Genesis of fenestral fabrics in tidal flat dolomites of Cambrian Stage 4: Evidence from Western Henan, Southern North China Block

Abundant fenestral fabrics are widely developed in microcrystalline dolomite, algal clump dolomite, and stromatolitic dolomite of Cambrian Stage 4 on southern North China Block. The paper aims to elucidate the genesis of fenestral fabrics and their relevance to palaeoenvironmental and palaeoclimatic conditions. The fenestral fabrics comprise micron- to millimetric (bird's-eye structures) and centimetric voids, predominantly infilled with calcite and gypsum, with minor occurrences of dolomite. Raman spectral data indicate that organic matter is more concentrated at the edges of fenestral fabrics than in the dolomite matrix or the fabric cores, suggesting that these structures may represent potential accumulation zones for gas bubbles, possibly linked to microbial decomposition. The dolomite sediments consolidated under the exposed, arid, and hot climate conditions, preserving these gas bubbles as micropores. Increased evaporation led to the desiccation and shrinkage of the dolomite sediments, which in turn enlarged the original micropores into larger voids, thereby creating the necessary space for the formation of fenestral fabrics. The calcite and gypsum crystals in fenestral fabrics were interpreted as products of significant Ca2+ loss from the dolomite matrix during the transformation of high-magnesium calcite to dolomite, coupled with the Ca2+ precipitation from high-salinity brine into fenestral fabrics. This rarely documented fabric type, defined by a dolomite matrix with calcite infill, contributes novel petrographic and geochemical evidence that refines current models of fenestral fabric formation, and supports new interpretations of palaeoenvironmental and palaeoclimatic conditions in ancient evaporitic systems. • Microbial gas escape drove fenestral fabric genesis in Cambrian dolomites. • Arid climate and high-salinity brines shaped unique fenestral fabric features. • Organic decomposition and desiccation led to pore evolution in tidal dolomites.