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

GREAT ANTIQUITY OF THE PRECAMBRIAN ROCKS

Establishing the age of a rock, though likely to be a complex problem, is seldom an end in itself. Such a determination is an essential first step in solving problems in structural geology, which in turn must be resolved before the geologic history of an area can be reconstructed. Much time and energy, therefore, are spent by many highly trained specialists in both private and public research institutions in working out and refining the age relations of key rock formations.

In practice, the "absolute" age of a rock is seldom determined. Relative age is more readily obtained, and for most purposes is generally sufficient. This may be done by carefully noting the field relations that one rock bears to another or by studying the fossils in the rocks. Thus, one rock cutting across another affords clear evidence of relative age—just as a given stratum must be younger than the rock on which it lies, so also a crosscutting dike must be younger than the rock it cuts across. Inclusions of gneiss or schist in a granite must be older than the enclosing granite. Relations such as these help establish relative age.

Paleontology is a major tool in relatively dating fossiliferous rocks. It also has great value in correlating rock formations regionally. But it cannot be applied to the crystalline rocks of the Precambrian System, in which fossils do not exist. Radiometric methods have come to the fore in the past several years as a means of obtaining "absolute" ages for such rocks. These methods utilize known rates of decay of radioactive isotopes in various rock-forming minerals such as mica, certain feldspars, and zircon.

Most, if not all, Precambrian rocks in the Black Canyon are more than a billion years old. The oldest rocks are the metamorphic gneisses, schists, and quartzites which accumulated as sands and muds on an ancient sea floor, were hardened into firm rock, and were metamorphosed to nearly their present form before most of the igneous rocks of the canyon were emplaced. Several episodes of emplacement followed, and field relations indicate the following general succession.

First to be injected were early formed pegmatites, which to some extent shared the severe deformation of the gneisses, schists, and quartzites. Several generations of pegmatite injection may have followed. At any rate, lamprophyre dikes were injected after the early formed pegmatites, and they in turn were followed by renewed but diminished deformation. Quartz monzonite then was intruded, probably over a long span of time.

The minor quartz monzonite bodies of the national monument area seem to be older than the Vernal Mesa Quartz Monzonite, which in turn seems to be older than the Curecanti. The radiometric age of the Vernal Mesa Quartz Monzonite is 1.22 billion years.

At about the same time that the Curecanti Quartz Monzonite was intruded the greater part of the pegmatite in the canyon was intruded. Again, a long span of time probably was involved, totaling perhaps millions of years.

The youngest rock in the Precambrian sequence is diabase. Dikes of this rock cut across all other Precambrian rocks. Its Precambrian age is not firmly established, and conceivably it could be as young as Pennsylvanian—perhaps little more than 200 million years old! All that can be said with assurance is that it definitely is older than the unconformity at the canyon rim and, hence, is plainly pre-Jurassic. But it probably is pre-Pennsylvanian, and most likely is Precambrian.

FIGURE 15.—Black Canyon of the Gunnison National Monument and vicinity, Colorado. (click on image for an enlargement in a new window)

THE ANCESTRAL UNCOMPAHGRE HIGHLAND—AN ANCIENT LAND SURFACE BURIED AND EXHUMED

During early and middle Paleozoic time—in the Cambrian, Ordovician, Devonian, and Mississippian Periods—a moderately thick blanket of strata accumulated intermittently in the Black Canyon area. These strata were completely removed by erosion in late Paleozoic time, but they still exist in adjacent areas, and their former existence in the Black Canyon area can be inferred from careful restorations of Paleozoic geography.

In Pennsylvanian time sharp uplift ended deposition and started a long period of erosion. During this period the earlier formed strata were removed. Even the Precambrian rocks were deeply scoured. Ultimately, the uplifted mountain mass itself was eroded away, but because its roots are preserved, its former extent can be inferred. Geologists refer to it as the ancestral Uncompahgre highland (fig. 16), not to be confused with the modern, much smaller Uncompahgre Plateau.

FIGURE 16.—Ancestral Uncompahgre highland just before Entrada Sandstone was deposited. The flanks of the old highland have been buried by earlier Mesozoic strata. Vertical scale is greatly exaggerated.

The ancestral Uncompahgre highland extended across southwestern Colorado from central-eastern Utah into northern New Mexico. Debris swept from its crest formed thick deposits in basins to the northeast and the southwest. These deposits are mostly red beds. They are well preserved near Aspen, Colo., where they form the picturesque Maroon Bells, at Ouray, where they form the colorful canyon walls, and at many other localities.

By Triassic time, the ancestral Uncompahgre highland was reduced to a low featureless plain. As shown in figure 16, it had the form of a broad flat arch or dome that sloped gently eastward and westward from an apex near the town of Gunnison. In the Black Canyon area it sloped westward at a rate of about 4 feet per mile.

In Late Triassic time, the ancestral Uncompahgre highland began to subside, and sediments began to encroach upon its flanks. These deposits, resting on the old Precambrian basement, are well displayed near Grand Junction in Colorado National Monument, where they comprise the Chinle, Wingate, and Kayenta Formations. But in the Black Canyon area the crest of the old highland remained exposed to erosion until Late Jurassic time.

In the Black Canyon area the first formation to be deposited on the old surface was the Entrada Sandstone. This formation thins abruptly eastward and wedges out against the slope of the surface near the east boundary of the national monument. The next overlying formation, the Wanakah, also thins abruptly eastward, but it reaches the vicinity of Sapinero before wedging out, about 20 miles beyond the edge of the Entrada. The Junction Creek Sandstone, which overlies the Wanakah, and the succeeding Morrison Formation overtopped the old highland and extended unbroken across its crest.

The planed-off surface of the ancestral Uncompahgre highland is well preserved in the Black Canyon as the conspicuous unconformity at the rim of the canyon. (See frontispiece.) This surface is one of the most striking features of the monument. It has been partly exhumed by erosion, the softer overlying formations having been removed; it forms the summit of Vernal Mesa on the southwest rim and the benchlike top of Mesa Inclinado on the northeast. Thin patches of Entrada Sandstone mantle both mesas.