Orbital control on cyclical organic matter accumulation in Early Silurian Longmaxi Formation shales

Abstract

High resolution (939 samples) total organic carbon content (TOC) analyses were conducted on the Shuanghe Section of ∼152.6 ​m in the Changning area, Sichuan Basin. The sampling section was divided into two units considering the distinct-different deposit environment and sediments accumulation rate. The lower part (Unit 1) and the peer part (Unit 2) with high resolution sample spacing (0.08–0.4 ​m) enables the identification of the precession cycle in two sedimentary sequences with distinct different sedimentary accumulation rate. MTM Power spectral analyses on untuned TOC series reveals significant peaks exceeding above the 95% confidence level and shows that both Unit 1 and Unit 2 have recorded Milankovitch cycles of 405 kyr long eccentricity, short eccentricity, obliquity and precession. The floating astronomical time scale (ATS) was constructed on the Shuanghe Section in the Early Silurian (∼439.673–444.681 ​Ma), and which was calibrated by 405 kyr long eccentricity cycles. The total duration of the Wufeng and Longmaxi shales is 5.01 Myr. The floating ATS used for estimating the duration of the graptolite zones and each stage in the study interval. Finally, we postulated two models that could verify the linkage between orbital cycle and organic accumulation. To make sure whether productivity or preservation is the main factor that under long eccentricity control, the phase correlation between the obtained filtered signal and the theoretical orbital solution should be made clear in the further research.