Guided by the analysis of source-to-sink system,this study investigates the Paleogene Oligocene Lingshui Formation in the Qiongdongnan Basin by comparing the geological characterizes in land and sea areas and integrat...Guided by the analysis of source-to-sink system,this study investigates the Paleogene Oligocene Lingshui Formation in the Qiongdongnan Basin by comparing the geological characterizes in land and sea areas and integrating outcrop,core,drilling,logging and 3D seismic data,to systematically analyze the characteristics of the source,transport pathway,and sink during the deposition of Lingshui Formation,and reveal the patterns,controlling factors and petroleum geologic significance of the source-to-sink systems.The results are obtained in five aspects.First,during the fault-depression transition,the Qiongdongnan Basin received sediments from the provenances presenting as segments in east-west and zones in north-south,primarily with the Indosinian granites in the Shenhu Uplift in the east and the Yanshanian granites in the west.Overall,the sources are young in the southern and northern parts and old in the interior of the basin.Second,three types of sediment transport pathways are identified:paleo-valleys,fault troughs and trough-valley transitional zones.Third,based on differences in sediment supply modes,the unique source-to-sink systems during the fault-depression transition in marine rift basins are categorized into three types:exogenous,endogenous and composite.Fourth,the characteristics of these source-to-sink systems are primarily controlled by provenance,paleogeomorphology,and sea-level changes.Provenance lithology and scale dictated the composition and volume of sedimentary deposits.Paleogeomorphology influenced erosion intensity in the provenance and the development of paleodrainage systems,thereby affecting the distribution and types of sedimentary systems.Additionally,sea-level changes decided the extent of the provenance,but also regulated the sediment distribution patterns through oceanic processes such as waves and tides.Fifth,the exogenous source-to-sink systems may form large-scale reservoir bodies,the endogenous systems develop secondary pores due to presence of soluble minerals,and the composite systems demonstrate reservoir properties varying from area to area.展开更多
The North Sulawesi arc(NSUA)constitutes the northern arm of Sulawesi Island and is characterized by complex Cenozoic records of magmatism and tectonics.Zircon U-Pb geochronological and Hf-O isotopic data,whole-rock ma...The North Sulawesi arc(NSUA)constitutes the northern arm of Sulawesi Island and is characterized by complex Cenozoic records of magmatism and tectonics.Zircon U-Pb geochronological and Hf-O isotopic data,whole-rock major oxides,trace elemental,and Sr-Nd-Pb isotopic data of the high-silica granites from the NSUA document their petrogenesis and tectonic setting.Zircon elemental analysis of the granitic samples shows a juvenile oceanic crust origin and the U-Pb geochronology indicates their Oligocene ages between 30.4 and 27.3 Ma.The samples have high SiO_(2)(75.05 wt.%-79.38 wt.%)and Na_(2)O(4.48 wt.%-5.67 wt.%),low K_(2)O(0.15 wt.%-1.34 wt.%)and MgO(0.07 wt.%-0.91 wt.%)contents,belonging to calc-alkaline I-type high-silica granites.They have enriched LREE and LILE,and depleted HREE and HFSE,showing significant Eu,Sr,Nb,and Ta negative anomalies.These high-silica granites have low(^(87)Sr/^(86)Sr)_i ratios(0.704412-0.704592),positiveε_(Nd)(t)values(from+5.1 to+6.6),positive zirconε_(Hf)(t)values(from+10.1 to+18.8),low zirconδ^(18)O values(4.20‰-5.02‰),and similar Pb isotope compositions to the Indian Ocean MORB.Such signatures suggest that these high-silica granites were derived by partial melting process of the juvenile arc crust in an intra-oceanic setting.The felsic magmatism in the NSUA was likely driven by mantle upwelling and decompression melting during the Oligocene,in response to slab roll-back linked with the convergence of the East Sulawesi ophiolitic crust or the microcontinental fragments.展开更多
The Lake Tana area is located within a complex volcano-tectonic basin on the northwestern Ethiopian plateau.The basin is underlain by a thick succession of Oligocene transitional basalts and sub-alkaline rhyolites ove...The Lake Tana area is located within a complex volcano-tectonic basin on the northwestern Ethiopian plateau.The basin is underlain by a thick succession of Oligocene transitional basalts and sub-alkaline rhyolites overlain in places,particularly south of the lake,by Quaternary alkaline to mildly transitional basalts,and dotted with Oligo-Miocene trachyte domes and plugs.This paper presents the results of integrated field,petrographic,and major and trace element geochemical studies of the Lake Tana area volcanic rocks,with particular emphasis on the Oligocene basalts and rhyolites.The studies reveal a clear petrogenetic link between the Oligocene basalts and rhyolites.The Oligocene basalts are:(1)plagioclase,olivine,and/or pyroxene phyric;(2)show an overall decreasing trend in MgO,Fe_(2)O_(3),and CaO with silica;(3)have relatively low Mg#,Ni and Cr contents and high Nb/La and Nb/Yb ratios;and(4)show LREE enriched and generally flat HREE patterns.All these imply the origin of the Oligocene basalts by shallow-level fractional crystallization of an enriched magma sourced at the asthenospheric mantle.The Oligocene rhyolites:(1)are enriched in incompatible while depleted in compatible trace elements,P and Ti;(2)show a strong negative Eu anomaly;(3)contain appreciable amounts of plagioclase,apatite,and Fe-Ti oxides;and(4)show clear geochemical similarity with well-constrained rhyolites from the Large Igneous Province(LIP)of the northwestern Ethiopian plateau.Low-pressure fractional crystallization of mantle-derived basaltic magma in crustal magma chambers explains the origin of these rhyolites.Our study further shows that the Oligocene basalts and rhyolites are co-genetic and the felsic rocks of the Lake Tana area are related differentiates of the flood basalt volcanism in the northwestern Ethiopian plateau.展开更多
