Lejopyge armata and associated trilobites from the Machari Formation (Middle to Late Cambrian) of Korea and their stratigzaphic significance: a preliminary study  

HONG, P., LEE, J. G. AND CHOI, D. K 

The Machari Formation of Korea has been well known to yield abundant and diverse trilobites of Middle to Late Cambrian age with some brachiopods and gastropods, the `Machari fauna' (Kobayashi, 1962). At present, nine trilobite zones have been established in the formation. The Tonkinella Zone is the only zone representing the Middle Cambrian and occupies anapproximately 15-m-thick succession of dark gray argillaceous limestone and thick-bedded bioclastic grainstone to packstone beds at the basal part of the formation. These beds are succeeded by an unfossiliferous sequence (ca. 45-m-thick) of dark gray slate, dolomitic limestone, and lime breccia, that collectively forms the lower part of the formation. The overlying middle part is characterized by dark gray to black laminated shale and shale-parted limestone facies, and yields diverse trilobites of Late Cambrian age. These trilobites provide a very precise biostratigraphic zonation for the Upper Cambrian of the Machari Formation: i.e., the Glyptagnostus stolidotus, Gbptagnostus reticulates, Proceratopyge tenue_, Hancrania brevilimbata, Eugonocare longifrons, Eochuangia hana, Agnostotes orientalis, and Pseudoyuepingia asaphoides zones in ascending order (Lee, 1995). The upper part is an altemating sequence of light gray dolomitic limestone and laminated black shale beds showing a characteristic "zebra" pattern, but is poorly fossiliferous.

The Tonkinella Zone is characterized by the dominance of Tonkinella, Olenoides, Kootenia, Pemnopsis among others and is convincingly assignable to the middle Middle Cambrian in age. The next higher biozone in the Machari Formation, the Glyptagnostus stolidotus Zone, represents an early Late Cambrian age. Therefore, no late Middle Cambrian trilobite faunas have so far been known in the formation. This study attempted to find, if any, acid-resistant fossils by dissolving the samples collected from the putatively unfossiliferous dolomitic limestone and lime breccia beds of the lower part.One of the samples yields an interesting trilobite assemblage comprising Lejopyge armata, CyclolorenZella (?) sp., Peronopsis sp., ágnostoid gen. and sp. indeterminate, and polymeroid gen. and sp. indeterrninate. Aside from Peronopsis, all of the remaining taxa have been documented for the first time in Korea. The occurrence of Lejopyge armata is particularlysignificant, as it is an index species reported widely within the uppermost Cambrian Lejópyge laeuigata or correlative zones (Robison, 1984). On the other hand, the genus CyclolorenZella is exclusively known in China and has a stratigraphic range (Damesella/Yabeia to Drepanura zones) across the MiddleUpper Cambrian boundary (Yang, 1978; Zhang and Jell, 1987). In short, this acid-resistant trilobite assemblage indicates an uppermost Middle Cambrian in age and supplements the biostratigraphic scheme of the Machari Formation, filling partially the gap between the Tonkinella and the Glyptagnostus stolidotus zones.  

References  

Kobayashi, T, 1962, The Cambrian-Ordovician formations and faunas of South Korea, Part 9, Palaeontology 8, The Machari fauna. Journal of the Faculty Science, University of Tokyo, Section 11, 14,1-152.

Lee, J. G.,1995, Late cambrian trilobites from the Machari Formation, Yongwol, Korea. Unpublished Ph.D. Thesis Seoul National University, Seoul, 418 p.

Robison, R. A.,1984, Cambrian Agnostida of North-America and Greenland, Part I, Ptychagnostidae. The University of Kansas, Paleontological Contributions, Paper 109, 59 p.

Yang, J., 1978, Middle and Upper Canbrian trilobites of western Hunan and eastem Guizhou. Professional Papers of Stratigraphy and Palaeontology, Chinese Academy of Geological Sciences, no. 4, 1-82. (in Chinese)

Zhang, W and Jell, PA., 1987, Cambrian Trilobites of North China. Science Press, Beijing, 459 p.