Anatomy of the seaward steps and seaward termination of the Brent clastic wedge

Abstract

ABSTRACTThe northward and seaward outbuilding of the Middle Jurassic Brent delta in the Northern North Sea, comprising from the base and upwards Rannoch, Etive and Lower–Middle Ness Formations, displays a stacking pattern of coastal facies tracts described as ‘steps and rises’. Each ‘step and rise’ is formed in response to stratigraphic base‐level (A/S ratio) changes and defines high‐frequency cycles within the Brent Group. During a decreasing A/S ratio regime shoreface progradation dominated and formed the wave–storm‐dominated Rannoch facies tract with a rather flat shoreface trajectory curve resembling a step. A succeeding change to an increasing A/S ratio regime led to a transgression and stratigraphic rise (step‐up) due to aggradation of tidal bay and lagoonal–barrier complexes (Etive facies tract) on top of the previous deposited shoreface. This pattern was formed both due to sediment volume partitioning comprising spatial displacement of depocentres through time and facies differentiation involving changes in coastal geomorphology and dominant deposited facies tract. In the northern areas, where the Brent seaward stepping and the entire Brent clastic wedge terminates seaward, a vertical stack of Etive and Rannoch facies tracts occurs. The Rannoch facies tract has the northernmost extension and the termination of the Brent clastic wedge comprises a vertical stack of shoreface and shelf deposits. The aggradational nature of the tidal bay–barrier sandstone complex of the Etive facies tract explains the seaward thinning from 30 to 40 m to zero sandstone thickness within a short distance (a few kilometres). Facies tract mapping, architectural relationships and mass balance considerations do not indicate that the Etive facies tract was formed due to ‘forced regression’ or ‘lowstand’ developments. Instead, the basal surface of the Etive facies tract was probably formed by transgressive erosion. The embayments created by the transgression were filled by aggradational tidal bay and barrier complex deposition of the Etive facies tract.The seaward stepping, lower Brent Group, shows similarities to outcrop analogues of the Cretaceous, Mesa Verde Group in the Western Interior Seaway and the Palaeogene, Battfjellet Formation on Spitsbergen, supporting a model with facies differentiation within high‐frequency cycles, and that the large‐scale stratigraphic development is controlled by the stacking of these cycles.