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Abstract


Pangea: Global Environments and Resources — Memoir 17, 1994
Pages 175-197
Regional Paleogeography and Tectonics

The Upper Carboniferous-Permian Oslo Rift; Basin Fill in Relation to Tectonic Development

Snorre Olaussen, Bjorn T. Larsen, Ron Steel

Abstract

The Permo-Carboniferous Oslo Rift, an en echelon graben system, was created by east-west extension partly caused by dextral movement along the Sorgenfrei-Tornquist-Teissure Zone. This zone was most likely activated by a late compressional event in the Variscan segment of the Hercynian Orogenic cycle. The Oslo Rift is thus an example of local suturation during the creation of the Pangea super-continent.

The north-south trending Oslo Rift system consists of the onshore Oslo Graben and the offshore Skagerrak Graben. The basin-fill of the onshore Oslo Graben is characterised by large volumes of volcanics and only minor sedimentary rocks, for which a six phase development model from pre-rift to the termination phase has been derived. The pre-rift and initial rift phases (Late Carboniferous) coincided with the late Variscan stage of the Hercynian Orogeny. The basin fill which accumulated prior to the rift basin formation, the pre-rift phase, consists of a thin alluvial and deltaic succession with a marine limestone of (?) Moscovian age. The initial rift phase is recorded in the Vestfold Graben with more than 1500 m of alkali basalt lavas, while in the north, close to the transfer zone in the initial Oslo Graben, the basin fill comprises Gzelian lacustrine delta and volcaniclastic alluvial fan deposits capped by a single tholeiitic basalt. The synrift phase (Early Permian) is divided into two sub-phases that were characterized by plateau lavas, fissure eruption and normal faulting as well as formation of central volcanos and caldera collapse. The basin fill in the syn-rift phase, besides volcanics and thin but widespread continental deposits that are intercalated with the plateau lavas, developed as thick alluvial fan deposits along the master fault or locally inside the rift valley along escarpments bounding a volcanic accommodation zone, and as thick eolian and wadi deposits in the northern part of the Oslo Graben. The two last recorded phases (late Early and Late Permian) of graben development were the emplacement of young syenitic and granitic batholiths.

The highly volcanic Oslo Rift is comparable with the Kenya Rift of the East African rifts and has some similarities to the Rio Grande Rift. In many periods the high production rate of extrusives in the Oslo Graben was enough to fill the relief created by extension, thus explaining the small amount of syn-rift sediments.

Although only known from seismic and geophysical data, the basin-fill development of the offshore Skagerrak Graben is similar to the Oslo Graben, but probably with less volcanism.

A post-rift sedimentary cover as a response to thermal cooling has probably been developed in the offshore Skagerrak Graben. A previous existence of post-rift basin fill in the onshore Oslo Graben is unclear. Post-rift sediments may also have been developed there, but at different scale and time in comparison to the Skagerrak Graben due to variations in duration and activity of the magmatism. The assumed post-rift infill is dated as Late Permian (offshore graben) and Triassic/ Early Jurassic (onshore graben) by onshore age determination and by seismic/well correlation between neighbouring basins. Subsequent uplift and erosion of the Oslo Rift and surrounding areas are suggested to have occurred in Early Triassic and “mid” Jurassic, with stripping of the post-rift basin fill in the offshore Skagerrak Graben taking place in Early Triassic, whereas “mid” Jurassic uplift removed the possible post-rift basin fill and much of the syn-rift basin fill of the onshore Oslo Graben.

Potential source rocks are burned out in the Oslo Graben. However the Skagerrak Graben may yet prove to be hydrocarbon bearing.


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