Porsgrunn Formation (Cocks & Worsley 1993)
Above the Vik Formation and below the early Wenlock reefs of the Braksøya Formation there are a variety of rocks, which are well exposed in the north of the Oslo Region (the Bruflat Formation of Toten, Hadeland and Ringerike) but poorly exposed from the Oslo‑Asker districts southwards. These latter rocks have been grouped hitherto within the Skinnerbukta Formation (Worsley et al. 1983, p. 30), but in the present study it has become apparent that they are better divided into two separate formations: (a) the true Skinnerbukta Formation of Malmøya and Bærum, which are thinbedded dark shales and calcareous turbidites now known to be of Wenlock age; (b) a new Porsgrunn Formation, which consists of a variety of deeper shelf, largely bioturbated chiefly mudstones, but with subsidiary siltstones and occasional nodular limestones, which are all of late Llandovery age. The stratotype of the new formation is a series of discontinuous exposures along Øyekastveien, Porsgrunn (from Grid Ref. NL 369554 to 370555). The base is defined at NL 369554, where a 1.5 m transitional muddy limestone is seen from the underlying Vik Formation followed by massive mudstones with occasional shelly bands (e.g. Collection 22 at 5 m above the base). These are followed by siltstones and mudstones with thin carbonate horizons, much bioturbated, followed by a greater proportion of mudstones with (near the top) some thicker siltstones and larger limestone nodules developed, in which are seen numerous Palaeocyclus (Fig.4). The junction with the overlying Braksøya Formation is clearly exposed, and the total thickness of the Porsgrunn Formation at its type section is estimated at 52 m.
Dating of the Porsgrunn Formation has also been achieved by the new finds of crenulata Zone graptolites together with Eocoelia sulcata in the Holmestrand district and by the association of Erinostrophia (Fig. 8f) and Palaeocyclus in the upper part of the formation in the Skien and Porsgrunn districts, indicating that the whole formation probably lies within the crenulata Zone. The faunal assemblages indicate a general shallowing upwards from the deeper‑water Eoplectodonta bearing beds near the base to the Palaeocyclus‑Erinostrophia horizons near the top of the formation.