Geological Survey Bulletin 1191 Black Canyon of the Gunnison: Today and Yesterday

THE GUNNISON UPLIFT

The dominant structural feature of the Blank Canyon area is the Gunnison uplift. Though it is fundamentally a Laramide structure and hence is relatively young in a geologic sense, its outlining structural elements began to take form eons before in Precambrian time. In the main, it is outlined by old Precambrian fault blocks which readjusted themselves to renewed stresses during the Laramide orogeny. The uplift, therefore, as shown by figure 19, is mainly a tilted up-faulted block rather than an updomed fold. It is gently flexed across the crest, however, and is sharply folded locally along faults.

FIGURE 19.—Generalized section across Gunnison uplift showing restored form. Vertical scale is exaggerated about 2-1/2 times.

If minor fractures and displacements are ignored, the Gunnison uplift is thus seen as a broad tilted block, now capped in most places by Dakota Sandstone but stripped down in some places to the Precambrian basement and overlapped in others by younger volcanic rocks. The sloping tops of Fruitland Mesa, Grizzly Ridge, Pine Ridge, and Vernal Mesa are manifestations of this great block. The steep south slopes of Vernal Mesa and Poverty Mesa coincide with lines of faulting. Far to the east, the southward rise of the old Uncompahgre peneplain in the Lake Fork area is a further manifestation of the Gunnison uplift. This rise brings Precambrian rocks from river level at Sapinero to mesa-top level on Willow Creek Mesa, an average rise of about 240 feet per mile.

FRACTURES IN ROCKS

Joints

Joints are fractures in rocks, along which there has been little or no displacement of one side of the fracture past the other. They contrast with faults, along which there has been appreciable or even great displacement. Probably the most prevalent causes of jointing are tensional or shearing forces set up by crustal movements within the earth. Fractures form when the imposed stress exceeds the strength of the rock. Under certain conditions, the rock may fail by bending or even by solid flowage rather than by fracturing. Failure sometimes takes all three forms. In the Black Canyon, joints are among the more spectacular manifestations of past earth stresses. Large well-formed joints contribute greatly to the character of the canyon scenery, particularly in the steep-walled sections of the national monument. (See figs. 5, 20, and frontispiece.)

Two main sets or families of steeply dipping joints extend throughout the area. One set trends northwestward, the other northeastward. Each set contains innumerable individual fractures, all about parallel, some large, and some small. The northwesterly set is the more prominent in the monument, and excellent examples are at every hand. Cross Fissures View in particular, as the name suggests, looks out along a zone of very large joints. Widened by weathering, these joints form great open fissures hundreds of feet deep and thousands of feet long. The same zone passes below Spruce Tree Point where it is visible to the north toward Big Island and to the east across the canyon. Deep fissures controlled by the northwesterly joint set also indent the face of Painted Wall (frontispiece) and the cliffs at Cedar Point and Dragon Point.

Being natural planes of weakness, joints materially aid weathering and erosion by providing access for moisture or frost. By the gradual enlargement of such openings, masses of rock become separated from the main canyon wall, eventually toppling to the floor below. Large isolated monoliths such as Big Island are products of weathering and erosion along master joints. Island Peaks on the opposite rim have formed in about the same manner, although they are partly bounded by dikes and foliation planes, as well as by joints. Countless smaller pinnacles and spires along the canyon walls are partly bounded by joints (fig. 20).

FIGURE 20,—Isolated pinnacles of quartzitic gneiss, bounded by vertical joints (in shade) and foliation planes. Sapping by frost is chief cause of deep clefts along joint planes. Pinnacles are about 1,600 feet high on northeast rim opposite Pulpit Rock.

Low-dipping joints also cut the canyon walls, but they are far less abundant in most places, and certainly less spectacular, than steeply dipping ones. They are most evident, and perhaps most abundant, in the Curecanti Quartz Monzonite pluton in the upper part of the canyon as shown in figure 11, but they are conspicuous also in such places as the north wall of Poverty Mesa. In the national monument a few large ones can be seen from the rim overlooks.

Faults

Two major faultlines and many subordinate ones pass through the Black Canyon area. The major ones and several subordinates trend east-southeastward in a subparallel manner. Other faults trend northward to northeastward. One of the major faults is called the Red Rocks fault. It trends 20 miles or more from the head of Red Rock Canyon to Fitzpatrick Mesa west of Blue Creek. It forms the escarpment south of High Point, trends up Jones Draw toward the South Rim Campground, and continues southeastward beyond East Portal. This fault is marked by a wide zone of intensely shattered rock; hence, in many places it forms the bottom of ravines. Its displacement is visible from Tomichi Point, looking southeast up the river.

The other major fault is called the Cimarron fault for its exposure near Cimarron, Colo. Named by J. Fred Hunter in 1925, it has been known to geologists since the 19th century. It arises near Bostwick Park and trends at least 40 miles east-southeastward to Powderhorn, perhaps farther. Key places where the fault is well shown are the imposing escarpment west of Cimarron (fig. 21) and the narrow gap of Blue Creek south of Half Way House.

FIGURE 21.—Cimarron fault, looking north across valley of Squaw Creek, just east of Cerro Summit. Precambrian rocks in upthrown block above fault trace and Mancos Shale in downthrown block below. Fault here has a vertical displacement of perhaps 5,000 feet. Poverty Mesa forms skyline.

