The cryptic stratigraphic record of the syn- to post-rift transition in the offshore Campos Basin, SE Brazil

Abstract

Rift basins typically comprise three main tectono-stratigraphic stages; pre-, syn- and post-rift. The syn-rift stage is often characterised by the deposition of asymmetric wedges of growth strata that record differential subsidence caused by active normal faulting. The subsequent post-rift stage is defined by long-wavelength subsidence driven by lithospheric cooling and is typified by the deposition of broadly tabular stratal packages that drape any rift-related relief. The stratigraphic contact between syn- and post-rift rocks is often thought to be represented by an erosional unconformity. However, the late syn-rift to early post-rift stratigraphic record is commonly far more complex since (i) the associated tectonic transition is not instantaneous; (ii) net subsidence may be punctuated by transient periods of uplift; and (iii) strain often migrates oceanward during rifting until continental breakup is achieved with crustal rupture. Previous publications on the Eastern Brazilian marginal basins have not historically used the tripartite scheme outlined above, with the post–pre-rift interval instead being subdivided into rift, sag and passive margin tectono-stratigraphic stages. In addition, the sag stage has been previously described as late syn-rift, early post-rift or as a transition between the two, with the passive margin stage being equivalent to the classically defined post-rift, drift stage. Two (rather than one) erosional unconformities are also identified within the rift-to-sag succession. In this work, we use 2D and 3D seismic reflection and borehole data to discuss the expression of and controls on the syn- to post-rift transition in the shallow and deep water domains of the south-central Campos Basin, south-east Brazil. We identified three seismic–stratigraphic sequences bounded by unconformities, named lower and upper pre-salt and salt. The lower pre-salt interval is characterised by wedge-shaped packages of reflections that thicken towards graben and half-graben-bounding normal faults. This stage ends with the development of an angular unconformity, inferred to form as a result of the onset of the oceanward migration of deformation. The upper pre-salt is typically defined by packages of subparallel and relatively continuous reflections that are broadly lenticular and thin towards fault-bound basement highs, but that locally contain packages that thicken against faults. The pre-salt to salt contact is defined by an erosional unconformity that is largely restricted to basement highs, and which is inferred to have formed due to base-level fall and uplift associated with local fault reactivation, resulting in the formation of channels of possible fluvial origin. Based on its geometries and seismic facies, we conclude that the lower pre-salt interval is syn-rifting and syn-tectonic, deposited during active continental extension and upper crustal faulting affecting the entire evolving margin, whereas the overlying upper pre-salt is syn-rifting and post-tectonic in the Campos Basin, deposited when extension and faulting had migrated seaward to the future location of the spreading centre. The results of our study support the arising notion that the syn-rift sequence does not only display syn-tectonic sedimentary packages, and thus the tripartite tectono-stratigraphic model for rift development is too simplistic and cannot be applied when assessing rifts in the context of the regional development of continental margins.