Current Articles
2024, Volume 42, Issue 6
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2024,
42(6):
1849-1875.
doi: 10.14027/j.issn.1000-0550.2024.101
Abstract:
Objective Amalgamation and fragmentation cycles of the global supercontinents from the Proterozoic to Early Paleozoic are profoundly related to the geological evolution and distribution of massive economic resources in South China Block. Methods In this study, we examined the tectonic framework of the South China Block and related processes of geodynamic evolution and their impact on resource formation and environmental development in the context of assembly and breakup of the global supercontinents. [Results and Conclusions] The Yangtze Block of South China experienced similar tectonic events to the Laurentia breakup during the Early Paleoproterozoic, implying their close affinity before the Columbia （Nuna） assembly. Although the impact of tectonic evolution exists in different parts, the Yangtze Block recorded the assembly and breakup of the Columbia supercontinent, whereas the Cathyasia Block only shows the records of subduction-collision related to the supercontinental assembly. Before ca.820 Ma, a regional unconformity developed along the southeastern margin of the Yangtze Block, and the onset of sedimentary overlap represented a new cycle of basin evolution since the breakup of the Rodinia supercontinent. Subsequently, the South China Block experienced five evolutionary periods, represented from old to young by （a） the embryonic-to-young rift （820⁃800 Ma）, （b） mature rift （800⁃720 Ma）, （c） early glacial （720⁃660 Ma）, （d） interglacial （660⁃650 Ma）, and （e） late glacial periods （650⁃635 Ma）. The South China Block experienced an Early Paleozoic orogen that was closely associated with the Gondwana assembly and was close to Australia or India. Critical geological events in South China such as the Neoproterozoic Snowball Earth are considered tightly linked to the Rodinia breakup, whereas Precambrian giant economic deposits of Cu, Fe, and Mn were largely controlled by the magmatic-sedimentary events. Furthermore, successive uplift-subsidence processes promoted the formation and evolution of Paleozoic sedimentary basins with high-quality source rocks, large-scale reservoirs, and seals, which are critical for the development of hydrocarbon resources.
Objective Amalgamation and fragmentation cycles of the global supercontinents from the Proterozoic to Early Paleozoic are profoundly related to the geological evolution and distribution of massive economic resources in South China Block. Methods In this study, we examined the tectonic framework of the South China Block and related processes of geodynamic evolution and their impact on resource formation and environmental development in the context of assembly and breakup of the global supercontinents. [Results and Conclusions] The Yangtze Block of South China experienced similar tectonic events to the Laurentia breakup during the Early Paleoproterozoic, implying their close affinity before the Columbia （Nuna） assembly. Although the impact of tectonic evolution exists in different parts, the Yangtze Block recorded the assembly and breakup of the Columbia supercontinent, whereas the Cathyasia Block only shows the records of subduction-collision related to the supercontinental assembly. Before ca.820 Ma, a regional unconformity developed along the southeastern margin of the Yangtze Block, and the onset of sedimentary overlap represented a new cycle of basin evolution since the breakup of the Rodinia supercontinent. Subsequently, the South China Block experienced five evolutionary periods, represented from old to young by （a） the embryonic-to-young rift （820⁃800 Ma）, （b） mature rift （800⁃720 Ma）, （c） early glacial （720⁃660 Ma）, （d） interglacial （660⁃650 Ma）, and （e） late glacial periods （650⁃635 Ma）. The South China Block experienced an Early Paleozoic orogen that was closely associated with the Gondwana assembly and was close to Australia or India. Critical geological events in South China such as the Neoproterozoic Snowball Earth are considered tightly linked to the Rodinia breakup, whereas Precambrian giant economic deposits of Cu, Fe, and Mn were largely controlled by the magmatic-sedimentary events. Furthermore, successive uplift-subsidence processes promoted the formation and evolution of Paleozoic sedimentary basins with high-quality source rocks, large-scale reservoirs, and seals, which are critical for the development of hydrocarbon resources.
2024,
42(6):
1876-1889.
doi: 10.14027/j.issn.1000-0550.2024.089
Abstract:
Objective The Neoproterozoic Era marks a crucial period for continental generation, reworking, and reshaping of the South China Block （including the Yangtze Block to the northwest and Cathaysia Block to the southeast）. However, Neoproterozoic rocks are widely developed in the block and constitute the leading basement sequences of Phanerozoic rocks and ore deposits. Studying the formation of Neoproterozoic rocks and crustal construction processes of the South China Block is important for exploring regional resources and environment impacts, the assembly and dispersal behaviors of Neoproterozoic supercontinents and the subsequent “Cambrian explosion.” Methods Here, we reviewed key geological records in the South China Block during the Early-to-Middle Neoproterozoic and suggested the existence of a multi-terrane （or multi-microcontinent） structure in South China and nearby regions during the Neoproterozoic. [ Results and Conclusions] This multi-terrane structure may have paved the way for a long-lived ocean-continent interaction in Neoproterozoic South China, as well as consequent arc magmatism, subduction-zone metamorphism, and terrane accretion. Furthermore, the terrane/microcontinent boundaries could be roughly defined by a series active- and passive-continent-margin rock units. Specifically, we speculated the multi-terrane structure was established in the Late Mesoproterozoic and persisted during the Early-to-Middle Neoproterozoic. This conclusion is based on the occurrence of a series of Late Mesoproterozoic to Neoproterozoic oceanic lithosphere relics, including island arc rocks （e.g., Dahongshan, Tongmuliang, Changba, Shuangxiwu arcs） and ophiolites （e.g., Shimian, Miaowan, Huashan, Sanchazi and northeastern Jiangxi ophiolites）. Late Mesoproterozoic continent passive-margin sedimentary rocks （e.g., Shennongjia, Macaoyuan, Kunyang, and Huili Group） that are extensively preserved in the South China Block may correspond to a successful continental rift and support a multi-terrane structure. In the Early Neoproterozoic, we suggest passive continental margins transitioned active ones, and extensive convergent processes operated within the South China Block and its nearby regions. This conclusion is supported： （1） linearly-distributed continental-arc calc-alkaline magmatism along western and northern Yangtze margins （i.e., the Panxi-Hannan Belt）； （2） rapid endogenic recycling processes induced by ocean-continent subduction recorded in the Huangshuihe migmatites； （3） a range of 880-750 Ma subduction-zone metamorphism reported in the Panxi-Hannan Belt and South Qinling； （4） Neoproterozoic extrusion-related deformation documented within the basement sequences of the Jiangnan orogen, southeastern Yangtze margins； （5） numerous accretionary rock complexes from the Neoproterozoic that witnessed accretion of oceanic sediments and arcs, ophiolites, outboard terrane （e.g., eastern Jiangnan orogen, Dahongshan Group and Huangshuihe Group, Changba complex）. In this regard, we propose the South China Block experienced a prolonged Neoproterozoic accretionary orogeny and consequent terrane/microcontinent amalgamation, accompanied by possible outward migration and reorganization of active continental margins. These Neoproterozoic subduction-accretion processes played an important role in shaping the current South China Block. More importantly, these Neoproterozoic terrane-boundary domains could be major sites for Phanerozoic crustal differentiation and element recycling that led to the formation of a range of ore deposits within the South China Block. For example, W-Sn and rare earth ores in southern Hunan and southern Jiangxi provinces situate in boundary domains between Nanling-Yunkai and Wuyi terranes. Additionally, W and Cu-Au ores in northeastern Jiangxi province are located at boundary domains between Huaiyu terrane and southeastern Yangtze margin. This potential connection between Neoproterozoic basement and Phanerozoic ore-forming processes requires further investigation in the future.
Objective The Neoproterozoic Era marks a crucial period for continental generation, reworking, and reshaping of the South China Block （including the Yangtze Block to the northwest and Cathaysia Block to the southeast）. However, Neoproterozoic rocks are widely developed in the block and constitute the leading basement sequences of Phanerozoic rocks and ore deposits. Studying the formation of Neoproterozoic rocks and crustal construction processes of the South China Block is important for exploring regional resources and environment impacts, the assembly and dispersal behaviors of Neoproterozoic supercontinents and the subsequent “Cambrian explosion.” Methods Here, we reviewed key geological records in the South China Block during the Early-to-Middle Neoproterozoic and suggested the existence of a multi-terrane （or multi-microcontinent） structure in South China and nearby regions during the Neoproterozoic. [ Results and Conclusions] This multi-terrane structure may have paved the way for a long-lived ocean-continent interaction in Neoproterozoic South China, as well as consequent arc magmatism, subduction-zone metamorphism, and terrane accretion. Furthermore, the terrane/microcontinent boundaries could be roughly defined by a series active- and passive-continent-margin rock units. Specifically, we speculated the multi-terrane structure was established in the Late Mesoproterozoic and persisted during the Early-to-Middle Neoproterozoic. This conclusion is based on the occurrence of a series of Late Mesoproterozoic to Neoproterozoic oceanic lithosphere relics, including island arc rocks （e.g., Dahongshan, Tongmuliang, Changba, Shuangxiwu arcs） and ophiolites （e.g., Shimian, Miaowan, Huashan, Sanchazi and northeastern Jiangxi ophiolites）. Late Mesoproterozoic continent passive-margin sedimentary rocks （e.g., Shennongjia, Macaoyuan, Kunyang, and Huili Group） that are extensively preserved in the South China Block may correspond to a successful continental rift and support a multi-terrane structure. In the Early Neoproterozoic, we suggest passive continental margins transitioned active ones, and extensive convergent processes operated within the South China Block and its nearby regions. This conclusion is supported： （1） linearly-distributed continental-arc calc-alkaline magmatism along western and northern Yangtze margins （i.e., the Panxi-Hannan Belt）； （2） rapid endogenic recycling processes induced by ocean-continent subduction recorded in the Huangshuihe migmatites； （3） a range of 880-750 Ma subduction-zone metamorphism reported in the Panxi-Hannan Belt and South Qinling； （4） Neoproterozoic extrusion-related deformation documented within the basement sequences of the Jiangnan orogen, southeastern Yangtze margins； （5） numerous accretionary rock complexes from the Neoproterozoic that witnessed accretion of oceanic sediments and arcs, ophiolites, outboard terrane （e.g., eastern Jiangnan orogen, Dahongshan Group and Huangshuihe Group, Changba complex）. In this regard, we propose the South China Block experienced a prolonged Neoproterozoic accretionary orogeny and consequent terrane/microcontinent amalgamation, accompanied by possible outward migration and reorganization of active continental margins. These Neoproterozoic subduction-accretion processes played an important role in shaping the current South China Block. More importantly, these Neoproterozoic terrane-boundary domains could be major sites for Phanerozoic crustal differentiation and element recycling that led to the formation of a range of ore deposits within the South China Block. For example, W-Sn and rare earth ores in southern Hunan and southern Jiangxi provinces situate in boundary domains between Nanling-Yunkai and Wuyi terranes. Additionally, W and Cu-Au ores in northeastern Jiangxi province are located at boundary domains between Huaiyu terrane and southeastern Yangtze margin. This potential connection between Neoproterozoic basement and Phanerozoic ore-forming processes requires further investigation in the future.
2024,
42(6):
1890-1902.
doi: DOI:10.14027/j.issn.1000-0550.2024.111
Abstract:
Objective An extensive suite of Neoproterozoic diabase dykes is exposed in the Jinkouhe area of western Sichuan province, offering crucial insights into the tectonic evolution of the western Yangtze Craton. These diabases bear key implications for understanding the early tectono-magmatic history of the Yangtze Craton and its relationship with the assembly and fragmentation of the Rodinia supercontinent. Methods Sensitive High-Resolution Ion MicroProbe（SHRIMP） zircon U-Pb geochronology and whole-rock geochemistry analyses were performed on the Jinkouhe diabases. Results SHRIMP zircon U-Pb dating of the diabase samples yields an age of 823±10 Ma （MSWD=1.30, n=10）. The diabases are classified as a tholeiitic basalt series, displaying Nb-Ta depletion and negative Eu anomalies. Their geochemical signatures are indicative of both island arc and intraplate basaltic affinities. Conclusions The Jinkouhe diabases are interpreted to have originated from partial melting of a spinel-garnet lherzolite mantle source, which was metasomatized by subduction-related melts. Combined with regional geological data, these diabases were most likely formed within a post-collisional extensional setting, reflecting tectonic processes associated with the final amalgamation of the Yangtze Craton within the context of the Rodinia supercontinent assembly.
