2009 Portland GSA Annual Meeting (18-21 October 2009)

Paper No. 4
Presentation Time: 9:00 AM-6:00 PM


HILTON, Richard P., Earth Science, Sierra College, 5000 Rocklin Road, Rocklin, CA 95677, rhilton@sierracollege.edu

It appears that some porous and permeable tuffaceous rocks, diatomite, and occasional deep soils atop volcanic rocks in NW Nevada and SE Oregon have been influenced by permafrost during wetter and colder Pleistocene conditions. On relatively flat surfaces as well as gently sloping surfaces (sometimes parallel to current drainages), there are hundreds of strange rimless oblong closed depressions, as well as circular closed depressions. Good examples can be seen in the Butcher Flat area of the High Rock (Soldier Meadow) caldera, Nevada. Occasional circular closed depressions have rims and appear to be pingo scars. One such double scar (41˚ 31' 02.04” N and 119˚ 22' 27.39” W) is ~140 m in length, 110 m in width, and 6 m deep with a rim 2 m above the surrounding surface. In today's desert climate all the depressions are usually dry but stand out clearly on Google Earth as their lower, more watered interiors have a pronounced increase of greener vegetation (sage and grasses). The depressions lie mostly within a 75 x 118 km rectangle between 42˚ 10' 38.61” N and 41˚ 06'54.89” N, by 119˚ 39'54.94” W and 118˚ 57' 45.94” W. Present elevations (and during the Pleistocene) of the depressions range from ~1635 to ~2060 meters allowing for the colder temperatures necessary for thermokarst production during the Pleistocene. These depressions are so numerous and cover such a broad area they are not likely to be either magma-induced explosion pits or slump features, although some slump features may exist. Meteor impact has been all but ruled out because of the apparent absence of meteoritic material, the depressions are in relatively soft substrate, and the patterns do not suggest impact of a fragmented body. Because the rhyolite tuffs, diatomite and local soils are not highly soluble they are probably not collapsed solution pits. No piping has been found. Hence, the most likely hypothesis is that these strange depressions are indicative of Pleistocene thermokarst.