Objective Trace elements play an important role in the reconstruction of the paleoredox states of marine sediments. Among them， molybdenum（Mo） has become an ideal proxy for the reconstruction of the redox state of ancient water bodies because it is mainly enriched in the form of autogenetic chemical deposition and less affected by terrigenous debris. However， compared with the ocean， lakes are more susceptible to terrigenous detritus and have more complex sedimentary environment， and the difference in the environment affects the settlement， distribution， and enrichment of Mo. In this study， the surface sediment of Qinghai Lake， the largest lake in China is the main research object. The enrichment mechanism and controlling factors of Mo element in alkaline， suboxic， and brackish water lakes were identified to provide a scientific basis for the applicability of Mo element in the reconstruction of lake paleoenvironment. Methods The planar distribution characteristics of Mo in the surface sediments of Qinghai Lake were determined by high density sampling and systematic testing， including X-ray fluorescence spectrometer （XRF） and inductively coupled plasma-mass spectrometry （ICP-MS）. In addition， the coupling relationship between Mo content and terrigenous input， particle size， element content， total organic carbon （TOC）， and depositional environment was established to determine the main controlling factors of Mo enrichment in the surface sediments of Qinghai Lake. Results The preliminary results show that Qinghai Lake is a brackish water lake with alkaline and weak oxidation. The planar distribution of Mo in the surface sediments of Qinghai Lake increases from the coastal shallow water area to the deep water area in the lake， and Mo is slightly enriched in the deep water area. Under sulfidic conditions， the soluble Mo in the water body is converted to particle reactive thiomolybdates. Because the overall water body of Qinghai Lake is in a state of oxidation， the enrichment of Mo in the sediments is unrelated. The enrichment of Mo in the surface sediments of Qinghai Lake may be controlled by the adsorption effect， that is， it is adsorbed and precipitated by clay minerals， Fe-Mn oxyhydroxides， organic matter， and other substances. The correlation analysis between Mo content and terrigenous input， particle size， element content， and TOC in the selected two transects shows that Mo has a positive correlation with TOC， and the change is similar， whereas Mo has a weak correlation with Al₂O₃ and Zr， which represents the terrigenous detritus input， and the change is significantly different. There was no correlation with clay minerals and Fe-Mn oxyhydroxides. This indicates that the influence of terrigenous detrital input on the distribution of Mo content in Qinghai Lake sediments is weak. Affected by the pH of water and the redox state of surface sediments， clay minerals， Fe-Mn oxyhydroxides have little influence on Mo enrichment in this environment， and TOC is the main controlling factor of Mo enrichment. Conclusions For alkaline and brackish water lakes with weak oxidation of bottom water， such as Qinghai Lake， organic matter adsorption and preservation are the main controlling factors for Mo enrichment in sediments. The type of organic matter could also affect the application effect of Mo as a proxy for redox identification of palaeolakes. Although the type of organic matter may influence the identification of redox conditions by Mo enrichment degree， Mo enrichment in weakly oxidized lake sediments is mainly controlled by the adsorption and preservation of organic matter， and Mo enrichment in sulfidic lake sediments is controlled by both the adsorption and preservation of particles such as organic matter and the reaction with H₂S in the water and the final preservation in the sediment （MoS₂）； thus， the Mo enrichment capacity is stronger. Therefore， Mo is an effective proxy for judging the redox state of palaeowater in the lake basin.

