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The Tanum Formation, named after Tanum in Asker, comprises mainly lightcoloured conglomerates and calcareous sandstones, with or without thin shale partings. Up to about 5.5 m of fossiliferous, greenish or reddish shales with arenaceous intercalations occur within the upper half of the formation in the hillside above Semsvatn in Asker. The fragments in the conglomerates are usually less than 10 cm in diameter, but some exceeding 20 cm have been found. The fragments are subrounded to rounded with a medium to high sphericity and consist mainly of quartz and quartzite (commonly coated or pigmented throughout by hematite) and to a small extent also of quartz‑mica schist, limestone and shale (Dons & Györy 1967). Current bedding, graded bedding, scouring surfaces and channel fillings are common. Volcanic rock fragments have been identified in the Staverhaugen drill‑core (Elder & Kanes 1966). The Tanum Formation is the most widespread of the 3 formations and is up to 20 m thick in Asker.

The rather rich fossil flora and fauna in the section above Semsvatn consist of land plants (i.a. fern‑like leaves, Calamites spp., mostly C. undulatus, Asterophyllites equisetiformis, Cordaites principalis, Samaropsis holtedahli, Lebachia piniformis, Ernestiodendron filiciforme, E. arnhardtii), fresh water fishes (i.a. Pleuracantbus sp., Megalichtys sp., Amblypterus sp.) and fresh water bivalves (Palaeanodonta spp.) and conchostracans. (For references to fossil descriptions, see first page of this article). The fossil assemblage indicates a middle Early Permian age. Less well preserved fossils have been found elsewhere in Asker. About 0.5 km north of Holmestrand railway in Vestfold, poorly preserved plant fossils (Cordaites sp., Calamites sp.) occur between the earliest lava flows of the basal basalt (B1), but could be of the same age as those in the Tanum Formation.

The remarkable change in lithology from the Kolsås Formation to the Tanum Formation may reflect a change in topography, and as assumed by Dons & Györy (1967) this may have been caused by fault movements connected with the initiation of volcanic activity. These authors suggest that certain pebbles in the conglomerates came from the Precambrian Telemark area to the west, perhaps by a stepwise transport. Considerable precipitation, possibly intermittent, must have occurred in the source areas and possibly in the Oslo Region itself ‑ again perhaps related to the volcanic activity. According to Elder and Kanes (1966), the Tanum Formation lithology in the Staverhagen core suggests deposition in a point‑bar environment.