Ordovician Unit, Oslo Region
(Previously termed: 4a-β-4b-α, Robergia Beds, Hovinsholm Shale).
Main Lithology. Shale (Hovinsholm Shale, Hovinsholmskiferen).
Basal Stratotype. Mjøsa (Nes-Hamar): Hovinsholm Farm, Helgøya, (PN 098 327).
Definition. The Hovinsholm Formation is restricted to the Mjøsa districts and comprises grey shales alternating with calcarenites and scattered limestone nodules. The top of the formation is transitional to the overlying Furuberget Formation (Skjeseth 1963 pp.64,74). The thickness of the formation cannot be determined in the type area (Skjeseth 1963 p.65) but some 78m of the unit including the transition to the overlying unit at Holmen in Brummundal, Ringsaker was reported by Størmer (1953 pp. 109-110) and Skjeseth (1963 p.74).
Fauna and Age. As Skjeseth (1963 p.64) noted, these strata were termed the 'fossil-barren beds' by Holtedahl (1909) but following the discovery in Ringsaker of specimens of the remopleuridid Robergia microphthalma (Linnarsson) (see also Nikolaisen 1983 pp.294-5), Størmer (1953 p. 109) renamed them the Robergia Beds. The only fossils known from the Hovinsholm Formation in Nes-Hamar are in the upper transition beds (Skjeseth 1963 p. 65) but the formation as a whole has yielded a diverse shelly and graptolitic fauna at Herram and Holmen in Ringsaker (Størmer 1953 pp.109-110; Skjeseth 1963 p.72; Nikolaisen 1983 p.295). Shelly fossils from these localities include trilobites, ostracods and graptolites and indicate a correlation with the Andersö Shale in Jämtland (upper Llanvirn - lower Caradoc). The graptolite fauna was identified by Bulman (in Størmer 1953 pp.109-110) with that from the upper part of the Holmen section considered to be indicative of an horizon 'somewhat higher than the zone of Nemagraptus gracilis'. An early Caradoc age for the top of the formation is thus suggested.
(From Owen et al., 1990: The Ordovician Successions of the Oslo Region, Norway. NGU Special Publication 4. Figs. refer to the printed version.