The Weber Deep represents a widespread crustal extension system in eastern Indonesia with a huge submarine normal fault system,the Banda Detachment,related to the slab rollback tectonic model(Spakman and Hall,2010;Cum...The Weber Deep represents a widespread crustal extension system in eastern Indonesia with a huge submarine normal fault system,the Banda Detachment,related to the slab rollback tectonic model(Spakman and Hall,2010;Cummins et al.,2020).However,the cause of tectonic extension remains debated(e.g.,Audley-Charles et al.,1972;McCaffrey,1988;Das,2004;Spakman and Hall,2010;Pownall et al.,2013;Cummins et al.,2020;Hutchings and Mooney,2021;Shah et al.,2023).展开更多
Geochemical studies of crude oil and source rock play an important role in future exploration in Zhanhua Depression.In this study,thirty-one oil samples collected from Shahejie Formation in Zhanhua Depression,Bohai Ba...Geochemical studies of crude oil and source rock play an important role in future exploration in Zhanhua Depression.In this study,thirty-one oil samples collected from Shahejie Formation in Zhanhua Depression,Bohai Bay Basin,NE China have been geochemically analyzed and their organic geochemical characteristics have been applied to differentiate groups of oils.These oil samples can be classified into two families based on multiple biomarker proxies and stable carbon isotopic values.FamilyⅠis characterized by a low ratio of pristane over phytane(Pr/Ph<0.7),a relatively high ratio of phytane over n-C18(Ph/n-C18),varying ratios of gammacerane over C30 hopane(Ga/C30H)and C22/C21 tricyclic terpane,and a low ratio of C19/C23 tricyclic terpane.FamilyⅡis marked by a relatively high Pr/Ph ratio(0.7-1.6),relative low ratios of Ph/n-C18 and C22/C21 tricyclic terpane,and avarying ratio of C19/C23 tricyclic terpane.Both familiesⅠandⅡwithin these crude oils can be subdivided into two families based on different values of stable carbon isotopic composition of individual n-alkanes.Moreover,the potential source rocks of oil samples in FamilyⅠand FamilyⅡwere likely derived from the upper Es4 member and Es3 member,respectively,based on the correlation of organic geochemical characteristics of the oils and source rocks.The results of oil-source rock correlation provide insight into the process from oil generation to migration and to final accumulation,providing a better understanding of factors controlling oil-gas distribution for prediction of sweet spots.展开更多
Dehydration melting of metasupracrustal rocks at mid-to deep-crustal levels can generate water undersaturated granitic melt.In this study,we evaluate the potential of~1.89–1.88 Ga metasupracrustal rocks of the Precam...Dehydration melting of metasupracrustal rocks at mid-to deep-crustal levels can generate water undersaturated granitic melt.In this study,we evaluate the potential of~1.89–1.88 Ga metasupracrustal rocks of the Precambrian of southern Finland as source rocks for the 1.86–1.79 Ga late-orogenic leucogranites in the region,using the Rhyolite-MELTS approach.Melt close in composition to leucogranite is produced over a range of realistic pressures(5 to 8 kbar)and temperatures(800 to 850℃),at 20%–30%of partial melting,allowing separation of melt from unmelted residue.The solid residue is a dry,enderbitic to charnoenderbitic ganulite depleted in incompatible components,and will only yield further melt above 1000–1050℃,when rapidly increasing fractions of increasingly calcic(granodioritic to tonalitic)melts are formed.The solid residue after melt extraction is incapable of producing syenogranitic magmas similar to the Mid-Proterozoic,A-type rapakivi granites on further heating.The granitic fraction of the syenogranitic rapakivi complexes must thus have been formed by a different chain of processes,involving mantle-derived mafic melts and melts from crustal rock types not conditioned by the preceding late-orogenic Svecofennian anatexis.展开更多
In the design of building structures,joint efforts must be decided to resolve the depth of competent layers across the intended site to safeguard the durability of civil engineering structures and to avert the disastr...In the design of building structures,joint efforts must be decided to resolve the depth of competent layers across the intended site to safeguard the durability of civil engineering structures and to avert the disastrous consequences of structural failure and collapse.In this study,an integrated methodology that employed DC resistivity tomography involving 2-D and 3-D techniques and geotechnical-soil analysis was used to evaluate subsoil conditions for engineering site investigation at Okerenkoko primary school,in the Warri-southwest area of Delta State,to