Carbonate Platforms in Non-marine Rift System in the Early Cretaceous (Pre-salt) of the Campos Basin, Brazil

The Campos Basin is a passive margin basin formed during break up of Gondwana and sea-floor spreading in the South Atlantic. During the Early Cretaceous an oblique extension system resulted in a complex structural style and the southern Campos Basin can be divided into three distinctive tectonic domains, each with characteristic structures, stretching factors and subsidence rates. These indicate long-lived and progressive rifting that developed basinward.

This structural template plays an important role in controlling depositional patterns of the lacustrine carbonate platforms in the Campos Basin. The proximal domain is characterised by half-graben with fluvio-deltaic sediments in hangingwall basins and carbonates on structural highs. The central domain is dominated by carbonates over horst blocks and graben. The distal domain has an isolated, fault-bound horst that accumulated platform and slope carbonates.

Characterisation of facies and depositional environments is based on a proximal to distal transect of wells providing core, sidewall core, image resistive (FMI), and gamma ray logs. Bioclastic carbonates vary in texture from wackestones to rudstones and comprise bivalves, gastropods, ostracods, peloids and/or oncoids. The molluscs are endemic to the southwest Atlantic and are interpreted to inhabitat lacustrine environments. Autochthonous floatstone to rudstone facies occur in deeper lake margin areas with shorelines characterised by reworked molluscan rudstones (coquinas). Microbialites comprise stromatolites and laminites with associated ostracods, bioclasts and ooids, and occur in more distal locations.

Decimetric-thick depositional sequences are recognised in the coquinas that start with flooding surfaces overlain by ostracod-rich siltstones and shales. These grade upwards into autochthonous and parautochthonous bivalve-rich floatstones to rudstones that become thicker-bedded and more allochthonous up-section. An upward increase in energy is indicated by increasing fragmentation and abrasion interpreted to relate to shallower lacustrine settings. Subaerial exposure surfaces on these fragmented coquinas are brecciated, iron-stained and reworked.

Overall, these pre-rift lacustrine carbonates show an upward trend from ostracode to bivalve to gastropod dominated bioclastic facies that are overlain by post-rift microbialites. Together, they form the carbonate reservoirs of the pre-salt sequences in the Campos Basin.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California