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1.
Ruiliang Wang Shuichang Zhang Simon Brassell Jiaxue Wang Zhengyuan Lu Qingzhong Ming Xiaomei Wang Lizeng Bian 《International Journal of Earth Sciences》2012,101(5):1397-1406
Stable carbon isotope composition (δ13C) of carbonate sediments and the molecular (biomarker) characteristics of a continuous Permian–Triassic (PT) layer in southern China were studied to obtain geochemical signals of global change at the Permian–Triassic boundary (PTB). Carbonate carbon isotope values shifted toward positive before the end of the Permian period and then shifted negative above the PTB into the Triassic period. Molecular carbon isotope values of biomarkers followed the same trend at and below the PTB and remained negative in the Triassic layer. These biomarkers were acyclic isoprenoids, ranging from C15 to C40, steranes (C27 dominates) and terpenoids that were all significantly more abundant in samples from the Permian layer than those from the Triassic layer. The Triassic layer was distinguished by the dominance of higher molecular weight (waxy) n-alkanes. Stable carbon isotope values of individual components, including n-alkanes and acyclic isoprenoids such as phytane, isop-C25, and squalane, are depleted in δ13C by up to 8–10‰ in the Triassic samples as compared to the Permian. Measured molecular and isotopic variations of organic matter in the PT layers support the generally accepted view of Permian oceanic stagnation followed by a massive upwelling of toxic deep waters at the PTB. A series of large-scale (global) outgassing events may be associated with the carbon isotope shift we measured. This is also consistent with the lithological evidence we observed of white thin-clay layers in this region. Our findings, in context with a generally accepted stagnant Permian ocean, followed by massive upwelling of toxic deep waters might be the major causes of the largest global mass extinction event that occurred at the Permian–Triassic boundary. 相似文献
2.
Jin Liu Zhenghong Liu Changqing Yin Chen Zhao Xinyuan Yu 《International Geology Review》2020,62(6):754-780
ABSTRACTThe Changchun-Yanji belt recorded widespread Permo-Triassic magmatism, but their origins remain unclear, inhibiting a comprehensive understanding of the magmatic response to the final closure of the Paleo-Asian ocean in the eastern Central Asian Orogenic Belt (CAOB). Here, we present new geochronological, geochemical, and Hf isotopic data for the Permo-Triassic plutons from Northern Liaoning province, NE China. Combined the published ages with our new data, the Permo-Triassic magmatism in the eastern CAOB can be divided into five episodes: early Permian (293–274 Ma), middle–late Permian (270–257 Ma), latest late Permian–Middle Triassic (255–242 Ma), Late Triassic (240–215 Ma), and latest Late Triassic (209–200 Ma). The middle Permian and Late Triassic mafic plutons (i.e. ~266 Ma Mengjiagou gabbro–diorite, ~240 Ma Jiancaicun gabbro and ~224 Ma Shudetun gabbro-diorite) contain relatively high TFe2O3, MgO, Cr and Ni contents with positive εHf(t) values (+1.2 to +7.2), suggesting a depleted mantle origin. These mafic rocks together with the coeval granitoids make up typical bimodal associations, suggesting that they were formed under an extensional environment. The conclusions are also supported by occurrence of A-type granites during 270–257 Ma and 240–215 Ma. By contrast, the granitoids of 255–242 Ma in the eastern CAOB, including the Jianshanzi (~251 Ma) and Daganhe (~242 Ma) monzogranites, show typical geochemical features of adakitic granites, with high Sr/Y ratios and negative εHf(t) values (–8.6 to – 22.0), suggesting that the magmas were generated through partial melting of thickened ancient lower crust. Combined with previous studies, a four-stage tectonic evolution scenario was proposed: (1) active continental margin stage during 293–274 Ma; (2) continuing subduction resulted in the initiation collision, moderate crustal thickening, and slab break-off during 270–257 Ma; (3) final closure of the Paleo-Asian Ocean associated with continued crustal thickening occurred during 255–242 Ma; (4) lithospheric delamination in a post-collisional extensional environment occurred during 240–215 Ma. 相似文献
3.
