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

Gravitational collapse structures may range in scale from centimetres to hu ndreds 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 l istric and concave upwards detachment surface. The degree of deformation in creases in the direction of sliding. If movement of the sliding rocks is su fficiently slow, several intact slump blocks may be identified within the s lide area. The movement of blocks may be translational or rotational. Two t ypes of gravitational collapse structures are identified. In Type A, the ne wly-formed detachment reaches a free surface at the toe of the slide. In Ty pe B, however, the listric detachment fault follows a weak layer and its di splacement is accommodated by simultaneous slip along a major, steeper faul t. This results in a ramp-flat-ramp fault geometry. Gravitational failure is observed along the east flank of the Statfjord Fie ld, northern North Sea. The triggering mechanisms were probably earthquakes and high fluid pressures. Listric faults detached within soft shales and a re associated with several rotated slump blocks that decrease in size away from the break-away zone. The slumping occurred in several phases. First, p arts of the Brent Group failed. The detachment surface was within shales of the Ness Formation. Next, the slumping cut into the Dunlin Group and detac hed 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 cr eated by the Dunlin slumping. In the final stages of gravitational failure, slumping reached into the Statfjord Formation and detached within shales a t the base of the unit or within shales of the uppermost Hegre Group. The r elief created at the head (break-away zone) of Statfjord slumping caused re newed slumping of the Brent and Dunlin Groups. A study of gravitational fai lure analogues demonstrates several similarities in geometry in spite of di fferences in scale, lithology, degree of consolidation, and triggering mech anism. (C) 1999 Elsevier Science Ltd. All rights reserved.