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Geosciences and Environmental Change Science Center

North Park-Medicine Bow Mountains Project

Task 3: Neogene landscape evolution

Regional geologic sketch map showing distribution of late Cenozoic volcanic, intrusive, and sedimentary units. Intrusive centers in the Rabbit Ears Range and Never Summer Mountains were probably linked to contemporaneous volcanoes between about 32 Ma and 28 Ma, which have now been eroded. (larger version).

geologic map of study area

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.

dikeComposite intrusive porphyry dike in the Rabbit Ears Range, emplaced about 30 Ma (photo J. Cole)

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).

archive photoDiverse late Oligocene volcanic rocks east of the Never Summer Mountains preserved in a tilted fault block. This area is known as Little Yellowstone in Rocky Mountain national Park (photo W. Lee, USGS, 1916)

archive photoCoarse, andesitic volcanic breccias in the Little Yellowstone area east of the Never Summer Mountains (photo W. Lee, USGS, 1916)

schematic diagram of the Braddock Peak CompleSchematic diagram showing stratigraphic relationships and ages of volcanic rocks in the Never Summer Mountains (Braddock Peak complex of Cole and others, 2008) (Larger view)

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).

Geologic sketch map of the Never Summer Mountains region showing locations of intrusive and volcanic rocks of the Braddock Peak complex (Cole and Braddock, 2009). Faults that offset these late Oligocene rocks are normal faults with north and northwest trends. (larger version)

geologic map of study area explanation of geologic map

Structural sketch map of Oligocene-Miocene volcanic and tectonic features related to the Braddock Peak intrusive-volcanic complex in the northern Never Summer Mountains. Geologic features shown: oval gravity low (dashed, hachured line enclosing buried granite porphyry at Jack Creek (JC) and Mount Cumulus (MC) stock); tuff-breccia ring in Upper Illinois River (UI) drainage; Colorado River fault (CRF); Michigan River fault (MRF); North Fork thrust (NFT); Never Summer thrust (NST). Geographic features shown: Braddock Peak (BP); Chambers Lake (Cl); Cameron Pass (CmP); Clark Peak (CP); Kawuneechee Valley (KV); Little Yellowstone (lV); North Park (NP); Owl Mountain (OM); Specimen Mountain (SM); Seven Utes Mountain (SU).

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.

Iron Mountain ashflowsSubhorizontal rhyolite flows at Iron Mountain on the timberless skyline. Two flows are preserved (each with a dark, rubbly cliff at its base formed by well-lithified rhyolite). These flows probably erupted from a vent complex near Iron Mountain and spread northward over a gentle landscape that was previously covered by the Thunder Mountain rhyolite ash-flow tuff (28.0 Ma).

rhyolite ash-flow tuffPink, pumice-bearing rhyolite ash-flow tuff preserved on top of eroded White River Formation and Coalmont Formation in the North Park syncline. This tuff is a distal remnant of the Thunder Mountain rhyolite ash-flow tuff (informal) erupted at 28.0 Ma. It is overlain by boulder gravel of the North Park Formation (photo J. Cole)

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.

layers in North Park FormationFluvial boulder gravel and coarse sand of the North Park Formation, lying immediately above the Thunder Mountain rhyolite ash-flow tuff on the south flank of the North Park syncline (photo J. Cole)

heterolithic boulder gravel and pebbly sandstoneCharacteristic heterolithic boulder gravel and pebbly sandstone of the North Park Formation in the middle of the North Park syncline. This outcrop is located at the headquarters of the Arapaho National Wildlife Refuge along the Illinois River (photo J. Cole)

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

Blue RidgeRemnants of a coarse boulder gravel deposit along Blue Ridge in the southern Never Summer Mountains. The clasts are primarily Precambrian crystalline rocks, but include some Mesozoic(?) quartz sandstone, possibly from the Dakota Sandstone. This gravel currently lies above 10,000 ft on the landscape and is interpreted to have filled a paleovalley in late Oligocene-early Miocene time that was elevated by Miocene-Pliocene uplift of the Southern Rocky Mountains.

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.

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