Objective An extensive suite of Neoproterozoic diabase dykes is exposed in the Jinkouhe area of western Sichuan province, offering crucial insights into the tectonic evolution of the western Yangtze Craton. These diabases bear key implications for understanding the early tectono-magmatic history of the Yangtze Craton and its relationship with the assembly and fragmentation of the Rodinia supercontinent. Methods Sensitive High-Resolution Ion MicroProbe（SHRIMP） zircon U-Pb geochronology and whole-rock geochemistry analyses were performed on the Jinkouhe diabases. Results SHRIMP zircon U-Pb dating of the diabase samples yields an age of 823±10 Ma （MSWD=1.30, n=10）. The diabases are classified as a tholeiitic basalt series, displaying Nb-Ta depletion and negative Eu anomalies. Their geochemical signatures are indicative of both island arc and intraplate basaltic affinities. Conclusions The Jinkouhe diabases are interpreted to have originated from partial melting of a spinel-garnet lherzolite mantle source, which was metasomatized by subduction-related melts. Combined with regional geological data, these diabases were most likely formed within a post-collisional extensional setting, reflecting tectonic processes associated with the final amalgamation of the Yangtze Craton within the context of the Rodinia supercontinent assembly.
2024,
42(6):
1903-1917.
doi: 10.14027/j.issn.1000-0550.2023.122
Abstract:
Objective The southeastern part of South China underwent the strong Kwangsian orogeny during the Early Paleozoic. However, there is still considerable controversy surrounding its nature. To understand the geodynamic mechanism of the Early Paleozoic Kwangsian orogeny in South China, detrital multi-mineral U-Pb geochronology was used to reconstruct the paleogeographic position of the Yangtze and Cathaysia Blocks in the Early Paleozoic and restore the relative displacement of the terranes. Methods U-Pb geochronology and hafnium isotopic data of detrital zircon and detrital monazite from Early Paleozoic strata of South China were used to trace the potential sources. Results （1） Detrital zircons from the Cambrian to Ordovician strata in western Yangtze yield predominantly age peaks at 850-750 Ma and 550-500 Ma, with minor age peaks at 1 000-900 Ma, 1 900-1 800 Ma, and 2 550-2 400 Ma. Combined with the regional lithofacies evolution, provenance analysis demonstrates that the abundant 550-500 Ma detrital zircons with positive εHf（t） values reflecting the significant involvement of juvenile crustal materials were likely derived from the Cadomian Arc Belt along the Iran-Turkey margin. The Neoproterozoic detrital zircons preserved in the Cambrian to Ordovician strata of the western margin of the Yangtze Block were derived from the Neoproterozoic igneous rock in the western Yangtze Block and the recycled materials from the underlying Neoproterozoic strata in western Yangtze. Provenance analysis reveals that the Yangtze Block and Iran have a close provenance linkage； （2） U-Pb ages of detrital zircon and monazite from the Cambrian to Ordovician strata in the Cathaysia Block exhibit predominant age peaks at 1 000-900 and 550-500 Ma. The former aligns with the age of the magmatic and metamorphic zircon/monazite in the East Ghats belt of NE India and Rayner belt in East Antarctic, whereas the latter corresponds to the U-Pb ages of the zircon and monazite from the Kuunga orogen in NE India； （3） In the age spectra of the Silurian succession of southeastern Yangtze, peaks older than 440 Ma are identical to those of the pre-Silurian strata, indicating that the provenance of detritus of those ages was recycled materials from the underlying Cambrian to Ordovician strata. The age peak of 460-410 Ma coincides with the timing of the massive 440-400 Ma syn-orogenic granites in Cathaysia Block. Conclusions The result of provenance analysis favors an oblique distribution of the Yangtze and Cathaysia Blocks along the northeastern margin of East Gondwana. In particular, the Yangtze Block was situated in northeast Iran, whereas Cathaysia occupied the northeastern margin of India during the Cambrian to Ordovician. It is crucial to note that the Kwangsian orogeny is an oblique strike-slip convergence triggered by the final assembly of the supercontinent Gondwana during the Late Neoproterozoic to Cambrian. The oblique strike-slip convergence of the Yangtze and Cathaysia Blocks after the Late Ordovician （~460 Ma） resulted in the formation of the Wuyi-Yunkai orogenic belt （460-420 Ma）.
Objective The southeastern part of South China underwent the strong Kwangsian orogeny during the Early Paleozoic. However, there is still considerable controversy surrounding its nature. To understand the geodynamic mechanism of the Early Paleozoic Kwangsian orogeny in South China, detrital multi-mineral U-Pb geochronology was used to reconstruct the paleogeographic position of the Yangtze and Cathaysia Blocks in the Early Paleozoic and restore the relative displacement of the terranes. Methods U-Pb geochronology and hafnium isotopic data of detrital zircon and detrital monazite from Early Paleozoic strata of South China were used to trace the potential sources. Results （1） Detrital zircons from the Cambrian to Ordovician strata in western Yangtze yield predominantly age peaks at 850-750 Ma and 550-500 Ma, with minor age peaks at 1 000-900 Ma, 1 900-1 800 Ma, and 2 550-2 400 Ma. Combined with the regional lithofacies evolution, provenance analysis demonstrates that the abundant 550-500 Ma detrital zircons with positive εHf（t） values reflecting the significant involvement of juvenile crustal materials were likely derived from the Cadomian Arc Belt along the Iran-Turkey margin. The Neoproterozoic detrital zircons preserved in the Cambrian to Ordovician strata of the western margin of the Yangtze Block were derived from the Neoproterozoic igneous rock in the western Yangtze Block and the recycled materials from the underlying Neoproterozoic strata in western Yangtze. Provenance analysis reveals that the Yangtze Block and Iran have a close provenance linkage； （2） U-Pb ages of detrital zircon and monazite from the Cambrian to Ordovician strata in the Cathaysia Block exhibit predominant age peaks at 1 000-900 and 550-500 Ma. The former aligns with the age of the magmatic and metamorphic zircon/monazite in the East Ghats belt of NE India and Rayner belt in East Antarctic, whereas the latter corresponds to the U-Pb ages of the zircon and monazite from the Kuunga orogen in NE India； （3） In the age spectra of the Silurian succession of southeastern Yangtze, peaks older than 440 Ma are identical to those of the pre-Silurian strata, indicating that the provenance of detritus of those ages was recycled materials from the underlying Cambrian to Ordovician strata. The age peak of 460-410 Ma coincides with the timing of the massive 440-400 Ma syn-orogenic granites in Cathaysia Block. Conclusions The result of provenance analysis favors an oblique distribution of the Yangtze and Cathaysia Blocks along the northeastern margin of East Gondwana. In particular, the Yangtze Block was situated in northeast Iran, whereas Cathaysia occupied the northeastern margin of India during the Cambrian to Ordovician. It is crucial to note that the Kwangsian orogeny is an oblique strike-slip convergence triggered by the final assembly of the supercontinent Gondwana during the Late Neoproterozoic to Cambrian. The oblique strike-slip convergence of the Yangtze and Cathaysia Blocks after the Late Ordovician （~460 Ma） resulted in the formation of the Wuyi-Yunkai orogenic belt （460-420 Ma）.
2024,
42(6):
1918-1935.
doi: 10.14027/j.issn.1000-0550.2024.094
Abstract:
Objectives Following the Early Paleozoic Wuyi-Yunkai orogeny, eastern South China experienced a tectonically quiescent period in the Late Paleozoic, with large-scale marine transgression and thick sediment deposition. Specifically, the nature of the strata in the Lower Devonian to the Kungurian stage of the Early Permian, such as isopach changes and lateral variation of lithofacies, are important for understanding the post-orogenic topography of eastern South China. Methods Stratigraphic thicknesses and lithological data for the region were collected and strata isopach maps and lithofacies maps were compiled for six main epochs/ages： the Early, Middle and Late Devonian, the Early Carboniferous, the Late Carboniferous to the Sakmarian age of the Early Permian, and the Artinskian age to Kungurian age of the Early Permian. The tectonic subsidence rate of the basin at five different localities was calculated for each epoch/age. Results The subsidence of the Late Paleozoic basin in eastern South China is divided into two stages： （i） a high subsidence rate （about 25 m/Ma） in the Devonian； and （ii） a low subsidence rate （about 10 m/Ma） in the Carboniferous to Kungurian age of the Early Permian. Conclusions Taking the regional widely-distributed and large-scale normal faults into consideration, it is speculated that the first stage of basin subsidence was mainly influenced by tectonic activity inherited from the post-Wuyi-Yunkai orogenic collapse and possibly the separation of South China from Gondwana during the Devonian. The second stage was controlled by the icehouse climate together with the sediment load in the basin during the Carboniferous to Early Permian.
Objectives Following the Early Paleozoic Wuyi-Yunkai orogeny, eastern South China experienced a tectonically quiescent period in the Late Paleozoic, with large-scale marine transgression and thick sediment deposition. Specifically, the nature of the strata in the Lower Devonian to the Kungurian stage of the Early Permian, such as isopach changes and lateral variation of lithofacies, are important for understanding the post-orogenic topography of eastern South China. Methods Stratigraphic thicknesses and lithological data for the region were collected and strata isopach maps and lithofacies maps were compiled for six main epochs/ages： the Early, Middle and Late Devonian, the Early Carboniferous, the Late Carboniferous to the Sakmarian age of the Early Permian, and the Artinskian age to Kungurian age of the Early Permian. The tectonic subsidence rate of the basin at five different localities was calculated for each epoch/age. Results The subsidence of the Late Paleozoic basin in eastern South China is divided into two stages： （i） a high subsidence rate （about 25 m/Ma） in the Devonian； and （ii） a low subsidence rate （about 10 m/Ma） in the Carboniferous to Kungurian age of the Early Permian. Conclusions Taking the regional widely-distributed and large-scale normal faults into consideration, it is speculated that the first stage of basin subsidence was mainly influenced by tectonic activity inherited from the post-Wuyi-Yunkai orogenic collapse and possibly the separation of South China from Gondwana during the Devonian. The second stage was controlled by the icehouse climate together with the sediment load in the basin during the Carboniferous to Early Permian.
2024,
42(6):
1936-1947.
doi: 10.14027/j.issn.1000-0550.2024.072
Abstract:
Objective The Ebian Group in the western Yangtze Block has traditionally been regarded as a representative lithostratigraphic unit of the Late Mesoproterozoic in South China. The sedimentation age and provenance characteristics of the Ebian Group can provide important constraints for reconstructing the Meso- Neoproterozoic tectonic evolution of the Yangtze Block under the background of the assembly of the global Rodinia supercontinent. Methods In this study, petrological and detrital zircon U-Pb geochronological analyses were conducted on metasedimentary rocks of the Ebian Group. Results The detrital zircon ages of two metamorphic fine-grained litharenite sandstones from the Jiadanqiao Formation of the Ebian Group range from 1 950 Ma to 900 Ma. The main relative probability peaks are ca. 910, 1 570, and 1 730 Ma, with secondary peaks at ca. 1 130, 1 390, 1 840, and 2 500 Ma. The maximum depositional ages are 908±8 Ma （MSWD=0.10, n=5） and 905±5 Ma （MSWD=0.52, n=9）, respectively. Conclusions Combined with regional geological data, the depositional age of the Jiadanqiao Formation of the Ebian Group is constrained as 910⁃860 Ma. The sediment source was magmatic rocks in the western Yangtze Block, with a predominant direction towards the south. Thus, formation likely occurred during the Early Neoproterozoic micro-block assembly and can be associated with the global assembly of the Rodinia supercontinent.
Objective The Ebian Group in the western Yangtze Block has traditionally been regarded as a representative lithostratigraphic unit of the Late Mesoproterozoic in South China. The sedimentation age and provenance characteristics of the Ebian Group can provide important constraints for reconstructing the Meso- Neoproterozoic tectonic evolution of the Yangtze Block under the background of the assembly of the global Rodinia supercontinent. Methods In this study, petrological and detrital zircon U-Pb geochronological analyses were conducted on metasedimentary rocks of the Ebian Group. Results The detrital zircon ages of two metamorphic fine-grained litharenite sandstones from the Jiadanqiao Formation of the Ebian Group range from 1 950 Ma to 900 Ma. The main relative probability peaks are ca. 910, 1 570, and 1 730 Ma, with secondary peaks at ca. 1 130, 1 390, 1 840, and 2 500 Ma. The maximum depositional ages are 908±8 Ma （MSWD=0.10, n=5） and 905±5 Ma （MSWD=0.52, n=9）, respectively. Conclusions Combined with regional geological data, the depositional age of the Jiadanqiao Formation of the Ebian Group is constrained as 910⁃860 Ma. The sediment source was magmatic rocks in the western Yangtze Block, with a predominant direction towards the south. Thus, formation likely occurred during the Early Neoproterozoic micro-block assembly and can be associated with the global assembly of the Rodinia supercontinent.
