High resolution gamma-ray spectrometry logging applied in the interpretation of sedimentary processes and environments of the Itararé Group (Permo-Carboniferous), Paraná Basin, Santa Catarina state

Abstract

Outcrop gamma-ray spectrometric logging in the upper strata of the Campo Mourão Formation and lower strata of the Taciba Formation were used to investigate: (i) the influence of lithological variation on gamma-ray patterns; (ii) its potential to discriminate and identify lithotypes; and (iii) sedimentary processes and related environments of these Permo-Carboniferous strata. The gamma-ray logs were subdivided into three gamma-ray units, according to the total count, concentrations and contents of K, eU and eTh. These gamma-ray units were associated to three depositional units: Glaciolacustrine system with thin-bedded turbidites in the Campo Mourão Formation; and two submarine fans systems (channels, overbanks and lobes) with thick-bedded turbidites systems in the Taciba Formation. Gamma-ray spectrometry allowed distinguishing 'common' shales from black shales, which are often similar in the faciological description in fieldwork. Likewise, carbonate levels were identified as a function of the decrease in gamma-ray concentrations in black shales of the Campo Mourão Formation. The relationship between the radiometric data and the grain size indicated that, in the field, the lithological heterogeneity has a great influence on the ability of the gamma-ray spectrometer to distinguish lithotypes. The eTh/K and eTh/eU ratios were used to infer sedimentary processes and related environments. Through the eTh/eU ratio, a reducing environment, responsible for the precipitation of authigenic uranium in low sedimentation rate deposicional system, was evidenced, discriminating depositional units from different environments.