Paleoclimate Evolution of the Cryogenian Tiesi'ao Formation- Datangpo Formation in Eastern Guizhou Province:Evidence from the Chemical Index of Alteration
Presently two episodes of global glacial events could be distinguished in Neoproterozoic,when dramatic changes in climate happened globally.Two formations of strata distributed in Nanhua System (Cryogenian)around eastern Guizhou Province with typical characteristics of glacial sediments.The lower named Tiesi'ao Formation and the upper named Nantuo Formation correspond with Sturtian glaciation in 720Ma and Marinoan glaciation in 650Ma respectively,and the strata between the two formations are of Datangpo Formation representing deposition during the interglaciation.This paper discusses paleoclimate evolution during the late Sturtian glacial period to interglacial period with the main elements dated from samples in Tiesi'ao Formation-Datangpo Formation,Songtao county and Tongren city,combined with evidence from chemistry and petrology.This illustrates that transient glacial events occurred following the end of Sturtian age.From bottom to top,the chemical index of alteration(CIA)raises from 51 in late Tiesi 'ao period to 60 in early Datangpo period.However,the shape of curve drawn with date from 52 to 68 is not linear but a serrated upward trend,showing the occurrence of ephemeral glacial event.The average value of CIA from Carbonaceous shales in upper Datangpo Formation and silty shales in middle,lower Datangpo Formation are 66 and 67,both supporting a stable warm condition.However,the mean of CIA decreases suddenly at the top of the latter which may due to the beginning of the Marinoan glaciation.On the other hand,it is further confirmed with evaluations on stability of sedimentary condition and composition of source rocks by A-CN-K triangle graph that the instability of climate follows the Sturtian glaciation and the stability of warm climate during the interglaciation.Thus,the fluctuations of glaciation-interglaciation paleoclimate evolution may be related to"Datangpo" type manganese mineralization.