基于贵州织金剖面的二叠系-三叠系界线附近的岩石薄片鉴定及古生物化石识别,划分出了2种碳酸盐岩沉积相、2种亚相和5种微相;有机质碳同位素组成测试结果表明晚二叠世时期有机质的δ13Corg值（-27.7‰）向早三叠世时期的δ13Corg值（-28.3‰）逐渐偏负,反映了有机质类型由高等植物向低等微生物变化的过程;全岩的δ13Crock值由晚二叠世（-24.2‰）向早三叠世（-20.9‰）变大,反映了二叠纪末期生物大灭绝事件造成了海洋生物的消失,进而引起12C在海洋中强烈富集;Fe-HR/Fe-T、Fepy/Fe-HR以及微量元素V/（V+Ni）与V/（V+Cr）比值大小,反映了二叠纪末期该区域的海水为氧化环境或弱还原环境,而三叠纪初期海水处于还原与铁化环境,在盆地内部没有发生硫化作用;V/Ni的值从晚二叠世的0.97~1.97向早三叠世的1.67~7.33明显变大,表明水体逐渐加深的过程,该结论与沉积微相变化相符;在生物演化上,从二叠纪末期的有孔虫-藻屑-三叶虫生物组合演变到三叠纪初期小型介壳-微生物组合,反映了该时期中国南方统一的两幕式生物灭绝与演化样式。 相似文献
4.
Interpreting global consequences of the Permian-Triassic (P-Tr) extinction requires examination across paleogeographic realms of Pangaea. The Spiti Valley in India, remnant of the peri-Gondwanan shelf, preserved trails of this environmental catastrophe in the Neo-Tethys Ocean. We document new sedimentological observations and high-resolution trace element concentrations and carbon, oxygen, lead isotope data across the P-Tr boundary in Spiti. Framboidal pyrites, fossils and laminated lithology of the Late Permian shales indicate deeper anoxic depositional environment while δ13Corg excursions of 2.4‰ and 3.1‰ in Atargu and Guling outcrops, respectively, identify the P-Tr transition across a clayey, partly gypsiferous ferruginous layer. Sedimentological similarities of this layer to other Neo-Tethyan sections from Transcaucasia and Iran indicate subaqueous oxidation of shallow marine sediments on a regional scale. Light Rare Earth Element – enriched Late Permian shales with conspicuous Ce–Eu anomalies reflect their source from the adjacent Panjal Trap basalts (ca. 289 Ma) of Kashmir. Continental crustal Nb–Ta and Zr–Hf anomalies appear at the P-Tr boundary sediments, and prevail through the overlying Early Triassic carbonates. Original Pb isotope ratios, along with an increasing Pb abundance closer to the P-Tr boundary, distinguish the volcanic source of the Late Permian shales from the continental crustal siliciclastic signature of the Early Triassic carbonates. Our δ13Corg, trace element and Pb isotope record from Spiti indicate catastrophic changes in sediment sources and facies, with effects on carbon cycle and are consistent with an abrupt episode of marine regression and erosional forcing, also observed elsewhere along northern Gondwanaland. Simultaneous eruption of Siberian volcanics and bolide impacts in Parana basin of Brazil and elsewhere implicating impact-triggered volcanism, left catastrophic regional-global imprints on sea level, climate, marine anoxia and tectonic stability that connected the P-Tr crisis across terrestrial and marine realms worlwide. 相似文献
5.
