Geological Journal最新文献

The Salina Cruz and Puerto Ángel beach areas in the Gulf of Tehuantepec, Mexican Pacific coast represent an important economic sector of the region. In this study, the mineralogy and geochemistry of bulk sediments, and geochronology of 400 detrital zircons recovered from the beach sediments were analysed to investigate their origin. The sediments are abundant in quartz, feldspar, ilmenite, cordierite, aragonite and anorthite. The chemical index of weathering revealed a moderate to intense weathering in the source area. The chondrite normalized REE patterns of bulk sediments are similar to the found in the Upper Continental Crust, suggesting the derivation of sediments from felsic igneous rocks. The REE patterns of zircons and the trace elemental ratios reveal a continental crust origin. Zircon U–Pb ages in the Salina Cruz beach were represented by Proterozoic (~545.1–1314.1 Ma; n = 170) and Cenozoic (~0.01–66 Ma; n = 20). The Puerto Ángel beach was abundant in Proterozoic zircon grains (~600.9–1171.4 Ma; n = 109) and followed by Mesozoic grains (~73.78–246.9 Ma; n = 40). The comparison of zircon U–Pb ages of this study with probable source rocks reveals that the Oaxaquia Terrane and Chiapas Massif Complex were the major contributors of Proterozoic zircons to the coastal areas. Similarly, the results indicate that the Cenozoic zircons were contributed by the Chiapas Massif Complex, coastal batholith and Cuicateco Terrane. The Mesozoic zircons are very few, derived from the nearby Xolapa Complex and the Chuacús Terrane.

We present significant findings of the Kausani Granite Gneiss within the Inner Lesser Himalayan Sedimentary Zone (iLHSZ) just north of the North Almora Thrust (NAT). The Kausani Granite Gneiss body lies within the quartzite of the Someshwar Formation and has a tectonized contact and a discordant relationship on the north side. Detailed seismic profiling across the body also confirms a similar result. Cathodoluminescence images of zircon from the Kausani body show no inheritance of older cores. The U–Pb ages from the zircon populations separated from the Kausani body give a crystallization age of 1866 ± 3 Ma. Along with the Upalda granite gneiss and Toneta granite gneiss near the Alakhnanda Thrust in the Garhwal Himalaya and the Dungeshwari granite gneiss near the Dailekh Thrust in Nepal, the Kausani Granite Gneiss north of NAT Kumaun Himalaya forms a major terrain boundary. These gneissic bodies mark the southernmost extent of felsic magmatism at NAT, rather than the Main Central Thrust. An about 1.8 billion years-old magmatic event in the LHSZ suggests that it is a currently active continental margin inside the ‘Greater India’ region, now situated in the Himalayan domain. However, the Pb-loss modelling of the U–Pb zircon data reveals thermal events during the Himalayan Orogeny (~45 Ma).

The hydrocarbon activity in Pengyang area, situated in the southwestern Ordos Basin, is notably prominent. Investigation on the migration laws of hydrocarbons is imperative for comprehending the involvement in uranium mineralization. Based on the analysis of spatial distribution of hydrocarbon containing fluid and hydrocarbon generation conditions of sandstone in the Luohe Formation, the organic geochemical characteristics including hydrocarbon components, carbon isotopes and biomarker compounds were analysed. The research results indicate that: (1) hydrocarbon fluid activities in the Luohe Formation are predominantly observed in layers exhibiting higher uranium mineralization. The mudstone of the Luohe Formation had low organic matter content and low thermal maturity, which was not conducive to hydrocarbon generation. (2) Hydrocarbon-containing fluid in the sandstone of Luohe Formation not only contained reducing gases such as methane and hydrogen but also chloroform asphalt components. The carbon isotopes of hydrocarbon in sandstone inform Luohe Formation resemble oil and gas in the Mesozoic. The biomarker parameter inferred that the parent rock of hydrocarbons in the Luohe Formation was formed under reducing and freshwater conditions, and hydrocarbon generation occurred at the mature stage. As above mentioned, a comparison was carried out between the affinity of hydrocarbon-containing fluid in the Luohe Formation and different layers of hydrocarbon source rocks. The migration behaviour of hydrocarbon-containing fluid in the Pengyang area has been summarized, and the involvement of hydrocarbon-containing fluid in uranium mineralization has been discussed. The main concepts are as follows: the sedimentary environment and thermal evolution conditions of hydrocarbons in the sandstone of Luohe Formation resemble those of the primary hydrocarbon source rocks in the Yanchang Formation. The main hydrocarbon charging events in the Luohe Formation occurred before the Late Cretaceous period, which is primarily related to two hydrocarbon generation events from 130 to 100 Ma in the Yanchang Formation and fault conduits connecting the Triassic to the Cretaceous Strata. The hydrocarbon-containing fluid released from Yanchang Formation migrating to the Luohe Formation provides reducing conditions for the precipitation of uranium in oxygen-bearing water bodies.

