1 School of Earth and Environmental Sciences, NS80, Seoul National University, Seoul 151–742, Korea. E–mail: dkchoi@snu.ac.kr

Key words: Trilobites. Biostratigraphy. Cambrian. Ordovician. Korea.

In Korea, the Cambrian–Ordovician Choson Supergroup is well exposed in the Taebaeksan Basin (Figure 1). The Choson Supergroup, a thick (1400–2000 m) siliciclastic–carbonate sequence of late Early Cambrian to early Late Ordovician age, has been subdivided into the Taebaek, Yongwol, Yongtan, Pyongchang, and Mungyong groups (Choi, 1998). The Cambrian–Ordovician boundaries in Korea have been traditionally placed at the boundary between the Hwajol and Tongjom formations in the Taebaek Group and between the Wagok and Mungok formations in the Yongwol Group (Kobayashi, 1966). Recently the global boundary stratotype section and point for the Cambrian–Ordovician boundary has been proposed for the level of the first appearance of the conodont Iapetognathus fluctivagus in the section at Green Point, Newfoundland (Cooper et al., 2001). At present, little information has been known on the conodont assemblages across the putative Cambrian–Ordovician boundary intervals in Korea. On the other hand, recent studies on trilobites were successful in documenting previously unknown trilobite assemblages in the Upper Cambrian and Lower Ordovician of the Taebaeksan Basin. In the following, the trilobite biostratigraphy of the Cambrian–Ordovician boundary intervals of the Taebaek and Yongwol groups will be dealt with separately, as their faunal contents are somewhat different from each other.

The Hwajol Formation has been known to comprise in ascending order the Prochuangia, Chuangia, Kaolishania, Dictyites, and Eoorthis zones (Kobayashi, 1966). Apparently, the uppermost Cambrian trilobite faunas are represented by members of the Dictyites and Eoorthis zones (Kobayashi, 1935), but poor preservation of the type material hampers the taxonomic assessment with certainty. Recently, we are able to locate fossiliferous horizons yielding well–preserved trilobites from the interval across the Hwajol and Tongjom formations: from old to young, the Quadraticephalus, Mictosaukia, Missisquoia–Onychopyge, and Richardsonella faunas. The Quadraticephalus fauna is dominated by the nominal genus along with Sinosaukia, Haniwa, Tsinania, Hamashania, and some agnostoids. The overlying Mictosaukia fauna includes Micragnostus, Mictosaukia, Haniwa and Pagodia, and is succeeded by the Missisquoia–Onychopyge fauna. The uppermost Richardsonella fauna is composed of Micragnostus, Pseudorhaptagnostus, Richardsonella, Yosimuraspis, and Platypeltoides. We provisionally consider that the Cambrian–Ordovician boundary in the Taebaek Group lies somewhere within the interval yielding the Richardsonella fauna, as Yosimuraspis, an earliest Ordovician trilobite endemic to Korea and China, occurs in this interval. The remaining part of the Tongjom Formation is poorly fossiliferous, while the succeeding Tumugol Formation comprises in ascending order the Asaphellus, Protopliomerops, and Kayseraspis zones (Kobayashi, 1934; Kim et al., 1991): the upper Tremadocian Asaphellus Zone occupying approximately the lower half of the Tumugol Formation yields Asaphellus, Kainella, and Hystricurus; the Protopliomerops Zone also contains a trilobite fauna of late Tremadocian age including Shumardia, Apatokephalus, Dikelokephalina, Protopliomerops, and Seisonia; and the Kayseraspis Zone has a relatively diverse Arenigian trilobite assemblage composed of Hystricurus, Kayseraspis, Illaenus, Dikelokephalina, Chosenia, Koraipsis, and Asaphopsoides.

Figure 1. Location maps. 1. Index map of the Korean peninsula showing the position of figure B. 2. geologic map of the Taebaeksan Basin, showing mainly the distribution of the Cambrian–Ordovician Choson Supergroup.

