Abstract: High-Resolution Stratigraphy of Neocomian, Deep Lacustrine and Deltaic Sediments from the Recõncavo Rift Basin, Brazil

Picarelli, A. T.; C.H.L. Bruhn - Petrobras/E&P & A.M.F. Figueiredo - Braspetro

This paper describes, using essentially subsurface data, the high-resolution stratigraphy and sandbody geometry of a Neocomian, deep-lacustrine to deltaic section from the Recõncavo rift basin, Brazil. Four high-order stratigraphic units (tectonosequences) were discriminated on the basis of sandbody geometry, stacking patterns, and unconformities recognized in the rift flexural margin; these tectonosequences were mostly controlled by tectonic activity during the rifting process. The tectonosequences comprise climatically-controlled, 3^rd- to 5^th-order depositional sequences; these in turn can be grouped into composite sequences, which were defined by distinct stacking patterns and erosion surfaces.

Tectonosequence I comprises an up to 700 m-thick succession of dark-colored, organic-rich mudstones, deep-water sandstones, and plataformal oncolitic calcarenites, deposited during the first stage of rifting evolution, associated with intense faulting and development of transfer zones. It is characterized by retrogradationally-stacked, fining- and thinning-upward successions.

Tectonosequence II comprises an up to 850 m-thick succession of interbedded mudstones, sandy and conglomeratic turbidites and debris flow deposits fed by flexural deltaic systems. It comprises two composite sequences, subdivided into seven fining- and thinning-upward sequences. The stacking patterns shown by Tectonosequences I and II suggest that tectonic subsidence exceeded sediment supply during their sedimentation.

Tectonosequence III contains an up to 850 m-thick succession of turbidites and debris flow deposits fed by axial deltas. It comprises two composite sequences, subdivided into four depositional sequences. The main structural control on sequence geometry was the contemporaneous development of a prominent transfer zone and the uplifting of the South Tucano half-graben. The stacking patterns shown by Tectonosequence III suggest that sediment supply exceeded tectonic subsidence during its sedimentation.

Tectonosequence IV (300 m-thick) comprises six sequences composed mostly of deltaic sediments, which are onlapping structural features that controlled the sedimentation of the underlying tectonosequences. Its stacking patterns suggest that the rates of sediment supply and tectonic subsidence were very similar.

Figures 1 and 2 depict the stratigraphic models for the sequences described for each one of the tectonosequences.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil