Geology and Eruptive History of the Late Oligocene Nathrop Volcanics, Central Colorado Volcanic Field

The Nathrop Volcanics consist of rhyolite lava and pyroclastic deposits located on the eastern shoulder of the upper Arkansas Graben in south-central Colorado and are part of the extensive late Eocene-Oligocene Central Colorado Volcanic Field. Deposits of the Nathrop Volcanics at Ruby Mountain consist of a lower lithic-rich lapilli tuff (ca. 3 m thick) with multiple layers that are reversely graded with respect to pumice clasts and are overlain by an approximately 30 m thick lithic-poor tuff breccia containing pumice blocks up to 1 m in diameter. The upper portion of the tuff breccia transitions up into a 5 m thick, moderately to densely welded tuff (vitrophyre), which in turn is overlain by a 20 m thick flow-banded rhyolite. A similar stratigraphic sequence is found at Sugarloaf Mountain (