MAPPING OF DISCONTINUOUS UNITS OF PYROCLASTIC PUMICE AND TUFF SOUTH OF MT. MAZAMA, OREGON

Within the Prospect South and Big Butte Springs, Oregon 1:24,000 USGS quadrangle, south of Mt. Mazama, samples have been collected of pumice clasts ranging from < 1 cm to 15 cm diameter, and from notably-weathered deposits of andesitic-to-dacitic volcanic tuff containing xenoliths up to 5 cm in diameter. These explosive deposits are both overlain and underlain by mafic flows originating locally. Notably, while these deposits all have calc-alkaline bulk compositions, the majority of the adjacent lavas are all tholeiitic or very nearly-so. Although no radiometric dates have yet been determined for any of these adjacent mafic units, nor for the pumice or tuff units directly, other local mafic flows suggest a maximum age in the Pliocene or Pleistocene, with a most-likely origin from Mt. Mazama, to the north.

The key challenges in determining the present and former lateral extents of these units are their broad discontinuity from one another, and the geochemical and textural differences both among the samples, and relative to other regional pyroclastic deposits, such as those erupted from Mt. Mazama. It is unclear to what extent these deposits have been spatially-fragmented due to syn-eruptive factors, later covering by lava flows, or various erosive processes. Concurrently, while geochemical differences among the samples may suggest a lack of true lateral continuity among the pumice or tuff deposits, it is unclear to what degree these differences are, in-part, a result of later alteration or contamination from adjacent units. Through this study, several potential extents have been proposed, with potential deposit volumes ascertained from 3D analyses presuming the respective presence or absence of original lateral continuity, and the singular or multiple eruptions responsible for producing these various pyroclastic materials.