The formation of early Eocene organic-rich mudstone in the western Pearl River Mouth Basin, South China: Insight from paleoclimate and hydrothermal activity

Abstract

Following the collapse of the Yanshanian orogenic belt, mantle magma uplift, crustal stretching and thinning, and multiperiod strong magmatic hydrothermal activities occurred during the Eocene in South China. As a complete early Eocene sedimentary record, the lacustrine organic-rich mudstone of the Wenchang Formation in the western Pearl River Mouth Basin (PRMB) provides an excellent opportunity to understand the early Eocene paleoclimate, paleoenvironment, and hydrothermal activities in southern China. By analyzing the organic petrology and geochemistry, the effects of paleoclimate and hydrothermal activity on organic matter (OM) accumulation in organic-rich mudstone in the early Eocene Wenchang Formation were revealed. The bulk geochemical analyses showed that these mudstones are characterized by high OM abundances (1.73–3.20 wt%), dominated by kerogen type II and in the early oil maturity window. Additionally, maceral analyses indicated that the OM sources were mainly lacustrine algae and terrestrial higher plants. The element ratios ((Fe + Mn)/Ti), rare earth element anomalies (δEu* and δCe*) and hydrothermal indicator discrimination diagrams (Zn-Ni-Co and (Cu + Ni + Co) × 10-Fe-Mn triangular plot, etc.) jointly revealed three episodes of hydrothermal activity during early Eocene Wenchang Formation deposition. Intense hydrothermal activity not only brought abundant nutrients and improved the primary productivity of the lake but also released reducing gas via hydrothermal eruption, which further aggravated the hypoxic bottom water conditions, thus promoting the accumulation and preservation of organic matter. In addition, the elemental ratios (Sr/Cu, Rb/Sr, and SiO2/Al2O3) indicated a warm paleoclimate in the early Eocene Wenchang Formation. In the period without hydrothermal activity, the humid paleoclimate promoted the prosperity of lake algae and peripheral terrestrial plants, and the high lake level provided excellent organic matter preservation conditions. Therefore, the warm and humid paleoclimate and intermittent hydrothermal activity jointly controlled the formation of lacustrine organic-rich mudstone in the early Eocene Wenchang Formation.