Pyrite Formation and Its Controls in Black Shales of the Buxin Formation (Lower Eocene) from the Sanshui Basin, Guangdong

Key words: 
• pyrite formation /
• controls /
• black shale /
• Buxin Formation /
• Lower Eocene /
• Sanshui Basin

Abstract: Analyses of TOC/S, TOC/DOP, S/Fe relationships and STOCFe multiple linear regression were carried out to determine limiting factors for pyrite formation in black shales of the Honggang Member (Lower Eocene Buxin Formation) in the Sanshui Basin. Submember A, showing lower organic carbon and higher iron contents than the other submembers, deposited in oxygenated bottom waters. Bacterial sulfate reduction took place below the sedimentwater interface, and pyrite was formed diagenetically and organic carbon was limited. Submember B and C are characterized by large variations in organic carbon content. Organicrich (TOC>4%) rocks deposited under anoxic environments, in which H2S presented above the sediment water interface as well as within the sediments. Pyrite was formed syngenetically and confined by the duration during which iron detritus reacted with H2S in the water column and by the availability of dissolved sulfate that decreased due to an increasing input of fresh waters. For organicpoor (TOC<4%) sediments, pyrite was of syngenetic and diagenetic. Organic carbonlimited diagenetic pyrite added to the earlier formed syngenetic pyrite, which was controlled by the time that iron minerals remained in contact with H2S in anoxic waters.