Oslofjorden Supergroup

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Oslofjord Supergroup consists of sedimentary rocks deposited from the middle of the Cambrian to the end of the Silurian in the area around the Oslofjord. These rocks stretches from Langesund and Porsgrunn in the south to lake Mjøsa and Brumunddal in the north. This area is also known as the Oslo Graben, a geological area with high volcanic activity in the late Carboniferous and during the Permian (310-250 Ma.). This volcanic activity produced a thick layer of hard igneous rock that covered the softer sedimentary rocks. With a hard cap of igneous rock above protecting the Oslofjord Supergroup from erosion, the sedimentary rocks survived the ensuing ice ages making it to today. This great variety in rock types and geological phenomena that can be seen in the Oslo Graben has made it famous among geologists worldwide.

The sedimentary rocks in the Oslo Graben was first described by Theodor Kjerulf in 1857. Kjerulf divided the marine part of the strata in 8 stages. Waldemar C. Brøgger furthered his work with a thorough description of the Ordovician stages, stages 3 to 5 in the figure, in 1882. In 1908, Johan Kiær described the upper part of the marine strata in detail. Kiærs descriptions resulted in a new total of 9 stages and multiple substages a, b, c, etc.. The Ringerike Sandstone was long belived to be part of the lover Devonian, but was eventually identified as upper Silurian and became stage 10. Dividing the strata in this way is called chronostratigraphy, a division based on the age of rocks. To achieve this they looked at the fauna present in the rocks, in other words which fossils is contained in the rocks. To make divisions for such a huge area using based on fauna is not without its challenges. At one point in history, a part of the Oslo Graben may have been shallow water whilst another part were deep ocean. Life in the deep ocean is generally not the same as that in shallow waters making it hard to pin down an age.  So in the modern division of the Oslofjord Supergroup stages has phased out and a lithostratigraphic division with formations have been adopted. The advantage with formations is that from the name one have an idea of where, and that any ages obtained from the formation of course will be valid for said formation. Even so the use of stages persists, as it is way more intuitive. As an example, we might be able to infer something about the lateral relation between the Solvik Formation and Solvang Formation based on their names, but not much else. And age is often one of the most important things. So if you instead are using stage 6 and stage 4bδ you instantly know that stage 6 is higher than 4bδ and therefore younger. Maybe you also know that the transition from stage 5 to 6 is the transition from Ordovician to Silur. A transition that marks one of the greatest mass extinctions events on earth. So the old use of stages is still used as a supplement to the formation.

The current definitions of the formations in the Oslofjord Supergroup are found in five articles. The oldest definition is from Turner (1974), who presents the three formations equivalent to stage 10. In 1983 Worsley et.al. described the marine formations from Silur. Whilst the Ordovician formations and the Alum Shale Formation were described in 1990 by Owen et.al. Formations from the lower Cambrian is only present at select locations in the Oslo Graben, as the ocean had yet to cover the whole area. Determining what is Cambrian and what is pre-Cambrian have been difficult, due to a high degree of deformation and a lack of fossils. Stages 1a and 1b was set by Vogt to be the first appearances by Discinella Holsti and Holmia Kjerulfi respectively (p. 362, 1924), two species of trilobites. In 2007, Nielsen & Schovsbo released an article describing Cambrian strata across southern Scandinavia. There they define the Ringstrand Formation in the lake Mjøsa district. The new Brennsætersag Member corresponds with Discinella Holsti and thus 1a, and Tømten Member with Holmia Kjerulfi and thus 1b. Tømten Member is in older articles referred to as «Holmia shale». Nielsen & Schovsbo also places the upper part of the underling Hedemark Group as Cambrian, making the colloquial use of Cambrian-Silurian strata as equivalent to Oslofjord Supergroup wrong. The latest update to be used in the figure on the right is from 2017 when Bockelie et.al. subdivided the Langøyene Formation into members.

Oslofjord Supergroup would be about 2,5 km thick if the entire series from lower Cambrian to upper Silurian where located in one place. The figure on the right shows the strata of Oslofjord Supergroup using the most known formations from each stage. There are however no place you can go and find the series as shown in the figure.  Most formations does not stretch across the entire Oslo Graben. Take for example Langøyene Formation an up to 55 m thick sandstone formation on the inner islands of the Oslofjord. This formation gradually thins towards the west. When it reaches Asker and the mainland it is completely replaced by the Langåra Formation. The Langåra Formation in the west is at most 35 m thick and yet it corresponds to both the upper part of Husbergøya Formation and most of Langøyene Formation in the east. So the difference in thickness can vary greatly as the sedimentation rates from area to area in often varied. Some places may also lack formations from a given stage, as there may have been no sedimentation at that time or later event may have eroded them away. In Oslo the Alum Shale Formation directly overlays pre-Cambrian gneiss and granites. Whilst in Lake Mjøsa district the Alum Shale Formation overlays the Ringstrand formation. Under The Ringstrand formation is the Hedmark Group, a sedimentary group mostly from the Proterozoic. In addition to varying sedimentation rates across the graben, the formations are also to varying degrees deformed from the Caledonian orogeny making it even harder to ascertain their thickness. 

Below is a timescale for the Oslofjord Supergroup. Ordovician times are taken from Nielsen et al. (2023). 

Cambrian times for the Ringstrand formation as it is defined by Nielsen & Schovsbo (2007) is estimated based on the literature I have found for its members. Høyberget et al. (2023) gives a time interval for the youngest member, the Skyberg member. Høyberget et al. (2015, 2023) places the Tømten member in the Holmia kjerulfi trilobite zone, which is consistent with older literature. Assuming these are equivalent in time, I have used the times for Holmia kjerulfi given by Peng et al. (2020) fig. 19.2 for the Tømten member. The Evjevik member fills the gap between Tømten and Skyberg members. Which is mostly equivalent with the Ellipsostrenua spinose zone as shown in Høyberget et al. (2023), but there is a gap between that trilobite zone and the stated start of Skyberg member so the boarder are uncertain. The last two members, the oldest ones, is shown as being partly in Schmidtellus mickwitzi zone by Høyberget et al. (2023), but the lower border is unknown. Again, I have assumed these members to equivalent with the entire trilobite zone and used the times given in Peng et al. (2020).

By using fig. 14 and 15 in Nielsen & Schovsbo (2015) I interpreted that the «Hawke Bay Event», a period of lower ocean levels, lasted too about 507,5 Ma in the north (Mjøsa Allochthon, rocks that originates from north of the coast of Møre). In the south by Slemmestad, the ocean don’t return until the Paradoxides paradoxissimus trilobite zone (ca. 505,5 Ma in Peng et al. (2020), but given as 506 Ma in Nielsen & Schovsbo (2015))

Silurian times is found by using the graptolites zones given by Bruton et al. (2010) in fig. 6 and the corresponding times given in Melchin et al. (2020) fig 21.2. This is done under the assumption of continuous deposition. For those graptolites in Brutton et al. (2010) that is not given in Melchin et al. (2020), I went via the British graptolites found in Zalasiewicz et al. (2009) to find the correct zones. Chronostratigraphy is from Cohen et al. (2013, updated v2024-12).