Framboidal pyrites in cherts of the Laobao Formation, South China: Evidence for anoxic deep ocean in the terminal Ediacaran.

Abstract

A detailed study of the size distribution of framboidal pyrites in cherts from the Laobao Formation (about 550～540Ma) at Silikou section in South China was conducted to evaluate the redox state of the terminal Ediacaran deep ocean. Framboidal pyrites are common in the cherts, but infdled framboids and euhedral grains are sparse. Framboids are dispersed in the cherts and show a narrow size range with maximum diameters range from 7.7 to 18μm. The mean values of framboidal pyrites diameters in Silikou cherts are less than 5μm for the lower seven samples and around 6.5μm for the upper three samples. In these chert samples, more than 85% of framboids of the lower seven samples are less than 7μm, and around 65% of framboidal pyrites are less than 7μm for the upper three samples. In addition, few framboids (fewer than 4% ) in most of the chert samples are 10μm. The small diameters and narrow size range of framboidal pyrites in Silikou cherts are consistent with that formed in euxinic sediments and imply that the terminal Ediacaran deep ocean was mainly anoxic. It is clear that mean values, median values and maximum diameter of framboidal pyrites in Silikou cherts are gradually increased from the lower to the upper section, which may indicate that the terminal Ediacaran deep ocean was gradually oxidized.