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

SEDIMENTARY ROCKS—CRETACEOUS

Three formations of Cretaceous age crop out in the Black Canyon area—in ascending order, the Burro Canyon Formation of Early Cretaceous age, the Dakota Sandstone of Late Cretaceous age, and the Mancos Shale of Late Cretaceous age. Cretaceous rocks younger than Mancos, such as the Mesaverde Formation, are present in adjacent areas but have been stripped from the Black Canyon area by erosion during Tertiary time. Lower Tertiary rocks, such as the Wasatch and Green River Formations, if ever deposited, have also been removed. The Mancos Shale is partly overlapped by volcanic rocks erupted in middle and late Tertiary time. A long interval of Late Cretaceous and early Tertiary time, therefore, is unrepresented by any rock formations, and inferences regarding events in this interval must be drawn from adjacent areas.

Burro Canyon Formation

The Burro Canyon Formation overlies the Brushy Basin Member of the Morrison Formation. It is recognized throughout most of the Black Canyon area, although in areas of poor exposure it is distinguished with difficulty from the overlying Dakota, which it resembles. It consists chiefly of crossbedded stream-laid pebble conglomerate and coarse-grained conglomeratic sandstone, but it also contains finer grained sandstone layers and shale. The pebbles in the conglomerate are mostly light- to dark-gray chert and light-gray to cream-colored quartzite, mostly less than half an inch across but rarely several inches. Maximum measured thickness of the formation is about 110 feet. Lithologic character suggests that the Burro Canyon Formation was deposited by meandering streams shifting their channels across a broad alluvial plain.

Casual visitors to the Black Canyon are not likely to see good exposures of the Burro Canyon Formation. In many places the formation crops out poorly and forms brushy rubbly slopes interrupted only locally by ledges. Its line of outcrop is about 100 to 200 feet below the rims of Fruitland Mesa and Grizzly Ridge on the north and east rims of the Black Canyon. Elsewhere, the formation is well exposed in the remote northern part of the area—along the west side of the Black Canyon, in excellent cliff exposures along Smith Fork (fig. 17), and along the north side of Red Canyon of Crystal Creek. Good exposures may also be seen near the head of the Black Canyon near the mouth of West Elk Creek.

Dakota Sandstone

Like the Morrison Formation, the Dakota Sandstone is very widespread in the western interior of the United States. It is well represented in the Black Canyon area, where it forms hogbacks, rimrocks, and dip slopes; it caps mesas to the north and east of the national monument including Fruitland Mesa, Grizzly Ridge, Poison Spring Hill, Dead Horse Mesa, and Pine Ridge. It also crops out between Bostwick Park and Vernal Mesa.

The Dakota consists mostly of crossbedded sandstone in beds a few inches to several feet thick. Beds are thinner and more uniform than in the underlying Burro Canyon. The Dakota also contains pebbly conglomerate, shale, mudstone, and locally coal.

Fossil ripple marks, worm burrows, petrified wood, impressions of twigs, and carbonized plant matter—all common in the Dakota—indicate deposition in shallow water. The coal accumulated in a swamp. The upper part of the Dakota interfingers with the lower part of the overlying Mancos Shale. Deposition of the Dakota, therefore, presaged the readvance of a seaway across the western interior of the continent.

Mancos Shale

The Mancos Shale also is widespread in the western interior of the United States. It flanks or encircles many mountainous uplifts of the region, including the Gunnison uplift through which the Black Canyon is cut. Being soft and easily eroded, it forms the bottoms of broad valleys and topographic basins. Such valleys are widely utilized for transportation routes, both highways and railroads. U.S. Highway 50, for example, traverses Mancos Shale almost from Cimarron, Colo., to Price, Utah; and the major towns between, including Montrose, Delta, Hotchkiss, Paonia, and Grand Junction, Colo., and Price and Green River, Utah, all stand on Mancos Shale.

Present access routes to the Black Canyon cross Mancos Shale. In ascending the slopes of the Gunnison uplift, one passes across progressively older rocks. Mancos Shale flanks the uplift south of Red Rock Canyon. Nearby, it underlies Bostwick Park and Cerro Summit south of the uplift and the Crawford, Iron Creek, and Crystal Creek areas north of it.

Excellent exposures are at nearly every hand in the Uncompahgre Valley west of the Black Canyon. There the shale presents a wearisome landscape of barren tan-gray slopes. But locally, it forms tracts of unworldly badlands likely to impress travelers looking for an offbeat change of scenery. In late afternoon, heightened by long shadows, the badlands are very photogenic. Extensive tracts just northeast of Montrose are easily reached by passenger car, except in wet weather when travel on the shale is precarious. Commanding views of the entire badlands area, and of the whole Uncompahgre Valley for that matter, are had from the summit of Flat Top Mesa, just 4-1/2 miles northeast of Montrose.

The full thickness of the Mancos Shale is undetermined in the Black Canyon area, as the top is eroded off. Partial sections penetrated by wells exceed 4,000 feet in thickness. As exposed in the Black Canyon area, the unweathered shale is dark gray, very soft, and somewhat fissile; weathered shale is lighter gray to tan. Much of it swells on wetting and contracts on drying; on moist hillsides, therefore, it is very susceptible to landsliding.

The Mancos contains large calcareous concretions, some of which enclose marine mollusks. It contains a few beds of fine-grained calcareous sandstone, also in part fossiliferous. The formation is gypsiferous, and in badland areas many of the slopes are littered with sparkly gypsum crystals.

Deposition of the Mancos Shale coincided with an encroachment of marine waters across the western interior of the United States. At one time an expanse of water reached from Arctic Canada to the Gulf of Mexico. The Mancos was deposited as mud on the shallow bottom of this sea.

As the Mancos accumulated, the sea bottom slowly subsided, so that the water maintained an essentially uniform depth. Finally, however, when the rate of subsidence diminished, deltal and beach deposits crowded the sea from the area. These deposits now comprise the Mesaverde Formation, still preserved at Grand Mesa north of the Black Canyon and at Cimarron Ridge to the south. Rank forests that flourished on the low swampy ground gave rise to extensive coal deposits in the Somerset, Carbondale, and Book Cliffs areas. Repeated expansions and withdrawals of the sea marked Mesaverde time and the remainder of the Cretaceous Period. At about the end of the Cretaceous Period, crustal warping on a very large scale lifted the area above the sea and drained off the marine waters for the last time. This period of up lift is known as the Laramide orogeny.