The Colatina Fracture Zone (CFZ) defines a distinct NNW–SSE-oriented linear zone of fractures and brittle faults that represents an inherited weak zone in the current crustal structure of the (Pre)Cambrian Araçuaí Orogen. In the Early Cretaceous, the CFZ was reactivated during rifting of West Gondwana and subsequent opening of the South Atlantic Ocean, as evidenced by the emplacement of dykes along its structural network and the development of major depocentres of the Campos Basin in the offshore segments of the CFZ. Previous thermochronological studies have demonstrated that the CFZ was also rejuvenated during the drift phase of the South Atlantic. However, a number of questions regarding differential surface uplift and basement exhumation between the CFZ and its surrounding areas, such as the Doce River Valley (DRV), are still unresolved. In this study, we aim to investigate the CFZ as a distinctive structure in the tectonic rejuvenation of the passive margin of south-east Brazil. Samples from the CFZ and the DRV were collected for apatite fission-track (AFT) analyses. In the DRV, samples yield AFT central ages from 87 to 97 Ma with mean track lengths (MTL) from 12.6 to 13.3 μm. In contrast, in the CFZ, AFT central ages from 70 to 83 Ma with MTL values from 13.2 and 13.4 μm are obtained. The correlation between AFT age and elevation suggests that the tectonic development of these regions was markedly different and uncoupled. The thermal history models from the AFT data further constrain this differential evolution. On the one hand, thermal history modelling for the DRV indicates a slower and protracted cooling since the incipient Atlantic rifting in the Early Cretaceous. On the other hand, the models for CFZ reveal a rapid cooling phase between the Late Cretaceous to the Palaeocene. In the DRV, the observed basement cooling was most probably triggered by erosion of the uplifted rift shoulder generated by Gondwana break-up. The more recent, Late Cretaceous–Palaeocene rock cooling, localized in the CFZ, was synchronous with a major phase of the Andean orogeny. This suggests that reactivations and erosional exhumation of the CFZ basement could be a consequence of far-field propagation of intraplate compressional stress. The higher susceptibility of the CFZ to reactivating over its surroundings shows that structural inheritance is a key factor in the differential tectonic evolution of passive margins. Further research on the Late Cretaceous–Palaeocene reactivation in the CFZ's offshore extension may be crucial for the exploitation of hydrocarbons in the Campos and Espírito Santos basins.

Mantle plumes related to Large Igneous Provinces have been linked to continental break-up and validated by the outpouring of mafic-ultramafic magmas that range from continental flood basalt magmatism to submarine plateau volcanism. This study presents a new set of geochemical and mineralogical data on mafic magmatic rocks from the Sylhet Trap of the Shillong Plateau, northeast India. The investigated mafic rocks (basalt and dolerite) are predominantly sub-alkaline-tholeiitic, composed of bytownite+labradorite and diopside+augite, with ophitic to sub-ophitic and glomeroporphyritic textures, the dark interstitial region of much finer grains consisting of opaque minerals and devitrified glass. The mafic rocks of Sylhet Trap show light rare earth elements enrichment with (La/Yb)_N ratio (1.92–2.86) and (La/Sm)_N ratio (1.11–1.40), an almost flat pattern of heavy rare earth elements along with mild europium anomalies (Eu/Eu*= 0.94–1.11). Trace element characteristics suggest their affinity towards enriched mid-oceanic ridge basalt and generated from low degree of partial melting of spinel source with minor involvement of crustal contamination. The similarity in geochemical characteristics of the investigated mafic rocks with the magmatism of Rajmahal Traps, eastern Peninsular India, Abor Volcanics, eastern Himalaya, along with Bunbury Basalt of western Australia and Cona Mafic exposed in southeastern Tibet, suggests their genetic linkage with mantle plume activities. Thus, we argue that the magmatic event of the Sylhet Trap is related to the Kerguelen mantle plume activity that played a significant role in the fragmentation of eastern Gondwana during the Lower Cretaceous period, giving rise to Greater India, Antarctica and northwest Australia.

Trillions of cubic meters of gas reserve have been found in the Sinian Dengying carbonate reservoirs with normal pressure in the central Sichuan Basin, while no industrial gas reservoir have been detected in the Sinian Dengying reservoir with normal pressure in the eastern Sichuan Basin. The pore fluid pressure of gas reservoir is usually closely related to total gas content. To investigate the pore fluid pressure evolution and its implication for gas reserve preservation in the Sinian Dengying reservoir of the central and eastern Sichuan Basin, we conducted a comprehensive analysis including fluid inclusion petrography, microthermometry and Raman spectroscopy. The timings of gas inclusions captured in the central and eastern Sichuan Basin occurred from 175 to 92 Ma and 191 to 183 Ma, respectively. The presence of two-phase vapour-solid bitumen inclusions with similar phase proportions in a single fluid inclusion assemblage of fluorite provides direct evidence of in situ oil cracking to gas. The widespread solid bitumen from the Sinian Dengying reservoir in the central Sichuan Basin indicates the existence of massive oil cracking, which results in the formation of overpressure in the reservoir. Pore fluid pressure evolution of the Sinian Dengying reservoir of the central Sichuan Basin experiences normal pressure stage (200–155 Ma), overpressure development stage (155–90 Ma) and overpressure release stage (90–0 Ma). The maximum pore fluid pressure and its corresponding pressure coefficient of the Sinian Dengying reservoir of the central Sichuan Basin are approximately 141.4 MPa and 1.95, respectively. The overpressure development stage reflects the processes of oil cracking and gas accumulation, and the overpressure release stage reflects the dissipation of some natural gas in the Sinian Dengying reservoir of the central Sichuan Basin. The pore fluid pressure of the Sinian Dengying reservoir in the eastern Sichuan Basin has maintained at normal pressure since 200 Ma, indicating that the gas reservoir was small during the oil cracking stage and natural gas completely leaked due to tectonic uplift.

