DECIPHERING LATE CRETACEOUS VOLCANISM WITHIN RIPLEY FORMATION BENTONITES USING U-PB DETRITAL ZIRCON GEOCHRONOLOGY

The Upper Cretaceous Ripley Formation (Fm.) is exposed throughout Pontotoc County, Mississippi (MS) and hosts local bentonite lenses that are attributed to regional volcanism. A particularly large lens, known as the Pontotoc bentonite, was strip mined in the 1950’s and the land has since been reclaimed. Recent investigations south and north of the site of the mined Pontotoc bentonite revealed previously unknown exposures. Based on litho- and biostratigraphy, the old and new bentonite beds are approximated to be younger than known regional volcanism (~70 Ma) within the Mississippi Embayment (ME). The purpose of this study is to 1) recover volcanogenic zircons for U-Pb zircon dating to better constrain the timing of late Cretaceous volcanic events and chronostratigraphy of the Ripley Fm. within the ME; 2) determine depositional patterns/environments using detrital zircon analysis; and 3) test if the newly found southern and northern bentonites correspond to the Pontotoc bentonite. Normal heavy mineral separations were conducted on six samples from the new southern bentonite exposure and were followed by Scanning Electron Microscopy (SEM) and Inductively Coupled Plasma Mass Spectrometry (ICPMS). Four samples underwent X-Ray diffraction (XRD) from the new southern and northern exposures in order to compare mineralogical compositions. U-Pb isotopic analysis provides an overall detrital zircon age range from ~1500 Ma – 300 Ma. Five of the six samples show a distinct age peak from ~1250 Ma – 950 Ma and a single sample exhibiting a major age peak from ~750 Ma – 300 Ma. Minor age peaks occur between ~750 Ma – 300 Ma and ~1500 Ma – 1300 Ma. XRD results reveal that the main constituents in in the southern and northern bentonites are quartz (39.9-56.9%), muscovite (9.7-16.5%), and kaolinite (9.0-21.9%) with smaller amounts of illite and montmorillinite. Geochronologic analysis suggests zircons were sourced primarily from the Grenville and Appalachian-Ouachita orogenies and secondarily from the Mid-Continent Granite-Rhyolite Province. Therefore, no Late Cretaceous volcanogenic zircons, ~75 Ma – 65 Ma, were recovered. XRD data, however, allow us to correlate at least the northern and southern bentonites based on similar mineralogical compositions.