The Bjorkåsholmen Formation (Tremadoc): a homogenous distribution of Trilobites throughout the Baltoscandian platform

• THE BJØRKÅSHOLMEN FORMATION (TREMADOC): A HOMOGENOUS DISTRIBUTION OF TRILOBITES THROUGHOUT THE BALTOSCANDIAN PLATFORMÅsa FriskUppsala University, Department of Earth Sciences, Palaeobiology, Norbyvägen 22, SE-752 36 Uppsala, Sweden.Extensive carbonate depositions initiate the Lower Ordovician succession of Baltoscandia forming the Tremadoc Bjørkåsholmen Formation, formerly the Ceratopyge Limestone, a distinctive unit corresponding to the trilobite zone of Apatokephalus serratus. In older studies a combined bio-litho stratigraphical concept of the unit has mostly been used, however a modern lithostratigraphical definition was given by Owen et al. (1990). The definition of the formation is based on the hypostratotype at Bjørkåsholmen in Slemmestad, Norway, and should be referred to instead of the historical synonym. The limestone succession has a broad regional distribution and its associated sediments were deposited across the Baltoscandian platform in a shallow water epicontinental sea. Similar depositional conditions are not known today. The unit is remarkable in its near homogenous facies, lithologic and faunal composition throughout the platform. The most recent revision of trilobites from the Ceratopyge fauna recognized 36 species assigned to 28 genera (Ebbestad 1999).In the present study sequences at Ottenby and Degerhamn on southern Öland, Sweden, were logged and compared for trilobite biostratigraphy. In both investigated localities the resulting trilobite abundance distributions are very consistent. The trilobite fauna from the Bjørkåsholmen Formation in the Oslo Region, Norway, and Öland are composed of the same typical Ceratopyge assemblage. Trilobite abundance data from both areas are nearly identical and demonstrate an upward declination of trilobite specimens. Comparison across the Baltoscandian platform between the westernmost occurrences in the Oslo Region and the easternmost outcrops on Öland therefore suggests widespread stable conditions of the Ceratopyge fauna during the sedimentation of the Bjørkåsholmen Formation.Ebbestad, J.O.R. 1999: Trilobites of the Tremadoc Bjørkåsholmen Formation in the Oslo Region, Norway. Fossils and Strata 47. 118 pp.Owen, A.W., Bruton, D.L., Bockelie, J.F. Bockelie, T.G. 1990: The Ordovician successions of the Oslo Region, Norway. Norges geologiske undersøkelse Special Publication 4. 54 pp.

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