THE AGE OF THE BEOTHUKA TERRANOVA RADIOLARIAN ASSEMBLAGE (ORDOVICIAN, WESTERN NEWFOUNDLAND)

The Beothuka terranova radiolarian assemblage was originally described from the Little Port Complex of western Newfoundland, where it was isolated from cherts, interpreted to originate from an ophiolitic succession of the Humber Arm allochthon. The rocks of the Little Port Complex yielded U/Pb Zirkon ages of 508 ±8 Ma and 505 ±3 Ma. These ages were used to determine the age of the Beothuka terranova assemblage as of basal Tremadoc age, based on the time scale of Harland et al. (1989).

Beothuka terranova occurs in the lower part of the Ollenidsletta Member of the Valhallfonna Formation of Spitsbergen, associated with a diverse, well-preserved radiolarian fauna. The importance of the material lies in the occurrence of the radiolarians in a well-dated graptolitic succession. The Beothuka terranova assemblage is tied to a precise graptolite biostratigraphy, indicating its origin from the lower part of the mid-Arenig Didymograptellus bifidus Biozone. The D. bifidus Biozone can be dated at approximately 475 Ma (Webby et al. 2004). However, radiometric dating is not available for the Spitsbergen succession.

The Spitsbergen assemblage includes a number of taxa found already in late Cambrian faunas, but also younger, Arenig faunal elements. A comparison of recently described Cambrian and early Ordovician radiolarians with the Beothuka assemblage of western Newfoundland shows that the Beothuka assemblage differs considerably from these assemblages and includes faunal elements more typical of younger Ordovician faunas.

The radiometric dating of the Beothuka terranova assemblage of the Little Port Complex and the biostratigraphic age derived from the Spitsbergen assemblage shows a time discrepancy of about 30 Ma that is unlikely to be the result of a long biostratigraphic range. As new radiometric data positions the Cambrian/Ordovician boundary at ca. 488 Ma, the supposed Tremadoc age of the Little Port Complex Beothuka terranova fauna may be questioned in the light of the Spitsbergen fauna. It is likely that this assemblage is not of the age reflected in the radiometric dating of the Little Port Complex, but, based on paleontological evidence, is considerably younger.