Jiuxi Basin Research Articles

Late Miocene tectonic forcing of climate transition in the northeastern Tibetan Plateau

Interactions among late Cenozoic global cooling, tectonic activity and regional climate evolution of the northeastern Tibetan Plateau remain elusive. To further understand the relative contributions of global climate change and tectonism, we conducted a comprehensive study of heavy minerals, rock magnetism, and branched glycerol dialkyl glycerol tetraethers (brGDGTs) in late Cenozoic sediments of the Laojunmiao section (ca. 12.7 Ma to 5 Ma) in the Jiuxi Basin. Our results reveal a notable increase in magnetic susceptibility (χ) and saturation isothermal remanent magnetization (SIRM) at 8.5 Ma, aligning with an accelerated decrease in temperature captured by brGDGTs. Combined with heavy mineral analysis, we propose that the profound changes in magnetic parameters and temperature are closely linked to the intensive uplift of the northern Qilian Shan since 8.5 Ma. Furthermore, this ongoing uplift has occurred simultaneously with persistent aridification in the Jiuxi Basin and its adjacent regions. The Qilian Shan might have reached a threshold elevation and began to act as a moisture barrier to northwestward moisture since 8.5 Ma. By synthesizing hydroclimate evolution records from the northeastern Tibetan Plateau, we deduce that changing topography resulting from tectonic uplift may have exerted a significant influence on regulating the moisture distribution in this region during the mid- to late Miocene.

Early Cretaceous wildfires and their palaeoenvironmental significance in the Jiuxi Basin, Gansu Province, Northwestern China

The occurrence of palaeowildfires in terrestrial ecosystems coincides with the colonization of plants, rendering it one of the most significant geological events and holding paramount importance for comprehending paleoclimate and paleoenvironmental changes. This study provides strong evidence of persistent palaeowildfire occurrences in the Jiuxi Basin, Northwestern China, during the Aptian to Albian period, considering the abundant, continuous, sedimentary charcoal fossils from the uppermost Chijinbao, Xiagou, and Zhonggou Formations of the Hanxia Section. Micromorphological studies based on the scanning electron microscopy indicate that substantial portion of the charcoal fossils originated from conifers. The fusinite reflectance values of the charcoal fossils range from 1.32% to 4.32%, indicating that the palaeowildfire temperatures may have been 330 °C to 700 °C, and can be divided into three stages, evolving from ground fires to surface fires and crown fires, and finally developing to ground fires. Furthermore, high atmospheric oxygen and seasonally dry climate promoted the frequent palaeowildfire environment in the Jiuxi Basin during the late Early Cretaceous. However, the persistent and repeated wildfires may have effected the palaeoecosystem of the Jiuxi Basin during the late Early Cretaceous, which is evidenced by the decline of coniferous trees and the increase of the early herbaceous angiosperms.

Orbital eccentricity and inclination metronomes in Middle Miocene lacustrine mudstones of Jiuxi Basin, Tibet: Closing an astrochronology time gap and calibrating global cooling events

Reconstructing Cenozoic lacustrine astrochronology is essential for understanding climatic and environmental change around northeastern Tibetan Plateau. Existing models attribute the driving mechanism of regional precipitation and aridification to the Tibetan Plateau uplift and global climate change. However, in northeastern Tibet lacustrine astrochronology has a gap from 17 Ma to 14 Ma. To bridge this gap, we investigate Milankovitch-forced cyclicity of green-red mudstone rhythms deposited during the early Middle Miocene Epoch in Jiuxi Basin. We construct color rank stratigraphic series from lacustrine sedimentary successions, and develop a 3.6 Myr-long astrochronology from 16.0 Ma to 12.4 Ma that partly covers the gap. Time series analysis reveals dominant 405-kyr cycles associated with the long orbital eccentricity cycle, and 173-kyr and 1.2-Myr cycles associated with amplitude modulations of the obliquity cycle. We propose that orbital eccentricity (405-kyr cycle) and inclination (173-kyr cycle) metronomes were recorded as climate-sedimentary threshold responses in the lacustrine setting. The maxima of 173-kyr and 405-kyr cycles correspond to green mudstones during lake expansions, and their minima to red mudstones during lake contractions. Anchoring the astrochronology to modeled orbital eccentricity and inclination metronomes further reveals that the recorded astronomical cycles are in phase with those of the marine archives, and suggests that lake evolution in Jiuxi Basin was paced with Middle Miocene cooling events.

