High-frequency Cycles and Paleogeomorphic Features of the Upper Member of the Lower Ganchaigou Formation in the Yingxi Area, Qaidam Basin

Key words: 
• salinified-lake basin /
• high-frequency sequence /
• west of Yingxiong ridge /
• Lower Ganchaigou Formation /
• evaporite

Abstract: In the Paleogene,a set of mixed sediments in a salinified-lake basin was developed to the west of the Yingxiong Ridge,controlled by regional tectonic movement and the local climate. The rock series have particular sed-imentary facies successions,which are very important in the study of high-frequency sequence division and paleogeo-graphic evolution of the salinified-lake basin. Upper member strata of the Lower Ganchaigou Formation consist of dark,fine-grained mixed sediments with complex mineral composition,mainly composed of terrigenous clasts,car-bonate minerals and evaporite minerals,which respectively correspond to the pre-salinization stage,the initial salini-zation stage and the final salinization stage of the lake water. In the study area,the strata are divided into seven fourth -order sequences (Psq1-Psq7). Except for Psq1 and Psq2,they occur in three parts:clastic rocks,carbonate rocks and evaporite. Theoretically,deepwater salt forms by the stratification of heavy brine as it sinks. As a result,salt rock in deep depression areas is thicker than in low uplift areas. Therefore,taking into consideration the fourth-order se-quence,the paleogeomorphology of the upper member of the Lower Ganchaigou Formation in the Yingxi area was able to be restored in accordance with the relative thicknesses of the salt rock strata. Psq1and Psq2 developed in the early stages of lake evolution,when the area covered by the salt-water lake was very limited. During the development of Psq3 to Psq5,the degree of salinization gradually increased,and the scope of salt-water lake continually expand-ed,with thick evaporite being formed in the depression area. Then,during the deposition of Psq6,the topography of the study area tended to be flatter due to filling and leveling of the previous stages,and the range of height differences was considerably reduced. Finally, during the deposition of Psq7, the western area of the lake in the study area gradu-ally became shallower and the sedimentary center began to migrate eastward.