Cyclostratigraphy of the Lower Jurassic (Toarcian) terrestrial successions in the Sichuan Basin, southwestern China

• Published in: Journal of Asian earth sciences 2023-07, Vol.250, p.105617, Article 105617
• Main Authors: Qiu, Ruoyuan, Fang, Linhao, Lu, Yuanzheng, Chen, Yuxuan, Huang, Renda, Lei, Wenzhi, Zhang, Pengyuan, Li, Mingsong
• Format: Article
• Language: English
• Subjects: Astronomical response; Cyclostratigraphy; Lower Jurassic; Sedimentary noise; Sichuan Basin
• ISSN: 1367-9120; 1878-5786
• DOI: 10.1016/j.jseaes.2023.105617

•Natural Gamma Ray datasets reveal the Milankovitch cycles of the Lower Jurassic strata in the Sichuan Basin.•An abrupt shift in astronomical response in the Da’anzhai Member suggests that the occurrence of strong obliquity cycle during the T-OAE could be globally present.•This study provides the first lacustrine record of sedimentary noise during the T-OAE in the Sichuan Basin.•Dramatic climatic perturbation probably controls lake evolution and sedimentary noise during the T-OAE. The Early Jurassic Toarcian was a key period that witnessed the global carbon perturbation and a series of climatic events. A reliable timescale is essential for obtaining a holistic understanding of astronomical response and environmental change. However, the astrochronology of the terrestrial basin at the Early Toarcian is still lacking. Here, utilizing high-resolution Gamma Ray (GR) datasets from three boreholes, we conduct a cyclostratigraphic analysis of the Lower Jurassic strata in the Sichuan Basin, southwestern China. Time series analysis reveals the most optimal sedimentation rates and evidence of the Milankovitch cycles. Tuning of these datasets to the interpreted eccentricity cycles yield three floating astronomical timescales of ∼ 2.65 Myr, ∼ 2.51 Myr and ∼ 2.33 Myr duration for the strata from the top of Ma’anshan Member to the base of Shaximiao Formation in the Sichuan Basin. Under high-precision floating astrochronology, we performed power decomposition analysis (PDA) and conducted sedimentary noise model for the tuned series. The enhanced obliquity signal in astronomical response in the Sichuan Basin is broadly consistent with the worldwide marine records, which implies that this signal could be globally present during the Toarcian epoch. In addition, the increased low-frequency sedimentary noise and sedimentation rate during a relative high lake-level period of the Da’anzhai Member may be related to the accelerated hydrological cycling associated with accelerated with Toarcian Oceanic Anoxic Event (T-OAE). These anomalies in both astronomical response and sedimentary noise probably suggest that the lacustrine system may be dominated by dramatic and rapid climatic perturbations during the T-OAE.