Burial‐diagenetic reticulate structures in carbonates: Bed‐confined tensile fracture networks from the Lower Triassic Feixianguan Formation, South China

ABSTRACT

Reticulate structures on carbonate bedding surfaces are commonly interpreted as depositional or early‐diagenetic shrinkage features, yet similar plan‐view geometries may also record burial fracturing and vein cementation. We integrate field and petrographic observations with mineralogical, cathodoluminescence, fluid‐inclusion, stable‐isotope and fracture‐mechanical analyses to reassess reticulate structures in thin wackestone–packstone interbeds of Member 2 of the Lower Triassic Feixianguan Formation, South China. They form polygonal networks at Dagouli (DGL) and two intersecting, near‐orthogonal fracture sets at Tuanshanbao (TSB). In cross‐section, both are steep to subvertical, bed‐confined calcite‐filled fractures that cut the matrix, bioclasts and mud clasts without obvious shear displacement or downward tapering, indicating fracture opening after compaction and early lithification rather than syn‐sedimentary shrinkage. Mechanically, fracture confinement to individual beds is consistent with mechanical‐stratigraphic control, whereas the contrasting geometries are compatible with near‐isotropic and anisotropic horizontal effective‐stress end members, respectively; both require elevated pore‐fluid pressure for tensile fracture opening. The vein calcite is blocky and sparry, dull to non‐luminescent and locally overprinted by recrystallisation and dolomitisation. Sparse aqueous fluid inclusions yield homogenisation temperatures of 136.5°C to 190.2°C. Together with the markedly lower δ ¹³ C and generally more negative δ ¹⁸ O values of the veins relative to the matrix, these data indicate calcite precipitation from moderate‐ to relatively high‐temperature burial‐diagenetic fluids carrying isotopically light carbon. Although the vein data do not directly link the vein‐forming fluid system to fracture opening, the required pressure may have built up through fluid generation by oil cracking and possible thermochemical sulphate reduction where enclosing mudstones restricted fluid escape. The structures are therefore interpreted as burial‐diagenetic, bed‐confined tensile fracture networks recording a post‐compaction fracture–fluid–cementation sequence. The Feixianguan case shows that bedding‐surface morphology alone is insufficient for genetic or palaeoenvironmental interpretation and provides a transferable framework for assessing morphologically similar structures in other carbonate successions.