Geosciences and Environmental Change Science Center
The rock units and landscapes of the CHB region show considerable evidence of late Cenozoic geologic events.
Erosion and relative stability seems to mark the late Eocene-early Oligocene time period following folding and faulting of the Coalmont-Middle Park Formations. Minor remnants of fine-grained, gray, tuffaceous siltstone and lacustrine limestone are preserved in a few scattered localities in the region; these deposits are correlated with the upper Eocene-lower Oligocene White River Formation. Steven (1960) mapped more extensive White River Formation within a south-flowing paleovalley system on the flank of the Medicine Bow Mountains north of Walden.
Late Oligocene time (about 32-28 Ma) was marked by considerable volcanic activity in this area. Dikes and small stocks of intermediate, porphyritic magmas were intruded in an east-west alignment beneath what is now the Rabbit Ears range, and volcanic breccias are preserved along the western side of the central CHB. Concurrently, diverse magmas intruded the Never Summer Mountains along a north-south alignment, producing cylindrical stocks, numerous dikes, lava-dome complexes, and fairly thick erupted deposits of volcanic ash, breccias, and lava flows. This igneous suite in the Never Summer Mountains has been designated the Braddock Peak complex, in honor of Bill Braddock (late geology professor from Univ. Colorado-Boulder).
The oldest magmas in the Braddock Peak complex are alkali basalts and andesites that erupted and flowed down paleovalleys that had been carved into the basement rock. Subsequent eruptions produced dacite and rhyodacite porphyries in addition to further mafic magmas, suggesting both evolution of magma chambers and replenishment by fresh mantle melts over time. Much of this geology is described in the guidebook for a Colorado Scientific Society field trip (Cole and others, 2008). See also the Geologic map of the Estes Park quadrangle (Cole and Braddock, 2009).
The youngest magmas erupted from the Braddock Peak complex consist of high-silica rhyolite characterized by prominent smoky quartz phenocrysts. The rhyolite erupted as ash-flow tuffs (at least two discrete flow units) that are preserved at Thunder Mountain, and locally spread many miles across the landscape into North Park and areas to the north. The Thunder Mountain rhyolite ash-flow tuffs erupted at 28.0 Ma. Slightly younger rhyolite flows covered the landscape northward across Iron Mountain.
Volcanic and intrusive rocks of the Never Summer Mountains and Rabbit Ears Range were faulted and rapidly eroded nearly as quickly as they were being deposited. Most faults that displace these late Oligocene rocks are normal faults, and most of them have northwesterly trends. Regional uplift and erosion of the volcanic highlands (northern Rio Grande rift signal?) led to deposition of the Oligocene-Miocene North Park Formation here and northward into Wyoming. These deposits typically contain clasts of volcanic rock derived from the surrounding igneous centers.
The North Park Formation rests directly on top of the Thunder Mountain rhyolite ash-flow tuff (28.0 Ma) in North Park and contains clasts of the rhyolite, numerous other varieties of volcanic rock, and Precambrian crystalline rocks. The distribution and sedimentology of the North Park Formation shows that it was transported by energetic streams and rapidly deposited. Finer-grained deposits correlated with the North Park Formation are present northward into Wyoming in a half-graben along the North Platte River in the Saratoga Valley
The volcanic landscape probably exerted a major influence on drainage systems that evolved in early Miocene time. High-level coarse gravel deposits are preserved at various places around North Park and they seem to have formed in paleovalleys that drained the volcanic landscape. They contain similar clast assemblages as the North Park Formation and they also locally overlie 28.0 Ma rhyolite ash-flow tuff.
Post-Miocene time is marked in this area by erosion, drainage integration, and locally significant stream capture and headwater diversion. Significant young faulting is not evident, and yet the persistent height of the northern Rockies in this area (significant areas in excess of 10,000 ft) and the rapid, deep incision of most major drainages indicates young uplift.