U-PB AND AR-AR AGE CONSTRAINTS ON MIOCENE TERRESTRIAL ROCKS: AN EXAMPLE FROM THE BARSTOVIAN LAND MAMMAL STAGE OF NORTH AMERICA

Part of the stratigraphic section of the middle Miocene Barstow Formation near Barstow, California serves as the reference section for the Barstovian Land Mammal Stage of North America. Current boundaries of 16.0 and 13.4 Ma are defined by magnetostratigraphy and K-Ar and ⁴⁰Ar-³⁹Ar ages from intercalated tuff beds. A key date of 14.8±0.01 Ma is from biotite separates from the Dated Tuff, which lies stratigraphically ~600 m above the base of the Barstovian. Certain horizons of the Barstow Formation contain carbonate deposits formed around subaqueous spring vents in the lake that filled the extensional basin. Fibrous calcite crystals from one of these deposits located stratigraphically ~300 m below the Dated Tuff yielded a concordant and precise U-Pb age of 14.7 ± 0.4 Ma. To resolve the difficulty of similar age in these two units, a third age was sought. Step-heating experiments of two aliquots of apparently unaltered fragments of the Skyline Tuff, a unit located 20 m stratigraphically below the Dated Tuff, yielded ⁴⁰Ar-³⁹Ar plateau ages of 14.170±0.016, with plateaus that extend over 80% of the released argon. The wide plateau and high precision suggests a homogeneous, undisturbed ⁴⁰Ar distribution in a single-phase source. XRD analysis of the bulk rock identifies sanidine as the dominant crystalline material. Single-step fusions of twelve individual sanidines from the Skyline Tuff give an average age of 14.07±0.4 Ma. The ⁴⁰Ar-³⁹Ar sanidine age of the Skyline Tuff is stratigraphically consistent with the U-Pb age of the calcite. The new dates suggest that the Barstovian Age might be almost one million years younger than previously thought. Furthermore, the study demonstrates that U-Pb dating can provide a precise method of dating lacustrine calcite in terrestrial sequences as young as Neogene. The U-Pb age, coupled with ⁴⁰Ar-³⁹Ar sanidine ages, also provides a basis from which to examine the calibration between these ages and ⁴⁰Ar-³⁹Ar biotite ages.