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AAPG Bulletin

Abstract

AAPG Bulletin, V. 89, No. 11 (November 2005), P. 1475-1491.

Copyright copy2005. The American Association of Petroleum Geologists. All rights reserved.

DOI:10.1306/06130505002

Deep-water Niger Delta fold and thrust belt modeled as a critical-taper wedge: The influence of elevated basal fluid pressure on structural styles

Frank Bilotti,1 John H. Shaw2

1Unocal EampE Technology; present address: Chevron ETC, 1500 Louisiana St., Houston, Texas 77002; [email protected]
2Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts

ABSTRACT

We use critical-taper wedge mechanics theory to show that the Niger Delta toe-thrust system deforms above a very weak basal detachment induced by high pore-fluid pressure. The Niger Delta exhibits similar rock properties but an anomalously low taper (sum of the bathymetric slope and dip of the basal detachment) compared with most orogenic fold belts. This low taper implies that the Niger Delta has a very weak basal detachment, which we interpret to reflect elevated pore-fluid pressure (lambdaasymp 0.90) within the Akata Formation, a prodelta marine shale that contains the basal detachment horizon. The weak basal detachment zone has a significant influence on the structural styles in the deep-water Niger Delta fold belts. The overpressured and, thereby, weak Akata shales ductilely deform within the cores of anticlines and in the hanging walls of toe-thrust structures, leading to the development of shear fault-bend folds and detachment anticlines that form the main structural trap types in the deep-water fold belts. Moreover, the low taper shape leads to the widespread development of backthrust zones, as well as the presence of large, relatively undeformed regions that separate the deep-water fold and thrust belts. This study expands the use of critical-taper wedge mechanics concepts to passive-margin settings, while documenting the influence of elevated basal fluid pressures on the structure and tectonics of the deep-water Niger Delta.

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