5.4.3. 1 climate and environment
Based on the analysis of the regional distribution characteristics of brownish-red calcareous soil, Guilin should be in a hot subtropical climate environment in the late Cretaceous. At present, geologists generally believe that the red bed is the product of arid climate, especially when it contains or contains evaporite, not to mention. There are red karst strata in the Late Cretaceous in Guilin, some of which belong to sporopollen fossils of plant species growing in arid climate, so some people infer that the Late Cretaceous in Guilin was a hot and arid climate.
The results of soil science research show that although calcareous soil is a kind of non-zonal soil, its formation is mainly the result of chemical leaching of carbonate rocks, so its formation and distribution are influenced by bioclimatic zones, showing certain zonal characteristics. At present, reddish brown calcareous soil is mainly distributed in tropical and subtropical regions (Table 5. 12). Due to the influence of monsoon climate, this area is generally characterized by high temperature and rain, alternating dry and wet seasons, but mainly damp and hot. Therefore, it is generally believed that the brownish-red calcareous soil produced in the weathering crust of carbonate rocks is mainly the product of chemical dissolution under the hot and humid climate conditions in tropical and subtropical regions. Because red calcareous soil is often distributed on the carbonate paleoweathering crust with gentle terrain, it is often considered that it has a long history of formation and is deeply influenced by paleoclimate. However, this ancient climate is also dominated by damp heat. For example, Mr. Sun Dianqing once pointed out that [2 1] Dagu-Lushan interglacial period was "an important reddening period, and the hot and humid climate affected the whole country, resulting in a large area of red soil weathering crust, which became an important symbol layer of this interglacial period". Another example is the discontinuous distribution of red weathering crust in some carbonate areas in northern China. For this phenomenon, Tian Jisheng believes that it is the result of dissolution and weathering of carbonate rocks under the hot and humid climate conditions between the first and second glacial periods of Quaternary Pleistocene [22].
On the other hand, in-depth study on lateritization and geochemical characteristics of iron and transformation conditions shows that ferrous iron with low valence can only be transformed into ferric oxide with high valence under the condition of water medium, and humid heat is a good climatic condition to realize this transformation.
Because the material composition of the late Cretaceous red calcareous mudstone in Guilin is ancient brownish red calcareous soil, Guilin should be in a hot subtropical climate dominated by damp heat in the late Cretaceous. The microfossil assemblage in calcareous mudstone is mainly composed of plant sporopollen fossils showing warm and humid climate, and also has water marks such as grain sequence layer and stripe layer. In addition, there is no evaporite found in Cretaceous karst construction in Guilin, which can also be used as an example. Although some Mesozoic-Cenozoic basins in neighboring areas such as Guangdong, Southwest Guangxi and Hunan have some gypsum and salt beds in Cretaceous, they are large and thin, dispersed or of secondary types such as veins and secondary fibers [23], which can only reflect that the basin is in a short-term arid climate environment during sedimentary diagenesis and cannot be used as the main index to restore the regional paleoclimate environment.
Sedimentary accumulation environment of red calcareous mudstone in 5.4.3.2
According to the analysis of soil-forming characteristics and conditions of brownish-red calcareous soil, the sedimentary accumulation environment in Guilin in the late Cretaceous was mainly small intermountain basins, karst fault basins and karst depressions. Although the remaining outcrops of the late Cretaceous red karst structure in Guilin are scattered on the plane, they are widely distributed and often intermittently distributed. In the vertical direction, it can reach the top of Monkey Mountain at an altitude of 580 meters, and can be seen in the Li Jiacun borehole near the sea level. Because of the extensiveness of this plane distribution and the difference of vertical distribution, it is easy to be mistaken for the late Cretaceous Guilin red bed deposition covering all karst terrain, including the current area with an altitude of 300 ~ 600 m. According to the analysis of soil-forming characteristics and conditions of red calcareous soil, it shows that it is difficult for Guilin to form such a large area of thick red bed coverage in the late Cretaceous.