2024,
42(6):
1948-1957.
doi: 10.14027/j.issn.1000-0550.2024.014
Abstract:
Objective The Huodiya Group is an important Precambrian stratigraphic unit in the northwestern margin of the Yangtze Block and an important graphite-bearing stratum in China. However, its depositional age, provenance, and tectonic attribution have long been debated. Methods In this study, laser ablation inductively coupled plasma mass spectrometry （LA-ICP-MS） zircon U-Pb dating was conducted on the sericite phyllite from the Shangliang Formation of the Huodiya Group, Wangcang area. [Results and Conclusions] The results show that maximum depositional age of the sample is 837.6±6.0 Ma （MSWD=0.60, n=5）. Combined with existing data, the depositional age of Huodiya Group in the Wangcang area is constrained between ca. 910 Ma and ca. 835 Ma, rather than previously-considered Mesoproterozoic strata. The age populations of detrital zircons are concentrated in four peaks of 832-843 Ma, 855-883 Ma, 895-936 Ma, and 952-973 Ma, and the detrital provenance is mainly from the southeast and northwest magmatic rocks. A synthesis of depositional age, provenance and regional geological background indicates that the Shangliang Formation of the Huodiya Group in the Wangcang area were likely deposited in a back-arc basin receiving provenance from both sides, which is a response to the late-stage convergence of the Rodinia supercontinent in the northwestern margin of the Yangtze Block.
Objective The Huodiya Group is an important Precambrian stratigraphic unit in the northwestern margin of the Yangtze Block and an important graphite-bearing stratum in China. However, its depositional age, provenance, and tectonic attribution have long been debated. Methods In this study, laser ablation inductively coupled plasma mass spectrometry （LA-ICP-MS） zircon U-Pb dating was conducted on the sericite phyllite from the Shangliang Formation of the Huodiya Group, Wangcang area. [Results and Conclusions] The results show that maximum depositional age of the sample is 837.6±6.0 Ma （MSWD=0.60, n=5）. Combined with existing data, the depositional age of Huodiya Group in the Wangcang area is constrained between ca. 910 Ma and ca. 835 Ma, rather than previously-considered Mesoproterozoic strata. The age populations of detrital zircons are concentrated in four peaks of 832-843 Ma, 855-883 Ma, 895-936 Ma, and 952-973 Ma, and the detrital provenance is mainly from the southeast and northwest magmatic rocks. A synthesis of depositional age, provenance and regional geological background indicates that the Shangliang Formation of the Huodiya Group in the Wangcang area were likely deposited in a back-arc basin receiving provenance from both sides, which is a response to the late-stage convergence of the Rodinia supercontinent in the northwestern margin of the Yangtze Block.
2024,
42(6):
1958-1970.
doi: 10.14027/j.issn.1000-0550.2023.095
Abstract:
Objective The Liantuo Formation is a key mid-Neoproterozoic stratigraphic unit in the Dahongshan area of the Yangtze Block, South China. Its deposition time coincides with the breakup of the Rodinia supercontinent, and it is the last sedimentary layer before the Sturtian Ice Age of the first snowball Earth. However, its deposition time and provenance need further study. Methods The study adopted U-Pb chronology of detrital zircons, paleoflow direction and gravel composition analysis from Liantuo Formation in this area. Results The sedimentation time is ca. 800-714 Ma, and major peaks of detrital zircons appear at approximately 2 500 Ma, 2 000 Ma, 880 Ma and 820 Ma, with secondary peaks at ca. 780 Ma. The provenance of the Liantuo Formation in the study area is from the depositional recycling of the Dagushi Group and Huashan Group in the lower strata, and from Archean to Neoproterozoic magmatic rocks in the northern Yangtze Block. Conclusions This suggests that the provenance of the Liantuo Formation is mainly from the northeast of the study area. Regions in western Hubei, southeastern Hubei to northwestern Jiangxi, southwestern Hubei to northern Hunan, and Dahongshan in northern Hubei, have similar detrital zircon peaks corresponding to Archean, Paleoproterozoic and Neoproterozoic ages. The distribution of magmatic rocks in the northern to southeastern Yangtze Block suggests that the provenance of the Liantuo Formation in the northern Yangtze Block produced a northerly to northeasterly supply. This implies that the paleogeographic pattern in the northern to southeastern margin of the Yangtze Block comprised high elevation in the north, low in the south, steep in the north and gradually sloping in the south.
Objective The Liantuo Formation is a key mid-Neoproterozoic stratigraphic unit in the Dahongshan area of the Yangtze Block, South China. Its deposition time coincides with the breakup of the Rodinia supercontinent, and it is the last sedimentary layer before the Sturtian Ice Age of the first snowball Earth. However, its deposition time and provenance need further study. Methods The study adopted U-Pb chronology of detrital zircons, paleoflow direction and gravel composition analysis from Liantuo Formation in this area. Results The sedimentation time is ca. 800-714 Ma, and major peaks of detrital zircons appear at approximately 2 500 Ma, 2 000 Ma, 880 Ma and 820 Ma, with secondary peaks at ca. 780 Ma. The provenance of the Liantuo Formation in the study area is from the depositional recycling of the Dagushi Group and Huashan Group in the lower strata, and from Archean to Neoproterozoic magmatic rocks in the northern Yangtze Block. Conclusions This suggests that the provenance of the Liantuo Formation is mainly from the northeast of the study area. Regions in western Hubei, southeastern Hubei to northwestern Jiangxi, southwestern Hubei to northern Hunan, and Dahongshan in northern Hubei, have similar detrital zircon peaks corresponding to Archean, Paleoproterozoic and Neoproterozoic ages. The distribution of magmatic rocks in the northern to southeastern Yangtze Block suggests that the provenance of the Liantuo Formation in the northern Yangtze Block produced a northerly to northeasterly supply. This implies that the paleogeographic pattern in the northern to southeastern margin of the Yangtze Block comprised high elevation in the north, low in the south, steep in the north and gradually sloping in the south.
2024,
42(6):
1971-1985.
doi: 10.14027/j.issn.1000-0550.2024.041
Abstract:
[Objective and Methods] This study aimed to systematically reveal the magmatic tectonic thermal events and crustal growth processes experienced in the Dabie area, U-Pb Hf isotope studies were conducted on zircons from three different river debris sediments in the Dabie area [Results] The results showed that zircon U-Pb ages can be divided into four groups： 2 676-2 424 Ma, 203-1 812 Ma, 890-748 Ma, and 140-120 Ma. The zircon Hf two-stage model age （tDM2） can also be divided into four groups： 3 371-3 241 Ma, 2 470-2 329 Ma, 1 858-1 751 Ma, and 1 132-1 072 Ma. [Conclusions] （1） The magmatic activity records in the Dabie region can be roughly divided into nine stages. The "quiet periods" are 3.00-2.90 Ga, 2.40-2.10 Ga, 1.80-0.90 Ga, and 0.70-0.14 Ga respectively, and the "explosive periods" are 4.00-3.00 Ga, 2.90-2.40 Ga, 2.10-1.80 Ga, 0.90-0.70 Ga, and 0.14-0.12 Ga. （2） The Dabie area has experienced four metamorphic events, which occurred in the Late Neoarchean （~2.50 Ga）, Middle Paleoproterozoic （~2.00 Ga）, Triassic （0.24-0.20 Ga）, and Cretaceous （0.14-0.12 Ga）. （3） The Dabie area mainly experienced four stages of crustal growth： the initial growth of the Earth's crust was approximately 4.4-4.1 Ga in the early stage of Earth formation； there was rapid growth and accumulation of crust from 4.0-2.8 Ga； 2.6 Ga to 2.0 Ga experienced long-term crustal growth, with strong the magmatic activity at 2.7 Ga to 2.4 Ga in the Dabie area, indicating that crustal reconstruction events were also extremely common during this period； crust grew slowly after 2.0 Ga. （4） Before 2.0 Ga, there were significant differences in magma, metamorphism, and crustal growth events between the Dabie and Huangling areas, indicating that the two were more likely independent micro blocks before this.
[Objective and Methods] This study aimed to systematically reveal the magmatic tectonic thermal events and crustal growth processes experienced in the Dabie area, U-Pb Hf isotope studies were conducted on zircons from three different river debris sediments in the Dabie area [Results] The results showed that zircon U-Pb ages can be divided into four groups： 2 676-2 424 Ma, 203-1 812 Ma, 890-748 Ma, and 140-120 Ma. The zircon Hf two-stage model age （tDM2） can also be divided into four groups： 3 371-3 241 Ma, 2 470-2 329 Ma, 1 858-1 751 Ma, and 1 132-1 072 Ma. [Conclusions] （1） The magmatic activity records in the Dabie region can be roughly divided into nine stages. The "quiet periods" are 3.00-2.90 Ga, 2.40-2.10 Ga, 1.80-0.90 Ga, and 0.70-0.14 Ga respectively, and the "explosive periods" are 4.00-3.00 Ga, 2.90-2.40 Ga, 2.10-1.80 Ga, 0.90-0.70 Ga, and 0.14-0.12 Ga. （2） The Dabie area has experienced four metamorphic events, which occurred in the Late Neoarchean （~2.50 Ga）, Middle Paleoproterozoic （~2.00 Ga）, Triassic （0.24-0.20 Ga）, and Cretaceous （0.14-0.12 Ga）. （3） The Dabie area mainly experienced four stages of crustal growth： the initial growth of the Earth's crust was approximately 4.4-4.1 Ga in the early stage of Earth formation； there was rapid growth and accumulation of crust from 4.0-2.8 Ga； 2.6 Ga to 2.0 Ga experienced long-term crustal growth, with strong the magmatic activity at 2.7 Ga to 2.4 Ga in the Dabie area, indicating that crustal reconstruction events were also extremely common during this period； crust grew slowly after 2.0 Ga. （4） Before 2.0 Ga, there were significant differences in magma, metamorphism, and crustal growth events between the Dabie and Huangling areas, indicating that the two were more likely independent micro blocks before this.
2024,
42(6):
1986-2005.
doi: 10.14027/j.issn.1000-0550.2024.036
Abstract:
Objective Determining the depositional age, sedimentary sources and tectonic background of Neoproterozoic sedimentary strata in Hunan province is one of the keys to understanding tectonic evolution in the middle segment of the Jiangnan orogenic belt. Methods Six clastic rock samples were collected from the Lengjiaxi Group and Banxi Group in the middle segment of the Jiangnan orogenic belt. Provenance and tectonic setting of the sedimentary basins were determined by studying the morphology, trace elements, and U-Pb/Lu-Hf isotopic composition of detrital zircons, and collating published data for detrital zircons and zircons from source areas. Results The results indicated that the Lengjiaxi Group was formed at about 852⁃825 Ma, and the Banxi Group was formed at about 820⁃720 Ma. The most important detrital zircon age peaks in the Lengjiaxi Group and Banxi Group occur at 920⁃790 Ma； age peaks also appear at 1 750⁃1 620 Ma and 2 500⁃2 450 Ma in the Lengjiaxi Group, and at 1 950⁃1 790 Ma and 2 420⁃2 330 Ma in the Banxi Group. The Hf isotopic characteristics show that the Lengjiaxi Group mainly received sediments from the Yangtze Block, and the Banxi Group received detritus from both the Yangtze and Cathaysia Blocks. Conclusions The U-Pb and Lu-Hf isotopic compositions of detrital zircons indicate that deposition in the Lengjiaxi Group and Banxi Group occurred before and after the amalgamation of the Yangtze and Cathaysia Blocks, respectively. The Lengjiaxi Group was deposited in the back-arc basin of a trench-arc-basin system which was closed by 825 Ma accompanied by strong folding in the sedimentary strata and a large number of S-type granite intrusions. The Banxi Group was deposited in an intraplate rift setting related to post-collision extension. Rifting-related magma erupted around 780⁃760 Ma, and gradually weakened along the sedimentation of the Banxi Group.
Objective Determining the depositional age, sedimentary sources and tectonic background of Neoproterozoic sedimentary strata in Hunan province is one of the keys to understanding tectonic evolution in the middle segment of the Jiangnan orogenic belt. Methods Six clastic rock samples were collected from the Lengjiaxi Group and Banxi Group in the middle segment of the Jiangnan orogenic belt. Provenance and tectonic setting of the sedimentary basins were determined by studying the morphology, trace elements, and U-Pb/Lu-Hf isotopic composition of detrital zircons, and collating published data for detrital zircons and zircons from source areas. Results The results indicated that the Lengjiaxi Group was formed at about 852⁃825 Ma, and the Banxi Group was formed at about 820⁃720 Ma. The most important detrital zircon age peaks in the Lengjiaxi Group and Banxi Group occur at 920⁃790 Ma； age peaks also appear at 1 750⁃1 620 Ma and 2 500⁃2 450 Ma in the Lengjiaxi Group, and at 1 950⁃1 790 Ma and 2 420⁃2 330 Ma in the Banxi Group. The Hf isotopic characteristics show that the Lengjiaxi Group mainly received sediments from the Yangtze Block, and the Banxi Group received detritus from both the Yangtze and Cathaysia Blocks. Conclusions The U-Pb and Lu-Hf isotopic compositions of detrital zircons indicate that deposition in the Lengjiaxi Group and Banxi Group occurred before and after the amalgamation of the Yangtze and Cathaysia Blocks, respectively. The Lengjiaxi Group was deposited in the back-arc basin of a trench-arc-basin system which was closed by 825 Ma accompanied by strong folding in the sedimentary strata and a large number of S-type granite intrusions. The Banxi Group was deposited in an intraplate rift setting related to post-collision extension. Rifting-related magma erupted around 780⁃760 Ma, and gradually weakened along the sedimentation of the Banxi Group.
2024,
42(6):
2006-2019.
doi: 10.14027/j.issn.1000-0550.2024.062
Abstract:
Objective Precambrian stratigraphy has undergone multiple tectonic events, complex tectonics, severe deformation, and metamorphism, making it inconvenient and difficult to compare the strata and establish an objective stratigraphic sequence. Methods This study selected the Taojinhe Formation, the oldest outcrop of the Neoproterozoic Fanjingshan Group in the western part of the Jiangnan orogen as a research object, and we collected sandstone samples carry out comparing detrital zircon morphology, U-Pb dating, rock, and mineral identification and analysis of the material origin and sedimentary geotectonic settings of this formation, as well as for multidimensional scaling analysis and comparison of the stratigraphy of the Fanjingshan Group. Results The sample rocks are mainly composed of detrital particles and fillers； the detrital particles are mainly quartz, feldspar and cutting and sub-angular to sub-rounded with average sortability. The fillers are mainly mica, silica, and iron, and the mica is mainly sericite and white mica. The zircon grains of the "Taojinhe Formation" are angular, representing short-distance transport, and the source rocks are granite, diabase, Syenite/Monzonite, and basalt, with detrital zircon peak ages of 875 Ma, 1 862 Ma, and 2 513 Ma. Based on the zircon geochemical data analysis, the "Taojinhe Formation" was deposited in a convergent background. U/Nb values of "Taojinhe Formation" zircon range from 25-700, with one mantle-sourced zircon and 98 arc-sourced zircons. Using U/Yb values to compare with Hf, all zircons were in the continental source range, and the Sc （average value 385.1） and Sc/Yb values （average 1.1） also show island arc magmatic features. Conversely, the zircons have high U/Nb, Sc/Yb, and U/Yb values, relatively low Nb/Yb values, and U/Nb > 20, suggesting that the zircons were formed in a subduction environment. In contrast, the zircons of the Yujiagou Formation are mostly elliptical, suggesting long-distance transport and deposition in a disseminated background, and the source rocks are granites, diorites, basalts, alkalites, and rarely kimberlites, with two distinct age peaks at 1 845 and 2 325 Ma. Conclusions The "Taojinhe Formation" and the Yujiagou Formation have different source rocks representing different material source areas, whereas the tectonic background is also different, and the maximum depositional age of the "Taojinhe Formation" is younger than that of the overlying Yujiagou Formation, which is not consistent with the stratigraphic sequences of the regional geological survey. Therefore, the original stratigraphic division may be erroneous. Comparison of zircon age diagrams shows that the Taojinhe Formation is similar to the Xiaojiahe Formation above the overlying Yujiagou Formation and has the same material source. Combined other stratigraphic features of the Fanjingshan Group, it is suggested that the "Taojinhe Formation" and Xiaojiahe Formation in the Taoshulin area may be the same stratigraphic units.
Objective Precambrian stratigraphy has undergone multiple tectonic events, complex tectonics, severe deformation, and metamorphism, making it inconvenient and difficult to compare the strata and establish an objective stratigraphic sequence. Methods This study selected the Taojinhe Formation, the oldest outcrop of the Neoproterozoic Fanjingshan Group in the western part of the Jiangnan orogen as a research object, and we collected sandstone samples carry out comparing detrital zircon morphology, U-Pb dating, rock, and mineral identification and analysis of the material origin and sedimentary geotectonic settings of this formation, as well as for multidimensional scaling analysis and comparison of the stratigraphy of the Fanjingshan Group. Results The sample rocks are mainly composed of detrital particles and fillers； the detrital particles are mainly quartz, feldspar and cutting and sub-angular to sub-rounded with average sortability. The fillers are mainly mica, silica, and iron, and the mica is mainly sericite and white mica. The zircon grains of the "Taojinhe Formation" are angular, representing short-distance transport, and the source rocks are granite, diabase, Syenite/Monzonite, and basalt, with detrital zircon peak ages of 875 Ma, 1 862 Ma, and 2 513 Ma. Based on the zircon geochemical data analysis, the "Taojinhe Formation" was deposited in a convergent background. U/Nb values of "Taojinhe Formation" zircon range from 25-700, with one mantle-sourced zircon and 98 arc-sourced zircons. Using U/Yb values to compare with Hf, all zircons were in the continental source range, and the Sc （average value 385.1） and Sc/Yb values （average 1.1） also show island arc magmatic features. Conversely, the zircons have high U/Nb, Sc/Yb, and U/Yb values, relatively low Nb/Yb values, and U/Nb > 20, suggesting that the zircons were formed in a subduction environment. In contrast, the zircons of the Yujiagou Formation are mostly elliptical, suggesting long-distance transport and deposition in a disseminated background, and the source rocks are granites, diorites, basalts, alkalites, and rarely kimberlites, with two distinct age peaks at 1 845 and 2 325 Ma. Conclusions The "Taojinhe Formation" and the Yujiagou Formation have different source rocks representing different material source areas, whereas the tectonic background is also different, and the maximum depositional age of the "Taojinhe Formation" is younger than that of the overlying Yujiagou Formation, which is not consistent with the stratigraphic sequences of the regional geological survey. Therefore, the original stratigraphic division may be erroneous. Comparison of zircon age diagrams shows that the Taojinhe Formation is similar to the Xiaojiahe Formation above the overlying Yujiagou Formation and has the same material source. Combined other stratigraphic features of the Fanjingshan Group, it is suggested that the "Taojinhe Formation" and Xiaojiahe Formation in the Taoshulin area may be the same stratigraphic units.
2024,
42(6):
2020-2030.
doi: 10.14027/j.issn.1000-0550.2023.140
Abstract:
Objective The Neoproterozoic Struttin Glaciation （~717⁃660 Ma） developed widely across the world and was well recorded in the Nanhua Basin, but the study of carbon cycling during this period is lacking. To investigate the carbon cycle process during the Sturtian Glaciation, the carbon isotope compositions of the Tiesi'ao Formation precipitated during the glacial interval were systematically studied. Methods This study focuses on drillcore ZK2115, located in Gaodi Manganese Deposit, Songtao area, eastern Guizhou province, China. We analyzed high-resolution organic and inorganic carbon isotopes （δ13Corg and δ13Ccarb） from the syn-Sturtian Tiesi'ao Formation. Results The results show that the Tiesi'ao Formation is characterized by negative carbon isotope excursions, ranging between -9.29‰ and -3.37‰ （mean -7.24‰）. The δ13Corg values vary between -33.63‰ and -23.35‰ （mean -29.29‰）. There is a positive correlation between the δ13Corg and δ13Ccarb values, indicating that the inorganic carbon isotope compositions are not affected by diagenesis and can be used to reflect the original carbon isotope signals of the Dissolved Inorganic Carbon （DIC） reservoir in the Nanhua Basin. Organic matter preserved in the Tiesi'ao Formation was generated through photosynthesis, and the carbon fixed in the organic matter was sourced from the DIC reservoir in seawater. Conclusions Combined with the low Total Organic Carbon （TOC） content （mean 0.2%） of the Tiesi'ao Formation, our results indicate that photosynthesis still occurred under the extreme glacial conditions at a low rate, although only a small amount of organic matter was generated. The study of carbon isotope compositions in glacial sediments can be used to explore carbon cycling during the Sturtian Glaciation, and to provide evidence for the paleo-marine environment and biogeochemical cycle under extreme climatic conditions.
Objective The Neoproterozoic Struttin Glaciation （~717⁃660 Ma） developed widely across the world and was well recorded in the Nanhua Basin, but the study of carbon cycling during this period is lacking. To investigate the carbon cycle process during the Sturtian Glaciation, the carbon isotope compositions of the Tiesi'ao Formation precipitated during the glacial interval were systematically studied. Methods This study focuses on drillcore ZK2115, located in Gaodi Manganese Deposit, Songtao area, eastern Guizhou province, China. We analyzed high-resolution organic and inorganic carbon isotopes （δ13Corg and δ13Ccarb） from the syn-Sturtian Tiesi'ao Formation. Results The results show that the Tiesi'ao Formation is characterized by negative carbon isotope excursions, ranging between -9.29‰ and -3.37‰ （mean -7.24‰）. The δ13Corg values vary between -33.63‰ and -23.35‰ （mean -29.29‰）. There is a positive correlation between the δ13Corg and δ13Ccarb values, indicating that the inorganic carbon isotope compositions are not affected by diagenesis and can be used to reflect the original carbon isotope signals of the Dissolved Inorganic Carbon （DIC） reservoir in the Nanhua Basin. Organic matter preserved in the Tiesi'ao Formation was generated through photosynthesis, and the carbon fixed in the organic matter was sourced from the DIC reservoir in seawater. Conclusions Combined with the low Total Organic Carbon （TOC） content （mean 0.2%） of the Tiesi'ao Formation, our results indicate that photosynthesis still occurred under the extreme glacial conditions at a low rate, although only a small amount of organic matter was generated. The study of carbon isotope compositions in glacial sediments can be used to explore carbon cycling during the Sturtian Glaciation, and to provide evidence for the paleo-marine environment and biogeochemical cycle under extreme climatic conditions.
2024,
42(6):
2031-2041.
doi: 10.14027/j.issn.1000-0550.2022.162
Abstract:
Objective The early Cambrian was a pivotal period of oceanic environmental evolution in Earth history. To grain the insight into evolutions of oceanic redox condition and nitrogen cycle, which can favor to ascertain the dominant control on coeval organic matter enrichment. Methods The present study conducted the iron speciation, nitrogen and sulfur isotopes analyses for the Qiongzhusi Formation in the Weiyuan area. Results The oceanic redox condition experienced several transitions in response to coeval sea-level fluctuations during the early Cambrian. In the Cambrian Age 2, the deep euxinic watermasses expanded to the shallow-water shelf environment, and intense denitrification resulted in the persistent consumption of NO3 - 3 -
Objective The early Cambrian was a pivotal period of oceanic environmental evolution in Earth history. To grain the insight into evolutions of oceanic redox condition and nitrogen cycle, which can favor to ascertain the dominant control on coeval organic matter enrichment. Methods The present study conducted the iron speciation, nitrogen and sulfur isotopes analyses for the Qiongzhusi Formation in the Weiyuan area. Results The oceanic redox condition experienced several transitions in response to coeval sea-level fluctuations during the early Cambrian. In the Cambrian Age 2, the deep euxinic watermasses expanded to the shallow-water shelf environment, and intense denitrification resulted in the persistent consumption of NO
2024,
42(6):
2042-2053.
doi: 10.14027/j.issn.1000-0550.2023.130
Abstract:
Objective The early Cambrian explosion is thought to be related to the increase of nitrate concentration； however, a recent study suggested that no significant change occurred in the nitrate concentration during this period. To identify the influence of nitrate on the biological explosion, this study reconstructed the characteristics of the paleomarine environment and nitrogen cycle during this period. Methods The borehole core （well ZK0202） in the slope area of the southeast margin of the Yangtze was used as the anatomical research object, and the major elements, trace elements, rare earth elements, carbon-nitrogen isotopes, and other indices were analyzed. Results The early Cambrian Fortunian-middle Age 2 （>526 Ma） paleomarine environment was characterized by a stratified ocean with a deepened chemocline, and both nitrogen fixation and denitrification developed during the same period, with nitrogen fixation being the dominant effect. In the late Age 2-early Age 3 （ca. 526-518 Ma）, the paleomarine environment was characterized by strong anoxic-euxinic behavior, and nitrogen fixation was the most important nitrogen cycle pathway. The middle and late Age 3 （<518 Ma） paleomarine environment was dominated by suboxic-oxic behavior, and nitrogen fixation was still developed. The evolution of the redox state may be related to the high primary productivity induced by hydrothermal action, and the persistent nitrogen fixation shows that the nitrate concentration was maintained at a low level during the same period. Conclusions Therefore, the abundance of nitrate may not be the main controlling factor for the biological explosion, and ocean oxidation and the abundance of organisms at the base of the food chain may be important triggers. The above research results further enhance the degree of research on the paleomarine environment and nitrogen cycle in the early Cambrian slope area of the southeast margin of the South China Yangtze, providing new references for the correct understanding of the environment-biological co-evolution in this period.
Objective The early Cambrian explosion is thought to be related to the increase of nitrate concentration； however, a recent study suggested that no significant change occurred in the nitrate concentration during this period. To identify the influence of nitrate on the biological explosion, this study reconstructed the characteristics of the paleomarine environment and nitrogen cycle during this period. Methods The borehole core （well ZK0202） in the slope area of the southeast margin of the Yangtze was used as the anatomical research object, and the major elements, trace elements, rare earth elements, carbon-nitrogen isotopes, and other indices were analyzed. Results The early Cambrian Fortunian-middle Age 2 （>526 Ma） paleomarine environment was characterized by a stratified ocean with a deepened chemocline, and both nitrogen fixation and denitrification developed during the same period, with nitrogen fixation being the dominant effect. In the late Age 2-early Age 3 （ca. 526-518 Ma）, the paleomarine environment was characterized by strong anoxic-euxinic behavior, and nitrogen fixation was the most important nitrogen cycle pathway. The middle and late Age 3 （<518 Ma） paleomarine environment was dominated by suboxic-oxic behavior, and nitrogen fixation was still developed. The evolution of the redox state may be related to the high primary productivity induced by hydrothermal action, and the persistent nitrogen fixation shows that the nitrate concentration was maintained at a low level during the same period. Conclusions Therefore, the abundance of nitrate may not be the main controlling factor for the biological explosion, and ocean oxidation and the abundance of organisms at the base of the food chain may be important triggers. The above research results further enhance the degree of research on the paleomarine environment and nitrogen cycle in the early Cambrian slope area of the southeast margin of the South China Yangtze, providing new references for the correct understanding of the environment-biological co-evolution in this period.
2024,
42(6):
2054-2065.
doi: 10.14027/j.issn.1000-0550.2023.036
Abstract:
Methods A total of 47 shale samples from the Tianlin and Shaba sections, which were deposited in different regions of the Sichuan Basin during this period （23 samples from the Tianlin section, and 24 samples from the Shaba section）, were selected for organic carbon （δ13Corg） and pyrite sulfur isotope （δ34Spy） analysis. The system dynamics software Vensim7.3.5 was used to reconstruct the model and analyze the dynamics. Results The main results are as follows： （1） Both δ13Corg and δ34Spy exhibited synchronous positive shifts before the Hirnantian glacial period, reached the peak value in the Hirnantian glacial period, and then decreased. The synchronous positive deviation of C and S isotopes can occur when the burial fluxes of organic carbon were 1.5 to 2.0 times, and pyrite 1.5 to 3.0 times, that of the pre-glacial period. This indicates that the high primary productivity and widely developed anoxic water column of iron were beneficial to the burial of sedimentary organic matter and pyrite. During the Hirnantian glacial period, with the decreased primary productivity and increased oxygen content in water column, the burial flux of organic carbon and pyrite decreased gradually, and the carbon and sulfur isotopes were slightly negative. The intensity of volcanic activity mainly affects the migration of δ13C, and the weakened volcanic activity was beneficial to the positive deviation of δ13C, while the enhanced volcanic activity after the glacial period was the necessary condition for the significant negative deviation of δ13C over a short time （0.5-1.0 Myr）. （2） The lower seawater sulfate concentration （approximately 5 mM） in the early stage of the Hirnantian glacial period was the basis of the positive shift of δ34S, with an increase of pyrite burial flux, the ocean sulfate concentration gradually decreased to 3 mM and reached the lowest during the Hirnantian glacial period, while the increase of terrestrial sulfate input flux in the post-glacial period was the main factor leading to the negative deviation of δ34S, making the ocean sulfate concentration gradually return to 5 mM. Conclusions The results quantitatively evaluate the main influencing factors of δ13C and δ34S synchronous migration. A new idea and quantitative prediction model are provided for the study of the kinetics of C and S isotope fractionation throughout geological history. [Obective] The significant migration events of carbon and sulfur isotopes （δ13C, δ34S） in the global transition period from the Late Ordovician to the Early Silurian have been widely recognized, but the causes of these isotopes are still controversial. Quantitative numerical simulation is one of the ways to solve this problem.
Methods A total of 47 shale samples from the Tianlin and Shaba sections, which were deposited in different regions of the Sichuan Basin during this period （23 samples from the Tianlin section, and 24 samples from the Shaba section）, were selected for organic carbon （δ13Corg） and pyrite sulfur isotope （δ34Spy） analysis. The system dynamics software Vensim7.3.5 was used to reconstruct the model and analyze the dynamics. Results The main results are as follows： （1） Both δ13Corg and δ34Spy exhibited synchronous positive shifts before the Hirnantian glacial period, reached the peak value in the Hirnantian glacial period, and then decreased. The synchronous positive deviation of C and S isotopes can occur when the burial fluxes of organic carbon were 1.5 to 2.0 times, and pyrite 1.5 to 3.0 times, that of the pre-glacial period. This indicates that the high primary productivity and widely developed anoxic water column of iron were beneficial to the burial of sedimentary organic matter and pyrite. During the Hirnantian glacial period, with the decreased primary productivity and increased oxygen content in water column, the burial flux of organic carbon and pyrite decreased gradually, and the carbon and sulfur isotopes were slightly negative. The intensity of volcanic activity mainly affects the migration of δ13C, and the weakened volcanic activity was beneficial to the positive deviation of δ13C, while the enhanced volcanic activity after the glacial period was the necessary condition for the significant negative deviation of δ13C over a short time （0.5-1.0 Myr）. （2） The lower seawater sulfate concentration （approximately 5 mM） in the early stage of the Hirnantian glacial period was the basis of the positive shift of δ34S, with an increase of pyrite burial flux, the ocean sulfate concentration gradually decreased to 3 mM and reached the lowest during the Hirnantian glacial period, while the increase of terrestrial sulfate input flux in the post-glacial period was the main factor leading to the negative deviation of δ34S, making the ocean sulfate concentration gradually return to 5 mM. Conclusions The results quantitatively evaluate the main influencing factors of δ13C and δ34S synchronous migration. A new idea and quantitative prediction model are provided for the study of the kinetics of C and S isotope fractionation throughout geological history. [Obective] The significant migration events of carbon and sulfur isotopes （δ13C, δ34S） in the global transition period from the Late Ordovician to the Early Silurian have been widely recognized, but the causes of these isotopes are still controversial. Quantitative numerical simulation is one of the ways to solve this problem.
2024,
42(6):
2066-2078.
doi: 10.14027/j.issn.1000-0550.2024.119
Abstract:
Objective Although the end-Guadalupian extinction is not as well known as the end-Permian extinction, it is regarded as a critical event that resulted in considerable biodiversity loss and significant disruptions to ecosystems. Following the End-Guadalupian extinction, global biodiversity was progressively restored during the Late Permian. Despite this recovery, the intricate relationship between biological resurgence during this era and the redox conditions of palaeo-oceans is still not fully understood. Methods The Wuchiaping Formation located in the Shangsi section of the northern margin of the Yangtze Block with high-resolution time frame constraints was selected as the research object. Petrological and sedimentary geochemical studies were employed to reconstruct the marine redox environment associated with the Wuchiaping Formation in the Late Permian. By examining the rock composition and geochemical signatures of the rock samples, we established key indicators of redox conditions （Ce*； Ce anomaly）. Additionally, we selected several indicators, including Th, Sc, Y/Ho, and Mn/Sr, to assess whether the redox indicator was influenced by non-redox factors. Results The collected global carbon isotope from the Wuchiapingian of Late Permian consistently exhibited a significant positive bias in the early Wuchiapingian, followed by a notable negative bias in the middle Wuchiapingian changes, indicating that this carbon cycle disturbance event had a worldwide impact. Moreover, this transformation corresponds to changes in seawater redox conditions, showing a close correlation between carbon cycle disturbance events and alteration in redox environments. Through comparison of the stratigraphic framework, the disturbance was found to align with the waxing and waning of P4 glaciation. Based on the systematic changes in the Ce anomaly curve of carbonate rocks, the evolution of marine redox conditions during the Wuchiapingian of Late Permian can be divided into three stages, reflecting three distinct global shifts in climate and environmental conditions. The gradual resurgence of early organisms led to an increase in primary productivity and photosynthesis, and the cooler climate was conducive to ocean circulation and facilitated oxygen exchange, resulting in an oxygen-poor environment. As the climate gradually warmed and the Late Paleozoic ice age ended, the situation began to shift dramatically. The weakening of ocean ventilation, caused by alterations in temperature gradients and circulation patterns, coupled with rising sea levels contributed to the expansion of the minimum oxygen zone, consequently transforming the ocean into an anoxic environment. This shift had profound implications for marine life and the overall health of ocean ecosystems, setting the stage for significant biological upheaval in the ensuing geological periods. Conclusions The study developed a redox model of the shallow marine environment during the Wuchiapingian, providing insight into the interactions between various environmental factors. It highlights a significant connection between the oceanic hypoxia event and the simultaneous biological and geological tectonic activities occurring. By examining these relationships, the research enhances our understanding of how environmental factors influenced marine life and sedimentary processes during this critical period in Earth history.
Objective Although the end-Guadalupian extinction is not as well known as the end-Permian extinction, it is regarded as a critical event that resulted in considerable biodiversity loss and significant disruptions to ecosystems. Following the End-Guadalupian extinction, global biodiversity was progressively restored during the Late Permian. Despite this recovery, the intricate relationship between biological resurgence during this era and the redox conditions of palaeo-oceans is still not fully understood. Methods The Wuchiaping Formation located in the Shangsi section of the northern margin of the Yangtze Block with high-resolution time frame constraints was selected as the research object. Petrological and sedimentary geochemical studies were employed to reconstruct the marine redox environment associated with the Wuchiaping Formation in the Late Permian. By examining the rock composition and geochemical signatures of the rock samples, we established key indicators of redox conditions （Ce*； Ce anomaly）. Additionally, we selected several indicators, including Th, Sc, Y/Ho, and Mn/Sr, to assess whether the redox indicator was influenced by non-redox factors. Results The collected global carbon isotope from the Wuchiapingian of Late Permian consistently exhibited a significant positive bias in the early Wuchiapingian, followed by a notable negative bias in the middle Wuchiapingian changes, indicating that this carbon cycle disturbance event had a worldwide impact. Moreover, this transformation corresponds to changes in seawater redox conditions, showing a close correlation between carbon cycle disturbance events and alteration in redox environments. Through comparison of the stratigraphic framework, the disturbance was found to align with the waxing and waning of P4 glaciation. Based on the systematic changes in the Ce anomaly curve of carbonate rocks, the evolution of marine redox conditions during the Wuchiapingian of Late Permian can be divided into three stages, reflecting three distinct global shifts in climate and environmental conditions. The gradual resurgence of early organisms led to an increase in primary productivity and photosynthesis, and the cooler climate was conducive to ocean circulation and facilitated oxygen exchange, resulting in an oxygen-poor environment. As the climate gradually warmed and the Late Paleozoic ice age ended, the situation began to shift dramatically. The weakening of ocean ventilation, caused by alterations in temperature gradients and circulation patterns, coupled with rising sea levels contributed to the expansion of the minimum oxygen zone, consequently transforming the ocean into an anoxic environment. This shift had profound implications for marine life and the overall health of ocean ecosystems, setting the stage for significant biological upheaval in the ensuing geological periods. Conclusions The study developed a redox model of the shallow marine environment during the Wuchiapingian, providing insight into the interactions between various environmental factors. It highlights a significant connection between the oceanic hypoxia event and the simultaneous biological and geological tectonic activities occurring. By examining these relationships, the research enhances our understanding of how environmental factors influenced marine life and sedimentary processes during this critical period in Earth history.
2024,
42(6):
2079-2087.
doi: 10.14027/j.issn.1000-0550.2024.033
Abstract:
Objective To discover fossil habitats that can be compared with the Chengjiang biota in the lower Cambrian Qiongzhusi Formation, regional stratigraphic correlation, and further reveal the life evolution of early Cambrian organisms, important paleontological fossil evidence is provided. Methods The study focuses on the macrobiotic fossils developed in the dark gray sandy shale of the lower Cambrian Qiongzhusi Formation. Fine profile surveys, fossil identification, electron probes, scanning electron microscopy, X-ray fluorescence （XRF） testing, and other analyses were conducted to explore the geological significance of the newly discovered macrobiotic fossils. Results The main types of fossils in Daxinshan include Kunmingella douvillei, pest ichnology, Conotheca sp., Maotianshania sp., Kutorgina chengjiangensis, Branchiocaris sp., Vetulicola sp., Eoredlichia sp., and fossil leguminosae （unknown species）. In addition, suspected skeletal fossils have been discovered at the same level in the Pude area. XRF scanning shows that the fossils contain relatively high concentrations of Ca, Cr, and P and are significantly different from the composition of the surrounding rock （Fe, S, Ti）. Ca and P are the main constituent elements of skeletal fossils. The fossils in the Daxinshan Pude area are not only well preserved and relatively abundant, but also have a large number of individual and diverse species. Conclusions and Prospects Transportation to the fossil site is convenient. Moreover, the occurrence of the fossils is relatively low compared to the "Chengjiang biota," which has research significance and scientific popularization value. Thus, it is possible to establish a second early Cambrian biota in the Luquan area of central Yunnan： the Daxinshan Pude biota. This discovery provides important paleontological fossil research materials for studying the patterns, scope, recovery, paleoenvironmental evolution, and stratigraphic regional correlation of early Cambrian biological activities.
