Northern Qaidam Basin Research Articles

Anthraxolite Evolution and Vanadium Enrichment Mechanism in the Tanjianshan Group, Upper Ordovician in the Northern Qaidam Basin

Earlier researchers have examined the interaction between organic matter (OM) and metals, but the paragenesis of anthraxolite and mechanism enrichment of vanadium (V) need further study. Our recent geological field survey discovered that anthraxolite contains a substantial amount of V in the Member “a” of the Tanjianshan Group, Upper Ordovician in the northern margin of the Qaidam Basin. Different organic geochemical studies (fluid inclusions, zircon U–Pb dating, and petrochemistry) were conducted to examine the origin and evolution of anthraxolite, as well as the enrichment of V. The extractable organic matter (EOM) content of anthraxolite is low, while it contains a high amount of non-hydrocarbon and asphaltene. Anthraxolite is over-mature with its equivalent vitrinite reflectance being up to 4.9%. Anthraxolite is sourced from mudstone of Member “a” of the Tanjianshan Group. There were two hydrocarbon charging phases: (i) Late Devonian; and (ii) Carboniferous. Multiple magmatic thermal events took place after the Late Devonian period, resulting in hydrocarbon fluid cracking, metamorphism, and finally the formation of anthraxolite. Marine hot water deposition was irrelevant to enrichment of V. It is inferred that V originated during biological and biochemical processes, then migrated and accumulated with hydrocarbon formation and evolution, and finally enriched in anthraxolite.

The coupled evolution of mid- to late Holocene temperature and moisture in the southeast Qaidam Basin

The southeast Qaidam Basin (SQB) lies near the boundary between the modern-day Asian summer monsoon (ASM) and mid-latitude westerly jet, and the paleoclimate variation in this region can be affected by these two atmospheric circulation systems. Reconstructions of paleotemperature and palaeohydrology are therefore critical to constraining the driving forces of climate in this region, where the ecological environment is fragile. Here, we analyzed glycerol dialkyl glycerol tetraethers (GDGTs), microbial membrane lipids occurring ubiquitously in aquatic and terrestrial environments across the globe, in an aeolian sediment profile over the last 7000 years from Xiangride Town (XRD) of southeast Qaidam Basin (SQB), China. The temperature record was generated using the global calibration of MATmr based on branched GDGTs (brGDGTs), whereas the palaeohydrological condition was reconstructed from the relative abundance of isoprenoid GDGTs vs. brGDGTs (Ri/b), the cyclisation index of brGDGTs (CBT), and the brGDGT-based pH indices. The results show that the paleoclimate during the mid-Holocene was relatively warmer and wetter in the SQB. Afterwards it was a trend to a cold and dry climate. Temperature variation was highly coupled with the moisture change during the mid- to late Holocene, as opposed to some previous studies showing a warm-dry and cold-wet climate pattern in the northern Qaidam basin. The palaeohydrological evolution agrees with the precipitation pattern of the Chinese loess plateau but opposes to that in Northeastern China, the middle reaches of Yangtze river and Arid Central Asia (ACA), implying that the ASM strength is the driving force of precipitation evolution during the mid- to late Holocene in the SQB and the spatial heterogeneity of the mid- to late Holocene precipitation pattern across China is remarkable. A rapid cold and drought event at around 4 ka before present (BP) was identified, which might be a key factor that caused the decline in the agricultural civilization during the late Neolithic Age in northwestern China.

The sedimentary evolution of Da Qaidam Salt Lake in Qaidam Basin, northern Tibetan Plateau: implications for hydro-climate change and the formation of pinnoite deposit

Da Qaidam Salt Lake, northern Qaidam Basin, is known for its liquid and solid boron resource. More than that, pinnoite-bearing sediments, occurring in this lake, are rare in the world. However, little attention has been paid to the hydro-climate change and brine evolution correlation with the formation of pinnoite deposit. According to the multi-proxy records, as well as AMS 14C dating, the sediment profiles DCD02 and DCD03 from Da Qaidam Salt Lake were studied. The results revealed that the ore-forming age of three pinnoite deposits are 7.83–8.62 cal ka BP, 6.74–7.07 cal ka BP and 4.66–5.41 cal ka BP. The sedimentary evolution of Da Qaidam Salt Lake along with the pinnoite formation during the past 13,000 years can be divided into four stages, i.e., (1) relatively fresh and deep paleo-lake with low salinity during 12.98–9.00 cal ka BP; (2) seasonal variations between fresh and brackish paleo-lake during 9.00–8.62 cal ka BP; (3) salt lake stage with relatively high salinity and alkalinity during 8.62–4.66 cal ka BP, and (4) relatively shallow lake with high salinity since 4.66 cal ka BP. Under favorable sedimentologic and hydrogeochemical conditions, a series of boron deposits including pinnoite ore formed in different zones of Da Qaidam Salt Lake due to the different ore-forming mechanisms. Finally, under the differential effect of climate warming condition between the high mountain catchments and arid basin, the boron ore-forming model was established as a higher mountain–lower basin–shallower water model.

