Comparative study of inner shelf sediments (Plio-Pleistocene and present day cover) in the Western Portuguese margin

Abstract

A comparative study between Pliocene-Pleistocene and present day inner shelf deposits, considering both grain-size distribution and the present day hydrodynamic conditions, at west Portuguese Coastal Margin, is conducted. The modal grain-size analysis indicates that the west Portuguese Plio-Pleistocene inner shelf sediments are made of a mixture of several populations. The coarser populations (mode >0.5 mm, 1empty set) result from reworking of previous shoreface and beach deposits after storm events or are related with transgressive ravinement lags. The intermediate population, with modes around 0.125 to 0.250 mm (3 to 2empty set), should be in equilibrium with the hydrodynamic storm conditions in the inner shelf. This population is also the main constituent of the present day inner shelf cover between 10 and 25 meters depth. The finer population, with modes around 32 mu m (5empty set), is probably deposited from suspension during fair-weather periods. Under the high hydrodynamic winter storm conditions this population remains in suspension and tends to be mobilized to outer shelf locations by downward currents. It is expected that the mud size population proportion in bulk sediment should increases basinward, but this proportion may as well be influenced by post-depositional processes in ancient deposits.