MONO LAKE EXCURSION IN THE LAHONTAN BASIN, NV

There are reports beginning with Denham and Cox (1971) that the Mono Lake Excursion (MLE) is recorded in sediment and volcanic rocks (Guillou et al., 2010) at more than 20 localities worldwide. In western North America, the excursion is present in exposed lacustrine sediments in the Mono Basin, CA (Liddicoat and Coe, 1979; Liddicoat, 1992; Coe and Liddicoat, 1994), Lahontan Basin, NV (Liddicoat, 1992, 1996), and at Lake Chewaucan, OR (Negrini et al., 1984). Also in the Lahontan Basin, the excursion is in an 18-m core from Pyramid Lake (Benson et al., 2008).

In the Mono Basin, the MLE is recorded at several widely separated localities and consists of two parts, an older one of westerly declination and inclination that approaches negative 30˚ when the relative field intensity is reduced by nearly four fifths. During that part of the excursion, the Virtual Geomagnetic Poles (VGPs) followed from old to young form a large clockwise loop centered at about 70˚ N, 90˚ E. The younger part of the excursion is positive inclination of nearly 85˚ when declination moves rapidly to the east, forming a small counterclockwise VGP loop near the locality (Denham, 1974). The relative field strength returns to the pre-excursion value during the younger half of the excursion, and the entire excursion is estimated to last several thousand years. The Laschamp Excursion (LE) that preceded the MLE in the Pyramid Lake core (Benson et al., 2008) has not been found in the Mono Basin.

Because the MLE and LE are in the Pyramid Lake core, it is possible that both excursions are present in the bank of the wave-cut shoreline on the east side of Pyramid Lake near Anaho Island. I will report a new paleomagnetic investigation at that locality showing the position of the MLE and where the LE might be recorded.