Geometry and late-stage structural evolution of Central Graben salt diapirs, North Sea

Abstract Central Graben salt diapirs may have initiated along extensional faults and evolved into diapirs in Triassic times. The lack of primary rim synclines indicates that the structures became diapiric without going through a pillow phase. Diapirs grew mainly by downbuilding, probably with sporadic periods of subtle bathymetric relief created at diapir crests through to the Miocene period. Sea-bed relief controlled the deposition of Palaeocene turbidite sandstones, which thicken away from the diapir crests from 0 up to 300 m. However, highest-density turbidity currents flowed across diapir crests and good-quality channel sandstones were deposited across the tops of the diapirs. Radial faults at top Palaeocene level concentrate at the elongate terminations of elliptical diapirs, or above deeper cross-fault intersections above more circular diapirs. In map view, the radial faults tend to cluster in three to six main groups. These faults can be very productive zones in the chalk reservoir and some of them have been active from the Palaeocene to the Recent. Concentric faults also formed at Top Palaeocene, especially parallel to the main NW–SE regional fault trend. These disappear as the strain decreases upward and away from the salt structure, so that only radial faults are observed at Oligocene level and above. Diapir growth temporarily ceased at the end of the Palaeocene, with burial by Eocene-early Oligocene strata. Important unconformities were locally developed above the diapirs during Palaeocene and mid-Miocene times with high-angle (up to 5°) onlap reflectors above the unconformities, which indicate that high sea-bed relief (>300 m) was created rapidly due to reactivation of the diapirs. This rejuvenation is interpreted to be caused during compression, which is only recognisable around the diapirs. Compression is thought to be produced by downslope sliding of overburden above the Zechstein salt on the Eastern Platform area during late Palaeocene, and by regional Alpine shortening across the whole of the Central Graben during middle Miocene times.