Sequence stratigraphy and palaeogeography of the middle Jurassic Brent and Vestland deltaic systems, Northern North Sea

The regional development of the Middle Jurassic Brent Group (Northern North Sea) has been studied within a sequence stratigraphic framework, and four depositional sequences (Exxonian) have been recognized. The phases of lowstand, progradation, aggradation, retrogradation and drowning of the 'classical' Brent deltaic system and a renewed deltaic progradation (the 'Vestland deltaic system') are illustrated by cross-sections and regional palaeogeographic maps. The Bajocian Brent delta progradation and aggradation developed during a period of varying rate of relative sea-level rise, forming a continuous sheet-like package of delta front facies associations in front of heterogeneous delta plain deposits. The stacking pattern varies from strongly progradational near sequence boundaries at a minimum rate of sea-level rise to aggradational during periods of pronounced sea-level rise. No delta incision or lowstand deposition due to major relative sea-level falls is envisaged. The Early Bathonian delta retreat took place in retrogressive pulses, and was probably influenced by synsedimentary fault activity along the ancient Viking Graben structure, forming an estuary in the Southern Viking Graben and gradually drowning the deltaic system in the Northern Viking Graben. After the maximum flooding of the Brent deltaic system in the Early Bathonian a second, pronounced deltaic progradation (the 'Vestland deltaic system') took place from the Central Viking Graben to ca. 60 degrees 30'N. In the Middle Bathonian a tectonic phase initiated the retreat of the Vestland deltaic system and possible erosion of the Brent Group deposits on the crests of tilted fault blocks in the Northern Viking Graben. The deposition of the Brent and Vestland deltaic systems was terminated by a pronounced tectonic rifting phase in the latest Bathonian.