THREE-DIMENSIONAL MEASUREMENT OF COLUMNAR JOINTING IN THE BISHOP TUFF, OWENS RIVER GORGE, CALIFORNIA

The Bishop Tuff is a Pleistocene ignimbrite whose outflow sheet covers ~1300 km² in eastern California, extending from Mono Lake on the northern end to Bishop on the southern end. Owens River Gorge (ORG), 100-200 m deep, runs through the tuff, providing well-exposed cross sections that allow the study of columnar jointing. Whereas columnar jointing has been extensively studied in basalts, such as at the nearby Devil’s Postpile, the phenomenon is almost undescribed in rhyolite. Our study aimed to gain a greater understanding of rhyolitic columnar jointing and its relationship to fumarolic mounds that surround the gorge, by forming a geologic model based on aerial drone photography, satellite imagery, and scanning electron microscopy in the area south of LADWP Powerhouse No. 1.

There is high variation in width and orientation among joints throughout ORG. Columnar joint width is bimodally distributed. The lower, densely welded section contains joints with median width 15 m that reach from the base of the gorge about 3/4 of the way up, to a thin (<2 m) transition zone of chaotic jointing. Above the transition zone is a section of shorter joints with median width ~2 m; these commonly radiate from a common point in a manner resembling saguaro cacti. Rhyolitic columnar jointing in ORG appears to be a modification of the standard three-zone model of basaltic columnar jointing, with a lower colonnade, smaller entablature, and upper colonnade.

Putnam (1960) proposed that radiating columnar joints occur below fumarolic mounds and record heat flow away from fumarolic pipes. However, in our drone survey of a 1.5 km stretch of ORG we found only a weak correlation; there are many instances of radiating joint sets that do not underlie fumarolic mounds. The upper ORG section is sillar, and pumice samples taken from fumarolic mounds exhibit secondary mineralization in the form of grape-like clusters of ~200 μm spheres that are not found in pumice of off-mound samples. The spheres have a tridymite shell enclosing altered volcanic glass. This secondary mineralization likely cemented the fragile volcanic ash to form a more coherent, erosion-resistant rock. In this area, it appears that the river abandoned a meander so that it could avoid a well-cemented mound.