LATE MIOCENE BIRTH OF THE UPPER COLORADO AND GREEN RIVER SYSTEMS

⁴⁰Ar/³⁹Ar dating of basalt flows and detrital sanidine (DS) grains in ancient fluvial deposits provides an emerging temporal framework for the early evolution of the Colorado and Green River systems on the western flank of the Rocky Mountains.

During and following ~37-23 Ma San Juan Volcanic Field eruptions, rivers drained radially from volcanic highlands including the newly DS-dated <16 Ma Columbine Pass paleoriver. This Miocene river flowed NW along what is now the crest of the Uncompahgre Plateau and probably flowed onto the Colorado Plateau via Unaweep Canyon.

Late Miocene (10.8-9.6 Ma) basalt flows on Grand Mesa are underlain by ~11 Ma ancestral Colorado River deposits, which are ~1500 m above the Colorado River. The ancient Colorado River flowed west from the central Rocky Mountains, past present-day Grand Mesa, towards probable Colorado Plateau depocenters. The youngest extensive lava flows (Grand Mesa, Little Grand Mesa, Battlement Mesa) in the region range in age from ~9.5-8 Ma, which indicates that deep regional exhumation in the upper Colorado watershed began after this time.

In the headwaters of the Yampa River valley, which is locally cut into the Miocene Browns Park Formation, ~6 Ma basalt flows overlie Browns Park sediments. These stratigraphic relationships could suggest that excavation of the Yampa valley began after ~6 Ma, perhaps later in time than in the upper Colorado River.

New DS dating of Green River terrace flights located upstream of the Uinta Mountains in Browns Park, show that the river flowed through Browns Park, across the eastern Uinta Mountains, and onto the Colorado Plateau by ~2 Ma. Green River terrace remnants are inset into Browns Park Formation deposits that are as young as ~8 Ma. Collectively, these observations suggest that the Green River joined the Colorado River after 8 and before 2 Ma.

Provocative hypotheses that need further testing include: 1) internal drainage of the Colorado Plateau prior to Grand Canyon integration after 6 Ma. 2) Neogene uplift of the Rocky Mountains predated and helped to drive integration of the Colorado River system through Grand Canyon. 3) integration of the upper Green River across the Uinta Mountains where it joined the Colorado River system may have been as recent as ~2 Ma based on the oldest preserved terraces.