贵州紫云剖面P/T界面附近碳氧同位素的变化及生物绝灭事件 总被引：12，自引：0，他引：12
:贵州南部紫云剖面由上二叠统长兴组的碳酸盐台地边缘沉积体系、下三叠统罗楼组的碳酸盐浅海陆棚沉积体系、以及下三叠统紫云组的半深海深海沉积体系组成。在前两种沉积体系中选取 18块样品进行碳、氧同位素测试。测试的结果表明 :上二叠统长兴组δ13 C平均值为 2 4 8‰ ,保持了较高的正值 ,δ18O平均值为 - 4 19‰ ;在P/T界面附近 ,δ13 C ,δ18O值都有显著的降低 ,δ13 C值由 2 71‰陡降至 - 0 90‰ ,变化幅度达 3 6 1‰ ,δ18O值由 - 5 0 3‰降为- 9 4 6‰ ,变化幅度可达 4 4 3‰ ;下三叠统罗楼组下部 ,δ13 C值为负值及低正值 ,出现明显的δ13 C值低异常 ,而δ18O值则比较稳定 ,介于 - 4 6 6‰和 - 5 6 2‰之间 ,平均值为 - 5 2 8‰。罗楼组上部 ,δ13 C值有回升的趋势。由于生物对轻碳具有重要的分馏作用 ,因此 ,P/T界面附近δ13 C值的显著变化反映了一次生物大量绝灭的过程。由δ18O值的变化反映出的生物绝灭的外因主要有温度和盐度两方面。从δ18O值可计算出在P/T界面处古温度有 4～ 5℃的升降 ,这势必影响到狭温生物的生存 ;从δ18O值计算出的Z值来看 ,古盐度也有小幅度的变化 ,加速了狭盐度生物的绝灭。事实上 ,生物的绝灭可能是海平面变化、火山事件、温度变化及盐度变化多种因素的综合 相似文献
6.
Several studies of the marine sedimentary record have documented the evolution of global climate during the Permo-Triassic mass extinction. By contrast, the continental records have been less exploited due to the scarcity of continuous sections from the latest Permian into the Early Triassic. The South African Karoo Basin exposes one of the most continuous geological successions of this time interval, thus offering the possibility to reconstruct climate variations in southern Laurasia from the Middle Permian to Middle Triassic interval. Both air temperature and humidity variations were estimated using stable oxygen (δ18Op) and carbon (δ13Cc) isotope compositions of vertebrate apatite. Significant fluctuations in both δ18Op and δ13Cc values mimic those of marine records and suggest that stable isotope compositions recorded in vertebrate apatite reflect global climate evolution. In terms of air temperature, oxygen isotopes show an abrupt increase of about + 8 °C toward the end of the Wuchiapingian. This occurred during a slight cooling trend from the Capitanian to the Permo-Triassic boundary (PTB). At the end of the Permian, an intense and fast warming of + 16 °C occurred and kept increasing during the Olenekian. These thermal fluctuations may be related to the Emeishan (South China) and Siberian volcanic paroxysms that took place at the end of the Capitanian and at the end of the Permian, respectively. Vertebrate apatite δ13Cc partly reflects the important fluctuations of the atmospheric δ13C values, the differences with marine curves being likely due to the evolution of local humidity. Both the oxygen and carbon isotope compositions indicate that the PTB was followed by a warm and arid phase that lasted 6 Ma before temperatures decreased, during the Late Anisian, toward that of the end-Permian. Environmental fluctuations occurring around the PTB that affected both continental and marine realms with similar magnitude likely originated from volcanism and methane release. 相似文献
7.
Carbon and oxygen isotope compositions of Lower-Middle Ordovician carbonate rocks in the northwestern Russian Platform (eastern Ladoga Klint, Lynna River, and Babino quarry sections) are considered. In the studied section interval, average δ13C and δ18O values are 0 ± 0.5 and ?5 ± 0.5‰ (V-PDB), respectively. Two closely-spaced negative carbon isotope excursions with the amplitude of 2‰ are established near the Lower-Middle Ordovician boundary (between the Floian and Dapingian stages). The lower part of the Darriwilian Stage is marked by the gradual decrease in δ13C values to 1‰. Excursions of δ13C do not correlate with δ18O variations and can be considered as primary. The carbon isotope event defined at the Lower-Middle Ordovician boundary is traceable at the interregional level and represents a promising stratigraphic reference level. It may likely be explained by decrease in the relative rate of organic matter burial due to sea level fall and expansion of well-aerated shallow-water basins with a low primary production of phytoplankton. 相似文献
8.
9.
