Ediacaran glacial records identified from the southern Alxa Block in NW China: Evidence from stratigraphy and C-O isotopes

Abstract

The Ediacaran-age glaciation is the last of three major glacial events during Cryogenian-Ediacaran time. Unlike the earlier Cryogenian-age Sturtian and Marinoan glaciations that deposited worldwide diamictite and cap carbonate couplets, the Ediacaran-age glacial cap carbonates have rarely been documented, which hinders the understanding of Ediacaran glaciation and glacial stratigraphic correlations. Here, we newly identified an Ediacaran glacial cap carbonate overlying the stratified diamictites in the upper part of the Shaohuotonggou Formation in the Neoproterozoic Hanmushan Group along the southwestern margin of the Alxa Block. The stratified diamictite with unsorted clasts and widespread ice-rafted dropstones indicates its glacial origin. The absence of glacial-interglacial depositional cycle suggests that no glacier advance-retreat process occurred during this glaciation, which likely represents a short-lived glacial event with rapid deglaciation. Carbon isotope values of the cap carbonate fluctuated between ca. −2.6 ‰ and ca. + 1.5 ‰, comparable to other Ediacaran glacial cap carbonates (ca. 560 Ma) from Arabia, Qaidam, northwestern Australia and Iran. Taking into account recently discovered Marinoan-age diamictite and cap carbonate couplet at the basal Shaohuotonggou Formation, we suggest that the southern Alxa Block most likely preserves two Neoproterozoic glacial events, i.e. the Marinoan-age glaciation and the late Ediacaran-age glaciation. Thus, the Shaohuotonggou Formation most likely represents a late Cryogenian–late Ediacaran succession that contains two intervals of glaciogenic deposits. The occurrence of Marinoan and Ediacaran glacial cap carbonates thus merits a reconsideration of the paleogeographic and paleoclimatic evolution in the Alxa Block during late Neoproterozoic time.