Ordovician Unit, Oslo Region

VENSTØP FORMATION

(Venstøpformasjonen)

(Dahll 1857)

(Previously termed: 4b, 4f, 4c-α Venstöb-Schiefer, Trinucleusschiefer, Trinucleus Shale, Lower Tretaspis Shale, Frognøya Shale).

Main Lithology. Shale Venstøp Shale, Venstøpskiferen)

Neostratotype. Skien-Langesund; Langesundstangen (Harland 1980, Fig. 4) (NI--- 432 392).

Hypostratotypes. Oslo Asker: Nakkholmen (Williams & Bruton 1983, Fig. 4) (NM 949 404). Ringerike, N.W. Frognøya (Williams & Bruton 1983, Fig. 7) (NM 652 582).

Thickness Variation. Skien-Langesund: Langesundstangen (11.1 m). Oslo-Asker: Nakkholmen (7.4m) Nesbru (10.9m) (see Williams & Bruton 1983). Ringerike: N.W. Frognøya (26.0m). Modum (Sylling): Nysæterbrenna (> 1 4m).

Definition. The Venstøp Formation is a dark, locally graptolitic, shale which occurs in Skien-Langesund, Eiker-Sandsvær Modum (Sylling), Oslo-Asker and Ringerike. The original type area for the formation was a series of exposures around Venstøp (Dahll 1857). Harland (1980 pp.276-277), however, designated a neostratotype on the southern tip of Langesundstangen as the Venstøp localities have deteriorated and are unsatisfactory as type exposures. Harland noted (1980 pp.272, 276) that both the base and the top of the formation in Skien-Langesund are abrupt lithological changes, with pale coloured limestones occurring both above and below the Venstøp Formation. In its type development, the formation is composed of dark, iron-stained shales with scattered limestone concretions (Harland 1980 p.276, Fig. 4; 1981 pp.25-26). The top of the formation is here taken to include planar limestones which are also developed at various horizons, especially near the top, elsewhere in the Oslo Region. In all districts the Venstøp Formation is bounded by limestone formations. In Oslo-Asker there is a thin phosphorite conglomerate at its base. The formation here was described in detail by Williams & Bruton (1983) who informally subdivided it into a lower dark shale member with rare limestone nodules and an upper member of alternating pale shales and limestone. Limestone horizons are much more common in the thick development of the formation in Ringerike (Owen 1979, Williams & Bruton 1983) and the unit here was considered to represent a local member, the Frognøya Shale Member by Owen (1979, 1981). Now that the Venstøp Formation has been more fully documented from other districts of the Oslo Region, this local unit term is abandoned herein.

Fauna and Age. Graptolites of the Pleurograptus linearis Zone are abundant at some levels in the Venstøp Formation in Skien-Langesund (Harland 1980 p.277), Oslo-Asker (Williams & Bruton 1983) and Ringerike (Owen 1979 p.253; Williams & Bruton 1983), and together with the trilobites, notably species of Tretaspis, indicate an early Ashgill (Pusgillian) age. Williams & Bruton (1983) argued that the phosphorite conglomerate in Oslo-Asker represents an hiatus equivalent to the upper D. clingani and lowest P. linearis graptolite zones.

The shelly fauna is commonly one of high abundance but low diversity. Thus Harland (1980 p.277) reported bedding planes packed with brachiopods, notably species of Onniella and Hisingerella along with rarer specimens of Sericoidea, Dalmanella and inarticulates. A broadly similar association is being described by Harper (in prep.) from the formation in Eiker-Sandsvær. Harland also noted (1980 p.276) the scattered occurrence of species of the trilobites Tretaspis, Flexicalymene and Primaspis. This is also the case in Eiker-Sandsvær and Oslo-Asker and the lower part in Ringerike. In the upper part of the formation in Ringerike specimens of Tretaspis and Flexicalymene occur in great abundance at some horizons. Brachiopods (Owen 1979, Hanken & Owen 1982), bivalves (Toni 1975) and gastropods also occur here. With the exception of Flexicalymene, the trilobites of the Venstøp Formation were described by Owen (1980a, 1981). The formation contains a relatively higher proportion of articulated individuals than in other upper Ordovician units, suggesting quiet conditions of deposition. Orthocone nautiloids, gastropods, bivalves and conularids are also known from the formation and some horizons, especially bedded limestones, are bioturbated with Chondrites being particularly common.