The Early Cretaceous Jehol Biota And Stratigraphy In The Northern Great Xing'an Range And Adjacent Areas

1. The Lithostratigraphic division and correlations of the Xinganling GroupThe Late Jurassic to Early Cretaceous volcano-sedimentary successions are widely distributed in the northern Great Xing'an Range area. Scientists named them Xinganling Group, and author of current thesis considered that the basic definitions of this group are realizing thus should be used keep on as a formal Group-level lithostratigraphic unit.The meanings of the Xinganling Group used in current thesis are as flowing:a series of late Mesozoic volcano-sedimentary successions distributed in Great Xing'an Range area and adjacent areas. The group developed unconformablely above the pre-Late Jurassic strata, and covered by the coal bearing Zhalainuoer Group and other comparable units. The extent of the group is mainly in Great Xing'an Range area (including uplifted areas and basins of Dayangshu Basin, Hailar-Tamsag Basin, and Erlian Basin). The western and northern extent is restricted by the national boundaries. The eastern extent arrived at the western edge of Songliao Basin. The southern extent lasted until at area of the Xilamulun River, where they joined to the Jinlingsi Group and Jehol Group mainly developed at Yan-liao areas (Northern Hebei and Western Liaoning).For a long time, the divisions of the formations of Xinganling Group are under dispute, there are many plans and their correlation relationships are consusion. In order to resolve this problem, current thesis refered to previrous scientists'works on the division of stratigraphic district, and considered the litho-associations, ages, status of previous works, and outcrop conditions, current thesis established a stratigraphic minor region division model:①Longjiang-Huma (eastern slope of the Great Xing'an Range);②Arshan-Argun (western slope of the range);③Hailar Basin;④Erlian Basin;⑤Wengniuteqi-Wulanhaote (southern of the range). Different stratigraphic nomenclature and divisions were used in each minor region, and this should be useful to avoid dispute and chaos on who to name the units.Since the Late Jurassic-Early Cretaceous successions in Aershan-Argun Minor Region are more complete and better understanded, thus the stratigraphic division model in this minor region has been chosen as a rule to be used in regional stratigraphic correlations. The sequence of the successions is as flowing:Lower Subgroup, Tamulangou Formation (J2-3, Basalt), Jixiangfeng Formation and Murui Formation (J3-K1?, trachyte, rhyodacite, pyroclasts); Upper Subgroup, Walagan Formation (K1, andicite, basalt), Shangkuli Formation (K1, trachyte, rhyodacite, pyroclasts, shales), Yiliekede Formation (K1, coal-bearing basalts andesitic basalts). The Lower Subgroup (160-131 Ma) as a whole could be correlated with the Jinlingsi Group which developed in North Hebei-West Liaoning, and the Upper Subgroup (130-110 Ma) could be correlated with the Jehol Group, but these relationships don't meant isochronal correlation. The upper part of Lower Subgroup and the lower part of Jehol Group (Zhangjiakou Formation and Dabeigou Formation) have the close geological age, but it is feasible to put the prior into the Lower Subgroup, according to the Lithostratigraphic framework.The Xinganling Group could be subdivided into three volcanism cycles with each evolved from basic to acidic volcanics, lengthways. The basic and acidic volcanics of each cycle have a certain extent of finger interlude (synchrono-heteropic facies variety) relationships, hence, each composed of bimodal volcanic rock associations. The first cycle is correspond to the Tiaojishanian-Zhangjiakouan volcanic cycles in North Heibei-West Liaoning, therefore, would be possibly further subdivided into two cycles and each correlated to the first two late Mesozoic syn-rifting cycles in East Gobi Basin of Mongolia; The second cycle correlated to the Yixianian volcanic cycle in western Liaoning; The third cycle correlated the Daxingzhuangian volcanic cycle in western Liaoning and the Yingchengian volcanic cycle in the Songliao Basin. Each two in succession cycles could be divided sharply (completely), sometimes they have the transitional relationships.The Xinganling Group, defined and divided in current thesis, have a import character of composing many lacustrine sediments. Those lacustrine sediments are important oil-generation beds