The Upper Eocene–Lower Oligocene Qianjiang Formation of the Jianghan Basin in central China consists of a 4700-m-thick lacustrine succession, containing 1800 m of halite deposits. The maximum thickness of the formati...The Upper Eocene–Lower Oligocene Qianjiang Formation of the Jianghan Basin in central China consists of a 4700-m-thick lacustrine succession, containing 1800 m of halite deposits. The maximum thickness of the formation is 4700 m, and includes 1800 m of halite. We have identified eight third-order depositional sequences based on pinch-out and onlap stratigraphic patterns in 2-D and 3-D seismic data and well logs. The basin evolved from a deep to shallow under-filled lake during the Eocene–Oligocene interval. The main rock types are dark mudstones, halite, and siltstone/sandstone in the depocenter, and alternating mudstone and gypsum in shallower areas. The vertical succession indicates a strong sedimentary cyclicity. Depositional facies indicate the presence of two lake system types. Halite developed in a saline lake system, whereas clastic sediments were deposited in freshwater lake systems. The alternating sediment types indicate that the basin cycled repeatedly between saline and freshwater lake systems. This cyclicity was caused by availability of accommodation space that was controlled by a combination of climate change, tectonic subsidence and sediment supply; notably, the highest frequency cycles occurred at Milankovitch timescales controlled by the Earth＇s orbital variations. The cyclic halite plays an important role in generating and preserving oil in the Qianjiang Formation of the Qianjiang depression.展开更多
A significant geologic event occurred on the Oligocene/Miocene boundary at 23.8 Ma in the northern South China Sea, which is named the Baiyun （白云） movement in this article. This event strongly affected not only th...A significant geologic event occurred on the Oligocene/Miocene boundary at 23.8 Ma in the northern South China Sea, which is named the Baiyun （白云） movement in this article. This event strongly affected not only the South China Sea, but also East Asia. After the Baiyun event, the ridge of seafloor spreading of the South China Sea jumped southward and rotated counterclockwise, and a strong subsidence occurred in the Baiyun sag of the Pearl River Mouth basin. The shelf break shifted suddenly from the south to the north of the Baiyun sag, and the deposition environment in this sag changed from continental shelf with neritic deposition to continental slope with deep-water deposition. Sediment geochemistry study indicated that the Baiyun event played a key role in the rapid change of sediment provenance for the Pearl River Mouth basin. Between 32 and 23.8 Ma, the source of sediments was mainly from the granites in South China, while after 23.8 Ma some sediments might have come from the eastern Himalaya, as the Pearl River drainage extended westward after the uplift of Tibet since that time. The Baiyun event led to a great change in the drainage framework of the paleo- Pearl River, sediment types and the depositional environments in the Pearl River Mouth basin, and relative sea level of the northern South China Sea, as well as sedimentation and hydrocarbon accumulation in the area.展开更多
Sedimentological, cyclic-stratigraphic, paleomagnetic, and clay-mineralogical studies on the early Oligocene Yaxicuo Group in the Hoh Xil Basin, the largest Cenozoic sedimentary basin in the hinterland of the Tibetan ...Sedimentological, cyclic-stratigraphic, paleomagnetic, and clay-mineralogical studies on the early Oligocene Yaxicuo Group in the Hoh Xil Basin, the largest Cenozoic sedimentary basin in the hinterland of the Tibetan Plateau, provide abundant information of paleoclimate changes. A 350-m thick section in the middle-lower Yaxicuo Group was analyzed to reveal the climatic history that occurred in the Hoh Xil region during the early Oligocene interval 31.30-30.35 Ma, dated with the paleomagnetic chronostratigraphy. The results indicate that arid and cold climate dominated the Hoh Xil region during the early Oligocene in general, being related to the global cooling and drying events that occurred in the earliest Oligocene. Within this period, relatively warm and wet climate accompanied by strong tectonic activity occurred in the 31.05-30.75 Ma interval; while arid and cold climate and relatively inactive tectonics occurred in the 31.30-31.05 and 30.75-30.35 Ma intervals. Furthermore, spectral analyses of high-temporal resolution paleoclimatic records show orbital periods including eccentricity, obliquity, and precession. It is concluded that paleoclimate changes during the early Oligocene in the Hoh Xil region were forced by both tectonic activity and orbital periods.展开更多
Coals developed in the Oligocene Yacheng and Lingshui formations in the Qiongdongnan Basin have high organic matter abundance, and the dark mudstones in the two formations have reached a good source rock standard but ...Coals developed in the Oligocene Yacheng and Lingshui formations in the Qiongdongnan Basin have high organic matter abundance, and the dark mudstones in the two formations have reached a good source rock standard but with strong heterogeneity. Through the analysis of trace elements, organic macerals and biomarkers, it is indicated that plankton has made little contribution to Oligocene source rocks compared with the terrestrial higher plants. The organic matter preservation depends on hydrodynamics and the redox environment, and the former is the major factor in the study area. During the sedimentary period of the Yacheng Formation, tidal flats were developed in the central uplift zone, where the hydrodynamic conditions were weak and the input of terrestrial organic matter was abundant. So the Yacheng Salient of the central uplift zone is the most favorable area for the development of source rocks, followed by the central depression zone. During the sedimentary period of the Lingshui Formation, the organic matter input was sufficient in the central depression zone due to multiple sources of sediments. The semi-enclosed environment was favorable for organic matter accumulation, so high quality source rocks could be easily formed in this area, followed by the Yacheng salient of central uplift zone. Source rocks were less developed in the northern depression zone owing to poor preservation conditions,展开更多
