Proposing an Entirely Pennsylvanian Age for the Fountain Formation through New Lithostratigraphic Correlation along the Front Range

Abstract

The Fountain Formation is an important unit recording ancestral Rocky Mountain tectonics and paleogeography along the Front Range urban corridor. The age of the formation constrains timing of uplift of the ancestral Front Range and Ute Pass uplift and subsidence of adjacent basins. Yet, age models for the Fountain Formation are crude and varied. Specifically, available biostratigraphic data suggest an entirely Pennsylvanian age for the Fountain Formation, however historical lithostratigraphic assignment of the Lyons Formation atop the Fountain Formation south of Lyons, Colorado allows for a significant early Permian component of deposition. New stratigraphic and sedimentologic data recorded from the Ingleside Formation and the Lower Permian unit at Manitou Springs, Colorado demonstrate: 1) a conformable contact between the upper Fountain Formation and Lower Permian strata at both localities and 2) close grain size and framework mineralogy comparisons at both localities. These data suggest that the Lower Permian at Manitou Springs, Colorado best correlates to the Ingleside Formation, rather than the previously mapped Lyons Formation. This newly proposed lithostratigraphic correlation aligns with the available biostratigraphic data that the Fountain Formation is a Pennsylvanian unit with little to no Lower Permian component. An entirely Pennsylvanian age for the Fountain Formation indicates that active uplift of the ancestral Front Range and delivery of first-cycle arkose had ceased by the latest Pennsylvanian.