NEOPROTEROZOIC FOSSILS IN MESOPROTEROZOIC ROCKS - CHEMOSTRATIGRAPHIC RESOLUTION OF A BIOSTRATIGRAPHIC CONUNDRUM FROM THE NORTH CHINA PLATFORM

Siliciclastic rocks of the Ruyang Group, southern Shanxi, and the broa dly equivalent Gaoshanhe Group, Shaanxi, contain exceptionally well-pr eserved, large (empty set approximate to 150 mu m) acanthomorphic acri tarchs recently interpreted as late Neoproterozoic (Sinian, c. 800-544 Ma) in age. This biostratigraphic interpretation is based on the pres ence of large acanthomorphs in Sinian successions of South China and e lsewhere and the perceived absence of comparable forms in older rocks; it casts doubt on the long-accepted interpretation of Ruyang and corr elative rocks as Mesoproterozoic in age (1600-1000 Ma). In contrast, t hick marine dolomites in overlying units contain abundant radial fibro us fabrics and a narrow range of delta(13)C values (c. 0+/-1 parts per thousand vs. PDB), features which characterize unambiguously Mesoprot erozoic carbonates elsewhere on the North China Platform and on other continents. Age estimates based on petrofabrics and chemostratigraphy are corroborated by a U-Pb zircon age of 999 Ma (no recorded error) fo r granites which intrude overlying carbonates. Thus, in combination, t he available data constrain the Ruyang siliciclastics and overlying ca rbonates to be older than about one billion years, making Shuiyousphae ridium Yan and other large process-bearing acritarchs from these units among the oldest known distinctly ornamented eukaryotic microfossils. (C) 1997 Elsevier Science B.V.