| The Middle Cretaceous is an era of violent climate change,global warming,intensification of carbon cycle,oceanic anoxic events and other activities.The environmental upheaval is of great significance to the evolution of modern Earth system.Determining the time of specific environmental change is not only a prerequisite for the relevant research of Cretaceous,but also conducive to the analysis of future climate change.Cretaceous continental strata are widely distributed in China,with volcanic activity zone in the East,which is conducive to accurate isotopic dating;However,terrestrial strata in northwest China lacks volcaniclastic deposits,and its age is relatively scattered and has not been unified.In particular,the age of deposition of the Xiagou Formation needs to be further analyzed by other means.Among them,cyclostratigraphy is one of the important tools to establish the floating age,which is helpful to improve the strata age framework.Studies have shown that the carbon isotope records of the black shale of the Xiagou Formation in the Northwest Changma Basin respond to the drastic fluctuations of the black shale of the OAE1a period,but there is no precise age constraint on the Xiagou Formation in the Changma Basin,however,it still needs more chronological evidence to judge the existence of OAE1a events only by using chemical stratigraphic method for black shale segment.During the OAE1a period,black shales were widely deposited all over the world.The black shale of European low latitude（～20°N-30°N）marine strata showed obliquity signal.Theoretically,the climate curve in low latitude region showed precession signal.Large black shale deposits have been found in the lower member of Xiagou Formation in Nantaizi,Zhangye,at paleo-latitude～23°N,but they have not been thoroughly studied by predecessors.Therefore,the lower member of Xiagou Formation in the Binggou section is taken as the research area to further explore the relationship between the black shale、OAE1a and the orbital driving mechanism of the black shale by using the method of cyclostratigraphy and curve correlation.The understanding is as follows:1.Using the digital redness data of Binggou section,a floating age scale of 2.884Ma is established.By sliding correlation between the redness of this time domain and the high-resolution European marine carbon isotope curve of 114-125 Ma,the best fitting point is 121.027 Ma.Based on the floating age scale of 2.884 Ma and the best fitting of121.027 Ma,the absolute age range of the Binggou section is 121.027 Ma-123.911 Ma.2.Based on the chronological results of Binggou section,a high resolution carbonate stable isotope curve was established in the study area,comparing the carbon isotope curves of the study area with those of the European marine strata during the OAE1A period,the trend of the two curves is consistent,indicating that the LAE1a event exists in Binggou section.3.Several indexes(TOC,C/N,Ca CO3,L*,δ13C,Fe,P)were measured in Black Shale II.TOC and C/N indicated that the organic matter of the lake originated from internal sources.Ca CO3 and L*reflected the color change of the samples.TOC,δ13C,Fe,P and other parameters could reflect the lake productivity change in black shale II to some extent.The results show that the dominant period of this horizon is precession and short eccentricity period,which is different from the obliquity period of the European marine black shale at low latitude in the same period.In this study,the Cretaceous continental black shales in northwest China are compared with the European marine climatic records,which constrain the age of the black shales of Xiagou Formation in Zhangye,at the same time,lake anoxic events is identified,and the precession period of the black shale section in Zhangye is calculated,it is also helpful to understand the evolutional characteristics of Cretaceous OAEs and the orbital driving mechanism. |