adduce the phenomena responsible for the visible cracks/structural failure observed in the buildings.The results obtained brought to light the geological structure beneath the subsurface,which consists of four geoelectric layers identified as topsoil,dry/lithified upper sandy layer,wet sand(water-saturated)and peat/clay/sandy clayey soil(highly water-saturated).The deeply-seated peat/clay materials(ρ≤20Ωm)were delineated in the study area to the depths of 17.1 m and 19.8 m from 2-D and 3-D tomography respectively.3-D images presented as horizontal depth slices revealed the dominance of very low resistivity materials i.e.peat/clay/sandy clay within the fourth,fifth and sixth layers at depths ranging from 8.68-12.5 m,12.5-16.9 m and 16.9-21.9 m respectively.The dominance of mechanically unstable peat/clay/sandy clay layers beneath the subsurface,which are highly mobile in response to volumetric changes,is responsible for the noticeable cracks/failure detected on structures within the study site.These observations were validated by a geotechnical test of soil samples in the study area.Atterberg’s limits of the samples revealed plasticity indices of zero.Thus,the soil samples within the depth analyzed were representatives of sandy soil that does not possess any plasticity.The methods justifiably provided relevant information on the subsurface geology beneath the study site and should be appropriated as major tools for engineering site assessment/geotechnical projects.展开更多
The study presented in this manuscript aimed to relate the sedimentary strata imaged by the ground penetrating radar(GPR)method through numerical modeling with the mapping of sedimentary strata acquired through geotec...The study presented in this manuscript aimed to relate the sedimentary strata imaged by the ground penetrating radar(GPR)method through numerical modeling with the mapping of sedimentary strata acquired through geotechnical surveys.The study aimed to expose how obtaining subsoil information through noninvasive/destructive electromagnetic waves is beneficial,as they are reliable and less costly than drilling holes beyond what is necessary to have a subsurface mapping.In this sense,physical-geological modeling was carried out.The information on the type of sediments,acquired through simple recognition surveys carried out in the city of Belém-PA,helped to create a model of a sedimentary package with its respective intrinsic physical properties.The result shows that the GPR recovered with good vertical and horizontal resolution at the beginning and end of the layers of the sedimentary package studied,proving to be very effective for locating geotechnical sounding points and safely reducing costs.展开更多
To characterize mid-ocean microseisms in the Arctic Ocean and explore potential seismic ambient noise imprint of Arctic warming,this study analyzes seismic records from nine ocean bottom seismometers(OBSs)deployed alo...To characterize mid-ocean microseisms in the Arctic Ocean and explore potential seismic ambient noise imprint of Arctic warming,this study analyzes seismic records from nine ocean bottom seismometers(OBSs)deployed along the eastern Gakkel Ridge during the Joint Arctic Scientific Middle-ocean ridge Insight Expedition(JASMIn E)in August 2021.In the period band of single frequency microseisms(10–20 s),typically produced by ocean waves directly impacting coastlines,no prominent spectral peaks are observed.In the double frequency microseism(DFM)period band(2–10 s),spectral powers are far less energetic than those in the open oceans by approximately 20–40 dB,especially for OBSs deployed off the Gakkel Ridge axis.This dramatically weak DFMs can be attributed to the presence of the perennial sea-ice cover,which hinders atmosphere-ocean interactions and thus obstructs the generation of DFMs.Based on polarization analyses of Rayleigh waves and correlations of DFM power and ocean wave height,the weak DFMs recorded on the seafloor likely originate from the northern Barents Sea and adjacent regions.As an Arctic warming hotspot,the northern Barents Sea is experiencing reduced sea-ice import from the interior Arctic which enhances atmosphere-ocean interactions.In this region,wind-driven waves with highly variable directions as documented by oceanographic data and the presence of the perennial sea-ice cover may promote the formation of wave trains propagating in nearly opposite directions,which nonlinear interactions excite DFMs.These DFMs continuously lose power due to seismic attenuation during propagation,becoming so weak that the OBSs deployed off the ridge axis detect no noticeable spectral peaks in the DFM period band.The OBSs along the ridge axis,by contrast,reveal more energetic power in the shortperiod DFM band of 2–5 s,which can be attributed to local DFM amplification caused by the thick unconsolidated sediment layer.展开更多
