A sequence stratigraphic analysis of the Jurassic coal measures in Yuqia region of Northern Qaidam basin

The principle of the sequence stratigraphy in well logs and outcrops has been used to reconstruct sequence stratigraphic framework of the Jurassic coal measures in Yuqia area of Northern Qaidam basin.Four sequence boundaries have been identified,which divide the Jurrasic coal measures into 3 third-order sequences,corresponding to Dameigou Formation,Lower Member of Shimengou Formation and Upper Member of Shimengou Formation,respectively.The main types of sequence boundary in the research area include regional unconformity surface(paleoweathering crust),incised valley fill of distributary channels,paleosols in the interfluvial area and the shallowing-upward to deepening-upward transforming surface of the depositional systems.The distribution of thick coal seams in the Jurassic continental lacustrine sequence stratigraphic framework is various compared with the paralic coal measures.The thick coal seam,represented by Coal Seam 7,developed near the transgressive surface of Sequence I in the basinal area and the paleo-uplift area,but in the marginal fluvial delta plain such as in Sequence II,the thick coal seem(Coal Seam 5) developed around the maximum flooding surface.In overall,in the continental third-order sequences,the thickest coal seams tend to develop in the transgressive systems tract.