Abstract

The Decatur Limestone and overlying Ross Formation, exposed in west-central Tennessee, USA, represent continuous marine deposition from Late Silurian (Pridoli) into Early Devonian (Lochkov) time. These units were deposited in a broad, shallow seaway that connected the Illinois Basin to the north with craton margin or open oceanic environments to the south. Northern exposures include the Decatur and the Rockhouse Limestone Members of the Ross Formation, which are mostly skeletal packstone and grainstone that represent open marine, current- and storm- influenced deposition with minor influxes of terrigenous mud during latest Silurian into earliest Devonian time. To the south, time equivalent, sparsely fossiliferous, grey shale (Rockhouse Shale Member) represents deposition in a somewhat deeper water regime. During later phases of Ross deposition an apparently non-tectonic bathymetric reversal resulted from a slight deepening to the north that was accompanied by greater influx of terrigenous mud and episodic winnowing of bottom sediment by storms (Birdsong Shale Member) and a contrasting shallowing to the south that resulted from upbuilding of a carbonate dominated tidal flat complex (Ross Limestone Member) associated with shoal-water or tidal channel hematitic skeletal grainstone (Bear Branch Member).