Newly discovered Cryogenenian Diamicties and Cap-carbonate in the Neoproterozoic successions from the Tarim Craton, NW China

Abstract:

Neoproterozoic cap carbonates associated with glacial deposits have been the unique lithofacies for establishing regional or interregional correlations and understanding the environmental conditions in the postglacial oceans. Here, we report a newly discovered Cryogenian tillite (in Aksu section), and a diamictite and cap-carbonate couplet (DCC) (in Wushi section) from the northwestern margin of Tarim Craton, NW China. The Aksu tillite is represented by reddish conglomerates, comprising angular to subrounded, poorly sorted clasts with diameters of 5-30 cm. Notably, the glaciogenic features, such as striations on clasts can be observed. The DCC member in Wushi section, sits atop an angular unconformity, and represents the end-Cryogenian strata according to the regional sequence stratigraphy (at the base of the Sugetbrak Fm, redbeds with interbedded 615-Ma mafic magmatism). The upper cap-carbonate of the DCC member consists of a < 2m-thick pink dolomite, which directly and conformably overlies the lower red diamictite. Given the striking similarity in both sedimentological characteristics and stratigraphic positions, the Aksu tillites and the Wushi DCC member may represent the same glaciation, most likely the Marinoan glaciation. To further test the correlation, high-resolution C-O isotopes and representative samples for geochemistry (Strontium isotope and trace element ratios) from the cap-dolomite are measured. As for the Sr isotope analyses, an incremental leaching technique was employed to extract the information about the chemical compositions of post-glacial ocean water.

The δ¹³C_PDB results consistently show moderate negative values of ca. -2.5‰. The minimum ⁸⁷Sr/⁸⁶Sr ratio from the leachates of each sample defines the contemporaneous seawater value, with the Mg/Ca ratio close to 1 (0.8). The ⁸⁷Sr/⁸⁶Sr ratios of all representative samples vary from 0.70680 to 0.70855. All the chemostratigraphic and cap-dolomite characteristics may suggest that our newly discovered Aksu diamictite and the Wushi DCC member represent the ending of the Cryogenian Marinoan glaciation.