Quantitative analysis of Triassic extension in the northern Viking Graben

Abstract

It has been recognized for over a decade that large-displacement, pre-Jurassic faults are present in the Northern Viking Graben, part of the North Sea rift. These faults define a series of major fault-blocks below the more obvious Jurassic rift basin. We attempt here to quantify the amount of extension associated with this older rift event, which is probably of early Triassic age. Quantitative modelling of the Triassic rift and the succeeding period of thermal subsidence has been undertaken, using a combination of flexural backstripping and flexural-cantilever forward modelling. These techniques suggest that Triassic extension across the Horda Platform (Norwegian sector) reached c. 40% ( ß = 1.4). The consequences of this extension were deposition of a thick (>3 km) Triassic-Upper Jurassic syn-rift and post-rift sequence, prior to renewed, but minor, extension in the Late Jurassic-earliest Cretaceous. The thickness of the Viking Group reservoirs in the Troll area appears to have been almost entirely controlled by sediment loading during post-Triassic thermal subsidence. Jurassic extension on the Horda Platform was <5%, an order of magnitude less than the Triassic event. The Horda Platform is therefore principally an area of Triassic extension marginal to the main Jurassic rift further west. In the UK sector of the Viking Graben, Triassic structures are less obvious than those below the Horda Platform, because of Jurassic overprinting. They are, however, still present. Average Triassic extension across the East Shetland Basin was c. 15%, comparable with the magnitude of Jurassic extension in the same area. We believe that the Tern/Eider and Cormorant fault-blocks, with proven shallow basement, comprised a large eroded horst during the early Triassic, uplifted in the footwalls of major faults flanking the Magnus and Statfjord half-graben. During the Triassic, the Magnus half-graben was contiguous with the Unst Basin, now situated in the western footwall of the younger Jurassic basin. The presence of the Unst Basin suggests that Triassic extension occurred across the area that is now the northern Shetland Platform, continuing into the West Shetland area. Although the more obvious structures in the Viking Graben are Jurassic in age, the earlier Triassic event was equally as important in controlling the structural and stratigraphic history of the basin.