The sequence stratigraphy of the Middle Jurassic coal measures in the Laogaoquan region of the northern Qaidam basin

Sedimentology,sequence stratigraphy and coal accumulation of the Jurassic coal measures in the Laogaoquan region of the northern Qaidam basin were studied using borehole data.The distribution of coal seams in different sedimentary environments and in the sequence framework was analyzed.A total of 22 lithofacies was identified and three palaeogeographical units,including delta plain,littoral and shallow lake,and lacustrine sand bars,were reconstructed.Three third-order sequences and corresponding systems tracts were subdivided.Coal accumulation occurred in the TST and the HST of the third-order sequence,and the thick coal seams tend to develop around the third-order maximum flooding surface.It is suggested that the palaeomires developed from the lacustrine,the interdistributary bay of the delta plain,and the back swamp of meandering rivers are the main environments for coal accumulation in the Laogaoquan area,and the major coal seam(M7),is developed in the late transgressive systems tract and the early highstand systems tract,around the maximum flooding surface.