藏南札达盆地上中新统—上新统托林组介形类群落

Upper Miocene-Pliocene ostracoda communities of Tuolin Formation in the Zanda basin,southern Tibet

笔者在西藏南部札达盆地新生代沉积地层中获得了丰富的介形类化石,根据介形类动物群在地层剖面上的分布规律,建立了两个介形类组合带:(1)Ilyocypris bradyi-Cyclocypris orum-Leucocythere dorsotuberosa组合带;(2)Leucocytherella-Candoniella zadaensis-Leucocythere mirabilis组合带。通过对研究区介形类组合带与国内外其他地区相同层位的介形类组合对比研究,将札达盆地托林组的时代厘定为中新世晚期—上新世最晚期。根据介形类动物群在剖面上的分布规律,自下而上建立了7个介形类群落:Candona-Candoniella群落;Ilyocypris-Cyclocypris群落;Leucocythere-Candona群落;Ilyocypris-Leucocythere群落;Leucocythere-Leucocytherella-Candona群落;Leucocythere mirabilis-Candona群落和Ilyocypris-Leucocytherella群落。通过对介形类群落详细的特征分析并结合磁性地层年代学数据,将札达盆地9.5~1.7 Ma的古气候划分为6个期次:(1)9.5~8.4 Ma为凉湿期;(2)8.4~6.3 Ma为温湿期;(3)6.3~5.5 Ma为凉湿期;(4)5.5~4.4 Ma为温湿期;(5)4.4~2.8 Ma为冷湿期;(6)2.8~1.7 Ma为温暖偏干期。将研究区9.5 Ma以来的气候演化特征与全球气候演变对比认为:札达盆地9.5~6.3 Ma间的气候以暖湿为主,可能与来自印度的东南季风加强有关;6.3~3.6 Ma间札达盆地古气候分析显示为相对暖湿期,存在气候波动,可能与来自印度洋的东南季风再次加强有关;3.6 Ma以后由于受全球气候变冷、冬季风加强及青藏高原强烈隆升的影响,札达盆地气候向寒冷干旱的环境转变,在2.8~1.7 Ma气候却变得温暖潮湿。

The Cenozoic strata of the Zanda basin in southern Tibet have yielded abundant ostracoda fossils.Two ostracoda assemblages have been established based on the distribution patterns of the ostracoda fauna on the stratigraphic profile:①Ilyocypris bradyi-Cyclocypris orum-Leucocythere dorsotuberosa assemblage;and②Leucocytherella-Candoniella zadaensis-Leucocythere mirabilis assemblage.Through comparison of the ostracod assemblages of the study area with those found worldwide at the same stratum,the age of the Tuolin Formation in Zanda basin has been redefined to late Miocene to Pliocene.Based on the distribution characteristics of the fossil succession,the ostracoda fauna in the study area has been be divided into seven ostracoda communities in ascending order:Candona-Candoniella community;Ilyocypris-Cyclocypris community;Leucocythere-Candona community;Ilyocypris-Leucocythere community;Leucocythere-Leucocytherella-Candona community;Leucocythere mirabilis-Candona community and Ilyocypris-Leucocytherella community.Through detailed character analysis of ostracod communities and the magnetostratigraphic chronological data,the paleoclimate of the Zanda basin during 9.5~1.7 Ma was divided into six periods:①the cool and wet period(9.5~8.4 Ma);②the warm and humid period(8.4~6.3 Ma);③the cool and wet period(6.3~5.5 Ma);④the warm and humid period(5.5~4.4 Ma);⑤the cold and wet period(4.4~2.8 Ma);and⑥the warm and dry period(2.8~1.7 Ma).Through a comparison of the climate evolution characteristics of the study area with the global climate evolution since 9.5 Ma,it is postulated that the Zanda basin experienced primarily warm and humid conditions between 9.5 Ma and 6.3 Ma,likely related to the strengthening of the southeast monsoon from India.The paleoclimate analysis of the Zanda basin during the interval 6.3~3.6 Ma indicates a relatively warm and humid period,with climate fluctuations,possibly due to the re-strengthening of the southeast monsoon from the Indian Ocean.Subsequently,after 3.6 Ma,the influence of the global climate cooling,winter monsoon strengthening,and the strong uplift of the Qinghai Tibet Plateau caused a shift towards a cold and dry environment in the Zanda basin.However,the area experienced a transition to a warm and humid climate between 2.8 Ma and 1.7 Ma.