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18-Prothero Et Al (Massacre).P65 Lucas et al., eds., 2008, Neogene Mammals. New Mexico Museum of Natural History and Science Bulletin 44. 239 MAGNETIC STRATIGRAPHY OF THE MASSACRE LAKE BEDS (LATE HEMINGFORDIAN, EARLY MIOCENE), NORTHWEST NEVADA, AND THE AGE OF THE “PROBOSCIDEAN DATUM” IN NORTH AMERICA DONALD R. PROTHERO1, EDWARD BYRD DAVIS2 AND SAMANTHA S.B. HOPKINS2 1 Department of Geology, Occidental College, Los Angeles, CA 90041; 2 Department of Geological Sciences, University of Oregon, Eugene, OR 97403 Abstract—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 produced a single proboscidean tooth fragment. Magnetic stratigraphy was conducted on these beds, which 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 suggests that these early occurrences are simply the first places colonized by the immigrating proboscideans and not marked by a unique ecological or taphonomic history. INTRODUCTION Barstovian occurrences of gomphotheres in Ixtapa, Chiapas, Mexico, as In their landmark review of the late Oligocene to early Pliocene well as the North Coalinga specimen mentioned above. “At their first (Arikareean through Hemphillian) mammalian faunas of North America, occurrence, the Proboscidea are too rare to be used as a defining taxon for Tedford et al. (1987, p. 188) noted that the first appearance of Proboscidea the beginning of the late Hemingfordian, and their actual first appearance in North America seemed to mark the beginning of the late Barstovian. probably is late in the span, but it may be useful to include Zygolophodon Despite their large size and relatively easy fossilization, Proboscidea as part of the characterizing fauna of the early Barstovian and to regard were conspicuous in their absence from the richly fossiliferous early the midcontinent appearance of gomphotheriids as an event defining the Barstovian Olcott fauna of Nebraska, and the early Barstovian Pawnee beginning of the late Barstovian” (2004, p. 214). They also noted (2004, Creek fauna of Colorado. As Tedford et al. (1987, p. 168) noted, “despite p. 215) that “this proboscidean datum is reasonably isochronous in the intensive large-scale collecting in the many Olcott channel fills, no re- majority of instances and we will continue to regard it as the most useful mains of Proboscidea have been found.” At that time, no evidence of way to define late Barstovian time.” Proboscidea was known from the early Barstovian of the type Barstow Thus, the validity of the Proboscidean Datum has become less Formation in California, either. However, Tedford et al. (1987, p. 162) well supported as new data emerge. As the above list shows, early did note the presence of Proboscidea in probable early Barstovian faunas Barstovian occurrences are the norm, rather than the exception, and only from Oregon. two major regions still lack early Barstovian proboscideans: the Great The presence of Proboscidea thus became one of the key criteria Plains and New Mexico. Thus, it is worthwhile to review the for distinguishing early and late Barstovian faunas. Woodburne and Swisher chronostratigraphy of most of these early proboscidean occurrences, 40 39 (1995, p. 347) referred to this late Barstovian occurrence as the “Probos- using magnetic stratigraphy and Ar/ Ar dating wherever possible, and cidean Datum.” However, they also noted that Proboscidea were known refine the calibration of these various “Proboscidean Datums” across from the late Hemingfordian Massacre Lake fauna, as well as the early North America. For this reason, the detailed calibration of the earliest Barstovian of Florida (Bryant, 1991). Consequently, they indicated occurrence, the Massacre Lake l.f., is crucial to evaluating the time sig- (Woodburne and Swisher, 1995, fig. 4) a latest Hemingfordian Probos- nificance of the Proboscidean Datum. cidean Datum for the northwestern and southeastern United States, and GEOLOGIC SETTING a late Barstovian Proboscidean Datum for the interior of North America. They noted (p. 347) that “this event is subject to the vagaries of biofacies The most comprehensive work on the Massacre Lake l.f. is and should be used with extreme caution.” Morea’s (1981) unpublished dissertation, but some of the important Tedford et al. (2004, p. 215) updated the list of pre-late Barstovian taxa from his description of the fauna have been published (e.g., Morea proboscidean occurrences. They include not only the late Hemingfordian and Korth, 2002; Baskin, 2003). Morea (1981) presents a stratigraphic at Massacre Lake and early Barstovian of Florida mentioned earlier, but framework for the fauna, which is