Geological Survey Professional Paper 132—A Rock Formations in the Colorado Plateau of Southeastern Utah and Northern Arizona

TRIASSIC SYSTEM.

Moenkopi formation.—The Moenkopi formation, of Lower Triassic age, comprises thick beds of shale and sandstone that lie unconformably on the Permian rocks and are at most places unconformably overlain by the distinctive Shinarump conglomerate. As thus defined the Moenkopi consists of several members, which vary considerably in character and thickness from one locality to another. The total thickness ranges from 304 feet in the Circle Cliffs to 920 feet south of the Henry Mountains and along San Juan River. No marine fossils have been reported from the formation in southern Utah, but the lower beds exhibit a regularity and a uniformity of texture that suggest deposition in a large body of water. Farther west and southwest, in Arizona, southern Nevada, and southwestern Utah, the lower part of the formation is clearly of marine origin but the upper part is unmistakably of continental origin.

Along the San Juan the Moenkopi consists principally of red sandy shale and earthy sandstone, with a little gypsum. (See Pl. VI, B.) South and southwest of Piute Farms, on the river, a thin bed of cream-colored sandstone is found near the middle of the formation, and this layer thickens gradually toward the south, developing into a massive cross-bedded member which in Train Rock has an estimated thickness of about 200 feet. This sandstone is apparently the same as the sandstone in southeastern Utah called De Chelly by Gregory. He treated the De Chelly as a separate formation, because in most of the region he studied south of the San Juan he found it above Moenkopi shales and immediately underneath the Shinarump conglomerate. He also found that the "Oljato sandstone" of Woodruff is made up of the De Chelly and the Shinarump. (See Pl. I.) The De Chelly sandstone, according to Gregory, is present on the east side of the San Juan oil field, where it is red and thins out toward the north. He also states that the Shinarump conglomerate, wherever it is present on the east side of the oil field, rests on the De Chelly sandstone. It appears probable, therefore, that the upper shales of the Moenkopi were entirely removed in part of the Navajo country by pre-Shinarump erosion. An alternate hypothesis might be that these upper shales are replaced toward the south by the thickening of the De Chelly sandstone.

PLATE VI—A (top), VIEW LOOKING UP SAN JUAN CANYON FROM POINT NEAR MOUTH OF CLAY GULCH, UTAH. Platform is underlain by Moenkopi conglomerate, but in foreground terrace gravels and landslide material rest on Shinarump. Moenkopi formation is exposed in lower part of canyon walls. Photograph by Robert N. Allen B (bottom), VIEW LOOKING WEST TOWARD HIGH MESA AT CLAY HILL CROSSING OF SAN JUAN RIVER, UTAH. Coconino sandstone underlies level foreground; Moenkopi formation, next above, weathers with fluted edges; De Chelly (?) sandstone lentil of Moenkopi is absent; Chinle forms steep badland slope with dark and light bands; Wingate, Todilto (?) and Navajo formations form cliffs at top of mesa. Shinarump conglomerate is absent. Photograph by Robert N. Allen.

The sandstone that is here tentatively correlated with the typical De Chelly sandstone of northeastern Arizona disappears entirely near Piute Farms and Clay Hill Crossing on the San Juan, and it was not recognized in sections for 25 miles north of Clay Hill Crossing.

Along Colorado River below Cataract Canyon the Moenkopi again shows three distinct divisions. The lower division, about 400 feet thick, consists of red and maroon sandstone and sandy shale in regular beds, with a gray conglomerate layer at the base. Shaly beds predominate near the bottom of the section, forming a slope that steepens upward and merges into a cliff made of the more resistant red sandstone. In most sections this cliff is capped with a layer of cross-bedded gray or cream-colored sandstone, ranging in thickness from a few inches to 60 feet. On account of its persistence and its striking contrast with the underlying red beds this member forms an excellent horizon marker. Its stratigraphic position and lithologic character suggest its correlation with the De Chelly (?) lentil on the San Juan.

The upper division of the Moenkopi along this portion of the Colorado consists chiefly of chocolate-colored, red, and gray shale and sandstone ranging in thickness from 350 to nearly 400 feet. Thin-bedded sandstone predominates in the lower part, and the proportion of sandy shale increases upward. The division as a whole has a weak topographic expression, forming a concave slope below the Shinarump cliff. Thin flaggy sandstone layers with strong ripple marks and current marks recur at short intervals in the section. Lenticular layers of cross-bedded sandstone 15 feet in maximum thickness are also common and form the roofs of many old cliff dwellings that are built in reentrants at horizons of shale. All the sandstone beds show evidence of strong fluviatile action, and the shales were probably deposited on flood plains or in shallow temporary lakes. A persistent layer of conglomerate, from a few inches to 25 feet thick, forms the base of the upper division and fills erosion channels in the De Chelly (?) sandstone. This distinct erosional unconformity within the Moenkopi is seen in all sections along Colorado River and its tributaries from Cataract Canyon to Red Canyon, where the gradual westward dip carries the formation below the surface. (See Pls. V and VII, A.)

Near Lees Ferry, Ariz., the Moenkopi consists of about 500 feet of red sandy shale and thin-bedded sandstone with seams of gypsum, but 50 miles to the south it has numerous thick beds of red and buff sandstone. Near Jacobs Pools, 25 miles west of Lees Ferry, 12 feet of greenish-yellow sandy shale and 12 feet of yellow sandy and gypseous limestone occur at the top of the formation.

