Characteristics and formation mechanism of multi-source mixed sedimentary rocks in a saline lake, a case study of the Permian Lucaogou Formation in the Jimusaer Sag, northwest China

The Permian Lucaogou Formation in the Jimusaer Sag of the southeastern Junggar Basin in China is an uncommon set of mixed sedimentary rocks. The mixed sediment content of the Lucaogou Formation is unusual due to the large amounts of tuffaceous material, which makes the system more complex than the commonly mixed siliciclastic-carbonate rocks. In this study, the volcanic material acts as a third end-member component in a multi-source mixed sedimentary system. Therefore, a new type of mixed siliciclastic-carbonate-tuffaceous rocks has been introduced. Methods including core observations, thin section analysis, grain size measurements, and X-ray diffraction are used to investigate the characteristics of mineral components, sedimentary structures and lithological assemblages in upper and lower sections of the formation. These analyses are then used to determine the lateral distribution area of mixed sedimentation. Sedimentary mixing processes originally described for siliciclastic-carbonate systems are used and expanded here for the multi-source mixed rocks in a saline lacustrine basin. Controlling factors on mixed sedimentation included climate change, lake level fluctuation, tectonic activity and sediment supply rate mentioned in previous studies, as well as variations of energy levels in different parts of the lake. Mixed-depositional models for the lower and upper sections of the Lucaogou Formation in saline lacustrine basin have been established, respectively.