Neoproterozoic to Quaternary strata are exposed in this area. According to the image characteristics and mapping requirements, it is divided into 17 stratigraphic units. According to the image map of three stratigraphic sections (Figure 3.5) of Shanghengtang, Wu Jialong and Outangdi, combined with regional aerial remote sensing images, the image characteristics of each stratigraphic unit in the survey area are summarized as follows.
Figure 3.5 Strata Profile
◎ Quaternary (Q): loose alluvial and diluvial deposits. Distributed in alpine valleys, valleys and valley plains. Most of the images are occupied by cultivated land, mainly paddy fields, followed by dry land (orchards, vegetable gardens, tea gardens, etc.). ). Valley Plain (Xujiang) is concentrated in villages and towns except cultivated land. For example, Jiangshan City is located on alluvial and diluvial sediments of this landform type. The image features on the aerial photograph are very clear, with white-gray tone, typical mesh or grid shape, and the boundary with the underlying stratum is clear (Figure 3.6a).
Qujiang Group (K2Q): purplish red conglomerate and sandstone. Scattered in Jigongshan, Mulian Cave, Laohushan and the east of Waitongling. Aerial photos show that rugged peaks and low and gentle hills are formed due to the influence of uplift fault blocks. In the gentle hilly area, there are covered parts, mostly cultivated in dry land, with a relatively uniform gray tone and undeveloped vegetation. It is in angular unconformity or fault contact with the underlying stratum, and the boundary is clear.
Fig. 3.6 Image characteristics of strata units
◎ Dingjiashan Formation (P 1d): dark gray flint and grayish yellow siltstone. It is distributed in strips from the bottom of loquat pomelo to Litongling. The aerial photos show the potholes and shadows, forming part of the banded mountains. The boundary with the underlying stratum is clear.
◎ Shitoushan Formation (C2-P 1s): gray-dark gray thick massive limestone. Mainly distributed in Maquelingdi-Li Peng-Xia 'nantang area, which constitutes the core stratum of syncline. The main landforms are isolated hills and dissolved depressions. In karst depression, the soil cover is thick, and there are more paddy fields and dry land. The vegetation on the lonely mountain is undeveloped and gray-dark gray. There are also quarries of different sizes. The boundary with the underlying stratum is clear (Figure 3.6b).
◎ Outangdi Formation (C2o): The lithology of this formation is complex, mainly composed of yellow-white feldspathic chronological sandstone, conglomerate mixed with purplish red siltstone, limestone lens, gray-white to yellow-white chronological conglomerate, etc. Mainly distributed in Fangjiashan, Outangdi, Tangya to Hejiashan, with sporadic outcrops in northern Wang Jia and Litongling. Geomorphologically, it often constitutes a large-scale mountain, which is distributed in the northeast, with undeveloped gullies, rough shadow lines, pitted vegetation and uneven color tone, and has a transitional relationship with the underlying stratum (Figure 3.6c).
Yejiatang Formation (C 1y): purple argillaceous siltstone, gray-white glutenite, black-gray sandy shale, clayey rock and siltstone. It is distributed in the west of Outangdi Formation in the west of Xishan Mountain, with sporadic outcrops in the south of Litong Mountain. Geomorphologically, it forms a large-scale mountain together with the Outangdi Formation, but the exposed area is small and mostly distributed along the ridge. In some areas, the bedding is clear, the vegetation is developed, the color is dark, and the boundary with the underlying Changwu Formation is clear, showing a false integration contact relationship. The boundary often shows that one side of the ridge changes from steep to slow, and there are obvious differences in colors, landforms, vegetation, shadows and other signs on both sides of the boundary (Figure 3.6d).
Changwu Formation (O3c): grayish green and yellowish green shale mixed with fine sandstone and siltstone. It is mainly distributed in the area from the bottom of Outang to Peng Tao and from Tangtou to Changfeng, with a wide range. On the aerial photograph, the image features are obvious, showing gray tone; The degree of vegetation development is relatively low; Gully development, dish-shaped; It is a low hilly landform with a "ginger-like" plane shape. The edge of the mound is smooth, the top of the mound is round, and the radial micro-gullies above the mound are well developed, which is the main symbol to identify the stratum. The boundary with the underlying stratum is mostly clear (Figure 3.6e), and some sections need to be traced and determined.
