罗布泊盐湖“大耳朵”盐盘特征、成因及古环境意义

Sedimentary Features,Origin and Paleoenvironmental Significance of "Great Ear" Salt Pans in the Lop Nor Playa

干盐湖蒸发岩沉积是干旱内陆封闭盆地的主要成盐模式,其地表盐壳沉积特征、蒸发盐类矿物具有明显的分带性。罗布泊干盐湖,保存了完整的"耳轮"型环状沉积模式和演化序列,在世界干盐湖中实属罕见。通过对罗布泊"大耳朵"干盐湖区多级环状亚沉积环境下发育的盐壳地貌、沉积结构、龟裂形态、矿物组成以及地下卤水的地表排泄情况进行地面调查和实地采样,探讨了不同"耳轮"环带盐壳地貌成因、相互承接关系和发展演变过程;并概述了蒸发盐盘不同沉积阶段沉积特征,及盐壳龟裂结构发育演化过程与地下水位埋深的关系。

The majority of present-day evaporite deposition occurs in arid closed continental basins in which the playa lake is most common.The distribution of evaporite minerals and sedimentary features of salt crust are usually zonal in a playa lake.The integrated zonal configuration is preserved well in the Lop Nor playa,which is represented by a series of concentric rings that closely resemble a great human ear in satellite images.This type of natural phenomenon is very rarely seen in a playa lake.The Lop Nor basin consists of a broad,flat salt plain,and salt crusts covering approximately 5,500 km2 with the lowest parts at 780 m above sea level.After investigating the fracture texture of salt crust,sedimentary structures,evaporite mineral composition,and the condition of groundwater brine discharge in the multicyclic ＂Great Ear＂ rings,we can discuss the origin of different salt crust types,the connection between adjacent concentric zones,and the evolutionary process of the concentric configuration of the ＂Great Ear＂ salt pans.This research also gives a general overview of the sedimentary features in different cycles of salt-pan evaporite,as well as the relationship between the development of polygonal fissure structures of salt crust and the groundwater brine table.The types of salt-crust structure include polygon crack,well-developed honeycomb-shaped halite crusts,mound-shaped pattern and plate-shaped halite crusts.However,the sediment beneath the surface is typically saturated with concentrated brines and displacive evaporites.There is visual evidence of discharging groundwater in the numerous moist salt pans covering most of the salt plain,suggesting that the capillary fringe of the groundwater table is close to the surface of the salt pans.The development of salt crust structures were generally controlled by the groundwater table with a depth of 〈 2m in the Lop Nor playa.But it is also affected by many factors such as sediment permeability,brine density,evaporites,cracks and atmospheric conditions.