Geologic and physiographic highlights of the Black Canyon of the Gunnison River and vicinity, Colorado
Wallace R. Hansen
The Black Canyon of the Gunnison straddles the boundary between two physiographic provinces, the Southern Rocky Mountains to the east and the Colorado Plateau to the west. Inasmuch as the boundary there is ill defined, the area shares characteristics of both provinces—sharp ridges, broad mesas, precipitous canyons, complex geology, and vegetation communities that range from desert shrub to boreal forest. The ultimate cause of this dramatic setting is the Gunnison River which, down through time, has carved the Black Canyon out of the heights of the Gunnison uplift. The river and its reservoirs separate the West Elk Mountains on the north from the San Juan Mountains on the south (fig. 1).
Physiographically and geologically, the Black Canyon is divisible into three sections—lower, middle, and upper—which merge gradually with one another but have marked differences. Common to all three, and imparting a certain unity, is an inner gorge of crystalline rock renowned for its sheer walls, startling depths, and awesome countenance. At one time the entire canyon area was covered by a capping of middle Tertiary volcanic rocks. This cap- ping, which provided the means for subsequent superposition of the river, has been differentially eroded, so that none remains in the lower section of the canyon and most is gone from the middle, but in the upper it is well preserved on the high mesas above Morrow Point and Blue Mesa reservoirs.
In the lower section, flaring walls of bright-colored Mesozoic sedimentary rocks, 300 m thick and nearly free of soil, surmount a narrow inner gorge of dark Pitts Meadow Granodiorite (fig. 2). The lower section is eroded into the plunging crest of the Gunnison uplift, and resistant Dakota Sandstone forms the outer canyon rim, sloping away in long, barren dip slopes. Successively older strata— the Burro Canyon, Morrison, Wanakah, and Entrada Formations— underlie the Dakota above the inner gorge. At the top of the Precambrian, the well-exposed Uncompahgran unconformity truncates the roots of the old Uncompahgre highland, formed in Penn- sylvanian time with the rise of the Ancestral Rocky Mountains. In the middle section, which contains Black Canyon of the Gunnison National Monument and Crystal Reservoir, the rim view is dominated by a rather flat skyline formed of hard Precambrian rocks on the south rim and poorly exposed sedimentary rocks on the north. Most of the sedimentary capping has been stripped from the south rim, exhuming the Uncompahgran surface. The sedimentary section, though preserved on the north rim, is largely concealed by soil and shrubbery. Here the inner gorge attains its greatest depth and grandest development, particularly between Pulpit Rock and Painted Wall (fig. 3), where it is 11/2 times deeper than wide (fig. 4), rimmed by the highest cliffs in Colorado.
In the upper section, remnants of the once broader volcanic cover rest directly on the Precambrian basement and form the heights of the canyon walls, although a thin wedge of Mesozoic rocks intervenes toward the head of the canyon. The rim is uneven and indefinite, merging with higher rolling back country or, locally on the south, becoming a serrated ridge. But its chief attribute is a nearly continuous palisade of resistant welded Blue Mesa Tuff 15- 100 m thick, forming a line of cliffs 300-600 m above the canyon floor and rising toward the west (fig. 5). The canyon has the V- shaped profile of a typical, though exceptionally rugged, mountain stream valley modified by fjord-like Morrow Point Reservoir. At Morrow Point Dam, the north rim is 1,000 m above the river.
Clearly, the middle section is the most dramatic reach of the Black Canyon, and few gorges in the world are its peer. The more verdant upper section is impressive in its own right, especially in autumn as seen from Colorado Highway 92, when the frosted oaks, choke cherries, and aspens herald the coming of winter. So far as the rocks are concerned, the lower section is the most colorful, and it best displays the sedimentary capping of the Gunnison uplift, the geologic setting of the canyon, and the regional structural framework.
- Hansen, Wallace R., 1981, Geologic and physiographic highlights of the Black Canyon of the Gunnison River and vicinity, Colorado, in: Western slope Colorado--western Colorado and eastern Utah, Epis, Rudy C.; Callender, Jonathan F., New Mexico Geological Society, Guidebook, 32nd Field Conference, pp. 145-154.