Sylvie Crasquin-Soleau Sylvain Richoz Jean Marcoux Lucia Angiolini Alda Nicora Aymon Baud 《Comptes Rendus Geoscience》2002,334(7):489-495
Very few data on Early Triassic marine ostracods are available. In most of cases, the data concern the upper part of the Early Triassic. The Çürük da? section, located in the Western Taurides (Turkey), well stratigraphically constrained by conodonts and foraminifera, has been re-sampled for ostracod study. A significant fauna has been discovered in the Late Permian and the levels of the Earliest Triassic (Hindeodus parvus and Isarcicella isarcica staeschei zones). The ostracods of the basis of the Kokarkuyu Fm. are the oldest Triassic forms ever discovered. The occurrence of Palaeocopes in the earliest Triassic and similarity between the Permian and Induan assemblages suggest that the Lower Triassic Çürük da? ostracods represent a survival assemblage after the Permo-Triassic mass-extinction. To cite this article: S. Crasquin-Soleau et al., C. R. Geoscience 334 (2002) 489–495. 相似文献
10.
研究海相碳酸盐岩的碳和氧同位素已有三十多年,积累了数千个数据,其目的在于研究古海洋碳和氧同位素的演变。在此期间,一部分研究者认为,海相碳酸盐岩的δ13C值在0±2范围内变化,未表现出与地质时代相关的变化趋势(Clayton和Degens,1959;Degens和Epstein,1962;Keith和Weber,1964;Galimov,1965;Becker和Clayton,1972;Schidlowski等,1975)。但是,另一些学者,如Jeffery等(1955),Baertschi(1975),Compston(1960),Weber(1967),Garrels和Parry(1974)却认为,海相碳酸盐岩的δ13C值随地质时代而有规律地变化。 相似文献
11.
《International Geology Review》2012,54(8):1017-1028
The Palaeo-Tethyan tectonic evolution of central Tibet remains a topic of controversy. Two Permian to Late Triassic arc-like volcanic suites have been identified in the eastern Qiangtang (EQ) block of north-central Tibet. Three competing models have been proposed to explain the formation of these volcanic suites, with two models involving a single stage of long-lived subduction but with opposing subduction polarities, while the other model involves a two-stage subduction process. Here, we present new whole-rock geochemistry, including Sr–Nd isotope data, for late Permian felsic volcanics of the Zaduo area. These volcanics are mainly low to middle K calc-alkaline felsic tuffs and rhyolites with SiO2 concentrations up to 73 wt.%. In primitive mantle-normalized diagrams, the volcanics are typified by large ion lithophile element enrichment and high-field-strength element (e.g. Nb, Ta, P, and Ti) depletion, with slightly negative Eu anomalies. They have initial Sr ratios (87Sr/86Sr) i of 0.70319–0.70547, and ?Nd(t) values of +3.4 to +3.5, suggesting derivation by the partial melting of a depleted mantle wedge, followed by assimilation of crustal material. The available geochemical data indicate the presence of two distinctive igneous evolution trends within the Permian to Late Triassic volcanics of the EQ block, consistent with a two-stage subduction model. Permian to Early Triassic arc-like volcanics are formed during northward (present-day orientation) subduction, whereas the Late Triassic volcanics are related to southward (present-day orientation) subduction of mafic crust of the Garze–Litang Ocean. 相似文献
12.
The severe mass extinction of marine and terrestrial organisms at the end of the Permian Period (c. 251 Ma) was accompanied by a rapid (<100 000 years and possibly <10 000 years) negative excursion of c. 3‰ in the δ13C of the global surface oceans and atmosphere that persisted for some 500 000 years into the Early Triassic. Simulations with an ocean–atmosphere/carbon-cycle model suggest that the isotope excursion can be explained by collapse of ocean primary productivity, and changes in the delivery and cycling of carbon in the oceans and on land. Model results suggest that severe reduction of marine productivity led to an increase in surface-ocean dissolved inorganic carbon and a rapid, short-term increase in atmospheric pCO2 (from a Late Permian base of 850 ppm to c. 2500 ppm). Increase in surface ocean alkalinity may have stimulated the widespread microbial and abiotic shallow-water carbonate deposition seen in the earliest Triassic. The model is also consistent with a long-term (>1 Ma) decrease in sedimentary burial of organic carbon in the early Triassic. 相似文献
13.
