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

IGNEOUS ROCKS—PRECAMBRIAN

Igneous rocks are volumetrically less plentiful in the Black Canyon than metamorphic rocks, but they are very abundant nevertheless. They are evident everywhere even to the casual observer, for they are almost singularly responsible for the impressive "architecture" of the canyon walls. Most of the more precipitous parts of the canyon are due to the erosive resistance of igneous rock. In the form of dikes, intrusive sheets, or irregular masses, moreover, they have a buttressing effect on adjacent metamorphic rock, so that the steepness and grandeur of the canyon walls at any given place depend largely on the relative quantity of injected igneous rock. The Narrows, for example, consists of gneiss thoroughly buttressed by dikes of pegmatite. The great cliffs at Chasm View and far upstream at Curecanti Needle are carved wholly from igneous rock. Painted Wall, the highest cliff in Colorado and one of the finest cliffs in the canyon, consists chiefly of gneiss, but it is laced with dikes of pegmatite, and the gneiss is so thoroughly enriched with feldspar that it approaches granite in composition, appearance, and physical behavior. (See frontispiece.)

Several kinds of igneous rock of Precambrian age crop out in the Black Canyon. By far the greater volume consists of only two rock types—quartz monzonite (several varieties) and pegmatite. Subordinate types more rarely observed include diabase, aplite, and certain dark-colored rocks called lamprophyres.

Vernal Mesa Quartz Monzonite

Quartz monzonite is a rock similar to granite in general character and appearance but different in mineralogical and chemical composition. The chief basis for distinction, in brief, is the type of feldspar. Granite contains predominantly an alkalic feldspar (generally microcline or orthoclase), whereas quartz monzonite contains roughly equal parts of alkalic feldspar and plagioclase. Chemically, therefore, granite contains more of the alkali metals sodium and potassium and less calcium than quartz monzonite does. Granite also generally contains more silica. In the terminology of the stone industry, all such rocks and many others as well are referred to collectively as granite, but to the petrographer they are distinctly different rocks.

Two chief varieties of quartz monzonite crop out in the Black Canyon. One called Vernal Mesa Quartz Monzonite (fig. 10) is a dark-gray coarse-grained variety. The other called Curecanti Quartz Monzonite is light colored and finer grained. Both varieties were named many years ago by Hunter (1925). Both form large homogeneous rock masses which lend distinction and character to the canyon walls.

FIGURE 10.—Hand specimen of Vernal Mesa Quartz Monzonite, showing porphyritic texture with large phenocrysts of microcline. Groundmass feldspar is plagioclase. Dark splotchy material is mostly biotite. One-third natural size.

Vernal Mesa Quartz Monzonite forms several separate intrusive bodies in the national monument area. The largest body has an outcrop of about 3 square miles. It forms the sheer walls of the canyon from Rock Point downstream past Chasm View (fig. 5) to Cedar Point, Dragon Point, Sunset View, and High Point. Clearly, this reach of canyon is the most rugged in the entire area. The same body of rock underlies most of the rolling upland between Big Draw and High Point. Undoubtedly its extent is much greater under cover of overlapping sedimentary rocks. Smaller masses of Vernal Mesa Quartz Monzonite crop out at the head of Red Rock Canyon, southeast of the main body near Big Draw, on the east side of Signal Hill near the River Portal Road, and along the River Portal Road about 4-1/2 miles above its junction with U.S. Highway 50.

Rocks similar to the Vernal Mesa Quartz Monzonite—different in detail, but possibly related in origin—underlie most of the Black Canyon in the Black Ridge quadrangle.

