Ordovician Unit, Oslo Region

Herram Member

(Herramleddet)

(Skjeseth 1963)

(Previously termed: 3b (pars), 3b-ε Heramb kalk og skifer)

Main Lithologies. Grey mudstones with limestone beds and nodules.

Basal Stratotype. Mjøsa (Ringsaker): Herram (formerly spelt Heramb) (NN 989 485).

Thickness variation. Mjøsa (Ringsaker): Herram (1.5m). (Nes-Hamar): Helskjer (>6.4m) (Fig. 10).

Definition. The grey mudstones and shales with limestone beds and lenses constituting the Herram Member contrast markedly with the underlying black shales of the Tøyen Formation. The member is overlain by nodular limestones and shales of the Stein Member in West Toten and Ringsaker and by the compact limestone of the Svartodden Member in East Toten and Nes-Hamar. The boundary with the Svartodden Member is, at least locally, a distinct erosion surface (Skjeseth 1963, Fig. 20). The thickest described development of the Herram Member is in Snertingdal, N.W or Ringsaker, where A. Bjørlykke (1979, Fig. 18) recorded some 11.5m.

Discussion. Skjeseth (1952 pp.143-148; 1963 p.71) described the type development of the Herram Member in the context of the Tøyen Formation as a transition to the overlying Stein Limestone. He considered the Herram Member to be restricted to the northernmost part of the Mjøsa Area (Ringsaker) but it was subsequently identified by Skaar (1972) in the Toten and Nes-Hamar districts. Skaar (1972) also suggested that it is present in northern Hadeland but reexamination of this section at Hovodden shows that the lowest 50cm of these beds belong to the Hukodden Member of the Huk Formation and the remainder to the Lysaker Member. At Helskjer on Helgøya, (Nes-Hamar) Skaar (1972 pp.30-32, Fig. A9) assigned approximately 4m of interbedded limestones and shales above the black shales of the Tøyen Formation to the

Herram Member. These strata were not mentioned by either Størmer (1953 pp. 101 -102) or Skjeseth (1963 pp.62-63) who also discussed the section here. All three workers distinguished a tripartite division in the overlying 'Orthoceras Limestone'. Reinvestigation of the section, however, suggests that whilst the massive Svartodden Member can be recognised, there is insufficient differentiation of the underlying strata to recognise the Lysaker and Hukodden members. Consequently all the limestones and shales between the Tøyen Formation and the Svartodden Member of the Huk Formation are assigned to the Herram Member (Fig. 10).

Fauna and age. Skjeseth (1952, 1963) noted the mixed graptolitic/shelly fossil assemblage in the type development of the Herram Member. The trilobites are being studied by Mr A.T.Nielsen of Copenhagen University. Skjeseth identified the trilobites Megistaspis planilimbata, and Raymondaspis limbata. The former characterises the planilimbata Zone in Sweden but the latter occurs in the simon and limbata Zones (Tjernvik & Johansson 1980). The latter, younger, age is suggested by the work of Erdtmann (1965 p.523, Fig. 9) who recorded graptolites including Tetragraptus bigsbyi, Dichograptus octobrachiatus, Phyllograptus densus and P. aff angustifolius. He thus suggested that the member here may be equivalent to the upper part of the Galgeberg Member of the Tøyen Formation in Oslo-Asker. Kohut (1972 pp.435, 437-8) recorded conodonts from the Herram Member at Herram which he interpreted as possibly indicative of an overlap between the Baltoniodus navis and Paroistodus originalis zones. He suggested that there may be an hiatus at an equivalent level in Sweden. Kohut also noted a conodont fauna extracted by G.Hamar from what he termed the 'lowermost bed of the Orthoceras Limestone at Helskjær'. This would certainly lie within the revised interpretation of the Herram Member presented here and may even be from the base of the member. He interpreted the fauna as belonging to the Baltoniodus navis Zone and Dr J.A.Rasmusson of Copenhagen University informs us (pers. comm. 1989) that the whole of the member in Ringsaker belongs in this zone as defined by Lindström (1971).