The Neoproterozoic Quruqtagh Group in eastern Chinese Tianshan: evidence for a post-Marinoan glaciation

The Neoproterozoic Quruqtagh Group in eastern Chinese Tianshan consists of, in ascending order, the Bayisi, Zhaobisban, Altungol, Tereeken, Zhamoketi, Yukkengol, Shuiquan, and Hankalchough Formations. Diamictite units occur in the Bayisi-Altungol, Tereeken, and Hankalchough Formations. Our field observations confirm the glacial origin of the Tereeken and Hankalchough diamictites, but the glacial origin of diamictites in the Bayisi and Altungol Formations remain ambiguous. While siliciclastic strata dominate the succession, carbonate units are recognized between the diamictites and immediately atop the Tereeken and Hankalchough diamfictites. Carbonates in the Altungol Formation are characterized by highly positive delta C-13 values (up to + 10.4%(o) versus PDB), comparable to those of pre-Marinoan but post-Sturtian carbonates. High-resolution sampling of the pinkish cap dolostone (ca. 10 m-thick) overlying the Tereeken diamictite shows a consistent delta C-13 chemostratigraphic pattern (ca. -5%o at the base and slightly decreasing upsection) similar to that of Marinoan cap carbonates. This cap dolostone is also characterized by macropeloids similar to those in the Marinoan-age Raventhroat cap carbonate, Mackenzie Mountains. Chemostratigraphic and sedimentary features are suggestive of a Marinoan age for the Tereeken glaciation. Thus, the younger Hankalchough diamictite may represent a post-Marinoan glaciation. This interpretation is consistent with the occurrence of vendotaenid fossils in the Shuiquan Formation. The alternative interpretation-the Tereeken and Hankalchough diamictites are, respectively, Sturtian and Marinoan in age-is not consistent with the above evidence or the chemostratigraphic features of the Shuiquan Formation and the Hankalchough cap carbonate. A post-Marinoan age for the Hankalchough and other glaciogenic deposits indicates that the disappearance of Doushantuo-type acritarchs and the subsequent radiation of macroscopic animals are closely linked, in geochronological terms, with a Neoproterozoic glaciation. (C) 2003 Elsevier B.V. All rights reserved.