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

CRETACEOUS SYSTEM.

Rocks of known Cretaceous age, all probably belonging to the Upper Cretaceous series, are represented in southern Utah by exposures in the vicinity of the Henry Mountains, along the southern border of the High Plateaus in western Garfield and Kane counties, and in a southeastward projection, of which Kaiparowits Plateau is the chief part, extending into eastern Kane County. (See Pl. X, A and B.)

The total thickness of the Cretaceous section in this region is about 3,500 feet. The rocks consist of alternating divisions of bluish-drab shale and yellowish-brown sandstone, the former appearing in slopes and badlands, the latter in more or less prominent escarpments and hogback ridges. They are divisible into seven distinct units, the basal one of which is doubtfully correlated with the Dakota sandstone. To the rocks above this basal sandstone Gilbert applied the following local names, in ascending order: Tununk shale, Tununk sandstone, Blue Gate shale, Blue Gate sandstone, Masuk shale, and Masuk sandstone. These names were derived from geographic features in the Henry Mountains. As duplication of names is contrary to the practice of the United States Geological Survey, only Gilbert's names for the sandstones have been adopted, and in this report his names for the shales will be used in quotation marks, because of the doubt regarding the relations of this whole succession of sandstone and shale to named units to the north and east. In the present state of knowledge Mr. Moore tentatively correlates the "Tununk shale," Tununk sandstone, and "Blue Gate shale" with the Mancos shale of southwestern Colorado and east-central Utah. The Blue Gate sandstone he tentatively correlates with the Mesaverde formation, the "Masuk shale" with the Lewis shale, and the Masuk sandstone with the so-called "Laramie" sandstone of southwestern Colorado. Some geologists, however, believe that the whole succession corresponds to the Mancos shale.

Dakota (?) sandstone—As in most of the Cretaceous sections of the Rocky Mountain and Plateau region, there is present in southern Utah a basal sandstone and conglomerate which appears to be homotaxially equivalent to the Dakota sandstone. This sandstone attains a thickness of 100 feet in the escarpment of Kaiparowits Plateau, but in the Henry Mountains district it is much thinner and in places is absent. It evidently overlies the beds beneath it unconformably, for the upper surface of the McElmo is somewhat uneven, the thickness of the upper division of the McElmo is variable, and at one place a slight fold in the McElmo strata was found to have been truncated before the deposition of the sandstone. To these evidences may be added the conglomeratic character of the formation. The color of the Dakota (?) sandstone is yellowish brown to white. The bedding is irregular. Locally thin beds of lignite are present.

"Tununk shale."—The formation above the Dakota (?) sandstone consists of bluish-drab argillaceous and in part sandy shale, called "Tununk shale" by Gilbert. The shale is 900 to 1,000 feet thick in exposures east of the Waterpocket Fold. It is very sandy and grades at the base into extremely fossiliferous brown sandstone. The shale is also fossiliferous, containing especially large numbers of Gryphaea newberryi.

Tununk sandstone.—The sandstone overlying the shale just described was called Tununk sandstone by Gilbert. It has a maximum observed thickness of about 100 feet. It is well developed in the vicinity of the Henry Mountains and appears to be present in the Kaiparowits Plateau but was not found southwest of Table Cliff Plateau in western Garfield County. It is light yellowish brown, is irregularly bedded, and locally contains some lignite.

"Blue Gate shale."—Overlying the Tununk sandstone is a shale formation 1,100 to 1,200 feet thick, which resembles the "Tununk shale" in texture and color but lacks the numerous fossils that are characteristic of the "Tununk." To this shale Gilbert applied the name "Blue Gate shale."

Blue Gate sandstone.—A prominent creamy-yellow to light-brown massive and irregularly bedded sandstone succeeds conformably the shale just described. Its thickness in the Henry Mountains, where it was named the Blue Gate sandstone by Gilbert,³⁶ is 250 to 500 feet, but in the Kaiparowits Plateau and west of Escalante the thickness of this formation is at least 1,000 feet. The sandstone contains lignite, which locally is of very good grade and reaches a thickness of 4 feet west of Mount Ellen. No fossils were obtained from this sandstone, but Lee³⁷ has suggested that it is comparable with the Mesaverde. It is extremely improbable that this sandstone could represent the Ferron sandstone of Castle Valley, as suggested by Lupton.³⁸ The Ferron occurs about 600 feet above the base of the Mancos as there identified and is overlain by 3,000 feet of Mancos shale, whereas this sandstone is 2,200 feet above the base of the Mancos and, as seen in the Henry Mountains, is succeeded by only a few hundred feet of shale. The Cretaceous section of southern Utah appears to be intermediate in character between those of southwestern Colorado, northwestern New Mexico, and northern Arizona and those of central and northern Utah.

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³⁶Gilbert, G. K., op. cit., p. 4.

³⁷Lee, W. T., Relation of the Cretaceous formations to the Rocky Mountains in Colorado and New Mexico: U.S. Geol. Survey Prof. Paper 95, p. 50, 1915.

³⁸Lupton, C. T., Geology and coal resources of the Castle Valley, Utah: U. S. Geol. Survey Bull. 628, p. 32, 1916.

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"Masuk shale."—Above the Blue Gate sandstone is a light-drab, very sandy shale, which in the Henry Mountains area has a thickness of 500 to 700 feet and to which Gilbert applied the name "Masuk shale." Below Table Cliff Plateau, in western Garfield County, the thickness of this shale is undetermined but is probably more than 1,000 feet. The upper part of the shale contains thin beds and lenses of brown sandstone and appears to grade without break into the overlying formation, the Masuk sandstone.

Masuk sandstone.—The uppermost Cretaceous formation in the region is a massive yellowish-gray sandstone, which east of the Circle Cliffs appears in a prominent table-land bounded by high, sheer cliffs. Its thickness is about 300 feet according to measurements made at its west margin, but Gilbert³⁹ reports a thickness of 500 feet.

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³⁹Gilbert, G. K., op. cit., p. 4.

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