Jurassic sediments geochemical constraints on provenance, weathering process, and palaeoclimate variation of the north margin of Qaidam Basin, north‐eastern Tibetan Plateau

The Jurassic sequence in the northern Qaidam Basin can be represented by the lower coal‐bearing swamp‐lacustrine sediments unit and the upper reddish clastic deposits suite, which recorded dramatic transition of deposit environment changes. Whole‐rock geochemical data of this succession in Dameigou profile were used to reconstruct the provenance and chemical weathering history. Based on the major element discrimination diagrams, trace element ratios and chondritenormalized REE patterns, most sediments were derived from a similar source area with felsic‐intermediate rocks as dominant contributors. Multiple geochemical parameters and index displayed good correlation, via vertical variations sequence, Jurassic chemical weathering history of northern Qaidam Basin was established, and four stages were recognized from the profile. Phases I and III experienced intense chemical weathering, while Phases II and IV are characterized by mild to moderate chemical weathering. Based on this, long‐term Jurassic climatic evolution of the Qaidam Basin was reconstructed. Early‐middle period of Early Jurassic and middle period of Middle Jurassic developed warm and humid climate. On the contrary, late Early Jurassic‐early Middle Jurassic and late Middle Jurassic‐Late Jurassic was deposited under relatively semiarid to arid climatic conditions.

Origin and Sources of Minerals and Their Impact on the Hydrocarbon Reservoir Quality of the PaleogeneLulehe Formation in the Eboliang Area, Northern Qaidam Basin, China

The Lulehe sandstone in the Eboliang area is a major target for hydrocarbon exploration in the northern Qaidam Basin. Based on an integrated analysis including thin section analysis, scanning electron microscopy, X-ray diffraction, cathodoluminescence investigation, backscattered electron images, carbon and oxygen stable isotope analysis and fluid inclusion analysis, the diagenetic processes mainly include compaction, cementation by carbonate and quartz, formation of authigenic clay minerals (i.e., chlorite, kaolinite, illite-smectite and illite) and dissolution of unstable materials. Compaction is the main factor for the deterioration of reservoir quality; in addition, calcite cement and clay minerals are present, including kaolinite, pore-filling chlorite, illite-smectite and illite, which also account for reservoir quality reduction. Integration of petrographic studies and isotope geochemistry reveals the carbonate cements might have originated from mixed sources of bioclast- and organic-derived CO2 during burial. The quartz cement probably formed by feldspar dissolution, illitization of smectite and kaolinite, as well as pressure solution of quartz grains. Smectite, commonly derived from alteration of volcanic rock fragments, may have been the primary clay mineral precursor of chlorite. In addition, authigenic kaolinite is closely associated with feldspar dissolution, suggesting that alteration of detrital feldspar grains was the most probable source for authigenic kaolinite. With the increase in temperature and consumption of organic acids, the ratio of K+/H+ increases and the stability field of kaolinite is greatly reduced, thereby transforming kaolinite into mixed layer illite/smectite and illite. Within the study area, porosity increases with chlorite content up to approximately 3% volume and then decreases slightly, indicating that chlorite coatings are beneficial at an optimum volume of 3%. A benefit of the dissolution of unstable minerals and feldspar grains is the occurrence of secondary porosity, which may enhance porosity to some extent. However, the solutes cannot be transported over a large scale in the deep burial environment, and simultaneous precipitation of byproducts of feldspar dissolution such as authigenic kaolinite and quartz cement will occur in situ or in adjacent pores, resulting in heterogeneity of the reservoirs.

New insights into the provenance of Cenozoic strata in the Qaidam Basin, northern Tibet: Constraints from combined U-Pb dating of detrital zircons in recent and ancient fluvial sediments

Determining the provenance of Cenozoic strata in the Qaidam Basin is key to understanding the basin-mountain coupling history in the northern Tibetan Plateau (TP). However, the specific source areas of Cenozoic strata in the northern Qaidam Basin remain highly debated. Here, we combine analyses of detrital zircons UPb geochronology for recent and ancient fluvial sediments from the northern Qaidam Basin to trace source areas of Cenozoic strata and reconstruct related mountain-building processes. The results indicate that the diagnostic 270–240 Ma zircon UPb peak, which was previously recognized as the unique input of a southern source area, is widespread in the recent fluvial sediments of the northern Qaidam Basin. Given our new zircon UPb data, the source-to-sink transport processes of Cenozoic sediments in the northern Qaidam Basin can be summarized as follows. (1) The northern Qaidam Basin has received the eroded detrital material from a dominantly northern source throughout the Cenozoic. (2) The Qaidam BeiShan was uplifted already at least by the early Eocene and served as the single contributor of detritus to the Dahonggou (DHG) region, suggesting that far-field stress due to the India-Asia collision had been propagated to this region as early as the early Eocene. (3) An abrupt change in provenance is observed in the DHG region during 46.5–43.7 Ma. We interpret this middle Eocene source change as reflecting the onset of growth of the North Altyn Tagh Range, implying that the North Altyn Tagh Range was already serving as an important source area for the DHG region by the middle Eocene. (4) The South Qilian Range served again as the dominant source area for the DHG region after 24.6 Ma. The shift in provenance from the North Altyn Tagh Range to the South Qilian Range can be attributed to the uplift of the Saishiteng Shan.

Application of channel-belt scaling relationship to Middle Jurassic source-to-sink system in the Saishiteng area of the northern Qaidam Basin, NW China

Palaeodrainage basin, as an important component of the source-to-sink system, contains critical information on provenance and palaeoenvironment. Previous studies indicate that the scaling relationships of source-to-sink system components generally follow power laws, and channel-belt thickness represents a reliable first-order proxy for the drainage area. In this study, a database of borehole cores and geophysical well logs of the Jurassic coal measures from Saishiteng area in the northern Qaidam Basin was used to reconstruct the palaeogeography, and to identify single-story channel-belts. Three palaeochannels, namely, River A, River B and River C, were identified which were persistent throughout the Dameigou and Shimengou Formations during the Middle Jurassic. The mean channel-belt thicknesses of River A, River B and River C in the Dameigou Formation were 9.8 m, 8.9 m and 7.9 m, respectively, and those in the Shimengou Formation were 7.4 m, 6.2 m and 5.4 m, respectively. We estimate the drainage area of three major rivers by using scaling relationships between drainage area and channel-belt thickness. The drainage areas of River A, River B and River C in the Dameigou Formation were 63.0 × 103 km2, 50.1 × 103 km2 and 37.7 × 103 km2, respectively, and those in the Shimengou Formation were 32.3 × 103 km2, 21.2 × 103 km2 and 15.3 × 103 km2, respectively. The drainage basin lengths of River A, River B and River C in the Dameigou Formation were 300.4 km, 239 km and 180.2 km, respectively, and those in the Shimengou Formation were 154.3 km, 101.3 km and 73.1 km, respectively. For both the Dameigou and Shimengou Formations, River A showed the largest scale, followed by River B and River C succeedingly, which was mainly determined by the stretch direction of provenance in the southern Qilian Mountains. The variations of channel-belt thickness, drainage area and drainage basin length between Dameigou and Shimengou Formations are the response of source-to-sink system to the transformation from extension to compression depression during the Middle Jurassic in the northern Qaidam Basin.

Lake evolution and its influence on the formation of oil shales in the Middle Jurassic Shimengou Formation in the Tuanyushan area, Qaidam Basin, NW China

The non-marine Qaidam Basin is a petroliferous basin in northwest China. The Tuanyushan area is located in the Saishiteng Depression in the northern Qaidam Basin. Coal and oil shales are widely developed in the Middle Jurassic Dameigou and Shimengou formations in this area where the sedimentary sequence and controls on coal accumulation have already been well documented. However, the geochemical characteristics of lacustrine fine-grained sediments, including lacustrine oil shales, in the shale member of the Shimengou Formation and the main controlling factors of the formation of the oil shales are ambiguous. This paper aims to reconstruct the lake evolution history during the Middle Jurassic period and reveal its influence on the formation of the oil shales in this area. Oil shales and fine-grained sediments were systematically sampled to determine their mineralogical and geochemical characteristics (major, trace and rare earth elements; stable carbon and oxygen isotopes). Based on lithological variations and total organic carbon (TOC) contents, a complete third-order sequence is identified and can be further divided into four system tracts (lowstand system tract, LST; transgressive system tract, TST; highstand system tract, HST and regressive system tract, RST) that correspond to four lake evolution stages (A–D). Changes in the lake level show an initial shallow lake, followed by a continuous upward deepening trend, followed by a shallowing trend. Shallow lake facies developed in the LST, TST and RST, whereas semi-deep to deep lake facies developed in the HST. Stable carbon and oxygen isotopes indicate that the Shimengou Lake was semi-closed to closed in the Middle Jurassic. Therefore, the water properties responded strongly to climate changes. According to elemental and mineralogical analyses, a moist climate prevailed, except during the early stages of the TST (stage B1) and HST (stage C1) when there was a semiarid climate. The semiarid climate influenced the water properties and detrital input and was the major controlling factor for the formation of the higher quality oil shales within a saline water environment. In comparison, under the moist climatic condition in the HST, the stable semi-deep to deep-water environment was the major controlling factor for the formation of lower quality oil shales within a fresh water environment.

Neogene Expansion of the Qilian Shan, North Tibet: Implications for the Dynamic Evolution of the Tibetan Plateau

AbstractThe Qilian Shan, at the northeastern frontier of the Tibetan Plateau, is a key area for studying the expansion mechanism of the Tibetan Plateau. Although previous thermochronology and paleomagnetic studies indicate Neogene northward expansion of the northern Qilian Shan, there is a distinct temporal gap in knowledge relative to the tectonic history of the southern Qilian Shan. This has hindered a complete understanding of the Cenozoic deformation pattern of the entire Qilian Shan. To study the growth history of the southern Qilian Shan, apatite fission track (AFT) data have been acquired from Zongwulong Shan and the Huaitoutala section. AFT thermal history modeling from the former shows a rapid cooling episode occurred at ~18–11 Ma, which is interpreted as marking the onset of intensive exhumation in the southern Qilian Shan. Within the Huaitoutala section, detrital grain up‐section shows progressively decreasing peak AFT ages followed by an age increase from midsection, implying that a sediment‐recycling event occurred at approximately 7 ± 2 Ma. Together with a shift in paleocurrent directions, this change marks the onset of Late Miocene deformation of the northern Qaidam Basin. Combined with previous studies on the deformation time of the Qilian Shan, our findings suggest that both the northern and southern Qilian Shan region grew outward synchronously in opposite directions during the Neogene. This resulted in the formation of a flower structure, which had an important impact on the deformation pattern of north Tibet. The synchronous outward expansion may have been triggered by the removal of mantle beneath north Tibet.

Geochemical Characteristics of Oil from Oligocene Lower Ganchaigou Formation Oil Sand in Northern Qaidam Basin, China

Oil from the Oligocene oil sands of the Lower Ganchaigou Formation in the Northern Qaidam Basin and the related asphaltenes was analyzed using bulk and organic geochemical methods to assess the organic matter source input, thermal maturity, paleo-environmental conditions, kerogen type, hydrocarbon quality, and the correlation between this oil and its potential source rock in the basin. The extracted oil samples are characterized by very high contents of saturated hydrocarbons (average 62.76%), low contents of aromatic hydrocarbons (average 16.11%), and moderate amounts of nitrogen–sulfur–oxygen or resin compounds (average 21.57%), suggesting that the fluid petroleum extracted from the Oligocene oil sands is of high quality. However, a variety of biomarker parameters obtained from the hydrocarbon fractions (saturated and aromatic) indicate that the extracted oil was generated from source rocks with a wide range of thermal maturity conditions, ranging from the early to peak oil window stages, which are generally consistent with the biomarker maturity parameters, vitrinite reflectance (approximately 0.6%), and Tmax values of the Middle Jurassic carbonaceous mudstones and organic-rich mudstone source rocks of the Dameigou Formation, as reported in the literature. These findings suggest that the studied oil is derived from Dameigou Formation source rocks. Furthermore, the source- and environment-related biomarker parameters of the studied oil are characterized by relatively high pristane/phytane ratios, the presence of tricyclic terpanes, low abundances of C27 regular steranes, low C27/C29 regular sterane ratios, and very low sterane/hopane ratios. These data suggest that the oil was generated from source rocks containing plankton/land plant matter that was mainly deposited in a lacustrine environment and preserved under sub-oxic to oxic conditions, and the data also indicate a potential relationship between the studied oil and the associated potential source rocks. The distribution of pristane, phytane, tricyclic terpanes, regular steranes and hopane shows an affinity with the studied Oligocene Lower Ganchaigou Formation oil to previously published Dameigou Formation source rocks. In support of this finding, the pyrolysis–gas chromatography results of the analyzed oil asphaltene indicate that the oil was primarily derived from type II organic matter, which is also consistent with the organic matter of the Middle Jurassic source rocks. Thus, the Middle Jurassic carbonaceous mudstones and organic rock mudstones of the Dameigou Formation could be significantly contributing source rocks to the Oligocene Lower Ganchaigou Formation oil sand and other oil reservoirs in the Northern Qaidam Basin.

CLM4.5 Model Simulation of Soil Moisture over the Qinghai-Xizang Plateau and Its Performance Evaluation

The Community Land Model version 4.5 (CLM4.5) driven by atmospheric forcing data (CRUNCEP) is used to simulate spatial and temporal characteristics of soil moisture in the Qinghai-Xizang Plateau for the period 1981-2016. Compared with station data, two reanalysis products (i.e.,the ERA-Interim and GLDAS-CLM) and the microwave remote sensing FY-3B/MWRI data, it is found that CLM4.5 can well reproduce the spatial and temporal variations of soil moisture in the Qinghai-Xizang Plateau. The microwave data with high spatial resolution (0.1°×0.1°), which is derived from various satellite remote sensing in the Qinghai-Xizang Plateau, provides detailed description of the spatial features of soil moisture. The results show that the spatial and temporal distributions of simulated soil moisture are consistent with station observations.The simulation is significantly correlated with observations in all soil layers but with high systematic deviations. And the consistency between simulation and observations gradually decreases from the surface downward. The spatial distribution of soil moisture in the Qinghai-Xizang Plateau is characterized by gradual increase from northwest to southeast. The high-value areas are found in the Sanjiangyuan Wetland and the southeastern Plateau, while the dry regions are located in the Qaidam Basin and the Tarim Basin. Soil moisture generally increases from surface downward. Temporal variation of soil moisture presents an alternate dry and wet pattern. There is little difference in soil moisture variation in different layers. The simulation also reasonably reproducesmonthly changes in soil moisture in the summer. Soil moisture in most of the southwestern region including the southern periphery of the plateau obviously increases in the summer. The dry area in the northern Qaidam Basin also markedly shrinks to the north. The simulated soil moisture provides more detailed description of spatial distribution and monthly changes of soil moisture in the summer compared with reanalysis data and microwave remote sensing data.

Towards a clarification of the provenance of Cenozoic sediments in the northern Qaidam Basin

Towards a clarification of the provenance of Cenozoic sediments in the northern Qaidam Basin

HYDROCARBON POTENTIAL AND PALAEO‐DEPOSITIONAL ENVIRONMENT OF LACUSTRINE SOURCE ROCKS: MIDDLE JURASSIC SHIMENGOU FORMATION, NORTHERN QAIDAM BASIN, NW CHINA

The Middle Jurassic Shimengou Formation in the Qaidam Basin, NW China, includes coals and lacustrine source rocks which locally reach oil shale quality (i.e. yielding >3.5 % oil on low‐temperature distillation). In the present study, the palaeo‐depositional environment and hydrocarbon potential of the 84.5 m thick Shale Member of the Shimengou Formation are investigated based on bulk geochemical parameters, organic petrographic data, biomarker analysis, and stable isotope geochemistry of 88 core samples.The Shale Member was deposited in an anoxic freshwater lake which formed following the drowning of a precursor low‐lying mire. Variations in bulk geochemical parameters allow four informal units to be identified, referred to (from the base up) as Units 1 to 4. These contain intervals of oil shale of varying thicknesses. In Unit 1, mudstones in the interval referred to as oil shale Layer 1 (true thickness [TD]: 2.06 m) are OM‐rich as a result of algal blooms and photic zone anoxia, and correspond to an initial flooding event. Subsequently, productivity of aquatic organisms decreased, resulting in the deposition of organic‐lean mudstones in Unit 2. Oil shale Layers 2 (TD: 2.03 m) and 3 (TD: 8.03 m) near the base of Unit 3 were deposited during maximum water depths. As with Layer 1, high productivity by algal blooms resulted in photic zone anoxia in a stratified water column. The shales in the upper part of Unit 3 are characterized by high TOC contents and a gradual increased input of terrigenous OM, and were deposited in a stable semi‐deep lake. Finally, organic‐lean mudstones in Unit 4 were deposited in shallow lacustrine conditions. The reconstruction of depositional environments in thick, non‐marine shale‐rich successions by mineralogical, petrographic and inorganic geochemical methods may be challenging as a result of the homogenous composition of component mudstones. The results of this study indicate, however, that sub‐division and basin‐wide correlation of such intervals can be achieved by organic geochemical analyses.Oil shales and organic‐rich mudstones in Units 1 and 3 of the Shimengou Formation Shale Member are excellent oil‐prone source‐rocks with a Source Potential Index of 3.2 t HC/m2. Considering the large area covered by the Shimengou Formation in the northern Qaidam Basin (∼34,000 km2), the results of this study highlight the regional significance for future petroleum exploration. They indicate that variations in organic productivity and dilution by minerals are key factors controlling the abundance and type of organic matter in the formation. An understanding of these factors will assist with the identification of exploration targets.

