Late Cretaceous lithospheric gravity balance adjustment event

This is a geological event of rapid uplift, rapid cutting and rapid accumulation, forming a thick upper-class molasse formation, and it is also an underground event with high intensity. The inducement of the event is the gravity balance adjustment of the lithosphere. The super-thick molasses formation formed by this event is widely distributed from Hainan to Guangdong, Fujian, Jiangxi to Zhejiang, and its position and phase sequence in the tectonic-sedimentary cycle are also consistent. Although it is difficult to find fossils due to the limitation of ecological environment, it is also a good regional correlation marker layer of Upper Cretaceous in southeastern China. The Danxia landform formed by it is full of towering peaks and steep cliffs dotted with mang forests and ancient temples, and it has become a famous tourist and convalescent resort in southeastern China, such as Fangyan Mountain in Zhejiang, Chicheng Mountain in Fujian, Wuyishan Cave in Taoyuan, Danxia Mountain in Guangdong and Guifeng Mountain in Jiangxi.

(I) Overview of gravity balance adjustment events

Under normal circumstances, the static pressure per unit area on a certain surface in the deep part of the earth is the same, which is called gravity balance, and this surface is called gravity balance surface. However, due to tectonic movement, mantle convection, magmatic activity and other reasons, the mass distribution of the lithosphere will change and the original equilibrium state will be destroyed. The earth has the ability to automatically adjust the mass distribution and make it return to equilibrium, which is called "equilibrium adjustment".

the asthenosphere is a plastic layer in the upper part of the earth, and its top surface can be depressed or uplifted due to the load or unloading of overlying rock columns. The static pressure per unit area of the overlying lithosphere can be changed by the uplift and depression of this surface to achieve the purpose of adjustment. This surface is the balanced adjustment surface or the balanced compensation surface. The specific method of balanced adjustment is that the load-bearing area rises, and a part of the upper low-density Si-Al layer is pushed out of the erosion datum to be eroded, and added into the high-density Si-Mg layer. This process has caused a large number of foothills accumulation-molasse-like construction. By reducing the low density layer and increasing the high density layer, the static pressure on the whole equilibrium adjustment surface is the same, and the lithosphere equilibrium is restored.

the most common equilibrium adjustment occurs in the collision zone of plates. The subduction, superposition and collision and compression of the earth's crust will cause lateral migration of materials and gravity imbalance, which will be automatically adjusted immediately, and the collision orogenic belt will be lowered and the marginal foreland basin will be filled with molasses. This is a typical equilibrium adjustment phenomenon, but its distribution is limited to the collision zone of plates. Another kind of equilibrium adjustment has not been recognized by ordinary geologists, or has not been paid enough attention to, that is, the equilibrium adjustment of gravity imbalance caused by mantle overheating, crustal remelting and magmatic activity, that is, the Cretaceous gravity equilibrium adjustment movement in southeastern China.

(II) The application of balanced adjustment rock cover in solving the relationship among Yongkang Group, Tiantai Group and Qujiang Group

The balanced adjustment event originated in the middle of the late Cretaceous, that is, the end of Chaochuan, and a set of molasse-like formations with conglomerate-dominated piedmont accumulation facies was formed at the beginning of China-Dai period, including the lower part of Dongfang Formation in eastern Zhejiang, the lower part of Zhongdai Formation in western Zhejiang, the lower part of Taihō section of Nanxiong Formation in Guangdong, the lower part of Chongan Formation in Fujian and Guifeng Group in Jiangxi.

it is similar to the common alluvial fan in appearance, but it is completely different in origin. The distribution of alluvial fans or skirts is limited to the estuary area, and there are alluvial fans when there are mountains and ditches, which run through the whole river development stage in time without fixed horizons, nor are they products of the same period, and their spatial distribution is discontinuous; However, in the Late Cretaceous, this set of millstones was overwhelming, starting from Hainan Island, spreading northward to Guangdong, Fujian, Jiangxi and Zhejiang provinces, and reaching the Jiangnan Uplift. Its thickness and continuity have not been seen in other times, so it is difficult to cover it with the word "alluvial fan", so the author calls it "foothill accumulation", which is washed by rivers, gravity and surface runoff. The fundamental reason is that the balance adjustment makes the mountain rise higher and higher, and then it quickly cuts down after exceeding the erosion datum, and the mountain roots gradually disappear, while the sedimentary basin sinks deeper and deeper, and a huge thick molasse-like formation is deposited. This is a typical event sedimentary suite, which should be deposited at the same time in all provinces, but some geologists in Zhejiang believe that there are "three sets of conglomerate", which are "three structural sublayers". There are dozens of red beds basins in Zhejiang province, each of which has this set of conglomerate, but after decades of geological investigation, there are no three sets of conglomerate in the same section in any basin. What should be the explanation? Because all basins are isolated from each other, there is no connection of similar strata between different basins, but there are no fossils in the conglomerate, so it is difficult to determine whether these conglomerates in each basin have the same horizon. The only way is to prove it through the simultaneity of geological events.

