BIOSTRATIGRAPHY AND AGE OF THE LOWER FAIRPOINT MEMBER OF THE FOX HILLS FORMATION (MAASTRICHTIAN), BADLANDS AREA OF SOUTH DAKOTA

Palynomorphs, macrofossils, and lithologic data collected from a 30m interval of the Fox Hills Formation in the Badlands area of South Dakota allow us to interpret the geologic age of this interval and to infer its stratigraphic position relative to other Fox Hills sections. The study interval consists of a series of siltstones and sandstones, dark in color, massively bedded, and glauconitic at the base, and grading upward into finely laminated, buff colored sandstone toward the top. A 0.5 to 5 m thick disturbed zone (DZ), containing contorted sandstone beds, shocked quartz and melt glass spherules, occurs about 4 m above the base of the section, and has been traced over 1000 km² throughout the study area. The DZ is consistently bracketed by a reversed and normal magnetic polarity zone throughout the study region. The DZ itself is of indeterminant polarity. Based on biostratigraphic data, these magnetozones likely represent either chron 30 or 31. The study interval rests on gray Pierre Shale, and lies below a zone of large sandy concretions that Pettyjohn (1967) places in the middle of his Fairpoint Member. The study interval contains an impoverished molluscan fauna dominated by nuculid bivalves with smaller numbers of gastropods, scaphopods, and the ammonites Discoscaphites gulosus and Jeletzkytes nebrascensis. The molluscan fauna is most similar to the fauna of the upper Timber Lake Member as described by Speden (1970) and Landman and Waage (1993), and probably represents the lower part of the Jeletzkytes nebrascensis ammonite zone in the Badlands area. Palynomorphs recovered include dinoflagellate cysts, spores, gymnosperm and angiosperm pollen grains, especially of the triprojectate and oculate types, and colonial algae. Terrestrial palynomorphs are common, but dinoflagellates are also relatively abundant. The assemblages recovered contain dinoflagellates diagnostic for the lower upper to middle upper Maastrichtian (e.g., Triblastula utinensis, T. wilsonii, Isabelidinium cooksoniae, Phelodinium tricuspe). We infer from this that the DZ impactite within the study interval is not associated with the Manson impact (too old) nor the end-K Chixculub impact (too young), but instead represents either a hitherto unrecognized local impact event, or a distal correlative of the impactite reported by (Turner et al., 2003) from the Raton Basin.