New Mexico Geological Society Annual Spring Meeting — Abstracts

Cooney Tuff (ignimbrite) was erupted in the Mogollon Range in southwestern New Mexico about 34 Ma, where it caused the subsidence of the proposed Mogollon caldera. Unresolved problems remain, including the definition, distribution, and thickness of the tuff, its age, and its source.

Cooney Tuff includes Whitewater Creek Rhyolite and Cooney Canyon Dacite members. Whitewater Creek Rhyolite is a densely-welded, phenocryst-poor, simple cooling unit, known only in the Mogollon mining district in Cooney Canyon of Mineral Creek, and along the historic Catwalk Tourist Trail in Whitewater Canyon, where the tuff is 100-200 meters thick, with no bottom exposed. The overlying Cooney Canyon Dacite consists of multiple, thin, partiallywelded to densely-welded ignimbrites, each a few tens of meters thick, and commonly separated by thin intervals of dark-colored sandstone beds and andesitic sills or lava flows. The Cooney Canyon Member may be as much as 600 meters thick in Whitewater Canyon.

Cooney Tuff crops out mainly along the western and southwestern front of the Mogollon Range, from the Mogollon mining district, near Glenwood, almost to the southeast end of the range near Cliff. The outcrops are all relatively accessible, although large areas are within, but at the margins of, the Gila Wilderness. In addition to Cooney Tuff in the Mogollon Range, rocks believed to be Cooney Tuff crop out only at Clifton, Arizona, where it has been called Clifton Tuff by others, and east of Clifton in the Big Lue Mountains 15' quadrangle.

If Cooney Tuff, in and adjacent to the Mogollon mining district, is an intracaldera sequence, its source may be largely buried beneath the Alma basin, to the west. Alternatively, the Cooney Tuff exposed along the Mogollon Range front, as well as that in the Clifton area, may all be outflow and the Mogollon caldera was completely destroyed by collapse of the younger Bursum caldera.

Further study could lead to redefinition of Cooney Tuff, if necessary, and more specific location of its source cauldron.