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

Cyclostratigraphy and high-frequency sedimentary cycle framework for the Late Paleozoic Fengcheng Formation, Junggar Basin

“…Located in northwestern China (~45 °N, 85 °E), the Junggar Basin is a huge inland lake basin that is tectonically constricted by the Hala'alate and Zhayier Mountains to the northwest, the Kelamaili Mountains to the northeast, and the Bogda Mountains to the south (Figures 1A, B). Within the Junggar Basin, an approximately 1.5km-thick stratigraphic sequence adequately documents important long-term features of the tectonic evolution of Central Asia, in addition to the evolution of paleoclimate and paleoenvironmental conditions of the continental region at middle to high latitudes in the Northern Hemisphere from the Paleozoic to the Cenozoic (Huang et al, 2021;Tang et al, 2022). With its location in the northwestern part of the Junggar Basin, the Mahu Sag is a major oil-bearing depression of approximately 5,000 km 2 in area (Yu et al, 2018b).…”

“…Using null hypothesis significance levels < 0.01, the most prominent peaks occurring at 9.3 and 17 cm/kyr represent the most likely sedimentation rates in MY-1 well (Figure 4A). Previous cyclostratigraphic analyses using density logging data indicated that the average deposition rate of the P 1 f 1 and P 1 f 2 members of the Fengcheng Formation was approximately 9 cm/kyr (e.g., Tang et al, 2022). We therefore preferred the 9.3 cm/kyr peak as the optimal rate of sedimentation.…”

“…Stratigraphic correlation of the Mahu Sag reveals that the deposition of the Fengcheng Formation occurred at approximately 300 Ma, developing mainly in the Asselian stage of the Early Permian (Cao et al, 2020;Huang et al, 2021;Tang et al, 2022). In addition to stable long eccentricity, the precession and obliquity cycles also exhibited shorter periods (Berger et al, 1992;Laskar et al, 2004;Waltham, 2015).…”

“…The fourth-order sequence is a secondary lake transgression-regression sequence with highamplitude water depth changes corresponding to the mediumterm base-level cycle and controlled by the 405 kyr long eccentricity cycle (Boulila et al, 2011;Liu et al, 2022). The fifthorder sequence is a rhythmic transgression-regression sequence with low amplitude lake-level change corresponding to the short-term base-level cycle and controlled by the ~100 kyr short eccentricity cycle (Boulila et al, 2010;Boulila et al, 2011;Tang et al, 2022). Many scholars therefore use different orders of eccentricity cycles as a reference to divide the high-frequency cycle framework of lacustrine shale sequences (e.g., Cong et al, 2019;Du et al, 2020;Fang et al, 2023).…”

“…The Fengcheng Formation is a Lower Permian stratigraphic sequence within the Mahu Sag in the Junggar Basin that contains the oldest alkaline lake hydrocarbon source rocks on Earth (Zhang et al, 2018;Huang et al, 2021;Tang et al, 2022). Although previous studies have reported climatically forced evaporite-containing sediments (Cao et al, 2020), the detailed relationships between alkaline lacustrine development and orbital forcing remain to be determined.…”