--> Autochthonous and Allochthonous Salt Tectonics in Brazilian Sedimentary Basins, Mohriak, Webster U.; Szatmari, Peter, #90100 (2009)

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Autochthonous and Allochthonous Salt Tectonics in Brazilian Sedimentary Basins

Mohriak, Webster U.1
 Szatmari, Peter2

1E&P, Petrobras, Rio de Janeiro, Brazil.
2
CENPES,
Rio de Janeiro, Brazil.

This work discusses the main structural features and the geological constraints associated with the pre-Mesozoic and Aptian evaporites in the Brazilian sedimentary basins. The standard geological models for halokinetic structures in the intracratonic and passive margin basins emphasize the occurrence of autochthonous salt in beds, pillows and diapirs. These structures are generally associated with extensional or compressional tectonics, but this paradigm is broken in some basins displaying halokinetic features where salt is detached from the mother layer, forming salt teardrops, apophyses, overhangs and salt tongues.

Pre-Mesozoic evaporites are recognized in the Parecis, Amazonas and Solimões basins, the first with carbonate and anhydrite layers identified in exploratory boreholes, and the latter two basins characterized by complete evaporitic cycles ranging from carbonates to sulphates and chlorides. The NE Brazilian continental margin is characterized by different Aptian evaporite layers deposited in structural lows that were often controlled by basement-involved faults. Evaporites are also recognized in the deep water region of the
Sergipe-Alagoas Basin, forming isolated salt bodies, contrasting with the huge salt wall provinces observed in other segments of the margin. Most halokinetic structures observed in the Southeastern margin basins are associated with Late Cretaceous and Early Tertiary sedimentary depocenters, which are locally controlled by mobilization of the Aptian salt. In the deep water region the salt walls occur aligned along a NS direction, but locally the trend is strongly influenced by NW-SE structural lineaments.
Recently, re-interpretation and analysis of seismic lines in the Brazilian margin suggest the occurrence of allochthonous salt structures, covering younger stratigraphic sequences that may be sand-prone. Regional 2D seismic lines that extend towards the oceanic limit of the salt province indicate salt nappes overriding volcanic highs and thrusted over Cretaceous and Tertiary sediments.

Physical models for similar features in the
Gulf of Mexico and West African margin suggest that salt apophyses may be formed due to compression of previous salt diapir stems, resulting in canopies or mushrooms where salt is extruded above younger sediments, thus forming excellent seals for sub-salt siliciclastic reservoirs.

AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil