Lithostratigraphy and biostratigraphy of the Mungok Formation (Lower Ordovician), Yongwol, Korea

Abstract

The Mungok Formation in Yongwol area is divided into four members: i.e., Karam, Paeiljae, Chommal, and Tumok members in ascending order. The basal Karam Member (29–55 m thick) consists mainly of ribbon rock and grainstone to packstone with intercalation of thin limestone conglomerate beds and chert layers. The Paeiljae Member (28.6–35m thick) is a monotonous sequence of light gray to gray, massive to crudely-bedded dolostone. The Chommal Member (30–51 m thick) is characterized by alternation of ribbon rock and limestone conglomerate facies with occasional intercalation of grainstone to packstone beds in the upper part. The uppermost Tumok Member (41.2–77 m thick) comprises ribbon rock, grainstone to packstone, limestone conglomerate, and marlstone to shale. Trilobites occur in three stratigraphically separated intervals, which are herein referred to theYosimuraspis, Kainella, andShumardia zones, respectively. TheYosimuraspis Zone occurs at the lowermost part of the Karam Member and yieldsYosimuraspis, Jujuyaspis, andElkanaspis, indicating an early Tremadoc age. TheKainella Zone is established based on the occurrence ofKainella andLeiostegium from the basal part of the Chommal Member and is correlated with the middle Tremadoc of North America and Argentina. TheShumardia Zone has a relatively long stratigraphic range covering most of the Tumok Member and is dominated by cosmopolitan trilobite taxa with a few endemic species. This faunal assemblage is well correlated with the late Tremadoc faunas of North China and Australia.