Sequence stratigraphy of the last filling phase of an intracontinental rift: a model based on the Lower Barremian of the Recôncavo Basin

Abstract

The sedimentary deposits of the Buracica Stage (Lower Barremian) of the Recôncavo Basin are characterized by a cyclic succession of fluvial, deltaic and lacustrine sediments, included in the São Sebastião Formation. Outcrop, core and well log studies supported the proposal of a model that explains the architecture of the stratigraphic sequences and its subdivision in six 3rd order sequences and twenty 4th order sequences. The lowstand lake-level systems tract (LLST) is characterized by a broad fluviodeltaic progradation, whereas the transgressive lake-level systems tract (TLST) is characterized by a retrogradational architecture, mainly in the distal areas of the basin. An aggradational pattern can occur in the proximal areas, where the sedimentation rate equals the rate of creation of accommodation space. The highstand lake-level systems tract (HLST) is predominantly characterized by a lacustrine pelitic sedimentation, which is a continuation of the sedimentation of the TLST pelites. The broad deltaic progradation anticipated by the Sequence Stratigraphy for the highstand systems tract of the marine sequences does not apply to the studied interval. The forced-regression systems tract (FRST) is represented by thin sandstone beds. The identification of this systems tract is hard since its deposits can be easily eroded. In the proposed model, the sequence boundaries are erosive surfaces produced during falling lake-level periods, which are climatically controlled. The position of transgressive and maximum flooding surfaces are related to the rate of coarse-grained sediment influx, which is dependent upon the water influx. The sequences are deposited and preserved by the accommodation space created by subsidence.