Cenozoic ostracoda and palaeoenvironments of the northeastern Tarim Basin, western China

The Paleogene rocks of the northeastern Tarim Basin in western China are composed mainly of reddish gypsum-bearing coarse-grained sediments, and fossils are rare within these elastic 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 Mid- to Late Miocene; gypsum-bearing sand and mud sediments were also deposited widely 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 the 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, freshwater originated mainly from snow-melt. 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. (C) 1999 Elsevier Science B.V. All rights reserved.