Bentonites of the Irati Formation in the southern sector of the Paraná Basin

Abstract

This paper aims to identify and to present mineralogical and chemical arguments that demonstrate the existence of bentonite levels in the Irati Formation, found in outcrops to the west Aceguá, in southern Rio Grande do Sul (Brazil). These levels are thin, on average 4 cm thick, large in area, and are composed of grayish-white to greenish massive claystones that contrast, in the field, with the shales of the Irati Formation. The bentonite levels of the Irati Formation are predominantly composed of Ca-montmorillonite, which constitutes the fine matrix of the rock; and scattered primary or magmatic crystals not larger than very fine sand. Among the main primary minerals representative of volcanic setting, it is possible to identify β-quartz paramorphs, sanidine, biotite, zircon, apatite and ilmenite, in addition to quartz and feldspar shards (splinters). More rarely, fragments of meso and macrocharcoals are found within the bentonite layers, which contrast with the maturity and type of non-vegetal organic matter of the Irati shale. Therefore, the nature of the precursor volcanism is inferred on the basis of rock geochemistry and crystal chemistry of the neoformed montmorillonite in the bentonite levels. Both methodologies indicate that during this period the volcanic ashes that reached the Paraná Basin were generated by volcanism of intermediate composition, which is in accordance with what is known about the Lower Choiyoi Volcanic Province manifestations, which were synchronous with the sedimentation of the Irati Formation in the Paraná Basin.