According to the dissolution residual soil of carbonate rocks mentioned above, one of the characteristics of carbonate rocks is that the amount of soil is very small. Therefore, it is generally believed that it is inconceivable to form a thick red weathering crust on the towering carbonate peak without a stable tectonic environment, a thick parent rock to provide the source of soil-forming materials and a long-term hot and humid climate. From the previous discussion, it can be seen that the main soil-forming period can reach 65.438+0.2 billion years (J-K), of which the Late Cretaceous in Guilin lasted about 24 million years, dominated by hot and humid climate conditions, indicating that the region had the climate and time conditions to form thick red weathering crust at that time, but further analysis shows that the structural conditions and rock conditions are difficult to meet.
First of all, from the analysis of structural conditions, during the Cretaceous period, due to the influence of Yanshan movement, the multi-stage fault structure activity in Guilin was relatively strong, accompanied by block ups and downs. Although the research results are beneficial to the vertical infiltration of karst water into rocks, due to its strong activity, hydrogeological conditions often change, and a stable karst soil-forming environment cannot be formed. At present, the outcrop area of the remaining late Cretaceous red karst structure is small, and most of them develop along the fault structure, as evidenced by the gravity collapse and calcium-dissolved conglomerate (about 75%) accumulated at the foot of the slope.
Secondly, from the analysis of rock conditions, the breccia in the late Cretaceous red karst construction in Guilin is mainly carbonate rocks, indicating that the material source of this construction is mainly in-situ or near-source carbonate rocks. If the total insoluble content of red calcareous mudstone in Late Cretaceous is 70. 1 1% (according to Table 5. 13) and its bulk density is 2.5 1t/m3[5], the formula 5.4.1./kloc can be roughly applied. If the red karst stratum is composed of dissolved calcareous conglomerate, dissolved calcareous clastic limestone and dissolved calcareous mudstone, and the calcareous mudstone and calcareous iron mud cement in conglomerate and calcareous clastic limestone only occupy about 25% of the stratum, it can be roughly estimated that every 1m thick red karst stratum needs to dissolve 17m thick carbonate rock. According to this calculation, if the Late Cretaceous red karst construction overburden with a thickness of about 100m is to be formed in Guilin, carbonate rocks with a thickness of about 1700m need to be weathered and dissolved, which does not include the thickness of carbonate rocks needed to form breccia under construction. At present, the maximum thickness of carbonate rocks from the Middle Devonian Donggangling Formation to the Lower Carboniferous near Guilin is about 2000 meters. If the carbonate rocks with this thickness were denuded in the Late Cretaceous, the carbonate rocks in Guilin should have been completely denuded. However, at present, the Upper Paleozoic carbonate rocks are still exposed in a large area in Guilin, and the late Cretaceous red karst strata are in karst unconformity contact with the lower Carboniferous and middle and upper Devonian carbonate rocks, respectively, which not only shows that the sedimentary-accumulation topography itself is uneven, but also shows that the erosion thickness in the ancient soil-forming period cannot be so large.
From the analysis of the above conditions, it is impossible for Guilin to form a large-scale and extremely thick red karst building cover in the late Cretaceous. Judging from the occurrence and structural characteristics of the above-mentioned red karst strata, the sedimentary environment at that time was mainly some small ancient intermountain basins, karst fault basins, karst depressions and caves. Due to the influence of fault structure, the subsidence of karst fault basin is large, and it is a concentrated gathering place of surrounding dissolution residues, so the thickness of sediment-sediment is relatively large. For example, in Tannan karst fault basin, the late Cretaceous red karst strata with a total thickness of about 1.80m are developed, including conglomerate (>: 100m) and mudstone (more than 70m without roof). By studying the rock structure, composition characteristics, stratigraphic types and occurrence characteristics of red karst strata, the formation environment is discussed, and it is considered that it can be divided into five microfacies [5], namely: karst basin (depression) valley, karst fault (depression) valley, karst slope toe, karst depression (depression) land-karst cave-fracture.
