CASE STUDY FOR THE REDEPOSITION OF PIKES PEAK GRANITE IN THE EARLY PLEISTOCENE SMOKEY HILL FORMATION, CHEYENNE COUNTY, COLORADO

A case study is proposed to pose the question, “What depositional event had the power to deposit a coarsening upward section of a very coarse-grained sediment sourced 170 miles west of the study area in the Rocky Mountains region?” The study area covers outcrops along the Smokey Hill River, 15.4 miles due west of the Colorado/Kansas border in Cheyenne County. Sediment deposition occurs in trough cross bed sets with a flow direction of - 67 ⁰ (ENE). Grains commonly range in size from 5-15 mm in diameter but can be over 100 mm. Sediment grains are friable, but calcareous cement is present. Bulk mineralogy (and fragments) is consistent with sediments derived from Pikes Peak Granite. No detailed descriptions of exposures of this unit have previously been reported along the Smokey Hill River. The Smokey Hill section is best correlated with the early Pleistocene Ogallala Formation or the Pleistocene Nussbaum Alluvium. Drilling undertaken so far indicates that the Smokey Hill section overlies the Ogallala Formation. This case study argues for the deposition of sediments by significant flooding to the ENE in rapid, shallow rivers over a significant area of eastern Colorado. The Froude number has been estimated to be from 1.30 to 5.26 for an average grain size of 8 mm to 15 mm, respectively and parameters capable of material size suspension. Froude numbers calculated indicate that water flow was supercritical during deposition.

A change in fluvial sedimentation occurred as Pliocene-Pleistocene temperatures cooled and moisture increased. Temperature change contributed to greater discharge and carrying capacity of drainage systems out of the Rocky Mountains to the eastern plains (Stanley & Wayne, 1972). The source area of the paleo-headwaters is interpreted to be from the South Park region of Colorado, 170 miles due west of the Smokey Hill Formation outcrop. This latitude is parallel to the path of the present-day Arkansas River.

The proposed case study will determine if the SouthPark area basin was a regional reservoir during the Pleistocene. If so, did it rupture or outflow rapidly causing mass large particle movement from the Front Range area adjacent from the Pikes Peak region, depositing plutonic and older sediments across the plains of eastern Colorado.