A 10 MA SILICIC FALLOUT TUFF IN THE TEXAS PANHANDLE—COMPARISONS WITH THE PEARLETTE ASH

A clean 1 meter thick layer of vitric ash within the Miocene Ogallala Formation has been tentatively correlated with one of several ash beds in the Twin Falls Volcanic Field on the basis of major and trace element composition as determined by microprobe analysis and X-ray fluorescence spectrometry of glass shard concentrates. The most likely correlation for the West Amarillo Creek ash is a series of ashes in a narrow age range between 10.94 Ma and 10.13 Ma. Subsequent to this correlation a K-Ar age determination on glass concentrate of the West Amarillo Creek ash yielded an age of 9.5 +/- 0.3 Ma The West Amarillo Creek ash lies on the east bank of the creek on the highest terrace of the north-flowing West Amarillo Creek, a tributary to the Canadian River in Potter County, Texas. In the outcrop area the Ogallala Formation occupies a shallow but broad (50 m wide) paleochannel into the underlying Triassic Tecovas and Trujillo Formations. The exposure of ash is several meters in width and appears uncontaminated by silt or sand for a thickness of 1 meter. Above the clean ash layer, glass shards mixed with increasing amounts of fine and medium sand upward in the section comprise more than two additional meters of the Ogallala.

The straight-line distance from the source area to West Amarillo Creek is more than 1200 km in a southeasterly direction. This direction is similar to the paths followed by the Mt. St Helens ash and Yellowstone ashes into the southern High Plains reflecting the orientation of the jet stream during the eruption. The southeasterly direction suggests that the jet stream is more important that surface wind directions(predominantly out of the southwest) in the long-distance transport of fallout ash. Exposures of Yellowstone Caldera Lava Creek B ash in the Texas Panhandle number more than 10 and the thickness of the ash layers in those exposures ranges between 0.3 to 1.5 m. The one meter thickness of the West Amarillo Creek ash suggests that the original volumes of wind-blown ash for the Twin Falls and Yellowstone sources are similar. An order of magnitude estimate for the minimum original volume of windblown ash as represented by the ash layer on West Amarillo Creek is 350 to 700 km3 depending on the amount of postdepositional consolidation and the width of the ash plume.