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[Objective] The study of the sedimentology, geochemistry and zircon U-Pb dating of sand-gravel sediments is highly significant for determining stratigraphic division, river system evolution, mountain tectonic uplift history and regional tectono-magmatic events. However, little is understood regarding the Hulun Buir Bayan Tara sand-gravel profile, which has restricted its stratigraphic division and in-depth understanding of the regional surface process. [Methods] For this reason, analyses of the sedimentology, heavy mineral, geochemical (macronutrients, trace and rare earth elements) and zircon U-Pb chronology were performed on this section to determine the chemical weathering characteristics and sedimentary cycles, as well as the material sources, and to explore the stratigraphic delineation attribution as well as the tectonic context and indications of regional magmatic-tectonic events. This has provided a new basis for discussion of areas of sedimentary environment and stratigraphic division and correlation. [Results] The following results were obtained. (1) The gravels are poorly sorted and rounded, and have undergone a non-weak degree of weathering, with quartzaceous material (61.2%) and tuff (34.9%) dominating the gravel composition, followed by terrigenous clastics (2.6%) and rhyolite (1.3%). The heavy mineral composition is clearly dominated by white titanium (32.54%) and ilmenite (33.01%c), followed by magnetite (12.11%), zircon-tourmaline- rutile (ZTR) (6.53%), hematite (5.47%) and anatase (1.34%), with less than 1% chlorite, rutile and garnet. There are very few, if any, unstable minerals in the sediments, indicating that they have undergone strong chemical weathering. Combined with geochemical characteristics and geomorphological evidence, the comprehensive analysis shows that the maturity of the sediment in the Bayan Tara sand-gravel profile is low, and is the product of the first weathering cycle, with moderate chemical weathering after accumulation. Comparison with the age spectrum of detrital zircons in the northern section of the Greater Khingan Mountains indicates that this is the provenance area. (2) Combined with macro- and micro-perspectives (heavy minerals and geochemical indicators, etc.), it is considered that the sediments of the Bayan Tala sand-gravel profile are alluvial-proluvial deposits formed in a warm, dry oxidizing environment. Combined with the new research findings, various methods (sedimentology, mineral petrology and geochronology) were integrated and analyzed and compared with the standard sections of the Baitushan Formation in Longjiang county and Ping'an town at the eastern foot of the Greater Khingan Mountains. This indicated that the stratigraphic units are part of the Baitushan Group. (3) The discriminant diagram of the tectonic background indicates a passive continental margin, which is consistent with the fact that the Greater Khingan Mountains are located within the tectonic plate and their structure is relatively stable. Peak U-Pb ages of clastic zircons at 100-200 Ma and 250-350 Ma record multi-stage magmatic tectonic events triggered by the closure of the Paleo-Asian Ocean and subduction and retrogression of the Paleo-Pacific Ocean in the northern segment of the Greater Khingan Mountains. [Conclusions] The study provides new evidence for division and further categorization of Quaternary strata and for reconstructing regional surface processes in the Greater Khingan Mountains during the early Pleistocene. [ABSTRACT FROM AUTHOR]

The Harbin loess is located in the easternmost part of the Eurasian loess belt. It is an ideal material for reconstructing the history of aridification of the Songnen Plain and determining the eastward advance of inland Asia. However, research on aridification of the climate in the Songnen Plain is quite weak. The relationships between aridification, tectonism, climate and drainage evolution in the area are currently poorly understood. In this study, these sedimentary characteristics, grain-size, magnetic susceptibility, total organic carbon and Sr-Nd isotopes are presented, combined with grain size end-member and wavelet analysis, to examine the history and driving mechanism of climate aridification in the Songnen Plain. It was found that due to the high-latitude cold climate, deposition of the Harbin loess-paleosol sequence was significantly different from that in other regions. The gleization sedimentary environment is the main controlling factor for the low magnetic susceptibility of the paleosol layer; the appearance of the Harbin loess indicates the climate aridification of the Songnen Plain. The Songnen sandy land began to develop at about 460 ka; loess >63 im and the EM3 end member component together clearly indicate the occurrence of two aridification and sandy expansion events at 380 ka and 300 ka. The Sr and Nd isotopes and TOC indicators reveal that the aridification phenomenon continued until 180 ka, after which the climate of the Songnen Plain gradually became warmer and wetter. Global climate cooling (enlargement of ice areas), regional tectonism (uplift of the Songliao watershed), and drainage change (altered Songliao water system) were collectively responsible for aridification in the Songnen Plain. This study is of great significance to the sustainable developmentof the ecology and social economy of the Songnen Plain. [ABSTRACT FROM AUTHOR]