Spectroscopy of Permo-Carboniferous Siltstones and Shales

This paper shows the application of reflectance spectroscopy in the characterization of siltstones and organic shales in different lithostratigraphic units (e.g. formation, group) Naked eye observation and characterization can be difficult when describing and comparing these rocks in distinct units, since sedimentary rocks of fine granulometry presents very similar physical properties. Other factors could also aggravate its identification in field, especially in tropical regions that present a more humid climate, such as the degree of weathering of the outcrop or the contact between two formations. The spectral analysis of these lithologies is not yet widely studies, since the presence of organic matter makes the reflectance patterns in the spectral curve much lower and with no characterizing absorption features. Thus, a methodology that facilitates the recognition and classification of fine granulometry rocks represents progress in the field of sedimentary petrology, which currently makes use of microscopic, geochemical and geophysical techniques. The reflectance spectroscopy technique provides a quantitative measure of the spectral reflectance of materials that is obtained by the ratio between radiance and irradiance. By using the spectral signature, it is possible to identify the main characteristics of absorption features of the minerals found and thus recognize constituent elements and/or classify such rocks. The objectives of this research are: 1) to search for patterns that characterize each sample/ lithostatigraphic unit using its spectral behavior, also identifying the main absorption feature of the fine granulometry rocks that present organic matter in their composition; 2) to test the possibility of distinguishing and grouping together each formation by its spectral behaviour using a classifier. To do so, 45 samples of lithostratigraphic units were selected, representing part of the permo-carboniferious succession of the Parana Basin- Brazil, such as Rio do Sul, Palermo and Irati Formations. All samples were ground to a size smaller than the silt fraction (0.062mm) and the ground sample measurements were made with a SPECTRAL EVOLUTION field spectroradiometer, type SR-3500, which records the spectral pattern in the wavelength interval from 350 to 2500 nm. Initial results show that samples of each formation present distinct spectral responses, even when they present very similar physical characteristics. Spectral data obtained through Principal Component Analysis (PCA) show that the samples are classified into three distinct groups. This technique has shown to be effective for lithological differentiation, as the spectral reflectance is directly related to the chemical properties of the rocks, which are distinct for each formation and too subtle to be perceived by naked eye analysis.

Authors: Lais V. Souza, Marcelo Kehl de Souza, Mauricio Roberto Veronez, Francisco Tognoli, Luiz Paulo Luna de Oliveira & Luiz Gonzaga Jr.

Keywords: Reflectance spectroscopy, PCA, Parana Basin, sedimentary rocks, organic matter

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