Objective To discover fossil habitats that can be compared with the Chengjiang biota in the lower Cambrian Qiongzhusi Formation, regional stratigraphic correlation, and further reveal the life evolution of early Cambrian organisms, important paleontological fossil evidence is provided. Methods The study focuses on the macrobiotic fossils developed in the dark gray sandy shale of the lower Cambrian Qiongzhusi Formation. Fine profile surveys, fossil identification, electron probes, scanning electron microscopy, X-ray fluorescence （XRF） testing, and other analyses were conducted to explore the geological significance of the newly discovered macrobiotic fossils. Results The main types of fossils in Daxinshan include Kunmingella douvillei, pest ichnology, Conotheca sp., Maotianshania sp., Kutorgina chengjiangensis, Branchiocaris sp., Vetulicola sp., Eoredlichia sp., and fossil leguminosae （unknown species）. In addition, suspected skeletal fossils have been discovered at the same level in the Pude area. XRF scanning shows that the fossils contain relatively high concentrations of Ca, Cr, and P and are significantly different from the composition of the surrounding rock （Fe, S, Ti）. Ca and P are the main constituent elements of skeletal fossils. The fossils in the Daxinshan Pude area are not only well preserved and relatively abundant, but also have a large number of individual and diverse species. Conclusions and Prospects Transportation to the fossil site is convenient. Moreover, the occurrence of the fossils is relatively low compared to the "Chengjiang biota," which has research significance and scientific popularization value. Thus, it is possible to establish a second early Cambrian biota in the Luquan area of central Yunnan： the Daxinshan Pude biota. This discovery provides important paleontological fossil research materials for studying the patterns, scope, recovery, paleoenvironmental evolution, and stratigraphic regional correlation of early Cambrian biological activities.
2024,
42(6):
2088-2101.
doi: 10.14027/j.issn.1000-0550.2024.098
Abstract:
Objective The paleokarst reservoirs of the Sinian Dengying Formation in the central Sichuan Basin are key targets for marine natural gas exploration in China, characterized by long diagenetic history, multiple diagenetic events, and strong heterogeneity. These characteristics lead to the complexity of oil and gas accumulation results, which restricts the next large-scale exploration and development. Methods By diagenetic mineral sequences, U-Pb isotope dating and laser in-situ element measurement in the Dengying Formation paleokarst reservoirs, combined with physical property testing of cores and image software identification, reservoir type, diagenetic sequence, reservoir-forming and hydrocarbon accumulation effect are determined. Results Two-types reservoir are identified in the 2nd and 4th members of the Dengying Formation： a fracture-vug type and a pore-vug type. Nine-stages diagenesis-hydrocarbon accumulation events were identified. The diagenetic fluids were of three types： seawater, modified seawater and hydrothermal. Multi-stage diagenetic events including three-stages seawater cementation （606±21 Ma, 604±42 Ma and 590±15 Ma） and three-stages hydrothermal activity （403±30 Ma, 259.4±3 Ma and 199±12 Ma） had destructive effects on reservoir physical properties. Conclusions Reconstructed paleoporosity for the period of diagenetic evolution indicate that the fracture-vug type of reservoirs in the 2nd member of the Dengying Formation always enabled hydrocarbon migration and accumulation during the formation stage of paleo-oil reservoirs, also during the stage of crude oil cracking into gas and during the stage of gas reservoir adjustment. Pore-vug type reservoirs allowed migration and accumulation only in the latter two stages. In the 4th member, fracture-vug reservoirs enabled migration and accumulation in all three stages of hydrocarbon accumulation.
Objective The paleokarst reservoirs of the Sinian Dengying Formation in the central Sichuan Basin are key targets for marine natural gas exploration in China, characterized by long diagenetic history, multiple diagenetic events, and strong heterogeneity. These characteristics lead to the complexity of oil and gas accumulation results, which restricts the next large-scale exploration and development. Methods By diagenetic mineral sequences, U-Pb isotope dating and laser in-situ element measurement in the Dengying Formation paleokarst reservoirs, combined with physical property testing of cores and image software identification, reservoir type, diagenetic sequence, reservoir-forming and hydrocarbon accumulation effect are determined. Results Two-types reservoir are identified in the 2nd and 4th members of the Dengying Formation： a fracture-vug type and a pore-vug type. Nine-stages diagenesis-hydrocarbon accumulation events were identified. The diagenetic fluids were of three types： seawater, modified seawater and hydrothermal. Multi-stage diagenetic events including three-stages seawater cementation （606±21 Ma, 604±42 Ma and 590±15 Ma） and three-stages hydrothermal activity （403±30 Ma, 259.4±3 Ma and 199±12 Ma） had destructive effects on reservoir physical properties. Conclusions Reconstructed paleoporosity for the period of diagenetic evolution indicate that the fracture-vug type of reservoirs in the 2nd member of the Dengying Formation always enabled hydrocarbon migration and accumulation during the formation stage of paleo-oil reservoirs, also during the stage of crude oil cracking into gas and during the stage of gas reservoir adjustment. Pore-vug type reservoirs allowed migration and accumulation only in the latter two stages. In the 4th member, fracture-vug reservoirs enabled migration and accumulation in all three stages of hydrocarbon accumulation.
2024,
42(6):
2102-2118.
doi: 10.14027/j.issn.1000-0550.2024.043
Abstract:
Objective Reef-shoal reservoirs of the Changxing Formation are widely developed in the Kaijiang-Liangping trough, and contain abundant oil and gas resources. Core and seismic data on the eastern side of the trough are limited, and traditional geological qualitative methods have difficulty in accurately dividing and comparing the sequence stratigraphy, and consequently in predicting the locations of high-quality reservoirs. This in-depth study of the high-frequency sequence stratigraphy of the Changxing Formation helps to refine understanding of the distribution of sedimentary facies in the sequence framework and clarify the sedimentary evolution process on the eastern side of the trough. This improves the accuracy of favorable reservoir prediction and also provides a stronger basis for the exploration and development of reef-beach reservoirs. Methods This study was based on field profiles and core, logging and seismic data, together with Integrated Prediction Error Filter Analysis （INPEFA） logging cycle technology, to determine the characteristics of the high-frequency sequence stratigraphy and sedimentary system in the Changxing Formation in eastern Sichuan Basin. Results The Changxing Formation was found to be divided into two third-order sequences and five fourth-order sequences from bottom to top. Carbonate ramp sedimentation was developed in the early stage of the Changxing Formation, and carbonate platform sedimentation in the middle-to-late stage, subdivided into open-platform facies, platform-margin facies and slope-basin facies. The favorable reservoir belt of the Changxing Formation mainly consists of platform-margin reef facies. The Changxing Formation contains crystalline dolomite reservoirs, （residual） granular dolomite reservoirs and reef dolomite reservoirs. These are strongly heterogeneous, and their overall rating is low-porosity, producing medium and low permeability. Conclusions The favorable reservoir zones are distributed in a ‘Y’-shape, mainly concentrated in the areas of Puguang 5-Laojun 1-Qilibei 2-Qilibei 101-Huanglong 5-Huanglong 8-Huanglong 4 distributed in discontinuous strips or dots along the platform margin. The platform margin beach in the back row of Qilibei is a sub-favorable zone occurring in a strip within the study area. The spatial and temporal evolution of the sedimentary facies belt, and the predictability of the locations of favorable reservoirs in the Changxing Formation, provides a geological basis for further exploration and development of reef-shoal reservoirs.
Objective Reef-shoal reservoirs of the Changxing Formation are widely developed in the Kaijiang-Liangping trough, and contain abundant oil and gas resources. Core and seismic data on the eastern side of the trough are limited, and traditional geological qualitative methods have difficulty in accurately dividing and comparing the sequence stratigraphy, and consequently in predicting the locations of high-quality reservoirs. This in-depth study of the high-frequency sequence stratigraphy of the Changxing Formation helps to refine understanding of the distribution of sedimentary facies in the sequence framework and clarify the sedimentary evolution process on the eastern side of the trough. This improves the accuracy of favorable reservoir prediction and also provides a stronger basis for the exploration and development of reef-beach reservoirs. Methods This study was based on field profiles and core, logging and seismic data, together with Integrated Prediction Error Filter Analysis （INPEFA） logging cycle technology, to determine the characteristics of the high-frequency sequence stratigraphy and sedimentary system in the Changxing Formation in eastern Sichuan Basin. Results The Changxing Formation was found to be divided into two third-order sequences and five fourth-order sequences from bottom to top. Carbonate ramp sedimentation was developed in the early stage of the Changxing Formation, and carbonate platform sedimentation in the middle-to-late stage, subdivided into open-platform facies, platform-margin facies and slope-basin facies. The favorable reservoir belt of the Changxing Formation mainly consists of platform-margin reef facies. The Changxing Formation contains crystalline dolomite reservoirs, （residual） granular dolomite reservoirs and reef dolomite reservoirs. These are strongly heterogeneous, and their overall rating is low-porosity, producing medium and low permeability. Conclusions The favorable reservoir zones are distributed in a ‘Y’-shape, mainly concentrated in the areas of Puguang 5-Laojun 1-Qilibei 2-Qilibei 101-Huanglong 5-Huanglong 8-Huanglong 4 distributed in discontinuous strips or dots along the platform margin. The platform margin beach in the back row of Qilibei is a sub-favorable zone occurring in a strip within the study area. The spatial and temporal evolution of the sedimentary facies belt, and the predictability of the locations of favorable reservoirs in the Changxing Formation, provides a geological basis for further exploration and development of reef-shoal reservoirs.
2024,
42(6):
2119-2134.
doi: 10.14027/j.issn.1000-0550.2024.109
Abstract:
Objective The Sinian tectonic-sedimentary differentiation characteristics of the Middle-Upper Yangtze transition zone are similar to those of Mianyang-Changning area in central Sichuan. Clarifying the sedimentary system and sedimentary filling process in this area can provide guidance for the exploration of oil and gas and sedimentary minerals. In the transitional zone of the Middle-Upper Yangtze Block, the Sinian period exhibited a structural-sedimentary differentiation pattern akin to that observed in the Mianyang-Changning trough in central Sichuan. The sedimentary characteristics and filling processes provide essential insights for hydrocarbon exploration and the assessment of sedimentary mineral resources in this region. Methods The study focused on macroscopic sedimentary structures, depositional sequences, and microfacies in thin sections from outcrops and drilling data. Combined with the analysis of relevant tectonic settings and geodynamic causes, a detailed analysis of the development characteristics of sedimentary systems was conducted. On this basis, the vertical evolution and lateral comparison of depositional facies across the rift were dissected to reveal the changes in sedimentary systems and the structural-sedimentary filling processes during the development and evolution of the intra-continental rift basins of the Sinian period in the central Upper Yangtze transition zone. Results Influenced by regional transtensional tectonic activity, the rifting activity led to the formation of half-graben-style fault basin structures, with significant sedimentary differentiation in the carbonate gravity flows along the gentle slope margins of the rift and the deep-water continental shelf under a regional shallow marine sedimentation backdrop. During the Doushan period, a compensatory filling feature with overlapping centers of subsidence and sedimentation developed. In the Dengying period, structural-sedimentary filling characte-ristics of thickened shallow-water carbonate sedimentation and under-compensated deep-water sedimentation were observed. The deep-water sedimentation at the subsidence center, which developed during the Doushan period, possesses favorable hydrocarbon source rock conditions and potential for shale gas exploration； during the Dengying period, the high-energy beach bars at the edge of the prograding platform on the eastern side of the Upper Yangtze, controlled by faults. The high-energy beach bars at the edge of the prograding platform on the gentle slope of the Central Yangtze are high-quality facies belts for oil and gas reservoir development, and they form near-source hydrocarbon reservoir configurations with the hydrocarbon source rocks within the rift basin. The synsedimentary faults controlling the basin provide pathways for the upwelling of phosphorus-rich hydrothermal fluids. Concurrently, the western fault and eastern inclined fault basin structure provide favorable sedimentary topographic conditions for the deep-sea phosphorus-rich upwelling currents to enter the shallow water sedimentation area of western Hubei from the deep-water area of the fault basin. Conclusions The tectono-sedimentary filling process of the Middle-Upper Yangtze Transition Zone, Sinian period is under the control of depositional timing, fault basin activities, and the integrated action of the carbonate factory. The high-energy mound-shoal complexes at the platform margins, controlled by faults, along with the high-quality hydrocarbon source rocks within the rift basins, create favorable source-reservoir combinations, which is a new field deserving attention for exploration of hydrocarbon. Furthermore, the formation and filling processes of the basin create favorable tectonic-sedimentary environments for the source, migration, accumulation, and mineralization of sedimentary-type phosphorite deposits.
