--> Abstract: Salt Tectonics Control on the Distribution of Drift Sedimentary Facies in the Jequitinhonha Basin, Bahia, Brazil, by L. P. Magnavita, A. J. Santana, G. M. N. Menezes, and C. P. Pereira; #90933 (1998).

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Abstract: Salt Tectonics Control on the Distribution of Drift Sedimentary Facies in the Jequitinhonha Basin, Bahia, Brazil

Magnavita, Luciano P.; Armando J. Santana; and Gloria M. N. Menezes - Petrobras/E&P; Cicero P. Pereira - Consultant

The Jequitinhonha Basin is located in the northeastern part of the eastern Brazilian margin, offshore Bahia state. It occupies an area of about 10,100 km2, from which 9,500 km2 is offshore. Two supersequences form the stratigraphic architecture of the basin. The first one corresponds to a continental syn-rift phase which began in the Early Aptian. It is comprised of siliciclastics of the Mariricu Formation and evaporites of its Itaúnas Member. These latter represent a transitional deposit that is followed by marine transgressive sediments accumulated during a thermal subsidence which typifies the passive margin drift phase. These sediments are composed of Albian limestones and siliciclastic Later Cretaceous and Tertiary sediments. This marine supersequence initiates with neritic clastics and carbonates of the Barra Nova Group deposited from Albian to Coniacian. From the Upper Cretaceous to the Lower Tertiary, transgressive slope and deep basin shales and fine sandstones characterize the Urucutuca Formation. From then on, a regressive system composed by alluvial fans deposits interbedded with a carbonate platform dominates, originating the Rio Doce and Caravelas formations, respectively.

The drift sedimentary facies are highly controlled by salt tectonics. To the east of a shallow bench, a trough created by salt flow characterizes this part of the basin (Fig. 1). The trough was formed as a result of raft tectonics and is typified by a highly structured package, with down-to-basin and counter regional faults, resembling the turtle structures present in the Kwanza Basin in Western Africa. The Tertiary deposits tectonically rest over a salt weld. Further east, a disconnected raft with Albian limestones as well as Later Cretaceous turbidites represent an interesting play in the basin. Following that, salt diapirs indicate the huge mass that flowed away from the platform. The analysis of carbonate facies as well as turbidite facies suggest that basin physiography evolved from a ramp-type into a platform-type due to the onset of salt movement in the Later Cretaceous.

The present basin physiography started to be delineated when a ramp was implanted on a mixed siliciclastic-carbonatic shallow platform during the Albian. The continuous subsidence of this ramp associated with high rates of organic productivity and sedimentation gave rise to the deposition of a thick package of shallow marine/high energy deposits, alternated with registers indicative of deepening of the basin. During the Upper Cretaceous (Turonian), the beginning of salt movement basinward drastically changed the basin physiography, which changed from a ramp to a steep slope morphology. The consequence is the appearing of a platform-slope-basin configuration. Sea level fall associated with this steeper slope allowed erosion, sediment bypass and deposition of gravitational flows (turbidites) in the slope and deep basin.

The post-rift tectonic evolution of the basin is typified by a thin-skinned extension due to salt movement. The salt withdraw, triggered either by basement irregularities (e.g., fault) or tilt of the substratum because of thermal cooling, led to the development of an allochthonous block that has moved more than 10 kilometers eastward due to gravity gliding. Between the allochthonous and the autochthonous an elongated graben was created, allowing the deposition of a thick Tertiary section. This graben abruptly terminates southward, apparently against a transfer zone. Not much data is available within this depression. The age of the oldest sediment in the center part of the trough is Paleocene, which indicates the moment when the allochthonous block separated completely from its counterpart, thus becoming a raft.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil