TIMING OF LATE CRETACEOUS GULF COAST VOLCANISM AND CHRONOSTRATIGRAPHIC CONSTRAINTS ON DEPOSITION OF THE RIPLEY FORMATION FROM A NEWLY RECOGNIZED BENTONITE BED, PONTOTOC COUNTY, MISSISSIPPI

The Upper Cretaceous Ripley Formation in the Mississippi Embayment consists of ~240 feet (73 m) of fossiliferous clay, sand, and calcareous sand beds. Lateral facies variations have complicated historical stratigraphic correlations between locations, but recent stratigraphic revisions in Mississippi recognize a lower transitional clay facies, a limestone, marl, and calcareous sand facies informally named the Troy beds, a sandy upper Ripley facies, and the formally named Chiwapa Sandstone Member. Ammonite biostratigraphy places the contact between the Chiwapa and the overlying Owl Creek/Prairie Bluff at ~68.5 Ma. The upper Ripley facies contains the Pontotoc Bentonite, which was identified north of the town of Pontotoc, Mississippi, in 1943. No radiometric dates have been obtained from this bentonite, but litho- and biostratigraphy indicate that the bentonite is younger than known volcanism from the Jackson Dome and related structures. Recent investigations resulted in the discovery of a previously unknown bentonite bed south of the town of Pontotoc. The purposes of the present investigation are 1) to test whether the new bentonite bed is correlative to the Pontotoc Bentonite and 2) to recover volcanogenic zircons for U-Pb and Hf dating to better constrain timing of volcanism and chronostratigraphy of the Ripley Formation. Outcrops in an active sand pit in the field area expose ~2.5 m of fine sand that contains poorly preserved burrow boxworks, likely Ophiomorpha, which would indicate a high-energy, shallow marine setting. The sand has an upper gradational contact with an overlying ~2.5 m of sandy clay, which contains the bentonite bed. The top meter of the sandy clay is overprinted by pedogenic features associated with the current soil surface. Two trenches (~15 m apart) were excavated through the entire thickness of the outcrop. In each trench, a stratigraphic section was measured and bulk samples were collected for zircons. Sampling began in the lower bounding sand and continued upsection in ~1 m intervals, corresponding to the gradational contact with the bentonite, and 2 locations within the bentonite.