REVISED LATE ORDOVICIAN CORRELATIONS AND THE RICHMONDIAN INVASION IN THE NASHVILLE DOME, TN

The Richmondian Stage (Cincinnatian Series; upper Katian) records significant faunal volatility associated with the final phases of the Great Ordovician Biodiversity Event (GOBE) and onset of the great Hirnantian extinctions. In their type area, the Cincinnatian strata provide evidence for a "Richmondian Invasion" in which >50 "exotic" taxa appear abruptly in the Cincinnati Arch outcrop region. Recent study shows that the immigrants largely appear in transgressive facies of the second or Clarksville Mbr. of the Waynesville Fm, coincident with a positive C isotopic excursion. Was this pattern of the Richmondian Invasion strictly a local phenomenon? Here we present a revised correlation of the Nashville Dome in Tennessee, based on lithological stacking patterns, sequence stratigraphy, and preliminary isotope data, that suggests comparable and synchronous faunal patterns. In contrast with previous correlations, the lower Waynesville Formation correlates with the upper part of the "Arnheim Formation" in TN, whereas the overlying Fernvale Fm, characterized by low isotopic values, may be equivalent to the upper Richmondian. The Waynesville interval starts with a 50 cm thick, phosphatic, bryozoan-rich limestone, capped by a rusty hardground, which appears to correlate with the basal South Gate Hill submember of the Cincinnati Arch, marking the C4/C5 sequence boundary. Overlying this basal bed is a 3-4 meter-thick shale-rich succession, probably corresponding to the Fort Ancient Mbr (lower C5) of the Waynesville Fm, with a medial ~40cm gastropod-rich limestone (Bon Well Hill submember?). This interval yields abundant dalmanellids, but few exotics, as typical of the Fort Ancient. The overlying shell-rich packstone, which apparently corresponds with the Clarksville Mbr, shows the abrupt appearance of classic Richmondian Invasion taxa, including the brachiopods Eochonetes. Strophomena, and Lepidocyclus, and the coral Grewingkia. This preliminary study indicates that the Waynesville Fm. can be correlated into the Nashville Dome, even at the level of submembers, and the Richmondian Invasion shows very similar patterns. The mid-late Richmondian interval thus shows evidence of correlated regional patterns of environmental and biotic change.