Análise palaeoambiental e caracterização dos Folhelhos Negros da Formação Barreirinha utilizando análises Multiproxy

Abstract

The initial sedimentation phase of the Barreirinha Formation was associated with a rapid relative sea-level rise during a significant marine transgression event that flooded the Amazon Basin. These organic-rich shales outcrop along a narrow yet extensive belt located on the southern margin of the Amazon Basin. Few studies have specifically addressed the potential paleoenvironmental variations linked to the deposition of these shales. This is mainly due to the relative lithological uniformity of these rocks—composed predominantly of fine-grained sediments—and their economic relevance, which has directed most research toward the maturation of organic matter. To investigate the paleoenvironmental variations during the deposition of these fine sediments—focusing on sedimentary dynamics, the origin, and provenance of the organic matter—a multiproxy approach was applied, combining various quantitative, semi-quantitative, and qualitative techniques. The analyzed stratigraphic succession is mainly composed of gray to black shales, exhibiting facies variations related to coarse terrigenous input and episodes of bioturbation. These features suggest a deep, distal, anoxic marine depositional environment, with no evidence of carbonate sedimentation, typical of the Abacaxis Member of the Barreirinha Formation. Mineralogical cluster analyses indicate a dominance of kaolinite, characterizing the Kaolinite Facies, with subordinate quartz, sulfates, and sulfides in the lower portions. The base of the succession includes massive fine-grained sandstones with cross-bedding, correlated with the Ereré Formation, interpreted as deltaic to inner shelf deposits. The transition to laminated shales interbedded with sandstones and siltstones marks the onset of the Devonian (Frasnian) transgression, with substantial continental input evidenced by heavy minerals, pyritized plant remains, and tasmanites. The presence of dumpstones suggests glacial influence and ice-rafted debris deposition. Upper levels show more homogeneous shales, enriched in organic matter, lacking bioturbation and detrital minerals, indicating maximum anoxia during the peak of the transgressive event in the Amazon Basin. Diagenetically, the shales underwent compaction, fracturing, mineral substitution, oxidation, and intense pyritization, mainly as framboidal pyrite—typical of reducing marine environments. The mineralogy is dominated by kaolinite and quartz, with accessory minerals indicating alteration processes and possible Jurassic-Triassic igneous intrusions (Penatecaua magmatism), which contributed to increasing the thermal maturity of the kerogen. Rock-Eval pyrolysis and biomarker analyses reveal Type II-III kerogen with gas-generating potential, ranging from immature to post-mature depending on proximity to igneous intrusions. These findings reflect a transgressive system strongly influenced by environmental controls and regional thermal input.