THE READS MILL OFFSET OF THE COOSA DEFORMED BELT ALONG THE ANNISTON TRANSVERSE ZONE, APPALACHIAN THRUST BELT, ALABAMA

The regional Appalachian décollement in Cambrian (Rome and Conasauga Formations) shale underlies the regional stiff layer of Cambrian-Ordovician (Knox Group) carbonate. In contrast, along the trailing part of the Helena thrust sheet, the Coosa deformed belt (CDB) ramps up-to-northwest from an upper-level detachment near the top of the Knox. Southwest of the regional Anniston transverse zone (ATZ), the CDB includes three tiers of one or more thin imbricate thrust sheets. A brittle duplex of Knox in the trailing part of the Helena thrust sheet balances upper-level shortening in the CDB.

Along the ATZ, lateral ramps of both the regional décollement and upper-level detachment form the northwest-trending Reads Mill offset (RMO) of the CDB. In the northwestern RMO, lateral ramps dextrally offset the CDB frontal tier and terminate the intermediate tier. Ordovician redbeds stratigraphically distinguish the northeastern part of the frontal tier, as well as the offset frontal tier, but are unconformably absent to southwest and southeast. The southeast trailing cutoff of the offset frontal tier is the northeast-striking hanging-wall cutoff of the orthogonal, structurally high Angel Knox block (AKB). The leading hanging-wall cutoff of the AKB is in the Knox, but internal imbricates include Rome and Conasauga, indicating frontal ramps from the regional décollement. In a northwest-trending train of southwest-plunging thrust-related folds along the southwest side of the AKB, the Knox plunges southwest beneath younger strata; along the anticlines, thrust faults cut up section southwestward at up-to-southwest hanging-wall lateral ramps from the regional décollement beneath the AKB to the upper-level CDB. Upper-level thrust sheets extend along the trailing cutoff of the intermediate tier of the CDB southwestward from the plunging folds along the RMO. Crosscutting relations indicate episodic break-forward and break-back fault movements. Internal imbricates within the AKB suggest that up-to-southwest lateral ramps separate the AKB from the subsurface duplex of Knox in the trailing Helena thrust sheet southwest of the RMO. Southeast-dipping post-Knox strata with thrust-related southwest-plunging folds define the southeast side of the AKB. The lateral ramps of the RMO are within and characterize the regional ATZ.