Cyclic late Katian through Hirnantian glacioeustasy and its control of the development of the organic-rich Wufeng and Longmaxi shales, South China

Abstract

Deposition of the organic-rich shales of the Wufeng and lowermost Longmaxi formations (late Katian through Hirnantian) occurred in a relatively stable tectonic setting and commenced at approximately 4.5 Myr prior to the end-Ordovician glacial maximum. These organic-rich shales are important because they provide a continuous sedimentary record of high-frequency eustatic changes associated with ice-sheet expansion and shrinkage. Here, we carried out a high-resolution cyclostratigraphic study of Fe3+ series and associated geochemical analysis of the Wufeng and lowermost Longmaxi shales in order to better understand sea-level cyclicity in the late Katian through Hirnantian ocean. The organic-rich shales deposited during this interval in the EHD1 drill core (Yichang, China) record four major Myr-scale eustatic cycles, which were associated with third-order sea-level fluctuations. Within each of these major cycles, 3–4 minor eustatic cycles corresponding to 405-kyr long eccentricity astronomical cycles were recognized and representing fourth-order eustatic changes. The cyclostratigraphic age of this study is generally in agreement with the radiometric dates in GTS2012, although the duration of the Hirnantian stage is ~0.5 Myr longer than previously thought. This study also infers glacial controls on the depositional environment of the organic-rich shales, as enhanced cooling triggered ice-sheet expansion and concurrent sea-level fall, leading to higher thermohaline circulation intensity and enhanced paleoproductivity through upwelling and terrestrial influx. In contrast, warming intervals led to ice-sheet shrinkage and concurrent deepening characterized by water column stratification and reduced nutrient supply. The results provide evidence in support of continuous continental glaciation in the late Katian through Hirnantian.