The Ordos Basin, as the second largest petroliferous basin of China, contains abundant oil and gas resources, oil shale, and sandstone‐type uranium mineral resources. Chang 7 shale is not only the major source rock of the Mesozoic petroliferous system of the Basin, but is also crucial in determining the space‐time distribution relationship of the shale section for the effective exploration and development of the Basin's oil and gas resources. To obtain a highly precise age of the shale development section, we collected tuff samples from the top and bottom profile of the Chang 7 Member, Yishi Village, Yaoqu Town, Tongchuan District, on the southern margin of the Ordos Basin and performed high‐precision chemical abrasion (CA)–isotope dilution (ID)–thermal ionization mass spectrometry (TIMS) zircon U‐Pb dating on the basis of extensive laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) zircon U‐Pb dating data. Our results show the precise ages of the top and bottom zircon in the Chang 7 shale to be 241.06±0.12 Ma and 241.558±0.093 Ma, respectively. We first obtained Chang 7 age data with Grade 0.1‐Ma precision and then determined the age of the shale development in the Chang 7 Member to be the early‐Middle Triassic Ladinian. This result is supported by paleontological evidence. The deposition duration of the Chang 7 shale is 0.5Ma with an average deposition rate of the shale section being 5.3 cm/ka. Our research results provide time scale and basic data for further investigation of the basin–mountain coupling relation of the shale section, the sedimentary environment and volcanic ash and organic‐matter‐rich shale development relation, and the organism break‐out and organic‐matter enrichment mechanism.
Based on isotopic, lithologic and electrical data and logging cycle analysis technique, stratigraphic sequences in the Middle–Upper Cambrian Xixiangchi Group in the Sichuan Basin and its adjacent area are divided, and its sedimentary characteristics and evolution are analyzed. The Xixiangchi Group can be divided into 5 third-order sequences (Sq1–Sq5), of which sequences I to III (Sq1–Sq3) are relatively complete, sequences IV and V are denuded in the Late Cambrian because of the Dian-Chuan paleo-uplifts. Third-order sequences of the Xixiangchi Group in this area have the characteristics of thin in the west and thick in the east, showing that the Caledonian paleo-uplift is a synsedimentary paleo-uplift and the paleogeomorphology in the platform is a gentle slope. Sequence I develops high stand systems tract and transgressive systems tract. The other third-order sequences are dominated by highstand systems tracts, and the transgressive systems tracts last shortly in time and are limited in area. The basic features of evaporative–restricted platform of gentle slope type developed continuously in the sedimentary period of the Xixiangchi Group, its sedimentary environment of "high in west and low in east" and the change of micro paleogeomorphology in the platform control the continuous development of sedimentary facies. Open platform is developed only in sequence I and sequence II, and the inner beach of the platform and the edge beach of the platform are mostly developed in sequence II and sequence III. It indicates that there are two platform margin zones in the study area, a relatively stable, large-scale platform marginal zone in NE Guizhou–Western Hunan and Hubei, and a moving and small-scale platform marginal zone in North Chongqing–Western Hubei.
Trees on the side directly exposed to sunlight generally grow faster than on the opposite side, a phenomenon termed plant phototropism. There are in situ vertical trunks of silicified wood in the Xiadelongwan area of Yanqing County, north Beijing, where the first National Geologic Park of Petrified Wood of China has been built since 2002. A few trunks have well-preserved growth rings. One petrified stump from the formation shows a positive phototropism direction of SW230°. As compared with the modern normal growth stumps in Beijing plain area, which have a positive phototropism direction of SW210 ° ± 5°, the evidence of wood phototropism supports the conclusion of previous palaeomagnetic studies that the North China Plate has rotated clockwise since the Late Jurassic. The known petrified wood stumps in the Yanshan-Liaoning area are mainly found from the strata of 165–136 Ma, which corresponds to the main stage of the Yanshanian Movement.
The early Toarcian oceanic anoxic event(T-OAE)has been studied for nearly 30years.Many western researchers had made a package of discoveries by studying on the sedimentology,paleontology and geochemistry around the West Europe and Mediterranean areas.Since the beginning of the 21st century,the Toarcian event has turned to be one of the international hot topics with regard to the Mesozoic geology,which has been extended beyond the Europe,approaching the Southern Hemisphere.The intensive studies in marine strata illustrate a rising eustatic sea level,high sea surface temperature,anoxic low waters and the eutrophic surface waters in that period,which is associated with the wide distribution of black shales and the abnormity of carbon isotope,oxygen isotope,strontium isotope,osmium isotope,nitrogen isotope and Mg/Ca ratios,and mass extinction.Occurrence of the T-OAE is not only confined to the Boreal Ocean and Tethys Ocean but also the Pathalassa Ocean and the records from both the Northern Hemisphere and Southern Hemisphere demonstrate that it is a global event.However,current studies are generally limited to marine sediments,while the impacts of and responses to the T-OAE in the terrestrial ecological systems are quite poorly known.According to reviews on the previous works on the terrestrial Jurassic system in northern China and the other parts of the world,we summarize the responses in the terrestrial ecological systems as follows:(1)obviously waning or ceasing coal-bearing deposits,emerging red sediments or dark lacustrine shales;(2)distinctively declining in the previous floras,thermophilous plants increasing in floras and immigrating to higher latitude areas;high contents of Classopollis pollen occurring in palynolopical assemblages;(3)temperature rising and aridity turning up,boundary of climatic zones moving towards higher latitude.China yields one of the best Jurassic terrestrial strata across the world.Especially,during the Early-Middle Jurassic in northern China,the climate generally kept warm and humid and thus several large lacustrine basins continuously occurred.The outcrops of Lower-Middle Jurassic are complete and rich in variety of fossils,which provide the ideal places and materials for studying on the impacts of and responses to the T-OAE in the terrestrial ecological systems.With more in-depth studies,scientists are expected to make important scientific contributions in this field.
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