Sedimentary Response to the Carnian Pluvial Episode and Astronomical Forcing in the 7th Member of the Yanchang Formation, Southwestern Ordos Basin

The Carnian Pluvial Episode (CPE) was a pivotal climate-environmental perturbation event during the Late Triassic, characterized by prolonged global rainfall, climatic warming, and substantial environmental changes that profoundly impacted both terrestrial and marine ecosystems. Establishing a high-resolution and high-precision Astronomical Time Scale (ATS) for the CPE, integrating cyclostratigraphic data with sedimentological findings, is of great significance for constraining the temporal framework of this event. This study investigates the sedimentary facies, the regulatory role of astronomical cycles on sedimentary processes, and the relationship between sedimentary records and the Carnian Pluvial Event, focusing on the 7th member of the Triassic Yanchang Formation in the Bazhu Shangliyuan area, southwestern Ordos Basin. The main findings are as follows: (1) Typical gravity flow sedimentary characteristics were identified in the 7th member of the Yanchang Formation in the study area. Based on sedimentary structures and lithological assemblages, a lithofacies classification scheme was established. (2) Gamma ray (GR) logging data analysis revealed a close relationship between paleoclimate and astronomical cycles, demonstrating that long and short eccentricity cycles controlled the development of lithofacies and their assemblages. (3) Based on the astronomical response of lithofacies and their assemblages, a climatic and sedimentary cyclicity pattern dominated by eccentricity was established. The results indicate that long eccentricity cycles influenced sand body deposition by modulating climate. The 7th member was deposited during the Carnian Pluvial Event under an overall humid climate. The gravity-flow sand bodies in the study area resulted from the combined effects of the Carnian Pluvial Event and astronomical forcing, providing a theoretical basis for deep-water oil and gas exploration in this region.