Evolution and geometries of gravitational collapse structures with examples from the StatfJord Field, northern North Sea

Abstract

Gravitational collapse structures may range in scale from centimetres to hundreds of kilometres and affect both loose sediments and consolidated rocks. The area affected by gravitational failure will commonly be amphitheatre-like in map view, whereas a cross-sectional view will typically display a listric and concave upwards detachment surface. The degree of deformation increases in the direction of sliding. If movement of the sliding rocks is sufficiently slow, several intact slump blocks may be identified within the slide area. The movement of blocks may be translational or rotational. Two types of gravitational collapse structures are identified. In Type A, the newly-formed detachment reaches a free surface at the toe of the slide. In Type B, however, the listric detachment fault follows a weak layer and its displacement is accommodated by simultaneous slip along a major, steeper fault. This results in a ramp-flat-ramp fault geometry. Gravitational failure is observed along the east flank of the Statfjord Field, northern North Sea. The triggering mechanisms were probably earthquakes and high fluid pressures. Listric faults detached within soft shales and are associated with several rotated slump blocks that decrease in size away from the break-away zone. The slumping occurred in several phases. First, parts of the Brent Group failed. The detachment surface was within shales of the Ness Formation. Next, the slumping cut into the Dunlin Group and detached within the lower parts of the group (shales of the Amundsen Formation). Renewed slumping of the Brent Group occurred at the new break-away zone created by the Dunlin slumping. In the final stages of gravitational failure, slumping reached into the Stattjord Formation and detached within shales at the base of the unit or within shales of the uppermost Hegre Group. The relief created at the head (break-away zone) of Statfjord slumping caused renewed slumping of the Brent and Dunlin Groups. A study of gravitational failure analogues demonstrates several similarities in geometry in spite of differences in scale, lithology, degree of consolidation, and triggering mechanism.