LANDSCAPE MODIFICATION BY LATE-HOLOCENE CIRQUE GLACIATION IN THE SIERRA NEVADA AND NORTH CASCADES

Holocene cirque glaciers are significant geomorphic agents in high alpine regions of the western U.S., yet their effects on sediment production are largely unstudied. We attempt to constrain the erosion rates of two small glaciers in the Palisades (Sierra Nevada, CA) and the Enchantment Lakes (North Cascades, WA) by estimating rock flour production from lake coring and moraine mass from surface maps.

Palisade Glacier is the largest extent glacier in the Sierra Nevada; a detailed lake coring study of the rock-flour outflow from the glacier demonstrates that it first formed in the Holocene about 3200 cal yr B.P., then progressively grew through time, reaching maxima at ~2200, 1600, 700, and 200 cal yr. B.P. The last maximum was the largest advance of the Holocene, roughly coincident with the global Little Ice Age maximum. First-order estimates of sediment volume in the three paternoster tarns based on the core records indicate they contain roughly 72,000-96,000 m³ of Holocene rock flour from the glacier. Assuming a density of 1250 kg m^-3, this silt represents the equivalent of ~33,000-44,000 m³ of granite eroded from the cirque. Averaged over the glacierized area, this would amount to ~2.1-3.3 cm of surface lowering during the Holocene. The Neoglacial moraine, although voluminous, is largely ice-cored, and most debris in it appears to have originated as rockfall from the cirque headwall, and thus primarily represents headwall retreat rather than direct glacier erosion.

In the Enchantment Lakes basin, WA, sediment coring indicates glaciers there also formed ~3200 cal yr. B.P., and culminated in the last ~200 yrs. Estimates of sediments in six paternoster lakes suggest a total of ~70,000-100,000 m³ of Holocene rock flour, mostly from the Snow Creek Glacier. Averaged over the glacierized area, this amounts to ~2.9-4.1 cm of surface lowering. These findings represent a first cut at quantifying erosional effects of Holocene cirque glaciers in the western U.S.