Graduate Thesis Or Dissertation

Stratigraphy and genesis of soils from volcanic ash in the Blue Mountains of Eastern Oregon

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  • Investigations were conducted to (1) determine the original pattern of deposition and thickness of ash in the Ochoco and Blue Mountain areas of Oregon, (2) determine if discontinuities exist in the parent material of these soils, (3) determine the degree of mixing of ash with other materials and evaluate reworking or stability of volcanic ash bodies over the landscape since deposition, and (4) evaluate changes due to weathering and genesis of soils developed from Mazama ash. Primary emphasis was placed in three study areas: Ochoco Butte, Day Creek and Tollgate. With the exception of overthickening of small portions of the windward edges of vegetated areas of the Ochoco Butte and Day Creek areas, the thickness of ash in the center of the ash bodies in all three study areas, is generally uniform. The upland thickness of ash in the Ochoco Butte and Day Creek areas is in the order of 55 cm, while in the Tollgate area the ash thickness is in the order of 67 cm. The thickness observations in the Tollgate area contrast to the generally accepted view that the thickness of the deposit decreases with increasing distance from the source. The uniform overthickening of the Tollgate area is attributed to several factors; the most significant are the presence of about nine cm of St. Helens Y ash in the area and high precipitation bringing down more ash at the time of original ash fall as suggested by the occurrence of accretionary lapilli. The observed thicknesses are greater than predicted by Williams and Goles (1968). The difference is attributed to the location of the study areas along the NE axis of the main lobe, whereas the projected thickness by Williams and Goles is along a north axis. Chemical composition, mineralogy, morphology, and particle size show a discontinuity in the parent material of ash profiles at the boundary of Cl with C2 horizons. The upper layer (A, AC, and Cl horizons) and the lower layer (C2 horizon) behave as two different stratigraphic units suggesting that the layers represent different showers from Mt. Mazama. Two major kinds of mixing identified in the study areas are mixing of the two ash layers (C2with C1) and mixing ash with non-ash. The C2 layer is residual in some places, mixed with Cl in others, and missing in still others. The amounts and distribution of >2 mm rock fragments, biotite, lithic material, and quartz in the ash profiles indicate that the amount of non-ash mixed in ash decreases with depth and that the total amount of mixing of non-ash with ash is small. The absence of ash on most south slopes, its presence in alluvial and colluvial ash fans along streams and canyons, and its "patchy" distribution on land surfaces suggests that considerable erosion, movement and redeposition of ash has taken place. However, the study areas on high ridge tops under vegetative cover show no drastic reworking of ash deposits by wind or water in these topographic positions. This is indicated by decreasing grass opal content with depth, presence of 5300 year old charcoal in a straight line in the profile, and general uniformity in thickness of the ash deposit. The major amount of mixing is attributed to tree throw. The only observable genetic feature of the ash profiles is the development of A and AC horizons. These horizons are developed in the upper layer. The weakly developed profiles, coarse silt minerology, distribution of particle sizes, and elemental composition indicate that soils are only slightly weathered even after 6600 years. Small differences in the chemical composition of A and AC as compared to Cl can mainly be attributed to weathering of glass, although may be due in part to the presence of paleosolic material in the A and AC horizons. The small amount of weathering of the ash soils may be due to the acidic (high silica) nature of the deposit and an unfavorable climate for weathering.
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