Reconnaissance geologic mapping of a Heavily-Forested shield area (Guiana Shield, Northwestern Brazil) / Fernando Pellon de Miranda.

Tese (Ph.D.)- University of Nevada, Reno, 1990

The part of the Guiana Shield situated in northwestern Brazil is one of the least geologically known regions of the country. It is characterized by a complex association of gneissic and granitoid rocks of middle Proterozoic age (Guianense Complex), which are not individually recognized as distinct units in the geologic map published by the RADAMBRASIL Project at 1:1,000,000 scale. The area is completely covered by rainforest. Lithologic contacts and geologic structures have no obvious geomorphic expression. In 1984, the Shuttle Imaging Radar-B (SIR-B) acquired digital data over the Guiana Shield. This L-band, HH polarization synthetic aperture radar system operated at a fixed incidence angle of 35.7 degrees on flat terrain. The semivariogram textural classifier (STC) was employed to discriminate and map flooded vegetation and water, which were considered the key cover types for the recognition of subtle topographic relief (up to 8.0 meters) in the study area. Linearly arranged floodplains were interpreted as linear geomorphic features. In addition, tonal linear features were identified on the SIR-B image using a contrast stretching technique (trackball linear mapping; TLM). A final map was developed including both STC-mapped and TLM-enhanced linear features, in order to display landscape patterns indicative of subtle structural discontinuities in the crystalline terranes of the Goiana Shield. The digital aeromagnetic data used in this research were obtained in 1987. Measurements were made using a proton-precession magnetometer with sensitivity of 1.0 nanoTesla (nT). Flight-lines were flown north-south at a 2.0-km spacing; tie-lines were flown east-west at a 20.0-km spacing. The survey was performed in a draped mode 150m above relatively flat terrain. The following products were derived as a result of digital data manipulation: (1) a grey-scale image of the reduced-to-pole magnetic anomalies, in order to highlight zones of distinct magnetic signature; (2) a terrace-magnetization map that delineates induced magnetization boundaries and theoretically outlines geologic structures and varying magnetic terranes; (3) a map showing the location of maximum values of the horizontal component of the pseudogravity gradient (boundary lines) that represent abrupt lateral changes in magnetization that are interpreted as steep structural or lithologic contacts. Comparison of the magnetic and remote sensing results with the geologic data acquired in the field by the RADAMBRASIL Project allowed broad terrane categories in the study site to be delineated for the first time: (1) areas characterized by intermediate brightness levels on the grey-scale magnetic image and by low to intermediate magnetization values on the terrace map (-0.00018 emu.cm-³) correspond to migmatites and metamorphic rocks devoid of muscovite with Rb-Sr radiometric age of 1640 ± 26 m.y. and mantle derivation (I.R. = 0.703 ± 0.001); (2) areas characterized by very bright signatures on the grey-scale magnetic image and by high terrace-magnetization values (greater than 0.00090 emu.cm-³) correspond to granitoids devoid of muscovite of unknown age cutting the mantle-derived migmatites and metamorphic rocks. Geologic faults with both topographic and magnetic expression of SIR-B linear features, terrace map, and magnetization boundary lines.