SPECTRAL REFLECTANCE-LITHOSTRATIGRAPHY OF TERRIGENOUS ROCKS OF THE PARNAIBA BASIN, BRAZIL: CHEMICAL COMPOSITION RELATIONSHIPS

Abstract

Functional associations of reflectance spectra with sedimentary rock constituents were analyzed for seven stratigraphic units from the Paleozoic Parnaiba Basin, northeastern Brazil. Twenty six core samples were selected for chemical analysis, thin section description and X-ray diffraction. Bidirectional Reflectance Factors (BRF) were obtained in the laboratory with illumination-viewing directions near nadir, in the visible and reflectedinfrared intervals (400- to 2,500-nm). The dependence of the BRF on chemical composition was evaluated for the following constituents: SiO2) A12 O3, total iron (expressed as Fe2 O3), K2 O, MnO, CaO, MgO, Na2 O, P2 O5, and TiO2. Additionally, we considered the average grain size of the sandstones, the total organic carbon content (TOC), and the SiO2 /Al2 O3 , SiO2 /Fe2 O3  and Fe2 O3 /Al2 O3  ratios. The best negative correlations of reflectance were obtained with A12 O3, Fe2 O3, organic matter and Fe26yAl2 O3 ratio, and positive correlations with SiO2, SiO2/Fe2 O3 and SiO2/ A12 O3  ratios. Even though major oxide constituents, such as SiO2 , A12 O3  and K2 O, do not produce absorptions in the range of 400- to 2,500-nm as iron ions and carbon do, the scrutiny of the scatter diagrams provided some clues to the role played by these interdependent variables in surface and very near surface light scattering and absorption. An increase in SiO2  is often associated with higher reflectance, while the reverse is true for A12 O3  and K2 O. This study indicates the need of empirical modeling for a better comprehension of the radiation transfer process.