GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 55-7
Presentation Time: 3:20 PM


SMITH, Eugene, ROWLAND, Stephen M., JOHNSEN, Racheal, REN, Minghua and FITCH, Shelby, Department of Geoscience, University of Nevada, Las Vegas, 4505 S. Maryland Parkway, Las Vegas, NV 89154-4010

Cryptotephra are small glass shards (<80 microns) of volcanic origin that occur invisibly in sediments and can be used to create precise isochrons in geological deposits and archaeological sites. We discovered two peaks of cryptotephra from a 100-cm interval of Member D2 of the Las Vegas Formation in Tule Springs Fossil Beds National Monument just north of Las Vegas, Nevada. Shards are sparse (<10 shards/gram) and small (60-100 microns) and display blocky and cuspate shapes. Samples were processed in the Cryptotephra Laboratory for Archaeological and Geological Research at UNLV.

Major element chemistry by electron microprobe indicates that the shards are high-silica rhyolite (>75 wt. % SiO2) with FeO < 1 wt. %, providing a unique major element signature that correlates to Wilson Creek tephra that erupted from the Mono Craters in eastern California. The Wilson Creek section contains 19 tephra layers that are indistinguishable using major elements but have distinctive trace element signatures. The upper peak of D2 shards provided high quality trace element data by LA-ICP-MS at Michigan State University and display light rare-earth element (REE) enrichment, a deep Eu anomaly, and a relatively flat heavy REE signature when normalized to chondrite. This signature (especially high La, Ce and La/Sm) indicates that the shards correlate with either Wilson Creek tephra units 7 or 15 (WC-7, WC-15) dated at 26.8+/-2.1 ka and 40.8 +/-1.9 ka respectively (238U-230Th isochron ages; Vazquez and Lidzbarski, 2012). The age of WC-7 is compatible with the age of D2 dated at 31.68 to 27.58 ka (Springer et al., 2017); therefore, we correlate the upper peak of D2 shards with WC-7. Chemistry is not yet available for the recently identified lower peak.

WC-7 has not been previously identified outside of the Mono Basin, but chemically it can be easily mistaken for WC-15, which is the most widely distributed of Mono Crater tephra. The discovery of this WC-7 in Tule Springs Fossil Beds National Monument greatly extends the fallout range of this tephra unit and suggests that it might be found at other sites in southern Nevada. The application of cryptotephra analysis to the Las Vegas Formation will allow for precise dating of fossiliferous horizons in this unit and among other Late Pleistocene localities.

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