AN ANALYSIS OF PUMICE-BEARING SOILS AND DEPOSITS NEAR PROSPECT, OREGON: IMPLICATIONS FOR POTENTIAL LOCAL VS. TRANSPORTED ORIGINS

Pumice clasts ranging from < 1 cm to 15 cm diameter have been collected from well-developed volcaniclastic soils and weathered deposits within the Prospect South and Big Butte Springs, Oregon 1:24,000 USGS quadrangle, at least 40 km south of Mt. Mazama. These deposits are both overlain and underlain by mafic flows originating locally. The clasts are aphyric, with a calc-alkaline bulk composition, while the compositions of the three adjacent lavas are all tholeiitic or very nearly-so. While no radiometric dates have yet been determined for any of these three units, nor for the clasts directly, other local mafic flows suggest a maximum age in the Pliocene or Pleistocene.

The soils were sampled from an arcuate quarry exposure 4-5 meters in height, while other deposits were presented as road cuts or adjacent to logging roads. The bulk chemistry and vesicularity of the clasts have been analyzed, and broadly correspond with those previously attributed to the climactic eruption of Mt. Mazama, though there is some textural heterogeneity among the samples. While the lavas throughout the surrounding region are primarily mafic, there are also massive tuff deposits found still further south, making a local origin for these clasts possible, but uncertain. Previous work (e.g. Klug, et al., 2002; Young, 1990; etc.) has documented a limited extent for pumice originating directly from Mt. Mazama, including the pumice desert to the north. While the highly-variable terrain between Mt. Mazama and this sampling location would seem to preclude direct transport by lahars or fluvial systems, this origin may not be fully discounted. An origin from Mt. Mazama would not only expand the field of available samples to assess the climactic eruption, but would suggest a far more-recently active eruptive history of the region south of Prospect, Oregon than has been determined by prior studies.