Objective The Sinian tectonic-sedimentary differentiation characteristics of the Middle-Upper Yangtze transition zone are similar to those of Mianyang-Changning area in central Sichuan. Clarifying the sedimentary system and sedimentary filling process in this area can provide guidance for the exploration of oil and gas and sedimentary minerals. In the transitional zone of the Middle-Upper Yangtze Block, the Sinian period exhibited a structural-sedimentary differentiation pattern akin to that observed in the Mianyang-Changning trough in central Sichuan. The sedimentary characteristics and filling processes provide essential insights for hydrocarbon exploration and the assessment of sedimentary mineral resources in this region. Methods The study focused on macroscopic sedimentary structures, depositional sequences, and microfacies in thin sections from outcrops and drilling data. Combined with the analysis of relevant tectonic settings and geodynamic causes, a detailed analysis of the development characteristics of sedimentary systems was conducted. On this basis, the vertical evolution and lateral comparison of depositional facies across the rift were dissected to reveal the changes in sedimentary systems and the structural-sedimentary filling processes during the development and evolution of the intra-continental rift basins of the Sinian period in the central Upper Yangtze transition zone. Results Influenced by regional transtensional tectonic activity, the rifting activity led to the formation of half-graben-style fault basin structures, with significant sedimentary differentiation in the carbonate gravity flows along the gentle slope margins of the rift and the deep-water continental shelf under a regional shallow marine sedimentation backdrop. During the Doushan period, a compensatory filling feature with overlapping centers of subsidence and sedimentation developed. In the Dengying period, structural-sedimentary filling characte-ristics of thickened shallow-water carbonate sedimentation and under-compensated deep-water sedimentation were observed. The deep-water sedimentation at the subsidence center, which developed during the Doushan period, possesses favorable hydrocarbon source rock conditions and potential for shale gas exploration； during the Dengying period, the high-energy beach bars at the edge of the prograding platform on the eastern side of the Upper Yangtze, controlled by faults. The high-energy beach bars at the edge of the prograding platform on the gentle slope of the Central Yangtze are high-quality facies belts for oil and gas reservoir development, and they form near-source hydrocarbon reservoir configurations with the hydrocarbon source rocks within the rift basin. The synsedimentary faults controlling the basin provide pathways for the upwelling of phosphorus-rich hydrothermal fluids. Concurrently, the western fault and eastern inclined fault basin structure provide favorable sedimentary topographic conditions for the deep-sea phosphorus-rich upwelling currents to enter the shallow water sedimentation area of western Hubei from the deep-water area of the fault basin. Conclusions The tectono-sedimentary filling process of the Middle-Upper Yangtze Transition Zone, Sinian period is under the control of depositional timing, fault basin activities, and the integrated action of the carbonate factory. The high-energy mound-shoal complexes at the platform margins, controlled by faults, along with the high-quality hydrocarbon source rocks within the rift basins, create favorable source-reservoir combinations, which is a new field deserving attention for exploration of hydrocarbon. Furthermore, the formation and filling processes of the basin create favorable tectonic-sedimentary environments for the source, migration, accumulation, and mineralization of sedimentary-type phosphorite deposits.
2024,
42(6):
2135-2143.
doi: 10.14027/j.issn.1000-0550.2024.065
Abstract:
Objective Several intraplatform deep-marine trough basins in the northern Yangtze Platform developed bedded chert in the Dalong Formation during the Late Permian. The organic-rich black rocks in the trough basins are shale gas exploration targets in the Sichuan Basin. The black chert deposition in the Dalong Formation started at 258.77 Ma, related to waning of the Emeishan mantle plume. However, its termination date and the cause of the disappearance of the intraplatform trough basins were still unclear. The results of this study help to better understand the evolution of these deep-marine troughs, offering a theoretical basis for potential assessment and exploration of Permian shale gas. Methods Zircon U-Pb LA-ICP-MS age dating and trace element measurement in the bentonite interbeds were carried out in the upper part of the Dalong Formation chert beds. Results The zircon grains are euhedral and show clear zoning, with Th/U ratios higher than 0.46. The zircon grains are enriched in heavy rare earth elements （HREE） with positive Ce anomalies and negative Eu anomalies, showing a rising trend from light REE to HREE in the chondrite-normalized REE patterns. The U-Pb LA-ICP-MS age of the zircon was 253.0±1.3 Ma. Diagrams of Th/Nb vs. Hf/Th and Th/U vs. Nb/Hf in the zircon show arc magma origin. Conclusions Chert deposition in the Dalong Formation in South China ceased in the mid-Changhsingian Stage of the Late Permian at 253.0±1.3 Ma. The arc volcanism eruption was not related to intraplate Emeishan mantle plume activity, but was caused by arc activity around the Yangtze Block, and was probably related to the disappearance of the deep-marine troughs in the northern Yangtze area.
Objective Several intraplatform deep-marine trough basins in the northern Yangtze Platform developed bedded chert in the Dalong Formation during the Late Permian. The organic-rich black rocks in the trough basins are shale gas exploration targets in the Sichuan Basin. The black chert deposition in the Dalong Formation started at 258.77 Ma, related to waning of the Emeishan mantle plume. However, its termination date and the cause of the disappearance of the intraplatform trough basins were still unclear. The results of this study help to better understand the evolution of these deep-marine troughs, offering a theoretical basis for potential assessment and exploration of Permian shale gas. Methods Zircon U-Pb LA-ICP-MS age dating and trace element measurement in the bentonite interbeds were carried out in the upper part of the Dalong Formation chert beds. Results The zircon grains are euhedral and show clear zoning, with Th/U ratios higher than 0.46. The zircon grains are enriched in heavy rare earth elements （HREE） with positive Ce anomalies and negative Eu anomalies, showing a rising trend from light REE to HREE in the chondrite-normalized REE patterns. The U-Pb LA-ICP-MS age of the zircon was 253.0±1.3 Ma. Diagrams of Th/Nb vs. Hf/Th and Th/U vs. Nb/Hf in the zircon show arc magma origin. Conclusions Chert deposition in the Dalong Formation in South China ceased in the mid-Changhsingian Stage of the Late Permian at 253.0±1.3 Ma. The arc volcanism eruption was not related to intraplate Emeishan mantle plume activity, but was caused by arc activity around the Yangtze Block, and was probably related to the disappearance of the deep-marine troughs in the northern Yangtze area.
2024,
42(6):
2144-2158.
doi: 10.14027/j.issn.1000-0550.2024.039
Abstract:
Objective This study examined the difference of sedimentary environment and tectonic evolution of the Wufeng Formation-Longmaxi Formation on the east and west sides of the Kangdian ancient land to support regional shale gas exploration and development and related research. Methods Based on a comprehensive geological survey and systematic mineralogical, petrological, and geochemical analysis, the provenance, paleoredox environment, and paleoclimate characteristics of black mudstone in the Wufeng Formation-Longmaxi Formation in the Zhaotong area on the east side of the ancient land and Yanyuan Basin on the west side were studied. Results The results show that the indicator parameters of terrigenous detrital such as Al2O3, TiO2, Sc, and Zr on the east side are more enriched than those on the west side, whereas the redox index parameters of MoEF, UEF, V/Cr, and U/Th are lower than those on the west side. There are clear differences in the sedimentary environment between the two sides. The eastern side is confined to marine shelf deposits, and the organic-rich shale is dominated by siliceous calcareous rocks. The west side is an open marine shelf, and the organic-rich shale is a siliceous rock series. Well XD2 on the east side is closer to the ancient land, the parent rock is mainly felsic igneous rock, the chemical weathering is stronger, the climate is warmer, and the sedimentary water is shallow and dominated by weak oxidation environment. The western side of the water body is relatively deep and dominated by anoxic reduction, with strong tectonic movement and more complex tectonic background, as well as active continental margin and island arc environmental properties, which may be related to the collision between the western ocean crust and Yangtze continental crust. Conclusions The formation of organic rich shale is the result of multiple factors together under different sedimentary evolution patterns. In comparison, the formation of organic rich shale on the western side of the ancient land is controlled by favorable preservation conditions of reducing water bodies, whereas the eastern side is controlled by extremely high paleoproductivity caused by the flourish of organisms in weakly oxidizing environments. Different sedimentary evolution patterns have created significant differences in the main controlling factors and distribution characteristics of organic rich shale formation on both sides of ancient land, which is of great significance for guiding regional shale gas exploration and development.
Objective This study examined the difference of sedimentary environment and tectonic evolution of the Wufeng Formation-Longmaxi Formation on the east and west sides of the Kangdian ancient land to support regional shale gas exploration and development and related research. Methods Based on a comprehensive geological survey and systematic mineralogical, petrological, and geochemical analysis, the provenance, paleoredox environment, and paleoclimate characteristics of black mudstone in the Wufeng Formation-Longmaxi Formation in the Zhaotong area on the east side of the ancient land and Yanyuan Basin on the west side were studied. Results The results show that the indicator parameters of terrigenous detrital such as Al2O3, TiO2, Sc, and Zr on the east side are more enriched than those on the west side, whereas the redox index parameters of MoEF, UEF, V/Cr, and U/Th are lower than those on the west side. There are clear differences in the sedimentary environment between the two sides. The eastern side is confined to marine shelf deposits, and the organic-rich shale is dominated by siliceous calcareous rocks. The west side is an open marine shelf, and the organic-rich shale is a siliceous rock series. Well XD2 on the east side is closer to the ancient land, the parent rock is mainly felsic igneous rock, the chemical weathering is stronger, the climate is warmer, and the sedimentary water is shallow and dominated by weak oxidation environment. The western side of the water body is relatively deep and dominated by anoxic reduction, with strong tectonic movement and more complex tectonic background, as well as active continental margin and island arc environmental properties, which may be related to the collision between the western ocean crust and Yangtze continental crust. Conclusions The formation of organic rich shale is the result of multiple factors together under different sedimentary evolution patterns. In comparison, the formation of organic rich shale on the western side of the ancient land is controlled by favorable preservation conditions of reducing water bodies, whereas the eastern side is controlled by extremely high paleoproductivity caused by the flourish of organisms in weakly oxidizing environments. Different sedimentary evolution patterns have created significant differences in the main controlling factors and distribution characteristics of organic rich shale formation on both sides of ancient land, which is of great significance for guiding regional shale gas exploration and development.
2024,
42(6):
2159-2173.
doi: 10.14027/j.issn.1000-0550.2023.041
Abstract:
Objective Deep-ultra-deep carbonate rocks have great potential in oil and gas exploration, and their reservoirs are characterized by strong heterogeneity and difficult prediction, which is closely related to their complex diagenetic alterations. Therefore, clarifying the beneficial diagenetic processes in reservoir development, that is, profitable diagenetic path and reservoir formation efficiency, is the key to revealing the genesis of deep carbonate reservoirs. Methods We have systematically compared the geochemical parameters, porosity, and permeability of the Sinian to lower Cambrian dolostone from the Sichuan Basin and Tarim Basin （including a small amount of data from the Oman Basin and Siberian Platform）,including carbon and oxygen isotopes （n=617）, strontium isotope （n=189）, rare earth elements （n=106）, fluid inclusion homogenization temperature （n=161）, cluster isotope temperature （n=117）, and porosity and permeability （n=1 557）. The geochemical feature templates of dolomite and late calcite of different origins in the two reservoirs were established. Results The results show that the geochemical characteristics of diagenetic minerals in the two reservoirs are similar, and both have undergone constructive diagenetic alterations such as dolomitization, meteoric water, fracturing, and hydrothermal fluids. Compared with the lower Cambrian dolostone reservoirs, the highest-quality reservoirs in the area （Dengying Formation） have experienced stronger seepage-reflux dolomitization, particularly the more intense hydrothermal alternations, suggesting that differential effective diagenetic alterations are the key to good carbonate reservoir formation. Conclusions Based on the above characteristics, the development of Sinian-Cambrian high-quality dolostone reservoirs in the Sichuan Basin and Tarim Basin is jointly controlled by the development of evaporite facies in the depositional period and the development of late structural hydrothermal fluids. The above findings can be used to predict deep-ultra-deep carbonate reservoirs, and have enlightenment significance for deep-ultra-deep oil and gas exploration.
