Shaanxi is located in central and western China and consists of three parts: the Loess Plateau in northern Shaanxi, the Guanzhong Plain, and the Qinling Mountains in southern Shaanxi. This is a modern geographical component, but do you know the geological evolution in the past, especially how Shaanxi has undergone earth-shaking changes between 300 and 250 million years?
Figure 1, Sauron preyed on ammonia in the Permian
I. Permian (290 million - 250 million years)
Discussing this issue, I have to say the Permian, because this time period is exactly the scope of the Permian. The Permian is the last epoch of the Paleozoic, and the bottom boundary is marked by the first manifestation of the tooth-shaped class streptognathodus isolatus, with an absolute age of (298.9±0.15) ma; the apex is marked by the first manifestation of the tooth-shaped class hindeodus parvus, and the absolute age is (251.902±0.024) ma.
Figure 2, the phosphate formation in the western United States is a typical formation of the Permian
The International Permian stratigraphic system is divided into 9 orders of 3 (Ural, Guadelupe, and Leping). Tooth fossils are the basis for the division of permian high-resolution biological strata, and are the symbol of all the permian global boundary strata (gssp, commonly known as "golden nails") that have been established and planned to be established.
Figure 3, Tooth-shaped stone of Lepingtong in The section of YangxinWujiachong, Hubei Province (Hao Shaobo, 2021)
Corresponding to our Ordos Basin in Shaanxi, the strata of this time period are artificially divided from bottom to top: lower system: Shanxi group, lower stone box group, middle system: upper stone box group, upper system: Shi Qianfeng group. The corresponding time is expressed in terms of period.
Figure 4, Oil Shale in Ordos Basin
Second, Shaanxi in the early Permian
(1) Early Permian Shanxi Period: Affected by the longitudinal migration and closure from west to east in the middle and late period of the Qinling Mountains and the Xingmeng Sea Trough in Craton, the overall elevation of the North China plot, the rapid withdrawal of seawater from the east and west sides of the Ordos Basin, the evolution of the nature of the basin from the land surface sea to the offshore margin basin, and the sedimentary environment changed from the marine facies to the continental sediments that partially retained the residual sea.
Figure 5, Early Permian Paleogeography of Shaanxi
At the beginning of the Shanxi period, the delta plain, the diversion channel and the inter-triage swamp were the main sediments. The delta plains shunt channel sand body is bird's claw-shaped, while several sedimentary swirling sandstones are superimposed and flowery. In the late Shanxi period, only delta phases developed, of which the north of Jingbian was the delta plain, and the south was the leading edge of the delta.
Figure 6, Okavango Delta
The shunt channel microphasic of the northern delta plains is the coarsest sediment, with an upwardly finening orthogonal order. The sand body forms striped and long lens-shaped, with a wealth of plant fossils and carbon chips.
Figure 7, orthogonal sequence of rocks in Tongchuan area
Overall, the east-west difference in the basin has basically disappeared, and the north-south difference has increased. As an underwater uplift, the central paleo-uplift further slowed down, and the segmentation effect on the western and eastern edges of the Ordos Basin was significantly weakened, the sea water was oversized to the south, and the Weibei uplift retreated to the south in a large area. The western margin fissure was filled in by the Benxi period and the Taiyuan period, and the depth of the depression was significantly shallower than that of the previous period. Only in the Alxa Left Banner and Guyuan areas there are deep depressions.
Figure 8, Bulge
(2) Early Permian Lower Stone Box Period: The central paleo-uplift range shifted significantly to the west, the Weibei uplift disintegrated, and the south of the Huating-Zhenyuan-Fuping line changed from the original relative uplift area to a relative depression area. The original relative uplift area of the southern edge of the North China Plateau has been transformed into a sedimentary area.
Figure 9, Permian amphibians bigaul
There is still a depression in the yinchuan area of the western rim, and the stratum north of the Uplift Wushen Banner in the north is gradually thinning, and the Hangjin Banner is the erosion belt. The climate evolved from warm and humid to hot and dry, and the vegetation was greatly reduced, forming a set of yellow-green continental clastic deposits.
Figure 10, Hangjinqi Kubuqi Desert
The northern source area continued to rise, and the river pushed further south. The early shoreline of the lake was in the Jingbian area, with the delta plain in the north and the delta frontier in the south. Later, the shoreline of the lake advanced to the north, still the delta plain in the north and the delta frontier in the south.
