Caracterização granulométrica de dunas sob-atuais costeiras: ilha de Algodoal/Maiandeua - NE do Pará.

Abstract

This work is part of integrated research (geology, geomorphology, botany, pedology, geochemistry) on Amazonian coastal ecosystems, developed by MPEGICNPq (DEL/1995-96). The study area is the APA Algodoal/Maiandeua (law n" 5.261/90), located between 00° 35' 03" to 00° 38' 29" LAT. S. and 47° 31' 54" to 47° 34' 57" LONG. W. Gr. The island constitutes a point between the bays of Marapanim and Maracanã, with a general direction NW -SE, and is formed by different geomorphological units (low plateaus, mangroves, dune fields, tidal plains and beaches). Our contribution constitutes the first step of the granulometric and mineralogical characterization of the sediments of the "internal dunes" and ecotone areas. The mid-term objective is to identify the source-area of the sands, testing, among others, the hypothesis of a provenance linked to the inner continental shelf, exposed to subaerial conditions during the last global eustatic cycle of the Terminal Upper Pleistocene/Holocene (EL ROBRINI et al, 1992; FARIA Jr, 1984; FARIA Jr et al, 1987; FRANZINELL, 1982; NASCIMENTO FILHO et al, 1993, etc ). The methodology used is classic in geomorphology and involves: (a) sieving of nineteen (19) samples collected along a plani-altimetric profile of 2 km length and E- W direction; (b) statistical analyses (mean diameter, degree of selection, asymmetry and kurtosis); (c) construction of histograms and simple frequency curves; and (d) textural classification of the sediments. A great granulometric homogeneity was observed along the studied profile, with a marked predominance of fine and very fine quartz sand, which contrasts with the first field observations that showed, on the reverse side of the dunes, an apparent heterometry of the surface sands. The homogeneity is maintained independently of the vegetation and dune modeling. This leads one to think of the existence of a large source area feeding both the beach formations, the present dunes, and the internal dunes. However, further research is needed (mineralogy, grain morphoscopy) to test this hypothesis.