Geomorfologia de uma área com alta sensibilidade ambiental na Bacia Potiguar (NE do Brasil)

Abstract

The study area is located on northern coast of Rio Grande do Norte state, NE-Brazil, inserted on the geological context of Potiguar Basin. This sedimentary basin is filled with rocks and sediments which ages range from Cretaceous to Quaternary. According to previous studies, the environmental sensitivity on this coastal zone is very high due to the industrial occupation of the natural environment. These industrial activities consist in hydrocarbon’s exploration, salt industries and shrimp farms. The constant activities of coastal hydrodynamic processes causing erosion on shoreline are also a strong contribution to classify such area as a very high environmental sensitivity coast. Such situation demands a constant environmental monitoring which can create a database of useful information for a better development of integrated coastal zone management. One of the most important knowledge to be achieved is the morphology of surface. This paper aims to apply a methodology of digital image processing to extract geological and geomorphological information about the study area. Optical images from satellites Landsat and CBERS were georreferenced using control points acquired on field with DGPS, using UTM-Zone 24 South coordinate system. The statistics were calculated and the images were processed using techniques like bands ratios and principal components analysis to create RGB color images. For the LANDSAT image the most efficient RGB compositions were 541, 531, 751 and the RGB composition between the ratios 7/3 – 5/3 and 4/3. CBERS color images used consist in RGB 431, 421, 432 and RGB with the principal components PC1-PC4-PC3. The selected RGB images were integrated with radar data from SRTM (Shuttle Radar Topographic Mission) and Radarsat, to create 2D and 3D hybrid images. The gain that this methodology provides in comparison to the individual analysis of each kind of image (optical and radar) consist on the joining of textural aspect of radar and the spectral information of optical images. Such hybrid images enhance the relief, aiding the interpretation of morphological features on surface. Geologic and geomorphologic maps were developed from the interpretation of hybrid images together with field data and information from previous studies. Litostratigraphy involves Jandaíra Formation (carbonaceous rocks), Tibau Formation (fine to conglomeratic calciferous sandstones), Macau Formation (basalts and gabbros), Barreiras Formation (conglomerates and coarse sandstones) and quaternary deposits. Geomorphological units are: tabular relief zones composed mainly by Barreiras Formation, described at Brazilian technical literature as “Tabuleiros Costeiros”, fluvial-marine flood plains, mangrove, fixed and mobile dunes and shoreline. Coastal tabular zones are composed by carbonatic, silissiclastics and magmatic rocks, with altimetry ranging from 7 to 119 m. Dunes are composed by sand and sandy soil (fixed ones), their altimetry reaches until 4 m. The fluvial-marine flood plain is under influence of tides and river floods, being composed by fine sandy and siltic sediments together with clay and organic matter and occur between 0 and 6 m above sea level. SRTM data was also used individually to obtain information about altimetry in meters, creating 2D digital elevation model and topographic profile. The altimetry ranges from 0 to 230 m, such values correspond to the elevation above relative sea level. The central part of the area consists on the AssuAssu river valley with elevation below 15 meters, placed between higher zones to west and to east. The western portion of the area correspond to Serra do Mel region, with elevation up to 30 meters and on the eastern region there’s a domic high known as Mangue Seco Dome, with the highest values of elevation (230 meters). Slope information was also extracted from SRTM, separating areas with very low slope (0 - 1°), low slope (1 - 1,5°), medium slope (1,5 - 2°), high slope (2 - 3,5°) and very high slope (3,5 - 7,7°). The area is generally very flat, but the highest slope zones consist in limits between features with higher elevation and lower zones. Aspect map was also created from SRTM data, this map displays the dip direction of surfaces, indicating the behavior of surface flow. Directional filters were applied on SRTM data and CBERS image aiming to enhance topographic lineaments on surface. Such lineaments were mapped and their orientation is parallel to regional fault systems of Afonso Bezerra (NW trend) and Carnaubais (NE trend), confirming the influence of such geological fault systems on coastal landscape which were already known from previous studies as being reactivated during the Holocene, so controlling coastal morphology and sedimentation. Comparing lineaments mapped from SRTM with those mapped from CBERS’s images is possible to note a decrease in lengths, however in both cases most lineaments are smaller than 5 km and some few are bigger than 10 km. Some outcrops have evidence of brittle deformation like faults, joints and silex formation, such structures are parallel to lineaments. This methodology created a cartographic database which can be used as some of the parameters to be considered during the development of coastal zone management plans. Such knowledge around morphology is important to the monitoring of industrial activities already installed on the coast, for the planning of new human occupation, and for a better development of emergency plans in cases of oil spills. Since, surface morphology influences directly on the interaction between coastal processes and human occupation. The geomorphology on the study area is deeply related to the nature of the geological units, tectonic factors and coastal processes. Forthcoming studies should apply the methodology used on this paper in detailed scales, using high resolution images and high resolution digital elevation models to provide the level of accuracy demanded by a so complex and sensitive region