Discovery of Late Triassic volcanic ash layers in the deep-water zone of the Nanpanjiang Basin (South China) and the possibility of Carnian Pluvial Episode correlation

TL;DR

This study identifies volcanic ash layers in the Late Triassic Nanpanjiang Basin, providing new radiometric ages that suggest a correlation with the Carnian Pluvial Episode and improve regional stratigraphic understanding.

Contribution

It presents the first U-Pb zircon ages for volcanic ash in the Niluo Member, establishing a Carnian age and linking slope-basin deposits to global CPE events.

Findings

Volcanic ash layers dated to ~229 Ma in the Niluo Member.

The Niluo Member records a Carnian depositional age.

Potential correlation of Niluo Member with the Carnian Pluvial Episode.

Abstract

This study presents new geochronological constraints for the Niluo Member within the slope-basin facies of the Late Triassic Nanpanjiang Basin, eastern Tethys. The basin underwent a significant marine-to-continental transition during this period. Previous biostratigraphic studies on platform facies were hindered by inconclusive conodont zonation, leaving the chronology of slope-basin deposits poorly resolved. To address this, we identified volcanic ash layers within the Niluo Member in the Wangmo area. Zircon U-Pb dating of these ashes yielded weighted mean ages of 229.9 Ma and 229.0 Ma, establishing a Carnian depositional age. This result is significantly younger than previous estimates and coincides with the CPE. The Niluo Member is interpreted as a period of slow, oxygen-deficient sedimentation, contrasting with the rapid turbidite deposition of the enclosing formations. This depositional hiatus likely facilitated the preservation of the datable ash layers. The Carnian age and unique lithology suggest the Niluo Member may record the CPE in the slope environment, potentially linked to increased terrigenous input that suppressed carbonate production. Concurrent conodont sampling was unsuccessful, validating the documented difficulties in applying biostratigraphy in these deep-water settings. These new radiometric ages provide a crucial anchor point for Late Triassic stratigraphy in South China, demonstrate the potential for preserving CPE records in slope facies, and offer a new basis for regional and global correlation.