Orbital cyclicity in a Turonian sequence of the Cotinguiba Formation, Sergipe Basin, NE Brazil

Abstract

The Turonian sedimentary succession of the Cotinguiba Formation in the Sergipe Basin, northeastern Brazil, has a well-defined cyclicity, which is evident in two different facies: thinly laminated (mm-thick ) varve-like beds and limestone/marlstone couplets (1–50cm thick). Both facies show periodic or quasi-periodic patterns associated with orbital-climatically driven Milankovitch cycles. Spectral analyses (Fast Fourier Transforms) were performed using the gamma-ray and sonic log data on the Turonian section of two wells. The periodograms revealed cycles interpreted to be related to eccentricity (100ka), obliquity (41ka), and precessional (23–19ka) cycles, and others that occur at time intervals different from the main Milankovitch frequencies. In addition, retrogradational–progradational cycles controlled by non-periodic tectonic-eustatic events can be recognized in the wells. The time span of these cycles estimated by the cyclicity analysis ranges from 0.36 to 2.8Ma, which is supported by calcareous nannofossil and foraminiferal biostratigraphic data. The Milankovitch cycles are inferred to have resulted from variations in organic matter, mud content and carbonate production on a carbonate ramp setting. These cycles probably reflect changes in primary productivity in response to run-off variations driven by climatic changes. Increases in amounts of organic matter, which is predominantly of phytoplanktonic origin, and in shallow-water carbonate production varied in response to a rise in nutrient levels and clay input during wet climatic phases.