Geological evidence for widespread faulting and fracturing of clay-rich sediments during initial consolidation

摘要

In the past decade, geophysical and geological evidence from many basins with thick successions of smectite-rich clays shows that this type of marine clay is intensely fractured and faulted during very early stages in its burial and consolidation history. In particular, 3D seismic data (spatial resolution of 12.5m) show that these fault patterns are polygonal in geometry, and this pattern cannot be explained by normal processes of tectonic faulting. It is argued that these polygonal fault systems are an intrinsic deformational response to fully three-dimensional consolidation and that they form during the earliest stages of pore fluid expulsion and void ratio reduction. They consist exclusively of small (metre-scale displacements) extensional faults. They are globally widespread, occurring in over 100 basins worldwide, in some cases covering areas in excess of 10~6 km~2. This large areal extent of the deformation is perhaps its most dramatic property: as an example, the offshore equivalent of the Eocene London Clay is pervasively deformed by faults spaced at 100's of metres over the entire southern and central North Sea Basin. The recognition of this new class of early deformational system has profound implications for engineering activities centred on smectitic clay soils, and poses some fundamental questions for the relationship between consolidation, stress history and strength.