Marinoan-aged red beds at Shennongjia, South China: Evidence against global-scale glaciation during the Cryogenian

Abstract

The continuous existence of the Marinoan Ice Age, a late Cryogenian panglaciation (Snowball Earth), is challenged by the occurrences of widespread glacial marine and nonglacial deposits consisting of stratified diamictite, laminated siltstone and claystone, and even carbonates, which indicate an episode of partial deglaciation. Red beds that commonly accompany these glacial marine and nonglacial deposits have not received enough attention to date. The middle Nantuo Formation of the Cryogenian Marinoan epoch in the Sanshengtai section, Shennongjia area, is composed of 6 lithofacies, i.e., Green Massive Diamictite, Green Laminated Lithic Feldspar Wacke, Green Laminated Siltstone and Claystone, Red Laminated Lithic Feldspar Wacke, Red Crudely Stratified Diamictite, and Red Laminated Siltstone. Among them, the last three constitute the red beds and are interpreted as glacial lacustrine and nonglacial deposits, which can be found and roughly correlated across several continents. High Fe2O3 and low Ce/Ce* values in the red beds suggest a probable association between the formation of red beds and the Neoproterozoic oxygenation events. Coincidences between the occurrences of red beds and high values of Al/Na, Ti/Na, and chemical index of alteration (CIA) indicate the favorability of warm intervals with high weathering intensities for the formation of red beds, whose presence indicates partial deglaciation during the Marinoan Ice Age and provides a new clue to contradict the worldwide Snowball Earth hypothesis.