Paleoenvironmental, paleoclimatic and stratigraphic implications of the mineralogical content of the Irati Formation, Paraná Basin, Brazil

Abstract

The Irati Formation documents a singular moment in the evolution of the Paraná Basin, when an effective restriction of water circulation between the syneclise and the Panthalassa Ocean finally established a favorable environment for the deposition of oil shales. The present study presents data from X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) for the determination of different mineralogical components in the entire interval of the Irati Formation. Seeking to correlate the varying mineralogical content of the formation with the paleoenvironmental, paleoclimatic and stratigraphic conditions, total organic carbon (TOC), total sulfur (S) and insoluble residue (IR) data were also utilized. The results showed that variations in mineralogical content occurred mainly in the interval where the greatest values of TOC, S and IR are found. The authigenesis of minerals such as analcime, pyrite, gypsite and barite are closely tied to the environment where they were formed, and these minerals bear within them characteristics of paleoenvironmental conditions, such as restricted water circulation, abundant organic matter, alkalinity and pelagic sedimentation. Variations in the mineralogical content of clay minerals point to an enrichment of illite in the upper interval of the Assistência Member, thus constituting a syndepositional process. The mineralogical data indicate a predominance of illite, chlorite and kaolinite in the Taquaral Member. However, clay mineral suite change in the different depositional domains of the Assistência Member: the internal platform has clay mineral suite dominated primarily by illite and saponite. Kaolinite increase in the intermediary platform and disappear in distal platform. The kaolinite increase probably is the result of intensified weathering due to increase temperature and precipitation. Kaolinite characterizes the transgressive systems tracts under warm and humid paleoclimatic conditions. In contrast, illite and saponite indicate warm and dry conditions in the highstand systems tracts.