Comparison of gravity‐driven deformation styles and behavior associated with mobile shales and salt

Abstract

Structures produced by salt and mobile shales are commonly similar; however, the material behavior of the two is different. Salt mobility is a fundamental material property, shale mobility only occurs if overpressured fluids are present. Dewatering of shales will stop their mobility, while renewed burial or the onset of an internal overpressuring process (e.g., diagenetic release of water or hydrocarbon generation) may renew mobility. Consequently, structures in mobile salt will envolve continuously until salt withdrawal produces touchdowns sufficient to stop salt mobility. Shale mobility may follow a deformation sequence similar to salt, or it may display a more episodic evolution reflecting critical overpressuring events. While salt mobility is confined to specific lithological units, the same is not the case for shale. The overpressuring of shales is strongly dependent on depth, so that mobile shale zones may cut across time/bedding boundaries. The differences in mechanical behavior lead to differences in structural style although many basic aspects of gravity tectonics remain the same. (1) Prekinematic structures and synkinematic deformation occur in both salt and shale tectonics. However, much of the prekinematic deformation in mobile shale‐dominated deltas may be lost by burial and conversion of the prekinematic sequence into mobile shales. (2) Fault‐dominated depocenters occur in salt and shale tectonics. In the Niger delta, overpressured shales vary in thickness from thin decollement zones to massive chaotic zones some 4–6 km thick. Fault‐controlled basins develop over mobile shales. Maximum basin depth is approximately the thickness of the mobile shale plus the thickness of the overlying pregrowth fault strata (i.e., some 4–8 km). (3) Diapirs are common to both salt and shale tectonics. Normal faults associated with reactive diapirism are common in both. Salt has the potential to almost completely evacuate a particular volume, resulting in local touchdowns or welds. Shale may also create touchdown areas, but complete collapse is uncommon because a large volume of immobile dewatered shale is usually left behind. (4) Salt nappes can cover extensive areas in the Gulf of Mexico. Some limited shale tongues occur in the Niger delta. They form imbricate thrusts that pass down dip into gravity flows and slumped blocks of shale derived from the exposed mobile shale.