UC Berkeley

Columnar structures have been exposed by preferential weathering in non-welded, non-vapor-phase-altered zones of the 1.2 to 1.8 Ma Bandelier Tuff ignimbrites around Valles Caldera, New Mexico. These long, largely vertical cylindrical features are similar in composition and texture to the host ignimbrite but represent areas that underwent alteration to make them more resistant to erosion. Analyses show that the column ignimbrite was altered by the addition of the zeolite mordenite as a mineral cement into pore spaces. This, and the presence of illite and, to a lesser extent, chlorite, suggests alteration at ~120–125°C. We map column geometry, spacing, stratigraphic location, and spatial distribution. Columns are mostly vertical, there is a correlation between the diameters and spacing, and they exist in linear to rounded clusters of a few to 30 columns. Based on the location within the ignimbrites, the shape and form of column groups, and the temperature of formation of the columns, we propose that slumping of over-steepened valley walls exposed friable, warm, unaltered ignimbrite that enabled ponded meteoritic water to permeate into the deposit, which allowed localized convective chimney cells to form. We develop an analytical model to show that infiltrating water is convectively unstable, and, for reasonable permeabilities, we can explain the observed column spacing and alteration temperature. This model permits columns to form within parts of the deposits perched in ignimbrite deposited against pre-existing valley walls. An abundant supply of water from high precipitation is implied, reflecting a syn-glacial, pluvial climate in the southwestern United States at that time (1–2 Ma ago).