A PENNSYLVANIAN AGE FOR THE FOUNTAIN FORMATION CONSTRAINED BY LITHOLOGIC COMPARISON OF OVERLYING EOLIAN UNITS

The Fountain Formation is poorly dated, but contains basal and upper biostratigraphic control at two different locations. The basal age of early Pennsylvanian is denoted by conodonts recovered at Manitou Springs, Colorado. The upper age of latest Pennsylvanian is denoted by fusulinids recovered north of Loveland, CO. However, at localities south of Denver, CO, the Fountain Formation is often portrayed as ranging into the early Permian largely because it is overlain by an eolian unit previously mapped as Permian Lyons Formation. This lithostratigraphic correlation of the Lyons Formation occurs over approximately 200 km through discontinuous and faulted outcrop. Yet, in northern Colorado, two eolian stratigraphic units occur: earliest Permian Ingleside Formation and slightly younger Lyons Formation. Here we present petrographic, sedimentologic and stratigraphic data that suggests that the upper constraining eolian unit at Manitou Springs more closely resembles the Ingleside Formation.

Near Loveland, Colorado, eolian and fluvial strata intercalate in the upper 50 m of the Fountain Formation before giving way into the predominately eolian Ingleside Formation. This gradational transition from largely fluvial/alluvial to predominantly eolian is also observed to the south at the Manitou Springs locality. Sandstone mineralogy and grain size distributions of the Ingleside Formation mimic the Manitou Springs eolian unit, whereas similar data from the Lyons Formation does not. Therefore, at Manitou Springs, we suggest that the eolian unit atop the Fountain Formation lithologically is best characterized as Ingleside Formation. Barring a significant southerly time transgression for the Ingleside Formation, this proposed change in nomenclature further suggests age control from the north can be used to constrain a minimum age for the Fountain Formation. Thus, an entirely Pennsylvanian age for the Fountain Formation is proposed.