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

SEDIMENTARY ROCKS—JURASSIC

Entrada Sandstone

The Entrada Sandstone is thin but well exposed at several places in the national monument along the northeast rim. It is visible from across the canyon as a thin light-colored band at the very rim of the canyon. On the southwest rim its outcrop is spotty because it has been stripped almost entirely away by erosion. The formation thins out completely east of a line joining Poison Spring Hill on the north and Coffee Pot Hill on the south, near the east boundary of the monument. East of this line the formation was never deposited, but it thickens abruptly to the west and on down the canyon to the north. Its most typical and best exposures are just west of the monument in the Red Rock Canyon area and farther north along the Black Canyon and its tributaries as far as Smith Fork (fig. 17).

FIGURE 17.—Stratigraphic section about 1,000 feet thick near mouth of Smith Fork. One of the most complete and best exposed Jurassic and Cretaceous sections in the Black Canyon area. Symbols: pC. Precambrian gneiss; Je, Entrada Sandstone; Jw, Wanakah Formation; Ji, Junction Creek Sandstone; Jms, Morrison Formation, Salt Wash Member; Jmb, Morrison Formation, Brushy Basin Member; Kb, Burro Canyon Formation; Kd, Dakota Sandstone.

At Red Rock Canyon the Entrada Sandstone is 85 feet thick and consists of fine-grained massive cliff-forming sandstone. Its colorful outcrop is visible from Montrose, 8 miles distant to the southwest. The greater part of the formation is bright reddish orange, but the upper 17 feet is pale yellowish orange. Farther north, the pale-color phase thickens to about 40 feet at the expense of the underlying darker color phase. In the thin sections in the eastern part of the monument the dark-color phase is completely lacking.

In most places the Entrada has a basal conglomerate about 1 to 4 feet thick. This conglomerate contains angular fragments of the underlying Precambrian rock—gneiss, pegmatite, and vein quartz—in a coarse-grained sandstone matrix. The underlying Precambrian rock was variably weathered to depths of as much as 10 feet during its long pre-Late Jurassic exposure. The old weathered zone is well preserved beneath the Entrada Sandstone. In many places the weathered zone grades indefinitely upward into the basal conglomerate of the Entrada; this relation is well shown in outcrops along the canyon rims.

Most geologists agree that the Entrada Sandstone was deposited on the margin of a shallow seaway, mainly as onshore dunes, but partly in a marine environment and partly, perhaps, under true desert conditions. The uppermost 5 feet at Red Rock Canyon is well bedded and thin bedded and contains oscillation ripple marks. It probably was deposited in shallow water under conditions transitional to the subaqueous environment of the overlying Wanakah Formation.

Wanakah Formation

The Wanakah Formation consists of gypsum, vari-colored shale and mudstone, friable sandstone, and gray cherty limestone. It accumulated in an arid landlocked seaway or brackish lake which flooded parts of southwestern Colorado and adjacent New Mexico after the Entrada Sandstone had been deposited. Seen from a distance, the overall aspect of the formation is that of a drab-gray slope separating brighter colored ledges of the Entrada Sandstone below and the Morrison Formation above. Where the Junction Creek Sandstone is present, the Wanakah directly underlies it.

Complete sections of the Wanakah Formation are exposed in the first large ravine west of Red Rock Canyon. Many good but less accessible sections are exposed farther north in the outer walls of the Black Canyon, in Red Canyon of Crystal Creek, and in the lower part of Smith Fork (fig. 17). Partial sections can be seen in the monument on the northeast rim, but completely exposed sections there are lacking.

Like the underlying Entrada Sandstone, the Wanakah Formation thins eastward. It overlaps the Entrada, but it, too, wedges out against the old slope of the planed-off Uncompahgre highland. It is about 150 feet thick at Red Rock Canyon, is about 55 feet thick at Sapinero, and wedges out completely a short distance east of Sapinero.

At the base of the Wanakah Formation in the more westerly parts of the Black Canyon is a distinctive limestone unit called the Pony Express Limestone Member. Outcrops of this unit are abundant along the northeast rim in the monument and can be seen from the north rim road near The Narrows.

The Pony Express was deposited in a brackish lake or an enclosed arm of the sea. Its eastern shoreline passed through the Black Canyon area and trended about south-southeast along a line from Red Canyon of Crystal Creek in Delta County to the south end of Dead Horse Mesa in Montrose County. East of this line the Pony Express Limestone Member never was deposited. Later and broader re-expansions of fresh water, however, are indicated by additional, more extensive limestones higher in the stratigraphic sequence. Some of these limestones contain the fossilized remains of fresh-water clams, snails, and algae.

