The Fabrics And Depositional Subsurface Hydrology Of Calcretes In Loess-palesol In Bailu Tableland,xi'an,China

The hardpan calcretes widely exsists in loess-paleosol but rarely in ravelly ground which has experienced certain diagenesis.The study of its origin is of great significance to the restoration of paleoclimate and paleo-subsurface hydrology in the region.By means of field survey,magnifying glass,polarizing microscope and X-ray diffraction annlysis,the paper studied the field attitude,macro-fabrics and micro-fabrics,mineral facies of the calcretes from the bottom of the 10th and 11th layers of paleosol(S₁₀,S₁₁)and its paleo-subsurface hydrology indications.The paper obtained the following results:(1)In the early Pleistocene loess-paleosol of Bailu Tableland in Xi'an,the hardpan calcrets generally exists at the bottom of paleosol layers,with a thickness of 0.3m-1m and exposed length of tens of meters to hundreds of meters.The hardpan calcretes have a sharp contact with the underlying and overlying loess-paleosol.The upper interface of hardpan calcretes is usually gentle,wavy and flat,while the lower interface is nodular.The degree of calcareous cementation between the upper and lower parts of the specimen is obviously different.The degree of calcareous cementation is high in the upper part but low in the lower part.In addition to pores and fissures,there are also needle-like pores in the hardpan calcretes which is the remains of paleosol.(2)The observation under polarizing microscope shows that the degree of calcifications of the micromass is high or low,the relative content of the 2 types of micromass is one of the main factors that determine the hardness of calcretes,the contact lines of the 2 types of micromass are irregular,showing the structure of silkworm erosion and harbor and concave or convex,which suggests that replacement may occurred during the transition from weak to strong calcification.The content of grains is low,mainly including quartz,feldspar,clay minerals and heavy minerals(rutile,zircon and pyroxene).In the strongly and weakly calcified micromass,the content of mineral grains is quite different,the quartz content in the strong calcified basement is generally lower than 5%,while in the weak calcified basement is 10%-15%,indicate that a portion of the quartz dissolves in the process from weak to strong calcification.The cement is mainly calcareous cement,including crystallitic and sparry calcite.The sparry calcite has cross ctenoid or mosaic texture,and generally can be seen for one to two generations.The pores are mainly intergranular pores of minerals and solution pores of quartz.The fissure is the weathered fissure of Paleosol.In addition,biological characteristics are rarely seen under polarizing microscope.(3)X-ray diffraction analysis show that calcite,quartz and clay are the main minerals in the powder samples of calcretes which accounting for 85.5%-94.5%of the total minerals,of which calcite accounts for 36.1%-70.2%,quartz accounts for 14.3%-27.8%and clay accounts for 9.1%-26.2%.The main clay minerals are mixed-layer of illite and montmorillonite(Occupying the total clay mineral 53%-75%)and illite(Occupying the total clay mineral 19%-41%),followed by a small amount of kaolinite(Occupying the total clay mineral 6%-8%).Compared with the weak calcified samples,the calcite content of the strong calcified samples is significantly higher.The results of a pore statistics method based on image processing show that there are 49%of the increased calcite of calcretes from the S₁₀ is caused by cementaion,51%is caused by replacement,there are 30.9%of the increased calcite of calcretes from the S₁₁ is caused by cementaion,69.1%is caused by replacement.By comparing the content of clay minerals and calcite,it is inferred that there may be clay minerals replaced by calcite during the formation of the hardpan calcretes,but the mechanism of replacement needs further study.The preliminary study showed that no transformation from montmorillonite to illite occurred.(4)This paper presents a view of the formation of hardpan calcretes formed in the groundwater environment,and the evidence includes:(1)The occurrence of the hardpan calcretes in the field is similar to that of the paleosol aquifer,and the site where it is exposed is probably the outlets for discharge of regional groundwater.(2)A large number of sparry calcite with a particle size of hundreds of microns were observed under polarizing microscope,and few biological characteristics were observed.(3)The thin loess with low Ca CO₃ content cannot provide sufficient material source for the formation of hardpan calcretes.(4)The hardpan calcretes in loess-paleosol lack the typical characteristics of pedogenic carbonate and palustrine carbonate(such as laminae,pisoliths,pendent,classic pedogenic profile etc.).On this basis,the paper puts forward the formation model of hardpan calcretes.According to the thickness of Paleosol S₁₀,S₁₁ and the ages of S₉ and S₁₂,the age of hardpan calcretes at the bottom of S₁₀ is estimated to be about 0.78Ma,and hardpan calcretes at the bottom of S₁₁ is about 0.83Ma.