Two cases of auroral substorms have been studied with the Polar UVI data, which were associated with solar wind pressure shock arriving at the Earth. The global aurora activities started about 1-2 min after pressure s...Two cases of auroral substorms have been studied with the Polar UVI data, which were associated with solar wind pressure shock arriving at the Earth. The global aurora activities started about 1-2 min after pressure shocks arrived at dayside magnetopause, then nightside auroras intensified rapidly 3-4 min later, with auroral sub-storm onset. The observations in synchronous orbit indicated that the compressing effects on magnetosphere were observed in their corresponding sites about 2 min after the pressure shocks impulse magnetopause. We propose that the auroral intensification and substorm onset possibly result from hydromagnetic wave produced by the pressure shock. The fast-mode wave propagates across the magnetotail lobes with higher local Alfven velocity, magnetotail was compressed rapidly and strong lobe field and cross-tail current were built in about 1-2 min, and furthermore the substorm was triggered due to an instability in current sheet.展开更多
Sedimentological and stratigraphic studies of seven stratigraphic sections of Permian Hongyanchi （HYC） and Quanzijie （QZJ） low-order cycles （LCs） in the Tarlong-Taodonggou half graben and Dalongkou area in Bogda...Sedimentological and stratigraphic studies of seven stratigraphic sections of Permian Hongyanchi （HYC） and Quanzijie （QZJ） low-order cycles （LCs） in the Tarlong-Taodonggou half graben and Dalongkou area in Bogda Mountains, NW China, demonstrate effective approaches and methodology in cyclo- and time-stratigraphic analyses of complex fluviallacustrine deposits in an intracontinental rift setting. A new synchronous stratigraphic unit, the lower QZJ LC is defined. The lower and upper boundaries of this cycle include a regionally correlative disconformity, erosional unconformity, and conformity, across which significant and abrupt changes in palaeoenvironments and tectonic and climatic conditions occurred. The lower boundary is an erosional unconformity and disconformity with a high-relief topography that juxtaposes lacustrine deposits of the underlying HYC LC with the overlying meandering stream deposits of the lower QZJ LC, and was caused by a regional tectonic uplift. The up- per boundary is a disconformity and local erosional unconformity and conformity, juxtaposing stacked paleosols developed on fluvial sediments with overlying fluvial and loessial deposits of the upper QZJ LC. The paleosols indicate landscape stability and a prolonged period of subaerial exposure and minimal deposition and suggest that climatic conditions were semiarid with strong precipitation seasonality in the Tarlong-Taodonggou half graben and subhumid in the Dalongkou area. The fluvial-loessial deposits indicate a renewed tectonic uplift and a change in the atmospheric circulation pattern. The newly-defined lower QZJ LC facilitates accurate palaeogeographic reconstruction in the study area during a period of major tectonic and climatic changes. The interpreted tectonic and climatic conditions provide a critical data point in the mid-latitude east coast of NE Pangea during the Mid-Permian icehouse-hothouse transition. The results demonstrate that a process-response approach is effective in time- stratigraphic analysis of complex fluvial-lacustrine strata in a highly-partitioned rift basin.展开更多
This study investigates the provenance of sedimentary rocks in Bogda Mountains, NW China, and reconstructs the lithology and unroofing history of the Eastern North Tianshan Suture. Petrographic point counting data of ...This study investigates the provenance of sedimentary rocks in Bogda Mountains, NW China, and reconstructs the lithology and unroofing history of the Eastern North Tianshan Suture. Petrographic point counting data of sandstones and compositions of conglomerates of upper Permian-lowermost Triassic Wutonggou low-order cycle from Zhaobishan, North Tarlong, Taodonggou, and Dalongkou sections in the southern and northern foothills of Bogda Mountains were used to interpret the temporal and spatial variations of lithology of the Eastern North Tianshan Suture, which is the sediment source area. Three compositional trends were identified. A trend of upwardincreasing quartz content and granitic pebbles in Zhaobishan section suggests a change from the undissected volcanic arc, accretionary wedge and trench setting to