Heggen Member
(Heggenleddet)
(New name)
(Previously termed: 4a-α Upper Didymograptus Shale, Ogygiocaris Shale).
Main Lithologies. Shales with limestone nodules.
Basal Stratotype. Modum (Vikersund): Road to Vikersund ski jump (Wandås 1984, Fig. 2) (NM 562 449).
Hypostratoptypes. Eiker-Sandsvær Rønningsfossen (NM 358 035). Skien-Langesund: Blekkebakken (Frierfjord) (Fig. 17) (NL 385 466) (NL 388 466).
Thickness variation. Modum (Vikersund): area around Vikersund ski jump (ca 120m). Eiker-Sandsvær: Rønningsfossen (>62m).
Definition. The base of the Heggen Member is defined at the first thick shale bed above the Helskjer Member. The unit is dominated by dark shales which weather brown. The shale beds are commonly of the order of 50 cm thick and are separated by horizons of dark nodular limestone. In Skien-Langesund, phosphatic horizons are developed at several levels Størmer 1953, pp.71-73). Calcarenites are not developed in the uppermost part of the formation in contrast to the succession in Oslo-Asker.
Fauna and Age. The Heggen Member contains a rich shelly and graptolitic fauna. Its distribution in Skien-Langesund, and Eiker-Sandsvær was outlined by Størmer (1953 pp.71-73, 78-80) and (Wandås (1982 p.6) has listed the trilobites of the lowest part of the member. Some of these extend up from the Helskjer Member but Megistaspis (Megistaspidella) maximus, Botrioides simplex (see Owen 1987) and Ogygiocaris all make their first appearance in the lowest 10m of the Heggen Member.
Berry (1964) described graptolites of the Didymograptus murchisoni Zone (but see comments above on its possible partial equivalence with the D. 'bifidus' Zone) from the base of the Heggen Member in Skien-Langesund and a level within the member in Eiker-Sandsvær
(From Owen et al., 1990: The Ordovician Successions of the Oslo Region, Norway. NGU Special Publication 4. Figs. refer to the printed version.