Cyclostratigraphy of Lower Triassic terrestrial successions in the Junggar Basin, northwestern China

The astrochronology for the Early Triassic has been constructed from well-preserved marine successions; however, astronomical analyses of terrestrial strata have continued to be rare and problematic. Physical logs (natural gamma ray logging data) from two wells in the Junggar Basin, northwestern China cover the entire Lower Triassic terrestrial Baikouquan Formation and have been shown to be a powerful tool in cycle analysis. Therefore, these logs offer an excellent opportunity to detect astronomically forced sedimentation in a terrestrial setting from mid to high latitudes. Power spectral analysis revealed that the sedimentary cycles had wavelengths of 31.3–35.9 m, 8.4–10.0 m, 4.0–4.6 m, and ~4.0 m. These are interpreted to correspond to the orbital eccentricity cycles of 405 kyr and ~100 kyr and to the obliquity cycles of ~51 kyr and ~34 kyr, respectively. We, thus, established an astronomical time scale for the studied interval, assuming that the 31.3–35.9 m cycles represent the 405 kyr eccentricity cycles. This astrochronology indicated that the duration of the Baikouquan Formation was 2.0 ± 0.1 Myr. Statistical evaluations of optimal sediment accumulation rates using the correlation coefficient approach showed that the rates of the investigated successions were approximately 6.0–12.0 cm/kyr. Orbitally controlled climate change was the major dynamic mechanism for the formation of Early Triassic fan delta successions in the Junggar Basin.