Tremadoc Trilobites from the Mungog Formation, Yeongweol, Korea

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Abstract
The Mungog Formation consists predominantly of carbonate with lesser amounts of shale, representing a shallow marine environment. It is divided into four members based on the association of dominant lithofacies such as ribbon rock, grainstone to packstone, flat-pebble conglomerate, and marlstone to shale facies. The basal member, ca. 45 m thick, consists mainly of ribbon rock and grainstone to packstone with intercalations of thin flat-pebble conglomerate beds. Trilobites occur at the lowermost several-m-thick interval of the basal member. The lower member is recognized by the occurrence of a thick (30-35 m in thickness) sequence of dolostone and no trilobites have been recovered from this member yet. The middle member, 35–60 m thick, is characterized by alternations of ribbon rock and flat-pebble conglomerate lithofacies with occasional intercalations of grainstone to packstone beds. Kainella, the only known trilobite from the middle member, occurs at the lowest bed (ca. 30 cm thick) of the member. The upper member, 50-60 m thick, comprises ribbon rock, grainstone to packstone, flat-pebble conglomerate, and marlstone to shale facies. This member yields relatively abundant and diverse trilobites along with brachiopods, crinoids, ostracods, and fossils of uncertain zoological affinity. Fossil occurrences in the Mungog Formation are confined to the three stratigraphically separated intervals, which are referred to the lower, middle, and upper fauna, respectively. The lower fauna consists dominantly of Yosimuraspis and subordinately of Jujuyaspis and Pseudokainella and indicates an early Tremadoc in age. Comparable faunas are well represented in North China. The middle fauna represented solely by Kainella can be correlated with the middle Tremadoc faunas of North America and Argentina. The upper fauna comprises mainly cosmopolitan trilobite taxa including Micragnostus, Asaphellus, Shumardia, Hystricurus, Apatokephalus, and Dikelokephalina, with some endemic species such as Hukasaw+aia cylindrica and Koraipsis spinus. It is closely comparable to the Dumugol fauna of the Duwibong sequence in Korea and late Tremadoc faunas of North China and Australia.

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