Late Cenozoic Sedimentation and Deformation in Northern Colorado and Adjoining Areas

Miocene sedimentary rocks in northern Colorado record evidence of late Cenozoic deformation, including folding, uplift, and normal faulting. Faults with late Cenozoic movements are localized along zones of Laramide faulting, and many have movements in an opposite direction from their Laramide movements. The Miocene formations in northern Colorado and their stratigraphic equivalents in Wyoming and Nebraska include the Browns Park, North Park, and Troublesome of northwest Colorado and the Arikaree and Ogallala of northeast Colorado. These formations, which formerly were much more extensive, are mainly nonorogenic eolian and fluvial siltstone and sandstone as much as 900 m thick. In the White River Plateau, Grand Mesa, State Bridge, and Middle Park areas, the sediments are interlayered with, or intruded by, basalts that are remnants of a much more extensive volcanic field than is preserved today. Deformation accompanied and followed deposition of Miocene sediments and basalts, as shown by (1) deposition of Miocene rocks in a paleovalley cut prior to 25 m.y. along the axis of the Uinta arch and normal faulting later than 9 m.y. ago in the eastern Uinta Mountains, (2) major uplift later than 10 m.y. ago of Miocene rocks of the White River Plateau and folding of Miocene basalt in the State Bridge area, (3) faulting of Miocene rocks on the west flank of the Park Range, (4) faulting of Miocene rocks indicating renewed deformation along the trace of the Williams Range thrust (Laramide ancestry) in Middle Park, (5) faulting of Miocene rocks along the Blue River, suggesting uplift of the Gore Range, and (6) sharp folding of Miocene rocks in the North Park syncline and faulting in Saratoga valley.