Sedimentary origin of fine-grained rocks in the Permian Lucaogou Formation in the southern Junggar Basin: implications from grain size analysis

A grain size analysis of a single lamina within fine-grained sedimentary rocks is an effective method to reveal its hydrodynamics. In this study, taking the clastic particle content as a parameter, the laminae of fine-grained rocks in the Permian Lucaogou Formation in the southern Junggar Basin are divided into clastic laminae, clay–clastic laminae, clastic–clay laminae and clay laminae. We counted the grain size of clastic particles in a single lamina. The grain size curve reveals four transport modes in the relatively deep-water environment: saltation, graded suspension, uniform suspension and wind transport. Among them, clastic laminae and clay–clastic laminae are developed in shallow lakes, and the sediment transport modes are mainly graded suspension, saltation and minor uniform suspension; these laminae are caused by flood and wave action under strong hydrodynamic forces. Clay–clastic laminae, clastic–clay laminae and clay laminae form in a semideep lake, and the sediment transport modes are graded suspension, uniform suspension and minor saltation; these laminae are caused by the alternating action of flood and static water under moderate hydrodynamic forces. Clastic–clay laminae and clay laminae form in a deep lake, and the sediment transport modes are mainly uniform suspension, as well as graded suspension, saltation and wind transport; these laminae are caused by stratified flow, static water and wind transport under weak hydrodynamic forces. We believe that the sedimentary genesis of the abovementioned fine-grained rocks with laminae is generally applicable to the genetic interpretation of fine-grained sedimentary rocks in continental lakes.