Ferns From The Early Cretaceous Changcai Formation In Yanbian Area, Jilin

The late Mesozoic terrestrial strata are well developed in Yanbian area of eastern Jilin, China. It yields abundant fossil flora and fauna, being an important part in the Mesozoic biota and non-marine deposits in China. Quite a number of Chinese and foreign paleontologists and geologists did lot of work on the division and correlation of the late Mesozoic strata in this area. For the first time, Yang Xuelin (1959) divided the coal-bearing strata in Yanbian area which overlying on the Tuntianying volcanics into two formations i.e. Xishangping Formation (Fm.) and Changcai Fomation. Later, since the two formations are difficult to distinguish each other, they were combined to be called Changcai Formation (s. l.). The geological age of the Changcai Formation (s. l.) is controversial due to the lack of strong evidences.Since 1950's, 42 species of 11 genera of ferns have been recorded from the Changcai Formation (s. l.) in Yanbian area (Yang Xuelin, 1959; Zhou Zhiyan et al, 1980; Zhang Wu et al, 1980; Shang Qinghua, 1992; Bureau of Geology and Mineral Resauces of Jilin Province, 1988, 1992) based on the gross morphology. There is no anatomic research on the fertile organs of these fossil fern leaves.There are some isotopic dating data on the early Cretaceous volcanic rocks in Yanbian area in recent years (Chen Yuejun, 2003; Li chaowen et al, 2007), but they are weak in confining the geological age of the volcano-sedimentary strata in this area due to the fact that either there is no biostratigraphic evidence to be coupled with the isotopic dating data, or the isotopic dating data is conflict with the biota.For better understanding the Upper Mesozoic sequences and the biota of the Changcai Formation, detailed work were undergone, including the field investigation and collection of fossil plants and isotopic dating samples, and the identification and description of the ferns from the early Cretaceous Changcai Formation. Finally, the characteristics of the fossil ferns and the age of the Changcai Formation were discussed systematically, and some achievements are as follows:1. The Upper Mesozoic strata in Yanbian area can be divided in ascending order into early Cretaceous Tuntianying Fm., Changcai Formation (s. l.), Quanshuicun Fm., Toudao Fm., Tongfosi Fm., early-late Cretaceous Dalazi Fm. and late Cretaceous Longjing Fm. The Lower Cretaceous Helong Group is characterized by a suit of medium volcanic rocks, volcano-clastic rocks and coal-bearing deposits, and composed of Tuntianying Fm., Changcai Fm. and Quanshuicun Fm. ascendinally. The Tuntianying Fm. is composed mainly of medium volcanics and volcanoclastic rocks, intercalated with black sandstone and shale. The Changcai Fm. (s. l. including the Xishanping Fm.) is composed mainly of riparian-lacustrine swamp coal-bearing deposits, intercalated with andicite and tuff. The Quanshuicun Fm. is mainly composed of medium volcanics and volcano-clastic rocks; some times, the sandstone and conglomerate at the bottom. These three formations are quite clear independent sequences marked by the coal-bearing strata of the Changcai Fm. The Helong Group is outcropped mainly in the Helong basin and the western part of the Yanji basin. While, the Yanji Group outcropped mainly in the eastern part of Yanji basin, unconformably overlying on the Helong Group, is composed of Toudao Fm., Tongfosi Fm. Dalazi Fm. and Longjing Fm, characterized by riparian-lacustrine clastic rocks.2. 