《International Geology Review》2012,54(1):116-129
The Arapuçandere Pb–Zn–Cu ore body is a typical vein-type lead–zinc deposit of the Biga Peninsula, and is currently being mined for lead and zinc. In the study area, Permian–Triassic metamorphic rocks, Triassic metaclastic and metabasic rocks, Oligocene–Miocene granitoids, Miocene volcanic rocks, and Quaternary terrigenous sediments crop out. The ore deposits developed as Pb–Zn–Cu-bearing veins along faults in Triassic metasandstone and metadiabase. Microscopic studies reveal that the veins contain galena, sphalerite, chalcopyrite, pyrite, marcasite, covellite, and specular hematite as ore minerals, and quartz, calcite, and barite as gangue minerals. Analysed sulphur-isotope compositions (δ34SVCDT) of galena, sphalerite, and chalcopyrite range from ? 5.9 to ? 1.9‰ (average ? 3.4‰), from ? 5.5 to ? 1.7‰ (average ? 4.2 ‰), and from ? 3.5 to ? 0.9‰ (average ? 2.6‰), respectively; that of H2S in the hydrothermal fluid was in the calculated range of ? 5.8 to +0.1‰ (average ? 2.5‰). These isotopic values suggest that magmatic sulphur dominates in sulphides, mixed with minor, isotopically light sulphur. Because no contemporaneous magmatic activity is associated with mineralization, it may be assumed that sulphur was leached from the surrounding Triassic units, mainly from metabasic, partly from metaclastic rocks. Lead-isotope studies indicate a model age of 114–63 Ma for the lead reservoir, in accord with possible sulphur-bearing local source rocks. Thus, the sulphur and lead deposited in the studied ore veins were probably leached from Triassic metabasic and metaclastic rocks some time during the Early Cretaceous to the Palaeocene. 相似文献
14.
Xi-An Yang Jia-Jun Liu Long-Bo Yang Si-Yu Han Xiao-Ming Sun Huan Wang 《Mineralogy and Petrology》2014,108(2):303-315
The Yangla copper deposit, with Cu reserves of 1.2 Mt, is located between a series of thrust faults in the Jinshajiang–Lancangjiang–Nujiang region, Yunnan, China, and has been mined since 2007. Fluid inclusion trapping conditions ranged from 1.32 to 2.10 kbar at 373–409 °C. Laser Raman spectroscopy confirms that the vapour phase in these inclusions consists of CO2, CH4, N2 and H2O. The gas phases in the inclusions are H2O and CO2, with minor amounts of N2, O2, CO, CH4, C2H2, C2H4, and C2H6. Within the liquid phase, the main cations are Ca2+ and Na+ while the main anions are SO4 2? and Cl?. The oxygen and hydrogen isotope compositions of the ore-forming fluids (?3.05‰?≤?δ18OH2O?≤?2.5‰; ?100‰?≤?δD?≤??120‰) indicate that they were derived from magma and evolved by mixing with local meteoric water. The δ34S values of sulfides range from ?4.20‰ to 1.85‰(average on ?0.85‰), supporting a magmatic origin. Five molybdenite samples taken from the copper deposit yield a well-constrained 187Re–187Os isochron age of 232.8?±?2.4 Ma. Given that the Yangla granodiorite formed between 235.6?±?1.2 Ma and 234.1?±?1.2 Ma, the Cu metallogenesis is slightly younger than the crystallization age of the parent magma. A tectonic model that combines hydrothermal fluid flow and isotope compositions is proposed to explain the formation of the Yangla copper deposit. At first, westward subduction of the Jinshajiang Oceanic Plate in the Early Permian resulted in the development of a series of thrust faults. This was accompanied by fractional melting beneath the overriding plate, triggering magma ascent and extensive volcanism. The thrust faults, which were then placed under tension during a change in tectonic mode from compression to extension in the Late Triassic, formed favorable pathways for the magmatic ore-forming fluids. These fluids precipitated copper-sulfides to form the Yangla deposit. 相似文献
15.
