Geological mapping of the Dantas Basin and the Role of Geology in the Study of the occurrence of Landslides
ISBN 978-85-88783-11-9
Authors
1Barros, L.; 2Vinagre, R.; 3Avelar, A.
1 Email: leandrorlbarros@gmail.com
2 Email: rodrigogeologo@yahoo.com.br
3 Email: andreavelar@acd.ufrj.br
Abstract
The region of the Dantas Basin, Nova Friburgo, Rio de Janeiro, Brazil, was affected by catastrophic events related to rainfall occurred in January 2011. Aiming to improve the understanding these events and why this concentration in this region, a study in detail the geology was done, since the existing studies in the literature have been performed on a large scale, not contributing enough to the solution of this question information. For this, the rocks of the region were mapped in detail scale and at least seven lithologic units were recognized. For a better understanding the structural architecture of the fractures in the area, three outcrops were chosen. Was applied the method of Davis (1984), which aims to quantify the fracture, so that it is possible to calculate the density of these same outcrops. Thus was observed that the sheet joint dip 20° to the NW when the rock has no tectonic foliation. However, when there are foliation the fractures ranges from 20° to 60°. Was noted that the longitudinal axis of the Dantas Basin is parallel to the strike of the tectonic fractures dipping to SE, coinciding with the direction of the three basic dikes that occur in the area. When comparing the longitudinal axes of the landslides was found that these obey the approximate direction of tectonic fractures, NW-SE and NE-SW directions. So there might be some kind of control of drainage networks made by the structures. By overlaying the landslides with the geological map, it is clear that what influenced landslides was the presence of the Orthogneiss granitic granodioric rock. Besides occupying most of the mapped area, its granitic composition produces a rich sandy soil in quartz with little clay matrix. However, other rocks of the mapped area also produce soils with similar characteristics, so there is still the question of why this lithological unit was the most influencing landslides. One explanation for this may be related to structural features, since it is one of the most deformed rocks. The occurrence of penetrative and continuous surfaces, such as foliation and fractures work for a large percolating water, thus accelerating the weathering and erosion. This hypothesis agrees with the data obtained regarding the density and penetrative of the fracturing occurring in that unit, since the other rocks with similar compositions are younger, little or nothing deformed and had less fractures, these being less penetrative. Another aspect that deserves attention is related to the equigranular granite fine. This unit unlike the Orthogneiss granitic granodioritic does not have many penetrative structures, has no tectonic foliation and the fracture rate is apparently lower than the Orthogneiss. Furthermore, it is one of younger rocks area. What could have influenced the grater production of soils and blocks, in such accelerated rate? One possible explanation could be in their mineralogical composition. Although being a granite rock, which should be quite similar to orthogneiss, the petrography showed a large change of the primary crystals (magmatic) of feldspars to secondary carbonates, which probably contributed to the landslide occurred at that point. The same, but in a less intense level, occurs with the Medium Porphyritic Granite rocks. This may partly explain why this unit has had a high incidence of landslides, as well as the orthogneiss.
Keywords
Dantas Basin; Tectonic foliation; Sheet joint; Method of Davis