23 species of 10 genera of ferns were identified from the early Cretaceous coal-bearing Changcai Fm. in the Helong basin and the western part of Yanji basin, they are Osmunda cretacea Samylina, Ruffordia goepperti (Dunker) Seward, Glenichenites cycadii (Schenk) Prynada, Acanthopters gothani Sze, Coniopteris concinna (Heer) Vassilevskaja, C. densivenata Deng, C. ermolaevii (Vassil.) Meng et Chen, C. longipinnata Deng, C. nympharum (Heer) Vachrameev, C. saportana (Heer) Vachrameev, C. setacea (Prynada) Vachrameev, C. vachrameevii Vassilevskaja, Athyrium asymmetricum Chen, A. cretaceum Chen, A. hailaerianum Chen, Asplenium changcaiumi Sun et Liu, A. popovii Samylina, A. tiefanum Deng, Arctopteris heteropinnula Kirichkova, Cladophlebis acuta Fontaine, C. lobulata Samylina, C. pseudolobifolia Vachrameev, Lobifolia novopokroskii Lebedev, Raphaelia prinadai Vachrameev. Among them, in situ spores of 6 species of 4 genera were observed, they are Glenichenites cycadii (Schenk) Prynada, Coniopteris longipinnata Deng, Coniopteris vachrameevii Vassilevskaja, Athyrium cretaceum Chen, Asplenium changcaiumi Sun et Liu, Asplenium popovii Samylina. Especially, the spores in situ of Glenichenites cycadii (Schenk) Prynada were observed for the first time, which might be one of the great progresses in present research.3. The ferns from the Changcai Fm. is characterized by the prosperous Dicksoniaceae (2 genera and 9 species), Athyriaceae (one genus and 3 species), Aspleniaceae (one genus and 3 species) and ferns incertain sedis (3 genera and 5 species). It shows the character of north subtropical-temperate zone warm and humid vegetation in the early Cretaceous Siberian-Canadian Phytofloristic Realm (Vachrameev B.A., 1964, 1988), or in the Boreal Realm (Sun Ge et al, 1995).4. All the ferns from the Changcai Fm. are early Cretaceous species in eastern Asian and the eastern part of Russia, only a few species can be found also in the late Cretaceous. Thus the geological age of Changcai Fm. is undauntedly early Cretaceous. Since most of the species are seen in Neoconmian-Aptian, and some are found in early Albian from the eastern part of Russia, the age of Changcai Fm. can be further inferred to Aptian. Besides the ferns, there are also some other taxa (Sun Yuewu et al, 2005; Wei Wenyan et al, 2005), such as Nilssoniopteris prynadae, Nilssoniopteris platyrachis, Neozamites verchojanensis, Ctenis lyrata, Ginkgo coriacea, G. sibirica, Phoenicopsis speciosa, Elatocladus manchurica. Compared with the Aptian (or Aptian-early Albian) Shansong flora (Li et al, 1980, 1986), the Changcai flora might be younger than the Shansong flora since there are no angiosperms, Chiaoheella and Chilina found in Changcai Fm. although there are Ctenis lyrata and Neozamites verchoyanensis in the Changcai flora. Therefore, the age of Changcai Fm. is not younger as Aptian.5. Pollen and spore assemblages of the Changcai Fm., i. e. Laevigatosporites– Cedripites–Clavatipollenites Assemblage is characterized by the frequent appearance of spores of Lygodiaceae and the high content of Lavigatosporites (Yu Jingxian, 1983; An Junyi, 1996). The pollen in this assemblage is dominated by Pinaceae, and a few pollen of Clavatipollenites were found. The age of Changcai Fm. was assigned to Aptian by Yu (1983). This is coincident with that of the mega fossil plants. Furthermore, there are pollen and spores from the Tuntianying Fm. which is directly underlying on the Changcai Fm. The pollen and spores is characterized by dominant gymnosperm pollen, a few spore represented by Cicatricosisporites and Fixisporites and no angiosperm pollen. The age of this pollen and spore assemblage is regarded as Berriasian (Li Wenben, 2001).6. The Dalazi Fm. of the Yanji Group, which overlying on the Changcai Fm., yields abundant fossils including mega fossil plants (with angiosperm), palynomorphs (pollen and spores, dinosysts and other algae), ostracods, conchostracan, bivalves, gastropods, insects and fishes, etc. indicates late Albian to early Cenomanian in age. Therefore, the age of Changcai Fm. should be older than late Albian.7. The andicite of the upper part of the Tuntianying Fm., sampled from the south of Tuntianying, Sandaowan Town of Longjing, Yanbian area of Jilin, undergo Zircon Uranium- Lead isotopic dating by using the LA-ICP-MS method. The weighted mean age of the andicite is 125.1±2.7Ma, about the bottom of Aptian. This indicates that the Changcai Fm. is unlikely older than Aptian.8. To sum up, the geological age of the Changcai Fm. is Aptian.