MONO LAKE EXCURSION IN MONO BASIN, CALIFORNIA, AND AT CARSON SINK AND PYRAMID LAKE, NEVADA

A restudy of the Mono Lake Excursion in the Mono Basin, California, using two new localities 20 km apart, shows that the palaeomagnetic record in the basin is the most complete and accurate for the entire excursion. The excursion is also recorded in exposed sediment from Pleistocene Lake Lahontan in northwestern Nevada, about 250 km north of the Mono Basin. Sampling of the Lake Lahontan sediment was done at Pyramid Lake and in the Carson Sink. Following the initial discovery of the Mono Lake Excursion by Denham & Cox (1971), Liddicoat & Coe (1979) showed that the excursion consists of two parts split by a volcanic ash bed. At Pyramid Lake and in the Carson Sink, the older half of the excursion is not recorded, but the younger half can be recognized. Two factors appear to have contributed to the partial recording of the excursion at Pyramid Lake and in the Carson Sink: (1) the relative strength of the palaeomagnetic field (low during the older half of the excursion, and high during the younger half), and (2) the slower sedimentation in Lake Lahontan, estimated to be about half the rate of deposition for the lacustrine sediment in the Mono Basin. Tephrochronology and carbon-14 dates indicate an age of 28 000 yr for the middle of the Mono Lake Excursion. On the assumption that the sedimentation rate was a constant 25 cm kyr-1 in the Mono Basin, the span of the full excursion is about 2000 yr.