HR: 17:50h
AN: H34A-06    [Abstracts]
TI: 40Ar/39Ar Dating of the Middle Miocene Skyline Tuff, Barstow Formation, CA
AU: * Cole, J M
EM: jcole@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Rt. 9W, Palisades, NY 10964, United States
AU: Rasbury, E T
EM: troy.rasbury@stonybrook.edu
AF: Stony Brook University, Department of Geosciences, Stony Brook, NY 11794-2100, United States
AU: Hemming, S R
EM: sidney@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Rt. 9W, Palisades, NY 10964, United States
AU: Machlus, M
EM: machlus@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Rt. 9W, Palisades, NY 10964, United States
AU: Swisher, C C
EM: cswish@rci.rutgers.edu
AF: Rutgers University, Department of Geological Sciences, Piscataway, NJ 08854, United States
AB: The Miocene Barstow Formation, located in the Mojave region of southern California, is famous for its abundant mammalian fossil record. It also contains a series of volcanic ash horizons intercalated within the sedimentary sequence. MacFadden et al. (1990, GSAB, v.102) and Woodburne et al. (1990, GSAB, v.102) used K-Ar and 40Ar/39Ar dating of these ashes to constrain the age of the stratigraphy, magnetostratigraphy and biostratigraphy of the type Barstow Fm. and to calibrate the early Barstovian North American Land Mammal Age (NALMA). One of the most recognizable layers of the Barstow sequence is the Skyline Tuff. The resistant Skyline Tuff stands in relief and forms the top of many of the cliffs in the Barstow Syncline, particularly in Rainbow Basin. Stratigraphically, it forms the base of the upper member of the Barstow Fm., and is situated between the "Dated" and Hemicyon Tuffs. Published geochronological work on this important marker bed is limited to K-Ar ages of (diagenetic?) K-feldspar, illite/smectite, and clinoptilolite cuts. Unlike many of the ashes in the Barstow Fm., including the "Dated" and Hemicyon tuffs, the Skyline Tuff lacks biotite. We have sampled the Skyline Tuff from three locations, two in the Rainbow Basin, and one in the Owl Canyon. Single sanidine crystals were co-irradiated along with an inter-lab monitor standard cross calibration. We anticipate that by using the ages of these samples, we can directly compare the Skyline Tuff age with the biotite derived ages on the "Dated" and Hemicyon Tuffs. Better age control on the Skyline Tuff may therefore have important implications for calibration of the Barstow magnetostratigraphy, biochronology of the early to middle part of the Barstovian NALMA, and will improve correlation with endemic Great Plains faunas of middle to late Barstovian age.
DE: 1040 Radiogenic isotope geochemistry
DE: 1115 Radioisotope geochronology
DE: 1145 Tephrochronology (8455)
DE: 8404 Volcanoclastic deposits
SC: Hydrology [H]
MN: 2007 Joint Assembly