Contrasting styles of Late Jurassic syn-rift turbidite sedimentation: a comparative study of the Magnus and Oseberg areas, northern North Sea

Abstract

Contrasting types of sediment gravity flow and hemipelagite deposition are recorded by the Late Jurassic syn-rift successions in the Magnus and Oseberg areas in the northern North Sea. In both areas the Late Jurassic rift interval was characterized by recurrent tectonic phases. During the Late Oxfordian to Early Volgian, the Penguin halfgraben in the Magnus area received a significant amount of coarse clastics and can be regarded as an example of a well-fed, deeper-marine riftbasin. Periods with high rates of extensional faulting are characterized by isolated sediment gravity flow lobes which stack in an uneven, albeit downdip shifting or backstepping manner. The stratigraphic motif of these intervals is overall mud-prone, with coarse clastics represented by thick conglomerate-sandstone-mudstone aprons along the basin-bounding masterfaults. In contrast, periods with low rates of extensional faulting are characterized by basinwide, sheet-like, parallel-layered sediment gravity flow packages which infill the riftbasin in a progradational manner. From the Middle Volgian, the Penguin halfgraben became sediment-starved and shows a similar development to the Oseberg area. In the Oseberg area, the Late Jurassic syn-rift succession provides a typical example of a sediment-starved, deeper-marine riftbasin. Sediment gravity flow deposition in this area is limited to periods of high rates of extensional faulting. These coarser clastic depositional episodes mimic their counterparts in the Penguin halfgraben, being characterized by mud-prone strata with localized, isolated sediment gravity flow lobes and footwall derived clastic wedges along the basin-bounding masterfaults. In contrast to the well-fed basins, the intervening stages of tectonic quiescence in the sediment-starved basins are characterized by basinwide blanketing of clay, commonly of high source-rock potential. In both basins, the rotational tilt packages can be subdivided into an early, a climax and a late syn-rotational interval based on the architecture of their contained facies tracts. These depositional episodes reflect the temporal evolution of the riftbasins' catchment areas and their linked depositional systems. It is mainly the nature and amount of sediment supply during the following tectonic quiescence or post-rift stages that determine whether the riftbasin remained sediment-starved or became infilled. Simplified sequence stratigraphic models are of limited use in the analyses and lithology prediction of riftbasin infills, as individual halfgrabens often have their own specific infill history and hence characteristic syn-rift stratigraphic signature. The main controls on the variability of these deeper marine, syn-rift sediment gravity flow-hemipelagite packages are the style and rate of extensional faulting, sediment supply, the source area(s) sediment yield potential and calibre, and the halfgraben configuration and location with respect to the main hinterland areas.