EVIDENCE FOR AT LEAST FIVE GLACIAL ADVANCES FROM MIDDLE TO LATE PLEISTOCENE IN THE NORTHERN SIERRA NEVADA MOUNTAINS: MOHAWK VALLEY, NORTHEASTERN CALIFORNIA

The northernmost Sierran ice cap extended northward into Mohawk Valley, where a long-lived lake occupied the basin. Outlet valley glaciers deposited moraines and outwash from more than 5 different glacial advances ranging in age from > 510-610 ka to < 26 ka. Short, coarse-grained outwash terraces and ice-contact delta plains extend from moraines basinward and presently grade up to 190-m-above modern base level transitioning basinward to delta sequences. Moraines and outwash deposited during the oldest three or more glacial advances extended into Mohawk Lake. After 175-235 ka, incision responding to the lowering of Mohawk Lake cut through and exposed the glacial deposits.

We generally characterized at least five generations of glacial deposits, some of which are only found in buried positions. The older three or more generations are distinctly oxidized (7.5YR to 2.5YR) throughout the exposed deposits (up to 30 m) and differ from each other by a hue. The oldest outwash is buried and grades below a 510-610 ka tephra bed. The youngest oxidized outwash overlies the same tephra bed and is < 510-610 ka and, based on the lake history, could be < 175-235 ka. An associated moraine has an A/Bt1-5/Btqm1-2 soil profile with >1.3-m-thick argillic horizon over > 80 cm of silica horizons.

The youngest two generations of glacial deposits are < 175-235 ka and both lack distinct oxidation (10YR to 2.5Y). The older of the two has better developed soils, with an A/Bw1-2/Bt1-2/Bqmt1-4/2Bqm/Cq soil profile, a 1-m-thick argillic, and 1.6 m of silica horizons. The youngest outwash deposits overlie a 26 ka tephra bed and have an A/Bw1-3/Cox soil profile with a 50 cm cambic horizon.

Based on our mapping, there are differences in the relative extents of moraines, a proxy for relative magnitude of climatic change. The lake, however, limited the downstream advance of ice during earlier advances complicating comparisons. The most extensive glacial deposits are associated with the highest lake levels. After the lake began to spill westward and lower its sill, subsequent glacial advances were less extensive. Based on the relative extents of moraines and our present understanding of their ages, this record is not necessarily similar to other glacial records in the Sierra Nevada. It may be that local effects from Mohawk Lake overprinted the regional climatic signal.