Geology of the Cement Mountain Area, Elk Mountains, Gunnison County, Colorado by Keri Nelson

Abstract Compressional forces during the Late Cretaceous to Early Tertiary Laramide Orogeny created a series of eastward directed low angle thrust faults and subsequent thick skinned deformation. Today’s Rocky Mountains are remnants of this deformation. Geologic mapping of central Colorado’s Elk Mountains, specifically the Cement Mountain area 10 miles southeast of Crested Butte, Colorado, has revealed folds and faults of Paleozoic carbonate rocks associated with the Laramide Elk Range thrust fault to the north. All structural features in the area trend to the northwest as does the more than 50 kilometers of thrust fault. Further detailed study of this fault will lead to determining its extents and relationship to the complexities that have lead to the present day position of the Elk Mountains. Introduction Cement Mountain is located in the Elk Mountains, southeast of Crested Butte, Colorado, on the southern side of Cement Creek. The Cement Mountain Area, including Rosebud Gulch and the southeastern portion of Deadman Gulch (Fig. 1), is composed of Precambrian igneous and metamorphic rocks, which are dominantly overlain by Paleozoic carbonate rocks. North of Cement Mountain, the Tertiary Elk Range thrust fault system, striking northwest-southeast, splays into two subsidiary faults (Tweto and others, 1976) (Fig. 2). The southern splay of this fault has caused uplift and deformation of the Paleozoic carbonate rocks that are present in the area.

No detailed geologic maps of the Cement Mountain Area have been published, leaving no information on the deformation in the area associated with the Elk Range thrust fault. An anticline and monocline were mapped along with several faults as part of research carried out in the Fall of 2003 for the Bartleson/Prather Excellence in Geology Scholarship offered through Western Sate College. Also observed and studied is a jasperoid unit (secondary microcrystalline quartz) that is apparently replacing portions of the Ordovician Mantiou Dolomite, possibly due to hydrothermal alteration.

N Figure 1: Texture map of the Cement Mountain area including Rosebud Gulch, Deadman Gulch, Cement Creek to the northwest, Highway 135 to Crested Butte in the southwestern corner, and Spring Creek in the southeast corner.

Figure 2: Geologic map of the northern and southern splay of the Elk Range thrust fault (in circle). The Pennsylvanian/Permian Maroon Formation (PPm, light blue) is being thrusted over younger Cretaceous units on the Pennsylvanian Belden Shale (Pmb, light gray). Cement Mountain is in the south central section of the map. From Tweto (1979).

Geologic Setting During the Late Cretaceous to the Early Tertiary, compressional forces during the Laramide Orogeny caused thick-skinned deformation creating the present day Rocky Mountains. Eastward directed thrust faults and folds are remnants of the Laramide Orogeny in the Elk Mountains. Geologic mapping of the Cement Mountain area has revealed an anticline, forming Cement Mountain with a slightly northwest-southeast trending axis, and a monocline (not present on map, farther east), in the southeastern most portion of Deadman Gulch and trending northwest, which are most likely associated with the Elk Range thrust fault. Tweto (1979) mapped the more than 50Km of the Elk Range thrust fault on the Geologic Map of Colorado (fig.1) and the southern splay of the fault, north of Cement Mountain, has been approximately mapped on the detailed map of the Cement Mountain Area (plate 1). As seen on the Geologic Map of Colorado (Tweto, 1979), the hanging wall, composed of the Pennsylvanian/Permian Maroon Formation and

older units has been thrusted over younger Cretaceous units, moving on the decollment of weak Lower Pennsylvanian Belden Shale (fig. 2 and 3).

Figure 3: Northwest view from top of Cement Mountain of dipping Maroon beds as they are thrusted up by the Elk Range thrust fault on Double Top Mountain. Sawatch Range mountains are in the background and Cement Creek drainage is running down the center. Stratigraphic Analysis The stratigraphy of the Cement Mountain area includes the Cretaceous Dakota Sandstone, Mississippian Leadville Limestone, Devonian Chaffee Group, Ordovician Freemont Dolomite, Ordovician Harding Sandstone, Ordovician Manitou Dolomite, Cambrian Sawatch Quartzite, and the Precambrian granitic rocks. Upper Cretaceous Dakota Sandstone The Dakota Sandstone consists of gray to white sandstone and quartzite with a basal chert pebble conglomerate. The only exposure mapped on the Cement Mountain geologic map (plate 1) is in the central eastern portion of the map near Highway 135. Its total thickness in the area ranges from 120-300feet (Prather, 1999).

