MICROBORING ORGANISMS ASSOCIATED WITH WAULSORTIAN CARBONATE BUILDUPS FROM THE LOWER MISSISSIPPIAN FT. PAYNE FORMATION OF SOUTH-CENTRAL KENTUCKY AND TENNESSEE

The Lower Mississippian Fort Payne Formation in south-central Kentucky contains enigmatic carbonate mud-mounds that are similar to so-called “Waulsortian” mounds known from other localities of the same age world-wide. Mode of deposition and paleoenvironment of the carbonate mounds present persistent problems for interpretation. Localities in south-central Kentucky near Lake Cumberland are more proximal to the shallow Borden Delta platform while more distal, deeper settings are found along DaleHollow Lake in Tennessee. This study reports discovery of the first known microendolithic borings from green shales associated with the mounds that provide new evidence for paleobathymetry. Calcareous fossil materials were embedded in epoxy and etched with dilute HCl. Microendoliths were revealed as resin casts in platyceratid gastropods, articulate brachiopods, bryozoan zoaria, and coral skeletons, but are notably absent from a variety of abundant echinoderm clasts. The Dale Hollow locality provided exquisitely preserved casts in natural molds of fenestrate and cryptostome bryozoans, and brachiopods. Preliminary analysis using SEM has identified the autotrophic cyanobacterial boring Fascichnus frutex and an abundant unclassified endolith with affinity to the cyanobacterial ichnotaxon Eurygonum sp. Macroboring heterotrophs include possible phoronid borings in platyceratid gastropods. Bathymetric distribution of microendoliths in Recent marine environments provides a guide to paleoenvironmental interpretation. The relative abundance of autotrophic forms in the Fort Payne facies suggests photic depths for sites more proximal to the Borden Front as well as more distal sites in Tennessee that lack well-developed mud-mounds. This is consistent with previous documentation of dasycladacean algae in the more proximal, upslope facies. A photic depth for the more distal settings in Tennessee may be indicative of lower sea level above a flooding surface during initial phases of transgression. This is the first recorded occurrence of micro- and phoronid borings from the Lower Mississippian, adding a chapter to the geologic history of microscopic euendoliths.