Os arenitos de praia de Itaipuaçu e Jacone - RJ: origem, idade, distribuição e influência no fluxo de sedimentos

Abstract

The beachrock of Itaipuaçu is found on the upper shoreface at depths which varies from 2.5 m at the west side to 7.3 m at the east, and represents an evidence of a lower sea-level as the present one. It is related to the oldest Holocene barrier beach with a presumed age of about 7,000 years B.P. Contrary as in Itaipuaçu the beachrock of Jaconé outcrops in the intertidal area and is associated to the more modern barrier beach with an presumed age of about 3.700 years B.P. Both the beachrocks were formed in the interior of the sedimentary body of the beach, below the beach berm. The cement of the beachrocks is a low magnesium calcite, typical of a fresh water environment. Hence they are "pseudo beachrocks". ln Itaipuaçu the beachrock forms a submarine barrier which influences the interchange of sediments between the beach and the upper shoreface by retaining the fine sand fraction at the ocean side of the barrier which causes an additional coarsening of the beach sand.