predominantly transitional volcanic arc and subordinate accretionary wedge and trench, in the eastern part of the Eastern North Tianshan Suture. In North Tarlong and Taodonggou sections, however, the lithic content decreases and the contents of quartz and granitic pebbles increase up sections. These trends indicate that the western part of the Eastern North Tianshan Suture changed from an undissected volcanic arc to the transitional volcanic arc, accretionary wedge and trench. No clear trend in the lithic-rich sandstones of the Dalongkou section indicates that sediments were derived from the undissected volcanic arc in the Eastern North Tianshan Suture and local rift shoulders. Compositional variations of studied rocks suggest that the Eastern North Tianshan Suture was an amalgamated complex with great spatial and temporal heterogeneities in lithology and experienced persistent unroofing during late Permian-earliest Triassic. This study reconstructs a key element of the Chinese Tianshan Suture and serves as an example to understand the unroofing processes of ancient sutures.展开更多
Eustasy has commonly been invoked to explain peritidal carbonate cyclicity,but is difficult to explain cycles formed in a greenhouse climate when eustasy is minimal.We propose that peritidal cycles on an Early Triassi...Eustasy has commonly been invoked to explain peritidal carbonate cyclicity,but is difficult to explain cycles formed in a greenhouse climate when eustasy is minimal.We propose that peritidal cycles on an Early Triassic isolated carbonate platform in Guizhou,South China,were formed by hierarchical carbonate productivity variations.Most of the 149 shallowing-upward cycles are typically terminated by flooding over intertidal facies and contain rare supratidal facies and no prolonged subaerial exposure.Low-diversity benthos in the platform interior during the post-end-Permian biotic recovery were sensitive to environmental perturbations,which caused variations in benthic sediment productivity in the subtidal carbonate factory.The perturbations may be driven by changes in salinity and degree of eutrophication,or repeated platform mini-drowning by anoxic and/or CO2-charged deep water upwelled onto the banktop.They were modulated by Milankovitch orbitally-driven climatic and oceanographic factors as suggested by the hierarchical stacking pattern and spectral signals of these cycles.A one-dimensional conceptual model shows that hierarchical productivity variations alone may generate hierarchical peritidal carbonate cycles under conditions of constant subsidence and no sea-level fluctuation.展开更多
文摘The Weber Deep represents a widespread crustal extension system in eastern Indonesia with a huge submarine normal fault system,the Banda Detachment,related to the slab rollback tectonic model(Spakman and Hall,2010;Cummins et al.,2020).However,the cause of tectonic extension remains debated(e.g.,Audley-Charles et al.,1972;McCaffrey,1988;Das,2004;Spakman and Hall,2010;Pownall et al.,2013;Cummins et al.,2020;Hutchings and Mooney,2021;Shah et al.,2023).
基金financially supported by the Chinese NSF Grants[41903064]to Hong Lu。
文摘Geochemical studies of crude oil and source rock play an important role in future exploration in Zhanhua Depression.In this study,thirty-one oil samples collected from Shahejie Formation in Zhanhua Depression,Bohai Bay Basin,NE China have been geochemically analyzed and their organic geochemical characteristics have been applied to differentiate groups of oils.These oil samples can be classified into two families based on multiple biomarker proxies and stable carbon isotopic values.FamilyⅠis characterized by a low ratio of pristane over phytane(Pr/Ph<0.7),a relatively high ratio of phytane over n-C18(Ph/n-C18),varying ratios of gammacerane over C30 hopane(Ga/C30H)and C22/C21 tricyclic terpane,and a low ratio of C19/C23 tricyclic terpane.FamilyⅡis marked by a relatively high Pr/Ph ratio(0.7-1.6),relative low ratios of Ph/n-C18 and C22/C21 tricyclic terpane,and avarying ratio of C19/C23 tricyclic terpane.Both familiesⅠandⅡwithin these crude oils can be subdivided into two families based on different values of stable carbon isotopic composition of individual n-alkanes.Moreover,the potential source rocks of oil samples in FamilyⅠand FamilyⅡwere likely derived from the upper Es4 member and Es3 member,respectively,based on the correlation of organic geochemical characteristics of the oils and source rocks.The results of oil-source rock correlation provide insight into the process from oil generation to migration and to final accumulation,providing a better understanding of factors controlling oil-gas distribution for prediction of sweet spots.
