DETRITAL ZIRCON PROVENANCE AND CORRELATION OF TWO RIPLEY FORMATION (UPPER CRETACEOUS) BENTONITES, PONTOTOC COUNTY, MISSISSIPPI

Two previously unknown bentonite deposits in the Maastrichtian Ripley Formation of northern and southern Pontotoc County, Mississippi, offer an opportunity to improve regional chronostratigraphy and to reevaluate local mining potential. In Pontotoc County, the Ripley Formation is informally divided into a basal transitional clay and lower, middle, and upper intervals. The upper Ripley contains the Chiwapa Sandstone Member (CSM), which hosts the two new bentonites as well as bentonite beds that were previously mined in the central part of the county. The two new bentonites are located stratigraphically above the previously mined bentonites. This study utilizes scanning electron microscopy (SEM), detrital zircon U-Pb geochronologic dating, stratigraphy, lithofacies analysis, and X-ray diffraction (XRD) to determine and refine the possible sources and chronostratigraphy of the Chiwapa bentonites.

Detrital zircons ages from the two new bentonites range from Mesoarchean (~2,870 Ma) to Pennsylvanian (~305 Ma) and contain ~91% Appalachian source derived grains, including Appalachian-Ouachita, Gondwanan Terranes, and Grenville source terranes. Other igneous province source regions include the Mid-Continent Granite Rhyolite Province, Yavapai-Mazatzal, Trans-Hudson/Penokean, and Superior Province. These results suggest that the sediment feeding the northeastern Mississippi Embayment likely originated from the Appalachian foreland basin and Appalachian-Ouachita fold thrust belt. Preliminary results suggest three lithofacies present in the field areas representing shoreface to outer shelf environments. The correlation of bentonite deposits were based on U-Pb detrital ages and their stratigraphic positions (directly above CSM). However, XRD results indicate that the northern bentonite is kaolinite rich, while the southern bentonite is montmorillonite rich. Montmorillonite rich Ripley Formation clays have not been previously noted within the MSE and possibly indicate a volcanic source younger than known Late Cretaceous regional volcanism.