Facies analysis, petrography, and palynology of the Pirara Formation (Upper Jurassic-Lower Cretaceous) - Tacutu Basin (Roraima, Brazil)

Abstract

The Tacutu Basin formed from the rifts system that occurred in the Caribbean region during the Mesozoic and culminated with in the opening of the North Atlantic Ocean. It evolved as a half-graben inserted in the Guiana Shield north of the Amazonian Craton. The Pirara Formation corresponds to the rift phase of the evolution of this basin, however the studies characterizing this unit are very scarce and little accurate due to the absence of outcrops. Therefore, there is much debate about the origin and depositional environment of the Pirara Formation, as some authors argue that the basin was episodically connected to the sea during the rifting, whereas others claim that was a continental-lacustrine environment, under arid to semi-arid climatic conditions. This work was based on investigations carried out on recently drilled rock cores of the Pirara Formation, which provides substantial new information regarding lithological and sedimentological data. The main objective was to characterize the lithofacies petrography palynofacies and constrains the paleoenvironmental interpretation of this unit. Three main lithofacies were identified: marlstone, siltstone, and black shale, with the latter yielding TOC content between 5% and 13.7% and organic matter consisting of phytoclasts, palynomorphs, and amorphous particles. Highly hypersaline conditions are indicated by the presence of pseudomorphs of evaporitic minerals, occurring in the marlstone and siltstone facies. Conversely no marine components take place in the palynological assemblages. The results demonstrate that the Pirara Formation represents a playa lake environment that evolved in the Southern Caribbean region in the Late Jurassic-Early Cretaceous during the opening of the Central Atlantic Ocean.