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

METAMORPHIC ROCKS—PRECAMBRIAN

Gneiss

Many kinds of metamorphic rocks crop out in the Black Canyon. Gneiss, schist, and quartzite predominate and have a profusion of textural variations. Gneiss is the most common rock in the canyon, and fine-grained varieties of gneiss predominate in areas reached by trail or overlook in the national monument. Petrographers would call some of these rocks "quartz phyllonite." Good examples are seen at Tomichi Point, Pulpit Rock, Island Peaks, The Narrows, and Kneeling Camel, to mention but a few. Coarse-grained varieties predominate downstream below Warner Point, in the vicinity of Red Rock Canyon, and upstream from the monument near Blue Mesa Dam.

Most of the fine-grained gneisses are conspicuously layered. This layering is chiefly a relict feature preserved from ancient times before the rock was metamorphosed. The rock, in other words, accumulated layer by layer as sands and muds on the floor of a primordial sea. Compaction and crystallization, aided by burial, converted the sediment to rock. Shearing, granulation, recrystallization, and contortion deep within the earth imposed further changes. Strong uplift and deep erosion finally brought the rock to its present position at the surface of the earth.

Close examination of the fine-grained gneisses shows that they consist of mixtures of several different minerals. Though not at all obvious to the naked eye, these minerals are predominantly quartz and mica—chiefly clear mica or muscovite and subordinately black mica or biotite. In various proportions are feldspar and traces of other minerals. Feldspar predominates in many of the coarse grained gneisses, but quartz and mica are invariably present in appreciable quantity. Garnet, sillimanite, and, more rarely, staurolite and tourmaline are accessory minerals found in some gneisses of the Black Canyon. Magnetite, apatite, and zircon in trace amounts are present in most of them.

Many gneisses in the Black Canyon, especially in the upper part of the canyon near such places as Blue Mesa Dam and Pioneer Lookout Point, are "hybrid" rocks that contain about equal parts of igneous and metamorphic material. The igneous material was introduced at great depth into the preexisting rock as a hot fluid which permeated all available cracks, fractures, and other planes of weakness such as bedding or foliation planes. The resultant rock is variously called ribbon gneiss, injection gneiss, or migmatite. It consists generally of light-colored layers of quartz- and feldspar-rich material intermixed with darker colored layers of quartz- and mica-rich material. Gneiss of this sort (fig. 9) commonly shows evidence of severe deformation under high pressure, such as tight wrinkling of bedding planes or intricate folding and faulting on both a small and a large scale.

FIGURE 9.—Contorted gneiss, north rim of Black Canyon near Colorado State Highway 92. Contortions caused by flowage under great heat and pressure in the depths of earth's crust. Complicated structures such as these are confined to the Precambrian rocks. Light-colored bands are mostly feldspar and quartz, dark bands mostly quartz and biotite. Note fountain pen for scale.

Quartz-Mica Schist

Schist is much subordinate to gneiss in the Black Canyon, but it is plentiful in some places including parts of the national monument. Despite infinite variations in texture, grain, and mineralogy, only two basic types have wide extent—quartz-mica schist and hornblende schist or amphibolite.

Quartz-mica schist is found in the monument chiefly in the area between Warner Point and the mouth of Red Rock Canyon where it forms the core of a large anticline in the floor and wails of the Black Canyon. Farther north, it is found in Red Canyon of Crystal Creek and to the southeast at Sheep Knob, Morrow Point, Mesa Creek, and adjacent parts of the Black Canyon near Morrow Point damsite. In all these areas it is thought to be the same thick body of schist, repeated from place to place by folding and faulting.

The coarser grained schists are quick to catch the eye of most viewers. They have a glittery spangly appearance caused by countless flakes of muscovite and biotite in a matrix of quartz. Feldspar is present in most specimens, also, and with its volumetric increase the rock grades into gneiss. The finer grained schists are less arresting, having a silky or satiny look on fresh exposures. They gain character, however, from scattered small but well-formed bright-red crystals of garnet a millimeter or two across. Other minerals likely to be present in quartz-mica schist, but mostly in microscopic grains, are sillimanite, hematite, apatite, and tourmaline.

Amphibolite

Amphibolite is a term applied to foliated metamorphic rock composed chiefly of a mineral of the amphibole group (mainly hornblende) and a feldspar of the plagioclase group. Besides these constituents, the amphibolites of the Black Canyon commonly contain subordinate apatite, biotite, diopside, epidote, magnetite, quartz, sphene, and zircon—mostly in microscopic grains. The more strongly foliated amphibolites are sometimes called hornblende schist.

Dark-gray to nearly black masses of amphibolite are abundant in many parts of the Black Canyon, including the national monument where, however, they occur mostly outside the areas served by trails and overlooks. Thin amphibolite bodies crop out sparsely on the rim between Pulpit Rock and Gunnison Point in a zone that reaches across the canyon toward Kneeling Camel. Most of the larger bodies are associated stratigraphically with quartz-mica schist. Thick ones that crop out in the canyon walls between Warner Point and the mouth of Red Rock Canyon are visible at a distance from High Point and Sunset View; northwest from High Point some very dark colored ones can be seen 2 to 3 miles distant on the canyon wall below Green Mountain.

What probably is part of the same group of amphibolite bodies just described crops out north of Green Mountain in Red Canyon of Crystal Creek. Upstream from the monument, amphibolite is abundant in both walls of the Black Canyon a mile east of Morrow Point damsite and 2 miles west, farther east near the mouth of Round Corral Creek (Curecanti Needle quadrangle), and still farther east in the Sapinero area. Possibly the most accessible outcrops are near the south abutment of the U.S. Highway 50 bridge across Blue Mesa Reservoir 4 miles east of the dam. The best and most complete exposures, however, are those in Red Canyon of Crystal Creek.

Quartzite

Quartzite is subordinate volumetrically to gneiss and schist in the Black Canyon, but it has wide distribution and is an important member of the metamorphic sequence. Interbedded with either gneiss or schist, it grades into gneiss by an increase in feldspar content and into schist by an increase in mica. Even the purest quartzite in the Black Canyon contains some feldspar and mica.

Well-exposed quartzite crops out in the monument on the north rim along a primitive road half a mile east of the North Rim Ranger Station. Though thoroughly recrystallized, it shows well-preserved details of original bedding. These outcrops are the most easily reached good exposures of quartzite in the Black Canyon. The same beds trend southwestward to the steep brushy slopes across the canyon north of Gunnison Point. Thick sequences of similar quartzite, possibly the same strata, crop out near the southeast boundary of the monument on the north rim 1 to 2 miles or so east of East Portal. Similar quartzite is exposed 20 miles farther east at Lake Fork. Quartzite, grading into gneiss and interbedded with mica schist, crops out in the cliffs near the mouth of Cimarron Creek.