Integrated stratigra phic and geochemical study of the Santa Maria and Caturrita formations (Triassic of the Paranل Basin), southern Brazil

Several analytical techniques were applied in the study of the Alemoa Member (Santa Maria formation) and Caturrita formation to make an assessment of changing paleoenvironmental conditions through the Triassic succession of southern Brazil. This included a integration of petrography, X-ray diffractometry, scanning electron microscopy and chemical analysis of both major and minor elements, as well as uranium (U) and thorium (Th). The studied succession comprises part of the Triassic continental deposits of the Paraná Basin in South America and represents channel deposits surrounded by fine-grained flood plain facies. This stratigraphic interval is interpreted as an anastomosing fluvial system (Alemoa Member) that has gradually evolved into a braided system (Caturrita formation). The chemical index of alteration (CIA), which is determined through the rock chemical composition, was used to highlight weathering processes on primary minerals. The CIA follows an upward trend representing an increase on weathering conditions from the Alemoa Member to the Caturrita formation. The deep chemical alteration in the Caturrita formation suggests weathering under a humid, possibly tropical climate. These results are in agreement with U and Th analysis. The Th/U ratios rise toward the top of the section (Caturrita formation) suggesting an increasingly more humid climate. Thus, the gradual upward shift from an anastomosing to a braided fluvial style is ascribed to a climate change, and related increase in palaeorainfall. The existence of interstratified illite–smectite clay minerals, mud cracks and caliche concretions within the flood plain deposits suggests deposition under a hot climate with alternating wet and dry seasons. Therefore, the results of this work show that the integration of facies studies with mineralogical and geochemical analysis enhances our ability to perform detailed paleoenvironmental studies on continental successions.