Paleovolcanic and Hydrothermal Activity in Source Rock Formation: Insights From the Triassic Chang 7 Member, Ordos Basin, NW China

ABSTRACT

Volcanic ash input and lake‐bottom hydrothermal activity are commonly invoked to explain organic‐rich source‐rock formation, yet their respective geochemical signals and effects on organic matter enrichment remain difficult to distinguish in lacustrine basins. This study investigates the Triassic Yanchang Formation (Chang 7 Member) in the Ordos Basin using petrographic observations, scanning electron microscopy, organic geochemistry, and major‐ and trace‐element data from source rocks and tuffs. The results show that tuff occurrence and enrichments of Th, Pb, and Zr record the relative influence of volcanic input, whereas Mo and Cu enrichment, together with siliceous rocks and pyrite‐bearing hydrothermal‐associated textures, indicate lake‐bottom hydrothermal influence. The main innovation of this study is to separate the volcanic and hydrothermal signals and to evaluate their different roles in source‐rock development. Volcanic input appears to promote organic matter enrichment mainly in samples with total organic carbon (TOC) below 10%, probably through nutrient supply, but its effect weakens or becomes negative in high‐TOC black shales. In contrast, hydrothermal influence is more closely associated with high‐TOC source rocks, enhanced paleoproductivity, and reducing bottom‐water conditions. These findings provide refined criteria for distinguishing volcanic and hydrothermal effects in the Chang 7 Member and help explain spatial differences in organic‐rich source‐rock development in the Ordos Basin.