Recent paleobotanical investigations in Vietnam provide a good opportunity to improve our understanding of the biodiversity and paleoclimatic conditions in the geological past of Southeast Asia.Palms(Arecaceae)are a d...Recent paleobotanical investigations in Vietnam provide a good opportunity to improve our understanding of the biodiversity and paleoclimatic conditions in the geological past of Southeast Asia.Palms(Arecaceae)are a diverse family of typical thermophilous plants with a relatively low tolerance for freezing.In this study,we describe well-preserved fossil palm leaves from the Oligocene Dong Ho Formation of Hoanh Bo Basin,northern Vietnam.Characters of the fossil leaves,such as a fan-shaped costapalmate lamina,an unarmed petiole,a costa slightly enlarged at the base that then tapers distally into the blade,and well-preserved amphistomatic leaves with cuticles,suggest that they represent a new fossil species,which we herein designate Sabalites colaniae A.Song,T.Su,T.V.Do et Z.K.Zhou sp.nov.Together with other paleontological and palaeoclimatic evidence,we conclude that a warm climate prevailed in northern Vietnam and nearby areas during the Oligocene.展开更多
Cenozoic potassic-ultrapotassic igneous rocks are widespread in the southeastern Tibetan Plateau.Their petrogenesis and magmatic processes remain subject to debate in spite of numerous publications.Almost all of the C...Cenozoic potassic-ultrapotassic igneous rocks are widespread in the southeastern Tibetan Plateau.Their petrogenesis and magmatic processes remain subject to debate in spite of numerous publications.Almost all of the Cenozoic extrusive and intrusive rocks in the Yao’an area,western Yunnan Province,SW China,are geochemically shoshonitic,collectively termed here the Yao’an Shoshonitic Complex(YSC).The YSC is located in the(south)easternmost part of the ENE-WSW-trending,~550 km-long and~250 km-wide Cenozoic magmatic zone;the latter separates the orthogonal and oblique collision belts of the India-Eurasia collision orogen.Previously published geochronological and thermochronological data revealed that the rocks of the YSC were emplaced over a short timespan of 34-32 Ma.This and our new data suggest that the primary magma of the YSC likely was formed by partial melting of ancient continental lithospheric mantle beneath the Yangtze Block.This part of the continental lithospheric mantle had likely not been modified by any oceanic subduction.Fractionation crystallization of an Mg-and Ca-bearing mineral and TiFe oxides during the magmatic evolution probably account for the variable lithologies of the YSC.展开更多
Oil and gas exploration prospect for the Miocene and Late Oligocene strata in Qiongdongnan（琼东南） basin are evaluated.The structural-sedimentary and reservoir characteristics are discussed and helpful conclusions a...Oil and gas exploration prospect for the Miocene and Late Oligocene strata in Qiongdongnan（琼东南） basin are evaluated.The structural-sedimentary and reservoir characteristics are discussed and helpful conclusions are drawn.It was proved that there are mainly two reservoir and capping assemblages in Qiongdongnan basin by drilled wells,i.e.,one assemblage of the second and the third members of Lingshui（陵水） Formation,upper Sanya（三亚） and Meishan（梅山） formations.According to hydrocarbon evaluation from burial history and well data,all source rocks in Yacheng（崖城） and Lingshui formations have reached the mature-overmature stage.High temperature and overpressure provide dynamic conditions for oil and gas migration,while the long-term activity of basin controlling faults is adverse to their accumulation in upheaved areas.However,because the secondary faults in the basin ceased their activities at the end of the Sanya Period,and the Sanya Formation was later covered by the regional overlying layer of the Meishan Formation,the blocks and low uplifts near the secondary faults were favorable for oil and gas accumulation.Furthermore,as another important pointed area for gas migration,the highest position of a structural ridge should reasonably be a range that is covered by marine mudstone or sandy mudstone during the whole subsiding period,and therefore,the reservoir bed should not be in that position but at its inferior eminence or adjacent slope zone.Therefore,we can draw a conclusion that the inferior low uplifts and structural ridges rather than the upheavals or the top of a structural ridge,probably,have huge reservoirs,and should be chief exploration targets in the Qiongdongnan basin.展开更多
The present paper reports two new species of Buxus from the Oligocene Ningming Formation of Guangxi, South China, on the basis of several compressed fossil leaves.Buxus ningmingensis sp.nov.is characterized by an elli...The present paper reports two new species of Buxus from the Oligocene Ningming Formation of Guangxi, South China, on the basis of several compressed fossil leaves.Buxus ningmingensis sp.nov.is characterized by an elliptic to ovate shape; pinnate venation with marginal secondary, long hairs on the adaxial side of the midvein base; and amphistomatic, anomocytic stomata with a prominent outer ring.All of these characteristics suggest a closest affinity to the living Buxus microphylla subsp.sinica.Another new species identified as Buxus preaustro-yunnanensis sp.nov.is characterized by the diagnostic formation of its admedially branched tertiaries, the trunks of which are often strongly thickened by sclereides.This species is most similar to the extant Buxus austroyunnanensis.The co-occurrence of Buxus ningmingensis sp.nov.and Buxus preaustro-yunnanensis sp.nov.suggests that these two Buxus lived during the Oligocene under a warm, tropical to subtropical climate, similar to the current climate of Ningming.Our findings provide the earliest fossil evidence of Buxus leaves with detailed illustrations of leaf architectural and cuticular features.The occurrence of these two species indicates that Buxus began to diversify in Guangxi no later than the Oligocene.展开更多