Possibly the best place to view and examine the Vernal Mesa Quartz Monzonite at both close and long range is Chasm View overlook. Inasmuch as the rock differs little from place to place, one good exposure illustrates most of the general features. The most striking petrologic feature of the rock is its very coarse porphyritic texture. This term applies to igneous rocks in which certain mineral grains called phenocrysts greatly exceed the average grain size of the rock (fig. 10). In the Vernal Mesa Quartz Monzonite abundant phenocrysts of microcline feldspar 1 to 2 inches long form about 20 to 30 percent of the rock. Oligoclase, the groundmass feldspar, is less conspicuous than microcline, but it equals or exceeds microcline in total volume. Other mineral constituents in order of decreasing abundance are quartz, biotite, opaque iron minerals, epidote, sphene, hornblende, apatite, and calcite. Percent ages are about as follows:

Because of its coarse texture, the Vernal Mesa Quartz Monzonite has a mottled appearance—pink caused by microcline and gray caused by disseminated biotite in the ground mass. Viewed from a short distance, the overall color of fresh rock is light brownish gray. On some outcrops a deeper brown cast is caused by oxidation and weathering, and in many places the true color is masked by dark-brown to black encrustations of desert varnish.

Discerning viewers will notice that the rock has a fairly marked "grain" caused by a subparallel orientation of the microcline phenocrysts. Petrographic studies suggest that the rock was strained by slight viscous flowage after most of the mineral constituents had crystallized, but before complete solidification. During such flowage, phenocrysts tend to rotate mechanically into alinement with the direction of flowage. At the same time they may be damaged somewhat by rubbing one against another. Thus, most of the phenocrysts are fractured or abraded, and many of them have granulated rinds or borders. Further manifestations of viscous flowage are oriented inclusions of schist and gneiss torn from the walls of the quartz monzonite body during emplacement and frozen in place as the magma cooled. Though not abundant, such inclusions can be seen in nearly all good exposures of the rock.

Curecanti Quartz Monzonite

Curecanti Quartz Monzonite is widely exposed in the upper part of the Black Canyon where it forms one large central pluton and many smaller ones. It takes its name from Curecanti Needle, a spirelike monolith opposite the mouth of Curecanti Creek in a wildly picturesque section of canyon (fig. 11). Nearly-vertical walls of flesh-colored quartz monzonite exceed a thousand feet in height. The needle itself is nearly 800 feet high. Good views of the area are had from Pioneer Lookout Point on the Black Mesa Road and from the north rim of Fitzpatrick Mesa.

FIGURE 11.—Curecanti Needle. 6 miles downstream from head of Black Canyon. Thousand-foot cliffs of Curecanti Quartz Monzonite. left. Old roadbed and shed of narrow-gage railroad (left foreground) abandoned in 1940's.

The main Curecanti pluton is a thick lens-shaped body 3-1/2 miles across, west to east, and 2 miles across, north to south. It has a roof and a floor, both exposed in the canyon walls. In general shape it is similar to a flattish laccolith (fig. 12), but it differs fundamentally from a laccolith by being wholly discordant in relation to the intruded rock, which is contorted biotite gneiss. The western part of the pluton extends below drainage and appears to be deeply rooted. Physical conformation suggests that the quartz monzonite arose from depth beneath the western part of the pluton, gained access to a set of low-angle fractures in the country rock, then spread laterally by forcibly lifting its roof.

FIGURE 12.—Sterogram of the Curecanit pluton showing generalized restored form of the uneroded body. (click on image for an enlargement in a new window)

The quartz monzonite is a relatively uniform rock, light gray to orange pink and medium grained. Hand specimens from the main pluton are virtually indistinguishable from specimens from the minor plutons. A distinguishing mineralogic feature of the main pluton, however, is a scattering of clear pink garnets a tenth of an inch or so across. These garnets are seen in nearly every outcrop of the main pluton, but they are lacking from nearly all of the minor ones.

Average mineral composition of 15 samples of Curecanti Quartz Monzonite, each collected from a different locality, is as follows:

Minor Quartz Monzonite Bodies

Small elongate bodies of light-colored quartz monzonite crop out at a few places in the national monument area, mostly near to masses of Vernal Mesa Quartz Monzonite. They seem, therefore, to be related genetically to the Vernal Mesa Quartz Monzonite, although they differ greatly from it in appearance and composition. They are finer grained and contain far fewer dark-colored minerals. Oligoclase, quartz, microcline, and muscovite, in variable proportions, are the predominant minerals. Biotite, sphene, and thinly scattered garnets are subordinate.