The complex geological conditions in tunnels pose a huge challenge to the reliability of tunnel boring machine (TBM). However, existing reliability studies typically focus on core structures such as cutters and cutterheads, with less consideration given to the rest of the components that frequently fail. In this study, the reliability analysis and dynamic evaluation of TBM components with high failure rates are carried out relying on the Shanxi Central Yellow River Diversion Project. The life distribution and reliability variation characteristics of TBM components under different rock mass classes are investigated in terms of tunnelling time and tunnelling distance as two types of life data indexes. And the life index which is more suitable for the reliability evaluation of TBM components is identified by comparison. On this basis, a dynamic evaluation method for the reliability of TBM components under the condition of multi-classes surrounding rock is proposed. This method can quickly evaluate the current reliability of TBM components and serve as the basis for preventive maintenance. The results of this study play a certain role in supplementing the reliability research of TBM and also provide a scientific basis for optimizing the design and maintenance strategy of TBM components.

As more and more ancient sites are discovered around the world, protecting them in situ has become a challenge due to issues such as ground settlement and masonry wall leaks caused by groundwater fluctuation or rainfall. In this study, laboratory tests, borehole tests and field high-density resistivity detections are conducted to obtain information for numerical modelling, including design parameters. A complex three-dimensional hydrological–mechanical coupling model is then established to investigate ground settlement and wall deformation caused by groundwater fluctuation and rainfall. The seepage simulation results for the initial state are accurately verified by high-density resistivity imaging. Both measured data and numerical results indicate that changes in a single water head point mainly result in wall settlement. The pattern of wall deformation changes from settlement to lateral deformation with an increase in the drawdown rate of groundwater level. Furthermore, delayed rainfall and high-intensity rainfall can increase foundation settlement and wall deformation. Settlement deformation determines the upper limit of the global deformation when wall nodes are mainly affected. In contrast, if lateral spreading dominates wall deformation, it determines the lower limit of the global deformation. This study provides reference for in situ protection and foundation reinforcement of ancient sites.

Great progress has been made in marine shale gas of Wufeng–Longmaxi formations in the Sichuan Basin. However, shale gas exploration in the complex structural belt around the Sichuan Basin still faces great challenges. In this study, shales of Wufeng–Longmaxi formations collected from the northern Guizhou were taken as the studied target, organic matter (OM) characteristics, mineral composition, pore structure, methane adsorption capacity and in situ desorption gas content were measured, and the controlling factors of shale gas content were further discussed. The results indicated that the sedimentary facies of Wufeng–Longmaxi formations in north Guizhou varies from shallow-water shelf facies to deep-water shelf facies from south to north, and organic-rich shales are primarily distributed in Daozhen-Xishui areas, with a maximum thickness of about 80–100 m. Organic-rich shales are characterized by high total organic carbon (TOC) content, high thermal maturation and type I–II₁ kerogens, which can be comparable with those in commercially produced shale gas field in Sichuan Basin. High-quality shale gas reservoirs generally have a high content of brittle minerals, making them easier to be fractured. OM pores are the dominanted pore type in the studied shales, followed by intergranular pores associated with brittle minerals, dissolution pores within carbonate grains and microcracks, while clay mineral-related pores are poorly developed. The Wufeng–Longmaxi Formation shales generally have strong methane adsorption capacities, but these vary greatly across different areas. Shale gas adsorption capacity is primarily controlled by TOC content and thermal maturation level. Similarly, total gas content, including desorption gas and lost gas, varies greatly in different areas, and it is obviously lower than that in Fuling and Luzhou shale gas field, due to the loss of shale gas and low-pressure coefficient in the complex structural zone. It is worth explaining that shale gas is not always low in northern Guizhou, which is determined by burial depth and the distance of great fractures. Shale gas content is relatively high in LY1 well and DY1 well in Xishui-Daozhen area, and it is extremely low in TY1 well and AY1 well in Tongzi-Zheng'an area. Shale gas content in the same structural unit is primarily influenced by TOC content, OM pore development degree and water saturation. However, different structural units have different shale gas contents, due to the differences in preservation conditions. Shale reservoirs with good preservation conditions, that is, wide and gentle structure, far from a large fault and great burial depth, generally have high shale gas contents.