Sequence stratigraphy of the Middle to Upper Jurassic, Viking Graben area, North Sea
Sneider, John Scott
Vail, Peter R.
Doctor of Philosophy
The Middle to Upper Jurassic in the Viking Graben area was deposited during an overall transgression. Tectonics, eustasy, sediment supply and sediment source area control facies distribution. The Lower Toarcian to the base of the Cretaceous consists of seven major regressive-transgressive facies cycles. These regressive-transgressive (R-T) facies cycles are called second order and dominate facies distribution. Two maximum flooding surfaces bound a facies cycle which contains an unconformity internally. The first three cycles occur during a minor rift phase (Rift Phase 1) and the second four during a major rift phase (Rift Phase 2). The regressive phase of the second order cycles appears to correspond to periods of high fault displacement; however, the eustatic effect on these cycles is unclear. Each second order facies cycle is associated with a change in depocenters. Most major seismic discontinuities are tectonic in origin. Shelfal areas and local highs are often eroded during a second order regression and the deposition is shifted to the basin center. During transgression, deposition is more widespread; shallow marine sandstones often develop on the margins of the graben. The paleobathymetric profile created by regional and local tectonics dictates facies types. Maps and seismic profiles of Rift Phase 1 indicate that the relatively simple tectonics led to uniform sediment and facies distribution. During Rift Phase 2, the development of numerous sub-basins creates heterogeneous facies distribution. Fault escarpments prevent progradation into the graben; sediments accumulated on the edge of fault escarpments fail and are redeposited as gravity deposits at the base of the escarpment. The seven second order cycles are subdivided into fourteen minor facies cycles that are called third order cycles. These cycles can be bound by either unconformities forming sequences or maximum flooding surfaces forming regressive-transgressive facies cycles. The cause of third order facies cycles is unclear. Third order cycles are more sensitive to sediment supply. Source area dictates sediment types. Uplift and erosion of Paleozoic and Triassic sandstones yield sand-rich sediments, while erosion of Caledonian schist yields shale-rich sediments.