Interpreting beach sedimentary dynamics between Tróia and Sines (SW Portugal) using heavy minerals and textural analysis

Abstract

Knowledge of the littoral Troia-Sines (SW of Portugal) sand grain-size distribution and transparent heavy minerals rests on the study of 195 samples collected from beachface, berm, dune and sea-cliffs. The results show the dominance of coarse sand and a heavy mineral suite composed by tourmaline, muscovite, andaluzite, biotite, amphiboles, staurolite, pyroxenes, silimanite and kyanite. The textural sediment characteristics allow the definition of three different littoral long shore sectors: north, central and south. In the north sector there is a north-south increment in grain-size linked with clear evidences of a bioclastic supply from intertidal benthonic macrofauna communities. In the central sector, sea-cliffs are the source of heterometric sediments to the beach (from coarse sand to silt sized particles). In the southern sector, sea-cliffs also represent an important sediment source, nourishing the beach system with fine sands. The heavy mineral composition shows high quantities of mica close to the Sado inlet (north sector) and to the sea-cliffs (south sector). On the other hand, in the south sector near the Sines subvolcanic massif outcrops, some pyroxene and amphibole enrichment was detected. From these results, it was possible to deduce various sediment sources for this littoral area: inner shelf; Mio-Pliocene sea cliffs; Sines subvolcanic massif; bivalves communities existing in the Troia peninsula; and Sado estuary.