LATE PLEISTOCENE DROPSTONE RECORD OF THE WILSON CREEK FORMATION, MONO BASIN, CA

The levels of closed lake basins of western North America’s Great Basin are known to have increased substantially during the Quaternary glacial intervals. At Owens lake CA, published evidence suggests a general coincidence between Sierra Nevada glacier extent and higher lake levels. In the Mono Lake basin oversized clasts are preserved in the exposed deep lake sediments of the Wilson Creek Formation, interpreted by previous workers as ice-rafted dropstones. If they were dropped by icebergs, the presence of dropstones in the lacustrine sediments requires a coincidence between extreme glacial advance and lake level.

The Wilson Creek Formation has 19 ash layers that allow detailed correlation around the Mono basin (Lajoie, 1968, unpub. Ph.D. thesis, UC Berkeley). Eighty nine 5 cm intervals, each 20 meters long, were surveyed between stratigraphic markers Ashes 7 (~25 ka) and 19 (45 The highest abundance of dropstones (maximum number counted is 16 per 5cm level) is concentrated in the lowest 0.75m of the section, after which the frequency declines to zero by 1.3m; abundance averages 5 stones per 5cm level from 1.5 to 2.85m, followed by an average of 1 per 10cm level to Ash 7 (4.45m). The dropstone-rich level at the base of the section is likely of late Younger Tahoe age, consistent with geomorphic observations that the lake shore was relatively close to the terminus of at least one of the valley glaciers at that time (Putman, 1949, GSAB) and with stratigraphic evidence for lake level elevation (Lajoie, 1968). The upper part of the studied section is probably of Tioga age, and the lack of dropstones in that level supports the interpretation of relatively lower lake levels and smaller lateral moraines during the Tioga glaciation (Putnam, 1950, GSAB).