◎ Huangnigang Formation (O3h): brick red shale and silty shale. The thickness of this layer is small and stable, generally around 20M. The image on the aerial photograph is a uniform light gray narrow strip with smooth and delicate surface, often exposed on the hillside or saddle, and the vegetation is undeveloped. The boundary with the underlying stratum is clear (Figure 3.6f).
◎ Yanwashan Formation (O2y): grayish green and purplish red nodular argillaceous limestone. Its distribution is basically consistent with Huangnigang Formation, but it is also exposed in Jin Mu dock, Waipeng and Lianhuashan. Tectonically located in the core of anticline and its vicinity. The bedding image is clear and the vegetation is undeveloped. Located at the core of the anticline, it is mostly a valley or saddle terrain distributed in the northeast, and the dry land and villages between the valleys are concentrated. When it is a wing layer, it is a block hilly terrain distributed intermittently in the northeast. The boundary with the underlying stratum is clear (Figure 3.6g).
◎ Le Hu Formation (O 1-2h): thin black siliceous rocks, siltstones and shales. It is distributed in the line from Shanghengtang to Quanban and Grasshopper Mountain. The characteristics of the image are: dark tone, well-developed vegetation and dense pockmarked shadows; In geomorphology, ridges often form, perpendicular to the strike of rock strata, and gullies develop; The boundary with the underlying stratum is clear.
◎ Ningguo Formation (O 1n): dark gray black graptolite shale. Distributed in the area from Shanghengtang, Zhengjiawu to the west of Lingjiashan. The thickness of this layer is small, generally 20 ~ 40m. Because of the soft lithology and easy weathering, the terrain of this layer is typical saddle-shaped, with uneven vegetation development and gray color. The boundary with the underlying stratum is mostly clear (Figure 3.6h), and some sections need to be traced and determined.
◎ Yinzhubu Formation (O 1y): It is mainly composed of yellow-green variegated shale, which is distributed in strips along the southeast foot of the hilly land along the river. Gully development, uneven vegetation development and uneven color tone. Irregular massive mountains are distributed intermittently, and the micro-gullies without vegetation are well developed. The boundary with the underlying stratum is clear (Figure 3.6i).
◎ Edge group (): gray thin limestone mixed with marl or calcareous shale. Distributed in Jiangshangang and the northern part of Xishan. Due to the influence of structure, the rocks in this group are silicified to varying degrees, often forming tall mountains. The vegetation is well developed, and the hue is gray to dark gray. Gullies are scarce and quarries are distributed. It should be pointed out that the image characteristics of the edge groups in this area are quite different from those that are less affected by fault structures (Figure 3.6j).
◎ Tanghe Formation (): black-gray thin-layer siliceous rocks and carbonaceous shale. The exposed range is narrow, and only a small amount is distributed in the northeast section of Xishan Mountain. Topographically, it is located on one side of the ridge, with dense vegetation and dark gray tone. The contact boundary with the underlying stratum is not obvious.
Xifengsi Formation (Z2x): mainly grayish white dolomite. It is distributed in Jiangshangang and the northeast section of Xishan, along the ridge. The image color is dark gray, the vegetation is well developed and there are few gullies.
◎ Leigongwu Formation-Tang Zhi Formation (Z 1l-Z 1z): Leigongwu Formation consists of grayish green and grayish purple massive gravelly mudstone and silty mudstone. Tang Zhi Formation is mainly composed of light gray siliceous shale and silty shale. Although the lithology is quite different and the sedimentary environment is inconsistent, they are all located in the Xishan structural belt in the survey area, and the rocks are generally strongly silicified and difficult to distinguish, so they are merged into one stratigraphic unit.
