Seismic profile from the East Shetland Platform, with prominent reflectors probably from the  top Balder Formation, top Chalk/Shetland Group,  top Cromer Knoll Group and the top Kimmeridge Clay Formation.


Tertiary of the northern North Sea

At the opening of the Tertiary chalk sedimentation continued on the northern North Sea. Starting around 63 Ma, the East Shetland Platform was uplifted and tilted towards the south-east. Rapid erosion generated sands that were transported by large river systems to deep water submarine fans. These sands are up to 1 km thick in parts of the Viking Graben.

The Balder Formation records the most intense phase of Tertiary volcanic activity seen in the North Sea and may be the product of an outburst of volcanic activity triggered by the start of ocean floor spreading between Faeroe and Greenland.. In the lower part of the unit there are hundreds of individual volcanic ash layers, mostly only millimetres to centimetres thick but forming a total thickness of over 8 m at the northern end of the North Sea Basin, and known informally as the Balder Tuff. This ash unit is an important marker throughout the North Sea as it produces a distinctive gamma or sonic bow on well logs. The total ash thickness declines toward the south-east, but ashes are found as far away as southern England, Germany and Denmark. The ashes are of theoleiitic-basalt composition and were probably erupted from a large volcano, somewhere along the North Atlantic rift, north-west of Britain.

The cessation of volcanic activity in the early Eocene was accompanied by crustal cooling and subsidence. Sedimentation rates fell in the North Sea and muds replaced sands in the main sedimentary basins. Coastal facies represented by thin marine sandstones occur around the fringe of the East Shetland Platform. Uplift may have resumed around 3 Ma, with increased sediment supply from Scotland and Shetland indicating accelerating erosion under cooling climates.