GIORGI, Sara E. and ROBERTS, Michelle A., Geology Department, Humboldt State University, Arcata, CA 95521, seg18@humboldt.edu

Paulina Lake is one of two lakes which occupy Newberry Caldera in east-central Oregon. Terraces on the east shore of Paulina Lake suggest that lake level has fluctuated in the past. This study attempts to delineate the depositional and erosional history of the highest terrace, approximately four meters above present lake level. On the basis of field properties and XRF and thin section analyses, twenty eight units were described from a hand dug pit to a depth of 209 cm, and an auger hole extending to a depth of ~570 cm. Sediments from 179 to 570 cm depth are interpreted to be of lacustrine origin. Laminations, along with an abundance of rounded sand grains, are believed to have resulted from near shore depositional processes. Twenty different sandy and gravelly lacustrine units were identified. Gravels are more prevalant from the bottom of the auger hole to a depth of 390 cm. Lake level changes are inferred from the changing environments of a gravelly wavebase (up to 75% gravels) to a sandy shore.

Above a depth of 179 cm are seven tephra units. Ages of these deposits may eventually be estimated by calibration of tephras to known stratigraphic units. The only tephra tentatively identified thus far is an air fall from the Central Pumice Cone, at a 72 cm depth, with a calibrated age of ~6900-7200 years (Jensen, 2000). The Central Pumice Cone air fall, along with the other 6 tephras, were likely deposited on dry land suggesting that the burried terrace surface at 179 cm is older than about 7000 years. The lower terrace, 1 to 2 m above lake level, has an age of ~4860 cal. years (Robinson and Trimble, 1983). A submerged shoreline, 12 to 15 m below lake level, is dated at ~7500 cal. years (Reynolds, 2000) suggesting a dramatic fluctuation in lake levels between ~7500 years and today's lake level. The elevation difference from the top of the buried lake sediments at 179 cm depth to the present day lake level is ~2.2 m. This drop in elevation is essentially equivalent to the 2.4 m lake outlet lowering since Mazama time as reported by Chitwood and Jensen (2000). Thus, their infered age of ~4860 cal. years is possibly too young by ~2000 years.

Cordilleran Section - 98th Annual Meeting (May 1315, 2002)
Session No. 30--Booth# 18
Pacific Northwest Geology East of the Cascades: In Honor of George W. Walker (Posters)
LaSells Stewart Center: Agriculture
1:30 PM-5:30 PM, Tuesday, May 14, 2002

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