Paleoweathering, hydrothermal activity and organic matter enrichment during the formation of earliest Cambrian black strata in the northwest Tarim Basin, China

Abstract The earliest Cambrian black strata are widely deposited in many continents, and attracted increasing attention from scholars worldwide. Based on detailed investigations of strata from the Shiairike section in the Aksu area, northwest Tarim Basin, China, the effects of paleoweathering and hydrothermal activity on the organic matter enrichment of this black series was revealed. Combined organic and inorganic geochemistry evidence indicated that relatively strong weathering carried abundant nutrient elements and terrigenous detritus into the sea, which promoted a period of paleoproductivity and enhanced the sinking of dissolved organic matter. Moreover, the hydrothermal activity triggered the enrichment of abundant reducing gases (e.g. H2S) and minerals, whereby gases created euxinic benthic conditions that were favorable for the preservation of sedimentary organic matter, while minerals simultaneously diluted primary productivity. As a result, the low section of the black strata was characterized by dominant siliceous rocks with enrichments of P, Ba, Zn and Cu; whereas the upper section contained abundant organic matters that had the potential to generate and expel a remarkable volume of hydrocarbons. These results provide a theoretical basis for the international exploration of paleo-hydrothermal minerals within the early Cambrian black strata as well as that of the deep Cambrian oil and gas, particularly in the Tarim Basin, northwest China.