Carbonate Mud Bodies in Middle Mississippian Strata of Southern Indiana and Northern Kentucky: End Members of a Middle Mississippian Mud Mound Spectrum?

Abstract

Relatively small, lens-shaped carbonate mud bodies are common features in the Ramp Creek Formation and Harrodsburg Limestone (Mississippian) of southern Indiana and northern Kentucky. The outcrop dimensions of the lenses range from approximately 10 cm thick and 3 m wide to as much as 2 m thick and in excess of 100 m wide; their three-dimensional geometry is unknown. The lens cores consist of dolomitic mudstone that grades laterally and vertically into increasingly more fossiliferous wackestone to grainstone with fenestrate bryozoans and echinoderms being the dominant fossils. The great abundance of fenestrate bryozoan fragments surrounding the lenses suggests that lens evolution was controlled by the trapping of carbonate mud by the baffling action of bryozoans. Wisps of organic material preserved in the lens cores may be remnants of some form of non-calcareous algae that also baffled and trapped carbonate mud. These mud lenses are end members of a spectrum of Mississippian carbonate mud bodies ranging in size from these small lenses to the classical Waulsortian mounds that may be hundreds of meters thick and a kilometer or more broad. All of these carbonate mud bodies may have in part formed by baffling and localizing of carbonate mud by organisms andmore » in part by local production of carbonate mud. The major difference between large and small bodies is the water depth in which each formed. The Ramp Creek-Harrodsburg mud lenses may be miniature Waulsortian mounds that developed at or above fair-weather wave base on a relatively shallow carbonate platform rather than on shelf-to-basin slopes as proposed for the classical Waulsortian mounds.« less