ENIGMA ABOUT THE EXISTENCE OF LARGE NEOPROTEROZOIC FOSSILS IN HYANGSANNI DOLOMITE

We analyzed Sr, C and O isotopic compositions of Hyangsanni Dolomite in the Okcheon Metamorphic Belt (OMB). A minimum initial ⁸⁷Sr/⁸⁶Sr ratio of approximately 0.7071 and δ¹³C(PDB) values of 5-6 ‰ are interpreted to be associated with a glaciation of approximately 710 Ma, consistent with the occurrence of the neighboring Hwanggangni diamictite. The appearance of these Neopoterozoic glacial deposits and the detrital zircon U-Pb age distribution characteristic of NE OMB with prominent Mesoproterozoic peaks is very similar to the southern margin of North China Craton, suggesting that much of the Sino-Korean Craton has evolved as a single mass since Early Neoproterozoic.

In contrast, Lee et al. (1972) claimed that the Hyangsanni Dolomite was Cambrian, based on their discovery of a fossil specimen identified as Archaeocyatha, currently held at the Museum of Yonsei University. Recently, Ree et al. (2016) argued that it is not an Archaeocyatha but a deformation feature like a sheath fold. Yang et al. (2017) refuted that the specimen is a definitive fossil with the outer and inner walls connected by the septa organization and such forms cannot be made inorganically by deformation. If this oval specimen, about 22 cm in length and about 7 cm in width, is a fossil, it can be very important for understanding the evolution of life on Earth. However, fossils of similar shape and size have never been reported so far. The overall look is similar to Vendian Kimberella, but it is much larger and is estimated to be much older. We hope that such an enigma about the existence of large Neoproterozoic fossils in Hyangsanni Dolomite will be resolved soon.

References

Lee et al. (1972) Jour. Geol. Soc. Korea, 8, 191-197.

Ree et al. (2016) Geosc. Jour., 3, 285-294.

Yang et al. (2017) Jour. Geol. Soc. Korea, 53, 261-366.