| Abstrakt: |
The transition between Aptian and Albian stages of the Lower Cretaceous (-113 Ma) in the Parnaiba Basin (NE Brazil), represented by the Codo and Itapecuru formations, was a critical moment in the development of the proto-South Atlantic Ocean. During this time, deposition of lacustrine, shallow marine and fluvio-lacustrine sediments occurred in a large sag-type basin. The objectives of this study is the characterization of the organic matter (OM) of the succession of the 2-TV-l-MA borehole (270 m) for paleoenvironmental interpretation and evaluation of the thermal maturity of the OM by organic petrology and organic geochemistry techniques. The sedimentary succession was divided into 4 depositional systems according to the organic facies variability: (i) the base of the sequence represented by a freshwater lacustrine system (262.65-202.2 m); (ii) the lower middle section (201.75-159.6 m) a sabkha system with the deposition of microbial mats; (iii) the upper middle section (156.4-131.7 m) a deltaic system with incipient marine influence; and, (iv) the upper part of the borehole (112.7-28.1 m) with the emplacement of a fluvio lacustrine system. This study indicates that, in the study area, the deposition of the Codo Formation occurred, initially, in a lacustrine system, which evolved into a hypersaline lacustrine system and later into a fluvial system. The identification of dinoflagellate cysts, Subtilisphaera genus, and foraminiferal test-linings corroborated the hypothesis that the first marine incursions probably occurred at the initial stages of deposition of the Codo Formation, possibly from the Tethyan domain (Central Atlantic). The OM of the Itapecuru Formation is characterized by the occurrence of phytoclasts, corroborating previous palynological studies that suggest the development of vegetation during a significant period of its sedimentation, in this area. The organic matter is thermally immature, with vitrinite reflectance ranging from 0.37 to 0.52%VRr. [ABSTRACT FROM AUTHOR] |
