Late Carboniferous-Early Permian structural development of the Ringkøbing-Fyn High and adjacent Norwegian-Danish Basin

Abstract

A grid of 2D regional seismic lines from six different surveys together with Bouguer gravity anomaly data and borehole data sets have been used to get an insight of the structural development of the Ringkøbing-Fyn High and the nearby Norwegian-Danish Basin. The Ringkøbing-Fyn High consists of relatively shallow basement rocks as compared to the Norwegian-Danish Basin which is cut by the approximately north-south trending Horn Graben. The western fraction of the high is bounded by the Central Graben in the West and the Horn Graben in the East which is characterized by ESE-WNW to E-W to ENE-WSW to NE-SE trending extensional faults, displaying half-graben structural settings. Mainly normal faults belong to an early Permian rifting event based on the stratigraphic dating method. However, extensional faults related to a rifting phase older than Permian extension is limited to southwestern part of the Ringkøbing-Fyn High. The normal faults over the Ringkøbing-Fyn High and the adjoining Norwegian-Danish Basin can be classified as planar to slightly curved to listric in geometry. Mostly extensional faults are restricted below the top Rotliegend Group with minor reactivation in the Triassic and the mid-late Cretaceous. The Triassic reactivation is concentrated in the proximity of the Horn Graben. The thickness of the Palaeozoic sedimentary succession over the Ringkøbing-Fyn High is much greater in the northern half-grabens where it reaches up to 1500 (ms) twt while towards the southern half-grabens it reduces to 450 ms (twt). This varied thickness is accredited to differential uplift of the Ringkøbing-Fyn High. During uplift, maximum erosion of the Palaeozoic strata took place in the central part of the Ringkøbing-Fyn High displaying a dome shape while the northern side experienced relatively less erosion. Several regional unconformities have also been interpreted in the study area which include Rotliegend, Saalian, Base Jurassic and Base Cretaceous unconformities. Seismic interpretation shows that early Permian rifting and uplift of the Ringkøbing-Fyn High might have been contemporaneous in the study area. This positive structure remained relatively uplifted until early late-Triassic while the Norwegian-Danish Basin experienced subsidence after rifting which is inferred from the thick late Permian and Triassic succession. A thin late Triassic succession is found to be present at places over the Ringkøbing-Fyn High showing the start of deposition over the structure. The uplift of the Ringkøbing-Fyn High is attributed to the Skagerrak Centered Large Igneous Province (SCLIP) due to its best chronological relevance with stratigraphic dating of the faults. Later in late Cretaceous, less thickness of the Chalk Group over the Ringkøbing-Fyn High demonstrates that the structure has tolerated slight uplift during this time interval due to unknown reason. The study also constrains the northern extent of the Ringkøbing-Fyn High based on the Zechstein salt termination line and extensional faults on the northern flank of the Ringkøbing-Fyn High.