Tectono-Stratigraphic Model for the Aptian-Barremian Continental Rift Carbonates in the Southern Campos Basin, Brazil

Muniz, Moises C.¹; Bosence, Dan ²; Elders, Chris ²
(1)E&P-EXP, Petrobras, Rio de Janeiro, Brazil. (2) Earth Sciences, Royal Holloway University of London, Egham, United Kingdom.

This work proposes a tectono-stratigraphic model for the carbonate rocks of the Barremian - Aptian of the southern Campos Basin. These comprise the geologic record of the continental rift phase formed during the breakup of Gondwana and the formation of the South Atlantic passive margin. A complex arrangement of structures and features in a polygonal geometric style is the result of an oblique extension rifting system. Within this tectonic context, the late synrift bioclastic rocks (coquinas) overlie synrift siliciclastic rocks and basalt. Postrift microbialites underlie the Aptian salt.

Based on 3D seismic interpretation the structural styles of the pre-salt are characterized by a series of extensional structures. The major structural elements mapped are oriented NE-SW, some NNE-SSW, and are graben, half-graben and horsts. The area of study has been subdivided into three distinct tectonic domains, DI, DII and DIII. DI occurs in proximal areas and is characterised by half-grabens, bounded by extensional, planar, steeply dipping, normal faults, which are dominantly synthetic but also with antithetic polarity. Distally and basinwards in DII, the structural elements outline a polygonal, rhombohedral geometric pattern. Accommodation zones and a hinged margin with a sinuous NE-SW orientation which separates DII from DIII basinwards, are also developed. At greatest depth and to the east in DIII, the half-grabens form a linked large graben structure with more symmetric geometry, and the topographic high structures appears as relict of original horsts.

Each domain has a specific stretch factor and subsidence rate, which suggest long lived and progressive rifting developing basinward. The structural template of the syn-rift exerts a strong influence on depositional patterns.

Rock analysis indicate proximal to more distal lacustrine carbonate deposits with fluvial clastics in marginal areas. Within the Barremian succession, the dominant carbonate facies is molluscan rudstones and floatstones. Facies analysis and exposure surfaces indicate that these shallow lacustrine sediments accumulated in shallowing-upward cycles in response to changes in lake level. Microbialite facies, Aptian in age, appear to occur in the most distal locations. Both molluscan-rich facies (coquinas) and microbialites form the main carbonate reservoirs of the pre-salt sequences in the Campos and Santos Basins, Brazil.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.