The aim of the present study is to improve knowledge of the brachiopod fauna in the lower part of the Arnestad Formation (Caradoc) in the OsloAsker district and to document faunal changes prior to the Scoto-Appalachian brachiopod migration into the Oslo Region. This migration appears to have taken place during deposition of the upper Haljala and Keila stages (Lower Caradoc). Like the faunas in other parts of Baltica, the brachiopod fauna in the lower part of the Arnestad Formation, occupied an environment influenced by moderate current or wave regimes. It has a lower diversity than the more cosmopolitan brachiopod fauna in the upper part of the formation. The fauna in the lower part of the Arnestad Formation is, however, more diverse than the equivalent Swedish faunas in the Scanian Confacies Belt (“Outer facies belt”). The closest affinity, at the generic level, is with the faunas from the Livonian Tongue of the Central Baltoscandian Confacies Belt (“Middle facies belt”). The Onniella-Chonetoidea association, known from deep-water facies, appeared for the first time in the Oslo Region during the early Haljala stage. The lower boundary of the Arnestad Formation is diachronous, becoming younger towards the west. When comparing faunas and environmental indicators in the studied sections, it appears that the western most locality (Vollen) was located in a more shallow-water environment with a more marked current or wavedominated regime than that of the eastern locality.
Brachiopod bio- and ecostratigraphy in the lower part of the Arnestad Formation (Upper Ordovician), Oslo Region, Norway