PRELIMINARY STUDY OF A MIDDLE MISSISSIPPIAN (LATE OSAGEAN, EARLY VISÉAN) FORT PAYNE CARBONATE MUD-MOUND COMPLEX, CUMBERLAND COUNTY, SOUTH-CENTRAL KENTUCKY

Formation of the Lower/Middle Mississippian carbonate buildups found worldwide in siliciclastic basins are poorly understood. To better understand mound development, we examined in detail a 710-m-long exposure of a Fort Payne mud-mound complex exposed 8.5 mi northwest of Burkesville on Kentucky Route 61 in Cumberland County, south-central Kentucky. This mound complex consists of two superimposed mounds separated by a bed of glauconitic green shale. The complex sits with angular unconformity on apparently titled green shales, and is conformably overlain by more glauconitic green shales. This mound complex apparently developed on a sediment-starved structural high during a sea-level lowstand and was eventually enveloped by glauconitic muds; it is part of a unique chronostratigraphic glauconitic marker horizon called the Floyds Knob interval.

To provide a more contextual understanding of mound genesis, we mapped the distribution of facies in two dimensions, analyzed petrographic samples along a vertical section, and did stable-isotope analyses (δ¹³C_carb and δ¹⁸O_carb) on a series of samples. Facies mapping and petrographic analyses indicate that each of the superimposed mounds began with a bed of rudstone, packstone or grainstone that formed a firm mound substrate. On top of this, smaller, organic, carbonate buildups, indicated by clotted micrite with a laminar aspect, developed. The clotted texture reflects algal-produced carbonate mud interspersed with fragmented megafossils and underlying voids filled by early radiaxial fibrous calcite cement. This facies pattern was repeated in each period of mound growth. The δ¹³C_carb values (ranging from 2.5 to 4.5‰ VPDB) reflect those from Mii et al. (1999) and others for the Illinois Basin and are thought to represent changes in the dissolved inorganic carbon content of the oceans at the time. Two cycles recording concurrent decreases in δ¹³C_carb and δ¹⁸O_carb values correspond with each of the superimposed mounds and suggest possible glacioeustatic control on mound growth. The mounds appear to nucleate on debris-flow deposits at peak glaciation and expand following minor increases in sea level. The analyses conducted herein will aid future studies in understanding the role regional tectonics, paleoclimate and paleoceanography on carbonate mud-mound evolution.