Controls on Ash-Fall Deposit Preservation in Low-Energy Depositional Systems of the Rio Bonito Formation, Paraná Basin, Brazil

The Paraná basin is composed of stratigraphic units that record distinct paleoenvironmental settings, organized into six supersequences. The Gondwana I Supersequence  (Permian) records a transgressive-regressive cycle associated with tectonic and climatic changes, in which periglacial successions (Itararé Group), coastal and marine (Guatá Group), and continental deposits (Passa Dois Group) are preserved. These sedimentary units crop out along the eastern margin of the Paraná Basin in a complex structural configuration that reveals significant tectonic displacements attributed to normal faults, resulting in the lateral juxtaposition of stratigraphically distinct units. Due to this arrangement, volcanogenic deposits play a fundamental role as stratigraphic markers, as they allow the establishment of precise geochronological correlations. This study presents geochronological data obtained from a volcanogenic deposit at Morro dos Conventos outcrop, and compares it with a compilation of the ages of volcaniclastic sediments interbedded with sedimentary deposits from the Rio Bonito Formation, aiming to constrain the evolution of depositional systems that enabled the preservation of such volcanogenic deposits within this interval. The detailed stratigraphic section of the outcrop was conducted and samples were collected for geochronological analysis. U-Pb zircon ages were determined by LA-MC-ICP-MS from a volcanogenic layer. The results reveal a unimodal zircon population with a concordia age of 286 ± 1.4 Ma (N = 9; MSWD = 1.2), allowing correlation of the deposit with the Artinskian Stage. Sedimentological and stratigraphic analysis of the section indicates a paleoenvironment of storm wave-dominated shelf, with interbedded subsystems recording high-frequency cycles associated with changes in sea level or sedimentation rates within the second-order Permian transgressive sequence. Sedimentological and geochronological data suggest that the studied succession correlates with the upper portion of the Rio Bonito Formation, in a context of progressive drowning by the Palermo Sea. At the top of the section, a progradation of subsystems is observed, characterized by the arrangement of subaerial sequences under humid backshore conditions. A similar configuration has been documented in other areas of the basin during the Cisuralian, where pelitic successions associated with coal deposits preserve centimeter-thick intercalated volcanic ash layers. The preservation of these features is attributed to paleoenvironmental conditions of subsystems developed along the margins of subaqueous bodies, dominated by low-energy settings with limited reworking, favoring the deposition of fine-grained sediments. In the studied outcrop, the preservation of the volcanogenic deposit is interpreted as a result of deposition within fine-grained sediments characterized by redoximorphic structures, indicative of fluctuating conditions between dry and wet periods typical of subaerial environments influenced by aqueous systems. A similar preservation context is observed in volcanogenic deposits recorded both in CPRM wells (Brazilian Geological Survey) and in nearby outcrops of this stratigraphic interval. The coexistence of low-energy depositional systems and episodes of high-magnitude explosive volcanism along the western margin of Gondwana enabled the preservation of ash-fall deposits in the Paraná Basin stratigraphic record, commonly associated with the Choiyoi Magmatism during the proposed Rio Bonito Formation sedimentation interval.