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AAPG Bulletin

Abstract

AAPG Bulletin, V. 94, No. 9 (September 2010), P. 14011424.

Copyright copy2010. The American Association of Petroleum Geologists. All rights reserved.

DOI:10.1306/01051009080

New insights into deformation mechanisms in the gravitationally driven Niger Delta deep-water fold and thrust belt

Dominic Maloney,1 Richard Davies,2 Jonathan Imber,3 Simon Higgins,4 Stephen King5

1Center for Research into Earth Energy Systems (CeREES), Department of Earth Sciences, Durham University, Science Labs, Durham DH1 3LE, United Kingdom; [email protected]
2Center for Research into Earth Energy Systems (CeREES), Department of Earth Sciences, Durham University, Science Labs, Durham DH1 3LE, United Kingdom
3Center for Research into Earth Energy Systems (CeREES), Department of Earth Sciences, Durham University, Science Labs, Durham DH1 3LE, United Kingdom
4Statoil ASA, 4035 Stavanger, Norway
5BG Brazil EampP, Rua Lauro Muller, 116, Sala 1702, 22290-160, Rio de Janeiro, Brazil

ABSTRACT

We use two- and three-dimensional seismic reflection data from the deep-water Niger Delta fold and thrust belt to document evidence for two, discrete, postfaulting deformation mechanisms. An early phase of thrust-propagation folding is followed by folding caused by thickness changes within the basal shale detachment unit. The later phase of folding is caused by a lateral redistribution of the strata within the basal detachment unit. This example of late deformation occurred over a 4–5-m.y. period as a result of the displacement of approximately 590 km3 (sim370 mi3) of the underlying strata within the detachment unit. In another deformation event in the basal detachment unit, about 160 km3 (sim100 mi3) moved, laterally creating a synform in the overburden and parallel onlap fill, indicative of the relatively rapid creation of accommodation space. On the basis of seismic reflection data from the delta and a consideration of the volumes and rates of movement of sedimentary rock, we conclude that the poorly imaged succession commonly referred to as mobile shale cannot deform solely by ductile mechanisms as interpretations of shale tectonic provinces have commonly suggested but instead by brittle processes that involve thickening by thrust faulting and subseismic accommodation structures. Processes such as liquefaction, where a complete loss of shear strength is observed, had a minimal function. Therefore, the term ldquomobile shalerdquo in this setting is widely exaggerated. An awareness of postfaulting deformation mechanisms will be important for the successful exploration of gravitationally driven fold and thrust belts.

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