Paleoecology and Sedimentary Environment of Carboniferous in Qaidam Basin

The Carboniferous outcrops is mainly distributed in two areas of Northeastern and Southwestern margins of Qaidam basin.On the basis of uniform stratigraphic framework and correlated biostratigraphic sequences,comprehensive analysis of paleoecology and sedimentary feature provides convincing evidences for division of the sedimentary facies.The Lower Carboniferous is rich in fossil coral and brachiopod in the northeastern margin of Qaidam Basin.The coral evolved from small size of monomer toward big one,and the number of fossil genus and species varied from few to more with their abundance increasing,indicating that the sedimentary environment went stable and the water went deep.Meanwhile,two lithologic cycles of glutenite or inter-bedded sandstone and shale graded to limestone are developed,and the sedimentary environment evolved gradually from shoreland toward carbonate terrace.The biota of Upper Carboniferous is dominated by fusulinid and secondly by brachiopod and coral with flourished fossil plants in the lower part.Fossil brachiopod evolved toward smaller size and the abundance of coral is reduced,these changes are connected with shallow water and cold climate.The lithologies are dominated by alternating sandstones,shales and limestone,and the sedimentary environment based on shoreland tidal flat with swamp developed in the lower.The biota of Lower Carboniferous is dominantly composed of coral and brachiopod in southwestern margin of Qaidam basin.The biota of Upper Carboniferous is composed of dominantly fusulinid and secondly brachiopod,and the lithologies are mainly bioclastic limestone and oolitic limestone.Sedimentary environment of Carboniferous in southwestern margin based on alternating carbonate shoal of higher-energy environment and carbonate terrace of lower-energy environment.The analysis indicates that there existed transgressions twice in Early Carboniferous and continuous transgression in Late Carboniferous;the carbonate shoal facies favorable for oil accumulation is well developed in the southwestern margin,and the alternating marine and continental facies that is in favor of forming assemblage of oil source bed,reservoir and cap rocks is developed in the northeastern margin.