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Econ. Environ. Geol. 2021; 54(6): 615-627

Published online December 28, 2021

https://doi.org/10.9719/EEG.2021.54.6.615

© THE KOREAN SOCIETY OF ECONOMIC AND ENVIRONMENTAL GEOLOGY

Controlling Factors on the Development and Connectivity of Fracture Network: An Example from the Baekildo Fault in the Goheung Area

Chae-Eun Park, Seung-Ik Park*

Author Info. +

Department of Geology, Kyungpook National University, Daegu 41566, South Korea

Correspondence to : *Corresponding author : psi@knu.ac.kr

Received: December 2, 2021; Revised: December 18, 2021; Accepted: December 19, 2021

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided original work is properly cited.

Abstract

The Baekildo fault, a dextral strike-slip fault developed in Baekil Island, Goheung-gun, controls the distribution of tuffaceous sandstone and lapilli tuff and shows a complex fracture system around it. In this study, we examined the spatial variation in the geometry and connectivity of the fracture system by using circular sampling and topological analysis based on a detailed fracture trace map. As a result, both intensity and connectivity of the fracture system are higher in tuffaceous sandstone than in lapilli tuff. Furthermore, the degree of the orientation dispersion, intensity, and average length of fracture sets vary depending on the along-strike variation in structural position in the tuffaceous sandstone. Notably, curved fractures abutting the fault at a high angle occur at a fault bend. Based on the detailed observation and analyses of the fracture system, we conclude as follows: (1) the high intensity of the fracture system in the tuffaceous sandstone is caused by the higher content of brittle minerals such as quartz and feldspar. (2) the connectivity of the fracture system gets higher with the increase in the diversity and average length of the fracture sets. Finally, (3) the fault bend with geometric irregularity is interpreted to concentrate and disturb the local stress leading to the curved fractures abutting the fault at a high angle. This contribution will provide important insight into various geologic and structural factors that control the development of fracture systems around faults.

Keywords fracture system, geometry, topology, connectivity, controlling factors

단열계의 발달 및 연결성 제어요소: 고흥지역 백일도단층의 예

박채은 · 박승익*

경북대학교 지질학과

요 약

전라남도 고흥군 백일도에 발달하는 우수향 주향이동 단층인 백일도단층은 응회질 사암과 화산력응회암의 분포를 규제하며 복잡한 단열계를 수반한다. 본 연구에서는 백일도단층 주변의 상세 단열 지도를 기반으로 원조사법 및 위상기하 분석법을 통해 단열계의 기하 및 연결성의 공간적 변화를 파악하였다. 분석 결과 단열계의 밀도와 연결성은 화산력응회암에서보다 응회질 사암에서 더욱 높게 나타난다. 응회암질 사암 내 단층의 주향에 대한 구조적 위치에 따라 단열군의 방향 분산도, 밀도, 그리고 평균 길이가 변화한다. 또한 단층 굴곡 주변에는 단층과 고각을 이루거나 휘어진 단열이 집중되어 발달한다. 상세한 단열 관찰과 분석을 통해 본 연구에서는 다음과 같은 결론을 도출하였다. (1) 응회질 사암 내 단열계의 높은 밀도는 석영, 장석과 같은 취성광물의 높은 비율에 의해 제어된다. (2) 단열계의 연결성은 구조적 위치에 따른 단열군 방향의 다양화 및 연장성의 증가에 의해 향상된다. (3) 불규칙한 기하를 가진 단층 굴곡은 단층 주변의 응력을 집중 및 교란시켜 단층과 고각을 이루거나 휘어진 단열을 발생시킨다. 연구 결과는 단층 주변 단열계의 발달을 제어하는 여러 지질학적, 구조적 요인에 대한 이해를 증진시키는 데에 도움이 될 것으로 기대된다.

주요어 단열계, 기하, 위상기하, 연결성, 제어 요인

Article

Research Paper

Econ. Environ. Geol. 2021; 54(6): 615-627

Published online December 28, 2021 https://doi.org/10.9719/EEG.2021.54.6.615

Copyright © THE KOREAN SOCIETY OF ECONOMIC AND ENVIRONMENTAL GEOLOGY.

Controlling Factors on the Development and Connectivity of Fracture Network: An Example from the Baekildo Fault in the Goheung Area

Chae-Eun Park, Seung-Ik Park*

Department of Geology, Kyungpook National University, Daegu 41566, South Korea

Correspondence to:*Corresponding author : psi@knu.ac.kr

Received: December 2, 2021; Revised: December 18, 2021; Accepted: December 19, 2021

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided original work is properly cited.

