sediment distribution pattern suggests that in addition to the grains originated from the hinterland, the continental shelf constitutes an important sediment source for the beaches.