Cretaceous Sequence Stratigraphy And Basin Evolution In Duohemao Basin, Zeku County, Qinghai Province

Based on the study of Cretaceous sedimentary strata of Duohemao basin, the paper divided the facies into four categories, which include conglomerate facies, sandstone facies, mudstone facies and evaporate facies of which contain 17 lithofacies units. Vertically, the lower part of the sedimentary strata includes conglomerate facies, sandstone facies and mudstone facies while the top part is evaporate facies. So, the sedimentary strata can be divided into two Members:the first member and the second member of Duohemao formation. In space from the northern margin of Duohemao basin to the center of the basin, the Lithofacies units evolved radually from conglomerate facies to sandstone facies and to mudstone facies and finally to evaporate facies. The sedimentary environment was the fan delta-lacustrine depositional system.The study of sedimentary facies and sequence stratigraphy suggested the Cretaceous sedimentary strata of Duohemao basin can be divided into six third-order sequences: Sq1, Sq2, Sq3, Sq4, Sq5 and Sq6, and hence a high-resolution sequence stratigraphic framework of the whole basin were established. The water depth of Duohemao basin shifted from shallow to deep and then to shallow while accordingly, the base level rised in early stage with a cycle and then quickly decreased in the late stage. The main influence factors of the base level shiftment were, with their influence degree order: tectonic movement, climate, sediment filling and volcanic activity.The result of the basin volcanic rock facies and geochemical characteristics suggested the Duohemao basin to be an active rift basin which formed by the upwelling of hot material from the asthenosphere and later the formation of the nearly NS Duofutun—Duohemao fault zone.