Facies analysis and paleoenvironment of the Poti Formation in the Itaueira and Floriano region: Contribution to the Carboniferous depositional history of the Parnaíba basin

Abstract

The Parnaíba intracratonic basin exhibits sedimentation that reflects the different evolutionary stages of climate and paleogeography in the West Gondwana supercontinent. In this regard, we conducted a study comprising the Poti Formation in the southeast of Itaueira and Floriano regions in Piauí State, within the Parnaíba Basin. The facies analysis enabled the establishment of the depositional model for the study area, comprising ten sedimentary facies separated into two facies associations (FA) deposited in a shallow platform environment dominated by storm waves: Upper Shoreface (FA-1) and Lower Shoreface/Offshore transition (FA-2). It was possible to determine that the wave base line constantly varied, and a high-energy event provided the deposition of sediments with fine-to coarse-grained sandstone from FA-1 over those of FA-2, creating a first-order erosive surface (surf diastema) with a gradient varying from 40 to 45° that limits the two distinct facies association. This sedimentary event resulted in the excavation of the muddy offshore sediments by the lower shoreface, resulting in coarsening- and thickening-upward sandstones lying mostly under fine-grained sandstone with hummocky cross-stratifications. In the paleogeographic context, the cross-stratification paleocurrent indicates a direction towards the NW and, to a lesser extent, the SW. This direction suggests a connection between the epicontinental sea formed in the Parnaíba Basin and the paleosea of Tethys in the Late Carboniferous. Petrographic analysis classified the sandstones as subarkose and quartzarenite. This study employed petrography, X-ray diffractometry, architectural elements, and facies analysis to make inferences about the depositional system, climate, and geomorphology of the storm and wave-dominated deposits of the Poti Formation. The occurrence of kaolinite and illite in the studied deposits are typical clay minerals of humid and hot (dry) climatic conditions, respectively. Therefore, the overall data indicates a shift from retrogradational to progradational deposits that records changes in facies association and climate conditions in the Late Carboniferous.