Carbon and Strontium Isotope Profile on Neoprotozoic III at Three Gorges, China, and Possible evidence for ¡oPost Marinoan¡{+/-} Ice age

Neoprotozoic III and Lower Cambrian strata of the Three Gorges in China have been well documented on stratigraphy and paleontology. The carbonate-rich strata of the Upper Neoprotozoic III occur above two tillite formations (Gucheng Formation and Nantou Formation), and they are divided into two formations. The lower of these, the Doushantuo Formation, contains black laminated dolostone and shaly dolostone. The overlying Dengying Formation is dominated by pink to light grey dolomite in the upper part and contains fossils of macroscopic algae (Vendotaenids), the middle part outcrops limestone and base outcrops dolostone. The Gucheng tillite Formation is covered by interglacial deposits of Datangpuo Formation that are overlain by Nantou tillite Formation. The Lower Cambrian strata are grey fossiliferous limestone with some interbedded black shale contains complex small fossils zone. Carbon and strontium isotopes were measured by standard methods on a section from the top of the Nantuo Formation to the Lower Cambrian. d13C rises to +4 5¡ë from -4¡ë within the lower part of Doushantou Formation, and then drops dramatically to -8¡ë at the boundary of Doushantuo Formation and Dengying Formation. This is followed by a rapid rise to higher than 12¡ë from -8¡ë to +4¡ë at the base of Dengying Formation. Stable values of +3 to +4¡ë occur in the lower and middle part Dengying Formation, and are capped by a gradual drop from +2¡ë to -3¡ë at the top most part of Dengying Formation to the boundary of Cambrian as defined by complex small shall fossils. A short increase spans the boundary of Precambrian-Cambrian from -3¡ë to 2¡ë. 87Sr/86Sr rises from 0.7075 at the base and middle part of Doushantuo Formation to 0.7095 at the boundary of Doushantuo-Dengying Formation boundary, and then falls to 0.7083 at the lower part of Dengying Formation. Gradually, it rises from 0.7082 to 0.7092 at the middle to upper part of the Formation. After falling back to 0.7087 near the top of the Dengying Formation, 87Sr/86Sr rises again at the boundary of Precambrian-Cambrian synchronously with d13C. The carbon and strontium isotope record of the section is strikingly similar to those from the Congo craton in Namibia, which also has two tillite formations, the Chuos tillite Formation at the base, Abenab subgroup dolostone at middle, and Ghaub tillite Formation at upper, and Maieberg Formation of dolostone overlies Ghaub Formation. The isotope curves can be correlated with each other, Doushantuo Formation to Abenab subgroup, and Dengying Formation to Maieberg Formation, respectively. Similar lithological aspects of Three Gorges section and Congo craton support this correlation too, where the lower part of the sections mainly outcrops dark and laminated dolostone in Doushantuo Formation and Abenab subgroup, with pink dolostone in the Dengying Formation and Maieberg Formation. This suggests correlation of the Nantuo tillite to the Chuos tillite. Strong fluctuations of d13C and higher 87Sr/86 around the top of the Doushantuo Formation, may suggest the Ghaub Formation tillite can be correlated to the boundary of Doushantuo Formation-Dengying Formation. As the existence of two tillite formations of Neoprotozoic III in the Three Gorges (Gucheng Formation and Nantuo Formation), the Ghaub Formation tillite in Namibia may represents the third local glacial deposits that are so called ¡^opost Marinoan¡{+/-}.