文摘Dehydration melting of metasupracrustal rocks at mid-to deep-crustal levels can generate water undersaturated granitic melt.In this study,we evaluate the potential of~1.89–1.88 Ga metasupracrustal rocks of the Precambrian of southern Finland as source rocks for the 1.86–1.79 Ga late-orogenic leucogranites in the region,using the Rhyolite-MELTS approach.Melt close in composition to leucogranite is produced over a range of realistic pressures(5 to 8 kbar)and temperatures(800 to 850℃),at 20%–30%of partial melting,allowing separation of melt from unmelted residue.The solid residue is a dry,enderbitic to charnoenderbitic ganulite depleted in incompatible components,and will only yield further melt above 1000–1050℃,when rapidly increasing fractions of increasingly calcic(granodioritic to tonalitic)melts are formed.The solid residue after melt extraction is incapable of producing syenogranitic magmas similar to the Mid-Proterozoic,A-type rapakivi granites on further heating.The granitic fraction of the syenogranitic rapakivi complexes must thus have been formed by a different chain of processes,involving mantle-derived mafic melts and melts from crustal rock types not conditioned by the preceding late-orogenic Svecofennian anatexis.
文摘In the design of building structures,joint efforts must be decided to resolve the depth of competent layers across the intended site to safeguard the durability of civil engineering structures and to avert the disastrous consequences of structural failure and collapse.In this study,an integrated methodology that employed DC resistivity tomography involving 2-D and 3-D techniques and geotechnical-soil analysis was used to evaluate subsoil conditions for engineering site investigation at Okerenkoko primary school,in the Warri-southwest area of Delta State,to adduce the phenomena responsible for the visible cracks/structural failure observed in the buildings.The results obtained brought to light the geological structure beneath the subsurface,which consists of four geoelectric layers identified as topsoil,dry/lithified upper sandy layer,wet sand(water-saturated)and peat/clay/sandy clayey soil(highly water-saturated).The deeply-seated peat/clay materials(ρ≤20Ωm)were delineated in the study area to the depths of 17.1 m and 19.8 m from 2-D and 3-D tomography respectively.3-D images presented as horizontal depth slices revealed the dominance of very low resistivity materials i.e.peat/clay/sandy clay within the fourth,fifth and sixth layers at depths ranging from 8.68-12.5 m,12.5-16.9 m and 16.9-21.9 m respectively.The dominance of mechanically unstable peat/clay/sandy clay layers beneath the subsurface,which are highly mobile in response to volumetric changes,is responsible for the noticeable cracks/failure detected on structures within the study site.These observations were validated by a geotechnical test of soil samples in the study area.Atterberg’s limits of the samples revealed plasticity indices of zero.Thus,the soil samples within the depth analyzed were representatives of sandy soil that does not possess any plasticity.The methods justifiably provided relevant information on the subsurface geology beneath the study site and should be appropriated as major tools for engineering site assessment/geotechnical projects.
文摘The study presented in this manuscript aimed to relate the sedimentary strata imaged by the ground penetrating radar(GPR)method through numerical modeling with the mapping of sedimentary strata acquired through geotechnical surveys.The study aimed to expose how obtaining subsoil information through noninvasive/destructive electromagnetic waves is beneficial,as they are reliable and less costly than drilling holes beyond what is necessary to have a subsurface mapping.In this sense,physical-geological modeling was carried out.The information on the type of sediments,acquired through simple recognition surveys carried out in the city of Belém-PA,helped to create a model of a sedimentary package with its respective intrinsic physical properties.The result shows that the GPR recovered with good vertical and horizontal resolution at the beginning and end of the layers of the sedimentary package studied,proving to be very effective for locating geotechnical sounding points and safely reducing costs.
文摘为刻画北冰洋中部的地脉动特征,并探究北极变暖对地震背景噪声的潜在影响,本文分析了2021年8月北冰洋洋中脊国际联合考察计划在加克洋中脊东段布设的9台海底地震仪(ocean bottom seismometer,OBS)的记录.在通常由海浪拍打海岸产生的单频地脉动频段内(10~20s),并未观测到显著的能量波峰.在双频地脉动(double frequency microseism,DFM)的频段内(2~10s),其功率谱密度比其他开阔大洋低20~40dB,尤其是在加克洋中脊轴线以外布设的OBS,其功率谱更为微弱.这种极弱的DFM可归因于永久性冰盖的存在,它阻碍了大气与海洋之间的相互作用,从而抑制了DFM的产生.基于对组成地脉动的瑞利波的极化分析,以及DFM的功率谱与海浪波高之间的相关性结果,海底记录到的微弱DFM应产生于北巴伦支海及其周边海域.作为北极变暖的热点区域,北巴伦支海不断减少的海冰进口量加剧了该区的海气相互作用.海洋学数据显示,此处风驱海浪的方向存在高度变化性,加之存在的永久性冰盖,两者可能促使相反传播的波列形成,从而发生海浪非线性相互作用,激发DFM.由于传播过程中的能量衰减效应,DFM能量持续减弱,导致离轴布设的OBS无法观测到明显的能量峰值.相比之下,沿轴布设的OBS在2~5s的短周期DFM频段内观测到更强的功率谱,这与洋中脊轴内较厚的松散沉积层造成的DFM能量放大效应有关.
