Organic matter accumulation of the early Cambrian black shales on the flank of Micangshan-Hannan Uplift, northern upper Yangtze Block, South China

Abstract

The Micangshan-Hannan Uplift area is proved a new shale gas exploration area with massive potential of Niutitang Formation in the past three years. Nevertheless, the organic matter enrichment mechanisms of this set of organic-rich shales lack discussion. Here, eighty-two samples of a drilling well from the Micangshan-Hannan Uplift, Upper Yangtze Block, South China were investigated for the organic matter (OM) accumulation mechanisms by geochemical analysis. Specifically, paleoredox conditions are classified by V/Sc, U/Th, and Uau; primary productivity is evaluated by TOC, Cu/Al, Ni/Al and, P/Al; sediment flux is indicated by Ti/Al; and paleoclimate is inferred by CIA and Ga/Rb ratios. Vertically, the Kuanchuanpu dolomite, underlying the Niutitang black shales, with TOC contents from 0.34% to 1.18% (avg. 0.71%), was deposited in oxic and low productivity conditions under lower sea level and more humid climate. Upwards, the silica-dominated and clay-rich siliceous mudstone (Assemblage 1) of Niutitang Formation, characterized by high TOC contents from 1.31% to 4.66% (avg. 2.78%) was classified into three units. The OM enrichment of Unit 1 was controlled by both anoxic condition and high primary productivity resulting from rapid sea level rise and increasing aeolian flux under cold and arid climate. The OM enrichment of Unit 2 was manipulated by the dysoxic bottom water condition, although the primary productivity remained relatively high. Then, the OM enrichment of Unit 3 was dominated by increasing primary productivity of surface water due to higher aeolian input and the dysoxic with intermittent anoxic condition advantageous for OM preservation. At last, the mixed siliceous mudstone (Assemblage 2) of Niutitang Formation displayed much lower TOC contents (avg. 1.34%). Both of oxic condition as well as lower primary productivity due to the declining aeolian flux led to the less efficient OM accumulation. In sum, the favorable organic matter accumulation in Niutitang Formation was controlled by anoxic bottom water condition and high primary productivity, which was related to the Early Cambrian global transgression and increasing aeolian flux under cold and arid climate.