Paleoenvironmental evolution and organic matter accumulation of the lower Cambrian Maidiping marine black shales in the intracratonic basin, western margin of Sichuan Basin, South China

Abstract

ABSTRACT The Late Ediacaran to Early Cambrian marked a pivotal phase in paleoenvironmental and tectonic shifts. The Sichuan Basin, on the Yangtze platform’s western edge, developed the Mianyang-Changning intracratonic sag due to Xingkai taphrogenesis. The Maidiping Formation’s black shales, the initial fine-grained clastic deposits from the Early Cambrian, span the Sichuan Basin’s western margin. The influence of the intracratonic sag on organic matter accumulation and paleo-marine environment evolution remains unclear. Comprehensive petrological and geochemical analyses on the Qingping section revealed three distinct members within the Maidiping Formation: black shales and silicified dolomite with shales in a restricted basin (lower member), deep-water black shales (middle member), and phosphoritic turbidite sandstone with siliceous phosphorites (upper member). The lower member’s organic accumulation arose from restricted ocean circulation, leading to stratified waters and reduced oxygen at depth. The middle member’s organic build-up was influenced by enhanced intracratonic sag connectivity with the open ocean, sea-level rise, and increased productivity. The submarine hydrothermal activity might have induced anoxic conditions. The upper member saw a decline in organic matter due to phosphoritic debris and oxygenation. The Maidiping Formation augments the Lower Cambrian source rock, vital for oil and gas exploration in the Sichuan Basin’s western margin.