PROPOSING AN ENTIRELY PENNSYLVANIAN AGE FOR THE FOUNTAIN FORMATION EXPOSED ALONG THE FRONT RANGE, COLORADO

The Fountain Formation is an iconic clastic unit derived from Precambrian-cored uplifts of the Ancestral Rocky Mountains (ARM) and exposed along the modern Front Range today. The age of the formation constrains timing of uplift and adjacent basin subsidence of the ancestral Front Range and Ute Pass uplifts. Yet, scarce age constraints exist. Atokan conodonts recovered from near the base of the formation at Manitou Springs, Colorado and upper Virgilian fusulinids recovered at the top of the formation north of Loveland, Colorado. These age constraints have led to two different chronostratigraphic charts: 1) a chart for areas north of Lyons, Colorado that show the formation ranging from latest Morrowan to latest Virgilian, and 2) a chart for areas south of Lyons, Colorado that show the formation ranging from latest Morrowan to mid-Wolfcampian. The upper boundary of the southern chronostratigraphic chart is based solely on lithostratigraphic correlation of the mid-Wolfcampian Lyons Formation atop the Fountain Formation south of Lyons, Colorado.

New data suggests that at Manitou Springs, Colorado the overlying Permian unit is better lithostratigraphically correlated to the early Wolfcampian Ingleside Formation, rather than the currently mapped Lyons Formation. North of Lyons, Colorado the upper Fountain Formation exhibits intercalated fluvial-eolian strata that grade upward into eolian strata of the Ingleside Formation. The same relationship is observed in the upper Fountain Formation at Manitou Springs, Colorado. For further comparison, we sampled the Permian eolian strata at Manitou Springs, Colorado to compare with eolian strata of the Ingleside and Lyons formations exposed north of Lyons, Colorado. These results show that ratios of feldspar, grain size distribution and grain packing of the Manitou Springs, Colorado eolian strata is most similar to the Ingleside Formation. Thus, we propose a revised lithostratigraphy for Manitou Springs, Colorado.

Assuming minimal time transgression of the basal Ingleside Formation, the revision restricts the Fountain Formation to Pennsylvanian. Moreover, at Manitou Springs, Colorado, the upper ~300 m of the Fountain Formation post-dates late Paleozoic movement of the Ute Pass fault suggesting that local ARM tectonics here ceased before end of the Pennsylvanian.