PREVIOUSLY UNRECOGNIZED CENOZOIC STRUCTURAL CONTROLS ON RIVER COURSES ALONG THE EASTERN FLANK OF THE SOUTHERN ROCKY MOUNTAIN EPEIROGEN WITH A FOCUS ON “PIKES DOME”, AN INTERFLUVE HIGH BETWEEN THE SOUTH PLATTE AND ARKANSAS RIVERS – HOW THE FLORISSANT VALLEY DATES INITIAL RISE OF PIKES DOME TO THE LATEST EOCENE

The eastern flank of the Southern Rocky Mountain epeirogen (SRMe) (Eaton, 2009) is punctuated by 2^nd-order structural highs that control the flanking rivers. These interfluve highs express as east-plunging noses on a structure map of the Rocky Mountain Erosion Surface (RMES) made by combining data from a new compilation by the author across Colorado and parts of neighboring states with the base of the High Plains Aquifer (HPA) or Ogallala (Cederstrand and Becker, 1998). Magnitudes of the interfluve structures were derived from a residual showing the difference between the RMES and a regional trend structure map built using datums taken from lows in the RMES along the major rivers. The residual shows at least 500 m of relief on the major interfluve structures located from north to south between the South Platte and Arkansas rivers (Pikes Dome), and the Arkansas and Canadian rivers (Raton Dome).

Of these, Pikes Dome extends from the Thirtynine Mile volcanics of Colorado into western Kansas and shows 900 m of relief not counting Pikes Peak proper, which is a fault-bounded structure with an additional 1000 m of relief. The dome axis crossed several south- to southeast-flowing upper Eocene paleodrainages, thereby reversing or diverting their flow directions. The South Platte saw its southerly flow diverted first to the east to become the Ancestral Arikaree River (Pearl, 1971) by 200 m of uplift at the dome axis just south of Florissant, then to its current northeasterly course due to later uplift near Cripple Creek. Along the Castle Rock drainage, located between Coal Creek and Calhan in Colorado (Evanoff, 2007; Keller and Morgan, 2016), 425 m of uplift at the axis near Calhan replaced the river that once flowed to the southeast by the east-flowing Ancestral Arikaree River and finally a series of north-flowing creeks. In addition, the dome has been tilted east by the 1^st-order SRMe.

Dome timing can be constrained in several ways. The Miocene HPA has been deformed, so Neogene deformation is evident. However, given the Paleogene saw the advent of the SRMe, local volcanism, and realignment of the South Platte and Arkansas rivers, the rise of Pikes Dome likely started earlier. How much earlier is shown by water wells in the Florissant Valley that reveal a canyon over 150 m deep developed along the ancestral South Platte River at the rising dome axis. Canyon incision occurred between eruption of the Wall Mountain Tuff that blanketed the low-gradient, meandering river at 36.7 Ma, and the arrival of a lahar flow that blocked the canyon and created the Florissant Lake at 34.1 Ma.