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The First Discovery of Quaternary Fault in the Western Part of the South Yangsan Fault - Sinwoo Site

양산단층 남부 이서 지역에서 최초로 발견된 제4기 단층 - 신우지점

  • Choi, Sung-Ja (Geology Division, Korea Institute of Geoscience and Mineral Resources) ;
  • Ghim, Yong Sik (Geology Division, Korea Institute of Geoscience and Mineral Resources) ;
  • Cheon, Youngbeom (Geology Division, Korea Institute of Geoscience and Mineral Resources) ;
  • Ko, Kyoungtae (Geology Division, Korea Institute of Geoscience and Mineral Resources)
  • 최성자 (한국지질자원연구원 국토지질연구본부) ;
  • 김용식 (한국지질자원연구원 국토지질연구본부) ;
  • 천영범 (한국지질자원연구원 국토지질연구본부) ;
  • 고경태 (한국지질자원연구원 국토지질연구본부)
  • Received : 2019.06.10
  • Accepted : 2019.06.26
  • Published : 2019.06.28

Abstract

During the detailed geological survey around the southern Yangsan Fault, we newly found a Quaternary fault outcrop, which cuts unconsolidated sediments. The fault named the Sinwoo site, located in the Sinwoo pasture, Miho-ri, Duseo-myeon, Ulsan metropolitan city, is the first discovered Quaternary fault near the western part of the south Yangsan Fault. In this study, we provide information on characteristics of fault geometry and unconsolidated sediment at Sinwoo site based on the analysis data of topography, drainage, and lineament around the study site. The fault site is situated at pediment slope, but fan-shaped middle terrace, as well as thick sediment exposed at low terrace, indicates that the unconsolidated sediments have been deposited in the alluvial fan environment. The drainage develops to the third-order drainage system, and the first and the second drainage system meet at right angles to each other and form a radial drainage pattern. In addition, the NE-SW direction lineaments can be identified on the basis of the curvature of the river and the step of the topographic relief, running over the Sinwoo site. The fault of $N30-35^{\circ}E/79-82^{\circ}SE$ shows ~ 5.8 m apparent vertical offset and dominantly reverse-slip sense based on slickenline, rotation of pebbles, and drag folding at footwall. However, some discontinuous sediments observed in the footwall are interpreted as fissure-filling materials due to the strike-slip movement. Now, we are under multidisciplinary investigations of additional field survey and age dating in order to determine the evolution of Sinwoo site fault during the Quaternary.

양산단층 남부 지역을 대상으로 한 정밀 지질조사 수행 중에 울산광역시 두서면 미호리 신우목장 절개사면에서 미고결퇴적층을 자르고 있는 단층이 확인되었으며, 이는 양산단층 남부 인근 이서 지역에서 최초로 발견된 제4기 단층노두이다. 본 연구에서는 신우지점 주변의 지형, 수계 및 선형구조 분석 결과를 바탕으로 신우지점에서 관찰되는 단층의 기하학적 특성 및 미고결퇴적층의 특징 등의 정보를 제공하고자 한다. 단층이 발달하고 있는 지형은 침식 산록 완사면으로 보이나 저위면의 두꺼운 퇴적층과 중위면의 부채꼴(Fan) 양상은 충적선상지 지형 특징을 보인다. 수계는 3차 수계까지 발달하여 1차 수계와 2차 수계가 서로 직각으로 만나면서 방사상 형태를 보인다. 또한 하천의 휘어짐과 지형면의 단차를 통해 본 지점을 통과하는 북동-남서 방향의 선형구조들이 인지되었다. 신우지점의 단층은 $N30-35^{\circ}E/79-82^{\circ}SE$ 의 자세를 보이며, 약 5.8 m 의 겉보기 수직 변위가 관찰되는 역단층의 기하를 보인다. 단층면에서 확인된 단층조선과 단층 하반의 최상부 미고결퇴적층 내 역들의 방향성도 전형적인 역단층 운동감각을 지시한다. 하지만 단층의 하반 내 일부 단층대를 따라 불연속적인 퇴적물이 관찰되며, 이는 주향이동운동에 의한 열극구조를 상위의 미고결퇴적물이 채운 것으로 판단된다. 현재 절대연령 측정 및 추가적인 조사가 수행 중이며, 추후 이러한 다학제적 조사결과가 도출된다면 본 신우지점 제4기 단층노두에 대한 정확한 단층의 형성 및 발달사가 규명될 수 있을 것으로 사료된다.

Keywords

JOHGB2_2019_v52n3_251_f0001.png 이미지

Fig. 1. Simplified regional geological map of the eastern part of the Gyeongsang basin with the location of the study area, modified after Chwae et al. (1995) and Son et al. (2015).

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Fig. 2. Geomorphological map around the Sinwoo site based on hillshade images from LiDAR-derived DEM and topographic profiles across the piedmont; a green circle:location of the fault; orange: higher surface; light green:middle surface; pink: lower surface; red dashed-lines:lineaments; blue lines: drainages; A-A’ and B-B’: topographic profile line.

JOHGB2_2019_v52n3_251_f0003.png 이미지

Fig. 3. A panoramic photograph of the Sinwoo site fault and unconsolidated sediment units.

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Fig. 4. Photographs of the detailed sedimentary facies.

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Fig. 5. Photographs of the detailed fault-related structures at the Sinwoo site fault.

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