Astronomical calibration of a ten-million-year Triassic lacustrine record in the Ordos Basin, North China

The division of geological time is of general interest as this allows a better understanding of the geological processes and Earth's evolutionary history. Astrochronology provides a way to evaluate high-resolution timescales of Earth's climate changes. Astronomical tuning of orbitally forced stratigraphic records to construct astronomical-calibrated timescales has led to a progressive refinement of the geological timescale over the last decades. However, the Triassic remains a relatively weak link. A large lake with an area of 370 000 square kilometres formed in the Ordos Basin in North China during the Triassic Period. It has been demonstrated that the sedimentary deposits of the seventh Oil Member (Chang 7 Member) of the Triassic Yanchang Formation in the southern Ordos Basin responded to astronomically forced climate changes. However, the constructed astronomical-calibrated timescales failed to reach an agreement. This study presents natural gamma ray logs and geochemical climate record of Fe and Al concentrations from the southern Ordos Basin margin that are dated precisely from 239 to 229 Ma by combined U-Pb dating and astrochronology. Therefore, the Chang 7 Member was deposited during the Ladinian and Carnian (late Middle through to earliest Late Triassic), this knowledge will refine the Triassic astronomical scale. The distribution of detrital zircon dates shows that the Chang 7 Member in the southern Ordos Basin has multiple provenance characteristics. The southern Ordos Basin is far from this provenance area, which is reflected in the low sediment accumulation rate (0.5 to 2.0 cm/kyr). In contrast to the lower part of Yanchang Formation underlying the Chang 7 Member, the large tuffs and seismites from the bottom of Chang 7 Member indicate the initiation of tectonic collision between North Qinling orogenic belt and the southern margin of the Ordos Basin.

Automated summary

The division of geological time and the evaluation of Earth's climate changes are important for understanding geological processes and Earth's evolutionary history. However, the Triassic period remains a weak link in the geological timescale. This study presents new evidence to refine the astronomical scale of the Triassic by analyzing sedimentary deposits in the southern Ordos Basin. The findings suggest that the Chang 7 Member was deposited during the Ladinian and Carnian, which contributes to a better understanding of the Triassic period.