Establishment Of Standard Color System Of Red Beds In Xinjiang Basin,Jiangxi Province

A set of "lower ash and upper red" caprock rock assemblages were deposited in Xinjiang Basin,Jiangxi Province,which is located in the junction of Yangtze and Cathaysian plates,that is,the lower part is volcanic rocks of Early Cretaceous,and the upper part is continental red clastic rocks(red beds)from Late Cretaceous to Paleogene.Continental red clastic rock is the landscape formation of Danxia landform."The color of Danxia,the brightness of Danxia" is the main description of Danxia landform,and it is also one of the criteria for distinguishing this landform type.However,when comparing the main Danxia landforms in Xinjiang basin in China,it is found that the color of Danxia landforms has certain color difference.This color difference is influenced by factors such as natural light and visual perception,which leads to the subjectivity and experience of color description,and the lack of quantification and standardization.Therefore,in this paper,the red beds(Guifeng Group and Ganzhou Group)exposed in Xinjiang Basin,Jiangxi Province are selected as the research objects,and field investigation and sampling are carried out throughout the country.The reflectance of 243 rock samples is measured by SVC HR-768 portable ground feature spectrometer,and all spectral data are converted into CIE L*a*b* color space,and the simulated colors of all samples are obtained.The following results are obtained:(1)There are similarities and differences in the absorption and reflection of visible light in the Cretaceous red beds of Xinjiang Basin,that is,the reflection spectrum waveforms of different samples are similar,but the reflectivity values are different.The fresh samples rise rapidly in the spectral range of 551 ～ 608 nm,forming a reflection steep slope,while the samples with severe surface weathering rise slowly in this spectral range,and most of them have obvious absorption valleys near 682 nm.In addition,at 565 nm,there is an inflection point between the reflectivity of fresh samples and weathered samples.In the wavelength range of 350 ～ 565 nm,the reflectivity of fresh samples is lower than that of weathered samples,while that of fresh samples is higher than that of weathered samples in the wavelength range of 565 ～ 2500 nm,which may be related to the iron content.(2)The spectral data of the sample is converted into the corresponding simulated color(i.e.CIE L*a*b* color space value)through the spectral tristimulus value.It is found that the sample color and the simulated color have high consistency,which indicates that the conversion between spectral data and color system is feasible and accurate.In addition,the L*a*b* value of the sample is projected into CIE L*a*b* three-dimensional vector space,and all the data are located in the a*+～ b*+space quadrant,which proves that the red layer is reddish.The sample can be divided into five regions in CIE L*a*b*three-dimensional vector space,namely orange-pink series,tan series,brown-red series,gray-purple series and black-brown series.This color system provides a quantitative basis for the color division of red beds.The main colors of Hekou Formation are orange-pink and brown-red.Tangbian Formation mainly shows yellow-brown color and orange-pink color.(3)The simulated color of the sample is compared with the international standard color card one by one,and several color numbers closest to the simulated color are selected from the international standard color card.Finally,the corresponding relationship between the color numbers of the sample and the standard color card is determined by DE2000 color difference calculation method.The results show that the color numbers of Hekou Formation samples are mainly agate brown,rust,Van Dyke brown and Manyueberry red,among which conglomerate is mostly Van Dyke brown and Manyueberry red,and sandstone is mostly agate brown and rust.The color numbers of Tangbian Formation samples are mainly rust,agate brown,chestnut brown and golden brown,among which sandstone is mostly rust,chestnut brown and agate brown,siltstone and mudstone are mostly golden brown.(4)Combined with the field output characteristics of the samples,the color system of Hekou Formation shows the changing law of orange-pink system,brown-red system-orange-pink system-brown-red system from the northeast edge of the basin to the south edge of the basin,and the color system of Tangbian Formation shows the changing law of yellow-brown system,orange-pink system-orange-pink system,brown-red system-brown-red system from the east to the west of the basin.The change of color system of red layer may be related to lithology and weathering degree.With the increase of rock particle size,the L* value(brightness)of the sample tends to decrease,while a*/b* tends to increase,with the red tone gradually increasing and the yellow tone gradually weakening.Weathered samples covered with lichens will cause the red bed to darken,showing Somali brown,mocha black and grey red residue.Weathered samples are also related to mineral hydrolysis,resulting in the red bed becoming white,which reflects the change of iron oxide content to some extent.This project has certain reference value for the quantitative research of red bed color.