McIntosh Mountain

In the summer of 1999 the WSC Geology Department mapped two quadrangles for the National Park Service to help complete the digital map of the Black Canyon of the Gunnison National Park and Curecanti National Recreation Area. Mapping of the McIntosh Mountain and Little Soap Park Quadrangles completed 1:24000 scale mapping of the parks. As part of that study we discovered some interesting interactions between the large volume ash flow sheets of the San Juans and a sequence of through going streams that drained the Elk Mountains to the east. (Paleo- Gunnison, Taylor, and Tomichi drainages).

In the early Tertiary drainages in the Gunnison Basin must have been dominated by west and northwest flowing streams off of the high relief Elk Mountains to the east and the lower relief Gunnison Uplift to the southeast, both of which are Laramide structures. These structures form the southwest, south and eastern margins of the Piceance Basin in this area. These drainages were disrupted ~30-29 ma ago (Coven, et al.,1999 and Murphy et al. 2000) by lavas and debris flows from West Elk Volcano and the intrusion of the Gunnison Laccolith Cluster. This igneous activity blocked streams flowing west and northwest into the center of the Piceance Basin and produced the modern southward and then eastward flow of the Gunnison through Curecanti National Recreation Area and the Black Canyon of the Gunnison National Park. At ~28 ma ago drainage systems in the area were further disrupted by the emplacement of large volume ash flow sheets from both the western and central San Juan calderas that filled the topographic low between the San Juans and the West Elks.

The McIntosh Mountain Quadrangle is located in the area where these interaction are best exposed. There are numerous fluvial gravel expose between the tuffs and this is an example of the one of the largest and best exposed

The above is a detail from the geologic map of the McIntosh Mountain Quadrangle at 1:30000. This area is on the southwest slope of West Elk Volcano. The West Elk Breccia records this volcanism and makes up the lower slopes in the area. It consists of debris flows and coarse fluvial materials derived from the volcano (Twvg). Clasts in this unit are entirely volcanic, dominated by mafic and intermediate lavas. To the south, and just shown on this map, are fluvial gravels that record Paleo-Gunnison River channels (Twpg). These gravels contain Precambrian igneous and metamorphic rocks, Paleozoic and Mesozoic sedimentary rocks in addition to volcanics. West Elk Volcano was active for a short time (<0.5 ma) around 30 ma (Coven et al., 1999).

The slopes of the volcano were later modified by the emplacement of large volume ash flows from the San Juan, which filled in the Paleo-Gunnison River Channel and then ramped up onto the slopes of West Elk Volcano. The oldest ash flow on this map is the Sapinero Mesa Tuff, (27.9 ma; Bove et al. 2001), which was emplaced directly on the West Elk Breccia in this area. The emplacement of this tuff disrupted the Paleo-Gunnison River drainage, which was re-establised in channels that are recorded by polymict gravels that outcrop to the south of this area. Drainages off of West Elk Volcano were also re-established and filled with volcanic gravels at this time. One of these channels outcrops in this area (Tfvg).

These drainages were then modified by the emplacement of the Fish Canyon Tuff (27.6 ma; Lipman 2000). After this time the Paleo-Gunnison drainage was move to the north and cut a large relatively narrow channel into the tuff (Tfpg). The best exposure of the channel is in this area. The channel, where it crosses the mesa, is 175m deep and 580 across and completely cuts through the Fish Canyon Tuff down the Sapinero Mesa Tuff. At the time, this would have been a narrow gorge exposing light colored tuff and we have therefore named this the "White Canyon of the Gunnison". This channel is also unique in that it is perched on the side of West Elk Volcano about 350m above the trough formed by the top of the Fish Canyon Tuff between the San Juans and West Elks. The channel was filled by polymict gravels. The photos below show the outcrop and the view is marked on the map by black lines.

The gravels were then buried by the Carpenter Ridge Tuff (Tc), which was erupted 27.4 ma (Lipman, 2000). At that time the Paleo-Gunnison was again moved to the south and drainage off West Elk Volcano was re-established in this area and volcanic gravels were deposited on top of the Carpenter Ridge Tuff (Tmvg).

Here are two views of the "White Canyon of the Gunnison", one with and one without the geology superimposed. In this view the West Elk Breccia is overlain by Sapinero Mesa Tuff (27.9 ma) and Fish Canyon Tuff. (27.6 ma). The emplacement of the tuff disrupted the drainage and a channel was incised into the tuff and later filled with gravel. This was later overlain by Carpenter Ridge Tuff (27.2 ma). Gravels derived from the West Elks were later deposited over everything.

The map and photos below show the continuation of this channel to the west. This view is from high on a mesa looking down the paleo-slope of West Elk Volcano. The strong cliff forming unit in the middle distance is the Sapinero Mesa Tuff deposited directly on West Elk Breccia. The foreground is densely welded Carpenter Ridge Tuff with the Fish Canyon Tuff below. The mesa in the middle distance has remnant patches of Fish Canyon Tuff and two separate gravel containing channels. The nearer larger one is a continuation of the channel discussed above. The farther one is a trace of the Paleo-Gunnison River active between the emplacement of the Sapinero Mesa and Fish Canyon Tuffs. The water is Blue Mesa Reservoir which marks the modern Gunnison River. The high ridge in the background is in the LaGarita Wilderness area and the LaGarita (Fish Canyon) and Bachelor (Carpenter Ridge) Calderas are right behind the ridge.

Units are the same as above except Tspg, which is a polymict gravel that records a trace of the Paleo Gunnison River established between the emplacement ofthe Sapinero and Fish Canyon Tuffs.

Study of the gravels in this area is continuing in collaboration with Dr. Kurt Panter at Bowling Green University. Provenance studies of clast in the gravels is ongoing and preliminary results have been reported by Green et al. (2001).

References

Coven, B, Panter K and Stork A (1999) Ar/Ar age of the West Elk Volcano, Gunnison and Delta Counties, Colorado (abs.) Geol. Soc. America Abs. with Prog. vol.31, no.7, p.478

Bove et al. (2001), Geochronology and Geology of late Oligocene through Miocene volcanism and mineraliztion in the Western San Juan Mountains, Colorado, USGS Professional Paper 1642

Greene, L., Panter, K., Stork, A., Fillmore, R. (2001) Characterization of gravels interbedded with Oligocene ash-flow tuffs near Gunnison, Colorado: implications for ancient surface water hydrology and volcano-fluvial interaction: Geological Society of America Abstracts with Programs, vol. 33, no. 6. p. A-356.

Lipman P.W. (2000), Central San Juan Caldera Cluster, in Bethke P.M. and Hay R.L.(eds.), Ancient Lake Creede: Its volcano-tectonic setting, history of sedimentation, and relation to mineralization in the Creede Mining District: GSA Special Paper 346 p. 9-69

Murphy, R.T., Stork, A., Panter, K., McIntosh, W., and Esser, R., (2000), 40Ar/39Ar geochronology and petrogenesis of a Middle Tertiary volcano- laccolith complex, Gunnison County,Colorado: Geological Society of America Abstracts with Programs, v. 32, no. 7, p. 495.