New Ar-40/Ar-39 dates for Cretaceous Chauna Group tephra, north-eastern Russia, and their implications for the geologic history and floral evolution of the North Pacific region

Cretaceous nonmarine volcanogenic-terrigenous sedimentary rocks that contai n plant fossils are widespread in north-eastern Russia and phytostratigraph y has been a primary tool for regional mapping. The flora from the Chauna G roup is, however, compositionally anomalous, making its proper age assignme nt uncertain. It has been considered to be late Albian in age, coeval with floral Zone II of the Chandler River sequence in northern Alaska, despite c he presence of relatively advanced conifers and rare angiosperms typical of the Late Cretaceous. New radiometric data on extrusive volcanic rocks shar ply constrain the age of the flora showing that it is younger than some bio stratigraphers had suggested. Previous Rb-Sr dates from the Chauna Group ranged from 87 to 100 Ma. Howeve r, new Ar-40/Ar-39 (laser fusion and laser stepped heating) dates of plagio clase feldspar, biotite and amphibole all lie within 2 sigma errors of each other, close to 88 Ma, and all ages fall within the Coniacian stage of the Upper Cretaceous. Even with the younger age assignment the flora remains c ompositionally distinct and demonstrates the importance of accounting for e levation, edaphic conditions, and seral development when determining region al vegetation for stratigraphic and palaeoclimatic purposes. (C) 1999 Acade mic Press.