TRILOBITES FROM THE LEJOPYGE ARMATA ZONE (UPPER MIDDLE CAMBRIAN) OF THE MACHARI FORMATION, YONGWOL GROUP, KOREA

Abstract

Acid-resistant specimens of trilobites were recovered from the dissolved residues of lime mudstone collected from the lower part of the Machari Formation, Yongwol Group, Taebaeksan Basin, Korea. One of the samples yields a trilobite assemblage comprising Lejopyge armata (Linnarsson, 1869), Lisogoragnostus coreanicus n. sp., agnostid genus and species indeterminate, am- magnostid genus and species indeterminate, clavagnostid genus and species indeterminate, Cyclolorenzella sp., and Eoshengia? sp. The stratigraphic interval yielding the trilobite assemblage is herein designated as the Lejopyge armata Zone, which is correlated well with the upper Middle Cambrian biozones established elsewhere. This provides the first biostratigraphic reference point for the upper Middle Cambrian sequence of the Yongwol Group and fills partially the biostratigraphic gap between theTonkinella (middle Middle Cambrian) and the Glyptagnostus stolidotus (uppermost Middle Cambrian) zones in the Machari Formation. HE MACHARI Formation in Korea has been well known to yield diverse and abundant Middle to Upper Cambrian tri- lobites with some brachiopods and gastropods, the so-called Ma- chari fauna (Kobayashi, 1962). Based on the faunal assemblages, Kobayashi (1962) established biostratigraphic zones within the formation: i.e., in ascending order Olenoides (including Tonki- nella and Eochuangia faunas), Komaspis-Iwayaspis, and Olenus- Glyptagnostus zones; and the Hancrania fauna was considered more or less contemporaneous with the Olenus-Glyptagnostus Zone. However, the trilobite faunal successions of the Machari Formation have not been well understood due to the complicated geologic structure of the Yongwol area. Nonetheless, these bio- stratigraphic units have been widely employed in Korea, until recently Lee and Choi (1994, 1995, 1996) and Choi and Lee (1995) documented systematically the Upper Cambrian trilobites of the Machari Formation and, based on the new collections, pro- posed a profoundly modified biostratigraphic zonation for the Up- per Cambrian portion. This scheme includes in ascending order the Glyptagnostus stolidotus, Glyptagnostus reticulatus, Procer- atopyge tenuis, Hancrania brevilimbata, Eugonocare longifrons, Eochuangia hana, Agnostotes orientalis, and Pseudoyuepingia asaphoides zones (Lee et al., 1997). These Upper Cambrian bio- zones provided greatly improved biostratigraphic correlation of Korea with other parts of the world. To date, the Tonkinella Zone established by Kobayashi (1962) is the oldest zone within the Machari Formation representing a middle Middle Cambrian in age and is characterized by domi- nance of Tonkinella, Olenoides, Kootenia, and Peronopsis among others. The next higher biozone recognized is the Glyptagnostus stolidotus Zone of uppermost Middle Cambrian age. Therefore no upper Middle Cambrian trilobite faunas have hitherto been known in the Machari Formation. We recovered acid-resistant fossils from the putatively unfossiliferous lime mudstone beds that lie between the Tonkinella and Glyptagnostus stolidotuszones. They yield a trilobite assemblage comprising Lejopyge armata along with species previously undescribed in Korea. The occurrence of Lejopyge armata is particularly significant, as it is globally re- stricted within the upper Middle Cambrian (Robison, 1984). We propose the Lejopyge armata Zone for the interval yielding the nominal taxon and associated trilobites. The specific objectives of this article are to describe trilobites of the Lejopyge armataZone from the lower part of the Machari Formation and to address their significance in biostratigraphic correlation with other parts of the world. GEOLOGIC SETTING AND STRATIGRAPHY