Depositional model for a salinized lacustrine basin: The Permian Lucaogou Formation, Jimsar Sag, Junggar Basin, NW China

Abstract

Fine-grained sedimentary rocks are well developed in a salinized lacustrine basin in the Junggar Basin, China. Understanding the sedimentary characteristics and process of these fine-grained sediments is essential to find the exploration target for tight oil. In this study, the sedimentary characteristics and forming mechanism of fine-grained sediments in the Permian Lucaogou Formation within the Jimsar Sag were investigated based on core and thin section observation, SEM combined with XRD and geochemical data, and eleven lithofacies were identified, including (1) brecciated dolomitic mudstone, (2) intraclastic conglomerate, (3) mixed siliciclastic and intraclastic grainstone, (4) dolomitic siltstone, (5) Carbonate packstone and grainstone, (6) dolomitic mudstone, (7) laminated algae-bearing tuffaceous mudstone, (8) massive tuffaceous mudstone, (9) turbidite dolomitic mudstone to siltstone, (10) pebbled sandstone, and (11) green mudstone and siltstone. The organic constituents in the fine-grained sedimentary rocks include sapropelic, exinite, vitrinite, inertinite and secondary macerals, which are dominated by unstructured algae and asphaltene. The fine-grained sedimentary rocks are characterized by high-frequency cycles composed of three successive facies associations: profundal, littoral–sublittoral and palustrine, which recorded distinct lacustrine expansion and contraction. Developed mud cracks, paleosols and vertical dissolution fractures in the fine-grained sedimentary rocks indicate that these facies recorded the depositional process in an ephemeral to perennial lake in an arid climate. Fluvio-deltaic and aeolian transport were main modes for transporting the siliciclastic component that ultimately accumulated in a subaqueous and subaerial environment. The dolomite in the littoral zone might develop with the evaporitic concentration of the lake water during the early diagenetic stage. The depositional processes in the profundal zone included suspension and turbidity currents. Multiphase volcanic activities affected the lithofacies and prompted the accumulation of organic matter. Understating the depositional process of the fine-grained sedimentary rocks in the Lucaogou Formation in the Jimsar Sag is helpful to understand the depositional environment in the salinized lacustrine basin in the Junggar Basin.