Pedro Victor Zalán1
(1) Petrobras, Rio de Janeiro, Brazil
ABSTRACT: Growth Folding in Gravitational Fold-and-Thrust Belts in the Deep Waters of the Equatorial Atlantic, Northeastern Brazil
It is presently a well established fact in the petroleum industry that the deep/ultra-deep water realms of most continental margins are constituted by compression-dominated structural domains. This is due to gravity sliding processes that are widespread in the upper parts of the continental slopes and carry huge allochtonous piles of sediments down to the lower parts of the slopes/continental rises, via sub-horizontal decollement zones. When the detachment takes place inside overpressured shales (hopefully caused by petroleum generation processes) large gravitational fold-and-thrust belts (GFTB) develop due to contractional processes, engulfing huge volumes of shales and turbidite sandstones. They are part of linked extensional-compressional systems characterized by listric normal faults and rollover structures located updip, and systems of compressional strain downdip, distinguished by huge folds of diverse nature (detachment, fault-bend and fault-propagation folds), reverse and thrust faults. When salt is the lubricant, deformation is more complex and salt canopies develop. Important oil discoveries have been achieved in these compressional provinces in deep waters off Nigeria and in the GOM.
Although in some GFTB's the duration of the sliding/contracting processes seems to be short (e.g., Nigeria), in offshore northeastern Brazil GFTB's show evidence of long lasting, exclusively submarine, tectonism. As a consequence, sedimentation takes place while sediments are sliding downslope and contracting. Prime examples of growth folding presenting all the characteristic related structural and stratigraphic features are displayed in seismic sections. Potential traps include deformed and repeated turbidite sands below time-crossing unconformities and same age turbidites onlapping the same discordance.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado