Study On The Structure Of The Upper Cretaceous Clips In The Emei Area Of ​​Sichuan Province

Through detailed study on lithology, sedimentary structures, sedimentary sequences and trace fossils of Jiaguan Formation in E’mei area with sedimentary background, Jiaguan Formation is fluvial facies. Two Members are divided and four sedimentary microfacies of the lag deposits, the point bar, the natural levee and the floodplain are proposed.Fluvial deposits of the Upper Cretaceous Jiaguan Formation contain a low-diversity ichnofossils of invertebrate, vertebrate and plants in E’mei. Based on the ethological classification of trace fossils, which has been diagnosed ten ichnogenus and twelve ichnospecies. And they are Skolithos linearis、Skolithos verticalis、Arenicolites sp.、 Asterosoma sp.、Cystichnuim sp.、Scoyenia gracilis、Palaeophycus tubularis、Taenidium sp.、 Pelecypodichnus sp.、 Gordia sp.. Planolites sp.1、Planolites sp.2.Meanwhile, there are many vertebrate trackways found in Jiagua Fomation, and reckoning them as dinosaurs’ trackways. There always found plants root fossils in some stratums. Vertical dwelling burrows of opportunists are dominance in active channel whereas horizontal deposit feeding burrows, grazing burrows, and vertical dwelling burrows occur in embankment.Based on the trace fossils morphological characteristics and their tiering distribution, four types of ichnofabrics are divided:Arenicolites ichnofabric, Skolithos ichnofabrics, Scoyenia ichnofabric, Planolites ichnofabric and Palaeophycus ichnofabric. Palaeophycus ichnofabric is observed in the fluvial channel lag deposits with medium sandstone at the buttom of the fluvial, and the medium sandstone contains boulder clay with scoured structure. With a strong energy the trace fossils paralleled or inclined to the level. Arenicolites ichnofabric is preserved in the upper of the point bar with a middle-strong hydrodynamic condition, which occurres in the brown-red fine sandstone with parallel bedding, ripple bedding and trough or tabular cross bedding, and the hydrodynamic condition became weak up to the stratums. Skolithos ichnofabrics is preserved in the transition environment of point bar to embankment, whose energy condition is lower than Arenicolites ichnofabric and observed in both sandstone and mudstone. Planolites ichnofabric is formed in brown red sandstone with interbeded sandstones and mudstones on it. The trace fossils are distributed on the level, which is in a lower hydrodynamic condition in the natural levee micro facies. Scoyenia ichnofabric is preserved on the level of brown red sandstone, and the trace fossils have backfill structure with sandstone and mudstone discontinuous filled. The level occurs in the place where is interbeded sandstones and mudstones and interprets a natural levee with lower energy.By means of the semi-quantitative analysis on the ichnofabrics in the Jiaguan Formation in the study area, conclude that bioturbation index is low and numerous of factors work on the distribution of the ichnofaunas, including sediments feature, hydrodynamic condition, nutrition and so on. But the distribution of the ichnofabrics in Jiaguan Formation is consistent with the pattern of fluvial ichnofossils.