Paleo-sedimentary environment in relation to enrichment of organic matter of Early Cambrian black rocks of Niutitang Formation from Xiangxi area China

Early Cambrian organic-rich black rocks are extensively distributed in the southern China. In this study, we have discussed the relation of paleo-sedimentary environments with enrichment of organic matter. Based on TOC data, we divided the Niutitang Formation into three parts (Upper, Middle, and Lower). The amount of total organic carbon in the Lower (1.55 wt %) and the Upper part (1.35 wt %) of Niutitang Formation is significantly smaller than the Middle part (8.0 wt %) which suggest initial transgression, and regression stages of the sea level respectively. The Middle part from both sections (Longbizui and Sancha) contains higher TOC content, which is a due of intense transgression stage of sea level with high bio-productivity. Kerogen type index (KTI) using different macerals specify the organic matter in these black rock series is mainly type-I with a minor amount of type-II1. Moreover, these black rocks were deposited in weak to moderately restricted hydrographic conditions. The heterogenic mineral composition was also observed in these black rocks, including quartz, clay, gypsum, pyrite, barite, dolomite, and plagioclase. Quartz content in the Middle part is relatively higher than the Upper, and the Lower part displays a strong positive relation with TOC which suggests the main source of quartz during deposition of the Middle part was biogenic quartz. During Early Cambrian period due to extension between the Yangtze and the Cathaysian plate, the hydrothermal fluids rich in uranium, vanadium, molybdenum, zinc, barium, etc. from the deeper part of the earth crust and entered the ocean basin through extensional remnant fissures and cracks, and due to upwelling phenomena entered to the shelf area. These nutrient-rich deep crustal fluids at the surface of the sea enhanced the evolution and breeding of planktons and other marine life; meanwhile, it upsurges the bio-productivity. Additionally, it creates bottom water hypoxia, which is favourable for the preservation of organic matter in the sedimentary rocks. The main factors responsible for the enrichment of organic matter in these Early Cambrian black rock series of Niutitang Formation were a hydrographic restriction, hydrothermal activities, and paleo-redox conditions, respectively.