Paleoenvironment Interpretation of Early Cambrian Yurtus Formation, Tarim Basin, and Its Mechanism for Organic Carbon Accumulation

WANG Zhihong1,2, DING Weiming3,†, LI Jian1,2, HAO Cuiguo1,2, LIU Hui3, LI Tong3, DONG Lin3,†

1. Petrochina Research Institute of Petroleum Exploration & Development, Beijing 100083; 2. Key Laboratory of Gas Reservoir Formation and Development, CNPC, Langfang 065007; 3. School of Earth and Space Sciences, Peking University, Beijing 100871; † Corresponding authors, E-mail: wmding@pku.edu.cn (DING Weiming), lin.dong@pku.edu.cn (DONG Lin)

Abstract This study scrutinizes the petrological and geochemical signatures by systematically sampling three sections of Yurtus Formation, which deposited during the early Cambrian in the western margin of Tarim Basin. The redox conditions together with the intensity of primary productivity during Yurtus Formation deposition are reconstructed, which provide theoretical basis for the distribution of source rocks and evaluation of reservoir potentials. The extremely high Ba content (>1000 μg/g) and appearance of barite in the lower part of Yurtus Formation implies high primary productivity, which explains the substantive organic matter in the surface ocean and is also consistent with the abundant phytoplankton fossil record and high TOC content. The large positive excursion of δ15n (>8‰) in the lower parts of Yurtus Formation probably is resulted from denitrification or annamox, both of which occur only when O2 is depleted. It may indicate that organic matter transporting from the surface ocean to deeper ocean could consume dissolved oxygen by the process of degradation, which would drive the condition of deeper ocean more reduced. Meanwhile, low Ge/si values of cherts in the lower part of Yurtus Formation indicate normal seawater origin of Si input. Oversaturation state of silica promotes the organic matter preservation due to relatively fast cementation. Key words Yurtus Formation; Ge/si; Ba content; nitrogen isotope; primary productivity