Both the Red Rocks and the Cimarron faults are very ancient structural features, reactivated in Laramide time. Their earliest movements probably were Precambrian, and they haven't moved since Tertiary time. The high escarpments along both these faults are due to differential erosion of hard and soft rocks rather than to recent topographic displacements.

Subordinate faults are plainly visible at several points along the Black Canyon, where they displace the old Uncompahgre peneplain. Most of them die out upward or laterally into Jurassic and Cretaceous strata, their displacements passing into folds (fig. 22).

FIGURE 22.—Block diagram showing lateral and vertical passage of a fault into a monocline as exemplified in the Black Canyon area.

One of the best exposed fault zones in the entire Black Canyon area trends almost due northward along the lower part of the canyon, mostly in the west wall of the canyon. This zone is very conspicuous from many vantage points because dark Precambrian basement rocks are faulted against bright-colored younger sedimentary rocks. The zone contains several subparallel faults, most of which are upthrown on the west and pass laterally or vertically into monoclines (fig. 22). The longest individual fault of this zone has a surface trace of more than 6 miles, and it displaces the old Uncompahgre unconformity more than 800 feet. It arises from a monoclinal fold—visible from Montrose—at the first large ravine 3 miles downstream from Red Rock Canyon on the southwest side of the Black Canyon.

Trending northward, the fault crosses to the east side of the Black Canyon near the mouth of the tributary ravine, recrosses to the west side 2-1/2 miles farther north, then trends almost due northward parallel to the river for an additional 2-1/2 or 3 miles.

STRUCTURAL FEATURES CONFINED TO THE PRECAMBRIAN ROCKS

The Precambrian rocks, in their great antiquity, have been subjected to intensive repeated deformations unshared by the overlying younger rocks. They were intricately folded and fractured on both a small and a large scale before the younger rocks were deposited. Their folds range upward in size from microscopic wrinkles to great bends 5 to 10 miles across; their fractures range from hairline cracks to fault zones hundreds of feet wide and tens of miles long.

Small-scale folds are apparent in nearly every outcrop of metamorphic rock, but most particularly in the migmatitic gneisses (fig. 9). Intricate contortions in these rocks attest to the thoroughness of their deformation. Large-scale folds, though less evident to casual viewing, are demonstrated by detailed geologic studies—a case of the forest being hidden by the trees. The core of one major anticline, however, is well exposed to view from U.S. highway 50 near the mouth of Cimarron Creek. This fold was first described years ago by Hunter (1925), who observed that it is accentuated by large pegmatite dikes intruded along the layering of the folded metamorphic rocks and eroded differentially into sharp relief. Its form and magnitude exemplify other equally large folds less obviously exposed elsewhere in the Black Canyon.

Faults of the area, as noted before, are mostly reactivated Precambrian fractures. Some of them show clear evidence of early movement many times greater than their latest movement. Others have not moved at all since the old Uncompahgre peneplain formed.

Careful scrutiny of nearly any outcrop of metamorphic rock discloses abundant microfaults. These little fractures may have displacements of a few millimeters or several centimeters. Several generations of them may be discerned in a single outcrop. Scarce in the younger rocks, they further confirm the disparate complexity of the older rock structure.

CRUSTAL WARPING OF TERTIARY AGE AND ITS POSSIBLE EFFECT ON DRAINAGE

Warped volcanic ash flows in the mesas along the upper reaches of the Black Canyon indicate late Tertiary crustal adjustments in the area. Ash flows deposited as gently sloping layered sheets—sloping away from their eruptive source—now have the aggregate form of a broad shallow syncline. In section the syncline resembles a great shallow saucer. In plan its contour is more like a celery dish. In other words, it is elongate roughly east-west. It also is somewhat arcuate, concave northward. As shown by figure 23, its axis or keel slopes eastward and has a structural relief of more than 1,400 feet.

FIGURE 23.—Structure contour map, drawn at the base of the ash-flow tuff volcanic sequence, showing trend of the Gunnison River along syncline axis. (click on image for an enlargement in a new window)

Warping is not presently detectable beyond the eroded limits of the ash flows, although its effects may safely be assumed to extend much farther. Warped flows provide an easily recognized datum, but in more complex rock structure, warping is difficult or impossible to detect. And once the flow is eroded away, the evidence of warping is destroyed.

The effect of warping on the drainage history of the Black Canyon may have been considerable. In dipping strata of alternate hardness, drainage lines of downcutting streams tend to shift laterally in the direction of dip as soft layers are penetrated and hard ones are encountered. As the Gunnison cut downward through the ash flows in the initial stages of entrenchment, its course eventually became fixed along the axis of the syncline. On then encountering the hard crystalline rocks in the core of the buried Gunnison uplift, it continued to cut downward, but it ceased to migrate laterally.

A glance at a regional map shows that the Gunnison River has an arcuate course that is circumferential to the West Elk Mountains almost from its headwaters to its junction with the North Fork. The middle reach of this arcuate course coincides in trend with what remains of the curved axis of the ash-flow syncline. How much of its arcuate course beyond the eroded remnant of the syncline is due to structural controls since erased by erosion can only be guessed. The drainage pattern suggests that the syncline was formerly circumferential to the West Elk Mountains—in other words, a so-called ring syncline. Such a syncline might form by subsidence of the entire West Elk Mountain area, caused by removal of crustal support through eruption of a great volume of molten rock from the roots of the mountains. As previously noted, the volume of rock so erupted from the West Elk Mountains was very large. It may have equaled or exceeded 200 cubic miles.