Abstract
The Cananeia-Iguape system is a combined estuarine-lagoonal sedimentary system, located along the SE coast of Brazil. It consists of a network of channels and islands oriented mainly parallel to the coast. About 165 years ago, an artificial channel, the Valo Grande, was opened in the northern part of this system to connect a major river of the region, the Ribeira River, to the estuarine-lagoon complex. The Valo Grande was closed with a dam and re-opened twice between 1978 and 1995, when it was finally left open. These openings and closures of the Valo Grande had a significant influence on the Cananeia-Iguape system. In this study we present mineralogical, chemical, palaeomagnetic, and geochronological data from a sediment core collected at the southern end of the 50-km long lagoonal system showing how the phases of the opening and closure of the channel through time are expressed in the sedimentary record. Despite the homogeneity of the grain size and magnetic properties throughout the core, significant variations in the mineralogical composition showed the influence of the opening of the channel on the sediment supply. Less mature sediment, with lower quartz and halite and higher kaolinite, brucite, and franklinite, corresponded to periods when the Valo Grande was open. On the other hand, higher abundance of quartz and halite, as well as the disappearance of other detrital minerals, corresponded with periods of absence or closure of the channel, indicating a more sea-influenced depositional setting. This work represented an example of anthropogenic influence in a lagoonal-estuarine sedimentary system, which is a common context along the coast of Brazil.
Highlights
The Cananéia–Iguape estuarine-lagoonal system (Figure 1) is located in a large portion along the southern coast of the São Paulo State (Brazil), at latitude between 24◦50′ and 25◦40′ South and longitude between 47◦20′ and 48◦20′ West
Mesozoic intrusive alkaline rocks crosscut this basement complex and outcrop in various locations (Riccomini, 1995; de Souza et al, 1996). One of these outcrops forms the hill of São João, in the municipality of Cananéia, which is the part of land closest to our core location
A few positive peaks in natural remanent magnetization (NRM) and anhysteretic remanent magnetization (ARM) can be interpreted as an increase in the relative abundance of magnetic minerals, while negative peaks can be interpreted as a decrease
Summary
The Cananéia–Iguape estuarine-lagoonal system (Figure 1) is located in a large portion along the southern coast of the São Paulo State (Brazil), at latitude between 24◦50′ and 25◦40′ South and longitude between 47◦20′ and 48◦20′ West This region is characterized by a group of four islands, separated by tide channels, coastal lagoons, little estuaries, and the Ribeira de Iguape River that. One of these outcrops forms the hill of São João ( called the Cananéia Alkaline Massif), in the municipality of Cananéia, which is the part of land closest to our core location This hill is 137 m height and is surrounded by a coastal plain, filled with Pleistocene marine deposits (Cananéia Formation) and Holocene sediments of beach and mangrove swamp environments (see Figure 2) (Riccomini, 1995; Suguio et al, 2003)
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