Stratigraphy And Paleogeographic Characters Of Yixian Formation In Western Liaoning, China

The author has measured a series of typical sections of the Yixian Formation in west Liaoning and collected a lot of samples and fossils through the geological survey for 3 years. This paper makes a comprehensive research on the stratigraphy and paleogeographic characters of the formation by means of petrology, paleontology, structural geology, geochemistry and isotopic chronology. This research focuses on the Mesozoic basins yielding the Yixian Formation, including Yixian Basin, Jinlingsi-Yangshan Basin, Jianchang Basin and Lingyuan-Sanshijiazi Basin. Based on the characters of volcanic cycles, petrology and paleontology of the Yixian Formation, the author makes a systematic division and correlation of the Yixian Formation in the four basins. This paper discusses also the characters of paleostructure and lithofacies paleogeography. The study is significant to the correlation of fossil-bearing strata intercalated with volcanic beds in western Liaoning, and is helpful to the exploration and protection of relevant volcanic mineral resources. The discussion on the paleostructure and lithofacies paleogeography is useful to the study of geological history that is concerned about the origin and evolution of bird and angiosperm. Meanwhile, this will be beneficial for the study of paleogeography, volcanic activity and the evolution of organic-inorganic events.The new subdivisions and paleogeographic comparison of the Yixian Formation in western Liaoning are as follows:The Yixian Formation can be subdivided into four members. The first member of Yixian Formation is the member of sandstone-conglomerate and basic -intermediate-basic volcanic rocks, which are widely distributed in the study areawith the exception of sandstone-conglomerate, only occurred in the Yixian Basin and the Jinlingsi-Yangshan Basin. There are 4 fossil-bearing beds occurred in the first member of Yixian Formation. Two fossil beds with gastropods, bivalves, conchostracnas and ostracods were found in tuffite and tuffaceous sandstones of the Yixian Basin. Four fossil beds were found in the Jinlingsi-Yanshan Basin with the fossils of gastropods, bivalves, conchostraca, ostracods, dinosaurs and fossil wood occurred in the bottom sandstone and conglomerate, including Psittacosaurus occurred in upper part of tuffite, tuff and tuffaceous sandstones. The second member of the Yixian Formation is mainly composed of lacustrine volcanic-sedimentary rocks with broad distributions. There are lot of fossils belonging to the Jehol Biota occurred, including Sinosawopteryx, Confuciusornis and Archeafurctus liaoningensis. The author firstly discovered the pillow lava in the second member, which is interbeded into the fossil beds as "finger" shape. The isotopic data of the pillow lava provided important evidence for the age of the Yixian Formation and for the resolve of the fossil beds comparison problems. The third member of the Yixian Formation is mainly of intermediate-basic -intermediate-acidic volcanic rocks with wide distributions. The fourth member of Yixian Formation is of sedimentary rocks and acidic volcanic rocks that are mainly distributed in the Yixian Basin and Jinlingsi-Yanshan Basin. The upper part of the fourth member in the Yixian Basin, which is mainly of fish fossils, is also important fossil beds of Yixian Formation. It seems to have the characters of migration in the four members of the Yixian Formation from the west to the east influenced by volcanic activity center and the structural position indicated as the various strata thickness between different basins.Based on the study of petrology of volcanic rocks and the regularity of volcanism, the volcanic eruption cycle of the Yixian Formation is subdivided into four sub-cycles correspond respectively to the four lithologic members of the Yixian Formation. The sub-cycles can also further be subdivided into some smaller cycles. The research reveals the regularity for the migration of volcanic eruption in the sequence of basic - intermediate-basic - intermediate - acidic (alkaline) from early to late and from west to east. There is an obvious correlation between the sub-cycles. The Jianchang basin and Lingyuan-Sanshijiazi basin are situated in the transitional zone of the younger Mesozoic northern Hebei-west Liaoning volcanic rock province. In the Jianchang basin, along with the Jin-Yang and Yixian basins in the east, the sub-cycle migrates southeastwards. While in the Lingyuan-Sanshijiazi basin and the Pingquan basin in west, the migration is from southeast to northwest. Geochemical study shows that during the No.I sub-cycle, the magma in the Jianchang basin evolved from basic to intermediate; in No. IIsub-cycle, intermediate magma was dominated; while in No.IlI, it was mainly acidic-alkaline, with intermediate in the east. It is a calc-alkaline volcanic suite in general. In the Jin-Yang basin, the magmatic evolution in No.I sub-cycle was dominated by basic calc-alkaline rock series, the No. II sub-cycle was an intermediate-basic alkaline series, and the No. Ill was a basic calc-alkaline rock series. On the other hand, in the Yixian basin, magma evolved from basic to intermediate calc-alkaline/alkaline volcanic suite during the No.I sub-cycle; the No.II sub-cycle showed an intermediate-basic alkaline suite; in the No.III sub-cycle, it evolved from intermediate to alkaline, from calc-alkaline to alkaline suite; while the No.IV was in intermediate-acidic, calc-alkaline evolving to alkaline. Meanwhile, in the Lingyuan-Sanshijiazi basin, the magma evolution during the No.I sub-cycle presented intermediate and acidic; magmatism was not seen; and the No. Ill was acid. It is generally alkaline rock series. According to the structural environment differentiation by REE, the volcanic rocks were formed by the upward invasion of inner-plate basalt in the continental extension environment. It was derived from the HREE-enriched mantle, most likely from the same magma source. From No.I to No.IV sub-cycles, the hybridization became more and more intensive.By analysis on the paleostructure and lithofacies paleogeography of the Tuchengzi Formation before the Yixian Formation formed, it is concluded that, the regional principle stress field was NWW-SEE compression, which made the NE- or NNE-trending distributions of the Tuchengzi Formation in the studied area. Based on the distributions of the basin-controlling faults and main mobile faults during the developing stage of the Yixian Formation, as well as the regularity of volcanism in the Yixian Formation, the regional principle stress field in that time converted from NWW-SEE to S-N direction, which formed the NW-SE extension. The faulting becomes more intensive from west to east or southeast. From west to east, the types of the basins change from the depression-fault type (such as the Lingyuan-Sanshijiazi basin), though the semi-graben-fault-depression (such as the Jianchang and Jin-Yang basins) to the typical fault-depression type (the Yixian basin). Generally, the basin marginal faults on the east margin evidently control the distributions of the Yixian Formation.Synthetically applying the sedimentary environment, palaeobiological features, combined with the geochemical features of sedimentary rocks, the climate condition is regarded as warm temperate zone with seasonal arid or semi-arid climate condition during the Yixian stage. The climate is obviously different from Tuchengzi stage.The first member of the Yixian Formation mainly consists of fluvial facies...