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Pre-Pah Canyon Tuff bedded tuffs (Tpbt2) and non-to partially and moderately welded subzones of the Topopah Spring Tuff crystal-rich vitric zone (Tptrv3, Tptrv2

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High resolution magnetic susceptibility imaging is a new technique for studying the magnetic properties of geological thin sections. The two‐dimensional distribution of both remanent and induced magnetization can be determined with a spatial resolution (< 1.0 mm) that is similar to the size of phenocrysts in the sample. Amongst many problems in rock magnetism to which it could be applied, the technique holds great potential for understanding the origin of the anisotropy of magnetic susceptibility in pyroclastic flows. Preliminary tests on a single sample of ignimbrite indicate that secondary iron‐titanium oxide particles deposited within vesicle walls prior to their collapse are responsible for the bulk susceptibility.

Unit 5-Non-to partially welded, pumiceous pyroclastic-flow deposit Slightly elongated pumice clasts; bimodal distribution of pumice clast sizes; 20 to 30 percent pumice. Light colored prniice clasts; moderate reddish-orange to grayish-pink matrix. Base marked by thinly bedded fall deposits. Unit 4-Nonwelded, pumiceous pyroclastic-flow deposit Volcanic lithic clasts are large (20 to 70 mm), isolated or in swarms; prominent clasts of moderate reddish-orange tuff. Light colored pumice clasts; very pale orange to grayish orange-pink matrix. Lithic-poor sections appear similar to unit 2. Base mrrked by a heterolithologic sequence, of fall deposits. Unit 3-Nonwelded, lithic-rich pyroclastic flow deposit Lithic clasts comprise 5 to 10 percent, locally 10 to 30 percent (near the base and in several intervals within the unit); predominantly devitrified volcanic rocks with local obsidian. Grayish-orange to grayish-yellow or pinkish-gray matrix. The basal litHc-rich fallout is an excellent stratigraphic marker. Unit 2-Nonwelded, pumiceous pyroclastic-flow deposit 20 to 40 percent light colored pumice clasts; moderate pink or moderate orange-pink matrix. The fall deposit at the base of the unit contains porcelaneous ash layers. Unit 1-Nonwelded, lithic-rich pyroclastic-flow deposit 15 to 20 percent devitrified volcanic lithic clasts near base; lithics decrease upward to 3 to 7 percent. Light colored punice clasts; grayish orange-pink to light greenish-gray matrix; 7 to 12 percent phenocrysts. Bedded tuff unit Interbedded coarse-grained fallout deposits, pyroclastic-flow deposits (many reworked or with paleosols), and thinly bedded porcelaneous ash-fall deposits. Pyroclastic-flow deposits have 13 to 25 percent phenocrysts. Basal sandstone unit Massive to laminated, immature volcaniclastic sandstone; very pale orange to moderate red; medium to coarse grained; accumulations of argil lie pumice clasts and rare sedimentary structures including load casts, pinch and swell structures, and flame structures. Locally interbedded with reworked pyroclastic-flow deposits.