all preserved in a layer of yellowish also proboscidean trackways (but no bones yet) that occur in the early brown silty tuff that crops out on the opposing sides of two hills sepa- Barstovian of the type Barstow Formation (Reynolds, 1999; Reynolds rated by a drainage 10 km west of Massacre Lake in northwest Nevada and Woodburne, 2001). In addition, Tedford et al. (2004) listed the fol- (Fig. 1). Most of the specimens are preserved in silica-cemented nodules lowing early Barstovian occurrences of Proboscidea: North Coalinga that must be painstakingly prepared by hand. Careful preparation re- local fauna and Sharktooth Hill local fauna, California; Virgin Valley and veals teeth, bones, and some relatively complete skulls and jaws of small High Rock lake sites, northwestern Nevada; Mascall, Sucker Creek and animals and many fragmentary skeletal elements of larger animals. Among Skull Springs, southeast Oregon; Deep River and Madison Valley beds, the specimens collected from this locality is a single fragmentary loph of Montana; Burkeville l.f., Texas Gulf Coast; and Willacoochee Creek a proboscidean tooth, UCMP 75606, loc. V6161 (Fig. 2), collected by Fauna, Florida. Tedford et al. (2004, p. 214) noted that most of these Woodburne and a party from UC Berkeley in 1961. This tooth has been fossils, where identifiable, appeared to be of the mammutid Zygolophodon, tentatively assigned (Morea, 1981) to Zygolophodon on the basis of its and not of gomphotheres. However, they also noted apparent early small size, low crown height, and the apparent lack of conules in the 240 FIGURE 2. Proboscidean tooth fragment from Massacre Lake, UCMP 75606, loc. V6161. Scale = 1 cm. resistant agglomeratic ledges that marked the base of Morea’s (1981) unit 4, and correspond to UCMP V6160 and RV7043, the main fossil mam- mal localities. Another 10 m of section was covered and could not be sampled. The section is capped by the light gray rhyolitic volcanic ash- flow tuff that yields the 40Ar/39Ar date of 16.474 Ma (Swisher, 1992; but given as 16.2 Ma by Woodburne and Swisher, 1995). This same ash was originally was K-Ar dated at 15.6 Ma (Evernden et al., 1964). Samples were collected as oriented block samples in the field and subsampled with a drill press in the lab to produce a core. A minimum of FIGURE 1. Index map showing the location of the Massacre Lake fossil three samples was collected at each site, although site 8 had 5 samples localities and magnetic section in northwestern Nevada. (Table 2). Poorly consolidated samples were hardened with sodium sili- cate in the field. Samples that were too fragile to drill were molded into valleys between lophs. This is consistent with most of the early Barstovian cylinders using Zircar aluminum ceramic and dried in the magnetically faunas that contained Proboscidea. The remainder of the Massacre Lake shielded room. Samples were measured on the 2G cryogenic magnetom- l.f. is late Hemingfordian in character (Table 1). A number of previously eter at Occidental College, using a Caltech-style automatic sample changer. unrecorded species are present in the fauna, and currently appear to be After measuring NRM (natural remanent magnetization), each sample endemic to this area. However, Massacre Lake is the only late was treated with alternating field (AF) demagnetization at 25, 50, 75, and Hemingfordian mammal fauna known from the Great Basin, so these 100 Gauss to determine the coercivity behavior of the sample, and to endemics may not be truly site-specific, but instead reflect the faunal demagnetize any multi-domain grains before their remanence is baked in. character of the whole Great Basin at that time. A comprehensive re- Every sample was then thermally demagnetized at 50°C steps from 200° evaluation of the fauna reflecting the broader perspective gained in the 25 to 630°C to examine the demagnetization behavior in detail. This process years since its initial description would be an important contribution to removes any chemical remanent overprints due to iron hydroxides such the paleontology literature. as goethite (which dehydrates at 200°C), and shows how the samples In his unpublished dissertation, Swisher (1992) 40Ar/39Ar dated behaved as the Curie temperatures of magnetite (578°C) and hematite an ash flow tuff, referred to by Morea (1981) as the “light gray rhyolitic (630°C) were approached. ash flow tuff,” or “Unit 5” and by Swisher as the “Tuff of Big Basin,” at Results were graphed on orthogonal demagnetization (“Zijderveld”) 16.474 ± 0.035 Ma. This ash layer lies stratigraphically just above the plots (Fig.
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