Sharp transition from carbonates to cherts across the platform margin in western Hunan, south China during precambrian-Cambrian transition: Stratal-depositional patterns, mechanisms and implications
Wang Jianguo1 Chen Daizhao1 Yan Detian2 Wei Hengye1,3 Yu Hao1,3
1.Key Laboratory of Petroleum Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Neijing 100029;2.Key Laboratory of Tectonics and Petroleum Resources of Ministry of Education, University of Geosciences, Wuhan 430074;3.Graduate University of Chinese Academy of Science, Beijing 100049
The PrecambrianCambrian transition is a key period for the understanding of one of the most important intervals in Earths history，as it records the advent，extinction and accelerated diversification of metazoans coeval with sharp shifts in ocean geochemistry，a prolonged global oceanic anoxia.In order to investigate the trigging mechanism for these cooccurring processes，detailed crossingplatformstrike facies tracing from the transitional zone of platformbasin in western Hunan，were carried out to unravel different sedimentary facies and their spatial variation across this critical interval.A sharp transition from carbonates to cherts across the platform margin in western Hunan was identified in the earliest Cambrian，indicating syndepositional extensional faulting could have occurred along the transitional zone of platformbasin.Mounded chert，funnelized brecciated chert and vein chert which characterized by unique mineral compositions and internal structures in this transition zone，were formed by submarine hydrothermal venting on the faultcontrolled terraces along the margintoslope of Yangtze carbonate platform where syndepositional fault/fracture zone could have acted as conduits channeling the hydrothermal fluids heated up by the deepseated thermal source updip to the seafloor.The bedded chert，mainly distributed in the localities farther basinwards，implies enhanced influences of seawater away the vent fields during deposition.Considering widespread chert along the transitional zone of platformbasin in Yangtze area，intense hydrothermal venting might have occurred along the platform margin such that vast amounts of greenhouse gases，anoxic fluids rich in metal and nonmetal elements would have been released to the ocean/atmosphere，leading to climate warming，oceanic anoxia and oceanic eutrophication.As the hydrothermal activity waned，sea level would rise due to basin subsidence induced by crust cooling，abundant nutrient materials in the deep ocean were then brought to the ocean surface by upwelling currents，which could have increased primary productivity，and facilitated organic accumulation and preservation in the anoxic waters already during the Niutitang interval.
PrecambrianCambrian transition，Carbonate rock，Chert，Syndepositional fault，Hydrothermal activity
. Sharp transition from carbonates to cherts across the platform margin in western Hunan, south China during precambrian-Cambrian transition: Stratal-depositional patterns, mechanisms and implications[J]. Chinese Journal of Geology, 2011, 46(01): 27-41.