The whole basalt platform formed by the volcanic eruption of the Penghu archipelago. The erosion platform formed by the basaltic lava flow is the main landform, standing gently above the sea surface, and surrounded by steep cliff walls with a flat top and no obvious high mountain fluctuations. , Generally called "Fangshan". The surface geology of the Penghu Islands, except that Huayu is exposed as andesite, the remaining main rocks are basalt. Sedimentary rocks, volcanic tuff breccia, or a small amount of tuffy sandstone interbed with basalt, and a small amount of limestone.
The basalt flows are all horizontally distributed, and there is no tilt. The number of layers of basalt flow in different places varies, most places have one to three layers, there are sand and shale interlayer distribution between the streams, and a few places can distinguish up to four layers. Because each layer of basalt flow does not cover the whole area at the time of eruption, the formation of different locations becomes an independent small hydrogeological system. Existing geological drilling data shows that columnar joints and fissures of basalt, loose sandstone or limestone dissolution pores constitute the main groundwater layer in the Penghu area. The hydrogeological structure of the Penghu main island area is divided into three hydrogeological units within 250 meters below the surface, namely the upper and lower groundwater layers, and the middle water blocking layer. The upper groundwater layer is mainly surface soil layer, exposed and basalt layer close to the surface. Due to decompression and weathering, the rock layer cracks, columnar joints and pores are more developed, with higher porosity, and its thickness is about 10 meters . Below the upper groundwater layer is a basalt layer with good water resistance, with thin layers of tuff and sandstone between 20 meters and 120 meters thick. This basalt layer constitutes a water blocking layer that separates the upper and lower groundwater layers. The lower groundwater layer mainly lays beneath the sedimentary rocks of basalt. Its lithology includes coarse sandstone, fine sandstone, mudstone and limestone, with thin layers of volcanic breccia, tuff and basalt. It is currently the main water-drawing layer on Penghu Island. Groundwater stable hydrogen and oxygen isotopes and groundwater carbon 14 studies have shown that groundwater recharge in the upper groundwater layer of the Penghu area is affected by rainwater, and the groundwater in the lower groundwater layer is ancient water bodies. The source of recharge should be further studied.
Figure 14.2-1 Distribution map of geological drilling in Penghu area
Figure 14.2-2 Hydrogeological section from Fenggui Station to Huxi Station
2. Kinmen area
Ruichang Technology Co., Ltd. divided the groundwater into three subdivisions in February 1991 (see Figure 14.2-3). Taking Shangyi and Qionglin as the boundary, it is divided into the eastern and western peninsulas. The A zone of the western peninsula uses the Jinmen layer as the aquifer, with different thicknesses, and the maximum thickness can reach 60 meters. The eastern peninsula is bounded by Shishan and Taiwu mountains again. The granite gneiss rock outcrops in Taiwu and Shishan areas have few sediments and no underground aquifers. The eastern peninsula is further divided into two sedimentary basins, zone B and Area C is affected by granite outcrops. The thickness of the aquifer is only 20-30 meters, which belongs to the modern alluvial layer. Lieyu is regarded as the same area because of the small total area. Area A (Figure 14.2-4, Figure 14.2-5) has two clay layers with poor water permeability and three red soil layers and gravel layers with good water permeability. The first layer of permeable rock layer is a modern sedimentary rock and laterite conglomerate layer, the thickness varies with the terrain, the second layer is between two clay layers, the thickness is relatively fixed, about 10-30 meters, the third layer of permeable rock layer It extends to the base plate, and the thickness can reach more than 100 meters, but the thickness is different due to the depth of the base plate. The thickness of the A layer of the clay layer is about 5 meters to 10 meters, and some areas are sharply extinguished; the thickness of the B layer of the clay layer is thicker than the A layer. On the whole, there is a thicker trend in Qionglin, Shangyi and Guning. All are 10 meters thick. The uppermost layer of area B (Figure 14.2-6, Figure 14.2-7) is a modern sedimentary layer and red clay conglomerate layer with better water permeability, below which is a clay layer with a maximum thickness of less than 5 meters, which is not available in some areas Clay layer; below the clay layer is a layer of sandstone mixed with gravel, the thickness changes with the depth of the base plate. The sedimentary basins in area C (Figure 14.3-8, Figure 14.3-9) have two layers of clay, mainly distributed in the area near the sea of Liluo Bay, while the rest are less. The upper clay layer A is about 3-10 meters thick, and the lower clay layer B is thicker. The clay layer divides the permeable rock layer into three parts. The uppermost layer is less than 10 meters thick. Secondly, the loamy sand layer is between the two clay layers. In some areas, the thickness is up to 20 meters. The lowermost layer of sandstone contains gravel. Extending to the base plate, the rock layers are generally thicker. Use the data obtained above to further draw a hydrogeological raster (Figure 14.2-10). According to the grid diagram, it can be clearly shown that the permeable layer in the Jinxi area is thicker and more widely distributed than the two sedimentary basins in eastern Jinmen, and it is the main underground water storage area in Jinmen.
Figure 14.2-3 Jinmen area groundwater zoning map
Figure 14.2-4 Hydrogeological profile of Jinmen area-Columns map of Area A
Figure 14.2-5 Hydrogeological profile of Jinmen area-Rows map of area A
Figure 14.2-6 Hydrogeological Section of Jinmen Area-Columns Diagram of Area B
Figure 14.2-7 Hydrogeological Section of Jinmen Area-Rows Map of Area B.
Figure 14.2-8 Hydrogeological profile of Jinmen area-Columns map of Area C
Figure 14.2-9 Hydrogeological Section of Jinmen Area-Rows Map of Area C
Figure 14.2-10 Hydrogeological grid pattern in Jinmen area
3. Matsu area
According to the geological literature, the base rock layers of the islands of Matsu Island are composed of Mesozoic granites, and the surface is partially covered with the upper Jurassic rhyolite volcanic rock series; and there are late veins in the granite rock body And the penetration of small intrusive rock masses, and subject to dynamic crushing or thermal metamorphism, local granite bodies have been transformed into gneiss or thermal alteration and mineralization. The development of Matsu groundwater source, in addition to the public wide wells excavated by the Lianjiang County Waterworks, is also widely used by the people, but due to the lack of long-term monitoring and investigation data, the relevant information is insufficient.