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

THE LARAMIDE OROGENY— A TIME OF GREAT MOUNTAIN BUILDING

Near the end of the Cretaceous Period and during the early part of the Tertiary, widespread crustal disturbances in the western part of the United States outlined most of the major mountain uplifts and basins of the Rocky Mountain region. This critical interval in the history of the Rocky Mountain region is called the Laramide orogeny. The Gunnison uplift, into which the Black Canyon is cut, was formed at that time; renewed movements along old faultlines in the Precambrian basement elevated the Black Canyon area relative to what is now the Uncompahgre Valley to the west, the North Fork valley to the northeast, and the Cimarron Creek-Blue Creek drainages to the south. This uplift was caused chiefly by tilting of old fault blocks, but it was accompanied by some flexing of the sedimentary mantle and by considerable warping along the traces of the old faults. Locally, the beds were steeply tilted or even overturned.

Newly formed drainage systems collected runoff from the uplands and carried it to the intervening basins. An enormous lake formed northwest of the Black Canyon in an area called the Uinta-Piceance Basin. This lake probably covered the Black Canyon area also, but if it did, its deposits have since eroded away. Sediments that accumulated on its floor were derived partly from distant volcanic eruptions and partly from debris eroded off the adjacent uplands.

They now compose the Green River Formation, which is the source of the famous oil-shale deposits of western Colorado and eastern Utah.

Significant structural modifications at a later time followed the Laramide orogeny. For one thing, the great volcanic piles of the West Elk and San Juan Mountains were not erupted until considerably later in middle and late Tertiary time. They were preceded by a long period of erosion and were followed by renewed crustal warping.

EARLY TERTIARY DEPOSITION AND EROSION

Early Tertiary formations such as the Wasatch and Green River are preserved a few miles to the north of the Black Canyon at Grand Mesa. After the Green River lake disappeared—partly because of structural deformation and partly because of sedimentation—erosion attacked and destroyed the heights of the Gunnison uplift. Once again, the country was reduced to a flat plain, this time beveling the edges of the tilted Mesozoic formations and the reexposed Precambrian basement. Drainage that previously had flowed off the Gunnison uplift into adjacent basins now flowed at random across it, without regard to rock type or structure.

AN OUTBREAK OF VOLCANISM

After this long period of erosion, volcanic activity broke out in the West Elk Mountains to the northeast and in the San Juan Mountains to the south, drastically altering the appearance of the Tertiary landscape. One far-reaching consequence was a complete overhaul of the drainage pattern—a step essential, as we shall see, to the ultimate carving of the present Black Canyon. Before volcanism began, streams meandered freely across a broad flat plain. Flowing westward from the newly risen Sawatch Range, they left a train of gravels in their wake. The mounting piles of volcanic rock, however, encroached laterally on the drainage from both sides, forced the drainage first through the narrowing breach between the eruptive centers, and finally perhaps disrupted the drainage completely after burying the accumulated gravels. Between outbursts, drainage was intermittently restored as surges of water made their way across the thickening volcanic cover. Thus arose the ancestral Gunnison River.

Geologists recognize many types of volcanic deposits. Most familiar are the lava flows solidified from streams of molten rock. Flows of this type, however, are very subordinate in the Black Canyon area. Two distinct sequences of dissimilar volcanic material were erupted: an earlier sequence called the West Elk Breccia and a later sequence called the Alboroto Group.

West Elk Breccia

The West Elk Breccia contains flows of solidified lava, but by far the greater part of it consists of coarse fragmental material formed otherwise. No single mode of eruption can explain its diverse character. A large part seems to have resulted from the explosive destruction of previously formed volcanic cones. Some part probably formed from a slurry of mud and blocks. In any event, the volume of material produced was enormous. Remnants in the south half of the West Elk Mountains much exceed a thousand feet in average thickness. By conservative estimate, their original volume exceeded 150 cubic miles.

Good exposures of West Elk Breccia are abundant in the headward part of the Black Canyon, particularly in the Black Mesa-Soap Mesa area at such places as Curecanti Creek, Soap Creek, and West Elk Creek (fig. 18). Good exposures abound along Colorado State Highway 92, and fairly good ones are seen along U.S. Highway 50 on the northwest slope of Blue Mesa. Castle Peak and Cathedral Peak southeast of Crawford also consist of West Elk Breccia. The names of these peaks suggest the picturesque erosive habit of the formation. Cliffs, crags, pillars, windows, and boulder-capped hoodoos are characteristic.

FIGURE 18.—Craggy outcrops of West Elk Breccia near head of Black Canyon at mouth of West Elk Creek. Note embedded fragments and crude bedding. Crags are about 600 feet high.

As the West Elk eruptive cycle finally drew to a close, erosion began to attack the irregularities of the volcanic surface by planing off bedrock protuberances. Precisely when the trunk drainage of the area became organized into the Gunnison River as such is uncertain. Perhaps this did not happen until after volcanism had ceased, although drainage probably was channeled into an ancestral Gunnison soon after volcanism started. At any rate, drainage was never curtailed for any great length of time. It quickly reestablished itself after each eruption, as indicated by the interbedded gravels it left behind.

Renewed Volcanism—Ash-Flow Tuffs of the Alboroto Group

Some time after the West Elk eruptive cycle had ended and after erosion had planed off the irregularities of the West Elk surface, volcanism resumed in the area with the eruption of a drastically different type of material. In several distinct surges, tremendous volumes of hot incandescent ash were erupted from sources as yet unknown—sources presumed to be in the West Elk Mountains nearby, but possibly as far distant as the Silverton, Lake City, or Creede areas of the San Juan Mountains. Eruptions of this type are known variously as ash flows, glowing avalanches, or nuées ardentes. Their origins, geologic relations, and identification have recently been summarized in a definitive paper by C. S. Ross and R. L. Smith (1961).

In the Black Canyon area most ash flows were hot enough to firmly weld the individual ash particles together into a hard coherent rock called welded tuff. This rock is highly resistant to erosion. It forms the line of vertical cliffs above U.S. Highway 50 on Fitzpatrick, Blue, and Pine Creek Mesas and across the canyon on Soap and Black Mesas. Dillon and Sapinero Mesas further upstream are capped by the same formation. Though now much dissected by erosion, the several ash flows which compose the formation once covered at least 400 square miles, had a maximum thickness of more than 600 feet, and had a volume of perhaps 50 cubic miles. Similar possibly related deposits, thicker and even more widespread, cover adjacent areas to the south in the San Juan Mountains.

All available evidence indicates that drainage across the area resumed between ash-flow eruptions. Broadly speaking, the protracted volcanic activity athwart the course of the Gunnison River was but one episode in the still longer life history of the river. After each eruption, the river and its tributaries attacked the newly deposited volcanic rocks, only to be overwhelmed again by each new outpouring. When volcanism ended finally and irrevocably late in the Tertiary Period, the river at last began to cut the wonderful gorges of the present Black Canyon.