重庆中梁山地区二叠、三叠系界线附近生物更替的研究 总被引：2，自引：1，他引：2
<正> 二叠、三叠系界线附近海生无脊椎动物发生了明显的更替。许多古生代繁盛一时的类群在界线上消失,其中包括三叶虫、四射珊瑚、横板珊瑚、等。腕足动物长身贝目和戟贝目的大部分分子也于界线附近绝灭,只残存一些个体较小、壳质较薄的分子延续于三叠系的最底部。造成这些生物类群绝灭的原因目前认识很不一致。最近,国际地质科学工作者提出了“新灾变论”假说,并且利用这一理论来解释白垩纪末恐龙绝灭的原因,但是,在研究了重庆中梁山地区一些门类的生物组合后,得出的结论是这一更 相似文献
16.
V. R. Lozovsky Yu. P. Balabanov E. V. Karasev I. V. Novikov A. G. Ponomarenko O. P. Yaroshenko 《Stratigraphy and Geological Correlation》2016,24(4):364-380
The comprehensive analysis of the data obtained on terrestrial vertebrata, ostracods, entomologic fauna, megaflora, and microflora in deposits of the Vyaznikovian Horizon and Nedubrovo Member, as well as the paleomagnetic data measured in enclosing rocks, confirms heterogeneity of these deposits. Accordingly, it is necessary to distinguish these two stratons in the terminal Permian of the East European Platform. The combined sequence of Triassic–Permian boundary deposits in the Moscow Syneclise, which is considered to be the most complete sequence in the East European Platform, is as follows (from bottom upward): Vyatkian deposits; Vyaznikovian Horizon, including Sokovka and Zhukovo members; Nedubrovo Member (Upper Permian); Astashikha and Ryabi members of the Vokhmian Horizon (Lower Triassic). None of the sequences of Permian–Triassic boundary deposits known in the area of study characterizes this sequence in full volume. In the north, the Triassic deposits are underlain by the Nedubrovo Member; in the south (the Klyazma River basin), the sections are underlain by the Vyaznikovian Horizon. The Permian–Triassic boundary adopted in the General Stratigraphic Scale of Russia for continental deposits of the East European platform (the lower boundary of the Astashikha Member) is more ancient than the one adopted in the International Stratigraphic Chart. The same geological situation is observed in the German Basin and other localities where Triassic continental deposits are developed. The ways of solving this problem are discussed in this article. 相似文献
17.
Paleo‐fires and Atmospheric Oxygen Levels in the Latest Permian: Evidence from Maceral Compositions of Coals in Eastern Yunnan,Southern China 总被引：1，自引：0，他引：1
Inertinite maceral compositions of the Late Permian coals from three sections in the terrestrial and paralic settings of eastern Yunnan are analyzed in order to reveal the paleo-fire events and the atmospheric oxygen levels in the latest Permian. Although the macerals in the studied sections are generally dominated by vitrinite, the inertinite group makes up a considerable proportion. Its content increases upward from the beginning of the Late Permian to the coal seam near the Permian-Triassic boundary. Based on the microscopic features and the prevailing theory that inertinite is largely a by-product of paleo-fires, we suggest that the increasing upward trend of the inertinite abundance in the latest Permian could imply that the Late Permian peatland had suffered from frequent wildfires. Since ignition and burning depend on sufficient oxygen, a model-based calculation suggests that the O2 levels near the Wuchiapingian/Changhsingian boundary and the Permian-Triassic boundary are 27% and 28% respectively. This output adds supports to other discoveries made in the temporal marine and terrestrial sediments, and challenges the theories advocating hypoxia as a mechanism for the Permian-Triassic boundary crisis. 相似文献
18.
