Soft-sediment deformation structures in saline-lacustrine fine-grained mixed sedimentary sequences (southern Junggar Basin, NW China): Classification, lithofacies attribution, and forcing mechanisms

The primary sedimentary fabrics that trigger the formation and development of soft-sediment deformation structures worldwide are still not fully understood and even questionable. Little specialized investigation has been conducted on saline-lacustrine depositional settings characterized by multisource mixed sedimentary sequences displaying strong heterogeneity in fabrics and textures, lithofacies associations, and sedimentary structures. This work presents a systematic observation, classification, and interpretation of soft-sediment deformation structures developed in the saline-lacustrine fine-grained mixed sedimentary sequences of the middle Permian Lucaogou Formation. Here, seven soft-sediment deformation structure categories are depicted and explained in detail according to quantitative sedimentologic and petrographic constraints, including ductile (minor folds and convolute lamination, as well as load structures), brittle (minor faults, lithic breccias and debris-flow structures, boudinage structures, and sedimentary dikes), and hybrid brittle-ductile structures. Relationships were documented between soft-sediment deformation structures and lithofacies categories, emphasizing that dolomicrites and dolarenites are favorable for the formation of minor faults characterized by obviously higher average slip and dip angles of ∼0.30 cm and ∼64.0° and ∼0.20 cm and ∼50.6°, respectively, and dolomitic siltstones and mudstones are conducive to the development of both sedimentary dikes/clastic sills and also boudinage structures characterized by significantly higher averages of maximum width of 2.04 cm and 0.83 cm and 0.84 cm and 0.65 cm, respectively. Furthermore, both the feldspathic lithic silt to fine-grained sandstones and dolomitic lithofacies contribute significantly to the wide distribution of lithic breccias and debris-flow structures, yielding the highest maximum width of individual structures >0.45 cm. Potential triggering mechanisms for the differential development of soft-sediment deformation structures can be identified mainly through correlations between critical petrographic parameters (e.g., minor fault slip and dip angle, average maximum width of individual structures, etc.) and sandstone (argillaceous rock)/stratum ratios. This implies that there are crucial effects from terrigenous sandy input, argillaceous concentration, and brittle-ductile mechanical interaction on seismically induced liquefaction or fluidization. Finally, a conceptual model is proposed to illustrate the distribution pattern of soft-sediment deformation structures in the depositional architecture of saline-lacustrine fine-grained mixed sedimentary sequences, demonstrating that in the spatial variation from brittle to hybrid brittle-ductile and then to ductile types, the latter coincides well with decreases in both brittleness and arkosic concentration. Our findings demonstrate the importance of better understanding of the heterogeneous physical properties of saline-lacustrine shale reservoirs and their importance for guiding identification and fracturing development of oil resources, and this study provides a unique opportunity to explicitly highlight the broader scientific issues, including recognizing soft-sediment deformation structures as (1) geologic archives of paleoearthquake events, (2) seepage channels and accumulation spaces of strategic metals and mineral-metallogenic fluids, and (3) a unique mechanism for tracking continental tectono-dynamic and regional deformation processes occurring in multiple phases.