Objective The study of the relationship between climate proxies and climate factors in sandy surface sediments is highly significant when determining the reliability and applicability of climate proxies. Methods In this study， 32 samples of surface sediments were collected over a large spatial scale in Otindag sandy land. By analyzing correlations between chroma， Hm/Gt， magnetic susceptibility and modern climate factors （average annual precipitation and average annual temperature）， the relationships between each index and climate factors and their environmental significance are discussed. Results The results show that the variation range of a^* and b^* is small， and that the overall trend from east to west is increasing： the minimum value of a^* is 3.87， the maximum value is 8.26， and the average value is 6.09. The minimum value of b^* is 9.49， the maximum is 17.44， and the average is 14.24. a^* and b^* were negatively correlated with precipitation （correlation coefficients -0.88 and -0.77， P < 0.01）， and positively correlated with temperature （correlation coefficients 0.89 and 0.82， P < 0.01）， indicating that both a^* and b^* could be used as effective proxy indicators of climate change， with a^* being the more sensitive to climate factors. The variation range of L^* was small， and no obvious change rule from east to west was evident. The maximum value was 59.54， minimum value 48.80， and the average value was 54.69. The relationship between L^* and climate factors is not obvious， and its change is mainly directly affected by vegetation； it does not appear to have any obvious indicative significance for climate. The peak value of the first derivative of hematite ranges from 0.08 to 0.19， with an average of 0.14. The peak value of the first derivative of goethite ranges from 0.06 to 0.09， with an average of 0.07. The value of hematite/goethite （Hm/Gt） ranged from 1.20 to 2.73 （average 1.88）， and showed an increasing trend from east to west. The correlation between hematite and climate factors is higher than it is for goethite. The ratio of Hm/Gt is negatively correlated with average annual precipitation （correlation coefficient -0.85， P < 0.01）， and positively correlated with average annual temperature （correlation coefficient 0.84， P < 0.01）， which reveals the sensitivity of both hematite and Hm/Gt to precipitation and temperature. It effectively reflects changes of hydrothermal conditions in sandy land. In low-frequency magnetic susceptibility （χ_lf） and high-frequency magnetic susceptibility （χ_hf）， the variation direction of hf amplitude shows a constantly increasing trend from east to west. The percentage frequency magnetic susceptibility does not change significantly from east to west. The magnetic susceptibility of the region does not correlate well with climate factors. The source of the sand material may be the main factor in its variation， emphasizing the complexity and specificity of sand magnetic susceptibility changes. Conclusions a^*， b^*， and b^*/a^* are all effective alternative indicators for studying regional climate， whereas L^* is influenced by vegetation， resulting in ambiguous climate information. Hematite and Hm/Gt values correlate well with climate parameters and are ideal indicators of moisture and heat changes in sandy land. The surface magnetic susceptibility of sandy land is less affected by climate， reflecting the complexity and specificity of magnetic susceptibility changes in sandy land.

Objective By studying the geometric characteristics and formation mechanism of pyrite and tuff dikes in shales of the Qingshankou Formation in Gulong Sag， this paper discusses their formation time， sedimentary environment， sedimentary process， sediment state and diagenetic environment， and estimates the diagenetic compaction rate of shale， which provides reference for the formation mechanism and significance of the same type of dikes. Methods Based on detailed core observation， thin section analysis and dynamic calculation of dike formation， pyrite and tuff dikes are found in the Gulong shale， and their genetic mechanism and geological significance are explored. Results The pyrite and tuff dikes in shales of the Qingshankou Formation in Gulong Sag are small in scale， and pyrite dikes can be divided into two types according to their scale. One is the thick pyrite dikes with a width of 1-4 mm and a length of several centimeters； The other is a thin pyrite dike， which is less than 1 mm wide and several millimeters to several centimeters long. Tuff dikes are mostly less than 1 mm in width and several millimeters to several centimeters in length， and most of them are strongly bent into ptygmatite， erect， inclined or horizontally produced. It is speculated that gravity subsidence may be caused by density inversion， and the dynamic formation model of pyrite and tuff dike is established by calculation. Conclusions The shape of pyrite dike in shale oil reservoir of the Qingshankou Formation in Gulong Sag is very complex， which is formed by gravity subsidence sand dike after late pyritization， that is， there are silt dikes first and then pyrite dikes formation. The tuff formed by volcanic eruption settling on unconsolidated mud of lake， and then tuff dike formed due to gravity subsidence caused by density inversion， rather than overpressure filling from bottom to top caused by seismic liquefaction. The oblique pyrite dike and tuff dike are symmetrical at a distance of 180 degrees， which are subjected to bedding shear action. There are many silt-grade mud debris and large flame-like mud debris in the interior dikes， which are formed by the sand dike and tuff dike sinking into the unconsolidated or even muddy mud at the lower part and being captured.

