03 General Geology

Pumice Deposits of the Klamath Indian Reservation, Klamath County, Oregon

General Geology

The extensive pumice deposits in northern Klamath County, southern Deschutes County, and the northwest corner of Lake County are the result of violent eruptions about 5, 000 years ago from now-inactive volcanoes. The volcanoes include the ancient volcano known as Mt. Mazama (Crater Lake now occupies a caldera in this partially destroyed mountain), Mount Newberry (Newberry Crater), and one or two smaller volcanic cones. The pumice on the Reservation was derived almost exclusively from Mt. -Mt. Mazama. During the early stages of its eruption most of the ejecta consisted of small, gas-filled fragments of pumice, which were thrown high into the air and carried by the prevailing winds to the areas northeast, east, and southeast of the volcano. Later, less-explosive eruptions of glowing, gas-fluxed, flow-like avalanches, technically known as nuee ardentes, poured over the rim of the caldera and rushed down the flanks of the volcano into the valleys. Many of these flows coalesced to form the extensive pumice layer in the vicinity of Antelope Desert in the northeast corner of the Reservation and north of the Reservation near Beaver Marsh. In its waning stages of activity the volcano erupted a quantity of mafic scoria and cinders, which is intermixed with the upper pumice layers.

Two main types of pumice deposits resulted from these eruptions of Mt. Mazama; one derived from the settling of the fine pumice fragments that were carried by the wind, and the other from the glowing avalanches. Williams (1942) has classified the pumice deposits according to origin and has called them respectively “pumice fall” and “pumice flow”. In this report the names proposed by Williams have been used, though the flow and fall deposits have also been called, respectively, lump pumice and granular pumice by Moore (1937). For a more detailed description of the origin and the geology of the pumice and related rocks of the area, the reader should refer to the reports listed above and to the report by Diller and Patton (1902).

The bedrock on which the pumice is deposited varies from place to place. In some spots the pumice overlies Pliocene lake beds which are partly diatomaceous. The lake beds are approximately horizontal and generally are exposed only in road cuts and in river banks. In other places the bedrock consists of Pliocene basalt and, locally, flows of olivine basalt which are probably Pleistocene or Recent in age. The basalts include those at Boundary Butte and other smaller cinder cones in the west-central part of the Reservation. The basalt flows are horizontal and crop out through the pumice as rim rock along valley walls. The pumice occurs as sheet-like masses; the pumice flow locally overlies the pumice fall. Both pumice layers are essentially horizontal, but vary in thickness over short distances owing to the irregular surface on which they rest and to erosion and drifting of loose pumice on the upper surface of the pumice layers.

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