PALEOKARST DEPOSITIONAL SETTING OF THE ATTALLA CHERT CONGLOMERATE MEMBER IN THE DUNAWAY MOUNTAIN THRUST SHEET, APPALACHIAN THRUST BELT, ALABAMA

Rocks mapped as the Attalla Chert Conglomerate Member (ACCM) on Barker Mountain in the southwestern part of the Dunaway Mountain thrust sheet (Ashville 7.5-minute quadrangle) do not conform to the regional distribution of the laterally discontinuous member at the base of the Middle-Upper Ordovician Chickamauga Limestone, unconformably overlying the Knox Group. Instead, the map pattern of the ACCM on Barker Mountain shows an irregular, worm-and-patchwork distribution of chert conglomerate masses, <6 m thick, surrounded by Knox Group rocks. Clasts in the ACCM include white, subangular to rounded, cobble-sized chert and subrounded to rounded, coarse sand to pebble-size chert. The matrix is rounded, coarse-grained quartz sand and angular to subrounded chert sand. Explanations for the present outcrop distribution of the ACCM are: 1) out-of-place blocks that slid downhill from a stratigraphic position at the Chickamauga-Knox contact, which is high on Barker Mountain; 2) a Cenozoic conglomerate deposit; 3) a fault breccia associated with a mapped back thrust on the northwest side of the Dunaway Mountain thrust sheet; 4) an erosional remnant from a kink fold in the Dunaway Mountain thrust sheet; and 5) Middle-Upper Ordovician deposits on Knox paleokarst. Local lack of ACCM in stratigraphic position along the Knox-Chickamauga contact high on Barker Mountain argues against out-of-place blocks from that source. The exclusively chert-clast composition of the ACCM, lacking a more diverse clast population (e.g., sandstone from Red Mountain Formation and Hartselle Sandstone), is incompatible with detritus from a Cenozoic erosion surface. Bedding attitudes of units on Barker Mountain do not conform to the geometry of a kink fold of sufficient amplitude to leave an eroded remnant of ACCM. Rounded chert clasts and subrounded sand grains in the matrix are inconsistent with fault breccia. Middle-Late Ordovician erosion of chert detritus from the Knox Group and deposition in paleokarst sinkholes and caves provide an explanation for the irregular map pattern, the homogeneous clast lithology, and the irregular distribution of thick masses of the ACCM surrounded by Knox Group outcrops. Deposition of the ACCM on Knox Group paleokarst during the Middle-Late Ordovician is consistent with field observations and map patterns.