THE LOWER CAMBRIAN NIUTITANG FORMATION AT YANGTIAO (GUIZHOU, SW CHINA): ORGANIC MATTER ENRICHMENT, SOURCE ROCK POTENTIAL, AND HYDROTHERMAL INFLUENCES

Abstract

Organic‐rich marine black shales and cherts of latest Proterozoic to earliest Cambrian age are widespread in South China. In this study, 70 samples of the Early Cambrian basal Niutitang Formation from Yangtiao (Guizhou Province, SW China) were analysed using organic and inorganic elemental and isotopic methods in order to characterize the depositional environment and source‐rock potential of these lithologies. The sediments were deposited in a deep‐shelf environment subject to recurring anoxia, and contain organic material dominated by sapropelic kerogen derived from acanthomorphic acritarchs. The organic richness of the shales (average TOC = 6.7%) is due to a combination of high primary productivity (as shown by biogenic Ba, Cu, Ni and Zn) and enhanced organic preservation (as shown by correlation of TOC with more intensely reducing conditions). The chert intervals are interpreted to have been deposited during phases of relatively intense hydrothermal activity, when silica‐rich hydrothermal fluids were released into the marine water column. Shale intervals were deposited during phases of weak hydrothermal activity and therefore contain little silica of hydrothermal origin. Evidence of hydrothermal activity during chert formation may reflect watermass circulation from deep rift zones. Analysis of the organic‐rich facies of the basal Niutitang Formation demonstrates its considerable hydrocarbon‐generating potential as a function of its organic richness and source‐rock quality parameters (e.g. S1+S2). Intense tectonic deformation of the South China Craton during the Mesozoic has potentially caused leakage of early formed hydrocarbon accumulations, however, resulting in risks for hydrocarbon exploration in the study area.