Orbitally controls of climate recorded in a series of thin-multiple-layers coal seams in marine–continent transition environment during late Eocene

Abstract

Coal-bearing strata are not only good carriers of oil and gas resources and mineral resources but also record the characteristics of paleoclimate changes. The multilayer coal seams in the late Eocene Pinghu Formation (∼32–43.5 Ma) in the Xihu Sag of the East China Sea Shelf Basin have been found to be primary hydrocarbon source rocks. Many studies have reported the geochemical features and sedimentary facies of the coal-bearing rocks in the Xihu Sag. However, few studies have focused on the climate variation recorded in the coal seams, especially the astronomical cycles. In this study, we summarized the plan-view distribution of the coal seams during the deposition of the late Eocene Pinghu Formation and the associated sedimentary conditions. Through the time series analysis method, we established a floating astronomical time scale and a time–depth model using the gamma ray series of well K3 to perform high-frequency astronomical cycle analysis. The results show that the coal seams dominantly developed at the edge of the Xihu Sag during deposition of the Pinghu Formation and are characterized by multiple layers and thin thicknesses (average of ∼1 m). By comparing the coal seams to the obliquity and precession cycles, we found that the coal seams developed during the minimum period of precession and the maximum period of obliquity, corresponding to a climate with high seasonal contrast and an enhanced monsoon climate with abundant precipitation. Considering the climate factor and marine–continent transitional environment, we establish the development pattern of the late Eocene coal seams in the Xihu Sag. These results show the clear control of orbital cycles on the development of the coal seams and indicate the significant role of multiple thin coal seams in recording paleoclimate.