TEPHROCHRONOLOGY, 40AR/39AR DATING, AND MAGNETOSTRATIGRAPHY OF THE UPPER PART OF THE FURNACE CREEK FORMATION, DEATH VALLEY, CALIFORNIA

During late Miocene to middle Pliocene time, alluvial fan and lacustrine sediments of the Furnace Creek Fm. were deposited in a northwest-migrating basin; today the analogous depocenter is in Cottonball Basin, 10 km north-northwest of Furnace Creek Ranch. The uppermost part of the Furnace Creek Formation and overlying Funeral Fm. are well exposed along Zabriskie Wash, about 1 km east of Zabriskie Point. We dated the time-transgressive boundary between the formations using tephrochronology, precision ⁴⁰Ar/³⁹Ar dating, and magnetostratigraphy along a 115-m-thick unfaulted, fine-grained, tuffaceous section of the Furnace Creek Fm. On the basis of major-element composition, correlations with source-rock eruptions, paleomagnetic analyses, and new 40Ar/39Ar dating, the four tuff beds in this section are identified as: (a) Curry Canyon tuff (>3.35, <3.58 Ma); (b) lower Mesquite Springs tuff (3.29±0.01 Ma); (c) upper (pumiceous) Mesquite Springs tuff (3.29±0.10 Ma); and (d) Nomlaki Tuff (~3.30 Ma). The provenance of the Curry Canyon tuff is unknown, but is probably from the Walker Lane area; the Mesquite Springs tuffs are Bishop-like, but the eruptive center was likely closer, perhaps in northern Death Valley; and the Nomlaki tuff is from the southern Cascades of NE California. Paleomagnetic data indicate that the lower two tuffs are normal, whereas the overlying tuffs are magnetically reversed. We believe that the section includes the entire Mammoth paleomagnetic subchron: the normal-to-reverse polarity change between the Mesquite Springs tuffs is the 3.33-Ma base of the subchron, whereas a 100-m higher, reverse-to-normal polarity change is the 3.22-Ma top of the subchron. Lithologically, the section reflects a lacustrine transgression from conglomerates below the Curry Canyon tuff to sandstones to mudstones in which the Mesquite Springs tuffs were deposited. The section coarsens upward back to sandstones and conglomerates of the Funeral Fm. above the Nomlaki tuff, a result of syntectonic deposition of Funeral Fm. fanglomerates as the adjacent Funeral Mountains were being uplifted by oblique/dextral movement along the Furnace Creek fault. This fault has been largely inactive in the Quaternary, movement having shifted westward onto the Death Valley fault system.