ORIGIN OF A MIDDLE ORDOVICIAN EDRIOASTEROID FIRMGROUND, LEXINGTON LIMESTONE, CENTRAL KENTUCKY

More than 100 edrioasteroids of the species Cystaster stellatus were found on mostly convex-up, pedicle valves of the disarticulated brachiopod Rafinesquina alternata from a layer in the Middle Ordovician Sulfur Well Member of the Lexington Limestone. To understand the origin and paleoecology of the firmground, the edrioasteroid pavement layer and an underlying layer without edrioasteroids were examined in detail. A part of each layer was collected and described, isopachous maps of each were made, orientation and articulation styles of the brachiopods were placed on rose diagrams, and the numbers and positions of the edrioasteroids were counted and plotted to illustrate population composition, size-frequency distribution, and preferred location on host shells. Both layers were crudely graded, but the edrioasteroid-deficient firmground was scoured, had subtle mega-ripples, and exhibited few fossils, whereas the edrioasteroid-rich layer was thinner, subtly rippled, and contained many more fossils, with the brachiopod valves concentrated in ripple troughs. Both layers represent tempestites deposited below normal wave base in open-marine, intermediate-ramp conditions. The lower firmground, however, seems to represent more continuous, high-energy conditions, which made larval recruitment difficult, whereas the upper firmground probably reflected a less active, more distal environment that was more favorable for the recruitment of larvae from edrioasteroids and other benthic organisms. Presence of so many, single, convex-up valves indicates one-time deposition by a storm event, and the clustering of edrioasteroids near the central, domed part of each valve suggests later accumulation there for elevation into the water column for feeding. Most of the edrioasteroids are preserved intact and sharply overlain by a shale bed, suggesting rapid burial in mud fallout accompanying a later storm. Hence, because echinoderms typically disarticulate rapidly after death, the preserved edrioasteroids on this firmground were most likely still alive at the time of burial. Consequently, it is storm sedimentation that largely controlled the paleoecology of the firmground – both in creating the pavement substrate and in preserving the organisms that lived on it through rapid burial.