The Pseudoyuepingia asaphoides Zone of the Machari Formation was the youngest Cambrian zone recognized in the Yongwol Group (Lee and Choi, 1996), until Sohn and Choi (2002) reported the occurrence of Fatocephalus fauna from the Osangchon area. The precise stratigraphic position of the Fatocephalus fauna is however unclear, but definitely provides the first biostratigraphic reference point for the uppermost Cambrian interval in the Yongwol Group. The Fatocephalus fauna is dominated by Fatocephalus hunjiangensis, constituting more than 60% in abundance. Other trilobite genera include Micragnostus, Pseudorhaptagnostus, Koldinioidia, Hysterolenus, and Amzasskiella. The Yosimuraspis Zone of earliest Ordovician age occurs in the basal several–m–thick interval of the Mungok Formation. The Yosimuraspis Zone comprises Yosimuraspis, Jujuyaspis, Elkanaspis, and pliomerid gen. and sp. indeterminate (Kim and Choi, 2000a). The genus Jujuyaspis is an index fossil for the basal Tremadocian (Cooper et al., 2001) and provides a basis for locating the base of the Ordovician System in the Yongwol Group. The succeeding Kainella Zone yields Kainella, Leiostegium, and agnostid gen. and sp. indeterminate (Kim and Choi, 2000b). The Shumardia Zone contains nine trilobite taxa: namely, Micragnostus, Shumardia, Apatokephalus, Hystricurus, Dikelokephalina, Asaphellus, Hukasawaia, Koraipsis, and pliomerid gen. and sp. indeterminate (Kobayashi, 1960; Choi et al., 1994). The Kainella and Shumardia zones are assigned to the middle and late Tremadocian, respectively.

The trilobite faunas of the Cambrian–Ordovician boundary intervals of Korea can be well correlated with those of China and Australia (Figure 2). Although the Quadraticephalus and Mictosaukia faunas of the Taebaek Group have yet to be studied in detail, they show a close affinity with the uppermost Cambrian Quadraticephalus and Mictosaukia zones of North China (Chen et al., 1988). The Fatocephalus fauna of the Yongwol Group can also be correlated with the Mictosaukia Zone of North China and Mictosaukia striata–Fatocephalus Zone of South China (Peng, 1984). The Missisquoia–Onychopyge fauna of the Taebaek Group can be equated with the Missisquoia Zone of North China (Zhou and Zhang, 1985) and possibly North America (Ross et al., 1997). The succeeding Richardsonella fauna is likely contemporaneous with the Richardsonella–Platypeltoides Zone of North China, whereas the trilobite assemblages comparable to the Richardsonella fauna have not been known in the Yongwol Group, South China, and Australia. The early Tremadocian Yosimuraspis Zone is well represented in North China. Yosimuraspis has also been reported from South China (Peng, 1990), which facilitates the correlation of the Yosimuraspis Zone with part of the Apatokephalops yanheensis–Songtaioia cylindrica Assemblage Zone of the Yangtze Platform. Recent recovery of Yosimuraspis and Jujuyaspis in Australia (Shergold, 2000) suggests that Australia also had a close faunal affinity with Korea and North China during the early Tremadocian. Trilobite faunas comparable to the Kainella Zone have not been known in China and Australia. Interestingly, both Kainella and Leiostegium occur in the lower Tremadocian of Argentina (Harrington and Leanza, 1957) and North America (Ross et al., 1997), supporting a middle Tremadocian age for the Kainella Zone of the Taebaeksan Basin. The upper Tremadocian trilobite zones (Asaphellus, Protopliomerops, and Shumardia zones) of the Taebaeksan Basin are dominated by cosmopolitan taxa such as Asaphellus, Protopliomerops, Hystricurus, Apatokephalus, and Shumardia. On the other hand, Koraipsis and Dikelokephalina have a relatively restricted distribution in the upper Tremadocian of Korea, China, and Australia (Duan et al., 1986; Zhou and Fortey, 1986; Shergold, 1991).

Figure 2. Correlation of the trilobite biozones of the Cambrian–Ordovician boundary interval of Korea, China, and Australia. Columns 1–5 are based on the information from the following papers: 1, Kim and Choi, 2000b; Sohn and Choi, 2002; 2, Kobayashi, 1966; Kim et al., 1991; 3, Duan et al., 1986; Zhou and Zhang, 1985; Chen et al., 1988; 4, Peng, 1984, 1990; 5, Shergold, 1991, 2000.

Accepted: June 15, 2003