Junction Creek Sandstone

The Junction Creek Sandstone crops out near the upper end of the Black Canyon at Sapinero and continues eastward toward Gunnison. It forms discontinuous outcrops on the slope of Soap Mesa along Colorado State Highway 92 and is preserved locally on the slope of Black Mesa. It thins generally westward along the Black Canyon and is lacking or doubtfully present in sections near the national monument. Although discontinuous in the lower part of the canyon, it is well represented at the mouth of Smith Fork (fig. 17), where it is 81 feet thick in a section measured by T. E. Mullens of the U.S. Geological Survey. The greatest observed thickness in the area is about 90 feet at Blue Mesa damsite. The Junction Creek Sandstone is only about 25 feet thick at Sapinero but thickens abruptly east of Sapinero. Regionally, it thickens greatly to the south.

The Junction Creek consists of highly crossbedded pale-gray to pink fine-grained sandstone. It somewhat resembles the Entrada Sandstone, but the Entrada in the monument area is much finer grained and less crossbedded. The Junction Creek once was a great tract of dunes on the dried-out floor of the old Wanakah sea.

Morrison Formation

The Morrison Formation, renowned for its dinosaurs is one of the most widespread rock formations in the Rocky Mountain region. It flanks mountainous uplifts throughout Colorado and in parts of northern New Mexico, northeastern Arizona, eastern Utah, Wyoming, western South Dakota, southern Montana, western Oklahoma, and western Kansas. Its outcrop is extensive in the Black Canyon area, but it is poorly exposed in the national monument. Excellent exposures may be seen at many places downstream from Red Rock Canyon, at Red Canyon of Crystal Creek, and at Smith Fork (fig. 17). A good section is exposed near Sapinero at West Elk Creek. Most of the roadcuts along Colorado State Highway 347 between Bostwick Park and the monument boundary are in the Morrison Formation.

In the Black Canyon area the Morrison Formation has a maximum thickness of about 600 feet. It consists of bright-colored shale and mudstone, light-gray sandstone, and—in the upper part—light-gray mudstone and pebble conglomerate. On most outcrops the sandstone looks red, but this coloration is just a surface stain. Dinosaur remains, which are so characteristic of the Morrison Formation, have never been found in the Black Canyon area, but diligent search probably would discover some.

Two members of the Morrison Formation are recognized in the Black Canyon area—the Salt Wash Member, below, and the Brushy Basin Member, above. These members are widespread in southwestern Colorado and southeastern Utah, and they are readily distinguished from one another throughout the Black Canyon area, except where exposures are poor.

The Salt Wash Member consists of red, gray, and green shale and mudstone interbedded with crossbedded sandstone. Near the base of the member are a few thin freshwater limestone beds. Total thickness of this member is about 250 feet in the lower part of the canyon and about 180 feet in the upper part. The sandstone beds form conspicuous slope breaks—in most places three successive cliffs or ledges of sandstone are separated by slopes of shale, but the sandstone beds are lenticular and taper out laterally. The top of the uppermost cliff is the arbitrary top of the member, and the overlying beds are included in the Brushy Basin Member. The Salt Wash Member is the host rock for important uranium deposits in many districts of the Colorado Plateau, particularly in western Colorado and eastern Utah; uranium, however, has never been reported in the Black Canyon area.

The Brushy Basin Member, about 350 feet thick, is mostly light-gray shale or mudstone, but it contains considerable red shale, gray sandstone, and discontinuous but very distinctive conglomerate. Much of the shale contains impure bentonite, a clay derived from the alteration of volcanic ash. Conglomerate near the middle of the member or toward the top contains many small pebbles of red and green chert in a matrix of coarse poorly sorted sandstone. This distinctive rock is a reliable guide to the Brushy Basin Member in areas of poor exposure.

Studies by many investigators show that the Morrison Formation accumulated on a broad alluvial plain—the sandstones and conglomerates as fillings in the channels of ancient meandering streams, the mudstones and shales as overflow deposits on a flood plain, and the limestones in ephemeral lakes. Few formations so bestir the imagination: It is easy to visualize a flat dusty plain shimmering under a hot Jurassic sun. Clouds of ash from a distant volcano darken the sky. A sluggish stream meanders across grassy meadows, its backwaters and banks rank with strange vegetation, the scene teeming with reptiles in variety—in the water, on the land, in the air—some small, some large, some agile, some stuporous, all fierce. All the wonderment of little boys is embodied in this formation.