Fish Canyon Tuff, Colorado: the problem of two magnetic polarities in a single tuff

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The Fish Canyon Tuff, located in the central San Juan Mountains, southern Colorado, is currently recognized as one of the largest ash-flow tuffs in the world. Paleomagnetic samples from 21 sites have been obtained from a composite vertical section of the tuff. The two uppermost sites, from the otherwise normally magnetized tuff, exhibit stable but anitpodal (reversed) remanent magnetic (RM) directions. Field relationships and geochemical consistencies indicate that the entire section was emplaced relatively rapidly, probably in < 100 yr. Since it takes at least 4000 yr for the geomagnetic field to reverse completely, we do not attribute the two anitpodal sites to temporal differences between the top and the bottom of the tuff. Rather, we interpret the data to indicate that low-temperature alteration of the upper portion of the tuff, probably during emplacement of the overlying Carpenter Ridge Tuff, has altered the primary magnetic carrier, magnetite, and produced a chemically stable maghemite in its place. We conclude that this maghemite formed after a reversal in the geomagnetic field direction, which post-dates emplacement of the Fish Canyon Tuff, and is the carrier of the stable, secondary antipodal RM directions observed. © 1989.

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Physics of the Earth and Planetary Interiors

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