FOSSIL FUMAROLIC PIPES IN THE TSHIREGE MEMBER OF THE BANDELIER TUFF

The geology exposed on the walls (3000 m²) of a large pit in the Tshirege Member of Bandelier Tuff gives unparalled 3-D exposures of many structures that develop in thick deposits of pyroclastic flows. The subunits exhibit distinct rubble-filled fissures, or pipes, that range in width from centimeters to meters. The fissures exhibit zones of fines depletion, indurated wall structures, upward flaring geometry to the top of the host unit, and fissure-filling blocks of the host unit as well as rubble derived from overlying pyroclastic units. The fissures are regularly spaced at about 4.5 or 7.5 meters apart. All these field characteristics are indicative of fumarolic activity.

Petrographic, XRD, and XRF studies of distinct pipes were done to investigate the physical changes imparted to the tuff by the fumarolic activity. Petrography indicates that the pipe wall and pipe centers are enriched in tridymite and potassium feldspar. These minerals fill the void spaces in pumice and groundmass void spaces of the pipe wall rocks, imparting the indurated nature. Other mineralogic indicators of late stage fumarolic gas phase deposition are optically continuous, feathery overgrowths on sanidine phenocrysts and scapolite in pipe centers. Also, clinopyroxenes in the overlying rubble zones have oxidized rims indicative of highly oxidizing gases emanating from the fumarolic pipes below.

XRD analyses of rubble zones above the pipes show decreased cristobalite (4 wt. \%) and feldspar (9 wt. \%), and increased clay contents (12 to 26 wt. \%). This change from feldspar and cristobalite to clay suggests an acidic nature of fumarolic gases. The data indicate that there have been systematic changes in the geochemistry of the fissures readily attributable to the action of fumarolic gases.

Field relations, petrography, XRD, and XRF element analyses all indicate that the filled fissures are fossil fumarolic pipes. Fumarolic activity, established after deposition, must have continued after emplacement and induration of overlying units.