DETRITAL ZIRCON GEOCHRONOLOGY OF PENNSYLVANIAN STRATA IN COLORADO: EVIDENCE FOR APPALACHIAN-DERIVED SEDIMENT AND IMPLICATIONS FOR THE TIMING OF ANCESTRAL ROCKY MOUNTAINS UPLIFT

Pennsylvanian-Permian time in northern Colorado corresponds with uplift of the Ancestral Front Range and deposition of the Fountain, Ingleside, and Lyons Formations along its flanks. The Pennsylvanian in southwestern Colorado is characterized by deposition of the Molas and Hermosa Formations along the flanks of the Uncompahgre Highlands. We present new detrital zircon LA-ICPMS U-Pb data (120–150 zircons from each sample) from formations in northern and southwestern Colorado to understand sediment provenance and dispersal patterns. Sandstone samples were analyzed from the shallow marine lower and upper Ingleside Formation, the eolian Lyons Formation, the non-marine upper Molas Formation, and shallow marine lower Hermosa Formation. All sandstone samples display mixed Laurentian sources with age populations that record local and distal sediment derivation. All samples show 5%-10% concordant zircons ranging from 330–490 Ma, coinciding with high magmatic flux during the Taconic and Acadian orogenies in the Appalachian orogen. Appalachian-derived sources are also indicated by zircon age populations ranging from 500-750 Ma and 1000-1300 Ma that likely originated from accreted Peri-Gondwanan terranes and Grenville inliers, respectively.

This study detects the oldest appearance of Paleozoic zircons along the Ancestral Front Range, corresponding to deposition of the lower Ingleside Formation. We compare our data along the Front Range to prior detrital zircon studies from the Fountain Formation to conclude that a shift from non-marine to marine deposition across the Fountain-Ingleside transition was accompanied by decrease in local detrital zircons, likely marking the cessation of Ancestral Front Range uplift. Conversely, a shift from non-marine to marine deposition across the Molas-Hermosa contact was accompanied by increase in local detrital zircons, likely marking the initiation of the Uncompahgre uplift. Dominant populations of local zircons in the Pennsylvanian-aged Fountain and Hermosa Formations suggest that the Ancestral Front Range and Uncompahgre uplifts were restricted to Pennsylvanian time. Cessation of Ancestral Front Range uplift is interpreted to have begun in Late Pennsylvanian time, allowing for larger contribution of distal sediments in the Ingleside and Lyons formations.