Cyclostratigraphy of the Late Triassic Bolila Formation in the Qiangtang Basin, Tibetan Plateau

Interbedded deposits of terrigenous clastic rocks and carbonate rocks can develop in marine environments ranging from shallow to deep water depths. Such lithological rhythms may be controlled by multiple factors, including terrigenous input, carbonate content, ocean currents, sea-level fluctuations, sedimentation and subsidence, climate change, and astronomical forcing. Therefore, identifying the dominant driving forces behind lithologically rhythmic sedimentation is of great significance for reasonably interpreting sedimentary evolutionary processes. However, difficulties in reconstructing sea-level changes and the scarcity of high-resolution geochronological data have hindered progress in studying the linkage between astronomical forcing in shallow marine settings and lithological rhythms. Against this background, this study performs cyclostratigraphic analysis and sedimentary noise modeling using well logging data from QZ-16 Well in the Qiangtang Basin, Tibetan Plateau, to investigate the influence of astronomical forcing on lithological rhythms in the Upper Triassic strata of the region. Natural gamma-ray data from the Bolila Formation were tuned to the stable 405‑kyr long eccentricity cycle, establishing a high-resolution floating astronomical time scale spanning 3.8 Myr. Anchored by the age of the boundary between the Bolila Formation and the Bagong Formation (233.16 Ma), this study further constructed an anchored absolute astronomical time scale for the studied interval, constrained to the time range of 236.98 ± 1.42 Ma to 233.16 ± 1.37 Ma. In addition, sedimentary noise modeling based on the tuned gamma-ray data reveals the characteristics of relative sea-level changes during the deposition of the Bolila Formation, which are consistent with the trend of global sea-level changes during the same period. Particularly crucially, sea-level fluctuations exhibit distinct long-period obliquity cyclicity, which is highly consistent with global sea-level records and measured obliquity/total power curves. This indicates that long-period obliquity cycles acted as the primary driver of sea-level changes and regulated land-ocean water exchange during the Late Triassic greenhouse period, further verifying the aquifer-eustasy hypothesis. The reconstructed sea-level changes are also supported by regional sequence stratigraphic interpretations. Comprehensive analysis suggests that the rhythmic deposition of terrigenous clastic rocks and carbonate rocks was jointly controlled by insolation modulation and sea-level fluctuations. This study provides an important example for exploring the coupled relationships between astronomical forcing and changes in sea level, paleoclimate, and lithological rhythms at million-year time scales.