Palynological Study Of The Jurassic-Cretaceous Boundary: Correlation Between Terrestrial Deposits In China And Marine-Continental Transitional Strata In Japan

Defining the Jurassic-Cretaceous boundary is a controversy in stratigraphic study of the world. It has been widely accepted that this boundary can be defined at the bottom of Berriasian in Tethys, with the appearance of the ammonite Berriasella jacobi dating to ca. 145 Ma. However, it is difficult for the widespread terrestrial deposits in China to correlate with the international standard of marine facies. Moreover, local biostratigraphy imply that the continental J-K boundary in China is of 135 or 137 Ma age. It has a considerable discrepancy from the marine standard. The Somanakamura Group in Japan is represented by a succession of marine-continental transitional strata. It provides a bridge of marine and nonmarine stratigraphic correlation. The ammonite and radiolarian fossils preserved in this group suggest an age from Bajocian to early Valanginian. The J-K boundary was defined in or atop the Tomizawa Formation of the group according to the ammonite and radiolarian data.The present study bases on the fossil spores and pollen found from the Tomizawa and Koyamada formations. Three assemblages have been recognized. They are Assemblage 1(Cyathidites-Classopollis) from the upper part of the Tomizawa Formation, Assemblage 2(Cyathidites-Jiaohepollis) from the lower part of the Koyamada Formation, and Assemblage 3(Cyathidites-Spheripollenites-Ephedripites) from the middle to upper part of the Koyamada Formation. With the reference of ammonite and radiolarian evidence, as well as the characterastics of the sporopollen assemblages, the age of Assemblage 1 should be late Tithonian, the age of Assemblage 2 is probably Berriasian and the age of Assemblage 3 ranges from Berriasian to Valanginian. The Jurassic-Cretaceous boundary is between Assemblage 1 and 2.This palynological J-K boundary can be correlated with that of terrestrial sequence in China. Biogeographically, the distribution pattern of spores and pollen in southern China is in accordance with that in the Somanakamura Group. Assemblage 1 and 3 can respectively parallel the lower and upper layers of Tuchengzi Formation in northeastern China, which defining the age of Tuchengzi Formation from late Tithonian to Valanginian. Furthermore, sporopollen fossils from Dabeigou and Yixian formations show characteristics of Early Cretaceous progressively. Their ages may be younger than that of Assemblage 3, which are probably Valanginian or above it.Overall, it is suggested that the continental J-K boundary is located in the Tuchengzi Formation.