Abstract

The Baekildo fault, a dextral strike-slip fault developed in Baekil Island, Goheung-gun, controls the distribution of tuffaceous sandstone and lapilli tuff and shows a complex fracture system around it. In this study, we examined the spatial variation in the geometry and connectivity of the fracture system by using circular sampling and topological analysis based on a detailed fracture trace map. As a result, both intensity and connectivity of the fracture system are higher in tuffaceous sandstone than in lapilli tuff. Furthermore, the degree of the orientation dispersion, intensity, and average length of fracture sets vary depending on the along-strike variation in structural position in the tuffaceous sandstone. Notably, curved fractures abutting the fault at a high angle occur at a fault bend. Based on the detailed observation and analyses of the fracture system, we conclude as follows: (1) the high intensity of the fracture system in the tuffaceous sandstone is caused by the higher content of brittle minerals such as quartz and feldspar. (2) the connectivity of the fracture system gets higher with the increase in the diversity and average length of the fracture sets. Finally, (3) the fault bend with geometric irregularity is interpreted to concentrate and disturb the local stress leading to the curved fractures abutting the fault at a high angle. This contribution will provide important insight into various geologic and structural factors that control the development of fracture systems around faults.

Keywords fracture system, geometry, topology, connectivity, controlling factors

단열계의 발달 및 연결성 제어요소: 고흥지역 백일도단층의 예

박채은 · 박승익*

경북대학교 지질학과

Received: December 2, 2021; Revised: December 18, 2021; Accepted: December 19, 2021

요 약

전라남도 고흥군 백일도에 발달하는 우수향 주향이동 단층인 백일도단층은 응회질 사암과 화산력응회암의 분포를 규제하며 복잡한 단열계를 수반한다. 본 연구에서는 백일도단층 주변의 상세 단열 지도를 기반으로 원조사법 및 위상기하 분석법을 통해 단열계의 기하 및 연결성의 공간적 변화를 파악하였다. 분석 결과 단열계의 밀도와 연결성은 화산력응회암에서보다 응회질 사암에서 더욱 높게 나타난다. 응회암질 사암 내 단층의 주향에 대한 구조적 위치에 따라 단열군의 방향 분산도, 밀도, 그리고 평균 길이가 변화한다. 또한 단층 굴곡 주변에는 단층과 고각을 이루거나 휘어진 단열이 집중되어 발달한다. 상세한 단열 관찰과 분석을 통해 본 연구에서는 다음과 같은 결론을 도출하였다. (1) 응회질 사암 내 단열계의 높은 밀도는 석영, 장석과 같은 취성광물의 높은 비율에 의해 제어된다. (2) 단열계의 연결성은 구조적 위치에 따른 단열군 방향의 다양화 및 연장성의 증가에 의해 향상된다. (3) 불규칙한 기하를 가진 단층 굴곡은 단층 주변의 응력을 집중 및 교란시켜 단층과 고각을 이루거나 휘어진 단열을 발생시킨다. 연구 결과는 단층 주변 단열계의 발달을 제어하는 여러 지질학적, 구조적 요인에 대한 이해를 증진시키는 데에 도움이 될 것으로 기대된다.

주요어 단열계, 기하, 위상기하, 연결성, 제어 요인

    Fig 1.

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    Figure 1.A geological map of the Goheung area (adapted from Choi et al., 2002). The Goheung area, located in the southwestern part of the Gyeongsang Basin, is mainly composed of Precambrian and Jurassic crystalline basement and overlying Cretaceous volcano-sedimentary sequences.
    Economic and Environmental Geology 2021; 54: 615-627https://doi.org/10.9719/EEG.2021.54.6.615

    Fig 2.

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    Figure 2.A drone image displaying the Baekildo fault and regional fracture traces in the northeastern coast of the Baekil Island. White box indicates the location of the studied outcrop shown in Fig. 5. The strike of Baekildo fault is N70°W and the strike of fault bend in the studied outcrop is N79°W.
    Economic and Environmental Geology 2021; 54: 615-627https://doi.org/10.9719/EEG.2021.54.6.615

    Fig 3.

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    Figure 3.Outcrop photographs and micrographs of tuffaceous sandstone (a, c) and lapilli tuff (b, d) in the studied outcrop. Abbreviation - Qtz: Quartz, Lf: Lithic fragment.
    Economic and Environmental Geology 2021; 54: 615-627https://doi.org/10.9719/EEG.2021.54.6.615

    Fig 4.

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    Figure 4.(a) Felsic nodules displaced by a minor fault with a dextral sense of movement. (b) Micrographs of altered lapilli tuff. Abbreviation - Qtz: Quartz, Kfs: K-feldspar. (c) Calcite veins and curved fractures around fault bend.
    Economic and Environmental Geology 2021; 54: 615-627https://doi.org/10.9719/EEG.2021.54.6.615

    Fig 5.

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    Figure 5.Photograph of the studied outcrop displaying the location of grids and circles for fracture mapping, sampling, and analyses.
    Economic and Environmental Geology 2021; 54: 615-627https://doi.org/10.9719/EEG.2021.54.6.615

    Fig 6.