Types of palaeogeomorphic assemblage in 5.4.3.3
According to the analysis of the formation process and distribution law of brownish-red calcareous soil, Guilin karst developed in the late Cretaceous, with peaks and depressions as the main landform combination types. Because red beds are often regarded as the product of arid and hot climate, and red karst was formed in Guilin in the late Cretaceous, some people think that the development speed of karst in Guilin in the late Cretaceous slowed down. However, from the analysis of soil formation process and soil distribution law of carbonate rocks, the late Cretaceous should be the most intense period of karst development in Mesozoic.
According to the research of soil scientists, different subtypes of calcareous soil have different degrees of dissolution and weathering. From black to brown to red calcareous soil, the potassium removal effect of mica minerals gradually deepened (Table 5.14); The content of clay minerals increases gradually. All these show that the chemical weathering of insoluble residues in limestone is gradually strengthened from black to brown and then to red calcareous soil, and it also shows that brown-red calcareous soil has the longest weathering time and is the product of the formation of dissolved residual soil in carbonate rocks. Therefore, the red karst structure left in Guilin in the Late Cretaceous is not evidence of slow karst development, but can be used as one of the evidence of strong karst development in this area in the Late Cretaceous.
The research of soil scientists also shows that the development of calcareous soil is closely related to the development of karst landforms, and different types of calcareous soil reflect different karst landforms (Table 5. 12). To sum up, the calcareous soil formed in the Late Cretaceous in Guilin is mainly brown with a small amount of red calcareous soil. Therefore, it can be inferred from Table 5.2 and the distribution characteristics of calcareous mudstone that the karst landform in Guilin in the late Cretaceous was mainly composed of peak forests and depressions, and a small area of peak forest plains and karst basins developed in local karst catchments. This also shows from another side that the inference that the sedimentary environment is mainly small basins and depressions in the previous section is credible.
In addition, the distribution law of calcareous soil also gives us some enlightenment. Although the distribution elevation of late Cretaceous red calcareous mudstone is different now, it should be formed in relatively low-lying places, such as slope toe, depression (valley), solution gap, karst cave and so on. There are small-scale granular ordered layers and stripes in a considerable number of red karst buildings, and the amount of Cao and MgO contained in them is very high, which can prove this.
Some people may also ask this question. Since these red karst buildings should have been formed in low-lying places, why are they common at the top of the mountain now? This is because the karst stratum is composed of dissolution and weathering residues of carbonate rocks, so when it experiences karstification with the surrounding carbonate rocks which form normal topography after diagenesis, the dissolution rate is naturally lower than that of the surrounding carbonate rocks, which eventually leads to the mutual transformation of positive and negative topography. For example, according to Liu et al.' s latest research [24], many Cretaceous red beds in faulted or depressed basins such as Luoding Group, Nanxiong Group and Danxia Group in Guangdong Province were formed by a large number of limestone gravel and calcium-rich solutions provided by limestone mounds around the basin around 1 100 million years ago. Although the basic characteristics of this red bed karst landform are similar to those of ordinary limestone, the karst development is worse than that of ordinary limestone on the whole. Therefore, with the long-term dissolution and weathering, the positive carbonate peak gradually evolved into a negative karst depression, while the adjacent depression covered by the red karst group suffered from weak dissolution and gradually evolved into a positive carbonate peak (Figure 5. 19). In addition, the differential fluctuation caused by the faults at the edge of the basin and the depression may also uplift the red karst strata in the basin and the depression and form peaks. Because there is such an evolutionary relationship between the occurrence of red karst construction in late Cretaceous and the evolution of karst landforms, we must not simply make a historical and dynamic analysis when using red karst construction to restore ancient karst landforms and explore the law of karst development.
Fig. 5. 19 schematic diagram of the relationship between the outcrop position (occurrence) of red karst structure in late Cretaceous and the evolution of karst landform.
1.D2d-C 1 carbonate rock; 2.K2 red karst building; 3.q loose deposits; 4. Fractures and cracks
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