Temporal correlation of U. S. Great Basin lake sediments below the Mono Lake Excursion using paleomagnetic secular variation

Abstract:

Beginning nearly 50 years ago with a paleomagnetic study of exposed lacustrine sediments in the Mono Basin, CA (Denham and Cox, 1971), there have been subsequent studies to document paleomagnetic secular variation (PSV) in the basin and to establish a chronology for that record (Vazquez and Lidzbarski, 2012). We report a paleomagnetic secular variation (PSV) investigation of lacustrine sediments in the Mono Basin, CA, that extends the base of the PSV record of Lund et al. (1988) by about 20 percent. We did our investigation at two localities separated by about 4 km on the southeastern and eastern sides of Mono Lake: South Shore Cliffs (SSC) and Warm Springs (WS). The sampled interval at SSC is from 0.1 m above to 2.2 m below Wilson Creek Ash 19 in the tephrostratigraphy of Lajoie (1968), ending in loose sand. At WS, we sampled from Ash 17 to 1.0 m below Ash 19, a total of 2 m.

At SSC using back-to-back horizons 2-cm thick containing one to three samples each that were a.f. or thermally demagnetized, we found rapidly fluctuating PSV in the interval from ~ 0.3 to 1.0 m below Ash 19. The fluctuating PSV contains a change in declination of ~ 80˚ from 308˚ (n = 3, α-95: 6.1˚) to 29˚ (n = 3, α-95: 11.5˚) within a single hand sample that spans 14 cm. Inclination during that change in declination gradually rose from 56˚ to 63˚ and increased to 70˚ before reducing to a minimum of 29.9˚. The path of the Virtual Geomagnetic Poles when the declination is most westerly forms a narrow loop that reaches 49.7˚ N latitude near 170˚ E longitude. At WS the westerly swing in declination is absent, but the easterly declination and relatively steep inclination described above are recorded.

A study of the relative paleomagnetic intensity (RPI) shows that the maximum RPI is ~1.5 m below Ash 19 and decreases to a minimum ~6 cm above the ash. Distinct PSV and RPI features below the Mono Lake excursion correlate well between records from the periphery of Mono Lake and those from Pyramid Lake, NV (Benson et al., 2008) and Summer Lake, OR (Negrini et al., 2014). Also, the corresponding features in the Pyramid Lake record occur above the Laschamp Excursion. This supports the hypothesis that the sediments exposed around Mono Lake above the Wilson Creek Formation basal gravel layer and its equivalent sandy deposits are younger than the age of the Laschamp Excursion (~40 ka).