The topography of the Harlik Mountain has a strong impact on the formation of current arid climate in the Turpan-Hami Basin.However,it is still controversial if Harlik Mountain experienced significant exhumation durin...The topography of the Harlik Mountain has a strong impact on the formation of current arid climate in the Turpan-Hami Basin.However,it is still controversial if Harlik Mountain experienced significant exhumation during the Middle to Late Cenozoic according to the previous thermochronology studies.The features of the Oligocene to Miocene sediments in the foreland basin could provide productive information for resolving the debates.The peak ages of detrital apatite fission track analysis of the Oligocene–Miocene sandstone in the Turpan-Hami Basin are well comparable with the cooling age records of the Harlik Mountain rocks,indicating that the Oligocene–Miocene Taoshuyuanzi Formation in the basin was mostly derived from the Harlik Mountain.The stratigraphic sequence exhibits coarsening upward,reflecting that the source area was in a tectonically active period during the deposition process.Heavy mineral assemblages also suggest that the unstable minerals in the sediment increased significantly at the end of the deposition.Moreover,the proportion of apatite increased up-section,while the garnet content decreased significantly,indicating that the Carboniferous metamorphic rocks have been gradually eroded out and more intrusive rocks have been exposed to the surface.These observations suggest that the Harlik Mountain experienced exhumation during the Oligocene to Miocene,and the denudation depth afterward was probably less than 2.5 km according to the previously apatite(U-Th)/He data.The Oligocene–Miocene exhumation probably acted as one of the triggers for the heavy drought of the Turpan-Hami Basin during the Middle–Late Neogene.展开更多
The latitudinal richness gradient is a frequent topic of study on the modern landscape,but its history in deep time is much less well known.Here,we preliminarily evaluated the paleolatitudinal richness gradient of vas...The latitudinal richness gradient is a frequent topic of study on the modern landscape,but its history in deep time is much less well known.Here,we preliminarily evaluated the paleolatitudinal richness gradient of vascular plants for the Eocene(56-33.9 million years ago) and Oligocene(33.9-23 million years ago) epochs of North America north of IVlexico using 201 fossil floras.We calculated the direction and shape of the gradient using quadratic regression to detect linear and curvilinear trends.We performed regressions for the Eocene and Oligocene as well as for informal time intervals within the Eocene:early,middle,and middle + late.We found that quadratic models better explain the data than linear models for both epochs as well as for the early Eocene.A roughly linear trend in the middle and middle + late intervals may reflect limited sampling of high latitude floras for those times.The curvilinear relationship was weak for the Eocene and the model showed a peak in richness at 45.5°N.The curvilinear relationship was much stronger for the Oligocene and the peak occurred at 48.5°N.In the Eocene,the mid-latitude peak in richness may be explained by mean annual temperature,which was probably higher at some mid-latitudes than at lower ones.For the Oligocene,the peak in richness at midlatitudes may be explained by evolutionary diversification within the temperate zone or by increased aridity at low latitudes.We also assessed the latitudinal richness gradient of genera within modern floras in North America north of Mexico and we found a weak,curvilinear trend with a peak in richness at 31.5°N.Our results suggest that the latitudinal genus richness gradient of vascular plants in North America continued to develop into its modern structure following the Oligocene.展开更多
The Oligocene–Miocene carbonate record of the Zagros Mountains, known as the Asmari Formation, constitutes an important hydrocarbon reservoir in southern Iran. This marine carbonate succession, which developed under ...The Oligocene–Miocene carbonate record of the Zagros Mountains, known as the Asmari Formation, constitutes an important hydrocarbon reservoir in southern Iran. This marine carbonate succession, which developed under tropical conditions, is explored in terms of larger foraminiferal biostratigraphy, facies analysis and sequence stratigraphy in a new section at Papoon cropping out in the western Fars sub-basin, in the south-east of the Zagros belt. Facies analysis shows evidence of re-working and transport of skeletal components throughout the depositional system, interpreted here as a carbonate ramp. The foraminifera-based biozones identified include the Globigerina–Turborotalia cerroazulensis–Hantkenina Zone and Nummulites vascus–Nummulites fichteli Zone, both of Rupelian age, the Archaias asmaricus–Archaias hensoni–Miogypsinoides complanatus Zone of Chattian age and the ‘Indeterminate’ Zone of Aquitanian age. The vertical sedimentary evolution of the formation exhibits a progressive shallowing of the facies belts and thus the succession is interpreted as a high-rank low-order regressive systems tract. This long-lasting Rupelian–Aquitanian regressive event is in accordance with accepted global long-term eustatic curves. Accordingly, long-term eustatic trends would have been a factor controlling accommodation during the deposition of the Asmari Formation studied in the western Fars sub-basin.展开更多
基金Supported by National Natural Science Foundation of China Enterprise Joint Fund Project(U24B200849)National Natural Science Foundation of China(91528303).
