Acta Geologica Sinica

Abstract: Located in the middle segment of the Trans‐North China Orogen, the Fuping Complex is considered as a critical area in understanding the evolution history of the North China Craton (NCC). The complex is composed of various high‐grade and multiply deformed rocks, including gray gneiss, basic granulite, amphibolite, fine‐grained gneiss and marble, metamorphosed to upper amphibolite or granulite facies. It can be divided into four rock units: the Fuping TTG gneisses, Longquanguan augen gneisses, Wanzi supracrustals, and Nanying granitic gneisses. U‐Pb age and Hf isotope compositions of about 200 detrital zircons from the Wanzi supracrustals of the Fuping Complex have been analyzed. The data on metamorphic zircon rims give ages of 1.82‐1.84 Ga, corresponding to the final amalgamation event of the NCC, whereas the data for igneous zircon cores yield two age populations at ∼2.10 and ∼2.51 Ga, with some inherited ages scattering between 2.5 and 2.9 Ga. These results suggest that the Wanzi supracrustals were derived from the Fuping TTG gneisses (∼2.5 Ga) and the Nanying granitic gneisses (2.0–2.1 Ga) and deposited between 2.10 and 1.84 Ga. All zircons with ∼2.51 Ga age have positive initial ∍_Hf values from +1.4 to +10.9, suggesting an important crustal growth event at ∼2.5 Ga through the addition of juvenile materials from the mantle. The Hf isotope data for the detrital zircons further imply that the 2.8 Ga rocks are important components in the lower crust, which is consistent with a suggestion from Nd isotope data for the Eastern Block. The zircons of 2.10 Ga population have initial ∍_Hf values of −4.9 to +6.1, interpreted as mixing of crustal re‐melt with minor juvenile material contribution at 2.1 Ga. These results are distinct from that for the Western Block, supporting that the Fuping Complex was emplaced in a tectonic active environment at the western margin of the Eastern Block.

According to the adsorption‐desorption characteristics of coalbed gas and analysis of various experimental data, this paper proposes that the generation of secondary biogenic gas (SBG) and its mixing of with the residual thermogenic gas at an early stage inevitably lead to secondary changes of the thermogenic gas and various geochemical additive effects. Experimental results also show that the fractionation of the carbon isotope of methane of coal core desorption gas changes very little; the δ¹³C₁ value of the mixed gas of biogenic and thermogenic gases is between the δ¹³C₁ values of the two “original” gases, and the value is determined by the carbon isotopic compositions and mixing proportions of the two “original” methanes. Therefore this paper proposes that the study on the secondary changes of the thermogenic gas and various additive effects is a new effective way to study and identify SBG. Herein, a systematic example of research on the coalbed gas (Huainan coalbed gas) is further conducted, revealing a series of secondary changes and additive effects, the main characteristics and markers of which are: (1) the contents of CO₂ and heavy‐hydrocarbons decrease significantly; (2) the content of CH₄ increases and the gas becomes drier; (3) the δ¹³C and δD values of methane decrease significantly and tend to have biogenetic characteristics; and (4) the values of δ¹³C₂ and δ¹³Cco₂ grow higher. These isotopic values also change with the degradation degrees by microbes and mixing proportions of the two kinds of gases in different locations. There exists a negative correlation between the δ¹³C₁vsδ¹³Cco₂ values. The Δδ¹³Cc₂–c₁ values obviously become higher. The distributions of the Δδ¹³Cco₂–c₁ values are within certain limits and show regularity. There exist a positive correlation between the N₂ versus Ar contents, and a negative correlation between the N₂ versus CH₄ contents, indicating the down forward infiltration of the surface water containing air. These are important markers of the generation and existence of SBG.

Abstract Warm and humid climate and gentle hilly topography have provided favourable conditions for the development of the weathering crust of the granite intrusions in the Longnan area, Jiangxi Province. REE is mostly concentrated in an adsorption state in clay in the wholly weathered zone. The rare‐earth minerals enriched in the parent rocks provided the source material for the REE enrichment. Exchangeable REE accounts for 48‐86%. Extraction experiments and stable isotopic study of clay minerals suggest that the downward infiltration of meteoric water and increasing gradient of pH values have played an important role in the enrichment of REE during the progressive weathering. Slight fractionation of individual REE can not change their distribution patterns in the profiles, which are inherited from the parent rocks.

The Andaman Forearc Basin (AFB) is asymmetric in configuration and filled with a ∼6 km‐thick pile of Neogene to Recent sediments (∼4 s in two‐way travel time: TWT) with distinct zonation. It shows gradual thinning up to ∼3 km (0.8 s in TWT) towards the eastern end with a seabed gradient of 1:30. Thick deformed sediments ∼2 s (TWT) of the Outerarc are associated with intense faulting and occasional folding caused by recent tectonics. Development of a series of faults within the upwarped sedimentary column of Oligocene top to Recent is observed with a rotated fault block. These features are manifestations of Recent igneous intrusion, and reveal the presence of a mild N–S compressional regime. Its effect on the AFB resulted in further uplift of sediments, which can now be seen as the Invisible Bank. Forward gravity modelling supporting our seismic interpretation reveals that it is associated with igneous intrusion from the Moho (∼9 km depth), and also suggests that continental crust underlies the AFB. Strong Bottom Simulating Reflector (BSR)‐like features in the Miocene sediments of Outerarc and Forearc basin at a depth of 0.6 s below the seabed suggest the inferred probable occurrence of gas hydrates in the AFB.

