The Cretaceous period was marked by significant transformations on Earth; paleoclimatic and paleogeographic changes were important parameters for the tectonostratigraphic evolution of the Brazilian sedimentary basins. The Parnaíba Basin in northern and northeastern Brazil is a syneclise, with a central sedimentary succession exceeding 3 km in thickness, containing sediments ranging from the Silurian-Devonian strata to the Cretaceous. The northwestern sector of the Parnaíba Basin, known as the Grajaú Basin, evolved from the breakup of Gondwana, with sediments of the Aptian-Albian age. In the present study, faciological, stratigraphic and biostratigraphic studies were conducted in the vicinity of the town of Grajaú, where the Codó Formation comprises three sequences: sequence I (shales and mudstones with organic material, siltites, limestones and ostracodal limestones), sequence II (gypsites), and sequence III (limestones). The faciological and biostratigraphic analyses of these sequences revealed the first known occurrence of calcareous nannofossils and planktonic foraminifera derived from the epicontinental sea that encroached the continent through the opening of the Brazilian equatorial margin and confirm the occurrence of a marine ingressions in the Codó Formation, which is recorded in sequences I and III. The biostratigraphic data, based on the identification of the S. varriverrucata (P-270), H. aptiana and M. miniglobularis (P. rhori/M. miniglobularis), and D. grandiensis (O-11) biozones, indicate that at least the studied section of the Codó Formation was deposited in the late Aptian. The biota identified in the study section are of Tethyan origin, and the faunal association dominated by palynomorphs, planktonic foraminifera, and calcareous nannofossils confirms the Tethyan origin of the formation. In this context, the marine ingressions were a clear result of the evolution of the Brazilian equatorial margin.