Re-sedimented deposits in the rift section of the Campos Basin

Abstract

An integrated petrologic-sedimentologic-stratigraphic-seismic study of the Lagoa Feia Group, rift section of the Campos Basin, has shown that 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. Although in situ deposits of stevensite ooids and peloids occur dominantly at the base of the succession, 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. The common mixture of well-rounded basaltic rock fragments with angular, granitic-gneissic rock fragments and feldspars in the sandstones and conglomerates indicates recycling of epiclastic deposits from the early rift section, 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. Structures indicative of unidirectional or oscillatory flow 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 variable 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. Thus, due to syn-rift and mainly to post-rift erosion (the Neo-Aptian unconformity), the preserved rift section of the Lagoa Feia Group comprises mostly sediments deposited in the central troughs of the half-graben structures. Marginal sediments were extensively eroded and re-sedimented as gravity-driven mixed deposits. This new model, constructed form the integration of seismic, stratigraphic, sedimentologic and petrologic data, diverges substantially from the presently accepted model for the sedimentation of the rift section, opening new possibilities for the exploration of Campos Basin, as well as of similar settings, as in the adjacent Santos Basin.