Geological structure in Zhangxi 'an County, Gansu Province.

Zhangxi 'an County is located in the transition zone between Longxi Platform and Qinling Mountain in the Loess Plateau. The west Qinling Mountains account for 7 1.9% of the county's total area, and the Longxi Terrace accounts for 28. 1%. Geologically, it belongs to the western segment of Qinling tectonic belt.

(1) stratum

Represented by Gui Qingshan in the south and Yangzheshan in the southwest, the exposed strata are mainly Devonian, Carboniferous and Permian, especially the Permian strata are relatively complete, complete in sequence, rich in fossils and complete in biological categories.

The Upper Devonian Dacaotan Group is located in the south of Yangshan Mountain and the north of Gui Qingshan, and the named section is located between Dacaotan Village of Yangshan Mountain Hotel. Divided into upper and lower sections. The lower member is yellowish brown, purplish red timely sandstone and feldspar sandstone, mixed with siltstone, conglomerate and a small amount of purplish red iron-bearing timely sandstone, with a thickness of about 1600 m; The upper member is purplish red silty fine sandstone with feldspar sandstone, shale, limestone and sandy limestone, with a thickness of about 700-1500m.

The Upper Devonian-Lower Shiyan Wangjiadian Formation is distributed in Xijing and Wangjiadian areas. Dark gray and green gray silty sandstone, sandstone and shale, with a small amount of thin argillaceous limestone. This layer is in unconformity contact with the overlying Lower Permian and the underlying Xihanshui Formation. It contains the plant fossil Letophtoeum. Diamond, paleontological fossils. Short-winged orders and close-winged orders of Lepidoptera and other genera.

The lower part of Permian Li Shanchang Formation is flesh red, with light gray thick-bedded massive flash bioclastic limestone, and the upper part is light gray thick-bedded massive flash detrital biolimestone, which is deposited in the shallow facies of platform margin and contains a small amount of molting fossils. It is in contact with the underlying Dacaotan Group fault and locally unconformity with the underlying Wangjiadian Formation. The top is covered with Quaternary, and the horizon is below Dongzhakou Formation. The typical profile is located 2 kilometers east of Caozihe Township, Zhang Xi 'an County.

The lower part of Dongzhakou Formation of Lower Permian is purplish red massive gravelly argillaceous siltstone and grayish green medium-thick gravelly fine sandstone mixed with colored calcareous conglomerate. The upper part is dark gray medium-thick micrite limestone and sandy limestone mixed with marl, calcareous limestone and calcareous silty mudstone. There are abundant dragonfly fossils in the upper part, and only a few appear in the lower part. The bottom is in contact with the underlying fault of Dacaotan Group. The typical profile is located in Zhagoukou, Dongxia Village, Jinzhong Township, Zhang Xi 'an County.

The Permian Yaxia Formation is dark gray, gray-black with light gray thick layered bright limestone bioclastic limestone, containing bioclastic micrite limestone, see flint nodules and bands, with colored calcareous mudstone in the middle and mud gravel in some parts. It is dominated by open platform facies deposits, with semi-limited platform, platform tidal flat and shoal facies deposits. Dinosaur fossils are extremely rich, and a small number of coral fossils appear. Integrated contact with the underlying Dongzhakou Formation. The typical profile is located in Zhagoukou, Dongxia Village, Jinzhong Township, Zhang Xi 'an County.

Yangjiahe Formation of Lower Permian is grayish white and light gray, with middle-layer massive sparky bioclastic limestone, internal clastic limestone mixed with micrite bioclastic limestone, and the upper part is purplish red massive sparky organism-internal clastic limestone. It is dominated by platform margin shoal facies and semi-limited platform facies. Odonata and foraminifera are extremely rich. Integrated contact with Xiafuyaxia Formation. The typical profile is located in Yangjiahe, Caozihe Township, Zhang Xi 'an County.

The lower part of Lower Permian Jiangjiali Formation is yellowish brown thin mudstone, micritic bioclastic limestone mixed with yellow shale. The upper part is purple shale and silty micrite limestone, which gradually becomes calcareous fine sandstone. Mainly open platform facies deposits. The fossils are mainly decapod and coral, while Odonata and ammonites are rare. Integrated contact with Xiayangjiahe Formation. Cover the quaternary system. The typical profile is located in Jiangjiali Village, Caotan Township, Zhang Xi 'an County.

The Upper Permian Shiguan Formation is gray and purplish red, with thick bioclastic limestone, marl, oolitic limestone and argillaceous siltstone. It consists of open platform facies, marginal shoal facies and limited platform facies. Fossils are mainly brachiopods, as well as Odonata, corals and ammonites. The contact relationship with the upper and lower strata is unknown. The typical profile is located in Shiguan Village, Wuhu Bridge Township, Zhang Xi 'an County.

Tertiary is mainly brick red and purple loose conglomerate, sandstone, siltstone and a small amount of shale, which is deposited in inland lake basin. Quaternary eluvial deposits, slope deposits, alluvial deposits, proluvial deposits and cave deposits are mainly distributed on both sides of rivers, mountain slopes, valleys and caves.

Yanjing Formation According to the geological drilling data of Yanjing Salt Mine, the rock salt ore body is integrated in the third stratum of Linxia Formation of Neogene. The buried depth of salt roof is 70-176m, and the buried depth of floor is190-390m. As early as Cretaceous, this area was an inland saltwater lake with hundreds of square kilometers. After repeated drying for dozens of times, a large range of rock salt deposits were formed, which were buried underground with geological changes, forming an impermeable layer underground, and the salt deposits were preserved, belonging to inland lake deposits. There are 65,438+00 proven mining values. In the process of geological changes, the crust and salt mines broke along the riverbed of Zhanghe River, which caused the crust in Yanjing area to tilt up and the buried layer was shallow, which made the brine on the surface of rock salt seep out along the geological cracks, forming a large area of outcrop salt springs, and the rest were buried deep underground and difficult to find.

There are few intrusive rocks exposed in the stratum lithology, which are mainly dike-like.

The granite porphyry dike under the cliff is located under the cliff in Jinzhong Township. Geographical coordinates: east longitude10413 ′ 20 ″ north latitude 34 48 ′ 4 ″. Granite porphyry dike occurs in impure limestone with shale in upper Devonian-lower Devonian sandstone, and the dike intrudes along 30 strike fault. The dam is about 5 meters wide. The frequency of occurrence is 120 < 30-40. There are two copper-bearing barite veins in the sandstone fractures on the upper wall of the vein, which invade along the sandstone fractures. The length and width are 80m, 4.8 m, 104m and 1.88m respectively. The former has pure veins and contains 0.0 1-0.03% copper.

The zhengshang rock mass of Beigousi is located in Beigousi Village, Dacaotan Township, with the geographical coordinates of east longitude10413' 49 "and north latitude 34 4' 41". Gray-white syenite, massive structure, equigranular structure. Located on the north side of Dongping-Beigousi fault, it is in the form of a rock plant with a diameter of about 1400 m, which is in intrusive contact with the Ba Du Formation of Lower Carboniferous and the Li Shanchang Formation of Lower Permian, and in the southwest it is in fault contact with the Dacaotan Group of Upper Devonian.