Most of the individual minor quartz monzonite bodies are elongate parallel to the bedding or foliation of the enclosing wallrocks. Most of them are themselves somewhat foliated, and their foliation is parallel to that of the enclosing wallrocks.

Pegmatite

The term "pegmatite" applies to various intrusive igneous rocks of very coarse and variable grain size. Pegmatite is exceedingly abundant in the Black Canyon area, and countless bodies large and small contribute to the unique quality of the canyon scenery (fig. 13). Being highly resistant to erosion, as shown in figure 13, pegmatite crops out boldly where juxtaposed against less resistant rocks, such as schists and most gneisses. Against other igneous rocks, such as the Vernal Mesa Quartz Monzonite, it erodes less differentially, but it forms striking tonal patterns on the canyon walls. Painted Wail (frontispiece) is named for the festoonlike pattern of pegmatite in the sheer cliff below Serpent Point.

FIGURE 13.—Pegmatite dike intruding biotite gneiss in Black Canyon, about two-fifths mile downstream from mouth of Red Rock Canyon. Dike stands out because of superior erosive resistance. Note similar dikes in cliffs above.

Pegmatite—in the form of dikes, sills, and irregular masses—is seen from every rim overlook in the Black Canyon area. Particularly striking outcrops are across the canyon from Pulpit Rock, Chasm View, and Cedar Point. Several overlooks stand on pegmatite outcrops—Gunnison Point is a good example—and afford a close view of the rock.

Upstream from the national monument, many large and excellent examples of pegmatite crop out at Poverty Mesa, Cimarron Creek, Blue Greek (adjacent to U.S. Highway 50), and Pioneer Lookout Point. Countless dikes crop out in the walls of the canyon and along the floor.

The largest pegmatite bodies in the area are between River Portal Road and Big Cimarron Creek; many dikes are hundreds of feet across and thousands of feet long. The largest single mass of pegmatite is a stocklike body that forms the top and rugged east slope of Coffee Pot Hill. This mass is nearly 1-1/2 miles long, is more than half a mile across, and covers an area of more than half a square mile. Only slightly less impressive are the many large dikes on Poverty Mesa and Sheep Knob. Some of these dikes are plainly visible to travelers on U.S. Highway 50 south and west of Cimarron. Even from a distance their light color and rugged aspect set them apart from the country rock.

In close views the pegmatite is seen to consist chiefly of feldspar, quartz, and mica in coarse crystal intergrowths. The feldspar is mostly a pink variety of microcline, less commonly albite. The quartz is massive and milky, although scattered clear specimens have partly formed crystal faces. Feldspar-quartz intergrowths called graphic granite are very abundant. Muscovite is the predominant mica; biotite is subordinate. Some pegmatite dikes contain accessory magnetite or hematite in irregular partly faceted masses an inch or two across. More rarely, they contain well-formed trapezohedral garnets a few tenths of an inch across. A few pegmatite dikes that intrude amphibolite consist mostly of hornblende and plagioclase feldspar.

The coarse texture of the pegmatite is conspicuous to even the most casual viewer. Interlocking feldspar crystals in some places exceed 3 feet across—even 6 feet—and their lustrous cleavage surfaces fairly glitter in the brilliance of the western sun. Great masses of milky quartz are often several feet long. Books of pearly white mica are much smaller but no less arresting. These and most of them are rarely exceed a few inches across, even smaller.

Clear mica in large sheets—rare and valuable—is remembered as the isinglass window of Grandmother's oven door. Today, though no longer so used, clear mica has great industrial value, especially in the electrical industry, and is classed as a strategic mineral. The black variety is attractive but has no commercial value. It is less abundant than clear mica in Black Canyon pegmatites, but it is by no means rare. Bladed crystals as much as 6 feet long occur in some pegmatites.

