Depositional and Diagenetic Controls on Eyelid–Eyeball Limestones in the Middle Permian Maokou Formation, Sichuan Basin

The Mao-1 Member of the Middle Permian Maokou Formation in the Yongchuan area contains distinctive eyelid–eyeball limestones developed in an outer-ramp to intraplatform-basin transitional setting. The two components alternate as mud-rich laminae and carbonate-rich nodules or layers. Eyelid limestone contains 10–45% micritic carbonate mud and a mean total-grain content of 69.11%, whereas eyeball limestone contains <10% carbonate mud and 30.89% total grains. In the exploratory geochemical subset (five samples per facies), eyelid limestone has higher mean V, Mn, Fe, and Ba contents, but only Ba differs significantly between facies (Welch p = 0.036; exact Mann–Whitney p = 0.032). Bulk-rock rare-earth-element data are PAAS-normalized and used descriptively because concentrations are low and neither weak-acid leaching nor detrital correction was performed. The observations support a preferred interpretation involving primary depositional differentiation followed by localized early diagenesis, differential compaction, and pressure solution. Relative sea-level change may have modulated the alternation, but the available data do not resolve a unique cyclic driver. Core-scale pore data demonstrate facies-dependent heterogeneity rather than field-scale deliverability.