基金supported by the National Natural Science Foundation of China(Grant Nos.42330308,42406069)the Scientific Research Fund of the Second Institute of Oceanography,Ministry of Natural Resources(Grant Nos.QNRC2201,JB2401)the Zhejiang Provincial Natural Science Foundation(Grant Nos.LR24D060001,LZ23D060004)。
文摘To characterize mid-ocean microseisms in the Arctic Ocean and explore potential seismic ambient noise imprint of Arctic warming,this study analyzes seismic records from nine ocean bottom seismometers(OBSs)deployed along the eastern Gakkel Ridge during the Joint Arctic Scientific Middle-ocean ridge Insight Expedition(JASMIn E)in August 2021.In the period band of single frequency microseisms(10–20 s),typically produced by ocean waves directly impacting coastlines,no prominent spectral peaks are observed.In the double frequency microseism(DFM)period band(2–10 s),spectral powers are far less energetic than those in the open oceans by approximately 20–40 dB,especially for OBSs deployed off the Gakkel Ridge axis.This dramatically weak DFMs can be attributed to the presence of the perennial sea-ice cover,which hinders atmosphere-ocean interactions and thus obstructs the generation of DFMs.Based on polarization analyses of Rayleigh waves and correlations of DFM power and ocean wave height,the weak DFMs recorded on the seafloor likely originate from the northern Barents Sea and adjacent regions.As an Arctic warming hotspot,the northern Barents Sea is experiencing reduced sea-ice import from the interior Arctic which enhances atmosphere-ocean interactions.In this region,wind-driven waves with highly variable directions as documented by oceanographic data and the presence of the perennial sea-ice cover may promote the formation of wave trains propagating in nearly opposite directions,which nonlinear interactions excite DFMs.These DFMs continuously lose power due to seismic attenuation during propagation,becoming so weak that the OBSs deployed off the ridge axis detect no noticeable spectral peaks in the DFM period band.The OBSs along the ridge axis,by contrast,reveal more energetic power in the shortperiod DFM band of 2–5 s,which can be attributed to local DFM amplification caused by the thick unconsolidated sediment layer.
基金This work was supported by the National Natural Science Foundation of China (Grant Nos. 49974033 and 49634160).
文摘Two cases of auroral substorms have been studied with the Polar UVI data, which were associated with solar wind pressure shock arriving at the Earth. The global aurora activities started about 1-2 min after pressure shocks arrived at dayside magnetopause, then nightside auroras intensified rapidly 3-4 min later, with auroral sub-storm onset. The observations in synchronous orbit indicated that the compressing effects on magnetosphere were observed in their corresponding sites about 2 min after the pressure shocks impulse magnetopause. We propose that the auroral intensification and substorm onset possibly result from hydromagnetic wave produced by the pressure shock. The fast-mode wave propagates across the magnetotail lobes with higher local Alfven velocity, magnetotail was compressed rapidly and strong lobe field and cross-tail current were built in about 1-2 min, and furthermore the substorm was triggered due to an instability in current sheet.
