Delphine Rouby1, Stéphane Raillard2, François Guillocheau1, Renaud Bouroullec3, Cécile Robin4, Sebastien Castelltort1, Thierry Nalpas1
(1) University of Rennes, Rennes, France
(2) Total-Fina-Elf, Pau, France
(3) Imperial College, London, United Kingdom
(4) University of Paris VI, Paris, France
ABSTRACT: Rates of Deformation of a Growth Fault/Raft System (Offshore Congo, West African Margin) from Combined Accommodation Measurements and 3-D Restoration
We present an original approach combining sequence stratigraphy and 3-D restoration, to quantify the evolution, in space and time, of a growth fault structure located along the West African margin. It formed during the Albo-Cenomanian, by gravitational spreading of sediments above the décollement layer of Aptian evaporites.
From 3-D seismic data, we built a 3-D geometrical model of the system. We restored it to reconstruct its 3-D geometry at six incremental steps of its evolution. The kinematics of the fault system involved the rise and fall of a diapir between two rafts separated by regional extension. The sag basin evolved into a turtle back anticline bounded by listric faults.
From the sequence stratigraphy analysis of the logs of nine wells, we identified 11 genetic units sets (4th order cycles) with duration ranging between 0.5 to 2 Ma. We calculated variations of vertical space available for sedimentation (accommodation) for half-cycles of each genetic unit set and corrected it from eustasy to evaluate the corresponding subsidence.
Finally, from the reconstructed 3-D geometries, we measured rates of horizontal translation of the structural units (rafts and turtle back anticline) an show that they (1) translation increased throughout the Albo-Cenomanian rand (2) ranged between 3 to 30 times subsidence rates.
Thus, the main parameter controlling space available for sedimentation in the studied area was displacement on the faults (i.e. regional extension) with the superimposition of periodic (presumably eustatic) variations of 0.5 to 2 Ma. We discuss the relationships between accommodation variations and the structural framework.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado