2018年 29卷 第3期
Cambrian carbonates with abundant fossils of agnostoid trilobites deposited on the southern slope (Jiangnan slope belt) of the Yangtze Platform and in the Jiangnan deepwater basin are well exposed in the Wangcun Section of western Hunan, South China, and in the Duibian A Section of western Zhejiang, southeastern China, respectively. To better understand the response of carbon- isotope excursions to depositional environment changes, mass extinctions and eustatic events, we collected 530 carbonate samples in fresh roadcut exposures of the two measured sections for analysis of carbon and oxygen isotopic compositions. Data of δ13C from the Wangcun Section, western Hunan, South China, demonstrate that the Cambrian carbon-isotope profile includes three remarkable positive excursions CPEwc-1, 2, 3 in the Upper Series 2, in the Lower and in the Middle Furongian Series. Three distinctive negative excursions CNEwc-1, 2, 3 were separately tested in the Lower Terreneuvian Series, Lower Series 3 and in the Upper Furongian Series. Similarly, in the corresponding horizons in the Duibian A Section, Zhejiang Province, southeastern China, three positive excursions CPEdb-1, 2, 3 and three negative excursions CNEdb-1, 2, 3 also have been discovered. We interpret these significant carbon-isotope excursions as being associated with enhanced biogenic productivity, mass extinctions and eustatic events.
The Huichizi granite complex is the largest Paleozoic Ⅰ-type intrusion located in the North Qinling orogenic belt (NQB). In this study, we present systematic geochemical element data, zircon U-Pb ages, Lu-Hf isotopic data, and Sr-Nd isotopic data for the Huichizi granites. In terms of mineral and chemical compositions, these granites are biotite monzonitic and alkali-feldspar granites, both of which are characterized by high SiO2 and total alkali contents and low MgO, TiO2, and TFeO contents. These granites are weakly peraluminous (A/CNK values are 1-1.06 for biotite monzonitic granites and 1.04-1.09 for alkali-feldspar granites) and possess the geochemical characteristics of adakitic rocks, e.g., high Sr contents (319 ppm-633 ppm), Sr/Y ratios (18.5-174), and (La/Yb)N ratios (17.6-57) and low MgO (0.04 wt.%-0.83 wt.%), Y (3.0 ppm-17.2 ppm), and heavy rare-earth element (HREE) contents. This indicates that these rocks were most likely derived from the partial melting of a thickened lower crust. In situ zircon U-Pb dating of these granites yields Early Caledonian ages (437 Ma for biotite monzonitic granites and 424 Ma for alkali-feldspar granites), indicating that the Huichizi granitic complex is the product of multi-periodic magmatism. The positive but varying zircon εHf(t) values (+0.6 to +8.5) suggest that this thickened lower crust was mainly juvenile, i.e., accreted from depleted mantle during the Neo-Mesoproterozoic Period, but involved the ancient recycled crust. Biotite monzonitic granites formed during crust thickening at the extrusion stage, whereas the alkali granites formed during crust thickening at the extension stage (post extrusion). The Huichizi granite complex witnessed the process of extrusion to extension because of the collision between the NCB and the Qinling microcontinent in the Caledonian.
The Yangla Copper Ore Field in Northwest Yunnan Province, China, is a large region of deposits dominated by copper-bearing skarns whose origin remains debatable despite numerous studies over the past two decades. We have investigated the geological and geochemical characteristics of the skarns using field and microscopic observations combined with chemical analyses. The results show that the skarns fall into two categories. The first category is metamorphic skarn, which constitute the majority (> 90%) of skarns in the deposit and is characterized by stratiform occurrences conformable to Devonian host strata, anhydrous mineral assemblages such as diopside+hedenbergite+quartz, widespread banded structure, fine-grainsize (< 200 μm) and preserved tuff-like textures. Whole-rock major element compositions, REE and trace-element compositions resemble those of the country rock slates or schists. The skarn layers occur at variable distances (0-2 000 m) from Indosinian plutonic bodies. Fracture-filling veins and/or alteration halos are scarce or absent in or near the skarn layers. This category of skarn probably formed by isochemical contact metamorphism of fine calcareous clastic sediments or impure carbonate rocks during emplacement of the plutonic bodies with no significant material migration by hydrothermal fluids involved during the process, in which case metallic enrichment of the skarn layers was present in the protolith of the skarn. The second category is metasomatic skarn with relatively coarse-grained textures (200- > 1 000 μm) and volatile-reach assemblages such as diopside+tremolite+scapolite at or near the igneous contact zones, which constitutes only a minor proportion of ore compared with metamorphic skarn. Taking into consideration diverse existing opinions about the genetic type of the deposit, we suggest that the geological and whole-rock geochemical characteristics of the skarns are consistent with a metamorphosed and metasomatized SEDEX (sedimentary exhalative deposit) type rather than a hydrothermal metasomatic skarn type.
The Xiangshan mafic-ultramafic complex is one of the major Early Permian mafic- ultramafic intrusions in eastern Tianshan (Xinjiang, NW China), and consists of two major intrusive phases. The first intrusive phase is mainly gabbroic rocks hosting ilmenite mineralization, while the second intrusive phase is mainly lherzoilite associated with Ni-Cu sulfide mineralization. The Xiangshan ilmenite orebodies hosted in the Fe-Ti oxide-bearing gabbro occur along the contact between hornblende gabbros and leucogabbros. The hornblende gabbros and Fe-Ti oxide rich gabbros at Xiangshan are newly dated to be Early Permian (280.1 and 279.2 Ma, respectively). Major and trace element compositions of zircons and whole rocks from Xiangshan hornblende gabbro and Fe-Ti oxide gabbro have been measured by in situ excimer laser ablation ICP-MS. Zircon Ce4+/Ce3+ ratios based on lattice-strain model and Ti-in-zircon temperatures of hornblende gabbro and Fe-Ti oxide gabbro of the Xiangshan complex are calculated to evaluate the physicochemical variations during the ilmenite mineralization. Whole-rock geochemistry and zircon trace element geochemistry suggest that Fe-Ti oxide gabbros were formed from a basaltic parent magma which had undergone a transfromation from being H2O-rich to H2O-poor. During the magmatic evolution, primitive, H2O-poor basaltic melts may have been replenished into the system, increasing its solidus temperature and decreasing its oxygen fugacity and H2O contents. This may have supperessed the Ti-rich poikilitic hornblende fractionation and promoted the plagioclase fractionation, which consequently concentrated the ore-forming components in the residual melts and generated the ilmenite mineralization
The W-Sn-Mo-Cu-Pb-Zn polymetallic mineralization in the Huangshaping Deposit is associated with different types of Jurassic granitic intrusions such as quartz porphyry, granophyre and granite porphyry. In this study, we carried out detailed mineralogical, petrological, geochemical, zircon U-Pb geochronological and Hf isotopic studies on a newly found, Cu-mineralization-related fine- grained biotite syenogranite at Huangshaping. This syenogranite is mainly composed of quartz (40%), K-feldspar (30%), plagioclase (15%) and biotite (5%), enriching in SiO2 (75.58 wt.%-76.02 wt.%), K2O (5.24 wt.%-5.78 wt.%), Al2O3 (12.23 wt.%-12.46 wt.%), Rb (405 ppm-504 ppm), U (18.5 ppm-40.7 ppm), Th (47 ppm-52.3 ppm), Nb (49.6 ppm-57.5 ppm), and Cu (54 ppm-288 ppm) whereas depleting in P2O5 (0.01 wt.%), TiO2 (0.05 wt.%-0.06 wt.%), Ba (5 ppm-13.1 ppm), Sr (6.8 ppm-23.9 ppm) and Eu (0.04 ppm-0.05 ppm), with A1-type alkaline granite characteristics. Magmatic and hydrothermal zircons were separated from a Cu-mineralized syenogranite sample and yielded the 206Pb/238U concordant ages of 156.5±1.8 and 154.4±2.9 Ma, representing the intrusive age of the pluton and probable Cu-mineralization time at the Huangshaping Deposit, respectively. Zircon Hf isotopes suggest a crust-derived (partial melting of Paleoproterozoic basement) magma for the syenogranite, with some degree of crust-mantle interaction. We infer that the syenogranite is a transitional phase between the granophyre and the granite porphyry, not only contributed greatly to the Cu mineralization but also provided W-Sn polymetallic ore-forming metals for the Huangshaping Deposit.
Fatty acids were extracted from fluid inclusions, and analyzed for distribution and individual δ13C values to determine if there exists sulfate-reducing bacteria (SRB) and what substrates SRB depleted during U mineralization in the Dongsheng deposit. The 10-methylhexadecanoic acid (10Me16:0) has been detected from most of the samples using GC-MS based on relative retention time and co-injection of standard 10Me16:0 compound. Samples with higher 10Me16:0 concentrations, show higher U contents. The presence of 10Me16:0 along with iso-ω7-cis-heptadecenoic acid (i17:1ω7c) and ω7-cis- octadecenoic acid (18:1ω7c) in the fluid inclusions may indicate the occurrence of SRB during the U mineralization. Saturated fatty-acids and unsaturated fatty-acids detected in this study have individual δ13C values from -30.3‰ to -28.5‰ and -30.5‰ to -27.9‰, respectively. These values are close to those of n-alkanes of the associated oils and the bulk oils, but significantly distinct from methane, thus the oils are concluded to have been used as substrate for the microorganisms to survive on. This proposal is supported by oil biodegradation and ore-stage calcite cement with δ13C values from -1.4‰ to -17.2‰.
The Shuangwang gold deposit, located in the Fengxian-Taibai fore-arc basin in the western Qinling Orogen of Central China, has yielded over 70 tons of gold. It is an orogenic gold deposit occurring in an NW-trending breccia belt. Most of the ores are hydrothermal breccia type containing fragments of adjacent strata cemented by ankerite and pyrite. Pyrite is the most abundant metallic mineral and the major gold-bearing mineral in the ores. A total of 58 pyrite samples from main ore bodies of the Shuangwang gold deposit have been analysed for 44 trace elements by HR-ICP-MS. Sb, Ba, Cu, Pb, Zn, Bi, Mo, Co are selected as indicator elements to investigate the potential usefulness of trace elements in pyrite as an indicator in gold exploration. The results show that the supra-ore halo elements Sb and Ba, which may have been more active than other near-ore halo elements and sub-ore halo elements, are best to characterize the shape of ore bodies. Five target areas are pointed out for deep ore exploration based on a comprehensive study of supra-ore, near-ore and sub-ore halos. This study provides evidence that trace elements in pyrite can be used to depict the deep extension of ore bodies and to vector towards undiscovered ore bodies.
This study reports the sphalerite Rb-Sr age and LA-MC-ICP-MS in situ sulfur isotope analysis results of the Daliangzi Lead-Zinc Deposit in the Sichuan-Yunnan-Guizhou (SYG) triangle. Sphalerite Rb-Sr dating yields a Mississippian age of 345.2±3.6 Ma (MSWD=1.4), which is older than the published Late Triassic mineralization ages (230-200 Ma) of some other deposits. This indicates that at least two stages of lead-zinc mineralization have occurred in the SYG lead-zinc triangle. The first stage occurred in the Mississippian under an extensional environment, while the second stage occurred in the Late Triassic under a compressional environment. In situ sulfur isotope analysis of sphalerite growth zoning presents relatively large δ34S values of 11.3‰-15.2‰ with small variations. The large δ34S values indicate a reduced sulfur source of thermochemical reduction of seawater sulfates. Abundant organic matter in the black fracture zone possibly supplied reductants for thermochemical sulfate reduction (TSR) at the mineralization site. The small variation of δ34S values suggests a slow and stable TSR process that could prevent the sudden supersaturation of sphalerite in the fluid and the resulting of fast participation. This is consistent with the well-crystallized characteristic of the sphalerite of the Daliangzi Deposit.
Using well logs data only, the evaluation of shale gas hydrocarbon potential of Talhar Shale Member of Lower Goru Formation has been a challenge in Southern Lower Indus Basin in Pakistan. Well logs data analysis is helpful to evaluate the gas potential of source shale rocks. We introduced and applied empirical and graphical method to fulfil this task and derived geochemical parameters from well logs data. The method mentioned is cheap and fast. Talhar Shale has kerogen type III and type II which are montmorillonite clay and have potential to produce oil and gas. Talhar Shale has better sorption property. Empirical formulas are used to derive parameters, using well logs of porosity, density and uranium. Porosity and volume of kerogen, calculated from density log, give average values of 11.8% and 11.4%. Average value of level of maturity index(LMI) derived from log is 0.54, which indicates that it is at the early stage of maturity. Vitrinite reflectance is between 0.5%–0.55% as calculated by graphical method and empirical formula. Talhar Shale is at onset of oil generation, with main products of oil and gas. It is a good potential source in the study area.
Using well logs data only, the evaluation of shale gas hydrocarbon potential of Talhar Shale Member of Lower Goru Formation has been a challenge in Southern Lower Indus Basin in Pakistan. Well logs data analysis is helpful to evaluate the gas potential of source shale rocks. We introduced and applied empirical and graphical method to fulfil this task and derived geochemical parameters from well logs data. The method mentioned is cheap and fast. Talhar Shale has kerogen type Ⅲ and type Ⅱ which are montmorillonite clay and have potential to produce oil and gas. Talhar Shale has better sorption property. Empirical formulas are used to derive parameters, using well logs of porosity, density and uranium. Porosity and volume of kerogen, calculated from density log, give average values of 11.8% and 11.4%. Average value of level of maturity index (LMI) derived from log is 0.54, which indicates that it is at the early stage of maturity. Vitrinite reflectance is between 0.5%–0.55% as calculated by graphical method and empirical formula. Talhar Shale is at onset of oil generation, with main products of oil and gas. It is a good potential source in the study area.
The Bancheng Formation exposed along the Shiti Reservoir nearby Bancheng Town, Qinfang Basin, southern Guangxi, is mainly composed of thin-bedded chert. The radiolarian assemblages in the studied section suggest it was a pelagic setting and the age of the Bancheng Formation is Late Carboniferous to Early Permian. The detrital zircon U-Pb ages from the section are characterized by a Permian peak at ~282 Ma. Detrital zircon provenance analysis suggests that the Permian detrital zircons in the Bancheng Formation were likely from volcanic-magmatic arc rocks related to the subduction of the Paleo-Tethyan Ocean. The long-lived deposition (from Upper Devonian to Middle Permian, about 125 Ma) of the radiolarian cherts in the Qinfang Basin was comparable with that deposited in Ailaoshan Ocean. The radiolarian assemblages in Qinfang Basin show a Tethyan affinity. Together with the Permian subduction-related arc volcanic rocks and the E-MORB type basalts to the northwest of the study area, our data support the existence of a Permian arc-related basin in the Qinfang area.
Deep coalbed methane exists in high-temperature and high-pressure reservoirs. To elucidate the dynamic-change laws of the deep coal reservoir porosity and permeability characteristics in the process of coalbed methane production, based on three pieces of low- to medium-rank coal samples in the eastern Junggar Basin, Xinjiang, we analyse their mercury-injection pore structures. We measured the porosity and permeability of the coal samples at various temperatures and confining pressures by high-temperature and confining pressure testing. The results show that the porosity of a coal sample decreases exponentially with increasing effective stress. With increasing temperature, the initial porosity increases for two pieces of relatively low-rank coal samples. The increased rate of porosity decreases with increasing confining pressure. With increasing temperature, the initial porosity of a relatively high-rank coal sample decreases, and the rate of change of the porosity become faster. An exponential relationship exists between the porosity and permeability. With increasing coal rank, the initial porosity and permeability decrease. The change rate of the permeability decreases with increasing porosity.
Accommodation of continental convergence by crustal thickening and lateral transport is mainly featured as strike-slip faulting along the trends roughly orthogonal to the orientation of plate convergence. This style of faulting will affect seismicity of the involving areas which can be proved in low seismic zones by determining regional stress pattern using numerical methods. Accordingly, the stress distribution and deformation pattern of the South Sanandaj-Sirjan zone in the northeastern part of the Iranian-Arabian collision zone is investigated here using a three dimensional mechanical model. The modeled area is bounded between the Zagros thrust fault on the west and Dehshir-Baft fault in the east. The model is composed of three layers: the upper two layers represent the upper brittle and lower ductile crust of the collided continent and the lowest layer represents the lithospheric mantle. The upper crust behaves as an elastic material while the lower crust is considered as a non-Newtonian viscous fluid layer. The lithospheric mantle is taken as a low-viscosity material which is not allowed to move in any direction relative to the overlying layers. The Zagros thrust fault was treated with two different dip values saying 90 and 45 but Dehshir-Baft fault was modeled as a vertical fault and allowed to have a dextral movement regarding to the existing evidence. The driving mechanism applied to the western side of the model was chosen considering two different approaches including a kinematic approach (the Arabian-Eurasian convergence velocity; 35 mm/yr) and a dynamic approach (an external boundary force equal to 3.55E+17 N). The resulted stress field indicates an orogen-parallel component of right lateral shear along the Zagros fault implying a rotational deformation pattern within the modeled region that suggests a stress partitioning in the study area. The pattern also indicates a stress accumulation towards the south which could be a reason for the regional seismic quiescence between the two seismic Zagros thrust and Dehshir-Baft faults. Based on the present modeling results, it seems that high stress localization on the boundary faults can be a support of block structure approach or quasi-rigid blocks deformation within the study area. The resultant patterns of stress and displacement fields are generally totally comparable with plate boundary shear zones and have been proven by field data.
Four samples from a Permian reservoir in the Ordos Basin of North China were separated into twelve fractions in grain sizes of < 0.5, 0.5–1 and 1–2 μm. Using the 40Ar-39Ar step-heating method, all of the fractions essentially yielded plateau ages ranging from 172.5 to 217.1 Ma. These scattered plateau ages might not have been obtained from pure diagenetic illites but from mixed clay minerals, although the samples were disaggregated using a gentle freeze-thaw cycle to free them of non-clay minerals. A regional thermal event, as suggested by several proxies, led to intensive illitization as a distinct diagenetic process when the Yanshanian Movement triggered magmatism around the entire North China Block during the Jurassic to Cretaceous. Thermal illites formed during a short time period, whereas detrital illites came from various sources. The scattered plateau ages could have resulted from mixed degassing of thermal and detrital illites. Within one sample, the plateau ages decrease with the diminution of grain sizes, but it is difficult to extrapolate to the detrital-illite-free ages. Because the plateau age is a mixture of ages for thermal and detrital illites, this regression analysis studies the dependence of the plateau ages on the synthetic values of contents and ages of detrital illites instead of on the grain sizes. Comparing the samples to one another, the plateau ages show the same trend among the different grain sizes. Weighted by the contents and ages of detrital illites, linear regression analysis revealed the relationship between the plateau ages and the relative weight proportions. Based on iterated calculations, a thermal event age and a set of weight proportions were derived. The regressed thermal event age is 163.3±1.6 Ma, which coincides with regional thermal activities and links to gas accumulation.
China is one of the most important producers and suppliers of the chlorobenzenes (CBs), accounting for more than 50% of the world production. Although CBs contamination at the sites of chemical production factories is well-known, the knowledge on spatial distribution and contaminated degree of CBs are still limited. To better understand the occurrence of CBs, a total of 161 soil/sediment samples were collected from 36 sampling points within a 20 m deep from a site of decommissioned chlorobenzenes-producing factory in Wuhan City, Hubei Province, central China. The results showed that monochlorobenzene (MCB) is the predominant contaminants with the ΣCBs levels ranging from 0.05 to 8 600 mg/kg dry weight (dw) with a mean value of 384 mg/kg dw. The vertical distribution of CBs indicated that CBs may have clearly transported to deeper sections in the subsurface. Statistical analyses showed that there are no good correlations between the ΣCBs concentration with soil organic matter (SOM) content and the moisture content.
Dam construction for hydropower development is a very important subject all around the world, especially in developing countries due to energy crises. Filling of reservoir lakes may trigger or reactivate landslides in reservoir area. Active landslides in populated districts in the Xiangjiaba Reservoir area have become a striking problem for residents, local government and construction engineers. The key objective of this study is to analyze the role of reservoir filling and fluctuation in the activation/reactivation of the landslide as well as the mechanism of landslide from microstructures of pre-sheared slip surface. A large active landslide with a total volume of 1.25 Mm3 at the left margin of Jinsha River in Pingshan County of Sichuan Province, Southwest China, was selected as a case study. Field investigation, field monitoring and laboratory tests were carried out to find out the failure mechanism and the stability of the active landslide. The shear strength test and stability analysis confirm that the water level fluctuations have an adverse effect on slope stability. It is obvious from microstructure analysis that the clay minerals contribute to down-slope movement at micro-scale as well as presence of expansive minerals (e.g., montmorillonite) decreases the strength of soil due to water level rise.
Ocean bottom seismograph (OBS) is widely used in investigating deep crustal structure, which is characterized by a large amount of data information and abundant frequency components because of its multi-component acquisition. OBS is seldom used in deepwater oil and gas exploration and basin research due to the high cost. The complicated seismic wave field is caused by the complex seabed topography, basin and oil and gas structure in deepwater area, which increases the difficulty of image processing. In addition to reflection imaging, we utilize the multiple of OBS data to make accurate imaging and have achieved desirable results in a deep sea area in South China Sea in this paper. Firstly, the original P and Z components of OBS data are processed by wave field separation to obtain the upgoing wave filed and downgoing wave filed. Secondly, its image velocity filed is constructed. Finally, downgoing wave data is used to image (called mirror migration). Compared with conventional migration, the mirror migration can clearly image the seabed and provide better illumination for shallow layer below the seafloor in the case of sparse nodes, which is proved by the migration results of theoretical and real data in this paper.
Spectral analysis shows a low-frequency shadow under the BSR interface. Traditional low-frequency shadow analysis is based on stacked data. In order to understand the BSR low-frequency shadow more clearly, a frequency division analysis on stereoscopic observation seismic data based on the adaptive optimal-kernel (AOK) frequency analysis method is presented. It includes ocean-bottom seismometer (OBS) data (common receiver point data including vertical and horizontal components), vertical cable data (common receiver point data) and horizontal cable data (stacked section of different offsets). The OBS data frequency analysis gets a conclusion that vertical component has a significant effect on the low-frequency shadow, but the horizontal component did not. The vertical cable data shows that the low frequency band of vertical cable is wider than OBS. And then the horizontal cable data frequency analysis points out that the bigger the angle of incidence is, the more obvious the low-frequency shadow will be. The low-frequency shadow feature is shown in the stereoscopic observation field and the visual effect on common reception point data is better. The lateral reservoir distribution characteristics are predicted from low-frequency shadow feature analysis of the hydrate BSR based on stereoscopic observation.
