POSSIBLE RECORDING OF THE MONO LAKE EXCURSION IN CORED SEDIMENT FROM CLEAR LAKE, CALIFORNIA

We report the possible recording of the Mono Lake Excursion (MLE) in cored sediment from Clear Lake, CA. The locality (39.0˚N, 237.3˚E) is about 120 km north of San Francisco, CA, and 320 km northwest of the Mono Basin, CA, where the MLE first was discovered in North America (Denham and Cox, 1971). The field behavior at Clear Lake that might be the MLE is recorded in clay and peaty clay about 50 cm below the top of the lowermost 80-cm core slug (number 21) of a 21.6-m core. The samples (rectangular solids 21 mm on a side and 15 mm high) were measured in a cryogenic magnetometer after demagnetization in an alternating field to 35 milliTesla (Verosub, 1977). When followed from old to young, continuously-spaced samples record negative inclination of nearly 20˚ and northerly declination when relative field intensity (unnormalized) was reduced by an order of magnitude from the mean value. Those directions are followed immediately by positive inclination to about 50˚ and easterly declination of about 60˚ when the intensity is at a relative high. That pattern of behavior is recorded at three localities in the Mono Basin at the MLE (Liddicoat and Coe, 1979; Liddicoat, 1992). A path of the Virtual Geomagnetic Poles (VGPs) at Clear Lake form a clockwise-trending loop that is centered at 65˚N, 20˚E. The VGP that is farthest from the North Geographic Pole is at 29.3˚N, 337.1˚E, which is close to the path formed by the VGPs in the older portion of the MLE (Liddicoat and Coe, 1979; Liddicoat, 1992). The age of the sediment recording the anomalous paleomagnetic directions in Clear Lake slug 21 is about 30,000 years B.P. (Verosub, 1977). That age was determined from six (uncalibrated) radiocarbon dates, three of which are from near the base of the core (Sims and Rymer, 1975), and linear extrapolation between those dates and the three dates from higher in the core. The age of the possible excursion recorded in the Clear Lake sediment is in reasonable agreement with the age assigned to the MLE by Benson et al. (2003; 2008) and is nearly 10,000 years younger than the Laschamp Excursion (Guillou et al., 2004). We believe that this is a further indication that the anomalous field behavior known as the Mono Lake Excursion in the Mono Basin (Denham and Cox, 1971; Liddicoat and Coe, 1979) and possibly at Clear Lake is not the Laschamp Excursion.