Three‐dimensional magnetotelluric responses for arbitrary electrically anisotropic media and a practical application

ABSTRACTElectrical anisotropy in earth media increases the complexity of magnetotelluric responses. Magnetotelluric models based on anisotropic media must be developed to fully understand observed data. This paper presents a three‐dimensional algorithm for calculating magnetotelluric responses of arbitrary anisotropic media in the frequency domain. Using a staggered‐grid finite difference method, the model space is discretized into rectangular blocks with electric fields on the edges of each block and the magnetic fields normal to the faces of each block. The electric field Helmholtz vector equation that considers a full 3 × 3 conductivity tensor is calculated numerically under two orthogonal polarizations. In calculating the boundary values on the four sides of the three‐dimensional anisotropic model, we adopt different procedures for calculating the two‐dimensional responses on the sides in the x and y directions. The responses for a layered anisotropic model and a three‐dimensional isotropic model calculated with this algorithm are compared with the corresponding analytical and numerical solutions, respectively. The comparisons show that the algorithm's approximations are highly precise for a wide frequency band. A typical two‐dimensional anisotropic model and a general three‐dimensional anisotropic model were also constructed, and their responses were calculated. These anisotropic models have ordinary structures but can produce phase rolling out of quadrant magnetotelluric responses, which indicates that considering electrical anisotropy may improve our interpretation of observed data. Using this algorithm, we can model the observed data from the northern Qaidam Basin in northern Tibet, where ultrahigh‐pressure metamorphic rocks are exposed along an old suture, and seismic anisotropy was indicated in neighbouring areas. The phase tensors of the magnetotelluric sites at this location show large skew angles, and the corresponding phase splits are distinct in the off‐diagonal impedance elements. Although the isotropic three‐dimensional electrical structure can model the profile data well, the structure shows a sequence of conductive and resistive bodies in the mid‐lower crust of the north Qaidam Basin, which is very spatially inhomogeneous, and a simple intrinsic anisotropic body can also produce similar surficial responses. Using the three‐dimensional anisotropic algorithm, we found an equivalent anisotropic replacement for this area. The results of the three‐dimensional anisotropy modelling of the magnetotelluric data from the northern Tibetan Plateau show the valuable applicability of the three‐dimensional anisotropic algorithm in testing the qualitative presumption of electrical anisotropy.

Paleogene sedimentation changes in Lenghu Area, Qaidam Basin in response to the India–Eurasia collision

The Cenozoic India–Eurasia collision is evidently recorded in the Qaidam Basin, which is located in the north margin of the Tibetan Plateau. The present study describes a dextrorotary phenomenon at the end of the Eocene in Lenghu Area in the northern Qaidam Basin, discovered in sedimentary and tectonic records. This phenomenon is interpreted to have been a result of the levorotatory movement along the Altyn Tagh Fault (ATF) based on the following evidence. First, provenance, as analyzed by heavy mineral assemblages, was slightly deflected from southwestward to the westward by about 45°. Second, paleocurrent inferred from dip logging and seismic reflection changed clockwise by approximately 25°. Third, there is evidence of increases in fault activity in the area, especially in northwest–southeast-trending branches relative to older west–east-trending branches. Increases in faulting coincide with abrupt increases in sediment supply in the Oligocene, after earlier decreases based on total sand content during the Eocene. Our results demonstrate that the northern margin of the Tibetan Plateau synchronously responded to the initiation and termination of the India–Eurasia collision. The levorotatory strike slip of the ATF was immediately triggered by the complete collision at the end of Eocene, and the strike-slip movement caused the dextrorotary phenomenon in the Lenghu Area.

