Origin of Remanence Recording the Mono Lake Excursion In the Mono Basin, CA

Silt, sand, and volcanic ash deposited in the Mono Basin, CA, in Lake Russell is exposed on the margin of Mono Lake, which is the remnant of Lake Russell, and on Paoha Island in the lake. The silt records the Mono Lake Excursion (MLE)(Denham and Cox, 1971;Liddicoat and Coe, 1979) and several tens of thousands of years of paleomagnetic secular variation (Liddicoat, 1976; Lund et al., 1988).

The paleomagnetic directions and relative field intensity during the MLE are westerly declination (about 290) and negative inclination (about -30) during reduced intensity that are followed by easterly declination (about 100) and steep positive inclination (about 85) during high intensity at eight of 11 localities around Mono Lake. The three exceptions are at wave-cut cliffs on the east side of the lake where the westerly declination and negative declination are absent (Coe and Liddicoat, 1994). On Pahoa Island, the full excursion is recorded at one locality.

We explore reasons for the absence of part of the MLE at the wave-cut cliffs beyond the interpretation of Coe and Liddicoat (1994) that field strength is a controlling factor. Possibile reasons include the sedimentation rate -- at localities on the margin of Mono Lake the sedimentation rate is about 60 percent less than at the wave-cut cliffs, whereas on the island it is about 1.6 greater than at the wave-cut cliffs where the older half of the excursion is partially overprinted -- and lithology of the sediment. At Mill Creek on the northwest side of Mono Lake, the non-magnetic sediment fraction is coarser-grained than at the wave-cut cliffs by a factor of about two -- mean grain size 6.31 mm versus 3.70 mm, respectively -- and there is a similar difference in the calcium carbonate percentage by weight for the two localities (11.81 versus 6.56, respectively).