WORM-LIKE FOSSILS FROM THE NEOPROTEROZOIC SUCCESSIONS: IMPLICATIONS FOR THE EDIACARAN GLACIATION IN NORTH CHINA

The late Neoproterozoic successions in North China are rather incomplete and geographically restricted in the western and southern margins of the North China Block, but they typically consist of a diamictite (e.g., the Zhengmuguan diamictite in Ningxia Province) and overlying siltstone. This diamictite is traditionally correlated with the Marinoan-age (635 Ma) Nantuo diamictite in the Yangtze Block or regarded as representing a younger glaciation. This study attempts to use biostratigraphic data to test these alternative correlations. In the Helanshan area of Ningxia Province, abundant macroscopic worm-like fossils occur in a siltstone bed that appears to conformably overlie the Zhengmuguan diamictite. These fossils are preserved as compressions and are traditionally interpreted as trace fossils, with five described ichnogenera and six ichnospecies. Our recent study, however, suggests that they are body fossils of unknown affinities. We recognize three morphotypes. Type A is a centimeter- to decimeter-scaled, ribbon-shaped, Shaanxilithes-like fossil with clearly defined margins. The width of the ribbon is variable along its length, ranging from 1 to 3 mm. Annulations may or may not be present depending on preservation. Type B consists of aligned curved segments that form a centimeter-scaled linear structure. Like in Palaeopascichnus, the curved segments are not connected. Type C is similar to Horodyskia and consists of “chained beads”, each of which is 0.5-1.5 mm in diameters. Type A also occurs in the Zhoujieshan Formation, which conformably overlies the Hongtiegou diamictite in the Chaidam Block, and in the middle Dengying Formation of the Yangtze Block, which is constrained between 551 and 542 Ma. The stratigraphic occurrences of Shaanxilithes-like fossils in North China, Chaidam, and Yangtze Block indicate that the Zhengmuguan and Hongtiegou diamictites are probably younger than the Nantuo diamictite (635 Ma) unless a >80 Ma stratigraphic break exists between the Zhengmuguan and Hongtiegou diamictites and overlying Shaanxilites-like fossil occurrences. If so, sedimentary record of the Marinoan glaciation is absent from North China and Chaidam where Neoproterozoic diamictites may record an Ediacaran glaciation.