Crinoid distribution and feeding morphology through a depositional sequence: Kope and Fairview formations, Upper Ordovician, Cincinnati Arch region

Abstract

Crinoid columnals are major faunal components of interbedded shales and carbonates of the Upper Ordovician Kope to Fairview formations (Edenian-Maysvillian) of the Cincinnati Arch region. Six species can be identified on the basis of distinctive morphological characters of the columnals. Crinoid distribution was plotted from point-counted carbonate samples taken through a 68-m thick composite section of the Kope to Fairview formations in Campbell County, Kentucky. This section spans a shallowing-upward, third-order depositional sequence (C1), part of C2, and the Edenian-Maysvillian Stage boundary. The slender cladid crinoid Merocrinus occurs in the lowermost Kope below the base of this section. The slender disparids Cincinnaticrinus and Ectenocrinus occur throughout the section but are most abundant in the lower 25 m where the shale percentage is 60–90 percent. The larger, more robust disparid Iocrinus appears within the carbonate-rich Grand Avenue member of the Kope at 40–50 m, and the large, plated camerate Glyptocrinus first appears just above the Grand Avenue and becomes the dominant crinoid above the C1–C2 sequence boundary that lies just above the Kope-Fairview contact. The largest and most robust crinoid in this sequence, Anomalocrinus, occurs at the top of the Grand Avenue Member. Siliciclastic ratio and biofacies composition indicate that the occurrence of larger, more robust crinoid taxa is correlated with shallowing depth. Crinoid trophic niche differentiation is also correlated with decreasing depth and the concomitant increase in water movement caused by waves and currents. The deeper water disparids have a nonpinnulate filtration fan with low branch density and wider ambulacral grooves. The shallower water camerate Glyptocrinus has a pinnulate filtration fan with high branch density and narrower ambulacral grooves. These relationships are consistent with the predictions of aerosol filtration theory.