Objective Deep-ultra-deep carbonate rocks have great potential in oil and gas exploration, and their reservoirs are characterized by strong heterogeneity and difficult prediction, which is closely related to their complex diagenetic alterations. Therefore, clarifying the beneficial diagenetic processes in reservoir development, that is, profitable diagenetic path and reservoir formation efficiency, is the key to revealing the genesis of deep carbonate reservoirs. Methods We have systematically compared the geochemical parameters, porosity, and permeability of the Sinian to lower Cambrian dolostone from the Sichuan Basin and Tarim Basin （including a small amount of data from the Oman Basin and Siberian Platform）,including carbon and oxygen isotopes （n=617）, strontium isotope （n=189）, rare earth elements （n=106）, fluid inclusion homogenization temperature （n=161）, cluster isotope temperature （n=117）, and porosity and permeability （n=1 557）. The geochemical feature templates of dolomite and late calcite of different origins in the two reservoirs were established. Results The results show that the geochemical characteristics of diagenetic minerals in the two reservoirs are similar, and both have undergone constructive diagenetic alterations such as dolomitization, meteoric water, fracturing, and hydrothermal fluids. Compared with the lower Cambrian dolostone reservoirs, the highest-quality reservoirs in the area （Dengying Formation） have experienced stronger seepage-reflux dolomitization, particularly the more intense hydrothermal alternations, suggesting that differential effective diagenetic alterations are the key to good carbonate reservoir formation. Conclusions Based on the above characteristics, the development of Sinian-Cambrian high-quality dolostone reservoirs in the Sichuan Basin and Tarim Basin is jointly controlled by the development of evaporite facies in the depositional period and the development of late structural hydrothermal fluids. The above findings can be used to predict deep-ultra-deep carbonate reservoirs, and have enlightenment significance for deep-ultra-deep oil and gas exploration.
2024,
42(6):
2174-2190.
doi: 10.14027/j.issn.1000-0550.2023.019
Abstract:
Objective The carbonate rocks from the Sinian Dengying Formation in the northern Sichuan Basin experienced multi-stage fluid alteration, which has exerted a significant influence on the quality of the reservoir. Methods Laser in-situ carbon and oxygen isotopes, micro-area strontium isotopes, petrography, and cathodoluminescence were used to systematically analyze the petrology and diagenetic evolution of dolomite in the Dengying Formation. Based on this research, we clarified the diagenesis process of different diagenetic fluids and its influence on reservoir formation. Results The results indicate that： （1） The dolomite can be divided into three different lithofacies： microbial, granular, and crystalline. Growth-framework porosity of carbonates is the fundamental factor for developing high-quality reservoirs； therefore, laminated, stromatolite, and clotted dolomites with a large number of primary pores can be regarded as high-quality reservoir rocks. （2） Four diagenetic environments associated with the development of high-quality reservoirs were identified in the 4th member of the Dengying Formation, including evaporated seawater, shallow buried “trapped” seawater, medium-deep buried organic acid migration, and hydrothermal environments. Isopachous fibrous and blade cements precipitate in the evaporated seawater environment, and the cements have no cathodoluminescence. Their δ13C, δ18O, and 87Sr/86Sr values agree with the Sinian seawater. Fine-crystalline cements with moderate orange-red cathodoluminescence are precipitated in shallow buried “trapped” seawater environments, the δ13C, δ18O, and 87Sr/86Sr compositions of the cements are similar to the Sinian seawater, and samples have slightly lower δ18O than that of Sinian seawater. The medium-crystalline cements precipitate in the medium-deep buried organic acid migration environment, while the cements feature strong orange-red cathodoluminescence and a relatively more negatively biased δ18O. The 87Sr/86Sr value of the cements is sufficiently close to the Cambrian, indicating that the cement provenance may come from the Cambrian. Furthermore, the dissolution of buried organic acid exerts a positive effect on the reservoir evolution. Hydrothermal minerals such as saddle dolomite and quartz precipitate in the hydrothermal environment. Saddle dolomite is characterized by bright red cathodoluminescence, significant negative δ18O anomalies, and significantly high values of 87Sr/86Sr. With the influence of hydrothermal fluid, a large number of dissolution pores are formed, which greatly improve the property of the reservoir. （3） The development of high-quality reservoirs in the 4th member of the Dengying Formation in northern Sichuan was controlled and restricted by sedimentary environment, diagenetic evolution, and tectonic movement. Various sedimentary facies have laid a solid foundation for the reservoir development. The platform margin and interior mound-shoals are the predominant facies for the development of the reservoir. During their deposition, they feature powerful hydrodynamics and develop various combinations of microbial dolomite, forming in a significant number of primary pores. Despite being filled by various cements, they still serve as relatively favorable parts for pore penetration in the stratum, which has furnished a foundation for the modification of diagenesis in the later stage. Complicated diagenesis serves as a critical factor for the formation of high-quality reservoirs. Multi-stage cementation has caused the primary pores of the reservoir to basically disappear, whereas the secondary pores formed by buried organic acid dissolution and hydrothermal dissolution has turned into the primary spaces of the reservoir. Sichuan Basin has experienced multi-cycle tectonic movements, and multi-stage fractures have formed in the Dengying Formation, which have provided favorable conditions for hydrothermal activities. Among these, the favorable sedimentary facies belt is the fundamental character, multi-stage fluid dissolution is the key factor, and the faults formed by tectonic movement provide favorable the deep fluid fluxion. Conclusions The study of the identification and quantitative reformation degree of high-quality reservoir in the 4th member of the Dengying Formation provides a theoretical basis for sedimentary-diagenesis for predicting oil and gas exploration targets in this area.
Objective The carbonate rocks from the Sinian Dengying Formation in the northern Sichuan Basin experienced multi-stage fluid alteration, which has exerted a significant influence on the quality of the reservoir. Methods Laser in-situ carbon and oxygen isotopes, micro-area strontium isotopes, petrography, and cathodoluminescence were used to systematically analyze the petrology and diagenetic evolution of dolomite in the Dengying Formation. Based on this research, we clarified the diagenesis process of different diagenetic fluids and its influence on reservoir formation. Results The results indicate that： （1） The dolomite can be divided into three different lithofacies： microbial, granular, and crystalline. Growth-framework porosity of carbonates is the fundamental factor for developing high-quality reservoirs； therefore, laminated, stromatolite, and clotted dolomites with a large number of primary pores can be regarded as high-quality reservoir rocks. （2） Four diagenetic environments associated with the development of high-quality reservoirs were identified in the 4th member of the Dengying Formation, including evaporated seawater, shallow buried “trapped” seawater, medium-deep buried organic acid migration, and hydrothermal environments. Isopachous fibrous and blade cements precipitate in the evaporated seawater environment, and the cements have no cathodoluminescence. Their δ13C, δ18O, and 87Sr/86Sr values agree with the Sinian seawater. Fine-crystalline cements with moderate orange-red cathodoluminescence are precipitated in shallow buried “trapped” seawater environments, the δ13C, δ18O, and 87Sr/86Sr compositions of the cements are similar to the Sinian seawater, and samples have slightly lower δ18O than that of Sinian seawater. The medium-crystalline cements precipitate in the medium-deep buried organic acid migration environment, while the cements feature strong orange-red cathodoluminescence and a relatively more negatively biased δ18O. The 87Sr/86Sr value of the cements is sufficiently close to the Cambrian, indicating that the cement provenance may come from the Cambrian. Furthermore, the dissolution of buried organic acid exerts a positive effect on the reservoir evolution. Hydrothermal minerals such as saddle dolomite and quartz precipitate in the hydrothermal environment. Saddle dolomite is characterized by bright red cathodoluminescence, significant negative δ18O anomalies, and significantly high values of 87Sr/86Sr. With the influence of hydrothermal fluid, a large number of dissolution pores are formed, which greatly improve the property of the reservoir. （3） The development of high-quality reservoirs in the 4th member of the Dengying Formation in northern Sichuan was controlled and restricted by sedimentary environment, diagenetic evolution, and tectonic movement. Various sedimentary facies have laid a solid foundation for the reservoir development. The platform margin and interior mound-shoals are the predominant facies for the development of the reservoir. During their deposition, they feature powerful hydrodynamics and develop various combinations of microbial dolomite, forming in a significant number of primary pores. Despite being filled by various cements, they still serve as relatively favorable parts for pore penetration in the stratum, which has furnished a foundation for the modification of diagenesis in the later stage. Complicated diagenesis serves as a critical factor for the formation of high-quality reservoirs. Multi-stage cementation has caused the primary pores of the reservoir to basically disappear, whereas the secondary pores formed by buried organic acid dissolution and hydrothermal dissolution has turned into the primary spaces of the reservoir. Sichuan Basin has experienced multi-cycle tectonic movements, and multi-stage fractures have formed in the Dengying Formation, which have provided favorable conditions for hydrothermal activities. Among these, the favorable sedimentary facies belt is the fundamental character, multi-stage fluid dissolution is the key factor, and the faults formed by tectonic movement provide favorable the deep fluid fluxion. Conclusions The study of the identification and quantitative reformation degree of high-quality reservoir in the 4th member of the Dengying Formation provides a theoretical basis for sedimentary-diagenesis for predicting oil and gas exploration targets in this area.
2024,
42(6):
2191-2203.
doi: 10.14027/j.issn.1000-0550.2023.026
Abstract:
Objective In the Lower Yangtze area, thick Early Paleozoic carbonates are potential reservoirs for oil, gas and geothermal resources. In 2021, well Sure1, drilled by the Geological Survey of Jiangsu province in the Subei Basin, successfully detected high-temperature hot-dry rock geothermal resources in the Early Paleozoic carbonates, bringing the Early Paleozoic carbonate sedimentary microfacies and environmental evolution widespread academic attention. Methods A detailed microfacies analysis was conducted on well Sure 1 in Subei Basin and three field outcrops in the Ningzhen Mountains to reconstruct the sedimentary environmental evolution of the Early Paleozoic carbonates in the Lower Yangtze area and provide key basic geological information for the exploration of hot-dry rock geothermal resources in the study area. Results Thirteen microfacies, corresponding to peritidal and shoal environments in an inner carbonate platform setting with no significant change in paleowater depth, were identified by integrating sedimentological and paleontological observations. The Cambrian Mufushan Formation and Guanyintai Formation and Early Ordovician Lunshan Formation are mainly silty dolomites with micrite （MF1）, silty dolomite （MF2）, laminated silty dolomite （MF3）, scarce fenestrae bonded dolomite （MF4）, fine crystalline dolomite （MF5）, fine crystalline dolomite with micrite （MF7）. and breccia dolomite （MF8）. These strata lack fossils, but laminations, birdeyes, and fenestral structures indicate upper intertidal to lower supratidal environments. The Early Ordovician Honghuayuan Formation is mainly dolomitized peloidal limestone （MF6）, intraclastic dolomite （MF9）, dolomitized oolitic limestone （MF10）, oolitic dolomite （MF11）, intraclastic limestone （MF12）, and oolitic limestone with echinoderm fragments （MF13）, indicating a high-energy shoal environment. Conclusions The Cambrian Mufushan Formation, Paotaishan Formation, and Guanyintai Formation were dominated by an intertidal environment, while the lower part of the Early Ordovician Lunshan Formation and the Honghuayuan Formation were dominated by a shoal environment, and the ancient water depth was relatively deeper. The Cambrian Guanyintai Formation in the Lower Yangtze area is a favorable target for future exploration of hot and dry rock reservoirs.
Objective In the Lower Yangtze area, thick Early Paleozoic carbonates are potential reservoirs for oil, gas and geothermal resources. In 2021, well Sure1, drilled by the Geological Survey of Jiangsu province in the Subei Basin, successfully detected high-temperature hot-dry rock geothermal resources in the Early Paleozoic carbonates, bringing the Early Paleozoic carbonate sedimentary microfacies and environmental evolution widespread academic attention. Methods A detailed microfacies analysis was conducted on well Sure 1 in Subei Basin and three field outcrops in the Ningzhen Mountains to reconstruct the sedimentary environmental evolution of the Early Paleozoic carbonates in the Lower Yangtze area and provide key basic geological information for the exploration of hot-dry rock geothermal resources in the study area. Results Thirteen microfacies, corresponding to peritidal and shoal environments in an inner carbonate platform setting with no significant change in paleowater depth, were identified by integrating sedimentological and paleontological observations. The Cambrian Mufushan Formation and Guanyintai Formation and Early Ordovician Lunshan Formation are mainly silty dolomites with micrite （MF1）, silty dolomite （MF2）, laminated silty dolomite （MF3）, scarce fenestrae bonded dolomite （MF4）, fine crystalline dolomite （MF5）, fine crystalline dolomite with micrite （MF7）. and breccia dolomite （MF8）. These strata lack fossils, but laminations, birdeyes, and fenestral structures indicate upper intertidal to lower supratidal environments. The Early Ordovician Honghuayuan Formation is mainly dolomitized peloidal limestone （MF6）, intraclastic dolomite （MF9）, dolomitized oolitic limestone （MF10）, oolitic dolomite （MF11）, intraclastic limestone （MF12）, and oolitic limestone with echinoderm fragments （MF13）, indicating a high-energy shoal environment. Conclusions The Cambrian Mufushan Formation, Paotaishan Formation, and Guanyintai Formation were dominated by an intertidal environment, while the lower part of the Early Ordovician Lunshan Formation and the Honghuayuan Formation were dominated by a shoal environment, and the ancient water depth was relatively deeper. The Cambrian Guanyintai Formation in the Lower Yangtze area is a favorable target for future exploration of hot and dry rock reservoirs.