Objective The sedimentary environment of the Pearl River Delta is complex with abundant Perinereis trace fossils. The characteristics， distribution， and assemblage of Perinereis and their relationships with sedimentary environment and sediment properties are significant for the study of trace fossils and the palaeoenvironment. Methods Based on sedimentary and ichnological methods， grain size analysis， salinity， turbidity， total organic carbon （TOC）， X-ray scans， and three-dimensional （3D） reconstruction were applied to the modern biogenic sedimentary structures in different microenvironments of the Pearl River Delta tidal flat， discussed the composition， assemblage， and distribution characteristics of Perinereis in a tidal flat of the Pearl River Delta， and analyzed the relationship between the biological assemblage and the sedimentary environments. Results （1） The Perinereis mainly lives in the tidal flat area of the tidal channel and the interdistributary bay in the Pearl River Delta. （2） The remains of the Perinereis mainly have grazing trails and dwelling trails. However， the grazing trails on the layer are difficult to observe， the dwelling trails in the layer mainly have simple Y-， I-， and U-shaped and complex network structures， and there are bulges at the joints. （3） The Perinereis is suitable for living in the tidal flat or channel of the interdistributary bay with low hydrodynamic conditions， low salinity， low turbidity， and rich total organic carbon content. Conclusions The spatial distribution of Perinereis neoichnology is imbalanced， and multiple environmental conditions jointly control the abundance of Perinereis neoichnology. The habitat of Perinereis is similar to the trace fossil Polykladichnus， which can belong to both the Mermia and Glossifunites ichnofacies； the trace fossil assemblage of this relic mainly appears in the channel and tidal flat area， and this study supplements the biological data for the modern neoichnology of Perinereis.

Objective The discovery of lithium-mineralized bodies in multiple layers in the Permian to the Jurassic at the northern margin of the Yangtze Block is of great significance to the investigation， development and utilization of sedimentary lithium resources. This study aims to ascertain the nature and tectonic setting of the parent rocks in the source area of the lithium-bearing （lithium-rich） fine clastic rocks of the Jurassic Baitianba Formation， and to preliminarily discuss the influence of paleoenvironmental conditions on lithium enrichment. Methods Detailed petrological investigations were conducted on the typical lithologies within the formation， along with elemental geochemical analyses focusing on the lithium-bearing （lithium-enriched） fine clastic rocks. Results The oxide discriminant function graph of the main elements and the La_N/Yb_N-ΣREE， K₂O-Rb and La/Th-Hf diagrams indicate a mixed provenance of igneous， sedimentary and metamorphic rocks. The oxide discriminant function graph of the main elements， together with La-Th， Ti/Zr-La/Sc， La-Th-Sc， Th-Sc-Zr/10 and Th-Co-Zr/10 diagrams， show that the tectonic background of the sedimentary period was an active continental margin， and some source rocks were formed in a continental island arc environment. Lithium probably originated from arc magma material. Ga， Sr/Ba， Sr/Cu， C， CIA， U/Th， Ni/Co and other paleoenvironmental indicators indicate that the lithium-bearing rocks were deposited in a weakly oxidizing freshwater environment in a warm， humid climate. The A-CN-K and Th/Sc-Zr/Sc diagrams show that the parent rocks underwent moderate to intense weathering when the debris was first deposited. Conclusions The comprehensive study found that lithium enrichment was only superficially influenced by ancient salinity and ancient reduction-oxidation conditions. Rather， the warm humid climate was conducive to the formation of clay minerals， which increased the clay-mineral content of Li-bearing （Li-rich） fine clastic rocks and determined the extent of lithium enrichment.

