Extension and salt tectonics in the East Central Graben

Abstract

Mesozoic structures in the East Central Graben were formed in an extensional tectonic environment by the complex interaction of deposition, basin extension, halokinesis and subsequent erosion. Detailed analysis of seismic data on the eastern flank of the Forties-Montrose High indicates that the Triassic section exhibits rafting: large fault blocks were translated, rotated and distorted on a ductile Zechstein salt substrate. Rafts can be defined by an empirical structural relationship between the Base Cretaceous Unconformity and the Top Zechstein surfaces. The rafts exerted important controls on the distribution of Triassic reservoir rocks and younger reservoir and source rocks. Halokinetic movement was essentially a passive process related to eastward basin subsidence, with salt upwelling between rafted Triassic blocks. Subaerial exposure following uplift during the Early to Middle Jurassic and contemporaneous erosion and deposition led to the development of significant landscape topography. Onlap during the Middle to Upper Jurassic has preserved features such as cuestas with significant fault-line escarpments capped by Skagerrak sandstones. Adjacent to these escarpments deep erosion resulted in incised valleys, where significant angular unconformities occur. Physical modelling has been used to determine the processes and sequence of events that led to the present form of the East Central Graben. A structural model has been developed which relates the timing and rate of deposition and extension, salt movement and subsequent erosion. The interaction of regional extension, development of Triassic rafts and passive salt movement has not been described before. The model represents an alternative to the conventional interpretation that the observed structural features in the area were primarily the result of salt withdrawal.