SEDIMENTATION HISTORY AS AN INDICATOR OF RIFT INITIATION AND DEVELOPMENT - THE LATE BAJOCIAN-BATHONIAN EVOLUTION OF THE OSEBERG-BRAGE AREA,NORTHERN NORTH-SEA

The Tarbert Formation in the Oseberg-Brage area consists of shoreline sandstones and lower delta-plain heterolithics which basinward interdi gitate with offshore sediments of the lower Heather Formation and land ward with fluvio-deltaic deposits of the upper Ness Formation. The Lat e Bajocian-Tarbert and lower Heather Formations form three wedge-shape d, regressive-transgressive sequences which constitute offset, landwar d-stepping shoreline prisms. Initial gentle rotational extensional fau lting occurred during the deposition of the uppermost Ness Formation a nd resulted in basinfloor subsidence and flooding across the Brent del ta. Subsequent extensional faulting exerted the major control on the d rainage development, basin physiography, the large-scale stacking patt ern, i.e. the progradational-to-backstepping nature of the sequences, as well as on the contained facies tracts and higher-order stacking pa ttern in the regressive and transgressive segments. Progradation occur red during repetitive tectonic dormant stages, whereas the successive transgressive segments are coupled against intervening periods with hi gher rates of rotational Faulting and overall basinal subsidence. Axia l drainage dominated during the successive tectonic dormant stages. Tr ansverse drainage increased in influence during the intermittent rotat ional tilt stages, but only as small, local (fault block) hanging-wall and footwall sedimentary lobes. Rotational extensional faulting was a ccompanied by footwall uplift which resulted in erosion of older Brent Group deposits on the footwall highs. Several unconformity strands, w hich are coupled against intervening periods of rotational faulting or stepwise rotation, are mapped in the footwall of the Oseberg and Brag e Faults. The individual unconformity strands merge into a composite u nconformity in an updip direction on the fault blocks. The uppermost N ess, Tarbert and lower Heather Formations represent the initial to an early phase of the Middle Jurassic rifting. The intermediate-scale syn -rift sedimentary architecture of a single rift phase in such a mixed non-marine-marine rift basin is a threefold sandstone-mudstone-sandsto ne lithology motif. This motif also applies to shorter-term rotational tilt events superimposed on longer duration rifting events such as th e Late Bajocian-Early Bathonian rift phase.