A striking feature of pegmatite in the national monument area is its markedly different intrusive habit in different rock types. This difference is particularly noticeable between dikes that cut sheared or bedded schist and dikes that cut massive quartz monzonite. East across the canyon from Pulpit Rock, for example, an orderly array of straight-walled nearly vertical dikes "swarms" through the enclosing rock. At Cedar Point and Chasm View, however, low-dipping irregularly bounded dikes trend almost at random through the Vernal Mesa Quartz Monzonite; these dikes trend generally northwestward and dip unevenly to the northeast. Both in horizontal plan and in cross section, therefore, they depart widely from the orientation of dikes to the east. The difference in orientation is attributed to the differing fracture habits of the host rocks.

Aplite

Aplite is relatively scarce in the Black Canyon area and will not be seen by the average visitor. It is closely related to pegmatite in origin and composition but differs markedly in texture and grain size, its fine grain giving it a gray sugary appearance. Black Canyon aplites consist chiefly of quartz and feldspar with subordinate mica. Some contain garnets 1 to 2 mm across. Aplite forms straight-walled dikes, most being only a few tens of feet long in the Black Canyon area; most of them are only a few inches wide and rarely are more than a few feet wide.

Diabase

Diabase is another dike-forming rock that is uncommon in the Black Canyon area but is of much interest to the geologist for its petrologic and structural significance. Diabase is abundant in some parts of the United States, especially in the North Atlantic States, where it forms great intrusive sheets known commercially as traprock. Physically and petrographically this rock resembles the diabase of the Black Canyon. The Palisades of the Hudson are composed of diabase. The diabase of the Black Canyon area is a dark-colored fine- to medium-grained rock composed chiefly of feldspar and augite. The dominant feldspar is a calcic variety called labradorite. Common subordinate minerals in microscopic grains are quartz, orthoclase, hornblende, biotite, chlorite, iron minerals, and apatite.

Although uncommon in the Black Canyon, diabase forms by far the longest dikes in the area. In a zone only 1 to 3 miles across but more than 26 miles in length, several dikes extend discontinuously from lower Bostwick Park east-southeast beyond Lake Fork. The longest continuous dike crosses the canyon just upstream from Tomichi Point, has an exposed length of nearly 8 miles, and has a maximum width of more than 300 feet. Its full length is unknown, as it is truncated by faults. Other large dikes crop out near the east portal of the Gunnison diversion tunnel. Farther up the canyon a large dike crosses the river near Nelson Gulch (2 miles west of Curecanti Needle) and crosses Blue Creek about a mile above the mouth. This dike is about 200 to 250 feet wide and is exposed for nearly 3 miles. It probably is appreciably longer than 3 miles, as both ends are concealed by overlapping strata, and there is no evident tendency for the dike to taper out in either direction. It might well extend an additional 3 miles beneath cover. Other diabase dikes of the area are mostly a few tens of feet across and mostly less than half a mile in exposed length.

Despite their size, the diabase dikes generally crop out poorly. Where the diabase is fresh, it is hard, tough, and resistant to erosion, but the rock weathers rapidly by chemical decomposition, and the weathered rock crops out poorly. The best exposures are in craggy areas below the canyon rims. Back from the rims, where the rock is deeply weathered, diabase is poorly exposed or concealed by soil.

Lamprophyre

Rocks called lamprophyre have been observed mostly in the upper part of the Black Canyon, although they are uncommon even there. Their chief interest lies in their unusual composition and geologic occurrence. Several of them crop out at Pioneer Lookout Point. They are dark colored dike rocks, generally scores or hundreds of feet long but only a few inches to a foot or two wide. (See fig. 14.) They range rather widely in composition and appearance, but the predominant type is a dark greenish-gray fine-grained biotite porphyry. The matrix consists of hornblende, microcline, and oligoclase, with subordinate quartz, biotite, sphene, apatite, and zircon—all in microscopic grains. The lamprophyre dikes at Pioneer Lookout Point are cut by the Curecanti Quartz Monzonite and, hence, are clearly older than the quartz monzonite.

FIGURE 14.—Dikes of lamprophyre cutting across biotite gneiss in Black Canyon about 1,000 feet upstream from Curecanti Needle. Dikes of this type are scarce in the Black Canyon area.