6 / 2021-04-01 10:59:34

Mechanism for hyper organic matter enrichment of the Lower Guadalupian in northwestern Sichuan basin, South China

Hyper organic matter accumulation, High productivity, Euxinic, Upwelling, Sichuan.

Shale with hyper organic matter is deposited during the late Guadalupian in the eastern Paleo-Tethys Margin. As so far, few attention has been paid to the hyper organic matter enrichment mechanism. In this study, integrated study of lithology and geochemistry on the black shales from the top Maokou Formation in the Xibeixiang section in northwest Sichuan basin are carried out to investigate the hyper organic matter accumulation. The total organic content (TOC) data suggested that the black shales of the upper part of Maokou Formation in the Xibeixiang section can be subdivided into three Units (i.e., Unit 1 to 3 in ascender order) with hyper organic carbon contents ranging from 17.72 to 32.58% in Unit 2. The organic matters of the black shales are orange amorphous organic matter (AOM) and grey AOM. The high concentration of elements (Mo, U, V) suggested that the shales bearing hyper organic matter were deposited in an euxinic-sulfidic environment. High paleo-productivity in response to upwelling was postulated by the high concentration of elements (P: 360 to 1680 ppm; Zn: 234 to 1310 ppm). Sequence stratigraphy study shows that shales with hyper organic matter (Unit 2) were close to the maximum flooding surface. Detailed study demonstrated that paleo-productivity of Unit 3 was slightly higher than Unit 1. However, due to the redox differences between Unit 1 (anoxic) and Unit 3 (anoxic-suboxic), the TOC content in Unit 1 is about three times higher than that in Unit 3. Finally, we believe that the euxinic-sulfidic environment is the basic condition for the formation of hyper organic matter enrichment, and high paleo-productivity is the key factor in Unit 2. Besides, the anoxic-sulfidic environment and the occurrence time of high paleo-productivity in Unit 2 are approximately equivalent to the biological extinction period in the end of Guadeloupe, which has certain enlightenment for the study of biological extinction events in the end of Guadalupian.