GEOLOGIC RELATIONSHIPS AT THE SOUTHERN TERMINUS OF THE BREVARD ZONE IN ALABAMA

The Brevard fault zone (BFZ) is a structural/stratigraphic zone extending for >600 km from North Carolina (NC) to Alabama (AL). For ~300 km, from near Jacksons Gap (JG), AL, NE to NC, this zone maintains a nearly linear trend of ~N50°E. An internal stratigraphy (Jacksons Gap Group-JGG) with local structural complications is maintained within the BFZ over this distance. In eastern AL and western GA, the NW border/structural base of the BFZ is the Abanda fault (AF) and the SE border/structural top is the Katy Creek fault (KCF). Both of these structures are late in the Alleghanian kinematic sequence and have been described as having some combination of normal, thrust, or strike slip motion. South of JG the JGG curves from its regional strike to ~N05°E, and extends beneath the Coastal Plain (CP), curving around the hinge of the Tallasse synform (TS) to reemerge within the western Inner Piedmont’s (WIP) Opelika Group (OG) as the Loachapoka Formation (LF). In contrast to the fault bounded (AF) JGG on the NW flank (base) of the BFZ NE of JG, the structural base of the equivalent LF in the WIP is a stratigraphic contact with the underlying Auburn Fm. and is stitched by the ~450 Ma Farmville Granite. The structural top of the LF in the WIP is the Stonewall Line fault (SLF), a structure pre-dating the KCF in the kinematic sequence. Understanding the contrasting timing and geometric relationships between the JGG and its boundaries NE of JG, and its upper/lower boundaries in the WIP is critical for understanding the nature of the termination of the BFZ in AL and proposed significant regional movements associated with the BFZ. Previous proposed solutions for the AF are: 1) the AF tips out near JG, exhibiting negligible offset to the south and increasing net displacement NE; 2) south of JG the AF curves southeastward and continues south beneath the CP where it is truncated by the Towaliga fault, placing rocks of the E. Blue Ridge and equivalent OG in the fault’s hanging wall; and 3) the AF curves at JG, continuing beneath the CP, where it is folded around the hinge of the TS into the WIP OG. The KCF apparently tips out near JG, exhibiting negligible offset to the south, but increases net displacement to the NE where it replaces the SLF as the structural base of the WIP’s Dadeville complex. This implies that the fault at the structural base of the DC in the hinge of the TS is the SLF.