PREVIOUSLY UNRECOGNIZED NEOGENE 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

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 are clearly expressed 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 with the more regional 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 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 following rivers: 1) North and South Platte (Gangplank), 2) South Platte and Arkansas (Pikes Dome), and 3) Arkansas and Canadian (Raton Mesa).

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 monadnock or faulted-bounded structure with an additional 1000 m of relief. The dome axis crossed several south- to southeast-flowing paleodrainages, thereby diverting or reversing their flow directions. The South Platte saw its southerly flow diverted to the northeast (Epis et al., 1976) by 200 m of uplift at the dome axis just south of Florissant. 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 north-flowing creeks. In addition, the dome has been tilted east by the 1^st-order SRMe. Subtracting the tilt due to the SRMe in excess of typical river gradients and the dome magnitude gives a picture of the RMES topography prior to the both the 1^st- and 2^nd-order deformation and shows what the original drainage pattern looked like.

Timing of the doming is less understood. The Miocene HPA has been deformed, so Neogene deformation is evident. However, given that the Paleogene saw the advent of the SRMe, local volcanism, realignment of the South Platte and Arkansas rivers, and blockage and ponding along the South Platte immediately upstream of the dome axis, suggests the rise of Pikes Dome started earlier.