RE-EXAMINATION OF A MIDDLE MISSISSIPPIAN CARBONATE MUD-MOUND COMPLEX IN SOUTH-CENTRAL KENTUCKY

Lower–Middle Mississippian carbonate mud mounds are common occurrences in clastic-rich basins nearly worldwide. Despite their well-known occurrences, mound origin is still controversial. The mounds first appear after a major Late Devonian extinction wiped out most reefal, framework-building organisms. They were replaced by organisms like upright bryozoans, sponges, and stemmed echinoderms that had the ability to baffle and trap carbonate sediment, which contributed to the formation and composition of the mud mounds. Using microfacies and macrofacies analysis, as well as LiDAR and aerial surveys, we reexamined a 213-m-long exposure of a late Osagean (early Viséan) Fort Payne carbonate mud-mound complex, exposed near Burkesville on Kentucky Route 61 in Cumberland County, south-central Kentucky. The mound is at least 15-m high, sits with angular unconformity atop a rudstone, packstone or grainstone substrate on tilted, green, glauconitic shales, and is conformably overlain by glauconitic green shales. Facies mapping shows that the “mound” is really two mound complexes separated by a green-shale interval. Moreover, each mound complex is constructed of many smaller complexes of clotted micrite bound by thin shale partings that become erosional diastems. Many of these partings and seams are marked by layers of breccia.

The mound complex is wholly enveloped in upper Osagean glauconitic green shales of the Floyds Knob Bed, reflecting deposition during a period of sediment starvation at a time of sea-level lowstand. During this period of reduced sedimentation, mound complexes of clotted micrite, representing microbially produced carbonate, calcareous green algae, and carbonate muds accreted by baffling organisms, developed on firm substrates of reworked fossil debris. Fragmented megafossils are interspersed within the clotted micrites and generate underlying voids filled by early radiaxial fibrous calcite cement. Substrates of reworked fossil debris, fragmented megafossils, layers of breccia, and small mound complexes bound by erosional diastems indicate the influence of storms in mound development. Though typically ascribed to deep-water formation, mud-mound complexes like this probably represent more shallow-water development well within the photic zone and above storm wave base.