Mesozoic evolution of the Jæren High area, Norwegian Central North Sea

Abstract

The Jæren High area is located in a structurally complex area where the Central Graben, the Ling Depression, the South Viking Graben and the Witch Ground Graben converge. Wells drilled on the Jæren High indicate rapid changes both in facies and thickness of the Mesozoic sequence. The evolution of the region during Mesozoic times was considerably influenced by the thickness and the distribution of the Zechstein salt. Arches underlain by thick Triassic strata are separated by elongated grabens or troughs where mainly late Jurassic strata rest on ridges of Zechstein salt. At least three models are possible to explain the distribution of Mesozoic strata in grabens or troughs on the Jæren High. (1) Thin-skinned extension: a relatively thin Zechstein sequence overlain by Triassic strata was faulted, mainly in mid-Jurassic times. The salt started to pierce the footwalls resulting in the development of grabens separating the footwall and hangingwall blocks. These grabens trapped late Jurassic sands and shales, while the Triassic highs suffered erosion and non-deposition. (2) Salt dissolution: this model is based on the dissolution of Zechstein salt near to the surface or in the subsurface. During early to middle Triassic times salt started to move due to the weight of its overburden. Triassic depocentres and salt ridges developed. Middle and late Jurassic salt dissolution took place due to exposure of the salt at the surface, creating accommodation space for late Jurassic deposition of sands and shales. (3) Combined tectonism and salt dissolution: this model combines both tectonism and salt dissolution. Triassic and Jurassic extension resulted in uplift and rotation of the high. Subsequent erosion resulted in exposure of the salt along elongated ridges and salt dissolution took place due to salt exposure at the surface creating accommodation space for late Jurassic deposition. The third model, a combination of tectonism and salt dissolution, is the preferred model as it can explain the particular Mesozoic palaeogeography of the Jæren High.