The paleoecology of echinoderms of the Middle Ordovician Benbolt Formation of East Tennessee

The Benbolt Formation is part of the Middle Ordovician Chickamauga Group of eastern Tennessee. It consists primarily of cobbly-weathering argillaceous biomicrite and contains a diverse fossil fauna. Benbolt outcrops at Evans Ferry and Stanley Farm in Granger and Union counties, Tennessee, can be divided into three units based on weathering characteristics and fossil content; in ascending order, they are: (1) thinly-bedded biomicrite containing abundant fossils which include a diverse assemblage of echinoderms, (2) a massive-weathering biosparite containing large amounts of echinoderm debris but few identifiable fossils, and (3) a biomicrite interbedded with shale which contains few fossils. These three units represent an environment which changed as the Benbolt was deposited. The first of these was a stable shallow shelf with low current energy which was followed by a shoaling region or a skeletal sand bar which had higher current energy and finally by a quiet mudbank which was often inundated by fine-grained terrigenous elastics and possibly subaerially exposed at rare intervals. The major differences in faunal composition and lithology between Evans Ferry and Stanley Farm seem mainly to be the result of the probability that the water at Evans Ferry was slightly shallower than at Stanley Farm for most of the period of deposition of the Benbolt.

Specimens of 26 different species of echinoderm were collected for this study, mostly from the first unit which supported a far more diverse echinoderm fauna than the other two. These echinoderms belonged to the classes of crinoids, paracrinoids, eocrinoids, rhombiferans, blastoids, edrioasteroids, stylophorans and homiosteleans. Echinoderms were collected from approximately 30 horizons at each of the Benbolt localities and the occurrence of each species was plotted against petrologic data.