摘要
根据西安、山西保德等地第三纪晚期红土的土壤微结构、构造、粒度与化学成分等资料,讨论了该红土的成因、沉积环境和其中极为丰富的碳酸钙结核的物质来源和形成过程.研究表明,红土经历了土壤化,它主要是风成的其中的碳酸钙结核主要是在风化成壤过程中形成的.
This paper deals with the red soil of Neogene based on the data collected by field survey, chemical test and microtexture. The red soil is red and homogeneous in grain size, but it is not of stratification and grains larger than sand generally. The black-brown clay film and calcareous conceretion or conceretion layers can be seen often in the red soil. According to the characteristics of the red soil being of homogeneous grain size and not being of stratification, the origin of the red soil differs from that of normal aqueous deposit.
Through miscroscopc observation, we found a considerable amounf of optically oriented clay film that mainly occurs as flowing colloid in cracks. According to the studing results in pedology, optically oriented clay film is formed by soil-forming process and , if it developed well , is the characteristic of the soils of forest more intensely leached. Consequently it can be determined that the red soil is a kind of paleosoil that underwent morestrong soilformation. The part of the red soil rich in black-brown clay film and optically oriented clay film is a clay horizon of the soil and the calcareous concretion layer beneath the clay horizon is a illuvial horizon. Because the development of soil is in surficial condition, the author infer that the red soil under study is mainly formed in subaerial environment and is of eolian origin. In the light of the developed degree of the optically oriented clay film and the illuvium, It is considered that there were both the forest and the forest-steppe during the formation, the CaCO3content is 3-5% generally and the CaCO3occurs in the form of clastic grains or cementing material in the deposit of river-lacustrine facics. Though the CaCO3takes much amount ( 10-20% ) in the loess , the CaCO3content in recent windblown dust is only 3%. Hence a part of the CaCO3in Quaternary loess was formed after the loess was deposited. The CaCO3content in the red soil of Neogene is much more than that in the loess in Xian suburs. Why does the CaCO3occur in the form of concretion or concretion layers? These are the problems puzzied people. In the northwest China and the Hubci plain, the CaCO3takes 20% or so in recent soil , which indicates that a good deal of CaCO3can be produi in soil- forming process. There is a lot of calcareous concretion or concretion layer beneath the paleosol in the loess , whick demonstrates that soil-forming process is very favourable to producing calcareous concretion or concretion layers. For this reason, it can be deduced that besides a part of CaCO3transported by wind considerable and even most part of CaCO3was produced after the red soil deposited. Biology, rainwater and minerals weathered to release calcium resulted in CaCO3produced and concentrated. The deduction is consistent with the optically orentied clay film found in the red soil.
By chemicial examination of the soil , it is known that SiO2is prominent, about 60% , Al2O3the second , generally 15-16% or so, and thenFe2O3, 6-7%. The chemical composition of the red soil is similar to that of the loess , but there is a little defference in Al2O3that is more amount in the red soil than in the loess. The reason of why the contents of Al2O3and Fe2O3have more amount in the red soil than in the loess is that the accumulation of Al2O3Fe2O3removing much slowly in weathering and soil- forming process.
In the granulometric composition of the red soil, the grains from 0.05 to 0.005mm take 40- 45% in most of the samples, the grade more than 0.05mmis about 20%, and finer than 0.05mm takes 35-50% ia most samples. There is the similarity between the red soil and the loess in the grain size, but the fine grains obviously related to clayization in soil-forming process are more in the red soil than in the loess.
Although 0. 05- 0. 005mm grains of the loess are over 50% , this grade size is changeable in component. It can be changed into very fine grains under warm and moist climate to bring about a dominant grade size less than 0.05mm instead of 0.05-0.005mm. By present observation and measuremen
出处
《沉积学报》
CAS
CSCD
北大核心
1989年第3期113-120,共8页
Acta Sedimentologica Sinica
