NONMARINE BIVALVE ASSEMBLAGE IN THE ASKER GROUP, OSLO GRABEN AND ITS CORRELATION WITH A LATE PENNSYLVANIAN ASSEMBLAGE FROM NORTH-AMERICA

A non-marine bivalve band in the Asker (Group of Semsvik, Asker, usually regarded as being of Permian age, has been re-examined- The fauna, an assemblage of Anthraconaia protracta Eagar, is widely and continuously variable. The Semsvik assemblage is identical in shell height and length to A. protracta above the Benwood Limestone, middle Monongahela Group, of Pennsylvania. and to A. protracta 2700) km further west in central Arizona, with which the Pennsylvania band has been correlated. The ranges of variation within the Oslo, Pennsylvania and Arizona assemblages are closely similar. When their horizons are broadly correlated, the Semsvik band is placed in the upper Virgilian Stage of the upper Pennsylvanian and in the upper Stephanian C of Europe. This age, which is applicable to the thin succession of the Asker Group in general, pre-dates formation of the Oslo Graben and is in conformity with the radiometric age of the overlying basal basalt lava. The age of the Knabberud Limestone in the Asker Group is at present under discussion. The extremely widespread, evidently rapid dispersal of faunas of small Anthraconaia in Stephanian time may have been aided by palaeoniscoid fish.