Early-arrival waveform tomography (EWT) is one of the most promising techniques for building near-surface velocity model. Based on finite-frequency wave equation, EWT estimates velocities by matching calculated early-arrival waveforms with the observed ones. However, the objective function of EWT can easily converge to local minimum because of the cycle-skipping phenomenon. In order to reduce the cycle-skipping problem, a hybrid-domain early-arrival waveform tomography (HEWT) is proposed in this paper. The forward modeling of HEWT is realized in the time domain where early-arrival waveforms are easier to be selected from seismic data and less memory is needed than they are in the frequency domain. The inversion is implemented in the frequency domain where multi-scale strategy is more convenient to be realized than that in the time domain. Discrete Fourier transformation (DFT) is used to transform the time-domain wavefield to the frequency-domain wavefield. Test results show that HEWT is more competitive than EWT in both accuracy and computational time.
Bedding orientation measurement is a remote sensing application used in the field of geology, but the results are often unsatisfactory when the bed is dislocated along the strike direction. This paper proposes a new method called multiple-line method that is designed to improve bedding orientation measurement. This technique uses the bed as a vector, and fits the vectors as a plane to obtain the bedding orientation. This study employs the case of Yili Basin and measures 25 bedding orientations using the multiple-point and multiple-line methods, respectively. The result shows that the errors from the multiple-line method are mostly smaller than those from the multiple-point method when a bed is discontinuous.
The accurately determining the lake 14C reservoir age has a crucial significance for climatic reconstruction. In this study, the optically stimulated luminescence (OSL) dating method is employed to date samples from highstand lacustrine sediments, palaeoshoreline, fluvial terrace, and the alluvial fan of the Heihai Lake catchment. Accelerator mass spectrometry (AMS) 14C dating was also used to date fossil plants from highstand lacustrine sediments. Based on the calculations of linear regression with OSL against radiocarbon ages for same layers of two sections, the quantitative 14C reservoir ages were estimated to lie between 3 353 and 3 464 yr during the 1.8 to 2.4 ka, which showed temporal variation. The sources of old carbon are the dissolution of carbonate bedrocks distributed along the Kunlun Mountain. The OSL ages of the different members of the hydatogen sedimentary system at Heihai Lake catchment indicate that a stronger hydrologic condition occurred from 3.0±0.2 to 1.8±0.2 ka, with a maximum lake level of 9 m higher than present. This humid stage was widely recorded in different sediments on the QTP and Chinese Loess Plateau (CLP), indicating its broad synchronicity across the Asian Summer Monsoon region. The enhanced East Asian Summer Monsoon (EASM) and the Indian Summer Monsoon (ISM) resulted in the increase of moisture availability for the Heihai Lake area during this stage.
New high-resolution 3D seismic data image a submarine channel complex in the northern slope of the South China Sea. The channel complex stretches hundreds of kilometers across the slope and flows into the deepsea from the siliciclastic shelf margin, linking neritic environment to the pelagic plain. The evolution of the channel complex developed two sedimentary stages, stage Ⅰ (19.1–18.5 Ma) and stage Ⅱ (18.5–17.5 Ma), separated by erosional surfaces. In the first stage, the complex was filled with pure siliciclastic sediments, forming thick-massive sandstone intercalated by thin layers of mudstone. During the stage Ⅱ, the channel complex was deposited five carbonate-siliciclastic cycles. The unexpected channel-fill carbonate deposits present allochthonous characteristics, suggesting the siliciclastic channel was surprisingly used to transport carbonate sediment from the adjacent neritic carbonate platform. By analyzing the periodical carbonate sedimentary process in the siliciclastic channel complex, we infer that it was related to the in situ carbonate production of the neritic carbonate platform and was most likely to be controlled by the relative sea-level changes. Unlike line-source carbonate slope aprons or small-sized carbonate channels, the large-sized siliciclastic channel complex links directly neritic carbonate platform to deepwater basin and can transport large volumes of neritic carbonates to the pelagic environment in a short period. The new findings help to estimate the contributions of neritic siliciclastic shelf and carbonate platform to deepwater slope more accurately. This study suggests that channel systems are more complex than expected and have significant implications on the conceptual models describing the deepwater sedimentary theory.