文摘Guided by the analysis of source-to-sink system,this study investigates the Paleogene Oligocene Lingshui Formation in the Qiongdongnan Basin by comparing the geological characterizes in land and sea areas and integrating outcrop,core,drilling,logging and 3D seismic data,to systematically analyze the characteristics of the source,transport pathway,and sink during the deposition of Lingshui Formation,and reveal the patterns,controlling factors and petroleum geologic significance of the source-to-sink systems.The results are obtained in five aspects.First,during the fault-depression transition,the Qiongdongnan Basin received sediments from the provenances presenting as segments in east-west and zones in north-south,primarily with the Indosinian granites in the Shenhu Uplift in the east and the Yanshanian granites in the west.Overall,the sources are young in the southern and northern parts and old in the interior of the basin.Second,three types of sediment transport pathways are identified:paleo-valleys,fault troughs and trough-valley transitional zones.Third,based on differences in sediment supply modes,the unique source-to-sink systems during the fault-depression transition in marine rift basins are categorized into three types:exogenous,endogenous and composite.Fourth,the characteristics of these source-to-sink systems are primarily controlled by provenance,paleogeomorphology,and sea-level changes.Provenance lithology and scale dictated the composition and volume of sedimentary deposits.Paleogeomorphology influenced erosion intensity in the provenance and the development of paleodrainage systems,thereby affecting the distribution and types of sedimentary systems.Additionally,sea-level changes decided the extent of the provenance,but also regulated the sediment distribution patterns through oceanic processes such as waves and tides.Fifth,the exogenous source-to-sink systems may form large-scale reservoir bodies,the endogenous systems develop secondary pores due to presence of soluble minerals,and the composite systems demonstrate reservoir properties varying from area to area.
基金supported by the National Natural Science Foundation of China(No.42330302)the National Key Research and Development Program of China(No.2023YFF0803701)+1 种基金the Southern Marine Science and Engineering Guangdong Laboratory(Zhuhai)(No.SML2023SP206)the Australian Research Council(No.FL160100168)。
文摘The North Sulawesi arc(NSUA)constitutes the northern arm of Sulawesi Island and is characterized by complex Cenozoic records of magmatism and tectonics.Zircon U-Pb geochronological and Hf-O isotopic data,whole-rock major oxides,trace elemental,and Sr-Nd-Pb isotopic data of the high-silica granites from the NSUA document their petrogenesis and tectonic setting.Zircon elemental analysis of the granitic samples shows a juvenile oceanic crust origin and the U-Pb geochronology indicates their Oligocene ages between 30.4 and 27.3 Ma.The samples have high SiO_(2)(75.05 wt.%-79.38 wt.%)and Na_(2)O(4.48 wt.%-5.67 wt.%),low K_(2)O(0.15 wt.%-1.34 wt.%)and MgO(0.07 wt.%-0.91 wt.%)contents,belonging to calc-alkaline I-type high-silica granites.They have enriched LREE and LILE,and depleted HREE and HFSE,showing significant Eu,Sr,Nb,and Ta negative anomalies.These high-silica granites have low(^(87)Sr/^(86)Sr)_i ratios(0.704412-0.704592),positiveε_(Nd)(t)values(from+5.1 to+6.6),positive zirconε_(Hf)(t)values(from+10.1 to+18.8),low zirconδ^(18)O values(4.20‰-5.02‰),and similar Pb isotope compositions to the Indian Ocean MORB.Such signatures suggest that these high-silica granites were derived by partial melting process of the juvenile arc crust in an intra-oceanic setting.The felsic magmatism in the NSUA was likely driven by mantle upwelling and decompression melting during the Oligocene,in response to slab roll-back linked with the convergence of the East Sulawesi ophiolitic crust or the microcontinental fragments.
文摘The Lake Tana area is located within a complex volcano-tectonic basin on the northwestern Ethiopian plateau.The basin is underlain by a thick succession of Oligocene transitional basalts and sub-alkaline rhyolites overlain in places,particularly south of the lake,by Quaternary alkaline to mildly transitional basalts,and dotted with Oligo-Miocene trachyte domes and plugs.This paper presents the results of integrated field,petrographic,and major and trace element geochemical studies of the Lake Tana area volcanic rocks,with particular emphasis on the Oligocene basalts and rhyolites.The studies reveal a clear petrogenetic link between the Oligocene basalts and rhyolites.The Oligocene basalts are:(1)plagioclase,olivine,and/or pyroxene phyric;(2)show an overall decreasing trend in MgO,Fe_(2)O_(3),and CaO with silica;(3)have relatively low Mg#,Ni and Cr contents and high Nb/La and Nb/Yb ratios;and(4)show LREE enriched and generally flat HREE patterns.All these imply the origin of the Oligocene basalts by shallow-level fractional crystallization of an enriched magma sourced at the asthenospheric mantle.The Oligocene rhyolites:(1)are enriched in incompatible while depleted in compatible trace elements,P and Ti;(2)show a strong negative Eu anomaly;(3)contain appreciable amounts of plagioclase,apatite,and Fe-Ti oxides;and(4)show clear geochemical similarity with well-constrained rhyolites from the Large Igneous Province(LIP)of the northwestern Ethiopian plateau.Low-pressure fractional crystallization of mantle-derived basaltic magma in crustal magma chambers explains the origin of these rhyolites.Our study further shows that the Oligocene basalts and rhyolites are co-genetic and the felsic rocks of the Lake Tana area are related differentiates of the flood basalt volcanism in the northwestern Ethiopian plateau.
