REGIONAL POLYPHASE DEFORMATION ON THE NASHVILLE AND JESSAMINE DOMES, TENNESSEE AND KENTUCKY I: FAR-FIELD EFFECTS OF THE ACADIAN-NEOACADIAN AND ALLEGHANIAN OROGENIES

The unconformity beneath the Chattanooga Shale in the southern-central Appalachian foreland formed ~360 Ma as the distal Acadian-Neoacadian clastic wedge prograded southward from coeval southward uplifted source terrains in the internal parts of the chain. Ordovician and younger Pre-Chattanooga sequences are preserved beneath the unconformity in the Nashville and Jessamine domes in Tennessee and Kentucky, but the distribution of Silurian rocks is intermittent. The state geologic maps of TN (1966) and KY (1981) record numerous truncations of Lower Silurian rocks at the unconformity, and geologists have known for decades that there are numerous second- and third-order folds (and mappable small-displacement faults) across each dome, originally attributed to movement of basement blocks. Deformation affecting the rocks above the unconformity involves rocks as young as Early Permian, so that deformation is clearly Alleghanian. Structure contour maps constructed on horizons above and below the unconformity from published detailed 1:24,000-scale geologic maps in KY and TN reveal an array of Alleghanian folds above the unconformity, and also a separate array of pre-Chattanooga folds truncated by the unconformity that preserves Lower to Middle Silurian rocks in synclines. The geologic maps were scanned, georeferenced, and the contacts digitized in ArcMap 10.3.1 into a geodatabase for analysis and presentation. Subsurface data from state geological survey databases were also incorporated. That the pre-Chattanooga folds involve Lower to Middle Silurian rocks requires the deformation event be post-Middle Silurian (post Taconic) and pre-Late Devonian—possibly far-field effects of the Acadian-Neoacadian orogeny. These structures also provide another avenue for petroleum exploration in this region.