Paper Title: Astronomically forced cycles in Lower Carboniferous Luzhai Formation shales, Guizhong Depression, South China

Author: Siding Jin, Sibing Liu, Xiangfeng Wei, Chao Ma, Yicai Chen, Jingyu Hao, Kai Ni, Daojun Zhou

https://doi.org/10.1016/j.marpetgeo.2023.106427

Available online 26 July 2023

Introduction: Within the annals of the early Carboniferous period lies the enigmatic Luzhai Formation—a repository of continuous, substantial shales that hold the keys to hydrocarbon exploration and production in southern China. Yet, a high-resolution stratigraphic sequence framework and cyclostratigraphic studies into this organic-rich shale formation have been notably absent. In this groundbreaking endeavor, we present compelling evidence of Milankovitch cycles residing in the hallowed depths of two Luzhai Formation boreholes within the Guizhong Depression.

The Astronomical Revelation: Natural gamma-ray logging data pave the way for the establishment of a floating astronomical timescale (FATS), spanning approximately 5.9 million years. This FATS finds its foundation in the stable, long eccentricity cycles—a celestial waltz with a 405,000-year cadence. Anchored to the Devonian and Carboniferous boundary age (359.3 ± 0.4 million years), an absolute timescale emerges, casting its mantle over intervals ranging from 359.3 to 353.4 million years.

Quantitative Insights: Leveraging the might of correlation coefficients (COCO), evolutionary COCO, and the evolutionary TimeOpt method (eTimeOpt), we quantitatively align observed stratigraphic cycles with their celestial counterparts. These computations grant us a glimpse into the plausible range of sedimentation rates. In our quest to decipher orbital forces governing high-resolution sea-level changes, we invoke the power of the sedimentary noise model. Within this intricate analysis, we unveil the correlation between orbital forcing-controlled sea-level fluctuations and ∼1.2-million-year obliquity amplitude modulation (AM) cycles.

The Rhythmic Tapestry: As our investigation unfolds, the ∼1.2-million-year obliquity AM cycle emerges as the chief conductor orchestrating third-order sequence development. The ebb and flow of sea-levels etch their narrative upon the landscape, and the 3rd-order sequence boundaries find their stories intertwined with the zenith of DYNOT median curve values and the nadir of ∼1.2-million-year amplitude modulation (AM) curve values.

This chronicle of discovery takes us on a celestial journey through time, where Earth's geological rhythms resonate with cosmic symphonies. Within the confines of the Luzhai Formation, we unveil the intricate interplay of celestial bodies and the terrestrial realm, adding layers of understanding to the annals of Earth's history.