Structural development of the Fennoscandian Border Zone, offshore Denmark

Abstract

The Hans-1, Sæby-1 and Terne-1 wells are located within the Danish part of the Fennoscandian Border Zone and provide significant new data pertaining to the evolution of this important tectonic belt. All three wells encountered Palaeozoic and Mesozoic rocks. The Cambrian to Lower Silurian deposits can be closely correlated to sections known from Bornholm and Scania, and tectonic activity within the Fennoscandian Border Zone cannot be verified during this time interval. The thick Upper Silurian shales were probably deposited in a rapidly subsiding foreland basin, marginal to the speculative Caledonian deformation front which lies south of the Ringkøbing-Fyn High. A major hiatus comprises the Devonian and Early Carboniferous. Upper Carboniferous sediments were deposited before a phase of rifting. Upper Carboniferous intrusive and extrusive volcanic rocks suggest tectonic activities, heralding the rifting phase. Extensive block faulting occurred in the border zone during the Rotliegende and more than 650 m of reworked volcanic rocks were deposited in an alluvial fan environment. Syndepositional erosion created a major unconformity before the deposition of post-rift Zechstein sediments. During the Mesozoic the Fennoscandian Border Zone was divided into two zones: the Sorgenfrei-Tornquist Zone to the west and the Skagerrak-Kattegat Platform to the east. The Sorgenfrei-Tornquist Zone continued to experience tectonic activity in Triassic and Middle Jurassic times. The zone has experienced an uplift of 1700–2000 m during Late Cretaceous to Early Tertiary inversion tectonics and the Late Tertiary uplift of Fennoscandia.