The early to mid-Jurassic evolution of the northern North Sea

Abstract

Abstract The early to mid-Jurassic history of the northern North Sea is summarized as a complex transgressive-regressive-transgressive cycle controlled by variations in ‘long-term’ eustatic sea level rise, basin subsidence/margin uplift and sediment supply. The first of the transgressions occurred in the Hettangian when the Boreal Ocean spread southwards during a phase of fluctuating but generally increasing sea level rise. Offshore marine conditions developed in the north of the area during the Hettangian to Pliensbachian, while coastal to non-marine conditions developed around the ocean margins in the Unst Basin, the Beryl Embayment, the central Viking Graben and over parts of the Horda Platform. The coastal systems in the Beryl Embayment and central Viking Graben were drowned in the late Pliensbachian as a result of continued sea level rise. A widespread marine link was established with the Tethys Ocean to the south at this time. Sea level fall and/or basin margin uplift and erosion in the Aalenian led to the shedding of coarse clastic sediment transversely into the East Shetland Basin and the northern end of the Viking Graben. The main reservoir unit of the area, the Middle Jurassic Brent Group, formed in this way. A marine connection was maintained into the central Viking Graben area throughout the transverse progradation further north. Drowning of the progradational clastics and the start of the third major transgression began in the late Bajocian, but offshore conditions did not become fully established throughout the area until the Callovian or early Oxfordian.