Analysis of Milankovitch cycles and establishment of floating astronomical date scale based on well-logging data

The Chang 7 member in the Ordos Basin belongs to semi-deep to deep lacustrine depositional environment. The deposition process is stable and continuous, which recordes the Milankovitch cycles. This paper aims at converting the depth to the deposition time and establishing the stratigraphic time axis. First, we adopt the Fourier and wavelet transform to simulate and analyze the Earth orbital period signal, respectively, to identify Milankovitch cycles. The feasibility of the two methods is also demonstrated. Second, we establish the workflow and formulas of analyzing Milankovitch cycles, the depositional time and rate using spectrum analysis and wavelet deep-frequency analysis based on the logging data of the Chang 7 member. We obtain the transformation relationship between the wavelet scale and the Earth orbital period. Subsequently, we acquire Milankovitch cycles of the Chang 7 member using the logging data of Well B159. The average depositional rate of the Chang 7 member is subsequently calculated to be 5. 05 cm . ka(-1) and 5.07 cm . ka(-1) , respectively. Finally, we conclude that the stratigraphic cycle of the Chang 7 Member is mainly controlled by the long eccentricity period of 404-ka. The floating astronomical age scale of the Chang 7 member is established by reconstructing the wavelet coefficient curve of the long and short eccentricity period. The results confirm that there are 6 long eccentricity cycles and 20 short eccentricity cycles in the Chang 7 member, The depositional time limit of the Chang 7 Member is about 2.4 Ma within a depth interval of 124 m. The establishment of the floating astronomical time scale is conductive to accurately estimating the duration time of various geological events, which would benefit the deep-time Earth research.