Geochemical characteristics and depositional paleoenvironment of source rocks from the Lower Cretaceous Chijinbao Formation in Jiuxi Basin, China

The Lower Cretaceous Chijinbao Formation is one of the primary source rocks in the Jiuxi Basin in Northwest China. However, it is poorly studied due to the lack of a complete stratigraphic sequence exposed by drilling. In this research, an integrated analysis of palynofacies, organic geochemistry, and element geochemistry was systematically conducted on samples with longitudinal continuity collected from the Chijinbao Formation in the Hanxia Section of the Jiuxi Basin to determine the distribution of source rocks and the source and type of organic matter and to reconstruct the depositional paleoenvironment of the Chijinbao Formation. The lower part of the sequence, which is governed by poor source rocks with Type III organic matter, is characterized by palynofacies A (woody tissue–charcoal assemblage) grown in a proximal alluvial fan–shallow lacustrine environment. The excellent source rocks with mostly Type II 1 organic matter are considerable in the middle part of the sequence, which is marked by palynofacies B (amorphous organic matter assemblage), and occur in a distal semi-deep lacustrine–deep lacustrine environment. The upper part of the sequence is largely composed of poor and fair source rocks, represented by palynofacies C (amorphous organic matter–woody tissue assemblage), and a small amount of sporadically distributed good and excellent source rocks (approximately 1/3), which may be deposited in a proximal fan delta and a local semi-deep lacustrine–deep lacustrine environment. A semiarid–semi-humid climate and a freshwater–brackish water environment prevailed during the deposition of the Chijinbao Formation. The lower part of the sequence is characterized by the highest terrestrial detritus influx, strong weathering, humid paleoclimate, oxic water column conditions, and the lowest paleoproductivity. The middle part of the sequence has the lowest terrestrial detritus influx, relatively weak weathering, relatively arid paleoclimate, predominantly anoxic water environment, and the highest paleoproductivity. The climate became humid again during the deposition of the upper part, which is typified by an intermediate terrestrial detritus influx, strong weathering, fluctuations of oxic–anoxic bottom-water conditions, and moderate paleoproductivity. The correlation analysis of element proxies suggests that the increase in salinity during the deposition of the middle part is associated with an arid climate. Amorphous organic matter, which is derived mostly from aquatic organisms and degraded terrestrial higher plants, is the major hydrocarbon generation component in the source rocks of the Chijinbao Formation. It increases in abundance with an improvement in water reducibility and productivity, favoring the formation and preservation of excellent source rocks. • Excellent source rocks distribute in middle part of Chijinbao Formation. • AOM dominate organic components in excellent source rocks of Chijinbao Formation. • Excellent source rocks were deposited under anoxic and relative arid conditions. • High salinity correlates with arid climate during the deposition of middle part.

Shortening rate and kinematics of the active Huoshaogou Anticline, northern margin of the Tibetan Plateau, inferred from deformed fluvial terraces

Active thrusting and folding at the foot of the Qilian Shan Mountains has accommodated the northward growth of the Tibetan Plateau. However, the slip history of individual thrusts and fault-related folds over different timescales remain largely unknown. In particular, the growth of a series of active fault-related folds suggests a relatively young deformation history in the sedimentary basin of the Hexi corridor. Here, we show the kinematics of the active Huoshaogou Anticline (HSGA) through stratigraphic sequences, and the shortening rate of the anticline using a balanced cross-section in the Jiuxi Basin, located at the western end of the Hexi Corridor. The total shortening rate (0.91 ± 0.37 mm/a) was estimated from deformed fluvial terraces, including the shortening of folding (0.74 ± 0.34 mm/a) and the shortening of the Xinminpu Fault (0.16 ± 0.07 mm/a) within the HSGA region. The results show that the shortening of fold deformation is more significant than the shortening of the fault dislocation. This indicates that the northward growth of the Tibetan Plateau not only affects the mountain foot of the Qilian Shan but also cross the Hexi Corridor to the southern boundary of the Gobi-Alashan Block.

