Diverse origins and depositional patterns of phytoclast tufas in the Lower Cretaceous Jinju Formation, southern Korea

Abstract

Phytoclast tufas, allochthonous plant fragments encrusted by freshwater calcium carbonates, hold valuable clues to past environments and ecosystems. While these structures have been documented in various settings throughout geological time, detailed studies on the formative processes and paleoenvironmental implications of fossil phytoclast tufas, particularly in the Mesozoic, are scarce. This study investigates phytoclast tufas from the Lower Cretaceous Jinju Formation in the Gyeongsang Basin, southern Korea, to characterize their depositional patterns and paleoenvironmental context. Through the integration of macroscopic observations and detailed microfacies analysis, complemented by elemental composition measurements, we reveal a complex interplay of biological and abiotic factors in the formation of phytoclast tufas. Our findings highlight distinct differences in microfabrics and depositional patterns among phytoclast tufas from three localities (Geobuk-gil, Yonggung-ro, and Seonjeon-ri), despite their similar macrostructures. In the Geobuk-gil section, phytoclast tufas likely formed through microbial influence in paludal (marsh-like) setting and were deposited in the lake center. The Yonggung-ro phytoclast tufas presumably developed in a barrage system with both microbial and abiotic precipitation inducing tufa formation, and were transported to the lake margin and deposited by waves, resulting in fragmentation. In the Seonjeon-ri section, the phytoclast tufas may have formed in an abiotic precipitation-dominant lake margin environment and were subsequently transported by debris flows and deposited as a fan. These discrete phytoclast tufa horizons may represent episodic climatic events within the predominantly clastic succession. By demonstrating the diverse origins and depositional environments of phytoclast tufas within the same stratigraphic unit, our study contributes to refining models of tufa formation and underscores the importance of detailed microfacies analysis in paleoenvironmental reconstructions.