The Oslo Rift

During the Carboniferous period, Norway was part of the supercontinent Pangaea. Pangaea was formed during the Variscan orogeny, when Laurasia (North America, Baltica and Siberia) collided with the supercontinent Gondwana, between 370–290 Ma. This collision built up stress in the rocks surrounding the new mountain range, which resulted in rifting—such as the Oslo Rift. The Oslo Rift formed over a period of approximately 65 million years, from the late Carboniferous through the Permian and into the early Triassic. The rifting process has been divided into six phases (Larsen et al., 2008).

Phase 1 – Prerift (ca. 315–300 Ma)

Before rifting truly began, the area experienced subsidence, creating a basin in an otherwise relatively flat landscape (Sundvoll & Larsen, 1994). In this basin, the Asker Group was deposited. The oldest formation in the Asker Group is the Kolsås Formation, consisting mainly of siltstones and shales, interpreted as floodplain and shallow lake deposits (Dons & Györy, 1967; Henningsmoen, 1978). This was followed by the Tanum Formation, which contains more sandstones and quartz conglomerates, interpreted as floodplains, deltaic deposits, river channels (Dons & Györy, 1967; Henningsmoen, 1978) and marine beaches (Larsen et al., 2008).

The sediments are largely derived locally from the underlying Ringerike Groupof Silurian age, but there are also sediments that appear to be sourced from Avalonia. Avalonia was a smaller continent that became part of the Variscan Mountains when Pangaea formed. Today these rocks forms parts of the bedrock from southern England to northern Germany. It is likely that some rivers flowed northward from the mountains in the south and into the Oslo Rift (Dahlgren & Corfu, 2001; Kristoffersen et al., 2013). The quartz pebbles in the Tanum Formation conglomerates are believed to have originated from the west, around Telemark (Dons & Györy, 1967).

Fossils make it possible to estimate the age of the Tanum Formation. Based on fossil and mineral content in the deposited rocks, the sea appears to have encroached into the area late in the Bashkirian (323.4 ±0.4 to 315.2 ±0.2 Ma) and remained until late in the Moscovian (315.2 ±0.2 to 307.0 ±0.1 Ma), so approximately 318 to 308 Ma. Freshwater fossils from the upper parts of the Tanum Formation correlate with the regional Stephanian stage (approx. 307.5 to 300.5 Ma). I have not found any direct age estimates for the Kolsås Formation.

There are no signs of volcanic activity at the surface during this phase, but some of the earliest sillsintruding the Cambro-Silurian rocks are dated to this time. For example, some Mæanite sills in Oslo are dated to 304 ±8 Ma (Sundvoll et al., 1992).

Phase 2 – Initial rifting and basaltic lava (ca. 305–299.7 Ma)

Phase 2 is characterized by basaltic lava reaching the surface. Near Oslo, this first appears in the Skaugum Formation within the Asker Group. The Skaugum Formation mainly consists of volcanic sediments of basaltic origin. Specifically, much of the sediments are of alkaline olivine basalts found only in the southern Oslo Rift, suggesting northward transport (Larsen et al., 2008). Generally, rifting began in the south and progressed northward.

Some of the oldest dated basalts are the Brunlanes Basalt, from south of Larvik, with an age of 300.4 ±0.7 Ma(Corfu & Dahlgren, 2007). Around Oslo, the Kolsås Basalt [often referred to as B1] lies above the Skaugum Formation. It has been dated to 291 ±8 Ma (Sundvoll & Larsen, 1990), but since it lies below RP1, which is dated to 299.7 ±0.4 Ma (Corfu & Larsen, 2020), the Kolsås Basalt must be older than 299.7 ±0.4 Ma. The Kolsås Basalt is likely about 300 Ma.

Phase 3 – Climax, fissure volcanoes with rhomb porphyry lava (ca. 299.7–275 Ma)

The rhomb porphyry RP1, the Kolsås type, dated to 299.7 ±0.4 Ma (Corfu & Larsen, 2020), marks the beginning of the climactic phase of the Oslo Rift. During this period, the lavas of the Krokskogen Group (RP1–RP11) and RP12 in the Bærumsmarka Group were deposited, with minor basalts between some of the flows. Including subflows named a, b, c, x, the historically defined RP1–12 have been interpreted as 20 distinct lava flows. However, recent drilling shows that in just what was historically RP1–4, there are 38 separate lava flows(Svensen et al., 2024).

In addition to lava flows, sandstones and conglomerates are found between the flows. Some, such as the sandstone between the Kolsås Basalt and RP1, are interpreted as wind-blown dunes in a desert climate (Dons & Györy, 1967). There are also conglomerates and mudstones interpreted as deposits from alluvial fans and small lakes (Larsen et al., 2008). The thickest conglomerate is the Migartjern Conglomerate, a polymictic block conglomerate about 400 m thick, interpreted as a canyon filled with sediments (Larsen et al., 2008).

The Larvikite batholiths in Vestfold, which are the plutonic equivalents of the rhomb porphyries, have been dated to 299–288 Ma (Rämö et al., 2022).