吴家坪阶-长兴阶界线位于这两次生物大灭绝事件之间,科学界对该界线附近是否发生重大地质事件仍然知之甚少,有关该时期的环境变化、碳循环的研究也不深入.嘉禾大窝岭剖面牙形石生物地层以及碳同位素变化的研究,为进一步探讨吴家坪期-长兴期附近的生物及环境事件提供基础材料.晚二叠世时以深水盆地相沉积为主的大窝岭剖面位于湖南省嘉禾县袁家镇附近.该剖面大隆组出露较好,岩性主要为硅质岩、硅质灰岩、灰岩以及泥岩.在大隆组中共识别出牙形石1属3种(含一个未定种):Clarkina wangi,C. deflecta,C. sp.,并识别出牙形石C. wangi带和C. changxingensis-C. deflecta组合带.根据C. wangi的首现,将大窝岭剖面的吴家坪阶-长兴阶界线(Wuchiapingian-Changhsingian boundary,简称WCB)置于第11层底部.该剖面全岩无机碳同位素数据显示在13~15层发生了一次快速的负偏,从2.18‰负偏到-1.39‰,负偏值近3.50‰,可与浙江煤山及四川上寺等剖面进行很好的对比,揭示这次碳同位素负偏在华南区域上可具对比性. 相似文献
19.
AbstractThe Northern part of Great-India underwent an early rifting phase in the late Paleozoic, just at the end of the large scale Gondwanian glaciation. The beginning of the rifting processes is marked by large hiatus and discontinuities (para- conformities) between the early or middle Paleozoic sedimentary succession and the discontinuous middle-late Permian Traps and transgressive sediments. The Northern Indian passive margin consists of the present High and Lower Himalaya and a small part of the Indian craton and their sedimentary cover. The Permian rift shoulder is located in the Higher Himalaya, with part being in the underthrusted Lower Himalaya. The rim basin (landward of the shoulder) is well developed in the Pottawar- Salt Range area. From the rifting to the beginning of the drifting stages (early late Permian to late early Triassic time), the sedimentary evolution is characterised by three transgressive- regressive (T-R) second order cycles, two in the late Permian and one in the early Triassic. The break-up of the rift occurred during the second cycle (late Dzhulfian).In the Salt Range area, these three T-R cycles have been subdivided in eight third order sequences, five sequences for the upper Permian and three for the lower Triassic.At the end of Permian, hiatuses, gaps and local erosion of part of the margin are direct consequences of a first order relative sea-level fall; this is also the time of the largest extinction event of the Phanerozoic that deeply affected the carbonate productivity and the stratal patterns. With the following worldwide sea-level rise, a rapid and large scale transgression occurred in the early Triassic, well dated and recorded on the whole margin. High rate thermal subsidence gave way to generalized pelagic deposits about 2 My after the transgression.Profiles of whole rock inorganic carbon and oxygen isotopes from Guryul Ravine and Palgham sections in Kashmir, Nammal Gorge and Landu sections in Trans Indus Ranges (Pakistan), Thini Chu section in Kali Gandaki Valley, Central Nepal are presented in connection with the sequence stratigraphic analysis. The upper Permian record of high positive δ13C values are closely correlated with the second order T-R cycles and the third order sequences. The results presented in this study confirm the drastic drop of δ13C from the high positive values that characterised the upper Permian to lower values in the lower Triassic time. Stratigraphic correlation problems in the lower Triassic using carbon isotope geochemistry are briefly discussed. A positive δ13C excursion of 4–5% near the Smithian - Spathian substages boundary is observed for the first time. The δ18O values of samples from all the sections display major variations suggesting that the oxygen isotope record has been significantly affected by meteoric diagenesis, deep burial diagenesis or/and monsoon signature. 相似文献
20.
Paired organic and carbonate carbon isotope compositions of Late Permian Wujiaping (吴家坪) and Dalong (大隆) formations at Shangsi (上寺), Northeast Sichuan (四川) were analyzed by MAT 251. An abrupt negative excursion in the two isotope records was observed in the middle part of Dalong Formation, in association with a drop in the carbon isotope difference of the two records and an increase of total organic carbon (TOC) content. The negative drop of the paired carbon isotope records issuggestive of the input of 12C-enriched CO2. The molecular ratios of pristane to phytane and dibenzothiophene to phenanthrene indicate the anoxic condition in this interval. The enhanced TOC content is indicative of the elevated preservation of organic matter due to the anoxic condition. These isotopic and organic geochemical data probably infer the occurrence of the upwelUng in this interval. The additional contribution of volcanism activity observed in South China cannot be excluded to the input of 12C-enriched CO2 and the negative shifts in carbon isotope composition of bulk organic matter and carbonate. 相似文献