论洞庭湖区第四纪沉积及其下限

ON QUATERNARY SEDIMENT AND ITS LOWER LIMIT IN DONGTING BASIN REGION~*

The crust has been subsiding,which is resulted in by the swell of uppermantle in Dongting Basin region,since Indo-china movement.The strata of Pliocene E-poch and Quaternary Period have successively been deposited since the later stage of lat-er Cenozoic Era,the thickness of which is over 300 meters.There are HuxianshanGroup(Yanjing Group)(N_2),Miluo Group(Q_1~1),Xinkaipu Group(Q_1~2),BaishajingGroup(Q_2~1),Mawangdui Group(Q_2~2),Baishuijiang Group(Q_3)and Shuiluzhou Group(Q_4)that emerge from bottom to top.The lower limit of Quaternary System depends onthe belongingness of Huxianshan Group,that is to say,whether the group belongs toearly Pleistocene Epoch or not.The paleomagnetic measure of the Huxianshan Groupshows Gauss Normal Polarity Epoch,and the lithology and spore composition reflectsdamp-warm or arid environmental condition,while the paleomagnetie measure of theMiluo Group shows Matuyama Reversed Polarity Epoch,which means that the environ-ment changes into cold condition.According to these results,we think,it is more rea-sonable that the bottom of the Miluo Group be taken to be the lower limit of Quater-nary Systems,the age of which is about 2.5 million year.