LATE DARRIWILIAN (ORDOVICIAN) GRAPTOLITE BIOSTRATIGRAPHY OF THE ELNES FORMATION (S. NORWAY)

The Elnes Formation is a widely distributed lithostratigraphic unit in southern Norway, known for its richness in well-preserved graptolites. It consists mainly of fine dark to black shales with a variable amount of thin silt-beds in the lower part. In the higher part of the succession limestone beds and limestone concretion beds are common. In the Oslo Region, the best exposures for the lower part of the Elnes Formation and the contact to the underlying Huk Formation (Orthoceras Limestone) have been recognized in the Slemmestad area, SE of the city of Oslo. The higher part of the succession is presently extensively exposed in a number of sections on the shores of the island of Bygdøy. The Elnes Formation can be differentiated into the Helskjer, Sjøstrand, Engervik and Håkåvik members, overlain by the Upper Ordovician Vollen Formation. The Nicholosonograptus fasciculatus and Pterograptus elegans biozones have been described previously from the Elnes Formation, while the uppermost Darriwilian was unknown in graptolitic facies in Norway. In the Bygdøy sections, the Pseudamplexograptus distichus and Dicellograptus vagus biozones are well represented by rich and diverse graptolite faunas. The D. vagus Biozone replaces the old and widely known, but fairly imprecise Hustedograptus teretiusculus Biozone in Scandinavia. The D. vagus Biozone is defined by the FAD of the first dicellograptids, usually D. vagus, found in the Oslo Region and contains a diverse and highly endemic diplograptid graptolite fauna. Extremely rare proximal ends of Nemagraptus were found in the uppermost part of the Elnes Formation (Håkåvik Member), none of which shows any indication of the typical cladial branching of Nemagraptus gracilis. The Upper Ordovician (Sandbian) Nemagraptus gracilis Biozone, therefore, may not have been reached and is represented in the overlying Vollen Formation from which only a very poor and inconclusive graptolite record is available. The graptolite faunas of the Elnes Formation show a high endemicity and few elements can be found also in the North American equivalent, the Table Head Group of western Newfoundland, even though biostratigraphically important species are represented and enable the precise correlation.