Objective The end-Permian mass extinction event （EPME） led to a global decline in flora and biota. The thick coal seams prevalent during the Permian， no longer appear following this event， resulting in a prolonged coal shortage throughout the Triassic. In the Sichuan Basin， following the EPME， peat-forming spore plants that contributed to coal-forming period were lost in the lowlands， with no recorded subsequent coal seam development. This period is called "coal gap".The climate was hot during the Lower and Middle Triassic， and the turbulent sedimentary environment in the Sichuan Basin suppressed the growth and development of peat-forming plants. Coal seams re-appear in the Sichuan Basin during the deposition of the First member of the Upper Triassic Xujiahe Formation （T₃x₁）， characterized by thin coal seams and poor spatial continuity. During the middle to late sedimentation stages in the Xujiahe Formation， thicker coal seams developed with highly regular spatial distribution. Current studies of the Upper Triassic paleoclimate in the Sichuan Basin have mainly concentrated on the middle and late stages of Xujiahe Formation sedimentation， leaving a gap in research on paleoclimatic conditions during the initial coal-forming phase of the Late Triassic （early stage of the Xujiahe Formation）. To fill this gap， a focused study was conducted on the T₃x₁ （specifically the Gongnongzhen and Wangjialiang sections） in the northwestern part of the Sichuan Basin. Methods Conducted on the paleoclimate and coal-forming mechanism of the coal bearing strata （Gongnongzhen and Wangjialiang sections） of the Xujiahe Formation in the northwest Sichuan Basin based on field outcrop observation， petrological microscopy analysis， and principal and trace element analysis. Results It was found that the sedimentary facies of the T₃x₁ in the Gongnongzhen section are mainly delta front subfacies， whereas those in the Wangjialiang section are mainly delta plain subfacies. Weathering indices （CIA_corr， Rb/Sr） and climate indices （Sr/Cu and C values） of the T₃x₁ in the Gongnongzhen and Wangjialiang sections displayed an overall fluctuating trend. The humid and hot climate corresponds to strong chemical weathering intensity， whereas the period of warm semi-arid/semi-humid climate corresponds to moderate chemical weathering intensity. Conclusions The paleoclimate during the initial coal-forming period of the Upper Triassic Xujiahe Formation in the northwestern Sichuan Basin is of two types： （1） a hot， humid climate； and （2） alternating warm， semi-arid to semi-humid climates. Coal seams in the study area are associated with Type 1. The hot， humid conditions promoted the reproduction of peat-forming plants， providing abundant source material for the formation of coal seams. The paleoclimate alternating between dry to wet during the T₃x₁ in the region is closely related to super-monsoon activity.After the Middle Triassic， the South China Block gradually moved closer to the mid-latitude humid climate zone，amd the high concentration of pCO₂ combined with the influence of monsoonal activity in the study area jointly promoted the formation of a humid climate and consequent strong chemical weathering. Further research indicated that coal deposition requires both a suitable paleoclimate and an appropriate preservational environment. For instance， coal seams tend to be well-developed and preserved in the swamps of high-level system tracts.

Objective The aim was to update the sedimentary characteristics of the widely distributed Triassic dolomite-evaporite paragenesis system in the Sichuan Basin and clarify its development and distribution patterns and main influences， provide new insights for its study and guide the reconstruction of the regional paleogeography. Methods Drill core and seismic data were used to investigate the sedimentary characteristics in the study area from a number of viewpoints and determine the genetic mechanisms of the different types of symbiotic systems and reveal their spatiotemporal distribution patterns and main controlling factors. Results （1） Overall， four types of symbiotic systems are present in the Leikoupo Formation， northwestern Sichuan Basin： （i） thick layers of dolomite interbedded with thin layers of evaporate； （ii） interbedded dolomite-evaporate； （iii） thick layers of evaporite interbedded with thin layers of dolomite； and （iv） overlapping thick layers of evaporite. Types （i） and （iv） are widely distributed throughout the four sedimentary periods in the Leikoupo region. Stacking of thick evaporite and thick dolomite occurs most frequently in the central part of the Lei-3 and Lei-4 sedimentary periods. （2） The uplift of the Longmen Mountain island chain on the western side caused southeastern compression on the basin， and the Xuefeng Mountain uplift to the east restricted the southward migration of the ancient Luzhou Kaijiang paleo-uplift， and the contemporaneous Qinling Mountains southward subduction has been occurred.formed a structural pattern trending almost northeast–southwest in the Sichuan Basin， which controlled the overall northeast⁃southwest distribution of gypsum rocks and symbiotic systems. （3） The significant extent of the Luzhou Kaijiang paleo-uplift caused subsequent migration of the Leikoupo Formation subsidence center toward the west， and the gypsum sedimentary center and four coexisting systems also migrated westward from their earlier dispersed locations. Conclusions The sedimentary properties and distribution patterns of the symbiotic system resulted in four types of symbiotic system： gypsum-dolomitic lagoons， dolomitic gypsum lagoons， gypsum salt lakes， and gypsum salt basins. The paleogeography of each of the four sedimentary periods of the Leikoupo Formation was reconstructed from the distribution patterns of these types of symbiotic system.