After silicification, the lithology of this group of rocks is hard and its weathering resistance is enhanced, forming a belt-shaped mountain with steep east and gentle west. Rock joints and fissures are developed, and mixed coniferous and broad-leaved forests are flourishing and dark in color.
Shangshu Formation (Pt3s): mainly composed of dark purple massive acidic molten volcanic breccia, tuff and rhyolite. Distributed in the east of Jiangshan Port. It belongs to low hilly terrain, with brain-like or ginger-like landforms in plane, light gray-gray tone, dense distribution of dry land and villages, and developed gullies, and it is a dendritic water system.
Image characteristics of geological structures in 3.3.2.2.
Image characteristics of strata with different occurrences (1)
The horizontal strata show some unique image and geomorphological features on remote sensing images. In areas where the terrain is severely cut, the underlying ground is exposed, the newer strata are distributed on the top of the mountain or watershed, and the older strata are distributed in valleys or gullies. In the image, different tones or micro-geomorphic zones are closed concentric circles, shells, lace and other shadow patterns around the mountain or ridge. Due to differential weathering, stepped topography, table mountain, Pingdingshan and other landforms have been formed.
On the remote sensing image, the vertical strata appear as parallel lines or arc strips with different tones or micro-landforms. These belts are not affected by terrain cutting, and can cross valleys and mountains and extend in their own direction. The hard vertical stratum forms a ridge or ridged terrain with two symmetrical slopes; The weak strata form flat valleys and depressions. The combination of the two forms a "rib" terrain.
Inclined rock stratum is the most common rock stratum form. Due to the different degrees of occurrence and terrain cutting, various complex graphic features can be formed. In the flat area, because the terrain is uncut or very weak, the inclined strata show similar image characteristics to the vertical strata, so it is difficult to judge their dip angle and dip angle. However, in areas where the surface is severely cut, the inclined strata are characterized by a series of parallel broken lines, jagged lines, arcs and other image features in remote sensing images composed of different tones or micro-landform strips.
(2) Explain the occurrence of inclined strata
The method to explain the occurrence of inclined strata is to determine the relationship between the stratigraphic triangle and the direction of terrain slope through observation, analysis and judgment. Stratigraphic triangle refers to a triangular plane formed by connecting the high point (ridge point) on the outcrop line of the same inclined stratum with two adjacent points (valley points) on the remote sensing image. Its actual shape is influenced by lithology, terrain erosion degree, rock dip angle and other factors, and it can be triangular, semi-circular, half-moon, trapezoidal and other shapes in the image. Many stratigraphic triangles are often imbricate images along the dip angle of inclined strata, and are intermittently connected into wavy, serrated or irregular broken lines along the strike of strata. Stratigraphic triangle is the best indicator to judge stratigraphic occurrence.
There are many methods to judge the occurrence of strata by using the triangular surface of strata. Only one commonly used method-eye estimation method is introduced here.
Visual inspection method is to judge the occurrence of rock stratum according to the size of the vertex angle of rock stratum triangle on remote sensing images. When the dip angle of rock stratum is small, its vertex angle is also small (Figure 3.7). This method can only qualitatively distinguish the dip and dip of strata. Generally, the dip angles of horizontal, gentle, medium, steep and vertical strata are defined as < 5, 5 ~ 20, 20 ~ 45, 45 ~ 80 and > 80 respectively. When using aerial photos to estimate the occurrence, we should try our best to choose the rock stratum triangle in the center of the photos to reduce the error caused by the central projection.
Fig. 3.7 Distinguishing the dip angle of strata by using the image characteristics of strata triangle.
(3) Interpretation of fold structure
According to the interpretation marks of fold structure, two synclines and two anticlines can be explained in this area.
◎ Changtang syncline: the core is Changwu Formation, and the two wings are composed of strata such as Huangnigang Formation and Yanwashan Formation, which are distributed in the northeast. The triangular faces of the two-wing strata are clear, and the tips point in the opposite direction; The characteristic images of the two wings strata are symmetrically distributed, with the exposed width of the west wing strata being larger and the exposed width of the east wing strata being smaller, indicating that the axial plane tends to the southeast. According to the field investigation data, there are still some secondary folds in Changwu Formation of syncline core, which shows that syncline is a compound syncline.
