The Early Jurassic sedimentary records characteristics and geological significance in the northern margin of the Qaidam Basin, NW China: A study of Well Lengke-1

The Qaidam Basin began to accumulate continental deposits during the Early Jurassic period, when it was located at a geographical position of 8.8 °N latitude. The paleoclimate was characterized by a warm and humid climate zone, conducive to the formation of lakes and marshes. This ancient continental lake deposit record provides a robust indication of key paleoenvironmental and paleoclimatic information. According to the descriptions of rock debris in the basic geological research and oil & gas exploration, the drilling coring and electrical measurement data confirm that Well Lengke 1 is drilled through the Early Jurassic Huxishan Formation, and the location of Well Lengke 1 is the center of the lake basin, which can well reflect the Early Jurassic sedimentary characteristics and paleoenvironment restoration in Qaidam Basin. This study utilizes rock dating, thin section identification, biomarker compounds, and spore-pollen analysis to investigate the Early Jurassic sediments of the cold lake in the Qaidam Basin. The results confirm: (1) The sedimentary environment was identified as the origin of the sediments from 400 to 450 Ma and 250 to 300 Ma, and the development of algal bodies, algal wall keratinites, sporophytes and other organisms was found in thin section identification. (2) Biomarker analysis confirmed various compounds indicating that the second member of the Early Jurassic Huxishan Formation was rich in terrestrial higher plants, followed by aquatic lower organisms, freshwater lakes developed, and the depositional environment was partial oxidation; The first member of Huxishan Formation indicates low biological bacteria and algae (nC₁₄), terrigenous organic matter (nC₂₁-₂₉), and mixed organic matter. Gamma-cerane and paleosalinity analysis of sedimentary water suggest that black shale was formed in freshwater environment. (3) The palaeoclimate was humid, hot and rainy, the terrain was flat, the plants were flourishing along the marshes and shallow lakes, the plant group was flourishing gymnosperms, the sporogenous assemblage was Coniferae with cysts - piceites - spines, and the fern types were relatively simple. (4) The deep strata in this area have the characteristics of Paleozoic strata, and brachiopods, foraminifera, grid bryozoa, echinodermata and other marine fossils are found in the rock debris.