中国南方红土年代地层学与地层划分问题

南方红土是我国热带、亚热带以各类岩石和第四纪松散沉积物为母质发育的红色风化壳,也是我国分布最广的第四纪土状堆积。本文在探讨第四纪松散沉积物上红色风化壳的形成机制的基础上,根据风化壳发育程度将其划分为砖红土风化壳、红壤土风化壳和红化土风化壳3种类型。南方地区第四纪松散沉积物主要有河流相沉积、滨海相海滩砂和海岸风成砂——"老红砂"以及以下蜀土为主的风尘堆积。近年来在上述沉积物中多处发现旧石器遗址,促进了南方红土的地层年代学研究。根据前人及近期对南方红土的研究成果,讨论了南方红土的年代地层学问题,提出了以风化壳岩性特征为基础的岩石地层单位初步划分框架:老红砂划分为中更新统北海组、晚更新统晋江组;风尘堆积划分为中更新统宣城组、晚更新统下蜀组;河流相沉积只划出中更新统白沙井组,晚更新统留待以后研究再划出。

CHRONOSTRATIGRAPHY AND STRATIGRAPHIC DIVISION OF RED SOIL IN SOUTHERN CHINA

Red soil in southern China is distributed to the north till the south of the Qinling Mountain-Huihe River line,and to the west till the east of Tibet,with an area of about 200×104km2.It has the widest distribution in the form of Quaternary earthy deposits in China.Abundant information about geology,environment,climate,and Paleolithic culture has been available on it.It's significance can be compared favourably with loess deposits in North China.In this paper,we only focus on the red weathering crusts developed on the Quaternary loose sediments.The Quaternary loose sediments in southern China mainly consist of beach sand and coastal dune sand-the "Old Red Sand",fluvial sediments and eolian deposits including Xiashu loess.Recently,some Paleolithic sites have been found in these deposits,which facilitate study of the chronostratigraphy of the red soil.Firstly this paper focuses on exploring the forming mechanism of the red weathering crust on the Quaternary loose sediments.Secondly we provide the results of chronostritigraphy of red weathering crust in Baise Basin of Guangxi Province,Wenchang County of Hainan Province,Jingjiang Region of Fujian Province,and Xuancheng Area of Anhui Province.The accumulation period of the parent material and the ages of the red weathering crust were discussed.The contemporaneous zoning pattern of the red weathering crust during the climate change process was also discussed.Based on the above results and discussion,following general conclusions can be obtained:1.Red soil in southern China is a weathering crust from different parent rocks and Quaternary loose sediments in the tropical and subtropical zones.2.According to the degree of weathering,the weathering crusts in southern China can be divided into three types:① Laterite weathering crust.It was formed in strong weathering process of desiliceous and enriched bauxite,with formation of gibbsite.② red soil weathering crust.It was shaped during week process of desiliceous and enriched bauxite without formation of gibbsite.③ Reddish soil weathering crust in which only hematite was formed.The reddish colour was appeared on the upper part of the parent material.Vermiculated red soil was formed in the strong humid-warm stage when the red weathering crust was developed.3.Different general types of red weathering crust were occurred in different regions,as a result of climate changes along the latitude.From south to north with the weathering process become gradually weakened,laterite weathering crust,red soil weathering crust,and reddish soil weathering crust appeared in sequence.4.Middle Pleistocene was the weathering optimum period in southern China.The north boundary of the vemiculated red soil could reach the south of the Qinling Mountain and it became the paleosol S4～S5 in the Loess Plateau.5.Based on previous results of others and our recent investigations,a preliminary framework of division of rock-stratigraphic units of the red soil,characterized by lithological characters of the weathering crusts,is suggested:"Red Old Sand" deposits are assigned to the Middle Pleistocene Beihai Formation and Late Pleistocene Jinjiang Formation,the eolian deposits to the Middle Pleistocene Xuancheng Formation and the Late Pleistocene Xiashu Formation,fluvial sediments to the Middle Pleistocene Baishajing Formation.The division of the Late Pleistocene fluvial sediments will be determined in the future when more information is available.