基金supported by four student research grants from Geological Society of America, Ed Picou/GCSSEPM, American Association of Petroleum Geologiststhe Al Spreng Graduate Research Grant from the Geology and Geophysics Program of Missouri University of Science and Technology to Jonathan Obrist-Farnera research grant from University of Missouri Research Board and a research grant (No.2011ZX05008 004 053) from Institute of Geol-ogy and Geophysics of Chinese Academy of Sciences to Wan Yang
文摘Sedimentological and stratigraphic studies of seven stratigraphic sections of Permian Hongyanchi （HYC） and Quanzijie （QZJ） low-order cycles （LCs） in the Tarlong-Taodonggou half graben and Dalongkou area in Bogda Mountains, NW China, demonstrate effective approaches and methodology in cyclo- and time-stratigraphic analyses of complex fluviallacustrine deposits in an intracontinental rift setting. A new synchronous stratigraphic unit, the lower QZJ LC is defined. The lower and upper boundaries of this cycle include a regionally correlative disconformity, erosional unconformity, and conformity, across which significant and abrupt changes in palaeoenvironments and tectonic and climatic conditions occurred. The lower boundary is an erosional unconformity and disconformity with a high-relief topography that juxtaposes lacustrine deposits of the underlying HYC LC with the overlying meandering stream deposits of the lower QZJ LC, and was caused by a regional tectonic uplift. The up- per boundary is a disconformity and local erosional unconformity and conformity, juxtaposing stacked paleosols developed on fluvial sediments with overlying fluvial and loessial deposits of the upper QZJ LC. The paleosols indicate landscape stability and a prolonged period of subaerial exposure and minimal deposition and suggest that climatic conditions were semiarid with strong precipitation seasonality in the Tarlong-Taodonggou half graben and subhumid in the Dalongkou area. The fluvial-loessial deposits indicate a renewed tectonic uplift and a change in the atmospheric circulation pattern. The newly-defined lower QZJ LC facilitates accurate palaeogeographic reconstruction in the study area during a period of major tectonic and climatic changes. The interpreted tectonic and climatic conditions provide a critical data point in the mid-latitude east coast of NE Pangea during the Mid-Permian icehouse-hothouse transition. The results demonstrate that a process-response approach is effective in time- stratigraphic analysis of complex fluvial-lacustrine strata in a highly-partitioned rift basin.
基金partially supported by Alfred Spreng Graduate Research Grant from Geology and Geophysics Program of Missouri University of Science and Technology to DYZby a U.S. National Science Foundation grant (IES-1714749) to WY。
文摘This study investigates the provenance of sedimentary rocks in Bogda Mountains, NW China, and reconstructs the lithology and unroofing history of the Eastern North Tianshan Suture. Petrographic point counting data of sandstones and compositions of conglomerates of upper Permian-lowermost Triassic Wutonggou low-order cycle from Zhaobishan, North Tarlong, Taodonggou, and Dalongkou sections in the southern and northern foothills of Bogda Mountains were used to interpret the temporal and spatial variations of lithology of the Eastern North Tianshan Suture, which is the sediment source area. Three compositional trends were identified. A trend of upwardincreasing quartz content and granitic pebbles in Zhaobishan section suggests a change from the undissected volcanic arc, accretionary wedge and trench setting to predominantly transitional volcanic arc and subordinate accretionary wedge and trench, in the eastern part of the Eastern North Tianshan Suture. In North Tarlong and Taodonggou sections, however, the lithic content decreases and the contents of quartz and granitic pebbles increase up sections. These trends indicate that the western part of the Eastern North Tianshan Suture changed from an undissected volcanic arc to the transitional volcanic arc, accretionary wedge and trench. No clear trend in the lithic-rich sandstones of the Dalongkou section indicates that sediments were derived from the undissected volcanic arc in the Eastern North Tianshan Suture and local rift shoulders. Compositional variations of studied rocks suggest that the Eastern North Tianshan Suture was an amalgamated complex with great spatial and temporal heterogeneities in lithology and experienced persistent unroofing during late Permian-earliest Triassic. This study reconstructs a key element of the Chinese Tianshan Suture and serves as an example to understand the unroofing processes of ancient sutures.
文摘Eustasy has commonly been invoked to explain peritidal carbonate cyclicity,but is difficult to explain cycles formed in a greenhouse climate when eustasy is minimal.We propose that peritidal cycles on an Early Triassic isolated carbonate platform in Guizhou,South China,were formed by hierarchical carbonate productivity variations.Most of the 149 shallowing-upward cycles are typically terminated by flooding over intertidal facies and contain rare supratidal facies and no prolonged subaerial exposure.Low-diversity benthos in the platform interior during the post-end-Permian biotic recovery were sensitive to environmental perturbations,which caused variations in benthic sediment productivity in the subtidal carbonate factory.The perturbations may be driven by changes in salinity and degree of eutrophication,or repeated platform mini-drowning by anoxic and/or CO2-charged deep water upwelled onto the banktop.They were modulated by Milankovitch orbitally-driven climatic and oceanographic factors as suggested by the hierarchical stacking pattern and spectral signals of these cycles.A one-dimensional conceptual model shows that hierarchical productivity variations alone may generate hierarchical peritidal carbonate cycles under conditions of constant subsidence and no sea-level fluctuation.
