A petrographic and geochemical study of carbonate and silica phases from the Ediacaran Doushantuo Formation in the Three Gorges area of South China: Implications for diagenetic conditions

摘要

The carbonate of the Ediacaran Doushantuo Formation contains various silica phases in the Three Gorges area of South China. To better understand the diagenetic conditions under which these silica phases formed, we carried out a petrographic and geochemical study of the silica phases and the coadjacent carbonate phases at the Xiaofenghe section (XFH), in the Three Gorges area of South China. Five types of silica phases can be distinguished based on petrographic characteristics: S1 and S2 are composed of microcrystalline quartz, in chert nodules and disseminated in the host dolostone, respectively. S3 is composed of the megaquartz, while S4 and S5 are micro crystalline quartz and fibrous silica, respectively, both replacing calcite cement. Six distinct types of carbonate can be recognized, including the host matrix carbonate (C1), sparry calcite nodules (C2), calcite around chert nodules (C3), dispersed calcite cement within chert nodules (C4), fine micropeloidal calcite cements (C5) and calcite veins (C6). Sedimentary structures suggest that formation of the 51 through S5 silica phases and C2 through C5 carbonate phases occurred during early burial diagenesis, and therefore their formation recorded early diagenetic conditions.