Cyclostratigraphical Study On The Cretaceous Yixian Formation Of Well Liaofa D1 In Xiushui Basin,Liaoning Province

During the Cretaceous period,the Earth's climate and environment changed significantly,accompanying with large-scale biological events.The Cretaceous terrestrial sedimentary basins in northeast China record information on terrestrial ecosystems,which exerts the great advantages in understanding the co-evolution of climatic,biotic,and environmental systems.In this paper,environmental magnetism and cyclostratigraphy are studied two members of the Cretaceous Yixian Formation(the upper sedimentary strata depth range is 180.20–295.61 m,and the lower sedimentary strata depth range 549.47–652.34 m)of the well Liaofa D1 in Xiushui Basin in northern Liaoning.The temperature dependence of magnetic susceptibility curves(k-T)results and three-component IRM thermal demagnetization(Lowrie experiment)prove that the main magnetic minerals in the sedimentary rocks are magnetite,and a small amount of hematite and pyrrhotite.Using the high-resolution magnetic susceptibility data,cyclostratigraphy analysis was used to identify the potential Milankovitch cycles.Specifically,the sedimentary cycles of 19.54–24.76 m,5.36–8.07 m,and 3.18–4.60 m has been identified in the upper sedimentary layer,representing short eccentricity,obliquity,and precession cycles.The ratio of sedimentary cycles of 23.84–13.95 m,6.43–4.74 m,2.19–2.06 m,and 0.97–0.77 m in the lower sedimentary layer is 20: 5: 2: 1,which might be controlled by long eccentricity,short eccentricity,obliquity,and precession cycles.After the astronomical calibration to the eccentricity cycle,the spectrum analysis of magnetic susceptibility series reveals short eccentricity cycle with periods of 126 k.y.,113 k.y.,and 100 k.y.,obliquity cycles with periods of 48 k.y.,46 k.y.,and 33.5 k.y.,and precession cycles with periods of 25 k.y.,21.8 k.y.,20 k.y.,19.5 k.y.,and 18 k.y..According to the constructed "floating" astronomical time scales,the durations of the upper and lower sedimentary layer are estimated as 0.58 Myr and 2 Myr,respectively.There is a 4–5k.y.sub-Milankovitch cycle in the upper sedimentary layer of Well D1 in Liaofa,which may be a millennial-scale fluctuation of solar insolation,and the identified10–12 k.y.cycle may be the result of the sun passing through the tropical regions twice a year.