Lower Mississippian Leadville Limestone The upper approximate 100 feet of the Leadville Limestone consists of dark gray, thick bedded, fine to medium grained limestone and fine grained dolomite with abundant chert nodules in the top 20 feet (Fig. 4). The lower portion is dark gray, fine grained fossiliferous limestone which weathers to a light gray rough surface. The total thickness in the area is 190-200 feet (Zech, 1988).

Figure 4: West view from Rosebud Gulch of massive Leadville Limestone cliffs. To the right of the photo is Cement Mountain. Upper Devonian Chaffee Group The Chaffee Group consists of three separate units (Zech, 1988). The Gilman Sandstone consists of upper yellowish gray, weathering, slope forming, sandy dolomite, and lower yellow to gray, medium to coarse grained sandstone with some bioturbation and dolomitic breccia. The total thickness is 14-18 feet. The Dyer Dolomite consists of dark gray to yellowish gray dolomitic limestone which weathers to platy fragments and a sandy limestone basal layer. The total thickness is 119-134 feet. The Parting Formation consists of grayish orange, coarse to medium grained, poorly sorted sandstone with a

basal 5-20 feet of grayish-orange-pink, thin bedded “Marble Cake” dolomite. The total thickness is 69 feet. Upper Ordovician Freemont Dolomite The Freemont Dolomite consists of brownish gray, cliff forming, fossiliferous dolomite and dolomitic limestone. Tabulata, horn coral, and crinoids are all present. The total thickness is 45-75 feet (Zech, 1988). Middle Ordovician Harding Sandstone The Harding Sandstone consists of light gray to white, medium to coarse grained (bimodal), well rounded, bioturbated sandstone and quartzite, with abundant blue Heterostraci fish plates (Fischer, 1978). The total thickness is 7-15 feet (Zech, 1988). Lower Ordovician Manitou Dolomite The Manitou Dolomite consists of brownish gray, thin to thick bedded, sparsely fossiliferous, sandy dolomite and dolomitic limestone. The lower 50 ft. contains abundant chert nodules. It forms massive cliffs and is hydrothermally altered to a dark red Jasper in the eastern area of the map at the Rosebud-Deadman Gulch intersection (Fig. 5). The total thickness is 180-200 feet (Zech, 1988). Upper Cambrian Sawatch Quartzite The Sawatch Quartzite consists of light gray to white, massive cliff forming quartzite with a middle layer of brown to green, thinly bedded glauconite and a basal conglomerate layer. The total thickness is 130-300 feet (Zech, 1988). Precambrian The Precambrian in the area consists of the Taylor River Region granitic rocks in the 1,400 and 1,700 m.y. age group (Tweto, 1979).

Figure 5: Southwest view up Rosebud Gulch of Cement Mountain from the top of massive low dipping Manitou Dolomite cliffs forming the top of the Deadman Gulch monocline. Deadman Gulch is to the right of the picture. Jasperoid Jasperoid forms from the dissolution of carbonate rocks by silica enriched, hydrothermal water moving through a permeable rock unit (Lovering, 1972). In the southeastern portion of Deadman Gulch, the Manitou Dolomite and the Leadville Limestone are partially replaced by dark red, microcrystalline jasperoid. This replacement has been well documented in the Leadville Limestone and Manitou Dolomite in the Leadville Mining District in central Colorado (Lovering, 1972) and Goodknight (1981) documented replacement in the Leadville Limestone a few miles to the east in the Doctor Mine. Hydrothermal circulation in the area may have been initiated and carried out by several factors including; local intrusions, Laramide age fractures in rock units, and preexisting solution cavities (Lovering, 1972). Tall, jagged “chimney” deposits are characteristic of Jasperoid (Lovering and others, 1978) and two are present in Deadman Gulch (Fig. 6).

Figure 6: Southwest view of Cement Mountain from the top of the Deadman Gulch monocline. In the center of the picture is a Jasperoid “chimney” sticking up in the trees. Structural Analysis Structural features in the Cement Mountain area, including folds, faults, and beds, strike to the northwest. The beds dip towards the northeast, except where folded, and range from moderate dips of 4 degrees to greater dips of 30 or more degrees, depending upon deformation. To the west of Cement Mountain the Devonian Chaffee Group is placed next to the Cambrian Sawatch Quartzite and Precambrian by a down to the west fault. A general view of this fault on the Geologic Map of Colorado (Tweto , 1979) shows that it crosses Cement Creek and terminates to the northwest just east of Crested Butte and to the southeast just short of the Taylor River Canyon. It also appears to be a component of a series of en echelon faults trending southeast to the Tomichi Creek drainage southeast of Gunnison (Fig. 7).

