Organic facies applied in the paleoenviroment interpretation from sediments of the itapecuru formation (In the west-central part of the san luis basin-Brazil)

Abstract

The São Luís Basin, located in the Northwest of Brazil, is filled with sediments of the Pimenteira Formation (Devonian) and the Itapecuru, Codó, Grajaú and Bragança Formations (Cretaceous). The Itapecuru Formation, the target of this research, is the thickest sedimentary unit with 2.469 meters in the central-western part of the basin. The present work focuses on the study of the organic components and organic facies from this formation. The samples were collected from the stratigraphic well 2-ANP-5-MA and were analyzed for total organic carbon (TOC), insoluble residue (RI), and palynofacies. The data obtained from the counting of the palinofacies were processed using multivariate analysis (Clusters: Q-mode and R-mode) and extreme and quartile diagrams (bo-x-plots). The analyzed material has low TOC content (less than 0,50%), which reflects both the amount of organic matter deposited and the low degree of preservation and high degree of oxidation; the high values of insoluble residue (> 85%) suggested siliciclastic depostion. There was a positive correlation of TOC with cuticles, resins and continental palynomorphs (pollen grains and spores). The integration of these results indicates that the depositional conditions varied from a shallowfresh water setting with a marine influence, under river influence, in proximity of the source area of organic matter to a semi-restricted water body with variations in the column water and the energy regime increases, typical in fluvial systems such as oxbow lakes, channels and flood deposits. The warm arid to semi-arid climate prevailed, although local variations towards more humid conditions also occurred during deposition. Reworked palynomorphs from Devonian poitted out that the strata of the Pimenteira Formation were exposed during the deposition of the Itapecuru Formation. Based on the classification of organic facies from sensu Jones (1987), the organic facies corresponds to D that exhibits highly oxidized and redeposited organic matter for the Itapecuru Formation.