基金supported by the National Natural Science Foundation of China (No. 41322013)the Program for New Century Excellent Talents in Universities (No. NCET-110723)+2 种基金the National Key Basic Research Development Program of China (No. 2012CB822003)the Programme of Introducing Talents of Discipline to Universities (No. B14031)the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (No. CUG110611)
文摘The Upper Eocene–Lower Oligocene Qianjiang Formation of the Jianghan Basin in central China consists of a 4700-m-thick lacustrine succession, containing 1800 m of halite deposits. The maximum thickness of the formation is 4700 m, and includes 1800 m of halite. We have identified eight third-order depositional sequences based on pinch-out and onlap stratigraphic patterns in 2-D and 3-D seismic data and well logs. The basin evolved from a deep to shallow under-filled lake during the Eocene–Oligocene interval. The main rock types are dark mudstones, halite, and siltstone/sandstone in the depocenter, and alternating mudstone and gypsum in shallower areas. The vertical succession indicates a strong sedimentary cyclicity. Depositional facies indicate the presence of two lake system types. Halite developed in a saline lake system, whereas clastic sediments were deposited in freshwater lake systems. The alternating sediment types indicate that the basin cycled repeatedly between saline and freshwater lake systems. This cyclicity was caused by availability of accommodation space that was controlled by a combination of climate change, tectonic subsidence and sediment supply; notably, the highest frequency cycles occurred at Milankovitch timescales controlled by the Earth＇s orbital variations. The cyclic halite plays an important role in generating and preserving oil in the Qianjiang Formation of the Qianjiang depression.
基金supported by the National Natural Science Foundation of China (No. 40238060)
文摘A significant geologic event occurred on the Oligocene/Miocene boundary at 23.8 Ma in the northern South China Sea, which is named the Baiyun （白云） movement in this article. This event strongly affected not only the South China Sea, but also East Asia. After the Baiyun event, the ridge of seafloor spreading of the South China Sea jumped southward and rotated counterclockwise, and a strong subsidence occurred in the Baiyun sag of the Pearl River Mouth basin. The shelf break shifted suddenly from the south to the north of the Baiyun sag, and the deposition environment in this sag changed from continental shelf with neritic deposition to continental slope with deep-water deposition. Sediment geochemistry study indicated that the Baiyun event played a key role in the rapid change of sediment provenance for the Pearl River Mouth basin. Between 32 and 23.8 Ma, the source of sediments was mainly from the granites in South China, while after 23.8 Ma some sediments might have come from the eastern Himalaya, as the Pearl River drainage extended westward after the uplift of Tibet since that time. The Baiyun event led to a great change in the drainage framework of the paleo- Pearl River, sediment types and the depositional environments in the Pearl River Mouth basin, and relative sea level of the northern South China Sea, as well as sedimentation and hydrocarbon accumulation in the area.
文摘Sedimentological, cyclic-stratigraphic, paleomagnetic, and clay-mineralogical studies on the early Oligocene Yaxicuo Group in the Hoh Xil Basin, the largest Cenozoic sedimentary basin in the hinterland of the Tibetan Plateau, provide abundant information of paleoclimate changes. A 350-m thick section in the middle-lower Yaxicuo Group was analyzed to reveal the climatic history that occurred in the Hoh Xil region during the early Oligocene interval 31.30-30.35 Ma, dated with the paleomagnetic chronostratigraphy. The results indicate that arid and cold climate dominated the Hoh Xil region during the early Oligocene in general, being related to the global cooling and drying events that occurred in the earliest Oligocene. Within this period, relatively warm and wet climate accompanied by strong tectonic activity occurred in the 31.05-30.75 Ma interval; while arid and cold climate and relatively inactive tectonics occurred in the 31.30-31.05 and 30.75-30.35 Ma intervals. Furthermore, spectral analyses of high-temporal resolution paleoclimatic records show orbital periods including eccentricity, obliquity, and precession. It is concluded that paleoclimate changes during the early Oligocene in the Hoh Xil region were forced by both tectonic activity and orbital periods.
基金financially supported by the Major State Basic Research Development Program(973 Program)(Grant No.2009CB219402)
文摘Coals developed in the Oligocene Yacheng and Lingshui formations in the Qiongdongnan Basin have high organic matter abundance, and the dark mudstones in the two formations have reached a good source rock standard but with strong heterogeneity. Through the analysis of trace elements, organic macerals and biomarkers, it is indicated that plankton has made little contribution to Oligocene source rocks compared with the terrestrial higher plants. The organic matter preservation depends on hydrodynamics and the redox environment, and the former is the major factor in the study area. During the sedimentary period of the Yacheng Formation, tidal flats were developed in the central uplift zone, where the hydrodynamic conditions were weak and the input of terrestrial organic matter was abundant. So the Yacheng Salient of the central uplift zone is the most favorable area for the development of source rocks, followed by the central depression zone. During the sedimentary period of the Lingshui Formation, the organic matter input was sufficient in the central depression zone due to multiple sources of sediments. The semi-enclosed environment was favorable for organic matter accumulation, so high quality source rocks could be easily formed in this area, followed by the Yacheng salient of central uplift zone. Source rocks were less developed in the northern depression zone owing to poor preservation conditions,
基金supported by National Natural Science Foundation of China(NSFC)(31800183,41922010,42002020,41661134049)Yunnan Basic Research Projects(202001AU070137,2019FB026)+2 种基金Chinese Academy of Sciences"Light of West China"Program(2020000023)the CAS 135 program(2017XTBG-T03)Project“Study,collection of fossil woods in Vietnam for exhibition in System of Vietnam National Museum of Nature”(CT0000.01/19-21).
