New Mexico Geological Society Annual Spring Meeting — Abstracts

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 the stratigraphic sections.