Characteristics of the Micro-Nanopore System in a Pelitic Dolomite Reservoir: A Case Study of the Lower Cretaceous Xiagou Formation in the Qingxi Depression Based on Nuclear Magnetic Resonance Experiments.

The Lower Cretaceous Xiagou Formation is an important tight oil reservoir in the Qingxi Depression of the Jiuxi Basin. The micro-nanopore system within the reservoir requires a comprehensive analysis to improve the production of tight oil there. Nuclear magnetic resonance (NMR) experiments have been widely used for the petrophysical characterization of sandstones and carbonates. In the present study, the NMR experiment was applied to obtain the characteristics of the micro-nanopore system and permeability in the Lower Cretaceous Xiagou pelitic dolomite reservoir. According to the distribution shape of the transversal relaxation time (T₂) obtained under the 100% water-saturated condition (Sw), the samples are divided into four groups: (i) group I, two obvious peaks (P1 and P2); (ii) group II, an obvious high peak of P1 at 0.1˜1.0 ms and a relatively low peak of P2; (iii) group III, an obvious high peak of P2 and a relatively low peak of P1; and (iv) group IV, three peaks. In general, the distribution shape of T₂ under the initial condition (Sini) is unimodal, with all its peaks lower than those under the Sw condition. The NMR T₂ spectrum reflects the distribution of the rock pore radius. Most of the pore radius distributions are bimodal, and the main pore radius ranges from 10 nm to 70 nm. Three patterns can be identified and determined based on the distribution of the pore radius: I-unimodal distribution, II-bimodal distribution and III-trimodal distribution. The results indicate that the porosity in the Xiagou reservoir ranges from 1.17% to 6.89%, with an average of 3.33%. The permeability ranges from 0.03×10-3 μm² to 22.56×10-3 μm², with an average of 2.95×10-3 μm².

Late Quaternary Tectonic Activity and Slip Rates of Active Faults in the Western Hexi Corridor, NW China

The Yinwashan and Xinminpu faults are located in the Jiuxi Basin in the western end of the Hexi Corridor. The determination of their activity and slip rates is of great significance for understanding the eastward extension of the Altyn Tagh fault. Based on geological and geomorphologic field survey, trench excavation, optically stimulated luminescence dating, we define the fault geometry and kinematic properties of the two faults. Based on fault scarps measurement using differential GPS and 10Be surface exposure dating, we determined vertical slip rate of 0.09±0.01 mm/yr for the Yinwashan fault and 0.1±0.02 mm/yr for the Xinminpu fault. Using the dips observed in trenches and natural sections, we estimated horizontal shortening rates of 0.05±0.03 and 0.23±0.06 mm/yr, respectively. No significant strike slip motion is observed on these two faults, and we infer that this region was dominated by horizontal shortening in the Late Quaternary. Although the shortening rate is quite low on each individual fault, together with other faults in this area, these two faults have an essential role in transferring slip from the eastern end of the Altyn Tagh fault and in accommodating the northeastward growth of Tibetan Plateau.

Early Cretaceous palynology and paleoclimate of the Hanxia-Hongliuxia Area, Jiuxi Basin, China

The Jiuxi Basin is the earliest developed petroleum production area in China, and the Lower Cretaceous strata in this area are the main source rocks in this basin. Although the strata in this area are rich in fossils and have been researched for a long period, different interpretations of their geological age have been proposed. In this study, palynological samples collected from the Chijinbao Formation (Hanxia Section) and the Xiagou Formation (Hongliuxia Section) in the Jiuxi Basin were analyzed, and abundant fossil palynomorphs were identified. Based on detailed identification and statistical and comparative analyses, five palynological assemblages can be defined from the bottom to top of these strata. Among these, three assemblages are established in the Chijinbao Formation in the Hanxia Section. Their geological ages are Berriasian–Valanginian in the Early Cretaceous. Two assemblages are established in the Xiagou Formation in the Hongliuxia Section. Their geological ages are Hauterivian–Barremian in the Early Cretaceous. By analyzing the distribution characteristics and ecological environments of the different palynoflora types, we speculate that in the Berriasian–Valanginian, the vegetation of the basin mainly consisted of coniferous forests. During this time, ferns were scarce, and the climate was generally warm and humid. By the Hauterivian–Barremian, the vegetation mainly consisted of coniferous forests, ferns and cycads, and the climate was generally humid. The short-duration increase in Cheirolepidaceae reflects a short and slightly arid period in the early stage of the Hauterivian–Barremian. Furthermore, small-scale arid-humid climate fluctuations occurred during the deposition of both formations.

