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

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 pillo w phase. Diapirs grew mainly by downbuilding, probably with sporadic period s of subtle bathymetric relief created at diapir crests through to the Mioc ene period. Sea-bed relief controlled the deposition of Palaeocene turbidit e sandstones, which thicken away from the diapir crests from 0 up to 300 m. However, highest-density turbidity currents flowed across diapir crests an d good-quality channel sandstones were deposited across the tops of the dia pirs. Radial faults at top Palaeocene level concentrate at the elongate ter minations 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 t o the Recent. Concentric faults also formed at Top Palaeocene, especially p arallel to the main NW-SE regional fault trend. These disappear as the stra in decreases upward and away from the salt structure, so that only radial f aults 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 d uring Palaeocene and mid-Miocene times with high-angle (up to 5 degrees) on lap reflectors above the unconformities, which indicate that high sea-bed r elief (>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 Pla tform area during late Palaeocene, and by regional Alpine shortening across the whole of the Central Graben during middle Miocene times. (C) 2000 Else vier Science Ltd. All rights reserved.