RECRUITMENT OF A CARBONATE FAUNA TO A DELTAIC FACIES: THE NADA MEMBER CRINOID FAUNA OF THE BORDEN FORMATION, LOWER MISSISSIPPIAN, NORTHEASTERN KENTUCKY

The Nada Member of the Borden Formation in northeastern Kentucky records deposition of delta platform sediments during the middle Osagean ( Tn3b). New discoveries expand the known Nada crinoid fauna to 34 species (assigned to 27 genera). Nada crinoids are dominated by camerate crinoids, both in terms of abundance and diversity. The time and composition of the Nada fauna is significant. Late Osagean crinoid faunas had a separation of major crinoid clades into different sedimentary facies. Camerate crinoids dominated in carbonate facies; advanced cladids dominated in shallow-water siliciclastic facies; and disparids, primitive cladids, and flexibles dominated in deeper water siliciclastic facies. This relationship held through the Osagean, and a macroevolutionary change occurred during the Meramecian, after which advanced cladids typically dominated in all facies. In contrast, the Nada Member fauna is a fauna known previously to occur in the Burlington Limestone, the quintessential carbonate platform. The Nada crinoid fauna is typical of the Burlington Limestone in overall composition of the fauna (dominant camerates - both in abundance and diversity) and in the species composition, with most species known previously only from the Burlington Limestone. However, we regard this as only an apparent exception to the distribution of Osagean crinoid communities. The dominance of camerates in the Nada fauna stands in sharp contrast to the facies distribution of later Osagean crinoid faunas. This may reflect recruitment from the highly diverse and numerically abundant camerate species of the Burlington shelf. In fact those camerates in the Nada may represent the more adaptive Burlington camerate species that could inhabit a siliciclastic setting with reduced sedimentation or it may indicate those with greater larval dispersal abilities.