Palinofácies de folhelhos do devoniano superior da borda oeste da bacia do parnaíba palynofacies of upper devonian shales from the west edge of the parnaíba basin

Abstract

An outcrop section of rocks from the Upper Devonian, providing plant and animal microfossils that register a time of global post-anoxia and major marine flooding events, was investigated by applying palynofacies techniques and organic geochemistry. The primary purpose of this work is the palynofacies characterization of a Neo-Devonian section of the Parnaíba Basin, belonging to the Pimenteiras Formation. Thirteen outcrop samples were collected in the western edge of the Parnaíba Basin, Tocantins State - Brazil, where a possible contact between the Pimenteiras and Cabeças formations was identified. Through optical/visual analysis of organic content and organic groups (amorphous organic matter - AOM/Palynomorphs/Phytoclasts), as well as their relative abundance, associated with previous total organic carbon (TOC) data, it was possible to individualize four (4) distinct palynofacies throughout the section. The variation in the proportions of marine and continental palynomorphs suggests sea-level fluctuation during deposition, characterizing a transitional environment, varying from the most distal to the most proximal deposition platform. Based on the predominance of palynomorphs and phytoclasts, the kerogen types II and III were recognized, corroborating with the hydrogen and oxygen indexes obtained through pyrolysis data. The predominance of yellow to chestnut colors exhibited by the prasinophyte and acritarch components under ultraviolet light excitation indicates good organic matter preservation and thermal immaturity following the organic geochemistry data.