The Fengcheng Formation in the Mahu Sag of the Junggar Basin represents one of the oldest and most significant alkaline lacustrine systems, hosting abundant dolomite that serves as a key unconventional reservoir. However, the formation mechanism of dolomite remains unclear. This study integrates detailed petrography, geochemistry and cyclostratigraphy to elucidate the origin and distribution of dolomite. Petrographic characteristics indicate a penecontemporaneous origin for the dolomite, with no apparent hydrothermal influence. Mineralogical features exhibit a multi-zonation structure of dolomite, aligning with in situ Fe content, jointly indicating that a multi-stage formation process of dolomite from core to rim. Microbial methanogenesis likely played an important role in the dolomite formation. Spatially, dolomite is enriched in the transition zone but scarce in the depocenter zone, where sodium carbonate prevails. This distribution is primarily controlled by pH differentiation between the transition zone and the depocenter zone of the Mahu Sag. In the transition zone, orbitally driven wet–dry cycles regulated the lake-level change, which, in turn, controlled pH fluctuation, as revealed by the silica precipitation during humid phases and dissolution during arid intervals. In the depocenter zone, lake water remained at a high-pH state, which was unfavorable for dolomite formation. These findings highlight that pH dynamics, linked to orbital climate cycles, played a critical role in governing dolomite formation and distribution in this ancient alkaline lake, providing new insights for the formation of dolomite in alkaline lacustrine environments.
Yang et al. (Thu,) studied this question.