GEOCHEMICAL VERIFICATION OF THE 36.7 MA WALL MOUNTAIN TUFF IN THE FLORISSANT REGION, SOUTHERN FRONT RANGE, COLORADO: TECTONIC IMPLICATIONS

Erosionally isolated ignimbrite outcrops surrounding the highly fossiliferous late Eocene Florissant lake beds have been informally correlated with the 36.7 Ma Wall Mountain Tuff (WMT) ever since that unit was named in 1973. The rocks from Florissant were compared only on megascopic and petrographic features with the WMT in western South Park, proximal to its source in the Sawatch Range 85 km west of Florissant. But remnants of many megascopically similar Tertiary ignimbrites, some with caldera sources in the San Juan Mountains, have been dated in South Park; these have now been geochemically characterized and compared by McIntosh and Chapin (2004) using total alkalis vs. silica diagrams. Our 14 new analyses of ignimbrite samples from the Florissant area best resemble the analyses of the WMT from South Park, which are the highest in total alkalis and among the highest in silica in the entire Central Colorado Volcanic Field.

The Wall Mountain Tuff lies directly upon the extensive high-elevation low-relief Late Eocene Erosion Surface (LEES) which dominates the topography of much of central Colorado. WMT pyroclastic flows travelled unimpeded across the LEES in South Park and the Front Range as far as Castle Rock, a distance of 160 km. The Puma Hills, which now rise >800 m above the LEES at the eastern edge of South Park, would have almost certainly been a barrier to the eastward progression of the pyroclastic flows had they been in place at the time. We propose that uplift of the Puma Hills occurred after the WMT flows, likely during a period of Neogene block faulting that vertically displaced the LEES by hundreds of meters in other parts of the Front Range. This uplift would have likely been responsible for drainage re-orientation and stream incision, and it may have initiated debris flows which deposited giant boulders (up to 7 m diameter) on the erosion surface east of the Puma Hills.