Abstract: The large-area thick-layer sandstones in Cretaceous Bashijiqike Formation, Kuqa depression are important gas horizons of Tarim Oilfield. Disputes have always existed in research of the sedimentary environment of thick-layer sandstone. Brown muddy gravels and a small number of silty-fine sandy gravels are generally developed in thick-layer sandstones. The genetic mechanism and sedimentary environment of arenaceous pelitic gravel associated with thick-layer sandstone can be clarified to explore the sedimentary environment of large-area thick-layer sandstone from a brand new perspective. Based on field outcrops and core observation of multiple drilling wells, the mineral components and contents of muddy gravel are analyzed. On this basis, the types of muddy gravel and silty-fine gravel are divided according to the component, morphology and occurrence state. Based on component analysis, arenaceous pelitic gravel is judged mainly from gray silty mudstone adjacent to thick-layer sandstone. Using the uniformitarianism method, it is further discovered that muddy gravel sediments are developed in modern Tarim River (seasonal meandering river) and upper plain-lower plain of Ganjiang shallow water delta. Based on comprehensive analogy analysis in combination with previous research results, it is considered that the muddy gravel in thick-layer sandstone has the same genetic mechanism and development environment. This indicates that the muddy gravel and sandy gravel in Bashijiqike Formation are products under the environment of seasonal river or upper plain-lower plain of seasonal river shallow delta (influence area of intermittent lake), i.e., the products in the onshore sedimentary environment influenced by intermittent lake, rather than the products in the "underwater" delta front-shore-shallow lake environment for a long term. The genetic mechanism of sandy gravel includes two aspects, i.e., autochthonous deposit and short-distance transport after arenaceous pelitic collapse of river bank. Accordingly, it is believed that the thick-layer sandstone in Bashijiqike Formation is mainly developed in the sedimentary environment of seasonal river and upper plain-lower plain of seasonal river shallow delta.