龙街粉砂层形成时代及其古环境

本文从龙街粉砂层的颜色、岩性、沉积构造、相变关系及层序等多方面确认,龙街粉砂层足在元谋组上部洪积相的基础上发育起来的断陷湖盆沉积。依据龙街粉砂层的地貌位置、¹⁴C等方法确定其时间上限为10 000 a B.P.左右。依据元谋组与龙街粉砂层局部连续沉积关系,确定龙街粉砂层下限年龄为0.85 Ma B.P.左右（据古地磁测定的元谋组形成的上限年龄）。从而确定龙街粉砂层形成的时代为中更新世初到晚更新世末,地质年龄850 000—10 000a B.P.左右。

THE FORMATION TIME AND PALEOENVIRONMENT OF THE LONGJIE SILT BED

The Longjie silt Bed distributed in Yunnan Province, where the Jinshajiang River and the Longchuanjiang River joined together. The prevalent considerations on the genesis and the time of formation of the Longjie silt Bed are much different.Based on field survey and laboratory study the authors considered that the Longjie silt Bed may be formed from the beginning of middle Pleistocene to the end of Late Pleistocene, that is, about from 850 000-10 000 a B.P. with a paleoenvironment of subsided lake basin with arid or semiarid climate developed on the basis of the pluvial fan of the Yuanmou Formation.The Longjie silt Bed existing from 40 meters below to 70 meters over the water surface of the Jinshajiang River formed the basement of the second step of the Jinshajiang River terraces. On the Longjie section, the black clay layer 14 meters below the top was dated as 28 580 a B. P. (by Jiang Fuchu et al. with ¹⁴C method). The earlier sediments can be seen on the Bainiwan section, which had been dated by Jiang Nengren et al. with papleomagnetic method as the former stage of Brunhes Chron about 0.7—0.4 Ma B.P. in the Nangou ditch, Bainiwan, the authors discovered a-continity of sedimentation between the Yuanmou Formation and the Longjie silt Bed, then firmly indicates that the formation time of the Longjie silt Bed toughly corresponds to the end of the Yuanmou formation, e.g., about 0.85 Ma B. P. （Zhang Jinqi et al. with paleomagnetic method）. The Longjie silt Bed is homogeneous, stable and pure. Its mean deposit velocity is about 0.133-0.148 mm/a. According to this velocity, the Longjie silt Bed deposited at 0-825 or 0.743 Ma B.P.The Longjie silt Bed developed in a lacustrine environment is mainly composed of greengrey, yellow-grey and greywish silt, generally with horizontal laminations, has cross-stratification formed by wave and contains a lot of, about 10—20％——determined by microscope, carbonate minerals （in which the sedimentary structures showing monodirectional water flow has not been found）. At the edge of the Longjie basin and the foot of the hills around the basin, the Longjie silt Bed horizontally changed into pluvial and debris flow accumulations. Their underlying strata are the Lower Pleistocene Yuanmou Formation, which has generally been deformed by the Yuanmou movement and is unconformable with the overlying Longjie silt Bed. But, in low-lying land, the Longjie silt Bed and the Pluvial sediments of the Yuanmou Formation may be continuous.So, the whole process of the Quaternary geological evolution in this area has been successively and completely recorded in the sediments of Yuanmou-Longjie area.