TEXTURAL AND COMPOSITIONAL COMPARISON OF LOWER ORDOVICIAN (COTTER) AND LOWER MISSISSIPPIAN (BOONE) CHERTS WITH THE ARKANSAS NOVACULITE (DEVONIAN-MISSISSIPPIAN), ARKANSAS

Previous analyses of chert sampled from the Boone Formation (Lower Mississippian) exposed across the southern Ozark region, were compared with samples of the Arkansas Novaculite (Devonian-Lower Mississippian) taken at the famous Caddo Gap roadcut on the north flank of the Ouachita Uplift. Both of those analyses indicate a composition consistent with a volcanic ash source for the silica, likely provided by an island arc system associated with the Ouachita Orogen. Additional samples have been taken from the Lower Ordovician Cotter Dolomite in close proximity to previously sampled roadcut exposures of the Lower Mississippian Boone Formation in the southern Ozarks. Cotter samples were analyzed utilizing scanning electron microscopy (SEM) and energy dispersive x-ray (EDX) for comparison with previously analyzed Boone samples from the same area. Although representing different depositional settings and ages, each of the combined total of 19 samples comprises mostly the major elements silicon and oxygen, although aluminum, potassium, calcium, and sodium are also present in varying amounts. While all three sampled intervals have similar compositions and silica consistent with a volcanic source, the Cotter and Boone samples are texturally different from those of the Arkansas Novaculite, which was deposited in deeper water, and is coarser grained, suggesting closer proximity to the source, and possible recrystallization. Textures of the Lower Ordovician Cotter samples are similar to those of the later diagenetic chert that replaces the Upper Boone carbonates along bedding planes, suggesting a similar silica source and formational history.