Late middle Cambrian (Cambrian Series 3) trilobite faunas from the lowermost part of the Sesong Formation, Korea and their correlation with North China

Abstract

The Sesong Formation is a member of the Taebaek Group, Korea, which extends from late Cambrian Series 3 to middle Furongian in age. Recent studies on the trilobites of the Sesong Formation have contributed significantly to the revision of the biostratigraphy. However, trilobites in the lower part of the formation, which may include the “StephanocareZone”, have remained essentially overlooked since the establishment of the biozone, making it difficult to correlate with the equivalent biozones of North China. Here we report trilobite faunas from the lower part of the Sesong Formation in two different sections, the Seokgaejae and the Jikdong sections, which yield two species ofJiulongshaniaamong other species. Species ofJiulongshaniahave been known to occur successively in North China, so are useful for detailed correlation. Specimens ofStephanocare richthofeniare fragmentary and rarely occur in association withJiulongshania regularis, whileJiulongshaniaspecies occur throughout the studied intervals. Accordingly, it is reasonable to extend the previously establishedJiulongshaniaZone of the uppermost part of the underlying Daegi Formation into the lower part of the Sesong Formation. By doing so, theJiulongshaniaZone is correlated with theBlackwelderiaZone of North China with confidence. The lowermost part of the Sesong Formation in the Jikdong section yields a fauna includingJ. regularis, which implies that the boundary between the Daegi and Sesong formations is diachronous within the Taebaeksan Basin. The Daegi/Sesong formation boundary in Korea is comparable to the Zhangxia/Gushan boundary in North China in that it displays an abrupt change from a carbonate-dominant facies to a shale-dominant facies. The correlation employing theJiulongshaniaspecies indicates that the facies shift occurred significantly earlier in Shandong, North China than in the Taebaeksan Basin, Korea.