The floating astronomical time scale for the terrestrial Early Permian Fengcheng Formation from the Junggar Basin and its stratigraphic and palaeoclimate implications

Abstract

The Permian Fengcheng Formation of the Mahu Sag in the Jungar Basin is one of the most important hydrocarbon source rocks in Xinjiang oil field. The preservation of the geological records of Milankovitch cycles can be studied to understand the history of the Earth's evolution and sedimentary processes, and high‐resolution astronomical time scales (ATS) can be obtained by analysing (quasi) periodic sedimentary signals in geological records. In this study, the natural gamma ray (GR) and density (DEN) logging data are used to carry out a detailed spectrum analysis, including 2π‐Multi‐taper Method (MTM), Evolutive Harmonic Analysis (EHA), Correlation coefficient method (COCO) and DYNOT simulation. GR and DEN data analysis results show sedimentary cycles of 39.3, 9.1 m, 3.6–2.7 m and 2.1–1.7 m, 37.5, 9.7, 3.7 m and 2.4–1.9 m in MY‐1 well; MH‐15 well recorded the significant peaks at 81, 20, 8.6 m and 3.6–3.4 m and 66, 17.6, 6.8 m and 3.3–2.7 m; FN‐14 well revealed high‐confidence level frequencies with wavelength of 97, 23, 8.8 and 3.9 m and 88, 22, 8.5 m and 4–3.4 m respectively. The average sediment accumulation rates (SAR) are estimated at 9–9.7 cm/kyr for MY‐1, 14.5–19 cm/kyr for MH‐15, 13.5–14.5 cm/kyr for FN‐14 by using the COCO. In addition, we construct a continuous time scale for Fengcheng Formation, yielding a deposition duration of about 3.5 ± 0.2 Myr for these three drilling cores. Finally, we used the DYNOT sea‐level change model to analyse the GR and DEN logging series of three wells, for reconstructing the lake‐level changes of the Fengcheng Formation in the Junggar Basin.