Mineralogy, Correlation, and Grain-Size Distributions of Mazama Tephra and Other Postglacial Pyroclastic Layers, Pacific Northwest
Mazama air-fall tephra is a pumiceous pyroclastic deposit dispersed over almost a million square kilometers by an eruption about 7,000 yr ago of ancient Mount Mazama, the precursor of Crater Lake caldera, Oregon. The tephra layer is valuable as a stratigraphic marker and for what it reveals about the production and dispersal of tephra. The tephra is characterized by its minerals, which include plagioclase (73.1 ± 1.2 percent), hypersthene (9.4 ± 0.5 percent), magnetite (10.2 ± 1.1 percent), hornblende (3.9 ± 0.4 percent), and clinopyroxene (2.8 ± 0.3 percent). Mean refractive index of glass is 1.508 ± 0.001. Mineral abundances and refractive index are homogeneous, and over the region sampled the variances of these properties are no greater than in a single fragment of pumice. Within about 300 km of Crater Lake, the abundance of discrete mineral grains increases, but the proportion of heavy minerals does not change with increasing distance. Tephra at 14 localities can be correlated with Mazama tephra mineralogically. The localities are at Blue Mountains, Columbia Gorge, and China Hat Butte in Oregon; Curelom Cirque in Utah; and at archaeologic sites at Fort Rock Cave, Paisley Cave, Connley Cave, Three Sheep Rockshelter, Everyone, Hobo Cave, Wildcat Canyon, and Big Eddy in Oregon; and Night-fire Island in California. Other tephra layers in the region studied contain assemblages of minerals similar to the Mazama assemblage, perhaps because all have sources among volcanoes of the Cascade Range. This makes discrimination difficult, and no layers could be definitively correlated or identified with a specific source. Mazama tephra has heterogeneous size-frequency distributions that fit Rosin9s distribution. Heterogeneity is caused by intermingling of coarse pumiceous fragments and finer crystalline grains; however, if grain size is expressed as terminal velocity, then frequency distributions are log-normal and homogeneous; grains of disparate size or density occur together because they have equivalent terminal velocities. Phi median diameter varies curvilinearly with distance from Crater Lake, but the relation is not readily interpreted in terms of mechanism of deposition. Median terminal velocity, however, varies as V t * = K 3 · D − r , where V t * is median terminal velocity and D is distance. The parameters K 3 and r may characterize individual eruptions and facilitate understanding of the production and dispersal of air-fall tephra.