Extraordinarily high organic matter enrichment during the late Guadalupian in northwestern Sichuan basin, China

Abstract

Shale with extraordinarily high organic matter content (EHOMC) formed in the margin of the Sichuan carbonate platform during the late Guadalupian in the eastern Paleo-Tethys. So far, not much consideration has been given to the enrichment of EHOMC. Here, integrated lithological and geochemical analyses of the top Maokou Formation black shales in the northwest Sichuan basin were undertaken to investigate the EHOMC. The black shales were divisible into three units (i.e., Units 1 to 3 in ascending order); EHOMC occurred in Unit 2. Unit 1 was deposited during the early stage of the transgression under the small-scale upwelling in an anoxic environment with lower paleo-productivity than Unit 2, considering the moderate Cd/Mo ratios and the concentration of Mo, U, V, P, Cu, and Zn. Unit 2 exhibited high Cd/Mo ratios and high concentrations of elements (Mo, U, V, P, Cu, and Zn), suggesting that the shales with EHOMC were deposited through an intense upwelling in an euxinic-sulfidic environment with high paleo-productivity. Unit 3 demonstrated moderate paleo-productivity and anoxic-suboxic conditions, while the nutrient elements were derived through increased terrestrial detrital input. Unit 2 exhibited the highest total organic matter content (TOC) (17.72–32.58 wt%); it is inferred that the enrichment of EHOMC was mainly caused by a euxinic-sulfidic environment, while high paleo-productivity was another key factor. Besides, the euxinic-sulfidic environment and high paleo-productivity in Unit 2 occurred approximately during the intense upwelling, the occurrence of the high sea-level, and the biological extinction during the end of Guadeloupe. These factors together promoted the enrichment of EHOMC in Unit 2.