Journal of the Geological Society of Korea (지질학회지)

The Geological Society of Korea (대한지질학회)
권호 표지

The Mungog Formation (Lower Ordovician) in the Northern Part of Yeongweol Area: Lithostratigraphic Subdivision and Trilobite Faunal Assemblages

영월 북부지역의 문곡층(하부 오르도비스계): 암석층서적 세분과 삼엽충화석군

Park, Ki-Hyeon;Choi, Duck-Keun;Kim, Jeong-Hwan
박기현;최덕근;김정환

  • Published : 19940400

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Abstract

The Mungog Formation is divided into four informal members based on the association of four dominant lithofacies (ribbon rock, grainstone to packstone, flat - pebble conglomerate, and marlstone to shale): i.e., basal member, ca 45 m thick, composed mainly of ribbon rock and grains-tone to packstone; lower member, $30{\sim}35m$ thick, consisting exclusively of grainstone to packstone; middle member, $35{\sim}60m$ thick, characterized by alternations of ribbon rock and flat - pebble conglomerate beds; and upper member, $50{\sim}60m$ thick, comprising ribbon rock, grainstone to packstone, flat - pebble conglomerate, and marlstone to shale facies. Fossil occurrences are confined to the two stratigraphically separated intervals, which are refered to assemblages A and B, respectively. Assemblage A from the lowermost part of the basal member is dominated by kainellid and olenid trilobites and indicates an early Tremadocian in age, while Assemblage B from the lower part of the upper member is composed mainly of cosmopolitan trilobite genera representing a late Tremadocian age. In addition, we demonstrate that the subdivision of the Mungog Formation into members and associated faunal assemblages are helpful to understand the geologic structure of the Yeongweol - Machari area. In the western part of the Machari thrust, two thrust faults, the Mohari and Dumog thrusts, are recognized and confirmed by the occurrence of the basal member producing Assembalge A.

영월 북부지역에 분포하는 문곡층의 암상을 크게 4가지 -리본암, 입자암, 평력석회암 및 이회암-셰일 -로 구분하고 이들의 층서적 산출에 근거하여 문곡층을 4개의 비공식 멤버로 구분하였다. 최하부 약 45m 두께의 기저멤버는 주로 리본암과 입자암으로 이루어지며 부수적으로 양은 평력석회암층이 협재한다. 하부멤버는 두꺼운 (>30 m) 괴상의 암회색 돌로스톤으로 이루어지며, 중부멤버 (두께 $35{\sim}60m$)는 리본암과 평력석회암의 교호에 의하여 특징지워진다. 상부멤버 ($50{\sim}60m$)에는 리본암, 입자암, 평력석회암, 이회암-셰일암상이 모두 관찰되며, 이회암-셰일층이 빈번하게 나타남이 특징이다. 화석의 산출은 기저멤버의 최하부와 상부멤버의 하부에 국한되며, 이들을 각각 화석군 A와 B라고 명명하였다. 화석군 A에서 산출되는 삼엽충 Yosimuraspis, Jujuyaspis, Pseudokainella 등은 전기 트레마도시안의 시기를 지시하며, 화석군 B는 대부분 후기 트레마도시안 시기에 전세계적으로 넓게 서식하였던 삼엽충들로 이루어진다. 이 연구의 부수적 성과로 야외에서 문곡층의 멤버와 화석군을 정확히 인지할 경우, 영월 -마차리지역의 지질구조를 이해하는데 도움이 될 수 있는 예를 제시하였다. 즉, 마차리 드러스트 서부지역의 두 곳(기와골과 두목)에서 화석군 A를 산출하는 문곡층의 기저멤버를 확인함으로서 종래에 알려졌던 모하리 단층을 인정하는 한편, 새로운 드러스트 단층으로 두목 단층을 설정하였다.

Keywords

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