Figure 11, Delta Plain
The folds in the western section of the Xingmeng Trough were closed, and affected by its extrusion stress, the paleolu of Alxa and Yinshan in the northern part of the basin was further lifted, the slope became steeper, and the tectonic pattern changed significantly compared with the Shanxi Formation.
Figure 12, folds
3. Shaanxi in the Middle Permian
Stone box period in the middle of the Permian world: the climate became drier, with the acceleration of the base settlement rate, the sedimentary range continued to expand, and the Linyou Guanwu River in the south of the Ordos region and the Houzhou Gongmiao area of Qishan were overly covered on ordovician limestone, and the Upper Stone Box Formation was dominated by variegated sediments dominated by fine-grained clastic rocks.
Figure 13, Middle Permian Paleogeographic Map
In this period, the sedimentary system of rivers-deltas shrank and the range of lakes expanded. Affected by regional sea level rise and fall, North China is mostly affected by coastal intrusion events, and its direct manifestation is siliceous sponge rock. The sedimentary period of the Shangshi Box Formation formed a large area of shallow lakeside lakes and floodplain sediments.
Figure 14, Middle Soliosaurus preying on fish in the Permian ocean
The lithology is dominated by variegated mudstone sandwich layer of fine sandstone, with a single layer thickness of 5-40 m and a cumulative thickness of 80-120 m. Structures such as dry cracks, insect traces, and rain marks are common in mudstones, and the sand and mud are mixed and the sequence is unclear, which belongs to the intermittent sedimentation of flooding lakes. The lakes around Wuqi and Fuxian are relatively deep, and the horizontal stratification of mudstone is widely developed.
Figure 15, Horizontal stratigraphic mudstone
4. Shaanxi in the Late Permian
4.4. Late Permian Stone Thousand Peaks Period (Figure 5-8): The Xingmeng Sea Trough in the northern margin died due to the docking of the Siberian Plate with the North China Plate, and the subduction of the Qinling Sea Trough to the north intensified, causing the North China Plateau to rise as a whole and the sea water to withdraw from the Greater North China Basin.
Figure 16, Paleogeographic map of the Late Permian
The sedimentary environment of the basin has been completely transformed into a continental system, and the topography of the basin still maintains the steep and northern slowness of the south, the asymmetrical ji-shaped lake basin appearance of the deep south and the shallow north, and the delta-lake sedimentary progress shrinks. As shown in the figure below, a typical modern delta is shown.
Figure 17, Permian fern fossils
In the northern part of the basin, the area around Shizuishan-Hangjinqi-Jungarqi is occupied by the Quliu River and the Delta Plain, and the climate continues to be dry and hot, with few plants, which is a set of continental red strata, and the sedimentation of the lake in the southern part of the basin is mainly lakeside.
Figure 18, Red Formation
Due to the intensification of tectonic uplift in the south, the delta sedimentary system is advancing northward, the Pingliang-Qishan-Xingping-Weinan area is the delta plain development area, and the northbound to the West Peak-Linyou-Tongchuan-Hancheng area is characterized by the leading edge of the delta. The sedimentary area of the lake retreats to the south and southwest.
Figure 19, Lakes
In the area of Wuqi-Yan'an-Pucheng in central China, shallow lake sedimentation developed, but the overall lake became shallower and the lake gradually shrank. By the Time of Shiqian Peak, the original central paleo-uplift had completely disappeared, and the main tectonic pattern of the basin began to change from the previous east-west sub-belt to the north-south sub-belt, and the direction of the main tectonic line changed from north-south to east-west.
Figure 20, Lacustrine mudstone
The Permian period in Shaanxi mainly developed natural gas in the northern Shaanxi-Inner Mongolia region, and at present, the second gas production plant of Changqing Oilfield has been built in Yulin, the Sulige Command Center has been built in Wushen banner, and the Daniudi gas field has been built in Xiao trench rabbit.
Figure 21, Xi'an Changqing Oilfield Headquarters
In summary, the Permian period was also an era of strong differentiation of paleogeographic fauna, which made it difficult to compare between major regions. The contrast between China's continental and marine Permian is still lacking systematic research, and the high-precision dating of volcanic ash in the Continental Permian is the key to solving the relative ratio of land and sea. These studies will significantly change the traditional scheme of marine and continental stratigraphy comparisons.