華北東北部晚中生代陸相地層、生物群及其年代學(xué)研究

柳永清, 季強, 姬書安, 尤海魯, 呂君昌, 曠紅偉,江小均, 彭楠, 許歡, 袁崇喜, 王旭日

中國地質(zhì)科學(xué)院地質(zhì)研究所, 北京 100037

華北東北部晚中生代陸相地層、生物群及其年代學(xué)研究

柳永清, 季強, 姬書安, 尤海魯, 呂君昌, 曠紅偉,江小均, 彭楠, 許歡, 袁崇喜, 王旭日

中國地質(zhì)科學(xué)院地質(zhì)研究所, 北京 100037

華北東北部晚中生代陸相地層十分發(fā)育，從下至上為九龍山組、髫髻山組、土城子組、張家口組、大北溝組、義縣組和九佛堂組。它們主要的巖石組合為陸相火山巖熔巖、火山碎屑巖和間夾的河流-湖泊相沉積巖。九龍山組、髫髻山組（165-154 Ma）火山沉積巖系中發(fā)育以帶毛恐龍（近鳥龍）、哺乳動物、翼龍和昆蟲類等為代表的燕遼生物群（J2-J3），晚侏羅世末期土城子組時期(154-139Ma)該生物群大多數(shù)生物屬種消亡，并在早白堊世早期（張家口組、大北溝組、義縣組和九佛堂組）(144-120Ma)萌生新的生物群-熱河生物群（K1），以恐龍、哺乳類、被子植物、魚類和昆蟲等為特征。中侏羅世-早白堊世燕遼生物群和熱河生物群表現(xiàn)了明顯的演化關(guān)系，響應(yīng)著古環(huán)境變化與重大地質(zhì)事件發(fā)生的復(fù)雜地球表層圈層作用。

Late Mesozoic terrestrial strata are widely distributed across north-east China in western Liaoning, northern Hebei and southern Inner Mongolia. Due to the discovery of the famous Yanliao Biota ( J2-J3) (Ji et al., 2004), the Jehol Biota (K1) (Chen et al., 1999) and geological relationships with the destruction of the North Craton (Gao et al., 2004), Late Mesozoic strata and volcanic rocks in these areas have attracted the attention of paleontologists and geologists worldwide.

The Late Mesozoic terrestrial stratigraphic succession in north-east China contains two well-preserved terrestrial biotas and consists in ascending order of the Middle Jurassic Jiulongshan Formation and Tiaojishan Formation, the Upper Jurassic-Lower Cretaceous Tuchengzi Formation, and the Lower Cretaceous Zhangjiakou Formation, Dabeigou Formation, Yixian Formation and Jiufotang Formation (Fig.1). Both the Jiulongshan Formation and the Tiaojishan Formation have yielded fossils of the Yanliao Biota. The fossils of the Jehol Biota occur within the Lower Cretaceous Zhangjiakou Formation, Dabeigou Formation,Yixian Formation and Jiufotang Formation.

The Jiulongshan Formation, (called the Haifanggou Formation in Hebei Province), contains a fossil assemblage that includes conchostracans, bivalves, a number of insects and abundant plants. It has a thickness of more than 200 m and is mainly composed of sandstones, conglomerates, shales, interbedded pyroclastic rocks (tuffs) and, occasionally, some poor coal seams. There are more conglomerates in the lower part of the Jiulongshan Formation and the proportion of pyroclastic rocks increases upwards. The 200–800 meter thick Lanqi (Tiaojishan) Formation of western Liaoning(called the Tiaojishan Formation in Hebei and Inner Mongolia), mainly consists of rhyolites, basalts and andesites in the lower and middle part, acid tuffs, purple red tuffaceous siltstones, sandstones and conglomerates. Abundant fossils including cochostracans, insects, plants and several vertebrates, such as baby frogs, salamanders, pterosaurs, feathered dinosaurs and mammals, have been found recently in the Tiaojishan Formation of northern and eastern Hebei, western Lioaning and southern Inner Mongolia.

Fig. 1 Generalized stratigraphic column from Middle Jurassic to Lower Cretaceous in NE China

The Tuchengzi Formation is widely distributed in an E-W trend in the northern area of North China. It has a variable thickness, ranging from 160-4425 m, and is similar across western Liaoning, northern Hebei and central-southern Inner Mongolia. It mainly consists of purplish red conglomerates with interclations of tuffaceous sandstones, siltstones and tuffs in the lower member, grey-purple siltstones, sandstones and conglomerates intercalated with sandstones in the middle member and grey-purple conglomerate intercalated with green tuffaceous sandstones in the upper member. In addition, there are occasional intercalations of andesitics, rhyolitics, trachyandesitics and pyroclastic rocks in the middle and upper part.

The Tuchengzi Formation rests unconformably on the Tiaojishan Formation or Pre-Mesozoic units. Contact relationships between the Tuchengzi Formation and underlying volcanic rocks of the Tiaojishan Formation are generally described as disconformable, but field observations made during this study and previous evidence of age relationships in most localities indicate that there is no significant break in time between the underlying Tiaojishan and Tuchengzi episodes of deposition. Similarly, the Tuchengzi Formation is usually conformably overlain by silicic volcanic rocks of the Lower Cretaceous Zhangjiakou Formation although, occasionally, an angular unconformity is present.

