泥河湾的介形类化石与古环境变迁

EVOLUTION OF THE FOSSIL POPULATION OF OSTRACODES AND CHANGE OF ANCIENT ENVIRONMENT IN NIHEWAN

According to the variation of fassil population of ostracodes is the Yuxian Qipuyi well section, the Pumping station of Hongai saction and the Hutouliang section the huge thickness of river-lacustrine facies deposits of this area is divided into the Shixia Forma- tion (Pliocene), the Nihewan Formation (early Pleistocene), the Hutouliang Forma- tion (middle Pleistocene) , the Xishuidi Formation (late Pleisetocene) and the Dayuwan Formation (Holocene) . This division breaks antraditional ideas of the period of the Nihewan Formation. Based on the ecology of their members, of it is also inferred that the time variation of saltness of the ancient lake water is periodic and completes four periods from Pliocene to late Pleistocene in this area. Corresponding paleoclimate appears alternate change of cold and warm and dry and wet, and the total tendention is dry and cold.