Taphonomy Of Lycoptera Fossils And Its Paleoenvironment Analysis From The Cretaceous Liwaxia Formation In Liupanshan Area

The Cretaceous,as an important period in the process of earth evolution,has attracted much attention from many geologists,and it has also become one of the hot spots in the field of geoscience.Many geological anomalies occurred during this period,among which the mass extinction event and the Cretaceous anoxic event are the most famous,and most of these events have global effects.There are abundant animal and plant fossils in the Cretaceous lacustrine sedimentary strata.The formation of these fossils is related to the Cretaceous abnormal geological events,the fossil of the Jehol Biota middle-early Cretaceous lycoptera fossils is the most representative.In this paper,a large number of lycoptera fossils have been found in the Early Cretaceous lacustrine sediments of the Liupanshan group,rock samples and fish fossils of the Liwaxia Formation were collected from the Cretaceous outcrop in Wangjiagou,Huating County,Gansu Province.From these two types of samples,the rock samples were observed by under the microscope,whole rock X-ray diffraction analysis,major element and trace element concentration analysis,rare earth element concentration analysis,organic carbon isotope analysis,the lycoptera fossils were analyzed by micro-XRF analysis.The above methods are used to reconstruct and distinguish the paleoclimate environment,in this way,the possible causes of the formation of fish fossils are discussed.XRD analysis shows that the fossil-bearing is mainly composed of ankerite（42.70%）and quartz（41.50%）,while its overlying and underlying neighbors are mainly composed of quartz（average 40.40%）and feldspar（total average 15.98%）.The main elements of the fossil-bearing unit are Ca O（average 20.61%）and Mg O（average 10.52%）,while its neighboring units are enriched in Si O₂（average 54.58%）and Al₂O₃（average 14.79%）.According to the trace element analysis,Sr and U are enriched in the UCC,while Cu and Cr are depleted.Ba and V are similar to UCC.In the upper and lower rock samples,U is extremely enriched,Cu and Th are slightly enriched,Cr and Ni are slightly depleted,Ba and V are equivalent to UCC.The analysis of REE shows that the variation trend of light and heavy REE of the rocks in the fossil burial layers is basically the same as that of the rocks in the upper and lower layers,indicating that the sediments have the same provenance;the accumulation of light rare earth elements and deficit of heavy rare earth elements indicate that the deposition area is close to the source area.The organic carbon isotope analysis showed a negative and then positive drift ofδ¹³C_org from the bottom to the top of the outcrop in the study area.The XRF analysis of fossils showed that the Ca concentration on the fossil surface was the highest,followed by Fe,Si and P.In addition,there were a few As and Cr elements.The paleoclimate indexes MgO/CaO and Sr/Cu indicate a dry hot climate,the paleo-salinity index m value and Sr/Ba indicate a high salinity environment,and the paleo-redox environment indexes Th/U and V/（V+Ni）indicate a stratified aquatic environment with euxinic bottom water.The mass mortality of fishes was caused by a rapid redox change of the water body and toxicity of H₂S.Combined with the variation rule ofδ¹³C_org curve,it can also be regarded as the geological response of Cretaceous Aptian-Albian oceanic anoxia event 1a（OAE1a）in continental strata,and the dry hot climate and high salinity water were caused by the elevated atmospheric CO₂ concentration in the middle Cretaceous.Additionally,the euxinic bottom water provides favorable conditions for the preservation of fish fossils.