Submerged Tioga and Tahoe age Moraines at Meeks Bay, Lake Tahoe, Calif. Implications to Late Pleistocene Lake Levels

Details Submerged Tioga and Tahoe age Moraines at Meeks Bay, Lake Tahoe, Calif. Implications to Late Pleistocene Lake Levels Submerged Tioga and Tahoe age Moraines at Meeks Bay, Lake Tahoe, Calif. Implications to Late Pleistocene Lake Levels

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufmpp21c1595h&link_type=abstract

American Geophysical Union, Fall Meeting 2005, abstract #PP21C-1595

Physics

Geophysics

3000 Marine Geology And Geophysics

Scientific paper

At Meeks Bay, a well-preserved right-lateral morainal complex is constructed of till from the Tioga (>20.4 +/- 0.7 ka B.P.) and Tahoe (>62.0 +/- 1.3 ka B.P.; Howle et al., 2005) glacial advances. High-resolution digital bathymetry merged with the terrestrial DEM reveals that the onshore Tioga and Tahoe moraines extend offshore below the modern lake level of 1,899m. Longitudinal profiles show that the sub-aerial moraine crests grade evenly with the submerged crests and that the slopes of the sub-aqueous crests match the profile of the adjacent submarine canyon. In the digital bathymetry, the Tioga and Tahoe moraines are recognizable to 1,844 and 1,768m respectively. In May of 2004 and 2005 the moraines were imaged with the remotely operated submersible Triton deployed from the UC Davis research vessel, John LeConte. The Tioga moraine goes down to 1,817m, and rests upon Pliocene (?) lacustrine sediments. Above 1,817m the moraine is comprised of unsorted gravel, cobbles, and angular granitic boulders up to 3m across, typical of the Tioga age till on land. The moraine crest is continuous from 1,817m up to the shoreline promontory where the Tioga till on land enters the lake. Between 1,868m and 1,838m, the submerged Tahoe moraine contains well-rounded granitic boulders up to 2m across. The degree of rounding is consistent with the Tahoe boulders on land. The lowest occurrence of the Tahoe moraine was not imaged, but the glacial origin of the crest was confirmed. The submerged Tioga and Tahoe lateral moraines place upper limiting constraints on lake elevation, because lateral moraines do not form in water deeper than the ice thickness. Instead, when the relatively thin terminal ice cliff of an ablating alpine glacier interacts with water the glacier either melts, disintegrates by calving, or detaches along crevasses and floats when submerged to about 0.9 of the ice thickness. Reconstructions of ice thickness at the lower limits of the Tioga and Tahoe moraines here yield estimates of the maximum lake elevations at the glacial maxima. During the Tioga and Tahoe glacial maxima (>20 and >62 ka B.P., respectively) the lake elevation was <1,810m and <1,765m. These data indicate that in the Tahoe basin, late Pleistocene lake low-stands occurred during the glacial maxima. Asynchrony between glacial and lacustrine maxima has also been documented at Pleistocene lakes Russell and Lahonton (Lajoie and Robinson, 1982). Evidence at Meeks Bay for a post-Tahoe and pre-Tioga high-stand between 1,914 and 1,920m is provided by a gently sloping bench cut into the Tahoe moraine. This correlates well with reports of a Tahoe age high-stand at 1,926m. Birkeland (1963) proposed a Tahoe age ice dam at that may have raised lake levels to about 1950m. However, there is no evidence of this at 1950m in the Tahoe moraine at Meeks Bay, and therefore any such high-stand must have predated the Tahoe maximum. Available data suggests that large fluctuations in lake elevation have occurred from a mid Pleistocene (?) high-stand of about 2073m (Birkeland, 1963) to <1,765m at the Tahoe glacial maximum, up to about 1,920m after the Tahoe advance, down to <1,810m at the Tioga glacial maximum, and finally up to elevations between 1876 and 1899m during the Holocene (Schweickert et al., 2000). We thank the IEEE Oceanic Engineering Society for partial funding and commend the engineering students of Santa Clara University for designing, building, and deploying the Triton. Special thanks to Bob Richards and Brant Allen, captains of the LeConte.

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