The influence of syngenetic hydrothermal silica fluid on organic matter preservation in lower Cambrian Niutitang Formation, South China

Abstract

Early Cambrian shale-chert couplets are widely distributed in South China, within which abundant organisms have been detected. Cherts in early Cambrian are known to be influenced by hydrothermal activity. However, the influence of syngenetic hydrothermal activity on organism preservation has not yet been well evaluated. Litho-, chemo- and bio-stratigraphic data have documented that the syngenetic hydrothermal silica fluid activity from the Majiang-Yangtiao section of the early Cambrian Niutitang Formation in Guizhou province, SW China.Geochemical compositions [Fe/Ti, Al/(Al + Fe), Al–Fe–Mn ternary diagram and Fe/Ti vs. Al/(Al + Fe + Mn) diagram] indicated that the chert, siliceous shales and mudstones were strongly, moderately, and weakly influenced by hydrothermal silica fluids, respectively. Mudstones contained much more terrigenous elements (e.g. Al and Zr). The total organic carbon (TOC) content and concentrations of nutrient elements (Cu, Zn, P, Ba) were highest in siliceous shales, followed by mudstone and lowest in chert. This could be due to a high silica dilution from hydrothermal fluid in chert. Compared with the contiguous siliceous shales, chert was characterized by a high content of aerobic zooplankton (e.g. sponges and pogonophoras) with low V/Cr ratios. This represented a sedimentary facies altered by hydrothermal fluid input in the marine environment. Organisms were well preserved in chert with delicate ornamentation on the surfaces of spherical acritarch cells, and to a lesser extent in silica-rich siliceous shales, but less in mudstones. This suggested that hydrothermal silica fluid with rapid sedimentation provided a good environment for organism preservation. These well preserved organisms could provide organic carbon content with little or no hydrocarbon generation in the rocks, because the morphology were not changed and no solid bitumen were found surrounding these organisms. The study provided a clue for careful consideration in the context of hydrocarbon generation evaluation in early Cambrian source rock.