Stress sensitivity in naturally fractured reservoirs: a case study of the Lower Cretaceous Xiagou Formation, Qingxi Oilfield, Jiuxi Basin, northwestern China

Stress sensitivity is the variation of rock petrophysical parameters resulting from changes in effective stress. In fractured reservoirs, experimental methods exhibit a certain amount of error in the quantitative analysis of reservoir rock stress sensitivity. In addition, fracture-bearing experimental rock samples are difficult to obtain and prepare. Therefore, in the present study, reservoir rock stress sensitivity in naturally fractured reservoirs was investigated based on geomechanical modeling using a case study of the Lower Cretaceous Xiagou Formation in the Qingxi Oilfield. The results indicate that the Xiagou fractured reservoir experiences strong stress sensitivity with a fracture permeability damage rate reaching 94.38%. Natural fractures influence reservoir rock stress sensitivity. The degree of filling and type of filled minerals within natural fractures have great effects on the permeability damage rate. A higher permeability damage rate suggests stronger rock stress sensitivity. Generally, I) for reservoir rocks with unfilled natural fractures, the permeability damage rate is extremely high; II) for reservoir rocks with partially filled natural fractures, the permeability damage rate is high, and if the minerals within the natural fractures are insoluble, the permeability damage rate is slightly higher than if the materials are soluble; III) for reservoir rocks with completely filled natural fractures, the permeability damage rate is extremely low if the minerals within the natural fractures are insoluble; however, if the materials are soluble, the permeability becomes slightly higher with the increase of effective stress. Most importantly, this study provides a practical method for analyzing stress sensitivity in naturally fractured reservoirs.

Reconciliation of the Early Slip History of the Altyn Tagh Fault, Northern Tibetan

Whether the Altyn Tagh fault (ATF) had been extended beyond its current northeastern tip and linked with strike-slip faults in East Asia is a key to understanding the timing and mechanisms of crustal deformation in the northern Tibetan Plateau. We present Late Cretaceous dextral movement affected by Okhotomorsk Block-East Asia collision and a larger sinistral offset since Late Eocene along the ATF based on the provenance analysis of western Jiuxi Basin. Moreover, currently available estimates of offset based on displaced Paleozoic and Jurassic rocks could not represent the maximum offset due to late Cretaceous dextral offset.

Reorganization of sediment dispersal in the Jiuxi Basin at ~17 Ma and its implications for uplift of the NE Tibetan Plateau

The Tibetan Plateau offers an example for understanding the processes of continental collision and subsequent intracontinental deformation. The Jiuxi Basin, located at the northeastern margin of the plateau, bears crucial information on the growth processes of the NE Tibetan Plateau. Combined with published magnetostratigraphic constraints, integration of stratigraphic investigation and detrital zircon geochronological analysis on the sediments of the Cenozoic Hongliuxia section in western Jiuxi Basin suggests a major reorganization of the basin infill process at the period between depositions of the Baiyanghe and Shulehe formations at ~17 Ma. This finding is documented by (1) a generally upward-thinning sequence followed by an upward-coarsening sequence with corresponding changes in sedimentary facies, (2) shifting of the paleocurrent from the south to the north, and (3) shifting of the provenance from the Bei Shan-Kuantan Shan-Hei Shan in the north to the North Qilian Shan in the south. These results suggest that tectonic deformation arrived in the North Qilian Shan by ~17 Ma and lend support to the outward-growth model for the Tibetan Plateau.

