Análise petrográfica de arenitos e seus minerais pesados na formação barbalha em Nova Olinda - CE

Abstract

The Barbalha Formation is the first lithostratigraphic unit from Aptian-Albian stage, Lower Cretaceous, Araripe Basin. Mainly formed by sandstones intercalated with mudstones and shales, and occurrences of fine-layered conglomerates, Barbalha Fm. underlies the Crato Formation which is recognized by fossil-rich laminated limestones intercalated with sandstones and shales. This study aims to answer pending questions about these units e.g. correlation, provenance, reworking and diagenetic degree of sandstones through petrography and heavy mineral analyses. The petrography of samples from the top of Barbalha Fm. showed great resemblance with those from the bottom of the Crato Fm. with both sandstones being classified as quartz-wackes. However, the mineralogical composition of matrix showed divergence between those units where the Barbalha Fm. has a calcite composition whereas the Crato Fm. has a phyllosilicate composition. It suggests that pulses of fluvial discharge towards a lacustrine basin have interacted more conspicuously with the bottom and with alkaline waters in the Barbalha Fm sandstones. Through identification of heavy minerais from both units it was possible to notice that mineralogical assembly from Barbalha Fm. has larger amounts of altered minerais (alterite), minerais with unclear optical properties and obliterated grains, indicating a higher alteration degree in this unit. Establishing a garnet-zircon provenance índex ratio, the Crato Fm. samples showed higher values, indicating a larger degree of metamorphic contribution. The Barbalha Fm. showed more divergent values which suggest both a change in the provenance between and within the unit. Statistical analyses of the heavy minerais corroborate the lower affmity found between these units either by primary provenance or secondary weathering, suggesting that the studied sandstones had different drainage and depositional systems.