Bandelier Tuff

The Bandelier Tuff is a geologic formation exposed in and around the Jemez Mountains of northern New Mexico. It has a radiometric age of 1.85 to 1.25 million years, corresponding to the Pleistocene epoch. The tuff was erupted in a series of at least three caldera eruptions in the central Jemez Mountains.

The Bandelier Tuff was one of the first ignimbrites recognized in the geologic record, and has been extensively studied by geologists seeking to understand the processes involved in volcanic supereruptions.

Description
The formation is composed of ignimbrites produced by a series of at least three Quaternary caldera eruptions that culminated in the Valles Caldera eruption 1.256 million years before the present (Mya). The Valles Caldera is the type location for resurgent caldera eruptions, and the Bandelier Tuff was one of the earliest recognized ignimbrites.

The caldera lies on the intersection of the western margin of the Rio Grande Rift and the Jemez Lineament. Here magma produced from the fertile rock of an ancient subduction zone has repeatedly found its way to the surface along faults produced by rifting. This has produced a long-lived volcanic field, with the earliest eruptions beginning at least 13 million years ago and continuing almost to the present day.

Both upper members of the tuff show compositional zoning, in which the lower pyroclastic flows are more silicic and contain less mafic (magnesium- and iron-rich) minerals than the upper flows. This is interpreted as progressive eruption of a gravitationally zoned magma chamber in which volatiles are concentrated at the top of the chamber and mafic minerals have partially settled into the lower, hotter portions of the magma chamber.

The tuff contains up to 30% lithic fragments, which in the Otowi Member are estimated to have a total volume of 10 km3 and to be sufficient to quench welding through their cooling effect. The lithic fragments are 90% earlier volcanic rock, 10% Paleozoic sedimentary rock, and only traces of Precambrian rock, implying considerable flaring of the eruption vents. Some of the rock shows indication of contact metamorphism in the magma chamber walls with a magma rich in water and fluorine.

Members
The Bandelier Tuff consists of three members corresponding to at least three distinct caldera eruptions.

Much of the material in these deposits now forms the Pajarito Plateau, a scenic region of canyons and mesas on which Los Alamos is situated.

Economic geology
Pumice has been extensively mined from the Guaje Pumice Bed on the east flanks of the Jemez Mountains. Production was high enough in 1994 to help make New Mexico the second largest producer of pumice among the United States. The pumice itself is unconsolidated and easily removed once the overburden (typically Otowi Member ignimbrite) is removed. Much of the pumice was strip mined from public lands before reclamation bonds were required, leaving mining scars that are slowly revegetating.

History of investigation
The formation was given its name by H.T.U. Smith in 1938. The formation was divided into upper and lower units, which were recognized almost at once to correspond to separate caldera eruptions. In 1964, R.L. Griggs assigned the formal member names of Otowi Member to the lower unit and Tshirege Member to the upper unit, and gave the name Guaje Pumice to the basal pumice bed of the Otowi Member. In their paper establishing the stratigraphic framework for the Jemez volcanic field in 1969, R.L. Smith, R.A. Bailey, and C.S. Ross adopted Grigg's unit names and added the name Tsankawi Pumice for the basal pumice bed of the Tsherige Member.

In their 2011 map of the Valles Caldera, Fraser Goff and his coinvestigators formally added the La Cueva Member, informally known until then as the ignimbrite of San Diego Canyon, to the Bandelier Tuff.