Abstract: Salt-Tectonics Provinces across the Continental-Oceanic Boundary in the Lower Congo and Campos Basins on the South Atlantic Margins

Jackson, M. P. A. - Bureau of Economic Geology, The University of Texas at Austin; C. Cramez - Total; W. U. Mohriak - Petrobras/E&P

The Lower Congo Basin (northern Angola) and the Campos Basin (Brazil) were originally contiguous before separating in the Early Cretaceous. New ultradeep-water seismic data from these conjugate margins cross each continental-oceanic crustal boundary. Preliminary interpretation indicates that three salt-tectonic provinces each contain structures formed by superposed deformation ( 1).

1. Shelf-to-Upper-Slope Province. Thin Aptian salt overlies rifted continental crust in provinces (1) and (2). The cover extended in the Late Cretaceous by raft tectonics. An important structure along both conjugate margins is a seaward-flattening bend in the top of basement. This Tertiary-Holocene hinge may have been localized on a presalt rift margin. The hinge comprises (1) a major synclinal graben, (2) a wide, landward-younging salt weld, and (3) a small offset of the base of salt. This hinge formed by epeirogenic uplift of the continents and triggered a Tertiary phase of raft tectonics and accompanying contraction. Contraction is subtle in this province and has the form of buckling, inversion, and rejuvenation of triangular reactive diapirs and high-relief diapirs.

2. Mid-Slope Province. Major Late Cretaceous contraction formed a thrust belt. Subtle Cenozoic buckling followed. Salt tectonics may have initiated the 800-m-deep Congo submarine canyon. Flat-lying reflectors below the canyon convert in depth to a broad turtle structure. A keystone graben formed by stretching of the outer arc of the drooping turtle structure. The graben was excavated by the Congo Canyon. This spatial coincidence may be fortuitous, or the keystone graben could have initiated and focused erosion of the canyon.

3. Lower-Slope Province. Originally thick Aptian salt rests on oceanic or rifted, volcanic continental crust. Late Cretaceous contraction formed regularly spaced imbricate thrusts having variable vergence. Shortening continued spasmodically until the present day. That created growth folds, pinched off large diapiric walls, some of which have broad allochthonous overhangs, and raised their roof bathymetry. Buckle folds formed at the gradual seaward pinch-out of salt.

Contractional structures generally become younger seaward, climbing gradually to the bumpy sea floor. The seaward younging indicates that advance of a spreading sedimentary wedge caused most of the shortening. In contrast, only locally did contraction propagate landward as it piled up against a buttress created by salt pinch-out, horsts, or volcanoes.

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