文摘Recent paleobotanical investigations in Vietnam provide a good opportunity to improve our understanding of the biodiversity and paleoclimatic conditions in the geological past of Southeast Asia.Palms(Arecaceae)are a diverse family of typical thermophilous plants with a relatively low tolerance for freezing.In this study,we describe well-preserved fossil palm leaves from the Oligocene Dong Ho Formation of Hoanh Bo Basin,northern Vietnam.Characters of the fossil leaves,such as a fan-shaped costapalmate lamina,an unarmed petiole,a costa slightly enlarged at the base that then tapers distally into the blade,and well-preserved amphistomatic leaves with cuticles,suggest that they represent a new fossil species,which we herein designate Sabalites colaniae A.Song,T.Su,T.V.Do et Z.K.Zhou sp.nov.Together with other paleontological and palaeoclimatic evidence,we conclude that a warm climate prevailed in northern Vietnam and nearby areas during the Oligocene.
基金financially supported by the Ministry of Sciences and Technology of China(Grant No.2022YFF0800901)the Natural Science Foundation of China(Grant Nos.92055206 and 42163007)。
文摘Cenozoic potassic-ultrapotassic igneous rocks are widespread in the southeastern Tibetan Plateau.Their petrogenesis and magmatic processes remain subject to debate in spite of numerous publications.Almost all of the Cenozoic extrusive and intrusive rocks in the Yao’an area,western Yunnan Province,SW China,are geochemically shoshonitic,collectively termed here the Yao’an Shoshonitic Complex(YSC).The YSC is located in the(south)easternmost part of the ENE-WSW-trending,~550 km-long and~250 km-wide Cenozoic magmatic zone;the latter separates the orthogonal and oblique collision belts of the India-Eurasia collision orogen.Previously published geochronological and thermochronological data revealed that the rocks of the YSC were emplaced over a short timespan of 34-32 Ma.This and our new data suggest that the primary magma of the YSC likely was formed by partial melting of ancient continental lithospheric mantle beneath the Yangtze Block.This part of the continental lithospheric mantle had likely not been modified by any oceanic subduction.Fractionation crystallization of an Mg-and Ca-bearing mineral and TiFe oxides during the magmatic evolution probably account for the variable lithologies of the YSC.
基金supported by the Key Project of the National Eleventh Five-Year Plan of China (No. 2008ZX05023-004)
文摘Oil and gas exploration prospect for the Miocene and Late Oligocene strata in Qiongdongnan（琼东南） basin are evaluated.The structural-sedimentary and reservoir characteristics are discussed and helpful conclusions are drawn.It was proved that there are mainly two reservoir and capping assemblages in Qiongdongnan basin by drilled wells,i.e.,one assemblage of the second and the third members of Lingshui（陵水） Formation,upper Sanya（三亚） and Meishan（梅山） formations.According to hydrocarbon evaluation from burial history and well data,all source rocks in Yacheng（崖城） and Lingshui formations have reached the mature-overmature stage.High temperature and overpressure provide dynamic conditions for oil and gas migration,while the long-term activity of basin controlling faults is adverse to their accumulation in upheaved areas.However,because the secondary faults in the basin ceased their activities at the end of the Sanya Period,and the Sanya Formation was later covered by the regional overlying layer of the Meishan Formation,the blocks and low uplifts near the secondary faults were favorable for oil and gas accumulation.Furthermore,as another important pointed area for gas migration,the highest position of a structural ridge should reasonably be a range that is covered by marine mudstone or sandy mudstone during the whole subsiding period,and therefore,the reservoir bed should not be in that position but at its inferior eminence or adjacent slope zone.Therefore,we can draw a conclusion that the inferior low uplifts and structural ridges rather than the upheavals or the top of a structural ridge,probably,have huge reservoirs,and should be chief exploration targets in the Qiongdongnan basin.
