The Oligocene and Eocene Joy Tuff, named for the type locality at Joy townsite in the Drum Mountains, forms thick deposits within the Thomas caldera. The Joy tuff consists of two members: a lower, widespread unit of poorly to moderately welded crystal-rich tuff and a thin, local unit of highly welded black glass tuff.
SLIDE 23 shows the crystal tuff member near its base, in the type locality.
The crystal tuff member contains abundant broken phenocrysts of quartz,
sanidine, plagioclase, and biotite in a glassy to devitrified matrix of
shards. Typically, the tuff is moderately welded and contains black fiamme
near the base, as shown here. Lithic fragments (lower right) are derived
from the Mt. Laird Tuff.
SLIDE 24 shows a breccia of the Joy Tuff east of Topaz Mountain. The
tuff breccia consists of apparently randomly oriented masses of welded tuff
in a matrix of the same. The degree of welding is so consistent that the
brecciated nature of the tuff is not everywhere immediately evident. The
author recognized the breccia texture during a sudden shower, when wet outcrops
enhanced the subtle contrast between breccia and matrix. The tuff breccia
is interpreted as a possible vent or near-vent deposit. Lens cap shows scale.
SLIDE 25 shows the black glass tuff member of the Joy Tuff in the northern
Drum Mountains. The dark ledges immediately above the pediment are welded
zones. The black glass tuff is a local ashflow deposit, limited to the vicinity
of the proposed vent breccia east and south of Topaz Mountain. The tuff
contains three zones of differing degrees of welding, including an upper,
unwelded zone (light-colored layer). The unwelded zone is conformably overlain
by megabreccia of the northern Drum Mountains. The megabreccia, composed
entirely of chaotically oriented masses of carbonate rocks of Cambrian age,
was evidently emplaced as a landslide soon after eruption of the black glass
tuff. The rapid succession of tuff eruption and landslides would account
for the conformable contact between the two deposits. Cambrian formations,
exposed in a horst block west of the megabreccia, may have been the source
of the landslide. The view is looking north at the northeasternmost point
of the Drum Mountains.