GEOLOGY OF THE ALABAMA HALF-MILE SHOALS AREA IN THE SOUTHERN ALLEGHENIAN FOLD-THRUST BELT

Detailed geologic mapping, funded through the EDMAP Program by the USGS, of the Half-Mile Shoals 7.5-minute quadrangle in Alabama has been conducted to characterize lithologies of rock units present in the quadrangle and to reconcile geologic relationships with the three adjacent 7.5-minute quadrangles (Woodstock, McCalla, and West Blocton East quadrangles). Additionally, collected data will facilitate the construction of a balanced structural cross-section aimed at constraining the structural shortening in the Half-Mile Shoals, and adjacent Woodstock, areas. Stratigraphic units present in the quadrangle range from Middle to Late Cambrian to Late Cretaceous. These units are the Middle to Late Cambrian Conasauga Formation, Late Cambrian to Early Ordovician Knox Group, Middle Ordovician Chickamauga Limestone, Silurian Red Mountain Formation, Devonian Chattanooga Shale, Early Mississippian Fort Payne Chert, Late Mississippian Floyd Shale, Late Mississippian to Early Pennsylvanian Parkwood Formation, Pennsylvanian Pottsville Formation, and Late Cretaceous Coker Formation. These units are present in the adjacent quadrangles and are extended into the Half-Mile Shoals quadrangle area, which encompasses an area in the hanging wall of the Jones Valley thrust sheet in the Appalachian fold-thrust belt. Other major structural features in the area include a smaller displacement thrust fault with a northeast trending, tight, inclined syncline-anticline pair in its hanging wall and a cut-off syncline in its footwall. These structures are parasitic to the northwest limb of the Cahaba synclinorium, which occurs along the trailing edge of the thrust sheet and is truncated by the Montevallo fault to the southeast. Stratigraphic relationships constrain the deformation in the area to the Alleghenian Orogeny on the basis of unconformable Late Cretaceous strata overlying deformed Early Pennsylvanian strata. Geologic mapping and lithologic characterization of strata in the Half-Mile Shoals quadrangle have expanded the relationships established in the adjacent Woodstock, McCalla, and West Blocton East quadrangles. Based on our mapping, and detailed structural observations, we will construct a balanced cross-section to constrain kinematic development and structural shortening in the area.