The genesis of glaucony in the lower Ediacaran Doushantuo Formation, southern Shaanxi: implication to seawater redox condition

Glaucony is an important authigenic mineral in marine environment and has been widely used in palaeoenvironmental research.In this study, we report the occurrence of the glaucony in the lower Ediacaran Doushantuo Formation in southern Shaanxi Province for the first time.Integrated studies including petrography, in-situ microanalysis and X-ray diffraction(XRD)are carried out to constrain the diagenesis of the glaucony and the oceanic redox condition in the early Ediacaran of this area.Microscopic observations using polarizing microscope and scanning electron microscope(SEM)show that the glaucony mainly fills the pores of quartz, feldspar and other clastic minerals in the form of colloidal precipitates, suggesting authigenic precipitation during the early diagenesis.Since both Fe(Ⅱ)and Fe(Ⅲ)are simultaneously required for glauconitization, glaucony prefers to precipitate around the Fe redoxcline(suboxic conditions).Accordingly, the occurrence of glaucony in the lower Doushantuo Formation of study section implies suboxic pore water environments.The results of energy dispersive spectroscopy(EDS)show that the glaucony is characterized by high K₂O and Al₂O₃ but low Fe₂O₃ contents which are typical of Precambrian glaucony.The dissolution of detrital minerals along with ion exchange between the seawater and pore-water supplied the elements required for glaucony formation.Compared with contemporaneous glaucony-bearing strata in Weng'an area, Guizhou, the limited vertical distribution of glaucony in the study area indicates that the marine redox condition and paleogeography may have played a joint role in regulating the formation and spread of glaucony.