The Middle Ordovician of the Oslo Region, Norway. 34. The type Nakholmen Formation (upper Caradoc), Oslo, and its faunal significance

The Nakholmen Formation, at its type locality on the island of Nakholmen, Bunnefjord, consists of 13-14 m of dark shales with a few horizons of limestone nodules. The low-diversity but locally abundant fauna includes the brachiopods Hisingerella nana, Onniella cf. bancrofti and Sericoidea gamma, the trilobites Broeggerolithus discors and Lonchodomas aff. rostratus and the graptolites Amplexograptus rugosus, Climacograptus antiquus lineatus and Corynoides incurvus. The shelly and graptolite faunas suggest correlations with the Woolstonian-Actonian stages of the Caradoc and the lower part of the Dicranograptus clingani Zone respectively. The assemblage represents a deepwater, euxinic, biofacies offshore from the well-developed shelly associations in coeval strata in Asker, Ringerike and Hadeland. The brachiopod fauna is markedly dissimilar from those coeval associations elsewhere in the region, but the trilobite faunas have many elements in common.