Astronomical cycles and variations in sediment accumulation rate of the terrestrial lower Cretaceous Xiagou Formation from the Jiuquan Basin, NW China

Abstract

The Lower Cretaceous Xiagou Formation in Qingxi Sag consists of more than 1000 m of lacustrine sediments, including dolomitic mudstone/argillaceous dolomite and sandstone/mudstone, which is considered to be the most important source rock in the Jiuquan Basin. Power spectra, evolutionary fast Fourier transform (FFT), and correlation coefficient analyses of the gamma-ray (GR) series from four wells, including Liu 8 well, Long 14 well, Q2-36 well and Liu 9 well, through the Xiagou Formation reveal strong astronomical signals of long eccentricity (405 kyr), short eccentricity (100 kyr), obliquity (37 kyr) and precession (22 kyr). These provide an excellent opportunity to establish a floating astronomical time scales (ATS) then to determine variations in the sediment accumulation rate (SAR). Based on the floating ATS of the Xiagou Formation, the Xiagou Formation covers a 10.3 myr period depositional duration, and depositional duration of three members in the Xiagou Formation are 3.5 ± 0.1 myr, 3.1 ± 0.1 myr and 3.65 ± 0.15 myr, respectively. The high-resolution SARs of the Xiagou Formation vary from 8.5 cm/kyr to 19.8 cm/kyr. The different lithologies in the Xiagou Formation exhibit the same variation tendency in SARs, but the mechanisms of astronomical forcing that affect the SARs are different. In addition, the correlation between total organic carbon (TOC) and SARs can be used to investigate the impact of the SAR on organic matter abundance and conclude that relatively high SAR can protect organic matter and prevent oxidation, but rapid SAR can also dilute organic matter abundance of source rock.