Astrochronology of the Middle Jurassic Buqu Formation (Tibet, China) and its implications for the Bathonian time scale

The Buqu Formation in Qiangtang Basin in China was formed during the Bathonian Stage of the Middle Jurassic. The subject of this study is the formation at Well QZ11 at a depth of from 34 to 591 m, located in the Southern Depression of Qiangtang Basin. The optimal accumulation rate from the result of ASM analysis was used as the “target” to tune the GR (gamma ray) and DLR (double lateral resistivity) series in QZ11. Power spectra analysis in tuned GR and DLR series demonstrate the existence of a series of prominent astronomical periods within the Buqu Formation in the Qiangtang Basin. Then ~ 100 kyr bandpass filtered curve was used to establish a floating astronomical time scale for the Buqu Formation, and the maximum and minimum sedimentation rates without considering the factor of formation compaction were 150 m/Myr and 395 m/Myr, respectively. Using this scale, the lower limit for the duration of the formation is estimated to be approximately 2.09 Myr. This duration can serve as a reference for temporal and chronological calibrations of the Bathonian Stage.