MAGNETIC STRATIGRAPHY OF THE MASSACRE LAKE BEDS (LATE HEMINGFORDIAN, EARLY MIOCENE), NORTHWEST NEVADA, AND THE AGE OF THE “PROBOSCIDEAN DATUM” IN NORTH AMERICA

The “Proboscidean Datum” was proposed by Tedford et al. (1987) and Woodburne and Swisher (1995) as a marker of the late Barstovian (middle Miocene, about 14.5 Ma) in North America. Subsequently, a number of pre-late Barstovian proboscidean fossils have been reported, casting doubt on the validity of the Proboscidean Datum. The oldest of these is from the late Hemingfordian Massacre Lake local fauna of northwest Nevada, which produces a single proboscidean tooth fragment. Magnetic stratigraphy was conducted on these beds, which they yielded a stable remanence held mainly in magnetite that was entirely reversed in polarity. Based on the 40Ar/39Ar date of 16.474 ±0.035 Ma at the top of the section, we correlate the Massacre Lake beds with earliest Chron C5Cr (16.4-17.3 Ma). In addition, a number of other early Barstovian magnetostratigraphic sections with Proboscidea are reviewed, and quite a few yield fossils that date between 15.8 and 16.2 Ma. Our analysis of faunal data suggest that these early occurrences are simply the first places colonized by the immigrating proboscideans and not marked by a unique ecological or taphonomic history.