Stratigraphy of the Bandelier Tuff and Characterization of High-Level Clay Alteration in Borehole B-20, Redondo Creek Area, Valles Caldera, New Mexico

Abstract

Stratigraphy and alteration of borehole Baca 20 from the Redondo Creek area in the Valles Caldera have been investigated by binocular microscopic logging and X-ray diffraction techniques. Within the caldera, the Bandelier Tuff has thickened dramatically from documented exposures outside the caldera. The two principal members of the Bandelier, the Otowi and Tshrirege, have been recognized in Baca-20. Additional ash flow tuff cooling units have been delineated both below and above the Bandelier. The units above the Bandelier have undergone intense alteration with the formation of a variety of clay minerals, including pure Mg or Ca semctite, allevardite ordered illite smectite, kaolinite and illite. This alteration is apparently controlled largely by high original permeability of these rocks. The presence of the highly ordered illite smectite suggests that higher temperatures than presently prevail existed during the alteration process.