Ecological associations in a regressive sequence: the latest Ordovician of the Oslo-Asker District, Norway

Ten faunal associations are defined and documented from numerous collections made in the late Ashgill rocks of the Oslo-Asker district, Norway. The distribution of the associations is integrated with previous work on the sedimentology which demonstrated a regressive sequence caused by the eustatic fall in sea level due to the end-Ordovician glacial event. The associations in late Rawtheyan times were a Tretaspis Association in the deeper shelf, with an Onniella Association midshelf, which was also colonized in places by Palaeoporella thickets and by cystoids. The Onniella Association persisted into Hirnantian times, with the Hirnantia Association sensu stricto situated up-slope from it. Shallower still were the Dalmanella, Hindella-Cliftonia, and Holorhynchus Associations, with other associations dominated by bivalves and Monocraterion inhabiting the lower shoreface. More specialized Thebesia and Brevilamnulella Associations are also recognized, which colonized tidal channels and ooidal shoals at the top of the Ashgill sequence. The area was subsequently blanketed by the early Silurian transgressive deposits containing at their base specialized shelly faunas and then shales with graptolites, some of which can be attributed to the early Llandovery acuminate Zone.