HUK FORMATION
(Hukformasjonen)
(New name)
(Previously termed: 3c, 3c-α-γ.Orthoceratitkalkstein, Orthocerkalk, Orthoceras Limestone, Vækerø Formation (pars), Bestum Formation).
Main Lithology. Limestone (Huk Limestone, Hukkalken).
Basal Stratotype. Oslo-Asker: Huk (Fig. 7) (NM 938 410).
Hypostratotypes. Oslo-Asker: Engervik (NM 843 302), Vekkerø (NM 924 430), Skien-Langesund: Rognstrand (NL 409 411), Tangvalkleven (NL 397 429). Eiker-Sandsvær Råen (NM 459 176), Hals (NM 486 230). Ringerike: Smeden (NM 756 721). Hadeland: Hovodden (NM 785 952), Granvollen (NM 845 934). Mjøsa (Ringsaker) Herram (NN 989 485), (Nes-Hamar) Helskjer (PN 088 329), (Ringsaker) Stein (NN 919539).
Thickness Variation. Oslo-Asker: Killingen (6.8m), Vekkerø (6.17m), Bjørkåsholmen (7.7m). Skien-Langesund: Rognstrand (2.42m), Tangvalkleven (2.56m). Eiker-Sandsvær Råen (>11.55m), Hals (13.46m). Modum (Vikersund): composite value from Øvre Øren Hovland and Vikersund ski jump (>8.4m). Ringerike: Smeden (>8.8m). Hadeland: Granvollen (7.31 m), Mjøsa Helskjer (1 0.75m), Stein (40m).
Definition. The Huk Formation succeeds graptolitic shales of the Tøyen Formation in all districts except southern Eiker-Sandsvær, and Skien-Langesund. Here it overlies Cambrian Alum Shale; the contact being interpreted as a thrust plane in recent tectonic models of the Oslo Region by Ramberg & Bockelie (1981) and Bockelie & Nystuen (1985) but in a recent thesis, Ribland Nilssen (1985) has reinterpreted this 'Langesund hiatus' in terms of the southward transgression of the Ordovician rocks over the tilted Cambrian sequence. In most districts, the Huk Formation has a tripartite division with limestone members bounding a middle limestone-shale unit.
The top of the Huk Formation in most areas is a distinctive horizon rich in nautiloid cephalopods (Cycloendoceras). The overlying basal beds of the Elnes Formation are a transitional succession of limestones and shales (the Helskjer Member) passing up into the more typical dark shales of that formation. In its tripartite development, the Huk Formation maintains a fairly constant thickness although the middle member thickens somewhat in Eiker-Sandsvær thus giving an increased thickness to the formation there. The formation is thin in Skien-Langesund but in parts of the Mjøsa area (e.g. Nes-Hamar and E. Toten) the local bipartite division has a total thickness which is broadly comparable with that of the formation further south. However, in Ringsaker and W. Toten, the very thick Stein member is developed.
Discussion. The Huk Formation was the subject of an unpublished thesis by Skaar (1972) who termed the unit together with the overlying "Killingen Member" the Vækerø Formation'. This name, however, had been used earlier by Fjelldal (1966) for the Bjørkåsholmen Formation ('Ceratopyge Limestone') and overlying "Vestfossen Member" In view of the potential confusion, the name Vækerø Formation is not adopted here for either of these units. Harper (1986) and Owen (1987) used the term 'Bestum Formation' for the formation but this name was subsequently considered geographically inappropriate by the Norwegian Stratigraphical Committee.
Fauna and Age. In his study of the conodont faunas of the Huk Formation ('Orthoceras Limestone'), Kohut (1972) concluded that the entire formation with the possible exception of the uppermost part of the Stein Member lies within the upper part of the Didymograptus hirundo graptolite Zone. This conflicts with the correlation proposed earlier by Jaanusson (1960, pp.343-345) and Skevington (1965, p.70) who suggested that the base of the D. 'bifidus' Zone may lie at the top of or even within the middle member (Lysaker Member = 'Asaphus Shale') of the formation. Similarly whilst Kohut (1972) argued that the Huk Formation ranges in age from middle Volkhov to the Hunderum Substage of the Kunda Stage, Jaanusson (1960, Table 9) suggested a correlation with the upper Volkhov (Langevojan) to middle Kunda (Valaste) stages largely on the basis of the trilobite faunas. In the most recent correlation chart, Jaanusson (1982, Fig. 4) maintains this correlation between the conodont, graptolite and trilobite zones and the East Baltic stages. This is provisionally followed herein and is summarized on Fig. 8 along with Kohut's proposed correlation. A detailed re-study of the conodonts of the Huk Formation is being undertaken by Dr Svend Stouge and Mr Jan Rasmussen of Copenhagen University who have informed us (pers. comms. 1987 & 1989 and Rasmussen 1989) that their work largely confirms that of Kohut in the correlation of the typical tripartite development of the formation with the standard conodont zonation. This study complements one by Mr Arne Thorshøj Nielsen, also of Copenhagen, on the trilobites at the formation at Slemmestad and equivalent levels in Sweden and on Bornholm. The results of these studies should greatly clarify the correlation of the Huk Formation.
(From Owen et al., 1990: The Ordovician Successions of the Oslo Region, Norway. NGU Special Publication 4. Figs. refer to the printed version.