In-situ stress orientations in the Xiagou tight oil reservoir of Qingxi Oilfield, Jiuxi Basin, northwestern China

Knowledge of the present-day in-situ stress orientation is important for borehole stability, fluid flow in fractured reservoirs, and hydraulic fracture stimulation, etc. The Qingxi Oilfield of Jiuxi Basin, northwestern China, is an old oilfield; however, within which there were no systematic investigations into the present-day in-situ stress orientation prior this study. In the present study, the orientation of horizontal maximum stress (SHmax) in the Qingxi Oilfield was interpreted and analyzed based on 149 borehole breakouts (BOs) and 131 drilling induced fractures (DIFs) from electrical borehole imaging logs in 23 wells. Our interpretations revealed a prevailing ∼ NE-SW-trending (14.94°N∼52.74°N) SHmax orientation in the Xiagou tight oil reservoir of Qingxi Oilfield. However, the SHmax orientation indicated various tendencies both laterally and with burial depth within single wells, which were influenced by well-developed faults, natural fractures and bedding planes. The presence of these structures caused great contrasts of rock mechanical properties, influencing stress orientation variations. In addition, the effects of present-day in-situ stress orientation on natural fractures, hydraulic fracture stimulation and borehole stability were discussed. In the Qingxi Oilfield, ∼NE-SW-trending natural fractures indicated favorable contributions to subsurface fluid flow. Hydraulic fractures would propagate vertically following ∼ NE-SW-trending. Wells were more likely to experience borehole instability issues if they were deviated towards ∼ NE-SW-trending. The results can provide geological references for subsequent tight oil production in the Qingxi Oilfield of Jiuxi Basin.

Oligocene storm-induced lacustrine deposits in the Yaxi Area of the Jiuxi Basin, northeastern margin of the Tibetan Plateau

Storm-induced deposits in the sedimentary record can now be easily identified on account of a better understanding of processes of unidirectional and oscillatory flow. Thin-bedded sandstone, siltstone, and interlayered mudstone deposits of the Oligocene Jianquanzi Member in the Yaxi Area of the Jiuxi Basin along the northeastern margin of the Tibetan Plateau were formed by shallow lacustrine storm-generated flows. Petrologic analysis reveals that the sandstones exhibit moderate to high compositional maturity but are texturally sub-mature to immature. Based on investigation of drill cores, six storm-deposit lithofacies are identified, including massive or graded sandstone, parallel and trough cross-bedded sandstones, hummocky cross-stratified sandstone, wave-rippled sandstone and siltstone, massive or horizontal bedded mudstone, and structurally deformed sandstone and mudstone. In a vertical succession, these lithofacies occur as three fining-upward lithofacies associations, each of which represents different architectural elements (and depositional processes) of proximal to distal storm deposits. The relatively coarse proximal architectural element is formed by strong density flows and unidirectional-dominated combined flows; the transitional architectural element is dominated by unidirectional- and oscillatory-dominated combined flows to pure oscillatory flows, resembling Tb-e divisions of the classic Bouma sequence; the relatively fine distal architectural element is dominated by unidirectional-dominated combined flows with a very weak oscillatory flow component. The origin of storms responsible for these deposits is attributed to Asian winter monsoons associated with the paleoclimatic and paleogeographic setting.

Tectonic fractures in the Lower Cretaceous Xiagou Formation of Qingxi Oilfield, Jiuxi Basin, NW China Part one: Characteristics and controlling factors

The Lower Cretaceous Xiagou Formation of Qingxi Oilfield is a typical reservoir with low porosity and permeability in China. Fractures within the Xiagou Formation play an important role in the petroleum exploration and development. In the present study, analysis data of conventional drill core, thin section and imaging logging were used to determine the characteristics and controlling factors of tectonic fractures in the Xiagou Formation. The majority of tectonic fractures in the Xiagou Formation were unfilled, and their apertures were less than 1.0×10−4m. The dominant strikes of tectonic fractures were in the NE-SW and NW-SE directions. Approximately 68.8% of tectonic fractures were bedding fractures, followed by oblique fractures (about 18.2%) and vertical fractures (about 13.0%). In different places, the densities of tectonic fractures varied greatly. The controlling factors, including tectonic and non-tectonic factors, for tectonic fractures in the Xiagou Formation were confirmed, which have been analyzed qualitatively or semi-quantitatively. Analysis of tectonic factors indicated that tectonic fractures were more probably formed (or the fracture density was high) in regions with larger stress gradients and/or closer to faults. Within the same tectonic setting and stress field, non-tectonic factors of the lithology and mineral composition became the dominant factors governing the development of tectonic fractures. Analysis of non-tectonic factors showed that tectonic fractures were most developed in dolomitic mudstone. The fracture density was positively related to the proportion of brittle minerals in rocks of the Xiagou Formation.

