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Título

A PALEOGENE GILBERT-TYPE DELTA AT THE NORTHERN BORDER OF THE CABO FRIO FAULT ZONE, SANTOS BASIN, OFFSHORE BRAZIL

Texto do resumo

Gilbert-type deltas, named after G. K. Gilbert, who described such systems in 1885, are produced by river discharge and base level fluctuations at the border of active grabens and rifts. There are examples at the eastern border of the Basin & Range extensional province in western USA and along the southern border of the Corinth Gulf rift in Greece.
In this work we discuss a Gilbert-type delta observed during the interpretation and structural analysis of 3D seismic data in Santos Basin, at the intraplate continental margin in southeastern Brazil. The delta is positioned at the northern border of a Paleogene graben that deformed Cretaceous successions. The graben corresponds to a sector of an extensional fault system partially coupled to the basement, mapped in regional 2D seismic data. This regional fault system extends from the northern Campos Basin to the central Santos Basin and has been active since the Cretaceous. In the studied sector of the extensional trend, in Santos Basin, the fault system was previously denominated Cabo Frio Fault Zone (CFFZ), aligned to NE-SW. The CFFZ is parallel to the trend of adjacent onshore Paleogene rift basins of Taubaté, Resende, and Volta Redonda, which in turn follow the fabric of the late Proterozoic Ribeira Belt. The coupling degree of the fault system decreases from NE to SW, under the increasing influence of halokinesis towards SW, in the central Santos Basin. The geometry of normal faults changes accordingly, with a planar symmetric character to NE, where the Gilbert-type delta is positioned, and an antithetic listric morphology to SW, where sedimentary filling defines rollovers with section growth, eventually evolving to a touch-down pattern.
The Gilbert-type delta observed in the border of an active extensional depocenter at the CFFZ indicates that, during the Paleogene, the base level in Santos Basin was down at its position, around 120 km offshore, to the South of present shoreline. The offshore graben, at the Paleogene coastline, shares precisely both age and trend with the onshore rift system. In addition, there is a variation of mechanics and geometry along the regional fault system, with evident increasing basement coupling towards North. The studied structures suggest that a regional stress field, active both onshore and offshore, produced the observed extensional deformation at the intraplate continental margin in southeastern Brazil during the Paleogene. The deformation was also controlled by the basement fabric of the Ribeira Belt. The mapped fault trend shows activity during the Cretaceous, therefore we suggest that the observed deformation represents post-rift episodic reactivations along consistent intraplate stress trajectories conditioned by basement weakness trends.

Palavras Chave

Santos Basin; Gilbert Delta; Cabo Frio Fault Zone; Basement Tectonics; Paleogene Rift