of Hailar Basin and Erlian Basin in China, and East Gobi Basin in Mongolia. Some kinds of volcanis, like the stomatic basalts, are likely to be the good oil-gas bearing beds. The homochronous strata extend to Songliao Basin and eastern Heilongjiang are gradually changed into non marine volcanic-sedimentary coal-bearing strata and marine-terrigenous coal-bearing strata, and they have the potential for generation of natural gas.2. Biostratigraphy and ChronostratigraphyThe upper part of Lower Xinganling Subgroup yield the fossils of the first evolutionary phase of Jehol Biota, the Chonchastracan fossils formed the Nestoria-Sentestheria assemblage, which correspond to the Nestoria-Keratestheria fauna in North Hebei. The Upper Xinganling Subgroup yields the fossils middle and late phases of Jehol Biota.The fossils from northern Great Xing'an Range area are quite same of those in the core-region (North Hebei-West Liaoning) of the Jehol Biota. At the Longjiang-Huma region, the lower part of Jiufengshan formation yield the typical fossils of middle phase Jehol Biota, which is Eosestheria-Ephemeropsis trisetalis-Lycoptera assemblage.At the Arshan-Argun region, the Shangkuli Formation yield the Arguniella and Lycoptera which all belongs to the middle phase Jehol Biota. The Yiliekede Formation yield two fishes, Yanosteus sp. and Sinamia sp. This fish assemblage correlated to the fish assemblage of Jiufotang Formation in western Liaoning, and their strata also have the same age. Both the microfossils (spore-pollen) and mega-plants from the Yiliekede Formation represent a gymnosperm dominated flora, whose appearance looks quite like the flora described by Heer in 1876 from the Amurlands of Russia.There are 59 isotope ages from the Xinganling Group volcanics been stated and analyzed. The conclusion is that the whole of the group spans from 160 Ma to 110 Ma. The Lower subgroup are mainly between 160 Ma to 140 Ma, and the Upper subgroup concentrated within 130 Ma to 120 Ma. The volcanism achieved the peak at 125 Ma, and volcanics generated at this phase widely distributed at the whole region. The isotope ages of Xinganling Group have the same chronological framework of eastern China Mesozoic volcanism. Because the absolute age at the boundary of Jurassic and Cretaceous had not finally confirmed in the ISC (International Stratigraphic Chart), and the lithostratigraphic units have an all-pervading characteristic of diachronism, thus the two subgroups separately correspond to but not firmly constrained within Late Jurassic and Early Cretaceous.3. Paleobiogeography and Significances on Stratigraphical Correlation of Peipiaosteidae and SinamiidaeSinamiidae is an extinct clad of the Amiiformes, Holostei. They had been discovered only from the Early Cretaceous sediments in eastern Asia, and they were important members of the Jehol Biota. The family is composed of three genus and eight species, and the Sinamia has the widest distribution among the entire genus of Jehol Vertebrates Fauna. The geographic distribution of the Family ranges to the most northern part of NE China to the north, ranges to the eastern of Thailand in Southeast Asia to the south, ranges to the Jiuquan Basin of Gansu Province to the west, and ranges to the middle part of Japan to the east. The Sinamiidae occurred in several local characteristic fish faunas, they are:the Lycoptera-Peipiaosteus Fish Fauna (including the Jiuquanichthy Fish Fauna in Jiuquan Basin) in North China, Northeast China, and part of Northwest China, the Mesoclupea-Paraclupea Fish Fauna (including the Itoshiro, Wakino Fish Fauna in Japan and the Nagdong Fish Fauna in South Korea), and the Early Cretaceous fish fauna in the eastern of Thailand. The Family's wide distribution in the East Asia is one of the evidences to prove that the East Asia separately formed a paleobiogeographic region during Early Cretaceous. The Peipiaosteidae might exist only within the northern extent (northern Great Xing'an Range, Mongolia, Transbaikalia of Russia) and core-region of Jehol Biota, this implied that those areas ever had closer river systems and more similar paleoenvironment.As currently knows, Sinamiidae's geologic range is Barremian-Albian, and they extended to the widest range of their distribution during the Aptian. The wide distribution, concentrated geologic range, and the cleared diagnostic characters made this kind of fossils quite helpful in enhancing the precision