Since the West Pingdingshan Section in Chaohu was proposed as the candidate of the Global Stratotype Section and Point of the Induan‐Olenekian boundary in 2003, the Lower Triassic of Chaohu has been extensively studied. Based on the studies on the Lower Triassic of Chaohu, (1) a continuous conodont zonation is established, which has become an important reference for Lower Triassic stratigraphic correlation over the world; (2) the First Appearance Datum of conodont Neospathodus waageni was suggested and has been basically accepted as the primary marker to define the Induan‐Olenekian boundary; (3) a characteristic Lower Triassic excursion of carbon isotopes was brought to light and has been proven to be not only an excellent index for the stratigraphic correlation but also a unique indication for the perturbation of ecological environments in the aftermath of the end‐Permian mass extinction; (4) a magnetostratigraphic sequence is constituted with a certain biostratigraphic control in the low‐latitude region and it presents an important correlation to the Boreal sequence; (5) a cyclostratigraphic study provides an alternative method to constrain the age of the chronostratigraphic units; and (6) a scheme of the Olenekian subdivision is recently suggested to define the boundary between the Smithian and Spathian Substages. In addition, Chaohu is also the type locality of the Chaohuan Stage, the upper stage of the Lower Triassic in the China Chronostratigraphic System. Thus, the Lower Triassic of Chaohu is not only a classic sequence in South China, but also a key reference sequence to the investigation of the corresponding stratigraphy and geological events over the world. The recent achievements are viewed here for an overall understanding of the sequence. Then the current situation of the Induan‐Olenekian and Smithian‐Spathian boundaries is discussed to provide a reference for later works.

Abstract: Studies of lithology, sedimentary facies and the distribution regularity of SiO₂ and Al₂O₃ contents and Al₂O₃/SiO₂ ratio allow us to divide the Upper Pleistocene‐Holocene Series represented by the Milanggouwan section in China's Salawusu River valley into six segments: MGS1, MGS2, MGS3, MGS4, MGS5 and MGS6. The boundary ages for MGS1 (the Dishaogouwan and Dagouwan Formations), MGS2 (the upper Chengchuan Formation), MGS3 (the middle Chengchuan Formation), MGS4 (the lower Chengchuan Formation), MGS5 (most strata of the Salawusu Formation) and MGS6 (the bottom of the Salawusu Formation and the top of the Lishi Formation) correspond to those of MIS1, MIS2, MIS3, MIS4, MIS5 and MIS6, respectively, from deep sea sediments or continental glaciers. MGS5 can be subdivided into five subsegments (MGS5a, MGS5b, MGS5c, MGS5d and MGS5e) and the boundary ages of these subsegments correspond to those of MIS5a, MIS5b, MIS5c, MIS5d and MIS5e, respectively. Based on the paleoenvironment and paleoecology indicated by the primary chemical elements, fossil vertebrates, mollusks and pollen grains, we hypothesize that MGS1, MGS2, MGS3, MGS4, MGS5 and MGS6 and the subsegments of MGS5 match the corresponding stages for oxygen isotopes in the deep sea sediments and continental glaciers, and the substages of MIS5 in terms of climatic characters, further explaining the phenomena that determined the formation of the late Quaternary strata and the paleontology of the Salawusu River valley. These phenomena relate to fluctuations in the global climate (and particularly in the East Asian monsoon) during the glacial and interglacial periods.

Shale gas is a resource of emerging importance in the energy field. Many countries in the world have been making big financial investments in this area. Carboniferous shale in the eastern Qaidam Basin shows good exploration prospects, but limited research and exploration work for shale oil and gas resources has been undertaken. Geochemical analyses were performed on shale derived from the Upper Carboniferous Hurleg Formation in the eastern Qaidam Basin, Qinghai Province, and secondary electron imaging capability of a Field Emission scanning electron microscope (FE‐SEM) was used to characterize the microstructure of the shale. The reservoir and exploitation potential of the studied shale was assessed by comparison with research results obtained from the Barnett Formation shale in Fort Worth Basin, North America and the Basin shale of Sichuan province. The results indicate that the eastern Qaidam Basin Carboniferous shale is high‐quality source rock. There are four major microstructural types in the study area: matrix intergranular pores, dissolution pores, intergranular pores, and micro‐fractures. The size of the micropores varies from 6–633 nm, the majority of which is between 39–200 nm, with a relatively small number of micro‐scale pores ranging from 0.13–1 μm. The pore characteristics of the studied shales are similar to the North American and Sichuanese shales, indicating that they have good reservoir potential. No micropores are present in the organic matter, which is induced by its composition; instead we found an important lamellar structure in the organic matter. These micropores and microfractures are abundant, and are connected to natural visible cracks that form the network pore system, which controls the storage and migration of shale gas. This connectivity is favorable for shale gas exploitation, providing great scientific potential and practical value.