2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 13
Presentation Time: 1:30 PM-5:30 PM


CLARK, Peter U.1, BROOK, E.J.2, RAISBECK, G.3, YIOU, F.3 and CLARK, J.4, (1)Geosciences, Oregon State Univ, Corvallis, OR 97331, (2)Geology and Environmental Science, Washington State Univ, Vancouver, WA 98686, (3)CSNSM, Orsay, France, (4)U.S. Fish and Wildlife, Portland, OR, clarkp@ucs.orst.edu

In northern Labrador, classic weathering zones have long been interpreted to delineate successively limited glaciations of the Torngat Mountains by the eastern margin of the Laurentide Ice Sheet. In a preliminary study we used 10Be exposure ages for boulders and bedrock at the limit of, and immediately above, the youngest weathering zone to examine the timing of glaciation. Throughout much of the Torngat Mountains, large lateral moraines (the Saglek Moraines) occur at the break between the lowest zone and the next highest zone. We collected samples associated with these moraines in two regions: Nakvak Lake (58°38.9'N, 63°43.9'W) and an unnamed lake at 59°18.2'N, 64°22.8'W. Exposure ages of boulders (n=8) in moraines marking the limit of the youngest (Saglek) weathering zone at Nakvak Lake range from 12.6-20.2 ka, with one poorly determined outlying age of 43.7±13.4 ka. The uncertainty-weighted mean age is 14.1±1.4 ka (±1 sigma). At the unnamed lake, boulder ages in similar moraines range from 9.4-15.4 ka, and the mean age (n=5) is 12.8±1.9 ka. Ages for erratic boulders and bedrock 5-50 m higher in elevation than the Saglek weathering limit at both locations are similar or slightly higher than the moraine ages. At Nakvak Lake the age range for these samples (n=7) is 11.6 - 32.0 ka, with a mean age of 13.2±1.3 (excluding two old outliers). Similar samples at the unnamed lake (n=6) have an age range of 9.7-38.2 ka, with a mean age of 12.1±1.6 (excluding one old outlier). All ages were calculated using a sea level production rate of 5.1 at/g/yr (Stone et al., in prep.). In contrast to some previous suggestions, the exposure ages for the Saglek level moraines appear to postdate the global LGM (~21 ka). LGM ice may have been thicker, with the Saglek level representing a recessional phase of the deglaciation that occurred during the Bolling interstadial. This hypothesis is supported by the facts that most of the exposure ages immediately above the Saglek level are not significantly different than those for the Saglek moraines, and the oldest ages do not substantially predate the LGM. Alternatively, ice thickness may have been greatest at ~13-14 ka, but the ice margin position may have fluctuated sufficiently to create a vertical zone of ~ 50 m with approximately the same exposure history. These hypotheses can be tested with further work at higher elevations.