Astronomical constraints on deposition of the Middle Triassic Chang 7 lacustrine shales in the Ordos Basin, Central China

摘要

The Middle Triassic was a key period that witnessed the evolution of Earth system processes and the commencement of a terrestrial lake in the Ordos Basin, Central China. A high-precision stratigraphic framework is the key to understanding the nature and pattern of critical geological events. Detailed time series analyses of magnetic susceptibility (MS) data were performed on the deep lacustrine shale-dominated Chang 7 Member of the Yanchang Formation from the Y1011 well core. The results reveal well documented cyclic variations with wavelengths of 5.37 m, 1.39-1.78 m, 0.48 m, and 0.24-0.30 m, which are driven by long-eccentricity, short-eccentricity, obliquity, and precession in the Middle Triassic. The stable 405-kyr tuned floating astrochronological time scale (FATS) reveals that the depositional duration of the Chang 7 Member is approximately 5 Myr, and the sedimentation rates range from 0.90 cm/kyr to 1.69 cm/kyr. In particular, the lower part of the Chang 7 Member is characterized by an organic-rich, black shale, called "Zhangjiatan Shale", whose depositional duration can be estimated at about 1.7 Myr. Along with the published biostratigraphic divisions and U-Pb age constraints, our FATS further confirms that the Chang 7 Member mainly developed in the Ladinian Stage, and that the upper part of the Chang 7 spanned the Middle/Late Triassic boundary. The duration of the Chang 7 deposition suggests a temporal and genetic linkage between the responses of the Chang 7 shales in the Ordos Basin and the Indosinian Orogeny in the Qinling orogenic belt. The Ladinian Stage of the Middle Triassic probably witnessed a dramatic shift in the evolution of the regional geodynamic system.