title: Holocene glaciation of the Green River drainage, Wind River Range, Wyoming
Creators: name: Davies, Nigel
Publisher: name: Western Washington University
Year: 2011

description: <p>The Wind River Range (WRR) has long been the focus of glacial investigations, yet the Holocene record remains poorly understood. Moraines in the Green River Lakes drainage, on the northwest end of the Wind River Range, preserve a remarkably complete moraine record of late-Pleistocene recession, late-glacial and late-Holocene advances. At last glacial maximum (LGM) the study area supported large valley glaciers that extended beyond the rangefront; in historic times, however, glaciers are restricted to high alpine cirques. The largest remaining active glacier (Mammoth Glacier) has retreated to 2 km2 and is the primary source of meltwater and outwash to the Green River lakes. In addition to the active glaciers and pro-glacial lakes the study area contains abundant rock glaciers, glacially sculpted and polished bedrock, and post-Pleistocene glacial deposits. To assess the timing and magnitude of late-Holocene glaciation in the Green River Lakes drainage this projected used a combination of geomorphic mapping, lake coring and assessment of equilibrium line altitudes. As part of the funding through the United States Geologic Survey (USGS) EDMAP program, a product of this project was a 1:24,000 scale surficial geologic map of Green River Lakes drainage centered on the Gannett Peak Quad, and incorporating portions of Squaretop, Green River Lake and Dow Mountain Quads. The methods and units I used in this map follow the protocols of a similar mapping effort in the North Cascades National Park and other national parks in Washington (Reidel et al., 2007). I recovered twelve sediment cores from Upper and Lower Green River Lakes; these lakes trap the majority of the rock flour produced by the Mammoth Glacier. As a result, there is a continuous record of late Holocene sedimentation in the lakes that serves as a proxy record of glacial activity. Of the eleven cores recovered from the Upper Green River Lake, I focused analyses on the longest Livingston core (3.65m) and one shallow, high-resolution Glew core (75cm). The depth-age model for these cores are based on six AMS radiocarbon analyses (ranging from 430 - 4245 cal. yr B.P.) and 210Pb dates from the upper-most portion of the Glew core. The one high resolution Glew core (67 cm) from the Lower Green River Lake has a depth-age model that relies on two statistically indistinguishable AMS radiocarbon analyses (3060-3160 cal. yr B.P.). Rock flour input from Mammoth Glacier is recorded in visual stratigraphy (VS), organic content (OC), magnetic susceptibility (MS), and grain size distribution (GSD) in the Upper Green River Lake cores. The rock flour flux appears to have increased significantly shortly after ~1000 yr B.P., and rapidly after ~500 yr B.P, culminating at ~280 yr B.P. The increase in rock flour after 1000 yr B.P. likely records the rapid growth of Mammoth Glacier at the onset of the Little Ice Age (LIA). A similar rock flour signal is observed via VS, OC, MS and GSD in the Glew core from Lower Green River Lake at ~300 yr B.P.; however, the signal is muted compared to that in the upper lake. The LIA in the region (Schuster, 2000) is the largest Neoglacial advance as recorded in the moraine sequence and lacustrine stratigrarphy. The cores from Upper Green River Lake record the climatic LIA maximum at 280 yr B.P. A small, secondary peak in rock flour flux is recorded by a distinct green-gray sediment dated at ~260-240 yr B.P. This sediment appears to relate to an outburst flood from Scott Lake, a tarn immediately downstream of the Mammoth Glacier. Scott Lake's high water stand (10 m above modern shoreline - recorded by the "bathtub ring" of oxidation and the stranded delta), was stable through most of the Holocene. The breach may have been associated with an outburst from Mammoth Glacier, possibly induced by the draining of a side-glacial lake. Multiple ponding locations have been revealed by subsequent ice retreat from LIA maximum. The additional flux from Mammoth Glacier, produced a catastrophic debris flow into Scott Lake causing water to overtop the sediment dam. The down cutting of the dam initiated the breach, flooding the valley with ~ 2,000,000,000 m3 of water, suspended sediment, and debris. The coincidence of the outburst with the maximum rock flour flux suggests a causal link between the flood and the LIA maximum of Mammoth Glacier. The pre-LIA rock flour record in the Green River lakes is less clear, but consistently high MS and low OC values in sediments deposited between 1000-4500 yr B.P. suggests that the Mammoth Glacier was active but significantly smaller than its historical extent through much of that period. The Upper Green River Lake cores do not indicate any period in the last 4500 years when the Mammoth Glacier was absent entirely. To constrain temperature and precipitation conditions associated with the LIA and late-glacial glacier maxima, the glaciers of Green River Lakes drainage were modeled using end moraines, interpreted cirque headwalls and till coverage. These reconstructions enabled an assessment of the equilibrium line altitudes (ELA) which vary between 3050 m (late-Pleistocene -THAR) to 3610 m (modern - Mammoth Glacier - AAR). Using SNOTEL, PRISM and imported lapse rates from Colorado (Brugger and Goldstein, 1999) the best estimate of modern climatic conditions (average summer temperature and winter precipitation) at the modern Mammoth Glacier ELA are 3.78 oC and 68.48 cm SWE. Calculated conditions at the paleo-ELAs [LIA: (SWE); late-glacial: (SWE)] suggest only minor cooling and increased precipitation was required to cause these advances, consistent with Leonard's (2007) inference of the sensitivity of WRR glaciers to climate change.</p>