Mineralogy, petrography and geochemistry of an early Eocene weathering profile on basement granodiorite of Qaidam basin, northern Tibet: Tectonic and paleoclimatic implications

Weathering, as an important process in the earth surface system, can be significantly influenced by tectonics and climates over long time scales. Here, we use mineralogical, petrographic and geochemical data of a paleoweathering profile developed on basement granodioritic rocks of northern Qaidam basin, northern Tibet, to reconstruct early Eocene weathering conditions and to discuss how paleoclimates and tectonics dominated the weathering process. The results indicate that neoformed mineral phases in weathering products are dominated by smectite, and the profile has overwhelmingly low chemical index of alteration values (ca. 51–59) and significantly decreasing micropetrographic index values (from 25.0 to 0.2) from bottom to top. These findings suggest that the basement rocks experienced mild chemical weathering but relatively intensive physical weathering. We favor that non-steady-state weathering, in which mechanical erosion rates compare favorably with rates of chemical weathering, prevailed in northern Tibet during the early Eocene. The weathering conditions were likely an integrated response to active tectonism and dry climates at that time. Furthermore, chemical element mobility evaluation demonstrates that most of large ion lithophile elements and light rare earth elements (LREEs) of granodioritic rocks are quite active during weathering and can be easily leached even under mild chemical weathering conditions. Significant mass loss of Al and LREEs in upper weathered samples probably reflects acidic weathering conditions, which were likely due to extremely high atmospheric CO2 level during the early Eocene. This study, from the unique perspective of weathering process, suggests that intensive deformation and rapid tectonic erosion occurred in northern Tibet during the early Eocene, as a far-field response to the India-Eurasia collision. It also agrees with warm and relatively dry climates, which were likely attributed to the global greenhouse climates and the Paleogene planetary-wind-dominant climate system in Asia, respectively.

Revelation of organic matter sources and sedimentary environment characteristics for shale gas formation by petrographic analysis of middle Jurassic Dameigou formation, northern Qaidam Basin, China

The 7th member of Middle Jurassic Dameigou Formation (J2d7) in northern Qaidam Basin, China was proposed to have good hydrocarbon generating potential in previous studies. Here we apply an integrated petrographic and geochemical analysis to investigate the depositional environment, real thermal maturity and shale gas potential of organic-matter (OM)-rich assemblage. The relatively high content of C29 regular sterane (56.8%–62.8%) and vitrinite predominance in maceral composition suggesting plenty of higher-plant input, combined with the high pristane/phytane ratio (Pr/Ph)(2.8–8.2) and low gammacerane index (0.2–0.4) reveals oxic water column with in-situ depositional condition of carbonaceous-mudstone when the lower J2d7 was deposited. The maceral composition of oil shale at the upper part of J2d7 is mainly amorphous OM with small proportion of reworked vitrinite and inertinite, consistent with a mainly algea and small amount of exogenous OM input. When associated with low Pr/Ph (0.8) and high gammacerane index (4.3), the oil shale was deposited in an anoxic saline water column with mainly zooplankton OM source. The geochemical and petrographic result implies a transitional environment ranging from suboxic semi-saline to oxic fresh water environment with varying proportion of higher-plant and algae input by in-situ deposition and distant transportation for the mudstone at the middle part of J2d7. By use of random reflectance (Ro) distribution, two sedimentary environment microcycles which are confirmed by maceral composition, from the depth of 1983 m to 1962 m and 1954 m to 1931.9 m, are differentiated within the homogenous mudstone section. The variation of OM input condition and alternation of depositional environment led to accumulation and deposition of various kinds of abundant OM when J2d7 was deposited. Although the OM has actually low maturity at oil window with vitrinite reflectance around 0.6% according to the petrographic results, the liptinite inherited from higher-plants especially suberinite is the main maceral to generate gaseous hydrocarbons at the low maturity, which implies that the Dameigou formation (J2d7) of Qaidam Basin is of gas potential.

Late Cretaceous to early Eocene deformation in the northern Tibetan Plateau: Detrital apatite fission track evidence from northern Qaidam basin

