Cyclostratigraphic analysis of the Lower Cambrian shales in western Hubei and southern Guizhou

Key words: 
• Niutitang Formation /
• Jiumenchong Formation /
• Orbital forcing /
• Eccentricity /
• Sedimentary noise model

Abstract: The Lower Cambrian organic-rich shale is important marine shale exploration target in western Hubei and southern Guizhou. Understanding the chronostratigraphic relationships and their underlying causes of organic-rich shale intervals in different areas is crucial for shale gas exploration. In this study, we conducted cyclostratigraphic analyses using natural gamma-ray logging data from the Lower Cambrian Niedi-1 well in western Hubei and the Huangye-1 well in southern Guizhou, and determined the primary astronomical periods. Using previously reported zircon age as anchoring points, we constructed an astronomical time scale for the Lower Cambrian. Our results reveal that wavelengths of 36 m and 9 m in the Niedi-1 well, and 16.6 m and 4 m in the Huangye-1 well, correspond to orbital 405 kyr and 100 kyr cycles, respectively. Furthermore, we calculated the durations of the Niutitang Formaion in the Niedi-1 well and Jiumengchong Formation in the Huangye-1 well to be 3.3 Myr and 2.6 Myr, respectively, with age intervals falling within the range of 535.4±1.7 Ma to 536.82±1.7 Ma and 535.2±1.7 Ma to 537.84±1.7 Ma. Using a sedimentary noise model, relative sea-level changes in the Lower Cambrian were reconstructed. Through correlation between sea-level change curves (DYNOT and ρ1), eccentricity cycles and sedimentary cycles, our results indicate that sea-level changes correspond to 1.2 Myr obliquity modulation cycles and two sedimentary sequences (SQ1 and SQ2). In addition, comparing the total organic carbon content (TOC) of the Lower Cambrian organic-rich shale with orbital eccentricity, sedimentation rates and sea-level changes, our findings reveal that the intervals with high TOC value correlate with the maximum eccentricity of 405 kyr, while low TOC value correspond to minimum value in Niedi-1 well. Because of the chaotic behavior of the solar system, accurately predicting the mechanism driving organic-rich shale during the early Cambrian is impossible. Assuming that the maximum eccentricity corresponds to the maximum organic carbon content at that time, strong seasonal variation will, to some extent, trigger the enrichment of black shale over a more extended period. Furthermore, the comparison between relative sea-level changes and organic carbon content reveals no causal relationship between sea-level fluctuations and the enrichment of organic matter.