The Zhangjiakou Formation shows a north-east trending distribution in north-east China. The main lithologys of this formation, which varies in thickness from 200-1000 m, are rhyolites, dacite, rhyodacitic trachyandesitic tuffs and pyroclastic rocks with interclations of tuffaceous sandstones, siltstones and grey-purple shales, siltstones, sandstones and conglomerates.

The Zhangjiakou Formation either rests unconformably on Pre-Mesozoic units or shows a variably conformable or unconformable contact with the underlying Tuchengzi Formation. Field observations made during this study indicate that there is no significant break in time with the underlying Tuchengzi deposition episode. The Dabeigou Formation shows atemporal and spatial interfingering with the Zhangjiakou Formation and consists mainly of grey-purple shales, siltstones, sandstones and tuffaceous sandstones. It yields the conchostracan Nestoria, the mayfly Ephemeropsis trisetalis and the fish Peipiaosteus. The 790–1400 m thick Yixian Formation, which is rather variable in thickness and lithology, is composed mainly of andesites, basalts and interbedded tuffs, tuffaceous sandstones, shales, mudstones, siltstones and conglomerates. The Jiufotang Formation is 800–1200 m thick and is composed mainly of mudstones, siltstones, shales, sandstones and tuffs. It also shows a temporal and spatial interfingering with the Yixian Formation.

Fig. 2 Systematic geochronological calibration for the Mesozoic terrestrial stratigraphy of the Yanliao and Jehol Biotas in north-east China

These two well-preserved Mesozoic terrestrial biotas, the Yanliao Biota (J2-J3) and the Jehol Biota (K1), are widely distributed across eastern-northern Asia. Previous studies referred to fossils within the Jiulongshan and Tiaojishan Formations as the Yanliao Biota (and the well-known Daohugou Biota in Ningcheng, Inner Mongolia, which is equivalent to the Yanliao Biota) (Liu et al., 2006). This is mainly composed of terrestrial vertebrates including dinosaurs (the earliest feathered dinosaurs), the earliest swimming and flying mammals, transitional pterosaurs and salamanders together with plants and a large numberof well-preserved insects and freshwater invertebrates.

The Jehol Biota, defined initially as the Eosestheria-Ephemeropsis-Lycoptera assemblage, is widely distributed in eastern and central Asia. Most fossils of the Jehol Biota were recently excavated from the Yixian Formation and the overlying or interfingering Jiufotang Formation in north-east China. Fossils found so far include many vertebrates such as dinosaurs, feathered dinosaurs, birds, fishes, mammals, pterosaurs, turtles and other reptiles, together with plants and freshwater invertebrates, primitive insects, and conchostracans.

Fossils from these two biotas are exceptionally well-preserved in the Middle Jurassic or Lower Cretaceous interbedded fine-grained lacustrine sediments or tuffs. Both the Yanliao Biota and the Jehol Biota are among the most important of Mesozoic terrestrial biotas and have provided a rare opportunity to address questions about the origin and evolution of birds, mammals and dinosaurs, the evolution of feathers and flight, the early diversification of angiosperms, the evolution of insects and the timing of the radiation of placental mammals.

However, the geographic distribution of these two terrestrial biotas is very different. Members of the Jehol Biota occupied a much larger region than those of the Yanliao Biota. The habitat areas for the latter were confined to a limited number of locations in western Liaoning or north-eastern Hebei in China, although they have also frequently been reported from the Far East (Russia) and central Asia (Mongolia). Moreover, the Jehol Biota shows a different geographical extent and lateral extension through time in the geochronological episodes of 144-130Ma, 130-110 Ma and postdating 110 Ma.

Establishing a high-precision age framework for both the Mesozoic strata and the terrestrial faunas they contain is critical for understanding the behaviour, changes through time, transitions and evolutionary significance of the two biotas. However, geochronological studies and summarized results of geochronology for the Mesozoic strata and biotas they contain have rarely been systematic. Previously, absolute ages were generated by several different methods:40Ar/39Ar; U/Pb; Rb-Sr and K-Ar. As a result, accurate time points for the two biotas and a precise geochronological framework for these biotas, their fossils and the hosting strata remain to be established. Here, we present the results of new high-precision dating for these Mesozoic strata. In combination with previous results we have systematically clarified the ages of the biotas and their hosting strata and developed a systematic geochronological framework (Fig.2).

JI Q. 2004. Mesozoic Jehol Biota of western Liaoning, China (in Chinese). Beijing: Geological Publishing House.

CHEN P J. 1999. Distribution and spread of the Jehol Biota (in Chinese), Palaeoworld, 11: 1–5.

GAO S, RUDNICK R L., YUANG H L. 2004. Recycling lower continental crust in the north China craton. Nature, 432: 892-897.

LIU Y Q, LIU Y X, JI Shu-an, YANG Z Q. 2006. U-Pb zircon age for the Daohugou Biota at Ningcheng of Inner Mongolia and comments on related issues. Chinese Science Bulletin, 51(21): 2634-2644.

Late Mesozoic Terrestrial Stratigraphy, Biotas and Geochronology in North-east China

LIU Yong-qing, JI Qiang, JI Shu-an, YOU Hai-lu, Lü Jun-chang, KUANG Hong-wei, JIANG Xiao-jun, PENG Nan, XU Huan, YUAN Chong-xi, WANG Xu-ri

Institute of Geology，Chinese Academy of Geological Sciences, Beijing 100037, China