Introduction Evaluation Prospects











Type Section Information

The Kalamazoo Volcanics were first described by Young (1960) for exposures "from the Kalamazoo Pass-upper Kalamazoo Canyon area" of the north-central Schell Creek Range. Hose and Blake (1976) designated the uppermost of six members broken out by Young (1960) as the Tuff of Kalamazoo Creek, and correlated it with Unit B designated by Dechert (1967) in the Schell Creek Range.

Geologic Age

The Kalamazoo Volcanics are considered Early Oligocene in age, based upon a date of 34 Ma in the lower portion of the unit (Young, 1960).

General Lithology

Young (1960) divided the volcanics into seven map units in the Schell Creek Range, which are here described in ascending order. The lowest unit conformably overlies the Sheep Pass Formation and is composed of about 200 feet of dark brown andesite or basalt, which locally shows columnar jointing. Overlying the basalt is the distinctive 350 foot thick Tuff of Kalamazoo Creek (Hose and Blake, 1976). The tuff is composed of light to dark-gray weathering, vitropheric ash-flow tuff with black stretched pumice, and green-grey opaline amygdules with granitic zenoliths up to a foot in diameter. The base of the unit is composed of up to 300 feet of tan to lavender welded and flow banded tuff with fragments of pumice, volcanic flow rocks, and surrounding Paleozoic country rock.

Young (1960) informally named the overlying 400 to 800 foot thick unit the "brown volcanic member". It is a chocolate brown, crystal-poor, ash-flow tuff containing about 20 percent phenocrysts of quartz, plagioclase, potassium feldspar, hornblende, and biotite in a glass shard matrix. The base of the unit is a dark-gray vitrophere that is occasionally fragmental. Member 3 is about 300 to 500 feet of gray to lavender, rhyolitic to andesitic, volcanic agglomerate that is locally strongly flow banded, and has a basal black to brown glass at the base. Member 4 is a cliff forming, purple-gray andesite from 400 to 1,200 feet thick. The unit is locally flow banded as well as scoriaceous near the top, and contains visible phenocrysts of plagioclase hornblende and biotite.

The overlying unit, member 5, is the most easily recognized member of the Kalamazoo Volcanics according to Young (1960). It is composed of 400 to 600 feet of massive, light to dark brown, andesitic flow breccia with varicolored volcanic rock fragments in a glassy matrix. Thin green-gray water lain tuffs are interbedded with the unit, and the upper portion contains several thin glassy flows. The uppermost Member 6 is poorly exposed and is composed of several hundred feet of dark-gray andesite with highly contorted flow banding, gray ignimbrite, and a gray-green perlitic flow.

Average Thickness

The Kalamazoo Volcanics vary from 2,000 to 3,500 feet at their type locality in the Schell Creek Range (Young, 1960).

Areal Distribution

The Kalamazoo Volcanics are exposed in the Schell Creek Range and may correlate with volcanics exposed in the Antelope and Kern Mountains.

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Last modified: 09/12/06