Objective At present， the iron dolomite genesis mechanism of the Maosan-Wuyi in north-central Sichuan is not clear. Methods Based on the rock-mineral characteristics of the Maosan-Wuyi， we combined rock TIMA scanning， whole-rock X-ray diffraction analysis， microzonation in situ main trace elements， strontium isotopes fluid inclusions homogeneous temperature， and in situ U-Pb chronology of iron dolomite to study the mechanism of iron dolomite genesis. Results The study shows that： （1） the rock types of the Maosan and Wuyi are cherts， dolomites and tuffs. The tuffs mainly include mud crystal clastic， bright crystal clastic， and cloudy tuff； the dolomites are semi-autogenous-to-autogenous powdery-to-fine crystalline dolomite； the sedimentary tuffs are mainly composed of volcanic clasts， carbonaceous， calcium-magnesium， sand-grade clasts， and metallic minerals， among which the volcanic clasts are glassy clasts， basalt， and other clasts， and the calcium-magnesium mainly consists of dolomite and calcite. （2） The average values of trace elements in the dolomite at the top of Maosan and Wuyi show extremely high Fe content （10 678.40×10^-6）， high Mn content （822.95×10^-6）， Si content （1 929.81×10^-6）， Al content （394.11×10^-6）， and high Na+K content （362.38×10^-6）， indicating that the fluids in the closed environment are characterized by high salinity and alkaline metal content. This also indicates that the rock-forming fluids in the confined environment are characterized by high salinity and high alkaline metal content. The ⁸⁷Sr/⁸⁶Sr in the matrix of ankerite and mud crystal clastic tuffs are within the range of seawater from the same period in the Maokou and Wujiaping groups， and they also have low total rare earth element （REE） values and light REE （LREE） loss， consistent with the LREE loss pattern of seawater， indicating that the dolomitized diagenetic fluids and seawater of the same period have similar characteristics. （3） The U-Pb age of powdery-to-fine crystalline dolomite in the Maosan and Wuyi is 245.36±1.08 Ma， and the dolomitization mainly occurred in the Late Permian to Early Triassic. Conclusions The source of dolomitization fluids in the powdery-to-fine crystalline dolomite of the Maosan and Wuyi is the dewatering of clay minerals in the argillaceous tuffite and tuffaceous mudstone of the overlying Wuyi and Wuer. During this process， a large number of Mg²⁺， Fe²⁺， Al³⁺， and Si⁴⁺-rich ions were precipitated and transported to the granular tuffs at the top of Maosan and Wuyi to form iron dolomite using the seawater residual from the Maokou Formation to Wujiaping Formation during the same period as the carrier.

