Elemental Geochemistry Of Lacustrine Deposits From The Early Cretaceous Yixian Formation In The Sihetun Area,Western Liaoning,China

Lacustrine deposits of the Early Cretaceous Yixian Formation are widely exposed in western Liaoning Province,North China.Abundant feathered dinosaur and early angiosperm fossils were discovered in Yixian Foramtion of Beipiao city,which therefore has received extensive attentions in recent years.Systematic studies on Jehol Biota have been carried out by paleontologists and geologists and there are many research findings.However,the cause of death and preservation conditions of Jehol Biota still remain controversial.Some scholars believe that the death,transportation,burial and exquisite preservation of the Jehol Biota are related to volcanic eruptions and volcanic ash deposits around the lake basin.Other scholars believe that the fossils of the Jehol Biota are mainly preserved in shale and mudstone,and the preservation of the fossils is related to the anoxic environment of the water at the bottom of the lake.An important reason for the long-standing existence of this controversy is that we know very little about the sources,transportation and deposition processes of fine-clastic sediments in lacustrine shales and mudstones,Therefore,the elemental geochemical analysis of both shale/mudstone and volcanic ash layers deposited in the lacustrine facies of the Yixian Formation was conducted.This study aims to explore the tectonic setting of Yixian Formation,track the provenances of the lacustrine sedimentary,reveal the weathering,sorting and sedimentation process of sediment,and discuss the relationship between the volcanic ash layers and shale,mudstone after conducting a variety of element geochemical analyses,and then to explore the specific process and mechanism for the preservation of fossils of the Jehol Biota.The lacustrine sedimentary strata of the Yixian Formation in the Sihetun area of western Liaoning have been studied in detail in this study.A total of fifty-one shale and mudstone samples and nine volcanic ash samples were collected from the section of Sihetun Fossil Museum.By testing the concentration of major and trace elements in the samples and analyzing the geochemical indicators of various elements,we have obtained the following main conclusions:(1)Shale and mudstone samples are concentrated in the La-Sc-Th and Th-Sc-Zr/10 triangle maps,indicating the tectonic setting of the continental island arc and the active continental margin.This result is well consistent with previous studies on the tectonic settings of Early Cretaceous in western Liaoning.The section of the Sihetun Fossil Museum investigated in this study has preserved hundreds of layers of volcanic ash,indicating that there were frequent and intense volcanic activities in western Liaoning during the existence of the Early Cretaceous Sihetun Lake.The tectonic setting of the continental island arc and the active continental margin may be an important reason for the frequent volcanic eruptions in western Liaoning area.(2)Relatively high Chemical Index of Alternation(CIA)values of volcanic ash indicate a moderate degree of chemical weathering process,while low CIA values of shales and mudstones suggest a weak chemical weathering and probably reveal an arid condition at that time.The crossplots of A-CN-K,Zr/Sc vs.Th/Sc,and Th/U vs.Th demonstrate that most shale/mudstone samples have experienced little influence by weathering and sorting.By contrast,samples of the volcanic ash layer are relatively affected by weathering and sorting.This may be due to the fact that the volcanic edifice at that time were far away from Sihetun Lake,most of the volcanic ash was first transported by wind to the catchment basin of the Sihetun Lake and then transported to the lake by long-distance transporting and sorting in the rainy season.The double sorting of wind and flowing water experienced by volcanic ash may be an important reason for the relatively high CIA value.In consideration of that the volcanic edifice are far away from the lake,we speculate that the volcanic eruptions may not be an important cause of death of buried terrestrial vertebrates and aquatic organisms in lake sediments of the Sihetun area.(3)The data of trace elements(REE pattern,Hf vs.La/Th,Th/Sc vs.Cr/Th,and La/Sc vs.Co/Th crossplots)and major elements(A-CN-K diagram)reveal a distinct difference in compositions between volcanic ash and shale/mudstone.Specifically,the parent material of shale and mudstone in the Yixian Formation was mainly derived from felsic volcanic rocks,which is a normal background deposition;while the volcanic ash layer is mainly located in the area of intermediate-felsic igneous rocks,indicating that it is the product of intermediate-acid volcanic eruptions,which represents the occurrence of eventual deposition during the existence of Sihetun Lake,when the clastic material from the nearby volcanic eruptions has been deposited on the surface,and then it was transported to the lake by flowing-water.The significant difference in provenance between shale,mudstone and volcanic ash layer indicates that the volcanic ash originated from distant volcanic eruptions during lake development is not an important source of shale and mudstone deposits.In addition,most fossils are preserved in shales and mudstones,but few fossils are found in the volcanic ash layer.Therefore,we propose that there is no direct correlation between the death and burial of Jehol biota preserved in the shale/mudstone and volcanic activities.