Astronomical time scale of the Paleogene lacustrine paleoclimate record from the Nanxiang Basin, central China

Abstract

The chronostratigraphy of the Paleogene succession of the Nanxiang Basin, a petroliferous basin in central China with a predominantly lacustrine fill, remains controversial due to the lack of an accurate geological time scale. In this work, high-resolution gamma-ray (GR) logs from the B270 and BS1 boreholes from depocenters in the Biyang Depression of the Nanxiang Basin were used to analyze the cyclostratigraphy. Paleocene floodplain facies of red clastics and purple mudstone is conformably overlain by Eocene through Oligocene lacustrine facies of gray mudstone, oil shale, and clayey dolomite interbedded with siltstone-sandstone. The multitaper method of spectral analysis and Gaussian bandpass filtering were used to identify Milankovitch cycles. Based on the recognition of 405-kyr long-eccentricity cycles, the series was tuned to establish a 46-Myr floating astronomical time scale (ATS), which was then anchored to the Paleogene/Neogene boundary age of 23.03 Ma. The ATS enables the assignment of the ages to non-marine biozones, depositional facies and rifting episodes within the Nanxiang Basin. The subsidence and rates of sediment accumulation within the rift basin of the Nanxiang Basin has an inverse relationship to the convergence rates between Pacific plates and the eastern margin of Eurasia. The middle Eocene slowing of the convergence rate at ~50 Ma, coupled with the onset of the collision of the Indian and Eurasian plate, corresponds to the beginning of the climax stage of rifting, the development of deep lakes and the deposition of oil shales in the Nanxiang Basin. The organic-rich 3rd member of the Hetaoyuan Formation (Eh3) during this middle Eocene interval preserved a range of climatic cycles from annual varves to a ~1.2-Myr modulation of obliquity cycles, indicating the importance of climatic oscillations on the development of those hydrocarbon source rocks.