Marine red staining of a Pennsylvanian carbonate slope: environmental and oceanographic significance

Red-stained platform facies are a common feature of many carbonate settings throughout the geological record. Although the mechanisms involved in red staining of subaerially exposed or argillaceous, peri-platform limestones are reasonably well understood, the environmental and oceanographic significance of red carbonates often remains uncertain. Here, sedimentological, sequence stratigraphic, geochemical, paleontological, and quantitative bathymetric data from Pennsylvanian red intervals across a well exposed carbonate platform top and slope are documented and interpreted in a process-oriented context. On the upper slope (80–350 meters below the shelf break), red intervals alternate with gray, mainly microbial algal boundstones. On the lower slope (350–600 meters below the shelf break), redeposited red-stained mud builds matrix-supported breccia tongues interbedded with predominantly redeposited, clast-supported carbonate debris. The presence of large volumes of fibrous calcite biocementstones as well as firmgrounds point to low sedimentation rates or omission. In terms of sequence stratigraphy, red intervals occur within maximum flooding intervals and reflect near platform drowning. Elevated δ¹⁸O values (2–3‰) and an essentially cool-water, heterotrophic biotic association in red intervals on platform top and slope suggest deposition during sea-level highs, associated with colder water masses and high nutrient levels. The most likely drivers are an elevated thermocline and upwelling. The red staining is the result of iron oxidation which occurred during early diagenesis, likely by iron bacteria. These red intervals provide an important bathymetric benchmark against which other (mainly Paleozoic) red facies can be tested and calibrated.