Analysis of formation and depositional environment of Middle-Late Pleistocene slope-proluvial sediments in the Shihe River basin, Dalian

Slope-proluvial sediments, as a sedimentary product of both slope wash and flood processes, preserve substantial information on environmental changes. The Shihe River basin in Dalian City is situated within the transition zone between the low mountains and hills of the Liaodong Peninsula and the coastal alluvial plain. The exposed stratigraphy in this area includes the dolomite of the Ganjingzi Formation, limestone of the Nanguanling Formation, slate of the Changlingzi Formation, and Quaternary alluvial-proluvial sediments. Characterized by frequent heavy rainfall in summer, the region preserves multiple phases of slope-proluvial layers, which hold distinctive significance for interpreting regional environmental evolution. This study focuses on a 5.3m thick slope-proluvial sediments profile (39°19'23″N, 121°50'25″E; 68m above sea level) exposed along the Shihe River. The Shihe profile is divided into 8 layers from bottom to top, and seven optically stimulated luminescence(OSL) samples(SH-1 to SH-7) were collected for dating using the SAR-SGC method. Sixteen sedimentological samples(SH1~SH16) were collected for analysis of environmental proxies, including grain size, magnetic susceptibility, and geochemical elements. Grain size analysis was performed using a laser diffraction particle size analyzer, calculating the mean grain size(Mz), sorting coefficient(σ), skewness(Sk), and kurtosis(KG). Measurements were conducted on slope-proluvial sediments. Low-frequency mass-specific magnetic susceptibility(χlf), high-frequency mass-specific magnetic susceptibility(χhf), and frequency-dependent magnetic susceptibility(χfd) were determined using an MS-2 magnetic susceptibility meter. Major and trace element concentrations were analyzed using an X-ray fluorescence(XRF) spectrometer. Subsequently, the chemical index of alteration(CIA) and the weathering-leaching coefficient(Ba) were calculated. The results indicate that: (1) OSL dating reveals the profile was formed during the Middle to Late Pleistocene, with ages of 130.6±6.6ka to 186.6±9.0ka, and 18.8±1.7ka, respectively. (2) The sediment is predominantly composed of silt(mean content: 55.31%). The frequency distribution curves are polymodal, with a mean sorting coefficient of 5.36, indicating very poor sorting. The gravels are primarily composed of coarse and medium granules, exhibiting angular to subangular shapes. These characteristics are indicative of typical rapid deposition proximal to the source area. Based on the topographic features, the clastic materials are interpreted to be sourced from the western hills. Precipitation fluctuations during the Middle to Late Pleistocene provided the hydrodynamic conditions. Under the combined control of topography and climate, a slope-proluvial landform, characterized by the coexistence of sand, silt, clay, and gravels with slope deposits as the primary component and proluvial deposits as supplementary, was formed in the Shihe region. (3) During the late Middle Pleistocene, the relatively high average mass-specific magnetic susceptibility(135.63×10-8 m3/kg) and high Chemical Index of Alteration(CIA) value of 83.5, coupled with a low Ba ratio(0.25), collectively indicate a generally warm and humid climate with intense chemical weathering and ample hydrodynamic conditions. This led to the superposition of multiple phases of slope and proluvial processes. By the late Late Pleistocene, both magnetic susceptibility(128.47×10-8 m3/kg) and the CIA value decreased(75.5), while the Ba ratio significantly increased(0.41). This shift reflects a transition to a colder climate, weakened chemical weathering, and relatively reduced hydrodynamic energy. The characteristics of environmental change revealed in this study show spatial consistency with other Quaternary records from the Liaodong Peninsula and adjacent regions.