Resedimented Deposits in the Rift Section of the Campos Basin

Abstract

An integrated petrologic-sedimentologic-stratigraphic-seismic study in the Lagoa Feia Group has shown that Campos Basin rift sedimentation was dominantly intrabasinal (carbonate and stevensitic deposits), with siliciclastic deposits restricted to the proximity of graben border faults. The bivalve bioclastic rudstones (“coquinas”) that constitute the rift reservoirs show commonly limited abrasion of the bioclasts, and were deposited both on basement highs and lows throughout the rift section. In situ deposits of stevensite ooids and peloids are dominant in the basal rift section, but these particles are ubiquitous to the entire rift section, mixed in variable proportion with siliciclastic and carbonate sediments. The environmental conditions required for the formation of stevensite and the growth of bivalves are mutually exclusive, as stevensite forms only at pH greater than 10, while bivalves cannot tolerate pH greater than 9. Siliciclastic sandstones and conglomerates are rich in well-rounded volcanic rock fragments. Their common mixture with angular, granitic-gneissic rock fragments and feldspars indicates recycling of epiclastic deposits during erosion and extensive reworking of the basaltic rocks that constitute the basal rift, combined with first-cycle contribution from the plutonic basement. The studied cores show no evidence of subaerial exposure, and there is a lack of bioturbation, suggesting harsh environmental conditions. The rift deposits are dominantly massive or faintly-laminated, with diffuse facies boundaries. Unidirectional- or oscillatory-flow structures are subordinate. Integration of seismic, sedimentologic and petrographic evidence indicates that the Campos Basin rift section is formed mostly by re-sedimented gravitational deposits. The onset of the rift sedimentation occurred in synformal depressions, where bivalve banks or stevensite ooids were formed in shallow lacustrine environments under diverse alkalinity conditions. With the development of half-grabens and concentration of the tectonic activity along the border faults, recurrent tectonic events promoted the mixing and gravitational re-deposition of stevensitic, clastic and bioclastic sediments in deeper, fault-bounded troughs. Large scale units, hundreds of meters thick, were generated by major tectonic events, whereas compositional variations in the scale of meters were possibly a product of lake-level climatic fluctuations.

AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014