Yu Yunwen's plan of juxtaposing three groups was adopted in the report of 1 ∶ 25, map adjustment of Shengxian County in Zhejiang Province, that is, Yongkang Group, "Tiantai Group" and Qujiang Group are simultaneous. The synchronicity of the first two is correct, and the only difference lies in the strong volcanic activity in the east, where there are more volcanic interlayers in Tiantai Group, and there are few or no volcanic interlayers in Yongkang Group in the west. The viewpoint of juxtaposing Qujiang Group with Yongkang Group is hard to agree.

the formation of strata is obviously controlled by tectonic activity. Guantou period is a extensional subsidence period, forming a series of fault basins and uncompensated deposits represented by black clastic rocks; In the Middle Dai period, however, it was a uplifting tectonic environment, forming a super-compensated deposit represented by the red molasse-like formation; It's a bit thoughtless to regard two distinct stages of tectonic evolution as "the same period".

the only basis for comparing zhongdai formation with Guantou formation may be that the age value of basalt sandwiched in zhongdai formation is 15 Ma, which is similar to that of Guantou formation, but only one data makes it not representative enough. if there is a set of data, it can be believed. In addition, it is worth noting that the conglomerate of Zhongdai Formation is overlaid on Chaochuan Formation in places like Zhuliwu in Jinhua, Guantoutan in Longyou and Qingkou in Yiwu in the southern margin of Jinqu Basin, and the TPN mussel fauna assemblage has been found in some places in the latter. This fauna is only found in Guantou Formation and Chaochuan Formation. The lower volcanic rock series is a combination of Nakamura mussel-Ryukyu inkstone-Erguna mussel.

In recent years, dinosaur fossils have been found in Hengshan Formation under the conglomerate corresponding to Zhongdai Formation in Yangtze area, which proves that the age of the conglomerate is the Late Cretaceous, and the age is equivalent to that of Chaochuan Formation.

A dinosaur fossil, Jilantailong, was produced in the Zhongdai Formation, which can be found in the Early Cretaceous and the Late Cretaceous. According to the opinion of appraiser Dong Mingzhi, it is considered to be a relatively primitive species. Dinosaur fossils are usually produced in the Late Cretaceous in the region. Dinosaur eggs in the north of Tiantai County, dinosaur femur and dinosaur eggs in the west of Yongkang City are all found in the upper part of Chaochuan Formation, but there is no report of dinosaur fossils produced in Guantou Formation so far. From the data of event stratigraphy, the author advocates that Qujiang Group is juxtaposed with fangyan Group, but fangyan Group is only distributed in South China block, which is the Caledonian fold basement, with strong tectonic activity and large subsidence, and the later uplift and erosion are also large, so the thickness is larger than Qujiang Group on Yangtze block, and there are many denuded horizons in the upper part, often only the conglomerate in the lower part and a small amount of fluvial facies strata in the upper part remain, which is difficult to compare with Qujiang Group in an all-round way. However, the fangyan Formation in Laijiafangshan section of Tiantai (named Chichengshan Formation by Zhejiang Regional Geological Survey Brigade), which has a high degree of preservation, has a thick lacustrine layer equivalent to Jinhua Formation on the conglomerate, which is similar to Qujiang Group, but it is in pseudo-conformity contact with the basalt of Shengxian Group of Cenozoic, and the corresponding horizon of Quxian Group, which is fluvial in the upper part of Qujiang Group, is missing. This loss is also related to the structural ups and downs of South China block. The profile structure of Qujiang Group is exactly the same as Nanxiong Formation in Nanxiong, Guangdong, which is composed of piedmont conglomerates in the lower part, lacustrine clastic rocks in the middle part and fluvial clastic rocks in the upper part. However, Nanxiong Formation is rich in fossils, rich in late Cretaceous dinosaur fossils.

(3) Discussion on the inducement of the movement of equilibrium adjustment

As for the mechanism of gravity equilibrium adjustment in the late Cretaceous, no one has proposed it, because the origin of this molasse-like formation is considered as alluvial fan in various geological reports. This is the first time that it is considered to be the cause of balanced adjustment movement.

