Reconstructing early sedimentation patterns of the lower Codó formation (Early Cretaceous, NE Brazil): Evidence of marine influence and the onset of an alkaline hypersaline lake

The upper Aptian lower Codó Formation is an attractive stratigraphic unit within the Parnaíba and São Luís basins, northeastern Brazil, formed during the break-up of Western Gondwana. Located in a large sag basin, the formation records a dynamic depositional history of deltaic, lacustrine, and sabkha environments. This sedimentary evolution is reflected in a diverse lithological succession characterized by intermittent terrestrial input, episodic marine incursions, and restricted lacustrine phases culminating in sulfate evaporite precipitation. While the later stages of the formation have been extensively studied, the initial sedimentary processes and environmental conditions remain poorly understood. This research aims to reconstruct the depositional paleoenvironments during the initial stages of Codó sedimentation, elucidating the onset of the alkaline hypersaline lake environment and the earlier marine incursions in the study region. To achieve this, it was conducted a high-resolution facies analysis using core descriptions, trace fossil analysis, petrography, various types of X-ray fluorescence, scanning electron microscopy, X-ray diffractometry, and isotope analyses (δ¹³C, δ¹⁸O, and ⁸⁷Sr/⁸⁶Sr) on a ∼63-m-thick succession from borehole 2-TV-1-MA. Thirteen facies were recognized and grouped into brackish lake, prodelta, delta front, and alkaline hypersaline lake facies successions. The lower Codó Formation exhibits cyclic sedimentation, indicating fluctuating water dynamics associated with shifts between open and closed drainage systems controlled by paleoclimate. Tectonic subsidence drove the basin's evolution from a brackish lacustrine-deltaic environment with an incipient marine influence (balanced-fill basin stage) to a restricted system with the establishment of an alkaline hypersaline lake (underfilled basin stage).