The Crater Flat Tuff is herein revised to include a newly recognized
lowest unit, the Tram Member, exposed at scattered localities in the southwest
Nevada Test Site region, and in several drill holes in the Yucca Mountain
area. The overlying Bullfrog and Prow Pass Members are well exposed at the
type locality of the formation near the southeast edge of Crater Flat, just
north of U.S. Highway 95. In previous work, the Tram Member was thought to be
the Bullfrog Member, and therefore was shown as Bullfrog or as
undifferentiated Crater Flat Tuff on published maps. The revised Crater Flat
Tuff is stratigraphically below the Topopah Spring Member of the Paintbrush
Tuff and above the Grouse Canyon Member of the Belted Range Tuff, and is
approximately 13.6 m.y. old.
Drill holes on Yucca Mountain and near Fortymile Wash penetrate all three
members of the Crater Flat as well as an underlying quartz-poor unit, which is
herein defined as the Lithic Ridge Tuff from exposures on Lithic Ridge near
the head of Topopah Wash. In outcrops between Calico Hills and Yucca Flat,
the Lithic Ridge Tuff overlies a Bullfrog-like unit of reverse magnetic polarity that probably correlates with a widespread unit around and under
Yucca Flat, referred to previously as Crater Flat Tuff. This unit is here
informally designated as the tuff of Yucca Flat. Although older, it may be
genetically related to the Crater Flat Tuff.
Although the rocks are poorly exposed, geophysical and geologic evidence
to date suggests that (1) the source of the Crater Flat Tuff is a caldera
complex in the Crater Flat area between Yucca Mountain and Bare Mountain, and
(2) there are at least two cauldrons within this complex--one probably associated with eruption of the Tram, the other with the Bullfrog and Prow
Pass Members. The complex is named the Crater Flat-Prospector Pass caldera
complex. The northern part of the Yucca Mountain area is suggested as the
general location of the source of pre-Crater Flat tuffs, but a caldera related
to the Lithic Ridge Tuff has not been specifically identified.
