This abstract and its related poster was presented at the AGU Fall Meeting in San Francisco, December 15-19, 1996.
Click here for link to online version of the poster.
The caldera of Newberry Volcano in central Oregon has been the site of several silicic and mafic eruptions during the past 10,000 yr. Presumably, these eruptions were accompanied by episodes of rising, falling, and tilting of the caldera floor. We report evidence that the floor of the caldera near its center has been rising for part or all of the past two millennia.
During the summer of 1994, we discovered evidence indicating that the channel of Paulina Creek, which begins as outflow from Paulina Lake, endured a large flood sometime after the eruption of Mt. Mazama (6845 C-14 yr ago). All Mazama ash had been swept away, coarse gravel deposited, and bedrock channels eroded. The flood may have taken place about 1800 C-14 yr ago, based on an archaeologic excavation. Failure of a rock ledge at the lake's outlet probably caused the flood and allowed the lake's surface to drop up to 1.5 m.
We identified wave-cut terraces along the southwest and east shores of Paulina Lake that were abandoned when the lake's surface suddenly dropped during the flood. Following the flood, some terraces were tilted and displaced vertically upward with maximum displacement of 4 to 6 m at Little Crater Campground in the southeast corner of the lake. The average uplift rate of the campground terrace has been between 2 and 3 mm/yr. At the outlet in the southwest corner, little or no displacement has occurred, and in the northeast corner a wave-cut bench may have subsided significantly. To determine if uplift is continuing in modern times, data from at least three series of benchmarks (1931-1994), which completely transect Newberry Volcano and its caldera, should be examined.
Implications of these findings are important to current and future studies of volcanic and hydrologic hazards, archaeology, geothermal resources, knowledge of caldera unrest, and the geologic history of the caldera.
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