塔里木盆地东北部新生代介形类与沉积环境

CENOZOIC OSTRACODA AND PALAEOENVIRONMENTS OF THE NORTHEASTERN TARIM BASIN

塔里木盆地东北部库车以东地区下第三系化石罕见。该地区主要为红色含膏盐的粗碎屑岩沉积。中新世以来 ,由于喜马拉雅运动的影响 ,天山不断隆升 ,导致本区加速沉降 ,出现滨浅湖沉积环境 ,介形类逐渐繁盛。中新世中晚期半咸水介形类 Cyprideis爆发 ,广泛出现Cyprideis占绝对优势的半咸水浅湖环境含膏砂泥岩沉积 ,与东邻柴达木盆地阿尔金山以南地区相似。上新世以来 ,本区沉降速率猛增 ,堆积速率更快 ,基本上处于超补偿沉积 ,湖泊大规模萎缩 ,介形类数量锐减。由于喜马拉雅运动晚期的影响 ,天山山脉库车—库尔勒一线以北的山峰不断隆升 ,本区第四纪湖区规模比上新世大大扩展 ,湖泊范围比中新世显著向南迁移 ,Cyprideis再度繁盛并占优势 ,早更新世晚期距今大约不足百万年 ,本区尤其是库南 1井地区 ,除继续出现丰富的 Cyprideis外 ,突然出现淡水 -微咸水介形类 ,形成一个属种多样的高分异度介形类动物群 ,与柴达木盆地南部昆仑山北麓的介形类组合类似 ,主要有 Candona torosa,Candoniella lactea,Candona neglecta,Candona arcina,Cypridopsis vidua,Limnocythereinopinata,Cypris subglobosa,Ilyocypris gibba,Ilyocpyrisbiplicata,Eucyprisinflata,Candoniellaalbicans,Darwinula sp.,Stenocypris sp.等。

The Paleogene rocks of the northeastern Tarim Basin in western China are mainly composed of reddish gypsum bearing coarse grained sediments,and fossils are rare within these clastic facies.However,since the onset of the Himalayan orogeny in the Miocene,continuous uplift of the Tianshan mountains resulted in the rapid subsidence and appearance of a shallow lake environment.As a result of this,ostracods are widespread in the Late Cenozoic rocks of the Tarim Basin.The brackish water ostracod Cyprideis thrived from the Middle to Late Miocene;gypsum bearing sand and mud sediments were also widely deposited in the shallow brackish water lake environments.This situation is similar to that of the Qaidam Basin,which lies to the south of the Altun mountains.From the Pliocene onwards,the rates of subsidence and sedimentation increased.The resulting regression of the lake led to disappearance of ostracods from a large part of the basin.To the north of the Kuqa Depression,the Tianshan mountains underwent continuous uplift during the late Himalayan orogeny.As a result,the Quaternary lake deposits have a much greater extent than those of the Pliocene,and extend even further southward than those of the Miocene.Consequently,the brackish water ostracod Cyprideis became dominant again during the Quaternary.From thd end of the Early Pleistocene to present in this area,an abundant and high diversity fauna of fresh to brackish water ostracods appeared alongside abundant Cyprideis .This situation is similar to that of the southern Qaidam Basin,to the north of Kunlun mountains.The Quaternary assemblages mainly consist of Cyprideis torosa,Candoniella lactea,Candona neglecta,Candona arcina,Cypridopsis vidua,Limnocythere inopinata,Cypris subglobosa,Ilyocypris gibba,Ilyocypris biplicata,Eucypris inflata,Candoniella albicans,Darwinula sp.and Stenocypris sp..Under the extremely arid climate in the Tarim Basin,fresh water originated mainly from snow melted.This suggests that the Tianshan mountains to the north were uplifted rapidly to above the snow line during the last million years.The evolution of the ostracod assemblages indicates that the late Himalayan orogeny not only controlled the deposition of biota assemblages but also took an important role in the hydrocarbon accumulation and migration.