High-Resolution Petrological Characterization of Laminated Dolostones from the Upper Ordovician Saluda Formation, Cincinnati Arch Region, USA

Laminated dolostones are common in the rock record. However, their vertical petrological variability is largely ignored. The current study addresses this gap by conducting a detailed, multi-proxy investigation of dolostones in the Upper Ordovician Saluda Formation (southeast Indiana, USA). The laminae of the Saluda Formation are characterized as alternating green and tan dolostones containing desiccation cracks, low-angle ripple marks, and rip-up clasts. This study examines the textural, mineralogical, and geochemical variability of these laminated dolostones over 0.7 meters (n = 71), analyzed at 1 cm intervals. Thin section petrography, SEM imaging, and SEM-EDS elemental mapping show that dolomites in the tan laminae are coarser crystalline (50–120 μm), more porous, and have less intercrystalline clay. Green laminae dolomites, in contrast, are finer crystalline (20–50 μm), less porous, and have more clay. XRD analyses show that dolomite stoichiometry and cation ordering are not significantly different between the green and tan laminae. However, δ18O and δ13C values in the green laminae are heavier (-2.94 ± 0.18 VPDB and 1.00 ± 0.08 VPDB, respectively) than those in the tan laminae (-3.27 ± 0.20 VPDB and 0.71 ± 0.24 VPDB, respectively). Cross-cutting relationships show that the dolomites in the laminae formed prior to the clays, suggesting layer-by-layer syndepositional dolomitization. δ18O values for both laminae types are within the range of those expected for Upper Ordovician marine dolomites, suggesting dolomitization occurred in near-normal marine fluids. Slightly higher δ18O values in the green laminae are interpreted to reflect dolomitization in slightly more evaporative fluids relative to the tan laminae.