Southeastern Section - 64th Annual Meeting (19–20 March 2015)

Paper No. 2
Presentation Time: 1:25 PM


COOK, Brian S., Southwestern Energy Company, PO Box 12359, Spring, TX 77391-2359 and THOMAS, William A., Emeritus University of Kentucky, Geological Survey of Alabama, P.O. Box 869999, Tuscaloosa, AL 35486-6999,

In a clearly defined subrecess in the Appalachian thrust belt in northwestern Georgia, two distinct regional strike orientations of the interior (southeastern) structures intersect at nearly 90°; however, the leading (northwestern) structures cross the region with no deflection in strike. Between the nearly straight Kingston fault (northwest) and the sharply curved Rome fault (southeast), the Floyd synclinorium includes internal plunging folds that intersect at approximately 50°. In the subsurface beneath the folds within the Floyd synclinorium, seismic reflection profiles show tectonically thickened weak-layer rocks (mainly shale of the Cambrian Conasauga Formation) in a ductile duplex above basement rocks offset by low-magnitude basement faults.

An array of structural cross sections transecting the Floyd synclinorium supports measurement of extent and thickness of the ductile duplex. Contouring the thickness data yields a 3D model of the ductile duplex. A conventional line-length palinspastic restoration accounts for considerably less volume of the ductile duplex than that of the deformed-state ductile duplex in the 3D model. The significant mismatch in volume of weak-layer shale can be explained by a larger magnitude basement graben during Conasauga deposition and subsequent inversion of the graben to yield the present configuration of top of basement rocks.

The large initial depositional volume of shale of the Conasauga Formation in a subsequently inverted basement graben allows a volume balance of the resultant ductile duplex, which is essential for palinspastic reconstruction of the larger scale thrust sheets of the subrecess. The ductile duplex accounts for the pattern of structural interference shown in geological maps of northwestern Georgia, absorption of differences in shortening along the intersecting internal thrust sheets, and the geometry of the subrecess. The locally thick shale absorbed shortening along two intersecting azimuths, converting the nearly orthogonal trailing structures of the subrecess into the nearly straight leading structures. Regionally, the transition from the local Georgia subrecess into the larger scale Tennessee salient and Alabama recess is an effect of original thickness variations in the regional weak layer across basement faults.