TRILOBITE BIOFACIES AND LITHOFACIES OF THE UPPER ORDOVICIAN (SANDBIAN) LEBANON LIMESTONE, NASHVILLE DOME, TENNESSEE

The Late Ordovician (Sandbian) Lebanon Limestone of central Tennessee is a succession of subtidal carbonates that includes abundant, storm-generated bioclastic pack-, grain- and rudstone. The large lepiditicopid arthropod, Eoleperdita, is a conspicuous component of the rudstone facies, with bedding surfaces commonly crowded with disarticulated valves. The associated trilobite fauna is moderately diverse, including six to seven species, and is dominated by encrinurids and cheirurids. In the latter respect, it resembles more diverse biofacies of the younger Pooleville Member of the Bromide Formation of Oklahoma. Although the Lebanon Limestone has been interpreted by previous workers as a single depositional sequence (M3), a distinctive, regionally correlatable, sharp-based, meter-scale interval of grainstone near the middle of the formation, suggests a more complex stratigraphy. This grainstone unit, often referred to as the "Massive Member", may represent a TST of a second sequence, and is in turn overlain by a significant flooding surface and the deepest subtidal facies of the Lebanon. This deeper facies includes cm-thick lime mudstone layers with basal accumulations of graptolite rhabdosomes, some of which are current aligned, suggesting an origin as distal tempestites. The sparse, associated trilobite fauna is limited to low diversity assemblages of isotelines. Eoleperdita is also common in the graptolitic lime mudstone facies, indicating a broader bathymetric range than is typical for leperditicopid arthropods. Restriction of the group to shallow subtidal and peritidal settings apparently occurred later in its history.