MAGNETIC STRATIGRAPHY OF THE UPPER MIOCENE THOUSAND CREEK FORMATION, NORTHWESTERN NEVADA

The Thousand Creek Formation in northwestern Nevada yields an important late Miocene (early Hemphillian) mammalian fauna that has been studied for over a century. The age of this unit has been very confused, with the capping basalt giving a date of 9 Ma, but the underlying Rattlesnake Ash Flow Tuff giving 40Ar/39Ar dates of 7.05 or 7.26 Ma, and tephrochronology suggesting that the bottom of the section may be as old as 8 Ma. A magnetostratigraphic section was sampled on the longest exposed section along the roadcuts on the north side of Highway 140 just east of Thousand Creek. Combined alternating field and thermal demagnetization yielded a stable remanence held in magnetite with minor goethite overprints. Except for the stratigraphically highest site, the entire 40 m of section was of reversed polarity. Based on the dates on the Rattlesnake Ash Flow tuff at the top of the section, we correlate the Highway 140 roadcut section with Chron C3Br1n to C3Br (7.1-7.3 Ma). If the tephrochronology is correct, then the lower part of the section (too poorly exposed for paleomagnetic sampling) may be as old as 8.3 Ma.