The study of Lago Novo, considered the largest and of great economic potential for the state of Amapá, arose from the need for scientific information about the region, especially in the lacustrine system. For this purpose, this sedimentological and geochemical research was carried out. This lake is located in the innermost portion of the Coastal Plain of the State of Amapá, in the North Cape region. It is located in sediments of the Barreiras Group and recent sediments, 20 km away from crystalline rocks of the Guiana Shield and having an approximate area of 104 km². The waters of this lake show a very significant seasonal variation both in temperature (29.9 °C in the rainy season and 34.1 °C in the dry season) and in pH (5.65 in the rainy season and 7.95 in the dry season). dry season). The low electrical conductivity values, lower than 40 µs/cm, rule out a possible influence of the Araguari River estuary, while the analyzes of total dissolved solids (TSD) and suspended sediments indicate a small amount of ions and suspended sediments in the waters. of the lake. The lake bottom sediments are distributed in 5 granulometric classes: sand, silty sand, sandy silt, silt and clayey silt. The study of the faciological distribution of these sediments showed that the sediments of the sandy-silty and sandy-silt facies occupy about 47% of the lake area; silty facies, 44%; those from the sandy facies, 6% and those from the silt-clayey facies, 3% of the lake area. Quantitative analyzes defined the occurrence of 4 groups of clay minerals: kaolinite, smectite, illite and chlorite. Semi-quantitative analyzes allowed the establishment of mean values for the abundance of clay minerals in the lake sediments: kaolinite/chlorite with 72.6%; smectite 14% and illite 13.4%. According to the clay-mineral ratio found, 4 zones with distinct paragenesis occur: I) kaolinite/smectite ratio of 1.3/1 and kaolinite/illite ratio of 5/1; II) kaolinite/smectite ratio of 10/1 and kaolinite/illite ratio of 5/1; III) kaolinite/smectite ratio of 3.3/1 and kaolinite/illite ratio of 6/1; IV) kaolinite/smectite ratio of 26.4/1 and kaolinite/illite ratio of 8.2/1. The results of this research indicate that the elements that make up the Lago Novo system are products of the action of weathering agents acting on adjacent rocks and sediments, under the direct influence of the drainage network, geomorphology and seasonal climate changes.
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