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Flow Structure and Composition of the Southern Coulee, Mono Craters, California—A Pumiceous Rhyolite Flow

January 1, 1968

The Southern Coulee is the southernmost and largest of the four Recent pumiceous rhyolitic coulees, or stubby flows, of the Mono Craters, eastern California. It is one of the youngest volcanic deposits of the Mono Craters and is largely bare and uneroded. The coulee is 3.6 km long and averages 1.2 km in width and 75 m in thickness. It was protruded from a north-trending fissure beneath the crest of the Mono Craters ridge. About two-thirds of the lava flowed west and one-third flowed east.

The coulee has three main parts: the dome, located over the orifice, where flow was about vertical; the flow, where movement was lateral; and the talus slope, which surrounds the coulee and formed from the advancing steep-flow front. In addition, three small areas of air-fall pumice ash occur on the coulee and appear to be remnants of an ash eruption that took place during an early phase of the coulee eruption.

Three distinctive lithologic units based on rock density related to the degree of vesicularity have been mapped: a unit of lowest density (average ρ = 0.65); a unit of intermediate density (average ρ = 1.20); and a unit of highest density (average ρ = 1.75). The contacts between the units are abrupt despite the fact that core drilling has shown the coulee to be a jumbled mass of blocks down to a depth of at least 45 m. The two less dense units, which consist of highly inflated thick-bedded pumice, form two connecting boat-shaped bodies along the entire south margin of the coulee. These units are probably not over 25 m thick and are underlain by the unit of highest density, which appears to form the rest of coulee. The dense unit consists of thin- to medium-bedded dense pumice and lesser amounts of obsidian. The above distribution of the lithologic units in the coulee was probably caused by the eruption of rocks characteristic of all the units from, the southern part of the north-trending fissure, while only rocks of the unit of highest density erupted from the northern part. The protrusion of the coulee involved several subordinate and, in places, interfering streams that had slightly different courses and levels. This complex flow resulted in a modification or disruption of the original spatial relations of the lithologic units, as they erupted from the orifice.

The petrographic and chemical data indicate a uniform composition for the lava that belies its heterogeneous aspect. The lava is composed almost entirely of clear glass (average nD = 1.488 ± 0.001), and contains only trace amounts of microlites and cristobalite-sanidine spherulites. Eight chemical anlyses show a silica range from 74.7 to 76.2 percent and indicate a rhyolite composition of the sodipotassic subrang. This composition is characteristic of glassy-fluidal rhyolites.