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~*

印支运动以后,洞庭湖区地壳持续下降,至晚新生代晚期,连续沉积了上新世及第四纪地层,总厚度达300m以上.自老至新出露有湖仙山组(盐井组)(N_2)、汩罗组(Q_1~2)、新开铺组(Q_1~2)、白沙井组(Q_2~1)、马王堆组(Q_2~2)、白水江组(Q_3)和水陆洲组(Q_4).本区第四系下限的确定,取决于湖仙山组究竟归属于上新世或早更新世.湖仙山组古地磁测定属高斯正极性世,岩性及孢粉组合均反映湿热或干旱环境,而汨罗组属松山反极性世,气候转冷.故此,我们认为以汩罗组底界作为第四系下限较为合理,其年代在250万年左右.

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.