Deformational structures in the lower Barra Velha formation, Aptian, Atapu field, Santos Basin

Abstract

The Barra Velha Formation, an Aptian Pre-salt unit of the Santos Basin, southeastern Brazil, is well-known for containing an important hydrocarbon reservoir. In the Atapu field, the lower part of the formation consists of laminated lutite (carbonate, siliciclastic, and hybrid), microbialite, and tufa rocks. The succession records muddy and in situ carbonate deposition in a low energy and shallow alkaline lacustrine environment in an active rift basin. Millimeter to decimeter scale soft-sediment deformation structures, faults, and fractures are very frequent and recurrent along the lutite succession. Convolute folds, loop bedding, injectites, intraclastic breccia, diastasis cracks, and associated faults, record ductile-brittle deformation in semi-lithified sediments. Normal planar and listric faults and unconfined fractures register dominant brittle deformation in lithified sediment. Foliated cataclasite and rare reverse faults record downdip slip on the hanging wall of temporarily inactive listric faults. The set of structures, the geological model, and the absence of other evident trigger mechanisms indicate that both ductile and brittle deformation are related to seismic events. The soft-sediment deformation structures are interpreted as having evolved during seismic shaking, while foliated cataclasite probably registers inter-seismic creep deformation.