AN ANALYSIS OF PUMICE-BEARING SOIL HORIZONS NEAR PROSPECT, OREGON: IMPLICATIONS OF POTENTIAL TRANSPORT FROM MT. MAZAMA

Pumice clasts up to 4 cm diameter have been collected from well-developed volcaniclastic soils within the northern extent of the Prospect South, Oregon 1:24,000 USGS quadrangle. These soils 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.

These soils were sampled from an arcuate quarry exposure 3-4 meters in height. The bulk chemistry and vesicularity of the included clasts correspond well with those previously attributed to the climactic eruption of Mt. Mazama. Further, the broadly mafic composition of magmas throughout the surrounding region (almost exclusively basaltic to basaltic-andesite) would seem to make a local origin for these clasts unlikely. Notably, these samples were collected approximately 40 km south of Mount Mazama; previous work (e.g. Klug, et al., 2002; Young, 1990; etc.) has documented a far-more-limited extent for pumice originating directly from Mt. Mazama. 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.