In the Circle Cliffs the Moenkopi consists of 475 feet of chocolate-colored and yellow shale and sandstone with a few thin beds of hard limestone near the top. No gypsum was seen in the formation in this area or along the Colorado, but secondary veins of gypsum traverse the Moenkopi shales near Fruita, in Wayne County Utah. Only a little gypsum occurs in the Moenkopi of the San Juan region and near Lees Ferry, and the general scarcity of the mineral in the entire region under discussion is noteworthy in view of the high content of primary and secondary gypsum found in both the marine and continental phases of the Lower Triassic in Arizona, southern Nevada, and southwestern Utah.

From the foregoing account and from the columnar sections (Nos. 6 to 9 on Pl. I) it is apparent that the Moenkopi of Woodruff and Gregory in the San Juan region includes not only the typical or true Moenkopi but also the Coconino sandstone and the Supai (?) formation.

Shinarump conglomerate.—The remarkable persistence of the Shinarump conglomerate has excited the wonder of geologists who have studied Triassic sections in different parts of the Colorado Plateau. Within the area under discussion it is generally present at the base of the Upper Triassic, lying on the eroded upper surface of the Moenkopi formation. It is a massive unit of coarse cross-bedded sandstone, in which are interspersed lenses of conglomerate that contain small rounded pebbles of chert, quartzite, and silicified wood. Silicified logs are abundant, and many plant impressions were seen on bedding surfaces. The average thickness of the unit is probably less than 50 feet, but it fills depressions in the underlying beds and at some localities attains a thickness of more than 200 feet. Near the mouth of Nokai Creek on the San Juan the thickness is 220 feet.

The position of the Shinarump conglomerate between two thick shaly formations gives it a strong topographic expression on the sides of valleys, the edge forming a prominent cliff in front of a stripped bench and above a pronounced slope. (See Pls. IV, B; V, A; VI, A; and VII.) The formation is made still more conspicuous in many sections by its dark color on exposed edges; for although the fresh sandstone is ordinarily gray, it appears to acquire an especially dark coat of desert varnish with long exposure. This characteristic is so pronounced that the Shinarump is often referred to as "the black layer" by prospectors and others along the Colorado. Deposits of carnotite in the formation have been mined at several localities.

The exact conditions under which the Shinarump was deposited have been the subject of much discussion, and no conclusion appears to be generally accepted. Whatever the conditions were, it is evident that they must have been almost uniform over a very wide region, for in its essential characteristics the formation differs little in all the sections studied in Utah, Arizona, and Nevada. The uniformity extends even to its thickness, for the total section rarely exceeds 200 feet, and this maximum applies to very small areas. The lenticular and cross-bedded structure indicates the action of shifting streams with fluctuating volume, and the uniformly small thickness evidently indicates a nearly even surface with slight elevation. It is not probable that the region was a desert, for the abundance of silicified wood found in all sections testifies to a climate suitable for the growth of large trees.

Chinle formation.—The Shinarump is believed to be the "basal conglomerate" of the Upper Triassic, for the finer sediments of the Chinle formation in this region follow without any apparent break. Thick beds of marly shale, gray, pink, lavender, yellow, and variegated, form the most conspicuous part of the Chinle. Fine-grained sandstone, cherty limestone, and a few conglomeratic beds are intercalated with the shale. Large silicified logs are found in sandstone layers at several horizons, and there are other indications that the entire deposit is of continental origin. Along the San Juan the thickness ranges from 800 to 1,000 feet, and at Lees Ferry it is about 1,000 feet thick, but it is much thinner to the north, 300 feet along the Colorado above the Waterpockete Fold and 450 feet west of the Waterpocket Fold. This thinning may be due in part to pre-Jurassic erosion, for an erosional unconformity at the top of the Chinle is generally recognized, especially in the northern localities, although it does not appear to be profound.

Wherever the Chinle is exposed over areas of considerable size it gives rise to badland topography. Along the sides of valleys it forms long slopes, broken by low steps ate horizons of sandstone or limestone layers. (See Pls. IV, B; V, A; VI, B; VII; and VIII, B.)

PLATE VII—A (top), CASTLE BUTTE, NEAR MOUTH OF RED CANYON, UTAH. 1, Shale of upper member of Moenkopi formation; 2, Shinarump conglomerate; 3, Chinle formation; 4, Wingate sandstone. Photograph by C. R. Longwell. B (bottom), VIEW LOOKING NORTH ACROSS COLORADO RIVER AT LEES FERRY, ARIZ. Small rounded knoll to left stands on Shinarump conglomerate, which dips east. All the higher beds to the right form entire thickness of Chinle formation. Vermilion Cliffs in left background. Photograph by Robert N. Allen.

PLATE VIII—A (top), VIEW LOOKING NORTHWEST UP COLORADO RIVER TOWARD PROPOSED DAM SITE NEAR LEES FERRY, ARIZ. Canyon walls are formed by Wingate and Navajo sandstones. Todilto (?) formation is apparently absent. Photograph by Kirk Bryan. B (bottom), VIEW FROM POINT HALF A MILE SOUTH OF SPENCER CAMP, LOOKING SOUTH DOWN SAN JUAN CANYON TOWARD NAVAJO MOUNTAIN, UTAH. Canyon has been cut in Navajo, Todilto (?), Wingate, and Chinle formations. Photograph by Robert N. Allen.

Limestone is found in the upper part of the formation in beds that range from a few inches to 2 feet in thickness, intercalated with limy shales. The limestone is dense and hard and contains an abundance of chert in nodules or lenses. Layers of intraformational limestone conglomerate are not uncommon. In most places the limestone beds are thin and are limited to a section not exceeding 30 feet in thickness, but at a few localities the individual layers average nearly 2 feet in thickness and make up fully half of a section 50 or 60 feet thick. Sandstone increases in amount toward the top of the formation.