柴达木盆地新生代介形类动物群特征及环境意义

Features of the Cenozoic ostracod fauna and environmental significance in Qaidam Basin

柴达木盆地新生代介形类动物群依其属种组成及埋葬学特征可区分为河流、滨湖沼泽、浅湖（0～12m）、半深湖（12～25m）和深湖（〉25m）等多种组合。湖相介形类化石组合始见于始新世中期沉积的下干柴沟组下段上部，它们既不同于淡水湖介形类，也不同于溺湖介形类，而是典型的陆源半成水-成水湖介形类动物群；该组合的首次出现，是本区古近纪以来第一次大规模湖侵的标志。介形类动物群在柴达木盆地西部古近系中的纵向横向分布表明，在中、晚始新世（下干柴沟组上段-上干柴沟组下段沉积期）发生规模最大的湖侵，渐新世（上干柴沟组上段沉积期）的湖区虽仍继续向东扩大，但水体明显变浅，标志着湖侵已接近尾声。这一湖侵与邻区塔里木盆地西南始于古新世而结束于始新世末至渐新世初的古近纪海侵相比，发育时期大不相同，在起源上两者间应无因果关系。

Based on the generic-specific composition and the taphonomic characters, the Cenozoic ostracod fauna of the Qaidam Basin can be divided into fluvial, lake shorezone swamp, shallow lake （0-12 m）, semi-deep lake （12-25 m） and deep lake （ 〉25 m） assemblages. The lacustrine fossil ostracod assemblages occurred first in the upper part of the lower member of Xiaganchaigou Formation which was deposited during the Middle Eocene. They are neither identical to the fresh-water lake ostracod assemblages, nor the same as the lagoons＇. They are typical terrigenous brackish and salt water lake ostracod fauna. Their first occurrence in the area indicated the beginning of the first large-scale lake transgression in the Qaidam Basin since the Paleogene. The characters of their stratigraphic and geographic distributions in the Paleogene in the western Qaidam Basin also indicate that the maximum scale of the lake transgression was developed during the Middle and Late Eocene （during the deposition of the upper member of Xiaganchaigou Formation and the lower member of Shangganchaigou Formation） .During the Oligocene epoch （during the deposition of the upper member of Shangganchaigou Formation）, the lake level fell obviously, and the lake transgression in the area almost ended, although the shallow lake area was moving eastwards. Compared with the transgression that began in the Paleocene and terminated at the latest Eocene to the earliest Oligocene in the southwestern Tarim Basin, lake transgression in the Qaidam Basin was developed in a quite different time. There is no causal relationship between the origins for the transgression in these two areas.