Astronomical forcing in the coal-bearing Middle Jurassic Dameigou Formation, Qaidam Basin, northwestern China

Abstract

Coal seams, as a terrestrial carbon sink, are not only important sedimentary and energy mineral resources but also excellent recorders of paleoclimate change. The study of coal-bearing strata is particularly significant in exploring the relationship between geological records and orbital forcing. However, the understanding of the relationship between coal seams of different sources of organic matter and astronomical cycles is incomplete. This study focuses on the Dameigou Formation, a typical Jurassic coal seam in the Qaidam Basin, Northwestern China. Organic petrology analysis, biomarker analysis, and whole-rock mineral analysis were combined to reconstruct the vegetation types and paleolake water level changes during coal seam deposition. In addition, using a time-series analysis method and high-precision gamma-scanning data from Well D1, we established a floating astronomical timescale and a time-depth model for cyclostratigraphical analysis. The results show that the Dameigou Formation, as coal-bearing strata, preserves well-developed Milankovitch cycles, and the climatic conditions and vegetation types during coal formation vary among different coal seams. Different patterns of variation in eccentricity, obliquity, and procession determine the thickness of the coal seam. This study is of great significance for understanding the astronomical factors that influenced climate evolution and coal deposition during the Middle Jurassic.