AN  INTEGRATED PETROLEUM  EVALUATION OF NORTHEASTERN  NEVADA


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NORTHUMBERLAND TUFF

Type Section Information

The Northumberland Tuff is named for exposures along Northumberland Canyon in the Toquima Range (McKee, 1972).

Geologic Age

The Northumberland Tuff is Oligocene in age and erupted over a period of approximately 1 million years. The average K-Ar age is 32.3 +/- 1.0 Ma (McKee, 1976b).

General Lithology

The Northumberland Tuff is a thick pile of welded crystal-rich rhyolite tuff and minor pumice-rich tuff breccia and rhyolite lava flows. It encloses landslide blocks of Paleozoic cherts and shales as much as 1 mile across and is capped by water lain tuffaceous sediments along the caldera edge (McKee, 1976b).

The tuff is a gray to white crystal-rich rhyolite with 20 to 35 percent quartz and plagioclase phenocrysts, and minor amounts of Paleozoic chert and shale rock fragments in a devitrified shard-rich groundmass. The tuffs are highly altered and bleached with silicification and a honeycomb vein work of limonitic staining, particularly beneath Paleozoic slide blocks (McKee, 1976b).

Average Thickness

The Northumberland Tuff is about 2,000 feet thick within the Northumberland caldera, and is 100 to 400 feet thick in several exposures outside of the caldera in the northern Toquima Range (Kleinhampl and Ziony, 1985).

Areal Distribution

The Northumberland Tuff is only exposed in the northern Toquima Range.

Depositional Setting

The Northumberland Tuff was erupted about 33 Ma from vents in a shallow magma chamber south of Northumberland Canyon. Collapse of the volcanic center, after eruption of much of the low-energy tuffs, created a caldera. Paleozoic blocks as much as a mile across broke from the caldera rim and slid into the hot ash-flow sheets inside (McKee, 1976b). Continued volcanism filled the central part of the caldera with post-collapse ash up to 1,000 feet in thickness, and clastic sediments and landslide blocks nearly 300 feet in thickness.


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