Unraveling the growth of northern Tibet is crucial to understanding the geodynamic processes of the India-Eurasia collision, evaluating plateau uplift models and reconstructing associated paleoclimate history. However, pre-middle Miocene deformation history of northern Tibet remains poorly understood. We use detrital apatite fission track (detrital AFT) thermochronology of Mesozoic and Cenozoic sedimentary rocks from northern Qaidam basin to constrain the early growth of northern Tibet. Detrital AFT ages of the Mesozoic samples are younger than their depositional ages, indicating that the Mesozoic succession underwent two stages of exhumation after deep burial during 80–61 Ma and 54–47 Ma. Detrital AFT ages of the Cenozoic samples are older than their depositional ages and reveal that their source region experienced two periods of exhumation (peak ages in 86–59 Ma and 54–36 Ma). These results suggest that the northern Tibet successively experienced Late Cretaceous–early Paleocene and early–middle Eocene deformation. The Late Cretaceous–early Paleocene deformation implies that the extent of pre-collisional (India-Eurasia) deformation region in the plateau were much larger than previously known. The static detrital AFT peak ages (54–51 Ma) based on lag-time analysis for the Paleogene samples demonstrate the early Eocene deformation was a rapid and short-lived event, which was a far-field response to the India-Eurasia collision. Hence, we advocate synchronous deformation throughout the northern plateau at the collision time. Lag-time analysis results also demonstrate absence of Oligocene to early Miocene cooling ages and post-Eocene decreasing exhumation rates in the Cenozoic source regions. This suggests relatively quiescent tectonic settings in the Qilian Mountains during the Oligocene–early Miocene and agrees that the lateral strike-slip movement of the Altyn Tagh Fault was accommodated out of the plateau by the end of the early Miocene.

Dual Control of Depositional Facies on Uranium Mineralization in Coal‐bearing Series: Examples from the Tuanyushan Area of the Northern Qaidam Basin, NW China

AbstractThe uranium deposits in the Tuanyushan area of northern Qaidam Basin commonly occur in coal‐bearing series. To decipher the U‐enrichment mechanism and controlling factors in this area, a database of 72 drill cores, including 56 well‐logs and 3 sampling wells, was examined for sedimentology and geochemistry in relation to uranium concentrations. The results show that coal‐bearing series can influence uranium mineralization from two aspects, i.e., spatial distribution and dynamic control. Five types of uranium‐bearing rocks are recognized, mainly occurring in the braided river and braided delta sedimentary facies, among which sandstones near the coals are the most important. The lithological associations of sandstone‐type uranium deposits can be classified into three subtypes, termed as U‐coal type, coal‐U‐coal type, and coal‐U type, respectively. The coal and fine siliciclastic rocks in the coal‐bearing series confined the U‐rich fluid flow and uranium accumulation in the sandstone near them. Thus, the coal‐bearing series can provide good accommodations for uranium mineralization. Coals and organic matters in the coal‐bearing series may have served as reducing agents and absorbing barriers. Methane is deemed to be the main acidolysis hydrocarbon in the U‐bearing beds, which shows a positive correlation with U‐content in the sandstones in the coal‐bearing series. Additionally, the δ13C in the carbonate cements of the U‐bearing sandstones indicates that the organic matters, associated with the coal around the sandstones, were involved in the carbonation, one important component of alteration in the Tuanyushan area. Recognition of the dual control of coal‐bearing series on the uranium mineralization is significant for the development of coal circular economy, environmental protection during coal utilization and the security of national rare metal resources.

Chlorite chemical composition change in response to the Eocene-Oligocene climate transition on the northeastern Tibetan Plateau

The Eocene-Oligocene climate transition (EOCT) at ~34 Ma is a pronounced environmental reorganization that occurred in the Cenozoic era and has been extensively studied via marine archives. However, to date, there is still a less well-dated continental record across the EOCT, which can mostly be attributed to a lack of suitable proxies used in the varied continental settings in terms of sedimentary facies, provenance and climate, thus resulting in a poor understanding of the continental response to this transition. Located on the northeastern Tibetan Plateau, the Qaidam Basin is filled with continuous and well-exposed Cenozoic fluvio-lacustrine sediments. Cenozoic lacustrine sediment has been proven to be an ideal archive to study the continental climatic response to the EOCT. Here, we present the detailed mineralogical and elemental composition of chlorite, a widespread clay mineral in continental sediments formed by rock weathering, collected from a lacustrine sequence in the northern Qaidam Basin to constrain the onset of the EOCT and the related climatic response. The results show that chlorite abundance, crystallinity indices, intensity ratios of chlorite diagnostic peaks and chemical composition collectively exhibit remarkable changes at ~34.3 Ma. To be more specific, the well-crystallized Fe-Mg-rich chlorites were characterized by a high Fe content before the EOCT and turned into high-Mg-content Mg-chlorites with poor crystallinity. The chlorite chemical composition change is assumed to have been related to regional drying after the EOCT, thus providing new insights into the continental paleoenvironmental change based on this novel proxy.