THE ANNISTON TRANSVERSE ZONE IN THE WELLINGTON 7.5-MINUTE QUADRANGLE, APPALACHIAN THRUST BELT, ALABAMA

In the Wellington quadrangle in the Appalachian thrust belt in Alabama, the Anniston transverse zone (ATZ) crosses two important structures. The ATZ is a cross-strike alignment across the entire thrust belt of sites of along-strike change in thrust-belt structures.

On the northwest, the Coosa synclinorium is in the Helena thrust sheet, which is detached in the regional weak layer (Cambrian shale-dominated Rome and Conasauga Formations) below the regional stiff layer (Cambrian-Ordovician Knox Group massive carbonates). Trailing the Coosa synclinorium, frontal ramps of the Eden thrust sheet and Coosa deformed belt (CDB) rise from an upper-level detachment above the regional stiff layer, and Ordovician-Mississippian stratigraphy is imbricated in multiple thrust sheets. The Eden thrust sheet ends northeastward at the ATZ. The CDB includes three tiers of imbricate thrust sheets. The frontal tier is continuous along strike regionally but is offset dextrally by a lateral ramp at the ATZ. The intermediate tier ends northeastward along strike at lateral ramps in the ATZ. The alignment of lateral ramps forms the Reads Mill offset, which includes a northwest-trending array of southwest-plunging folds along an up-to-southwest lateral ramp on the southwest side of the Angel block of Knox Group rocks. The detachment beneath the Angel block is at the regional décollement but cuts up section southwestward to the upper-level detachment of the CDB. The upper-level Angel thrust sheets extend southwestward between the intermediate and interior tiers of the CDB. The interior tier of the CDB ends northeastward along strike at lateral ramps within the ATZ.

Southeast of the CDB, the northeast-striking frontal ramp of the Pell City fault bends abruptly to a northwest-striking dextral offset along an up-to-northeast footwall lateral ramp in the ATZ. Southwestward away from the lateral ramp, the Pell City fault cuts up section along strike from the Rome and Conasauga Formations into the lower Knox Group. Southwest-plunging folds in the hanging wall and the southwestward stratigraphic rise of the detachment level indicate an up-to-southwest hanging-wall lateral ramp of the Pell City fault. The hanging-wall lateral ramp of the Pell City fault is emplaced over the footwall lateral ramp along the ATZ in the Wellington quadrangle.