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 For-mation 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.
NSTL
Elsevier