Depositional mechanism of deep-lacustrine mudstones controlled by the coupling of sediment provenance and orbital-paced environmental evolution

Unraveling the sedimentary process and spatiotemporal distribution of deep-lacustrine mudstones is critical for clarifying the origin and distribution of shale-oil reservoirs in the Chang 7 3 sub-member (Ordos Basin). In this study, comprehensive analyses of petrology and cyclostratigraphy were conducted to decode the lithofacies and their provenance within a high-resolution astronomical framework. Subsequently, analysis of the paleoenvironmental evolution and its driving mechanism reveals the sedimentary genesis and depositional models of deep?lacustrine mudstones under the dual control of provenance supply and lake-level fluctuation. Within the Chang 7 3 sub-member, three primary lithofacies have been identified. Massive mudstones, derived from the Alxa Terrane, generally deposited in the northern part of the basin. In contrast, the other two types of binary laminated shales ("silt-sized felsic (SSFL) + organic-rich (ORL)" and "tuff-rich (TRL) + organic-rich (ORL)"), sourced from the volcaniclastics within Qinling orogenic belts, mainly deposited in the central and southern parts of the basin. Under the control of paleogeomorphology characterized by "steep-south and gentle-north", hyperpycnal flows prone to occurred within the southern part of the basin and carried volcaniclastics into the lake basin. Periodic lake-level fluctuations driven by obliquity cycle further controlled the transport distance of silt-sized particles to the center of the lake basin, inducing the alternate deposition of "SSFL + ORL" laminated shales (lowstand) and "TRL + ORL" laminated shales (highstand). Within the central part of the basin, the hyperpycnal flows were only able to transport volcaniclastics into this area during the period of low lake-level (low obliquity). High lake-level (high obliquity) led to the retreat of volcaniclastics, and the fine-grained sediments derived from the Alxa Terrane mainly occurred suspension settling, forming massive mudstones. Conversely, within the northern part of the basin, isolated from southern volcaniclastics inputs, the fine-grained sediments derived from the Alxa Terrane continuously occurred suspension settling to form massive mudstones.