The Goblin Colony: Spectacular Monoliths and Walls of Altered Bandelier Tuff South of the Valles Caldera, New Mexico

Abstract

Detailed geologic mapping combined with petrologic and geochemical analyses and a low-altitude aerial drone survey were used to investigate the development of monoliths and walls of altered Bandelier Tuff at Goblin Colony (GC) 11 km south of the southern Valles Caldera rim in New Mexico. The monoliths and walls consist of eroded spires, fins, spire and fin clusters, columns, and steam pipes of bright orange to tan tuff. Mapping shows that nonwelded to poorly welded unit 1 (Qbt 1 ) of the Tshirege Member of the Bandelier Tuff (1.23 Ma) filled a preexisting southwest-trending canyon cut into a complex sequence of eroded Miocene volcanic lava flows, domes, dikes, and volcaniclastic debris flows and sandstones. A dacite dike and plug zone about 1.5 km long formed a hydrologic boundary on the southeastern margin of the canyon before tuff (ignimbrite) emplacement. Thin sections of altered tuff show growth of feathery and blocky zeolite minerals in glass shards, pumice, and pore spaces and on feldspar (sanidine) surfaces. Patches of secondary hematite-limonite are also common, resulting in the intense orange colors of tuff at GC. X-ray diffraction (XRD) analyses reveal that the zeolites are mordenite with lesser clinoptilolite, with the zeolites forming primarily at the expense of rhyolitic glass. Scanning electron microscope (SEM) images verify that zeolites are growing primarily on glass and in voids. XRD also reveals that the ignimbrite contains exceptionally low quantities of vapor-phase minerals, tridymite, cristobalite, and excess alkali feldspar, even if unaltered by zeolites. Whole-rock chemical analyses of altered tuff are not as revealing as other techniques but show relative increases in water, Al 2 O 3 , total Fe 2 O 3 , MgO, CaO, K 2 O, and P 2 O 5 and decreases in SiO 2 and Na 2 O when compared with equivalent analyses of fresh tuff from the Pajarito Plateau. Concentrations of TiO 2 and MnO are unaffected. An aerial drone survey was flown over GC to examine the orientation of structures. Vertical spires, fins, and clusters of zeolite-altered tuff are broadly aligned in a northeast–southwest pattern on the upper (north) part of GC. However, the drone survey revealed that most spires, fins, and clusters developed along elongate or curving cracks 10 to 50 m long in random orientations. Unaltered tuff surrounds the various features. Two imposing walls of zeolite-altered tuff also developed in a general northeast–southwest trend, but the north wall displays a pronounced southward undulation, and the south wall is cut by en echelon faults or fractures. Between the walls is a modern eroded ravine about 35 m deep. Columns and steam pipes of zeolite-altered tuff are found in the upper to middle portions of GC and plunge 70° to 60° northwest. The columns display considerable zeolite alteration, are best exposed in the uppermost north part of GC, and are surrounded by unaltered tuff. Steam pipes protrude from the south face of the north wall; their rinds are zeolite-rich and their cores contain minor amounts of mordenite. The bottom of the GC sequence reveals a broad area of "tables" that display horizontal ledges of zeolite-altered tuff, and that can be traced for nearly a kilometer upstream of the main GC sequence. We compared low-temperature (zeolite-producing) hydrothermal processes at GC to other gas-release phenomena in ignimbrites, including some that occur elsewhere in the Bandelier Tuff. Our interpretation of the mapping and other data at GC is that the Tshirege ignimbrite that filled the paleocanyon there was cooled and probably gas-depleted due to distal flow over rugged topography.