Soft-sediment deformation structures in the Miocene deposits in the Luoyanshan area of the Qaidam basin, China: Implications for paleogeomorphology

Abstract

Based on well cores, nine types of lithofacies were identified in the deposits of the Miocene Lower Youshashan Formation in the Luoyanshan area of the Qaidam Basin. The preliminary interpretation of the sedimentary environment indicates that these deposits were formed in a shoreface to a deep lake and a delta environment. Soft-sediment deformation structures (SSDSs) within multiple layers were observed in five cores. These SSDSs can be divided into four categories: (1) load structures, (2) sand injections, (3) fault-graded beds, and (4) slumps. The detailed descriptions of these SSDSs allowed us to analyze and interpret the deformation mechanisms and driving forces. The deformation mechanisms reflected by most of the SSDSs in the cores are liquefaction and fluidization (load structures and sand injections). The remaining SSDSs exhibit plastic and/or brittle behavior (fault-graded beds and slumps). The corresponding driving forces are mainly unstable density gradients and gravitational body forces. The driving forces essentially reflect the original paleogeomorphological setting (i.e., the topographic slope gradient), the initial sedimentary column, and the biological, chemical, and/or physical processes acting on the sediments. After eliminating the interfering factors, the relationship between the different types of SSDSs and the paleogeomorphological settings were established using the driving forces as the intermediary between them. The formation of the symmetric load cast and flame structures was driven by the unstable density gradient of the sediments, indicating a flat topography. In contrast, the asymmetric load cast and flame structures, the fault-graded beds, and the slumps were driven by the gravitational body force, implying a sloping topography. Furthermore, the proportions of these four types of SSDSs in each well were statistically analyzed to qualitatively determine the paleogeomorphological setting of each well during the penecontemporaneous period and to support and refine the sedimentary analysis. The results show that the five cores contain sediments deposited in five types of lacustrine sub-environments: (1) a flat platform on a subaqueous beach-bar, (2) a flat shallow to deep lake, (3) the edge of a subaqueous beach-bar, (4) a position near and below the delta slope, and (5) a steep delta slope. The results of this study illustrate that there is a strong coupling relationship between the morphologies of the SSDSs and the penecontemporaneous paleogeomorphological setting. This correlation provides a new perspective for interpreting sedimentary environments.