DUNBAR, N.W.,, and KELLEY, S.A., New Mexico Bureau of Geology and
Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, New Mexico,
87801; GOFF, F., Department of Earth& Planetary Sciences, University of New Mexico,
Albuquerque, New Mexico 87131; McINTOSH, W.C., and HEIZLER, L.L., New Mexico
Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology,
Socorro, New Mexico 87801

Rhyolitic to trachyandesitic pyroclastic deposits exposed in three canyons (La Mosca, San
Mateo, and Water) around the Mt. Taylor edifice provide insight into early eruption processes at
this composite volcano. Measured sections coupled with geochemical analysis of glass allow
correlations between the canyon sections and, for some deposits, sources areas to be determined.
A 4 m thick pyroclastic surge and ashfall deposit at the base of the La Mosca Canyon section,
dated at 3.26±0.04 Ma, correlates chemically and chronologically with 3.3 Ma Grants Ridge high
silica rhyolitic deposits sourced 15 km southwest of Mt. Taylor. Overlying this deposit is a 1 m.
thick rhyolitic pyroclastic surge. Both deposits are absent in the nearby San Mateo Canyon,
where instead, a >28 m thick, complex sequence of rhyolite ignimbrites and pyroclastic surges is
found at the base of the exposed stratigraphic section. An ignimbrite in Water Canyon is
chemically similar to the upper part of the basal San Mateo pyroclastic sequence. Above these
pyroclastic surge and flow deposits in San Mateo Canyon, a series of small, chemically
correlated, pyroclastic fall deposits are found in San Mateo and La Mosca canyons. Two
rhyolitic ashfalls (one dated to 3.08±0.2 Ma), and a thin, dark-colored trachyandesite fall deposit
are present. The upper of the two rhyolitic ashfalls is also found in Water Canyon. Above this
sequence of ashfalls, a thick (~22 m.) rhyolitic pyroclastic flow sequence is present in San Mateo
canyon that chemically correlates with a thin (<1 m.) pyroclastic fall in La Mosca canyon. All
these pyroclastic beds are probably sourced from sites within Mt. Taylor.
The chemical trend of the pyroclastic sequences in the canyons ranges from high silica to low
silica rhyolite, consistent with derivation from a single, normally zoned magma chamber.
However, the presence of the trachyandesitic tephra layer in the middle of the sequence may
argue instead for multiple, small magma batches. Individual pyroclastic flow/surge deposits
appear to have restricted ranges, consistent with small volume, high aspect ratio events, possibly
related to dome collapse in early Mt. Taylor. In contrast, pyroclastic fall deposits, although
small, are more widespread, and are deposited in all three canyons, allowing correlation between
stratigraphic sections