Changes in lake sedimentary environment and characteristics of organic matter enrichment under the influence of geological events: The lower Jurassic (Sinemurian-Toarcian) organic-rich shale, Sichuan Basin, Upper Yangtze Plate

Abstract

Two sets of lacustrine organic-rich shale formations (Dongyuemiao and Da'anzhai members) have been developed in the Lower Jurassic (Sinemurian-Toarcian) in the Sichuan Basin of the Upper Yangtze Plate. The two strata have terrestrial sedimentary records of the Sinemurian-Pliensbachian boundary event (SPBE) and the Toarcian Oceanic Anoxic event (T-OAE). Based on the petrological and geochemical data, the paleoenvironment of lakes during the Dongyuemiao and Da'anzhai periods was reconstructed, and the organic matter enrichment characteristics under the influence of two geological events (SPBE and T-OAE) were systematically explored. From our analysis, it was indicated that two volcanic activities (the Central Atlantic Magmatic Province (CAMP) and Karoo Ferrar Large Igneous Province) triggered global geological events (SPBE and T-OAE) and global marine transgression during the Early Jurassic. As a result, the climate in the Sichuan Basin shifted towards dryness in the short term. During the middle of the Sinemurian period (Dong2 period), the Sichuan Basin was a freshwater lake formed under a warm-humid climate. The biological productivity of lakes is high, in terms of terrestrial debris input, and has an oxic bottom water environment. Under the influence of climate, higher biological productivity is the key to controlling the enrichment of organic matter. During the late Sinemurian period (Dong1 period), the greenhouse gas emissions from the Central Atlantic Magmatic Province (CAMP) caused the transformation of the climate in the Sichuan Basin into semi-dry to semi-humid. These changes induced a marine transgression in the eastern part of the basin. However, the impact of marine transgression on the enrichment of organic matter was limited, and the enrichment of organic matter still depends on biological productivity. During the Toarcian period (Da'anzhai period), the continuous eruption of the Karoo Ferrar Large Igneous Province caused the climate of the basin to transform into semi-dry during the maximum flooding period. This effect may have triggered the entire basin's marine transgression. High salinity, deeper bottom water, and dyoxic environments greatly promote the preservation of organic matter.