Objective Lithium rich claystone deposits with great potential for mineralization have been found in the bauxite-bearing sedimentary strata of Yunnan-Guizhou region in Southwest China， but the current understanding of the material sources and lithium enrichment mechanisms of these bauxite-type lithium rich claystones in these areas is still significantly insufficient. Methods In this study， two sets of bauxite-type lithium rich claystone systems， the Early Permian Daoshitou Formation in Yunnan and the Lower Carboniferous Jiujialu Formation in Guizhou， were analyzed in terms of their mineralogical compositions， major and trace elements， and the compositions of rare earth elements. Combined with the results of previous research， the study systematically explored the source attributes of two sets of bauxite-type lithium-rich claystone systems， which were formed in different epochs but have very similar petrographic characteristics， as well as determining the factors that led to the anomalous enrichment of lithium.［Results and Conclusions］ The results showed that the differences in source properties between the lithium rich claystones of the Jiujialu Formation in Guizhou and the Daoshitou Formation in Yunnan had a significant influence on their lithium content. In general， the development of the Jiujialu Formation， which has relatively low lithium content， is closely related to the weathering and denudation of impure dolomite in the underlying Loushanguan Group. Its original source may have been Neoproterozoic-Middle Proterozoic neutral acidic magmatism， which is significantly influenced by Mg-Fe compositions. The formation of Yunnan Daoshitou Formation claystones with relatively high lithium content was significantly influenced by the recycling of Ordovician sedimentary rocks， while the source connection with its underlying carbonate strata was weak. Their original source was dominated by neutral acidic magmatism. In addition， the lithium rich clay rocks in the Yunnan-Guizhou region were hosted by either kaolinite or illite， and the drainage of the watershed during the depositional period and the effect of coexisting ions in the water body played important roles in restricting the lithium enrichment in the clay. Analyzing the provenance of lithium rich clay rocks in the Jiujialu Formation in Guizhou and the Daoshitou Formation in Yunnan， along with the occurrence of lithium-bearing minerals， is of significant importance for understanding the ore-forming processes and controlling mechanisms of clay-type lithium deposits.

Objective The Chang 7 member of the Triassic Yanchang Formation in the Ordos Basin mainly developed semi-deep lake-deep lake subfacies； deep-water gravity flow deposits are widely developed. This study aimed to clarify its sedimentary characteristics and distribution rules which can indicate the direction of oil and gas exploration. Methods Taking the Chang 7 member in the Fuxian area of the southeastern basin as the research object， based on the fine description， grain size analysis and logging data of centimeter-level cores from 13 coring wells， the types and characteristics， distribution law and evolution model of gravity flow deposits in the Chang 7 member were clarified. Combined with oil and production test data， the control effect of gravity flows on reservoir distribution was analyzed. Results The results show that the Chang 7 member in the Fuxian area mainly developed three types of gravity flow sedimentary microfacies： sliding-slump， sandy debris flow， and turbidity current deposition. The sliding-slump deposits are characterized by the development of enclave bedding， crumpled deformation structures， and syn-depositional stepped small faults and sliding surfaces and are dominated by mixed deposits of sandstone and mudstone. Sandy debris flow deposits have developed thick massive sandstone， mudstone tearing debris， and mud-encapsulated gravel structure. The thickness of single sand body is 0.3-3.5 m， and the cumulative thickness can reach more than 10 m. Turbidity current deposits have developed flame-like structures， groove molds， other bottom mold structures， and incomplete Bouma sequences. Sand bodies are small in scale and limited in development. The thickness of single sand bodies is several centimeters to tens of centimeters. The Chang 7 member is divided into six typical logging facies combinations and 11 lithofacies types. The grain size probability accumulation curve is characterized by “one-stage suspension” and a “wide arch”. Most of the data in the C-M diagram are parallel to the C=M baseline. Conclusions Among the three kinds of gravity flow sedimentary microfacies in the Chang 7 member， the sandy debris flow has the largest scale of development， accounting for approximately 80%， and the drilling frequency is 60%， making it the most favorable reservoir. Sliding-slump and turbidity current deposits account for approximately 10%. The formation mechanism of the gravity flow in the Chang 7 member is a variety of effects of underwater slope break zones， volcanos， and earthquakes. The development scale of deep-water gravity flow deposits is large， and it forms a good “source-reservoir-cap” combination configuration with source rocks vertically， creating conditions for the near-source accumulation of tight oil. On the plane， the Chang 7₃ sub-member mainly developed turbidity current deposits， and the Chang 7₂ and Chang 7₁ sub-members mainly developed sandy debris flow and turbidity current deposits. The sandy debris flow sand body has a high oil level， mainly oil immersion and oil spots. In the sandy debris flow development area， the industrial oil flow wells account for 21% of the total number of test wells， making it the key target area for the next exploration.