Tectonic Fractures in the Lower Cretaceous Xiagou Formation of Qingxi Oilfield, Jiuxi Basin, NW China. Part two: Numerical simulation of tectonic stress field and prediction of tectonic fractures

In the Xiagou low permeability reservoir, tectonic fractures are important reservoir spaces for storage of hydrocarbon and can significantly improve the permeability; therefore, understanding and predicting the development and distribution of tectonic fractures in the Xiagou reservoir are important for both petroleum exploration and exploitation activities in the Qingxi Oilfield. In the present study, the Himalayan paleotectonic stress field, the period of time when the majority of unfilled tectonic fractures generated in the Xiagou Formation of Qingxi Oilfield, was simulated with a three dimensional (3D) finite element (FE) model. Estimations of rock failure criteria (RFC) and comprehensive rupture rate (CRR) were undertaken to quantitatively determine the development and distribution of tectonic fractures in the Xiagou reservoir. Horizontally, areas with well-developed tectonic fractures were primarily located in fault zones, regions among faults, around Well G107 and with changes in the orientation of faults. Vertically, the K1g3 layer showed a relative higher CRR, indicating that it was highly fractured. Factors of faults and the lithology controlled the development and distribution of tectonic fractures in the Xiagou reservoir of Qingxi Oilfield. Abnormal high fluid pressures in the Xiagou Formation can promote the development of fractures in reservoirs and the transformation from shear fractures to tensile fractures under certain conditions.

The Cenozoic growth of the Qilian Shan in the northeastern Tibetan Plateau: A sedimentary archive from the Jiuxi Basin

AbstractSedimentary deposits in Tibetan Basins archive the spatial‐temporal patterns of the deformation and surface uplift processes that created the area's high topography during the Cenozoic India‐Asia collision. In this study, new stratigraphic investigation of the Caogou section from the Jiuxi Basin in the northeasternmost part of Tibetan Plateau provides chronologic constraints on the deformation and northward growth of the plateau. Magnetostratigraphic analysis results suggest that the age of the studied ~1000 m thick section spans from ~24.2 Ma to 2.8 Ma. Detailed sedimentology and apatite fission track (AFT) analyses reveal that variations in the clast provenance, lithofacies, sediment accumulation rates, and AFT lag times occurred at ~13.5–10.5 Ma. We interpret these changes as in response to the initial uplift of the North Qilian Shan. In addition, paleomagnetic declination results from the section indicate a clockwise rotation of the Jiuxi Basin before ~13.5 Ma, which was followed by a subsequent counterclockwise rotation during 13.5–9 Ma. This reversal in rotation direction may be directly related to left‐lateral strike‐slip activity along the easternmost segment of the Altyn Tagh Fault. Combined with previous studies, we suggest that movement on the western part of the Altyn Tagh Fault was probably initiated during the Oligocene (>30 Ma) and that fault propagation to its eastern tip occurred during the middle‐late Miocene.

Late Quaternary Tectonic Deformation of the Eastern End of the Altyn Tagh Fault

AbstractThe Quaternary activity of the faults at the eastern end of the Altyn Tagh fault, including the Dengdengshan–Chijiaciwo, Kuantanshan and Heishan faults, was studied on the basis of interpretation of satellite images, trenching, geomorphologic offset measurements and dating. The Altyn Tagh fault has extended eastwards to Kuantanshan Mountain. The left–slip rates of the Altyn Tagh fault decreased through the Qilianshan fault and were transformed into thrust and folds deformation of many NW– trending faults within the Jiuxi basin. Meanwhile, under NE–directed compression of the Tibetan plateau, thrust dominated the Dengdengshan–Chijiaciwo fault northeast of the Kuantanshan uplift with a rate lower than that of every fault in the Jiuxi basin south of the uplift, implying that tectonic deformation is mainly confined to the plateau interior and the Hexi Corridor area. From continual northeastward enlargement of the Altyn Tagh fault, the Kuantanshan uplift became a triangular wedge intruding to the east, while the Kuantanshan area at the end of this wedge rose up strongly. In future, the Altyn Tagh fault will continue to spread eastward along the Heishan and Jintananshan faults. The results have implications for understanding the propagation of crustal deformation and the mechanism of the India–Eurasian collision.