◎ Li Peng syncline: The core strata are Dingjiashan Formation and Shitoushan Formation, and the two wings are composed of Outangdi Formation and Yejiatang Formation. The hub is distributed in the northeast, and the axial plane tends to the southeast. The strata in the west wing are exposed completely, the triangular surface of strata is clear, and the sharp points are opposite; Due to the influence of fault structure, the strata in the east wing are not completely exposed, and the stratigraphic sequence in some areas is reversed, but it is still symmetrical with the west wing. Karst landforms are widely developed in the core layer. Longitudinal, transverse and oblique faults are developed, and syncline is damaged to some extent.
Wu Jialong anticline: the core stratum is Yanwashan Formation, and the two wings are composed of Huangnigang Formation and Changwu Formation, and the hub is distributed in the northeast direction. The triangular faces of the two wings are clear, the tips point in opposite directions, and the stratigraphic images are symmetrically distributed. In the nodular limestone in the core of anticline, the longitudinal joints are developed, the rocks are easy to be weathered, and most of them form valleys or saddles along the axis, with abundant groundwater and concentrated villages.
◎ Xishan inversion anticline: it is distributed along the northeast of Xishan, and the core strata are Tang Zhi Formation-Leigongwu Formation and Xifengsi Formation. The two wings are Bianbian Formation, Yinzhubu Formation-Changwu Formation and Yejiatang Formation. The strata in the west wing of anticline are exposed in disorder, and some strata are inverted. For example, the western part of the northern part of Xishan Mountain and Waitongling can be seen that the older strata are covered with the newer strata. The east wing is a normal wing. Because the NE-trending and NW-trending fault structures in the anticline are well developed, silicified broken breccia belts are mostly formed in the core strata, which constitute the main body of Xishan Mountain, and the image characteristics of the two wings of the anticline are not obvious.
(4) Image characteristics of fault structure
Fault structures are developed in this area, which can be divided into four groups according to strike: northeast, northwest, northwest and nearly north-south. Their image characteristics are mainly manifested in the following aspects:
◎ Obvious tone abnormality often occurs along the fracture strike: when the fracture scale is large, a tone abnormal zone or tone interface with a certain width is often formed. For example, the NNW-trending fault at the bottom of the Lotus Pond and the NE-trending silicification fracture of the Xishan Mountain have obvious abnormal tone bands (Figure 3.8a); The fault on the eastern edge of Xishan Mountain is characterized by abnormal tone interface. When the crack scale is small, the abnormal tone line often appears along the crack direction, that is, the abnormal tone displayed along the crack exposure line is darker or brighter than the background tone.
Figure 3.8 Image Features of Fault Structure
◎ Loss, repetition or lateral dislocation of strata on both sides of faults: Faults with loss and repetition of strata are generally difficult to identify intuitively, and it is necessary to be familiar with stratigraphic sequence or stratigraphic lithologic combination to give a correct explanation. Generally, it is easy to identify the faults with staggered strata (rock mass), such as Wu Jialong, Changfeng-Lingjiashan, Lianhuashan-WU GANG and other NW-trending faults (Figure 3.8c, D). These faults stagger strata to varying degrees, resulting in obvious differences in tones on both sides of the faults.
◎ There are linear scarps (cliffs), ridges, depressions or valleys along the fault strike: due to the diversity of fault structure characteristics and properties, fault structures have different landforms. For example, the NE-trending reverse fault on the western edge of Xishan Mountain often has cliffs and cliffs along its strike (Figure 3.8b); The NE-trending reverse fault of Fangjiashan-Outangdi is almost distributed along the ridge line, forming a ridge-like terrain (Figure 3.8e). A large number of NW-trending linear depressions or valleys in the Ordovician distribution area are mostly related to NW-trending tensile and torsional faults.