Sequence Stratigraphy Of Middle And Lower Cambrian In The Central And Western Tarim Basin,Northwest China

Guided by the classical theory of sequence stratigraphy,based on the abundant data from outcrop,drilling core,seismic profile and carbon isotope,this paper tries to establish the sequence stratigraphic framework for the Middle and Lower Cambrian strata in the central and western Tarim Basin,northwest China.In this paper,the main factors that control the development of sequence stratigraphic are discussed.The characteristics and the plain distribution of the sedimentary facies under the control of sequence stratigraphic framework are clarified.Six third-order sequence interfaces were identified in the Middle and Lower Cambrian in the central and western Tarim Basin.The SB1 interface is an unconformity that has caused sedimentary discontinuity by tectonic movement(early Caledonian Episode I).On seismic profile,it can be identified by the downward and truncated contact above and below the T90 interface.The SB2 interface to the SB6 interface are sequence interfaces controlled by sea level rise and fall.The SB2 and SB3 interfaces are the ancient karstified surfaces in the outcrop,indicating that there are short-term exposure.The SB4 to SB6 interfaces are characterized by lithologic changes.Under the control of the sequence interface,the study area has a total of five third-order sequences.The first transgression event occurred in the Cambrian during the deposition of SQ1.The sediments are characterized by erosion above the water and sedimentation below the water and happened only in the northwestern margin of the Bachu uplift;The thickness of SQ2 sequence is rather stable thinned from the Bachu uplift toward the Tazhong uplift;The thickness of SQ3 sequence is the largest in the central part of the Bachu uplift.The second episode of transgression happened during forming of SQ4,forming stable but thin limestones.The SQ5 sequence was partially absent in the Bachu uplift,and the overall thickness was large.The sedimentary pattern of the Cambrian in the Tarim Basin is characterized by the platform in the west and basin in the east.The sedimentary system can be classified into the coastal tidal flat,the evaporation platform,the restricted platform and the open platform,and can be further divided into five subfacies and 21 microfacies in the Lower-Middle Cambrian.Controlled by sea level rise and fall,littoral tidal flat facies,restricted platform facies and evaporate platform facies are recognized one after another from SQ1 to SQ3,respectively.The open platform facies and the restricted platform facies are recognized in the Bachu and the Tazhong uplift respectively during the froming of the SQ4.Evaporative platform mainly developed in the SQ5.