Morphology, grain-size and faunistic composition of the macrotidal beaches of Tierra del Fuego

Abstract

Morphodynamic models have been applied to macrotidal beaches. They are based on unimodal grain sizes and average dynamic parameters as the wave height and period at the breaker zone. Bimodal gravel beaches do not fit to these models because a) grain size is subject to spatial segregations (gravel and granules at the upper beach zones and fine sand at the lower-tide terrace), and b) wave dynamics vary temporarily during the tidal cycle: the beach has a reflective behaviour during high tide and dissipative during low tide. Superimposed to these morphodynamical constraints, along the Atlantic coast of Tierra del Fuego inherited factors control the coastal morphology. In low-lying coasts (South-American plate portion), the Holocene sea-level fluctuation permitted the construction of beach ridge plains. On high-relief coasts (Scotia microplate), the same fluctuation have little enough sediment available, and therefore pocket beaches developed between capes and sand accumulations at river outlets. Two areas were found of interest by their heavy mineral content: the Chico river beach-ridge plain, and the coastline between Ewan and Ladrillero estuaries. With regard to the molluscan beach composition, rocky bottom specimens dominate at the capes.