Paleoenvironment Records Of The Upper Cretaceous In The Jiaolai Basin Of The Laixi,Shandong Province

During the evolution of the earth,the Mesozoic volcanic activity was frequent and the temperature was the highest in the Phanerozoic era.This era maintained a complete geological record.At that time,the paleogeographic pattern changed significantly,and the complete greenhouse climate cycle was developed closest to the human era.Especially the Cretaceous from 145 million to 66 million years ago is a relatively special era in the development of the earth,a period of relatively high temperature in the geological history,and a typical greenhouse climate period.At the same time,it is also a period of frequent volcanic eruptions,more major changes in the earth's circle and the biological world,and a significant rise in sea level.Marine strata were extremely developed during this period.There are terrestrial strata in China's Qinling Mountains,the Qinghai-Tibet Plateau and Xinjiang.Terrestrial deposits are formed on the interface between the atmosphere and the hydrosphere,biosphere and lithosphere,which can effectively record the paleoenvironmental information during the development and evolution of the earth.The scientific community generally recognizes that the reconstruction of paleoclimate records of terrestrial sediments can provide important evidence for a comprehensive understanding of global climate change,so the study of terrestrial sediments in the Cretaceous period has important scientific significance.The Jiaolai Basin has developed Cretaceous continental strata,and the stratum contains a large amount of paleontological fossils.A large number of studies have been made on the sedimentary environment and evolution of this basin through dinosaur fossils and sedimentary strata,but the paleoclimate evolution is relatively weakly understood.In this paper,the Late Cretaceous was reconstructed by using the sedimentary profiles of the continental sediments in the Laixi area of the Jiaolai Basin during the Late Cretaceous period.The total organic matter content?TOC?,carbonate content,Fe²⁺content,major and trace elemental composition characteristics of sediments were used to reconstruct the paleoclimate evolution characteristics of the basin during the Late Cretaceous period,which deepened the understanding of the evolution of the Cretaceous paleoenvironment in the Jiaolai Basin.In this paper,through the analysis and study of the Dagaolan section in the Laixi area of the Late Cretaceous Jiaolai Basin,the following insights were obtained:?1?The SiO₂ content in the profile sediment is the highest,ranging from35.36%to 77.65%,the Al₂O₃ content is from 10.58%to 17.49%,the Fe₂O₃ content is from 2.14%to 8.94%,the K₂O content is from 2.3%to 4.64%,and the MgO content is 1.19%⁴.23%,Na₂O content is 0.1%¹.9%,CaO content is 1.62%⁴3.08%,CaO is significantly enriched.The content of trace elements Cr,Co,Nd,etc.is relatively high,which is enriched in the profile,while Nb,Sr,P show a significant loss.The TOC content of the sediment is between 0.034%⁰.28%,the carbonate content is between 20.04%⁹4.3%,and the Fe²⁺content is 0.26mg/L¹.47mg/L.?2?The low content of Ni and Sr elements,the low Sr/Ba,Rb/K values,Ni/Co,V/Cr and the low value of the oxygen fugacity substitution index Cu/Zn,etc.,jointly reflect that the sedimentary period of the study area is land Phase oxidation environment.The low Rb/Sr value,high Sr/Cu value,high??CaO?/??Sr?value and extremely small C value indicate that the overall environment of the Late Cretaceous research area is relatively dry and hot,and the climate fluctuates significantly.The geochemical properties of most sediment samples in the Dagaolan profile show a low degree of maturity,which is the sediment of the first cycle.The chemical weathering alteration index CIA and A-CN-K triangle charts can reflect that the Late Cretaceous Laixi area experienced a primary-medium paleo-weathering process,which is related to relatively dry heat.?3?According to the characteristics of the major elements,the profile sediment mainly comes from the medium-acid volcanic rocks and the sedimentary rocks of the Bamu Formation in this area.Sediments are mainly transported by rivers over short distances.The element geochemical characteristics of sediments in the Dagaolan section are obviously controlled by the differentiation of river water media.?4?The sedimentary environment in the study area changed more drastically,and the water level in the later period was higher than that in the earlier period.There were short-term rapid changes in water inflow and water outflow,resulting in the phenomenon of alternating channels and floodplains.There is a coupling relationship between the changes of the sedimentary environment and the climate.The climate in the Laixi region has experienced a dry heat-high temperature and rain-warm and humidity,and eventually the evolution process towards drought development.?5?The dinosaur egg fossils found in the Dagaolan section indicate that the dinosaurs may live in a floodplain environment near the water.Humidity and temperature are conducive to dinosaur spawning and hatching.This fully shows the coupling relationship between biological events and sedimentary environment at a certain time and area.