Cenozoic Exhumation and Thrusting in the Northern Qilian Shan, Northeastern Margin of the Tibetan Plateau: Constraints from Sedimentological and Apatite Fission‐Track Data

Abstract:The Qilian Shan lies along the northeastern edge of the Tibetan Plateau. To constrain its deformation history, we conducted integrated research on Mesozoic–Cenozoic stratigraphic sections in the Jiuxi Basin immediately north of the mountain range. Paleocurrent measurements, sandstone compositional data, and facies analysis of Cenozoic stratigraphic sections suggest that the Jiuxi Basin received sediments from the Altyn Tagh Range in the northwest, initially in the Oligocene (∼33 Ma), depositing the Huoshaogou Formation in the northern part of the basin. Later, the source area of the Jiuxi Basin changed to the Qilian Shan in the south during Late Oligocene (∼27 Ma), which led to the deposition of the Baiyanghe Formation. We suggest that uplift of the northern Qilian Shan induced by thrusting began no later than the Late Oligocene. Fission‐track analysis of apatite from the Qilian Shan yields further information about the deformation history of the northern Qilain Shan and the Jiuxi Basin. It shows that a period of rapid cooling, interpreted as exhumation, initiated in the Oligocene. We suggest that this exhumation marked the initial uplift of the Qilian Shan resulting from the India–Asia collision.

Characteristics of rare earth elements of lacustrine exhalative rock in the Xiagou Formation of Lower Cretaceous in Qingxi sag, Jiuxi basin

The exhalative rock occurring in the Xiagou Formation of Lower Cretaceous in Qingxi sag, Jiuxi basin is a sort of a rare lacustrine white smoke type, rich in ferrodolomites and albites. This paper introduces the geological background, mineral association, and lithology of the exhalative rock, and discusses its REE geochemical characteristics and connection with hydrothermal environment. It is shown that the exhalative rock has basal characteristics of hydrothermal depositional formation of LREE>HREE, with positive δCe and negative δEu, which is different from the characteristics of marine exhalative rock. Since the REE pattern and exceptional distribution of δEu and δCe are highly similar to the characteristics of alkalescent tholeiitic basalt in the same layer, the exhalative rock of Xiagou Formation is considered to be closely related to the origin of basalt in the same horizon. The fact that the amount of REE of exhalative rock decreases outwards indicates that exhalative rock in the Xiagou Formation may be connected with lacustrine hydrothermal convection circulation.

Response of grain size of Quaternary gravels to climate and tectonics in the northern Tibetan Plateau

The widely distributed thick gravel deposits along the rim of the Tibetan Plateau have been long thought to be the product of rapid tectonic uplift of the plateau. However, this has been challenged by recent works that suggest these thick gravels may be the result of climate change. In this paper we carried out a detailed field measurement of gravel grain sizes from the Jiuquan and Gobi Gravel Beds in the top of the Laojunmiao section in the Jiuxi Basin in the northern margin of Qilian Mts. (northern Tibetan Plateau). The results suggest that the grain sizes of the Jiuquan and Gobi Gravel Beds over the last 0.8 Ma are characterized by nine coarse-fine cycles having strong 100-ka and 41-ka periodicities that correlate well with the loess-paleosol monsoon record and isotopic global climatic record from deep sea sediments as well as by a long trend of coarsening in gravel grain size. The coarse gravel layers were formed during the warm-humid interglaciations while the fine layers correspond to the cold-dry glaciations. Because the paleoclimate in NW China began to get dramatically drier after the mid-Pleistocene, we think the persistent coarsening of gravel grain size was most probably caused by the rapid uplift of the northern Tibetan Plateau, and that the orbital scale cyclic variations in gravel grain size were driven by orbital forcing factors that were superimposed on the tectonically-forced long-term coarsening trend in gravel size. These findings also shed new light on the interaction results of climate and tectonics in relation to the uplift of the Tibetan Plateau.