Eustatic Control of Gravity Tectonics: Concept, Mechanism and Limits

S. Raillard, P. Allix, G. Guerin, T. Mauduit, H. Lecanu, and J. P. Brun

Gravity tectonics over a ductile decollement characterizes deformation of the Albian to Recent section in the West African margin, from Gabon to Angola.

Largely studied during the past 20 years as a prolific petroleum play, it is now well known that three mains factors will control gravity driven deformation: the ductile layer, the slope as a response to the crustal activity and the overlying sedimentary loading.

For the West African passive margin, the slope effect at the first glance can be considered as constant and gravity driven deformation as a result of salt layer distribution (in time and space) and sedimentary loading.

If previous papers have already shown that the type and distribution of the deposits control the development of the classical structural domains: updip extensional to downdip contractional, this study will focused on the factor which control the sedimentary loading. Based on several natural examples combining seismic stratigraphy, sequential stratigraphy and structural studies and also analogical modelling, it is demonstrated that:

- as sediment distribution and then sedimentary loading is controled by relative sea level changes, thus sea level changes can be directly related to gravity driven deformation: a large sea level fall will provide an important increase of clastics supply which will enhanced gravity gliding. On the contrary relative sea level high will stop the deformation because of the decreasing amount of detritics and their homogeneous distribution.

- the salt downdip withdrawal during extension will enhance the eustatic effect.

- episodic crustal activity, materialized by westward tilting of the margin will interfere on this mechanism.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California