In Cretaceous, it entered the intraplate activity stage, especially after the Fujian-Zhejiang movement, and the geodynamic mechanism changed from the strong compression of Indosinian movement to tension after the stress rebound transition. Therefore, this set of conglomerate can't be a real molasse formation caused by uplift and denudation caused by collision and compression. The formation of many fault basins and the eruption of bimodal volcanic rock series all indicate that the geodynamic mechanism belongs to tension type. Fangyan Formation, Zhongdai Formation, Nanxiong Formation, Hekou Formation and Chong 'an Formation are all sedimentary formations of this type, which constitute Danxia landform everywhere. The characteristics of the latter, in addition to hanging peaks and standing upright, are flat formation occurrence. This feature shows that the uplift in this period was not caused by tectonic compression, but its wide distribution area could only be caused by balanced adjustment. The tensile stress started from the early Cretaceous and increased, and finally Taiwan Province and Hainan Island were separated from the southern landmass in the Cenozoic.

Generally, the time limit for equilibrium adjustment is 13 ~ 15 years, that is, it will reach equilibrium again within 1, years after the start of the movement. For geological history, this is only a short moment, but the piedmont accumulation facies is a rapidly accumulating rock, and the accumulation within this time limit will never be tens of meters, so the author really disagrees with Ma Wuping's classification of the tens of meters of conglomerate in Lishui Basin as "fangyan Formation".

Why on earth did the balancing movement take place at this time? What is its inducement? According to what happened here before the formation of the equilibrium adjustment mantle, the cause of the equilibrium adjustment can be traced back. This is undoubtedly attributed to the volcanic activity of the upper volcanic rock series. It is reasonable to think that the mantle overheating, crustal remelting and the formation of high-level magma chamber have reduced the density of the lithosphere, leading to the uplift and denudation of the crust and the formation of molasse-like formations in a wide area.

(4) Discussion on the reasons for the unbalanced adjustment of the rock suite along the coast

This set of molasse-like formations is widely distributed in the inland of Qiong, Guangdong, Fujian, Jiangxi and Zhejiang provinces, but to the east of Zhenhai-Lufengshen fault, this set of strata is completely absent. The phenomenon itself has attracted the attention of geological circles, but no one has yet sought an explanation for the phenomenon. Volcanic activity in this area is particularly strong. It is reasonable to say that these magma emplacements should cause gravity equilibrium destruction and uplift erosion, but there is no balanced adjustment of rock cover. Why? The author thinks it is probably related to structural control. Zhenhai-Lufeng fault zone is a collision and docking zone between Indochina-Hainan landmass and southern China landmass. The buried depth of the mantle on both sides is different, which is high in the east and low in the west, with a staggered distance of two or three kilometers. It can be seen from the Bouguer gravity anomaly map that this is a steep mantle dip zone with dense isolines in the east and positive sea area. The western continent is negative and the isoline is sparse. In other words, the inland gravity is low, and it must be uplifted and eroded in the equilibrium adjustment; The sea area and coastal areas have high gravity and will not uplift. This is the possible reason why there are millstone-like constructions all over the inland, but there are no traces along the coast. The coincidence of the distribution area of molasse-like rocks with the distribution of upper volcanic rocks just verifies the rationality of the speculation that the imbalance is related to magmatic activity.

The volcanic rocks distributed in the early Cretaceous are unprecedented geological bodies in the southeastern part of China, and the regional correlation is not a big problem, and most of the problems are in the upper volcanic rock series. However, the gravity balance adjustment event in the late Cretaceous and the fault depression event in the early Cretaceous each have their own distinctive marker beds. If the profiles are continuous, it is not difficult to make regional comparison and arbitration on the divergent opinions of predecessors. The above markers can form a Cretaceous framework, and the questionable strata can be filled into the stratigraphic framework according to the sequence, and the correlation relationship can be solved. For example, for the correlation of Hengshan Formation, predecessors thought that the horizon of Hengshan Formation was lower than Guantou Formation. From the Xiyuan section of Duntou Basin, it can be seen that it is between the volcanic event in the early Cretaceous and the equilibrium adjustment event in the late Cretaceous, and it is integrated with the upper-class molasses, which should be equivalent to the horizon of Guantou Formation and Chaochuan Formation. In recent years, dinosaur fossils have been collected in the upper part of Hengshan Formation, which further confirms that its horizon should be equivalent to Chaochuan Formation, and corrects the mistake of placing Hengshan Formation at the top of lower volcanic rock series in the past. Another example is the "Tangshang Formation" in each basin, which is between the balanced adjustment event and the volcanic event. It is necessary to re-understand the practice of putting its horizon on the fangyan Formation.