Estuaries provide food and habitat for a large array of benthic, and pelagic organisms, to primary consumers in the food chain and also to human beings. They act also as protection of coastal areas from floods and erosion. In this paper we studied the physical-chemical properties (interstitial pH, Eh and salinity) as well as the mineralogical and geochemical characteristics of sediments from the Cunani River-Amapá-Brazil, all in order to contribute to the geological knowledge of this remote area. The Cunani River is located in the northern domain of the coastal plain of Amapá State and it is one of a few estuaries that discharges into the Atllantic Ocean in a wide sandy beach. The tide influence reaches about 28 km away from the mouth of the river, with the saline front not exceeding 13 km. pH valoues of the interstitial fluids had vary between basic to slightly neutral, except in the sediments near to Cunani’s river mouth, where they were acidic. The negative Eh values, raging from -20 to -280 indicated a reducing environment. The dominant grain size is mud, accouting for 98% of the total sediments. Pearson’s r among major (Al2O3, Fe2O3, CaO, Na2O, K2O), trace elements (Nb, Sr, Th, Sc, Co etc) and REE (La, Nd, Tm, Sm etc) varied between medium and high correlation. We supposed that this linear relationship between this elements are related to the chemical structure of clayminerals as kaolinte, chlorite and illite. Mineralogically the sediments were composed by quartz, muscovite, smectite, kaolinite, albite and ilite. Two different transparent heavy minerals assemblies were distinguished, assembly A and B. Assembly A is formed by hornblende, epidote, topaz, staurolite, tourmaline, hypersthene, zircon, rutile, andaluzite, apatite and it’s representative of the muddy sediments of Cunani’s river. Assembly B is composed of zircon, hornblende, topaz, rutile, epidote and it is representative of coarsening upwards sediments. The study of heavy minerals revealed that these two assemblies had originated from differents sources. Assembly A had come from the Amazon river and assembly B had come mainly from Cunani Granite and also from Carnot Granodiorite and Cricou suite.
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