Astronomical constraints on the development of alkaline lake during the Carboniferous-Permian Period in North Pangea

Abstract

Astronomical forcing associated with Earth's orbital and inclination exerts a major control on climate as recorded in the sedimentary rocks. Alkaline lake deposits in the second member (F 2 ) of the Fengcheng Formation provided a unique geological record to explore the paleoclimatic conditions during the Late Paleozoic Ice Age (LPIA) in northern Pangea. In this study, we utilize high-resolution downhole logs (GR, DEN and CAL) to conduct a comprehensive cyclostratigraphic analysis of the Fengcheng Formation. Time series analysis reveals evidences for 405-kyr long eccentricity cycles in logs series, which is also confirmed by objective statistical approaches by modeling of optimal sedimentation rates. Tuning of these downhole logging datasets to the interpreted 405-kyr eccentricity obtained a ~ 5.0-Myr-long astronomical time scale (ATS). Afterwards, by anchoring the U-Pb age at the first member (F 1 ) and second member (F 2 ) boundary, obtaining an absolute age framework for the Fengcheng Formation that extends from 297.0 to 301.98 Ma. Under the high-precision time framework, we performed sedimentary noise model to reconstruct the lake-level changes in terrestrial Junggar Basin. Notablely, modeling of the tuned CAL series reveals high-frequency lake-level changes at 10 5 –10 6 kyr orbital time scale (i.e., ~1.2 Ma, ~450 kyr, ~180 kyr, ~116 kyr). These high-frequency lake-level variations were possibly teleconnected with the high-latitude ice sheets periodic waxing and waning and/or low latitude monsoonal circulation. Moreover, the development of rhythmic evaporite-containing mudstone in the F 2 was tightly linked to the basin's hydrological cycles, which was ultimately paced by the astronomical forcing. Finally, we propose a simplified model for illustrating the formation of the rhythmic evaporite layers and emphasize that the favorable paleotectonic, paleogeographic and paleoclimatic conditions were necessary for development of the alkaline lake. • Comprehensive cyclostratigraphic analysis of the Fengcheng Formation • Reconstruct the lake-level changes by using DYNOT simulation approach. • A novel alkaline development model associated with orbital forcing was firstly proposed.