BIOSTRATIGRAPHY OF AN EARLY ORDOVICIAN (TREMADOC) EPICONTINENTAL CARBONATE FACIES: THE BALTOSCANDIAN BJØRKÅSHOLMEN FORMATION

The Lower Ordovician in Baltoscandia was initiated by extensive carbonate deposition forming the Tremadoc Bjørkåsholmen Formation (formerly the Ceratopyge Limestone), a distinctive unit corresponding to the Apatokephalus serratus trilobite Zone. The unit is remarkable in its near homogenous facies, lithologic and faunal composition throughout the Baltoscandian platform, representing a shallow water epicontinental environment. Similar facies are recognized in the autochthonic Caledonides believed to have been deposited 400 km to the west of the present Norwegian shoreline, while the easternmost outcrops are found on the island Öland off the east coast of Sweden. This gives an east-west extension of nearly 2000 kilometres, while the north-south extension is less well known owing to lack of exposures. However, glauconitic sandstones replace the carbonates both in the Siljan District of Dalarna, Sweden, and in the easternmost baltoscandian platform in Estonia and the St. Petersburg area of Russia. Trilobites are the dominant fossil group, with 36 species belonging to 26 genera. For the ongoing study of trilobite biostratigraphy of the unit, the trilobite abundance distribution has been investigated for two localities on southern Öland, Sweden. In both investigated sections the resulting abundance distributions are consistent, with diversity declining upward. The abundancy distribution compares in detail with several sections studied earlier from the Oslo Region of Norway. A section in Västergötland, halfway between the Oslo Region and Öland, is currently also under study. However, preliminary comparison across the Baltoscandian platform between the Oslo Region and Öland, suggests an extensive stable environment and fauna during deposition of the Bjørkåsholmen Formation.