Phase 4 – Mature rift, central volcanoes and caldera formation (ca. 280–265 Ma)

As the rift matured, large central basaltic volcanoes developed. This stage is comparable to the current state of the East African Rift (Larsen et al., 2008). The East African Rift is also one of the few places outside the Oslo region where rhomb porphyries are found. As basaltic magma chambers emptied, the overlying rock collapsed, forming large calderas.

Eruptions of rhomb porphyries and basalts continued during this phase. Remnants near Oslo are preserved in the calderas, for example: The Bærumsmarka Group, including basalts B3 and RP12a–RP13bin the Bærum Caldera. In the Svarten and Øyangen Calderas, further north, The Vikseter Groupcontains basalts and RP13–RP17, plus ignimbrites and trachytes. In the east The Alnsjøen succession of lavas, ignimbrites, and sedimentary rocks are preserved in the southern part of The Nittedal Caldera (Naterstad, 1978).

Over time the eruptions became more felsic and explosive, resulting in many tuffs, ignimbrites, and tuff breccias in the calderas. The Alnsjøen succession in the southern Nittedal Caldera is perhaps the one of the best studied, see Whattam et al. (2024).

Granitic batholiths such as the Drammen Granite, dated to 287–272 Ma (Haug, 2007 in Corfu & Larsen, 2020), also formed during this period. Some gabbroic plugs are also dated to 266–265 Ma (Neumann et al., 1985).

Phase 5 – Aftermath, the syenitic batholiths (ca. 270–250 Ma)

Toward the end of the Oslo Rift, large alkaline batholiths were emplaced, especially north of Oslo. The Nordmarka–Hurdal syenite complexconsists of: Alkali feldspar granite(ekerite), syenites (Grefsen syenite) and Alkali feldspar syenite(nordmarkite). The nordmarkitesare the youngest rocks at 252 ±3 Ma, while the Grefsen syenites are slightly older at 255 ±4 Ma (Sundvoll & Larsen, 1990).

These rocks intruded into older lava sequences, but there is no evidence of surface eruptions. Any extrusive equivalents, if they existed, have likely eroded away by glaciations.

Phase 6 – Final phase, the last intrusive rocks (ca. 250–241 Ma)

The final phase of the Oslo Rift is marked by granitic intrusives and dikes. For example, Grorudite dikes date to 249 ±3 Ma (Sundvoll & Larsen, 1993).

Referanser

Corfu, F. & Larsen, B.T. (2020). U–Pb systematics in volcanic and plutonic rocks of the Krokskogen area: resolving a 40 million years long evolution in the Oslo rift. Lithos, 376–377(105755), 1-13. https://doi.org/10.1016/j.lithos.2020.105755

Dahlgren, S. Corfu F. (2001). Northward sediment transport from the late Carboniferous Variscan Mountains: zircon evidence from the Oslo Rift, Norway. Journal of the Geological Society, 158(1), 29–36. https://doi.org/10.1144/jgs.158.1.29

Dons, J. A., Györy, E. (1967). Permian sediments, lavas, and faults in the Kolsås area W of Oslo. Norwegian Journal of Geology, 47, 1. https://njg.geologi.no/publications/permian-sediments-lavas-and-faults-in-the-kolsas-area-w-of-oslo/

Henningsmoen, G. (1978). Sedimentary rocks associated with the Oslo Region lavas. In The Oslo Paleorift: A Review and Guide to Excursions (eds J. A. Dons & B. T. Larsen), pp. 17–24.

Universitetsforlaget, Trondheim, Norges Geologiske Undersøkelse Bulletin 337.

Kristoffersen, M., Andersen, T., & Andresen, A. (2014). U–Pb age and Lu–Hf signatures of detrital zircon from Palaeozoic sandstones in the Oslo Rift, Norway. Geological Magazine, 151(5), 816–829.

https://doi.org/10.1017/S0016756813000885

Larsen, B.T. (1978). Krokskogen lava area, i J.A. Dons and B.T. Larsen (Red.) A review and guide to excursions: Norges Geologiske Undersøkelse, Bulletin 337, 143–162.

Larsen, B.T., Olaussen, S., Sundvoll, B. & Heeremans, M. (2008). The Permo-Carboniferous Oslo Rift through six stages and 65 million years. Episodes, 31(1), 52–58. https://doi.org/10.18814/epiiugs/2008/v31i1/008

Neumann, E-R., Larsen, B. T. & Sundvoll, B. (1985). Compositional variations among gabbroic intrusions in the Oslo rift. Lithos, 18, 35-59. https://doi.org/10.1016/0024-4937(85)90005-2

Rämö, O.T., Andersen, T. & Whitehouse, M.J. (2022). Timing and petrogenesis of the PermoCarboniferous Larvik Plutonic Complex, Oslo Rift, Norway: New insights from U–Pb, Lu-Hf, and O isotopes in zircon. Journal of Petrology, 63, 1–29. https://doi.org/10.1093/petrology/egac116