Magnetic Characteristics Of Precambrian Red Beds In Taihang Mountains And Its Initial Paleoenvironmental Significance

In the northern part of China,especially in the Taihang Mountains,there are hundreds of meters thick of red sandstones and mudstones.Due to age-old and limitation in environmental proxies,the current knowledge on Precambrian environments is quite limited.Thick red beds developed in Precambrian in southern Taihang Mountains are conventionally speculated to be a littoral-shallow marine deposition,which are mainly based on it's the widespread ripple marks,mud cracks and cross bedding and other features.However,they can be formed in a variety of environments,such as lake,river or the Gobi environment.Moreover,the littoral-shallow marine environment is affected by seawater,which belongs to reducing environment or wet oxidation environment,which is contradictory to the fact that red sediment is mostly formed in well-drained oxidizing environment.In this study,detailed field investigation and indoor experiments on the red beds(Mesoproterozoic Era)in southern Taihang were carried out,which were mainly based on Guoliang section located in Huixian,Henan province.The red beds were divedied into two unconformity parts:the lower part is the red sandstones(150m thick),the upper part is interlayered by purple and yellow-grayish mudstones(40m thick),Field investigation found that the color gradient in red beds changes with corresponding bedding,indicating that the red color formed as the sediment deposited.Therefore,the red color in red beds is primary rather than secondary.According to their lithology,samples(n=324)were divided into three categories:red sandstone(n=264),purple red mudstone(n=22)and yellow-grayish mudstone(n=38).Samples were conducted by rock magnetic measurements in room temperature and high temperature,combined by chroma(L*,a*,b*),X ray diffraction and the content of calcium carbonate.Magnetic mineral types,contents and domain size in red beds were obtained.The correlations between magnetic properties and content of calcium carbonate,particle size and lanthanide elements were discussed as well.Results showed that the red quartz sandstones and red mudstones were mainly dominated by fine-grained hematite,while yellow-grayish mudstones displayed high content of paramagnetic minerals,coexisting with small amount of magnetite multi-domain.Red quartz sandstone is weakly magnetic,and the main magnetic mineral is hematite,which suggests that the paleoenvironment is mainly controlled by well-darined oxidizing environments.The ripple marks occur more frequently between depth 100m and 46m,indicating more frequent influence by water,which might attribute to magnetic declination,The interlayered purple red mudstone and gray mudstone may represent the alernative arid and moist conditions process of the period of alternate.The color of the red beds has obvious changes with the change of the layers so the stain and bedding/level to form over the same period,this evidence reflects the formation of red color is not secondary role,but in the long-term high temperature and low rainfall environment to form.The relative content of hematite were higher than that of Quaternary loess.With a multi solution environment of mud cracks and ripples,and the original red color limited conditons.We can exclude the possibility of the formation of shallow water environment and river lake environment.In a word,the red beds in Taihang Mountains likely formed in well-drained oxidizing environments in Mesoproterozoic Era.In the lower part of the profile,it was quite arid and fluctuated greatly with less influence by water.In the middle of the profile,it might become moist,indicated by frequent influence of water(e.g.ripple marks)whereas it was still in oxidizing environment.In the upper profile of clay layer,it was characteristic of alternation of arid and moist environments,forming interlayered purple red and yellow mudstone.It was reported that there were stromatolites and glauconite in part of red beds in Taihang Mountains.However,the profiles studied in this paper have not been found.To some extent,these sedimentary characteristics indicate that they have been affected by the water environment,but the significance of the characteristics of the sediments is more or less deterministic.It can be seen that the red beds in Taihang Mountains are characterized by complex sedimentary structures,which can not be explained by a single sedimentary environment.