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Figure 7: Geologic map of down to the southwest en echelon faults trending southeast towards the Tomichi Creek drainage (from Tweto, 1979), and a detailed geologic map of the northern most of these faults southwest of Cement Mountain. Cement Mountain itself is an asymmetrical anticline with the Sawatch Quartzite through the Leadville Limestone folded. Its axis trends slightly to the northwest and plunges to the southeast. The northeast limb is steeper (20 degree mean dips) than the southwest limb (12 degree mean dips) (fig. 8) (cross section A-A’).

Figure 8: Southwest view of Cement Mountain showing the dipping beds of the southeast limb of the anticline. On the northern side of the southeastern extent of Deadman Gulch (and off the map) is a northwest trending monocline. The Sawatch Quartzite through the Lower Pennsylvanian Belden Shale are folded most likely by an upraised fault block in the

Precambrian. The only limb to the north is near vertical with dips across the fold ranging from 4 degrees to 60 degrees. Discussion Knowledge of the southern extent of the Elk Range thrust fault and its associated deformation is limited to the large scale Geologic Map of Colorado by Tweto (1979). Analysis of this map suggests that the northern splay of the fault continues southeast across the Talyor River drainage just below the Taylor Park Reservoir and into the Fossil Ridge area near Cross Mountain. The southern splay could also take this route or may be expressed by a series of southeast trending en echelon faults (Fig. 9). Structural features indicative of thrust fault transfer and termination studied by House and Gray (1982) could possibly be present in this area. Future, more detailed study and mapping of this area of the Elk Mountains will be essential in understanding this fault.

Figure 9: Geologic map showing the possible route of the northern splay of the Elk Range thrust fault across Taylor River Canyon and into the Fossil Ridge area (in circle). To the southwest are the en echelon faults that may represent the southern splay of the Elk Range thrust fault. From Tweto (1979).

Conclusion Several deformational features associated with the Elk Range thrust fault are present in the Cement Mountain area and by mapping the geology of this area hopefully in the long run it will aid in further understanding of this fault. Large scale benefits of understanding this fault may include understanding its relationship to the Crystal Creek fault zone (a complex east-dipping high-angle Laramide thrust system that created the Sawatch Range in central Colorado) and how the Elk Mountains ended up 5-6 kilometers from their initial position relative to the Sawatch Range.

References Fischer, W.A., 1978, The habitat of the early vertebrates: trace and body fossil evidence from the Harding Formation (Middle Ordovician), Colorado: The Mountian Geologist, v. 15, no. 1, 26p. Gaskill, D.L., Mutschler, F.E., Kramer, J.H., Thomas, J.A., and Zahony, S.G., 1991, Geologic map of the Gothic Quadranlge, Gunnison County, Colorado, U.S. Geological Survey Quadrangle Map GQ-1689. Goodknight, C.S., 1981, Uranium in the Gunnison Country, Colorado, in Epis, R.C., and Callender, J.F., eds., New Mexico Society Guidebook, 32nd Field Conference, Western Slope Colorado, p. 183-189 House, W.M., and Gray, D.R., 1982, Displacement transfer at thrust terminations in southern Appalachians-Saltville Thrust as Example: AAPG Bull. , v. 66/7, p. 830-841. Lovering, T.G., 1972, Jasperoid in the United States-its characteristics, origin, and economic significance: United States Geological Survey Professional Paper 710, 151p. Lovering, T.S., Tweto, O., and Lovering, T.G., 1978, Ore deposits of the Gilman district, Eagle County, Colorado: United States Geological Survey Professional Paper 1017, 155p. Prather, T., 1999, Geology of the Gunnison Country, Second Edition: Gunnison, Colorado, B&B Printers, p. 22-23. Tweto, O., 1979, Geologic Map of Colorado, U.S. Geological Survey 1:500,000 Map MI-16. Tweto, O., Steve, T.A., Hail, J.W. Jr., and Moench, R.H., 1976, Preliminary Geologic Map of the Montrose 1 X 2 Quadrangle, Southwestern Colorado, U.S. Geological Survey 1 X 2 Quadrangle Map MF-761. Zech, R.S., 1988, Geologic Map of the Fossil Ridge Area, Gunnison County, Colorado, U.S. Geological Survey 1:24,000 Map I-1883.