of stratigraphical correlation or providing a good validation.Be compared with other extents of Jehol Biota at western China, southeastern China, Japan, and Korea, the northern extent have the much more similar vertebrate assemblage toward that from core-region. Actually, the geographic distance between northern extent and core-region is longer than other extents with less similar biota. Thus, biota's similarities would be controlled by their similar paleogeographic and paleoclimate backgrounds. For instance, they all belongs to the Late Jurassic to Early Cretaceous "Hinggan volcanic basins" and Early Cretaceous "N. China humid warm to temperate zone", Early Cretaceous "temperate-warm temperate floras", Late Jurassic? to Early Cretaceous "semi-tropical-warm temperate zone, semi-humidity", respondly with other extents not include those regions. However, the plaeoclimate might not such "warm" as they considered, according to some new evidences.4. A comparison of Early Cretaceous vertebrate faunas In the Early Cretaceous continental ecosystems, vertebrate faunas with unique taphnomic characters (Lagerstatten preservation quality) are distributed both on the Eurasia and Gondwana. They are Jehol Biota in North Hebei-West Liaoning; Xiagou Biota in Changma Basin, Gansu, which is a sub-biota belongs to Jehol Biota; Pietratoia Biota in Italy; Las Hoyas Biota in Spain; Santana Biota in Brazil; Lagarcito pterosaur fauna in Argentina, and most of them are exist within lacustrine environments. The Australian Victorian Polar Biota and Transbaikalian Baissa Biota (also a sub-biota of Jehol Biota) have the potential to be additional Lagerstattens.There was a long distance between Eastern Asian Jehol Biota and Australian Victorian Polar Biota during Mesozoic, and their biotas didn't have much close evolutionary relationships. However, similar Early Cretaceous continental ecosystem frameworks were developed in two areas separately. The main and holistic climate circumstance of the earth is warm during the Mesozoic, this made the Victorian Polar Biota, which located at the southern polar region during Early Cretaceous, under went a wormer climate like that at nowadays middle or middle-high latitude, but not the climate as cold as that of nowadays polar region. North Heibei-West Liaoning-Great Xingan Range-Transbaikalia areas might be a huge volcanic high-lands, their paleo-altitude may resemble to present-day Changbai Mountain area of NE China. So that, those places which located at the paleo-middle latitude missed the regularly worm climate that such latitude area should be, and a certain period or times of cold climate afected those regions. Therefore, the paleo-latitude and paleo-altitude contributed separately to the similar climate conditions of Victorian Polar Biota and Jehol Biota, and then the similar paleoecosystems represented at two areas. The establish of this paleoclimate driving model, and with the recently published data on the quantitative analysis on the paleoclimate, which indicate that the Jehol Biota had under went certain times and period cold climate, could explain the so said "contradiction" between the "Mesozoic eastern China Plateau hypothesis" and the paleoecology of Jehol Biota, thus such "contradiction" might not exist.The formation of those Early Cretaceous lacustrine Lagerstatten of vertebrates, might related to each special conditions and mechanisms. Toward to the different localities of Jehol Biota, should analyze them differently, to avoid use only one thoughts. Some sections in the northern Great Xing'an Range and Changma Basin in Gansu, ecorded the vertebrates mass mortality events in the early Cretaceous lakes, but the stratigraphic and sedimentary characters show no sign of volcanism affect, weather they formed under the condition comparable to the model of CO2-driven lake eruptions should be further investigated.The paleontological studies on Jehol Biota in the future, should concern not only the volcanic-sedimentary strata which had been proved of good conditions for fossils'accumulation and preservation, but also the sedimentary types of lacustrine carbonates which yield Lagerstatten like Santana and Las Hoyas biotas. There are abundantly lacustrine sediments exist within the Lower Cretaceous Xinminpu Group in Gansu and Liupanshan Group in Ningxia, western China, thus the future works should devote more attentions to find fossilifeous laminated carbonates.