Basal Miocene Ash in White River Badlands, South Dakota: GEOLOGICAL NOTES

Abstract

The white volcanic ash capping Sheep Mountain and other prominent landmarks in the White River Badlands is named the Rockyford Member of the Sharps Formation. It conformably overlies the middle and upper Oligocene Brule Formation, and may be correlated with the ash bed of the Gering Formation in Nebraska. The type section is on the S. end of Sheep Mountain, and the name is taken from the town of Rockyford. The member is 0 to 55 ft. thick, is of pinkish-gray ash with glass shards of rhyolitic composition (R. I. 1.492 - 1.504), and fragments of quartz, biotite, sanidine, andesine, clay and an amphibole. Prominent features are small pipe concretions, and many clastic dikes. Volcanic activity at the end of the Oligocene in Yellowstone and (or) the Colorado Rocky Mountains appears to be the only likely origin for the Rockyford ash. The unit shows no evidence of being substantially reworked except in the peripheral areas of the Grass Creek drainage; deposition, therefore, took place in a relatively short period of time.