A step toward a complete Triassic time scale: the proposal from China----Nanjing Branch of The Chinese Academy of Sciences

The Triassic Period (252-201 million years ago) is bounded by two mass extinctions, but the time scale of the Late Triassic still remains controversial. A new study published in Earth and Planetary Science Letters, a top journal of geoscience sheds light on this critical problem.

A research group led by Prof. HUANG Chunju at China University of Geosciences (Wuhan) and Prof. WANG Yongdong at Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences report their latest study on the astronomical cycles and magnetostratigraphy of the Late Triassic Xujiahe Formation of South China. Their study provides key evidence for resolving the decade-long controversy on the Late Triassic time scale; it also has significant implications on the global correlation of the end-Triassic mass extinction event and global climate and environmental changes.

The Milankovitch theory that quasi-periodic oscillations in the Earth-Sun position have induced prominent 10 to 100 thousand year variations in the stratigraphic record of climate is widely accepted. Reversals of the Earth’s magnetic polarity field are geologically rapid events. These provide the cornerstone for the high-resolution geochronology.

The astronomically tuned geomagnetic polarity time scale of the Newark Supergroup of North America provides a "global reference" for the Late Triassic time scale. However, the Newark reference scale has been challenged regarding its age scale and completeness. Therefore, an independent astronomical-tuned magnetic polarity zonation is required to verify the Newark reference scale.

Sichuan Basin is a large and typical Mesozoic terrestrial basin in eastern Asia. The dinosaur-track-bearing Xujiahe Formation has abundant and diverse fossils plants and fauna; it is one of the key resource beds for coal and gas. The Xujiahe Formation deposited in the lacustrine-fluvial environment ca. 200 million years ago. The chronology of the formation is quite uncertain that hampers global correlation with Upper Triassic marine sections.

In past years, the research team conducted an integrated study on the biodiversity and environmental change across the Triassic-Jurassic transition in the Sichuan Basin. Collaborated with international geologists, the team did a five-year long integrate study of astronomical cycles and magnetic stratigraphy from four sections in Xuanhan, Hechuan and Guangyuan regions of the Sichuan Basin from 2012 to 2016.

Variations in natural gamma-ray and magnetic susceptibility that reflect variable continental weathering in the source regions of the Xujiahe Formation are paced by Milankovitch cycles, especially the 100-kyr short eccentricity and 405-kyr long eccentricity. They compiled a multiple million years astronomical calibrated magnetic stratigraphy for the Xujiahe Formation that also has dating from detrital zircons and regional biostratigraphy.

The age of the Xujiahe Formation was accordingly assigned to the late Norian to the Triassic-Jurassic boundary 201.3 to 207.2 million years ago. For the first time, the cycle-calibrated magnetostratigraphy of the Xujiahe Formation is compared directly via the magnetic-polarity zones to the astronomical cycles of the Newark Supergroup.

The Sichuan-Newark time scale is compatible with the magnetostratigraphy from the candidate Global Boundary Stratotype Section and Point (called 'Golden Spike' in the geological community) for the Norian-Rhaetian boundary interval in Italy. A cooling event recovered from the oxygen isotope at the Italian Golden Spike is consistent with a similar event indicated from the fossil wood Xenoxylon at the Xujiahe Formation. Furthermore, age of the earliest dinosaur footprints in China is also dated to the middle Rhaetian (ca. 204 Ma).

This study greatly improves the chronologic resolution of the Xujiahe Formation. It also helps to resolve the controversy about the completeness and reliability of the Newark reference scale. This is the first solid evidence from China that provides critical constraint for the Late Triassic Time Scale. An international reviewer said this paper is “extremely concise and to the point, and presents interesting data that will be of use to the ongoing debate on the duration and subdivisions of the Late Triassic”.

This study was co-sponsored by the National Natural Sciences Foundation of China, Strategic Priority Research Program (B) of the Chinese Academy of Sciences, 973 program, 111 Project, China Scholarship Council, and Overseas Distinguished Teacher Program of the Ministry of Education. Prof. Chunju Huang and Yongdong Wang are two corresponding authors of the study.