Syn-rift sedimentary architectures in the Northern North Sea
Authors: Ravnås R. , Nøttvedt A. , Steel R. J. , Windelstad J. 
Published: April 2008

From Permian to Jurassic times the northern Viking Graben and adjacent platform areas experienced multiple rifting, the Permian-early Triassic and middle-late Jurassic rift episodes, separated by an intervening middle Triassic-middle Jurassic inter-rift period dominated by relative tectonic quiescence. The associated syn- and inter-rift strata show large variations in sedimentary architecture as a result of temporal and spatial variations in tectonic deformation and subsidence, sediment supply, climate and accommodation creation.

The Permian-early Triassic syn-rift succession is believed to consist predominantly of non-marine, arid to semiarid, aeolian, sabkha, alluvial and lacustrine strata, probably interbedded with marine strata on the Horda Platform and in the Viking Graben.

The middle Triassic-middle Jurassic experienced several subsidence stages which, together with climatic variations, exerted a major control on the periodic outbuilding and retreat of rift marginal, alluvial and shallow marine clastic wedges. Evidence for fault block rotation suggests that the subsidence was caused partly by minor extensional stages. As such, the middle Triassic-middle Jurassic does not fit the type of development assumed to be typical for either post- or pre-rift basins. Hence, the notation
inter-rift
is assigned to this period and the associated succession.

The middle-late Jurassic rift episode was characterized by multiple rift phases separated by intervening stages of relative tectonic quiescence. The syn-rift infill is mixed non-marine and marine and consists of fluvial through shallow marine and shelfal deposits to deeper marine sediment gravity flow and (hemi-)pelagic strata. At the larger scale, related to the entire middle-late Jurassic rift episode, the syn-rift infill in general shows a two-fold sandstone-mudstone lithology motif, typical of underfilled rift basins. At the intermediate scale, related to single rift phases, threefold sandstone-mudstone-sandstone, twofold sandstone-mudstone and single mudstone lithology motifs are present, typical of sediment overfilled/sediment balanced, sediment underfilled and sediment starved rift basins, respectively. The spatial and temporal variations in the syn-rift infill reflect relative distance to the rift basin hinterland areas (which had a large sediment yield potential) and overall increased tectonic subsidence and enhanced rift topography as the rift basin evolved.

This suggest that the tectonostratigraphic evolution of the northern North Sea rift basin can be viewed at several scales: at the largest scale the rift basin evolved through multiple rift episodes, which commonly had a duration of several tens of Ma. The rift episodes are separated by inter-rift periods. Rift episodes are subdivided into intervals representing distinct rift phases. These rift phases were separated by tectonic relatively quieter intervals, here referred to as tectonic quiescence stages. Inter-rift periods are subdivided into prolonged tectonic quiescence intervals separated by short-lived rift stages or minor rift phases. Distinct rift phases and inter-rift tectonic quiescence intervals commonly represent periods of a few to 10+ Ma, and correspond to second-order sequences or ‘megasequences’. At the smaller scale, syn-rift successions can be subdivided into packages related to distinct rotational tilt event or faulting events (deformation spans), representing hundreds of ka to few Ma and corresponding to third-order sequences. Solitary, large-magnitude faulting events (deformation clines) are likely to exert a major control on high frequency base- or sea-level fluctuations and thus on the development of higher-order sequences. However, such a control is difficult to prove and can probably only be recognized in sub-basins with abundant wells and a dense well spacing.