基金financially supported by the National Basic Research Program of China (973 Program) (No.2012CB822003)the Funds of Key Laboratory of Petroleum Resources, Gansu Province (No.SZDKFJJ20150603)+2 种基金the National Natural Science Foundation of China (Nos.41172022, 41272026)the Funds of Key Laboratory of Economic Stratigraphy and Palaeogeography, Nanjing Institute of Geology and Palaeontology, CAS (No.Y421140303)the Fundamental Research Funds for the Central Universities (No.lzujbky-2014-285)
文摘The present paper reports two new species of Buxus from the Oligocene Ningming Formation of Guangxi, South China, on the basis of several compressed fossil leaves.Buxus ningmingensis sp.nov.is characterized by an elliptic to ovate shape; pinnate venation with marginal secondary, long hairs on the adaxial side of the midvein base; and amphistomatic, anomocytic stomata with a prominent outer ring.All of these characteristics suggest a closest affinity to the living Buxus microphylla subsp.sinica.Another new species identified as Buxus preaustro-yunnanensis sp.nov.is characterized by the diagnostic formation of its admedially branched tertiaries, the trunks of which are often strongly thickened by sclereides.This species is most similar to the extant Buxus austroyunnanensis.The co-occurrence of Buxus ningmingensis sp.nov.and Buxus preaustro-yunnanensis sp.nov.suggests that these two Buxus lived during the Oligocene under a warm, tropical to subtropical climate, similar to the current climate of Ningming.Our findings provide the earliest fossil evidence of Buxus leaves with detailed illustrations of leaf architectural and cuticular features.The occurrence of these two species indicates that Buxus began to diversify in Guangxi no later than the Oligocene.
基金funded by the National Natural Science Foundation of China(Nos.41972208,42172251).
文摘The topography of the Harlik Mountain has a strong impact on the formation of current arid climate in the Turpan-Hami Basin.However,it is still controversial if Harlik Mountain experienced significant exhumation during the Middle to Late Cenozoic according to the previous thermochronology studies.The features of the Oligocene to Miocene sediments in the foreland basin could provide productive information for resolving the debates.The peak ages of detrital apatite fission track analysis of the Oligocene–Miocene sandstone in the Turpan-Hami Basin are well comparable with the cooling age records of the Harlik Mountain rocks,indicating that the Oligocene–Miocene Taoshuyuanzi Formation in the basin was mostly derived from the Harlik Mountain.The stratigraphic sequence exhibits coarsening upward,reflecting that the source area was in a tectonically active period during the deposition process.Heavy mineral assemblages also suggest that the unstable minerals in the sediment increased significantly at the end of the deposition.Moreover,the proportion of apatite increased up-section,while the garnet content decreased significantly,indicating that the Carboniferous metamorphic rocks have been gradually eroded out and more intrusive rocks have been exposed to the surface.These observations suggest that the Harlik Mountain experienced exhumation during the Oligocene to Miocene,and the denudation depth afterward was probably less than 2.5 km according to the previously apatite(U-Th)/He data.The Oligocene–Miocene exhumation probably acted as one of the triggers for the heavy drought of the Turpan-Hami Basin during the Middle–Late Neogene.
文摘The latitudinal richness gradient is a frequent topic of study on the modern landscape,but its history in deep time is much less well known.Here,we preliminarily evaluated the paleolatitudinal richness gradient of vascular plants for the Eocene(56-33.9 million years ago) and Oligocene(33.9-23 million years ago) epochs of North America north of IVlexico using 201 fossil floras.We calculated the direction and shape of the gradient using quadratic regression to detect linear and curvilinear trends.We performed regressions for the Eocene and Oligocene as well as for informal time intervals within the Eocene:early,middle,and middle + late.We found that quadratic models better explain the data than linear models for both epochs as well as for the early Eocene.A roughly linear trend in the middle and middle + late intervals may reflect limited sampling of high latitude floras for those times.The curvilinear relationship was weak for the Eocene and the model showed a peak in richness at 45.5°N.The curvilinear relationship was much stronger for the Oligocene and the peak occurred at 48.5°N.In the Eocene,the mid-latitude peak in richness may be explained by mean annual temperature,which was probably higher at some mid-latitudes than at lower ones.For the Oligocene,the peak in richness at midlatitudes may be explained by evolutionary diversification within the temperate zone or by increased aridity at low latitudes.We also assessed the latitudinal richness gradient of genera within modern floras in North America north of Mexico and we found a weak,curvilinear trend with a peak in richness at 31.5°N.Our results suggest that the latitudinal genus richness gradient of vascular plants in North America continued to develop into its modern structure following the Oligocene.
基金supported by the I + D + i research project CGL201560805-P (BIOGEOEVENTS)the Grup de Recerca Reconegut per la Generalitat de Catalunya 2017 SGR 824 "Geologia Sedimentària"
文摘The Oligocene–Miocene carbonate record of the Zagros Mountains, known as the Asmari Formation, constitutes an important hydrocarbon reservoir in southern Iran. This marine carbonate succession, which developed under tropical conditions, is explored in terms of larger foraminiferal biostratigraphy, facies analysis and sequence stratigraphy in a new section at Papoon cropping out in the western Fars sub-basin, in the south-east of the Zagros belt. Facies analysis shows evidence of re-working and transport of skeletal components throughout the depositional system, interpreted here as a carbonate ramp. The foraminifera-based biozones identified include the Globigerina–Turborotalia cerroazulensis–Hantkenina Zone and Nummulites vascus–Nummulites fichteli Zone, both of Rupelian age, the Archaias asmaricus–Archaias hensoni–Miogypsinoides complanatus Zone of Chattian age and the ‘Indeterminate’ Zone of Aquitanian age. The vertical sedimentary evolution of the formation exhibits a progressive shallowing of the facies belts and thus the succession is interpreted as a high-rank low-order regressive systems tract. This long-lasting Rupelian–Aquitanian regressive event is in accordance with accepted global long-term eustatic curves. Accordingly, long-term eustatic trends would have been a factor controlling accommodation during the deposition of the Asmari Formation studied in the western Fars sub-basin.
