1. science
2. geology

One-day field excursion in the Oslo region.
By
Professor Hans Arne Nakrem,
Natural History Museum, University of Oslo.
[email protected]
The Oslo region (“Oslo Graben”) is famous for its Lower Palaeozoic sedimentary succession. These
rocks are mainly composed of shales, sandstones and limestones, often rich in fossils. The Upper
Palaeozoic rocks consist of Permian-?Triassic lavas as well as an up to 90 m thick Late Carbonifeorus
fossil-poor siliciclastic unit. The excursion aims at visiting some of the Lower Palaeozoic units which
show interesting sedimentary successions, fossils, as well as tectonic features.
The Oslo Region (Oslofeltet) is a geological structure that varies in width from 40 to 70 km and
extends approximately 115 km both north and south of Oslo, and covers an area of roughly 10 000
km2. It is bordered by Precambrian rocks to the east and west, and by the Caledonian nappes to the
north. The Oslo Region also extends out into the fiord to the south (the Skagerrak Graben). The
Lower Palaeozoic succession is approximately 2 500 m thick.
The sedimentary package was folded and thrusted during the Late Silurian (Caledonian orogeny), as
well as rifted during the Late Palaeozoic rifting phase. Local and regional thermal metamorphism is
evident due to the Late Palaeozoic magmatic activity. During the Permo‐Carboniferous the
Caledonian foreland sediments were down‐faulted in the Oslo Graben and covered by plateau lavas
(basalts and rhomb porphyrs). Locally Permian intrusions are seen (batholiths, dykes, sills).
Towards the end of the Permian, when magmatic and, perhaps also, tectonic events had finally
ceased in the Oslo area, these processes moved westward where they continued into the Triassic.
The climate during the transition from the Permian to the Triassic continued to be extremely arid
warm.
Localities to be visited (change in programme may, however, take place).
Bygdøy, Huk, Oslo
What to see:
Lower-Middle Ordovician dark shales and massive limestones. Sedimentary rocks with fossils
(trilobites, graptolites and orthocone cephalopods). Huk and Elnes formations.
Caledonian compressional tectonics (thrusts and folding), Permian intrusions.
Kalvøya, Bærum
Upper Ordovician rocks, “incised valleys” with channel infill, corals, lithoclasts. Langøyene Formation.
Some Permian dikes, with effect on surrounding rocks (metamorphosis).
Slemmestad, Røyken
Slemmestad centre. Precambrian basement overlain by Middle Cambrian conglomerate, alum shale
and bituminous black limestone. Agnostid and paradoxid trilobites. Permian sill.
Slemmestad football arena
Middle Ordovician carbonate rocks (Huk Formation) with endoceratid cephalopods (hundreds), trace
fossils, hardgrounds/omission surfaces, shales with trilobites.
Bjørkåsholmen and Djuptrekkodden, Asker
Lower and Middle Ordovician succession including the upper Alum Shale, Bjørkåsholmen, Tøyen, Huk
and basal Elnes formations. Alternating dark/black shales, limestones and nodular limestones.
Palynological data is published from these localities in Tongiorgi et al. (2003) who identified a
microflora comprising 52 taxa of acritarchs from the Tøyen Formation (Tremadocian−Floian). The
preservation is generally poor but most taxa were identified to the species level. It was shown that
samples from the Hunneberg Stage contain a mixed cold-water (elements of the messaoudensistrifidum assemblage) and warmwater (Aryballomorpha-Athabascaella-Lua assemblage) microflora.
Samples from the Billingen to lower Volkhov Stages contain species recorded from the Yangtze
Platform (South China), which is considered to be part of the cold-water realm (or ‘Mediterranean
Province’) located at high southern latitudes around the margin of Gondwana (Tongiorgi et al. 2003).
However, because the samples from the Oslo area lack the typical cold to cool temperate-water
indicators from Perigondwana, such as Arbusculidium, Aureotesta, Coryphidium, Striatorheca and
Vavrdovella, the flora was regarded as compatible with a mid-latitude position of Baltica during the
Floian–Darriwilian. Presumed ocean currents caused a climate warmer than in China at this time. In
contrast, the late Dapingian to early Darriwilian (late Volkhov to early Kunda) Baltic microflora
belongs neither to the cold-water Perigondwana realm nor to a less well-defined warm-water realm.
Evolution in the composition of phytoplankton from a middle Floian ‘Mediterranean microflora’ to an
early Darriwilian ‘Baltic microflora’ occurs on both palaeocontinents (South China and Baltica). This
implies a reciprocal exchange within a mid-latitude realm controlled more by the pattern of
subtropical oceanic gyres rather than just latitudinal position. A similar evolution of acritarch
assemblages is also noted across the East European Platform in Russia and Poland.
Asker, Holmenskjæret
Upper Ordovician carbonate rocks (with Holocene glacial straitions), various fossils, incl, algae,
Palaeoporella. Langøyene Formation.
Further reading
Dorning, K.J. & Aldridge, K.J., 1982. A preliminary investigation of palynological assemblages from the
Early Silurian of Ringerike. Palaeontological contributions from the University of Oslo 278,
105-108.
Nakrem, H.A. & Rasmussen, J.A. 2013. Oslo district, Norway. In: M. Calner, P. Ahlberg, O. Lehnert &
M. Erlström (eds.): The Lower Palaeozoic of southern Sweden and the Oslo Region, Norway.
Field Guide for the 3rd Annual Meeting of the IGCP project 591. Sveriges geologiska
undersökning Rapporter och meddelanden 133, 58–85 (+references).
Smelror, M. 1987. Early Silurian acritarchs and prasinophycean algae from the Ringerike District, Oslo
Region (Norway). Review of Palaeobotany and Palynology 52, pp.137-159
Tongiorgi, M., Bruton, D.L. & Di Milia, A., 2003: Taxonomic composition and palaeobiogeographic
significance of the acritarch assemblages from the Tremadoc-Arenig (Hunneberg, Billingen
and lower Volkhov Stages) of the Oslo Region. Bollettino della Societa Paleontologica Italiana
42, 205−224.
Oslo Region, simplyfied geological map
Oslo Region stratigraphy
Slemmestad centrum, M. Cambrian on Proterozoic basement
Bygdøy, Huk
Slemmestad football arena
Slemmestad football arena, orthocones