Paleogeomorphic Restoration And Sedimentary Characteristics Of E₃² In Yingxi Area Of ??Qaidam Basin

The lithofacies types and cycles structures of mixed saline lacustrine basins are diverse and complex in origin,which are important response indicators of lake level rise and fall and palaeogeographic evolution.A set of saline Lacustrine basin mixed rocks is developed in the Upper Member of the Paleogene Lower Ganchaigou Formation(E32)in the Yingxi area of Qaidam Basin.Its mineral composition and rock structure occurrence are extremely complex,which leads to long-term controversy on petrology and sedimentary environment restoration in this area.There are divergent opinions on lithofacies classification and genesis judgement,and there is no unified standard.In addition,due to the influence of regional tectonic activities and the special climate of Paleogene,the migration of sedimentary subsidence centers and frequent changes of lake level in the basin also lead to the complex process of lake basin sedimentary filling,which increases the difficulty of palaeogeomorphological restoration.In view of this,based on the data of cores,thin sections,logging data and mineralogical analysis,this paper complex clarifies the lithofacies and cycle evolution characteristics of mixed rocks in the study area,and uses the thickness of gypsum-salt rocks to study the palaeogeomorphological restoration of sedimentary period,and then studies the evolution of sedimentary environment in Yingxi area.The lithofacies types of the Upper Member of the Paleogene Lower Ganchaigou Formation are complex.According to mineral composition and structural occurrence,12 lithofacies are identified,namely gypsum-bearing block gypsum limestone(dolostone),karst breccia,doloarenite(calcarenite),fine-grained sandstone,massive dolomitic(lime)siltstone,massive silty limestone(dolostone),massive argillaceous limestone(dolostone),massive dolomitic(lime)mudstone,layered gypsum-salt rock,thin-layeredsilty limestone(dolostone),thin-layered argillaceous limestone(dolostone)and thin-layered dolomitic(lime)mudstone.On the basis of interpretation template for lithofacies logging,according to deep-water salt-forming model and the characteristics and principles of high-density heavy brine gathering to low-lying areas to form salt beds preferentially,the thickness of gypsum-salt beds can be used to reflect the relative palaeogeomorphological fluctuation of sedimentary period.The results show that the landforms of the six oil groups all show the features of higher topography in the northwest than in the southeast and "alternation with uplift and depression".The underwater low-rise areas are also mostly distributed in the central and southeastern regions of the study area.However,due to the influence of tectonic movement in different periods and sedimentary filling,the specific locations of the subsidence areas and underwater low-rise areas have migrated and changed.In addition,the development position of soft sediment deformation formed by slumping coincides with the slope position in the palaeogeomorphological restoration results,which confirms the validity of the results indirectly.The comparative analysis of the relationship between palaeogeomorphology and lithofacies assemblage shows that the sedimentary palaeogeomorphology of alternation with uplift and depression and the lake level with frequent fluctuation control the complex and diverse lithofacies types and their assemblage sequences in this area.That is to say,the development and distribution of three lithofacies sequences,i.e.exposure type,shoal type and evaporation type,are controlled by three types of palaeogeomorphological units:underwater low uplift,slope and depression.Vertically,each lithofacies assemblage sequence overlaps and repeats frequently,which is characterized by the upward shallowing of multi-stage transient lake transgression-continuous lake retreat in the background of overall lake retreat.In plane,the thin-layered limestone(dolostone),massive limestone(dolostone)and gypsum-bearing block limestone(dolostone)developed in the main body of the study area have the characteristic that the thickness of them is larger in the middle-south than in the northwest.However,the dolomitic(lime)siltstone is more thicker in the northwest than in the southeast because of the influence of the provenance in Altun Mountain.Sedimentary microfacies distribution of six oil formations in Yingxi area is influenced by lake level changes and micro-geomorphology of the basin,which shows certain inheritance and an evolutionary process of gradual salinization.Based on this,a lake transgression-lake retreat sedimentary evolution model of the Upper Member of the Paleogene Lower Ganchaigou Formation in Yingxi area is established.