Os Solos do núcleo colonial do Tinguá

Abstract

The present article deals with the "Nucleo Colonial do Tinguá", in Distrito de Cava, Nova Iguaçu.

            The Nucleo is situated on the slopes of Iguaçu River with Utum and Piaba rivers drained in its direction and other small streams. 80% of the Núcleo total area is mountainous; the lowlands are found among the hills and are generally narrow and badly drained.

            The climate of this macro-region by the classification of Koppen is of the type Am, with the following characteristics.

            1) the average temperature of the coldest month is above 18° (July 18.5•C);

            2) the rainfall of the driest month is inferior to 60 mm (July 55.3);

            The climate Am is well accentuated what concerns the vegetal coverture; this is very exuberant; the vegetal aspect of the climate Aw the savanna is not found there.

            About the temperature we observed averages as 24.8, 24.6 and 23 C in the months of January, February and March, respectively. The averages of the coldest months (Jun, July and August) reach 19.1, 18.5 and 19.3°C, being the averages of the minimal 14.7, 13.9 and 14.3•C and of the maximal 31.2, 31.3 and 30.0•c.

            The annual rainfall in Tinguá is of 2 406.8 mm distributed as it follows = January 253.1; February 297.1 = March 311.7; April 183.7; May 109.5; Jun 63.6; July 55.3; August 72.7; September 134.2; October 223.4; November 251.7; December 350.7 mm.

            The geology in the area of the Núcleo Colonial do Tinguá is complex but very interesting. The "Tabuleiro Zone" situated at the eastern part of the Núcleo presents a mass of Archean rocks where we can distinguish laminate gneiss with abundant biotite sometimes chlorosised. Foiaitic rocks are also found on the headquarters of the Núcleo; at the western side (the range properly spoken) there are alkaline rocks which cross the fundamental gneiss mass of the crystalline complex, in different levels, forming a water-fall of 80 m, 580 m approximately of altitude over the level of the ocean.

            The gneiss of Tinguá is generally rich in biotite with potassium feldspar, acid plagiocrasio and accessory elements - veins of quartz, of hydrothermal origin, cut these rocks of the fundamental complex.

            In the lowlands we can see sediments from the Quaternary, of alluvial origin, sandy or

clayey ones, which form alluvial soils; they are among the expositions of the Archean and

the Jurassic.

            The soils are diversified by the phytogeological conditions and on account of topography also. In order to study them well it is necessary to consider the relief and the geological areas for a great part in from residual origin by the decomposition of the rocks in situ with greater quantity of colluvium or not, regarding the inclination of the ground.

            The soil becomes reddish on account of the oxidation of ferruginous elements of the rocks on the slopes of the hills where the gneiss appears or the soil can be roseate or also yellowish and whitish where the alkaline rocks appear.

            Thanks to the presence of the granules of quartz the soils of the crystalline complex are very permeable and physically much better than those originated from alkaline rocks. The anuvial soils from the lowlands besictes the bad drainage they present show also sandy and clayey sections; these last ones are very compact.