FOSSUM FORMATION
(Fossumformasjonen)
(Dahll 1857)
(Previously termed: 4a, 4b-d, 4a-4b-γ , Fossum Kalkstein, Ampyxzone, Chasmopskalk und Schiefer, Ampyx Limestone and Shale, Echinosphaerites zone, Ech i nosphae rites beds, Bryozoan zone, Bryozoan beds, Mastopora-Coelosphaeridium zone, Coelosphaeridium-Mastopora beds.
Main Lithologies. Rubbly, thinly bedded limestones and shales (Fossum Limestone, Fossumkalken).
Basal Stratotype. Skien-Langesund: Blekkebakken (Frierfjord) (Fig. 17) (NL 385 466 - NI--- 388 466).
Hypostratotypes. Eiker-Sandsvær Rønningsfossen (NM 358 035), Lindset (NM 355 021). Modum (Vikersund): Vikersund ski jump (NM 564 454).
Definition. The history of the terminology applied to this unit was summarized by Harland (1980 Table 1). The formation was originally included with the Steinvika Formation (see below) as the Fossum Kalkstein by (Dahll (1857). The Fossum Kalkstein and overlying 'Venstob Schiefer' together comprised Dahll's 'Fossum Gruppe'. The beds between the Elnes Formation and the Steinvika Formation ('Encrinite Limestone') were subdivided by Brøgger (1884), Størmer (1953) and Strand & Henningsmoen (1960) largely on palaeontological grounds but are here assigned to a revised version of Dahll's 'Fossum Kalkstein', the Fossum Formation.
As Størmer (1953 p. 73) noted, the base of the Fossum Formation along the shore of the Frierfjord in Skien-Langesund lies within a transition from the shales with limestone nodules of the Elnes Formation to rather rubbly thinly bedded limestones with thin shales of the Fossum Formation. This lithology typifies the whole formation although the shale horizons are thicker at some levels. A description of the lithofacies directly below the Steinvika Limestone was given by Harland (1981 pp.6-8). The formation is estimated to be of the order of 150m thick in Skien-Langesund and a similar thickness is probably present in Eiker-Sandsvær and Modum.
Fauna and Age. As noted above, although there is little lithological variation there is an upward change in the dominant fauna and flora of the Fossum Formation and this gave rise to the biostratigraphical subdivision of the unit best summarized by Størmer (1953 pp.7375, 80-81). The lowest part of the unit is characterized by trilobites including asaphids and raphiophorids, together with trinucleids (see Owen 1987) namely Botrioides and (a little higher up) Reeddlithus carinatus (Angelin). It is succeeded by a diverse shelly fauna including trilobites, brachiopods and several echinoderms (Bockelie 1981, 1981 a) including the cystoid Echinosphaerites aurantium (Gyllenhaal) and the eocrinoid Bockia grava Bockelie. Bockelie (1981 a pp.1 33-134) gave a full list of the echinoderms present and argued that the preservation of ambulacral appendages in E. aurantium indicates rapid burial by periodic influxes of fine terrigenous mud. The fauna also includes bryozoans which become more abundant in the succeeding more shaly beds. The uppermost part of the formation is characterized by the calcareous algae Mastopora and Coelosphaeridium, the latter occurring in great abundance at some levels. The boundaries between these various 'biozones' are gradational and it is not possible at the moment to define a formal biostratigraphical subdivision of the Fossum Formation.
The trilobites of the lower part of the Fossum Formation indicate a Llandeilo age and the formation probably extends well into the Caradoc.
(From Owen et al., 1990: The Ordovician Successions of the Oslo Region, Norway. NGU Special Publication 4. Figs. refer to the printed version.