An uppermost Cambrian trilobite fauna from the Yongwol Group, Taebaeksan Basin, Korea  

SohN, J. W', KiM, D. H. nrvv Choi, D. K.  

The Taebaeksan Basin was a continental margin-type depression, where the carbonate-dominated marine sediments were deposited in the early Paleozoic. The Yongwol Group is distributed in the western half of the Taebaeksan Basin and comprises the Sambangsan, Machari,Wagok, Mungok, and Yonghung formations in ascending orden. The lower three formations are assigned to the Cambrian and the upper two to the Ordovician. Middle Cambrian trilobites occur commonly in the Sambangsan Formation and the lower part of the Machari Formation. In particular, prolific occurrence of trilobites in the middle part of the Machari Formation allows the recognition of eight Late Cambrian biozones: i.e., Glyptagnostus stolidotus, Glyptagnostus reticulatus, Proceratopyge tenue, Hancrania brevilimbata, Eugonocare longifrons, Eochuangia bana, Agnostotes orientalis, and Pseudoyuepingia asaphoides zones (Lee, 1995). These zones are well correlated with the early Late Cambrian faunal successions elsewhere. However, the upper part of the Machari Formation and Wagok Formation are poorly fossiliferous and hence no late Late Cambrian trilobite faunas have hitherto been known in the Yongwol Group of Korea. The earliest Ordovician trilobite zone in Korea is the Yosimuraspis Zone of the Mungok Formation, the base of which marks the Cambrian-Ordovician boundary evinced by the occurrence of Jujuyaspis in the zone.

The trilobite fauna reponed in this study provides the first biostratigraphic reference point for the uppermost Cambrian interval in Korea. The fauna is composed of Micragnostus sp., Pseudorhaptagnostus (Machairagnostus) sp. cf. P. (M.) kentauensis, Oncagnostus sp., Fatocephalus hunjangensis, Koldinioidia sp., Hysterolenus sp., and Amzasskiella sp. It consists predominantly of Fatocephalus, constituting more than 60% in abundance. While agnostoid trilobites compose as much as 25% of the fauna, other polymeroids are less commonly observed. The genus Fatocephalus has so fan been known endemic to China and was reported from the uppermost Cambrian Mictosaukia Zone (Changia Subzone) of North China (Zhou and Zhang,1984) and Mictosaukia striata-Fatocephalus Zone of South China (Peng, 1984). Micragnostus and Koldinioidia occur widely in the Upper Cambrian to Lower Ordovician sequence of various parts of the world, whereas Hysterolenus and Amzasskiella are confined to the uppermost Cambrian sediments. Pseudorhaptagnostus (Machairagnostus) has been reported from the Upper Cambrian to Tremadoc sediments of Argentina, Bolivia, and Kazakhstan (Nielsen, 1997). The occurrence of Oncagnostus is rather unusual, as the genus was largely known from the Lower-middle Upper Cambrian. Of the seven taxa, five genera have been documented for the first time in Korea: they are Pseudorhaptagnostus (Machairagnostus), Fatocephalus, Koldinioidia, Hysterolenus, and Amzasskiella. Although some genera (Micragnostus, Oncagnostus, Koldinioidia, and Hysterolenus) are cosmopolitan in their distribution, the fauna as a whole shows an interesting biogeographic link with North China (dominance of Fatocephalus), South China (combining occurrence of Fatocephalus, Hysterolenus, and Amzasskiella), Kazakhstan (Pseudorhaptagnostus (Machairagnostus), Hysterolenus, and Amzasskiella), and even South America (Pseudorhaptagnostus (Machairagnostus). The frequent occurrence of pandemic taxa in the present collection suggests that this fauna may Nave inhabited in a somewhat deeper-water environment.  

References  

Lee, J. G.,1995, Late Cambrian trilobites from the Machari Formation, Yongwol, Korea. Unpublished Ph.D. Thesis, Seoul Nationat Univerrity Seoul, 418 p.

Nielsen, A. T.,1997, A review of Ordovician agnostid genera (Trilobita). Transactions of the Royat Society of Edinburgh: Earth Sciences., 87, 4G3-501.

Peng, S.,1984, Cambrian-Ordovicianboundary in the Cili-Taoyuan border area, northwestern Hunan with descriptions of relative trilobites. In: Stratigraphy and Palaeontology of Systemic Boundaries in China, Cambrian-Ordovician Boundary (1). Anhui Sci. Tech. Pubt. House, p. 285-405.

 Zhou, Z., and Zhang, J., 1984, Uppermost Cambrian and lowest Ordovician trilobites of North and Northeast china. In: Stratigraphy and Palaeontology of Systemic Boundaries in China, Cambrian-Ordovician Boundary (2). Anhui Sci. Tech. Publ. House, p. 61-163.