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    Figure 6.(a) A synthetic fracture network showing the classification of nodes and branches: I-node (circle); Y-node (triangle); X-node (diamond); I-I branch (green line); I-C branch (red line); C-C branch (blue line). (b) Ternary diagrams using different types of node and branch for evaluation of CB (the number of fracture per branch). Yellow dots represent the topology of the synthetic fracture network.
    Economic and Environmental Geology 2021; 54: 615-627https://doi.org/10.9719/EEG.2021.54.6.615

    Fig 7.

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    Figure 7.A detailed fracture trace map of the studied outcrop. The Baekildo fault, calcite vein, fracture, alteration zone, and unmapped area covered by sediment are shown in the map.
    Economic and Environmental Geology 2021; 54: 615-627https://doi.org/10.9719/EEG.2021.54.6.615

    Fig 8.

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    Figure 8.Rose diagrams representing fracture orientation and frequency at sampling sites 1 to 7. Four fracture sets were classified based on the rose diagrams.
    Economic and Environmental Geology 2021; 54: 615-627https://doi.org/10.9719/EEG.2021.54.6.615

    Fig 9.

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    Figure 9.Line graph showing the variation in intensity of each fracture set at sampling sites in tuffaceous sandstone (a) and lapilli tuff (b). The intensity of fracture set 1 in tuffaceous sandstone tends to decrease with the change in structural position from site 1 to site 4.
    Economic and Environmental Geology 2021; 54: 615-627https://doi.org/10.9719/EEG.2021.54.6.615

    Fig 10.

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    Figure 10.Line graph representing the variation in average length of each fracture set at sampling sites in tuffaceous sandstone (a) and lapilli tuff (b). Note that the average length of all fracture sets in tuffaceous sandstone tends to increase with the change in structural position from site 1 to site 4.
    Economic and Environmental Geology 2021; 54: 615-627https://doi.org/10.9719/EEG.2021.54.6.615

    Fig 11.

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    Figure 11.Bar graph showing the intensity of fracture system at each sampling site. Note that the fracture intensity in tuffaceous sandstone is higher than that in lapilli tuff.
    Economic and Environmental Geology 2021; 54: 615-627https://doi.org/10.9719/EEG.2021.54.6.615

    Fig 12.

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    Figure 12.Triangular plot of the proportion of node (a) and branch (b) type at each sampling site showing the variation of CB. Sampling site number is shown in the circles or triangles. The fracture systems in both rock types have high connectivity. Note that the fracture connectivity in tuffaceous sandstone gradually increases with the change in structural position from site 1 to site 4.
    Economic and Environmental Geology 2021; 54: 615-627https://doi.org/10.9719/EEG.2021.54.6.615

    Fig 13.

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    Figure 13.Triangular plot of the proportion of node (a) and branch (b) type at each sampling site after the removal of fracture set 1 showing the variation of CB. Sampling site number is shown in the circles or triangles. The higher the intensity of fracture set 1 increases, the more the connectivity decreases in the sampling sites.
    Economic and Environmental Geology 2021; 54: 615-627https://doi.org/10.9719/EEG.2021.54.6.615

    Table 1 . Intensity and average length data for each fracture set at sampling sites 1 to 7..

    Fracture attributesFracture setTuffaceous sandstoneLapilli tuff
    site 1site 2site 3site 4site 5site 6site 7
    Intensity (m/m2)Set 162.9351.3641.4937.9737.305.8821.09
    Set 214.5419.3220.0720.638.299.695.88
    Set 37.1111.9911.4618.825.0313.918.80
    Set 417.7426.9915.3016.465.6814.6918.21
    Average length (m)Set 10.1760.1700.2510.2210.2840.2220.222
    Set 20.0750.1100.1460.1870.0860.1950.140
    Set 30.0890.0570.0910.1340.0680.1730.143
    Set 40.1070.0910.1310.1450.0950.1740.190

    Table 2 . Topological parameters, including node and branch proportions and CB (the number of fracture per branch), at each sampling site..

    Rock typeSampling siteNode proportions (%)Branch proportions (%)Degree of connectivity
    IYXIIICCCCB
    Tuffaceous sandstoneSite 130.1333.1736.701.1919.4379.391.78
    Site 220.8238.1541.030.4812.9486.581.86
    Site 316.3734.9548.680.279.8389.911.90
    Site 411.4129.9558.640.126.5793.321.93
    Lapilli tuffSite 542.4828.1329.403.0228.7168.271.65
    Site 624.3341.0334.640.7215.5783.711.83
    Site 724.3433.5342.130.6915.2284.101.83

    KSEEG
    Aug 30, 2024 Vol.57 No.4, pp. 353~471
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