The Mancos/Tropic Shale Formation


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**Note: Formation descriptions are under heavy construction.**


The rocks assigned to the Tropic formation are exposed at the base of the high cliffs that extend from Kanarra Mountain northward to Lone Tree Mountain and eastward into the valleys of South Creek, Ellie Creek, Maple Creek, and other tributaries to Coal Creek. They crop out also in Fiddler Canyon. Generally in the topography the Tropic formation is represented by slopes of moderate inclination on which distintegration products and the debris of landslides are so thickly piled that exposures suitable for detailed study are rare. The formation as a whole is an unsystematic sequence of shale, sandstone, and coal, with subordinate limestone and conglomerate. Where typically exposed, the shale is thinly, regularly bedded, and consists of claylike material mingled with thoroughly sorted, spherical grains of glistening quartz (60+ percent) and some gypsum and calcite. Wherever observed, beds of this sort are interstratified with sandstone, carbonaceous shale, lignite, and limestone-in thin beds and nodules, commonly fossiliferous. The sandstones include hard massive beds and extremely friable beds, arranged as thick and thin, long and stubby lenses, 1 to 10 feet thick, some of them crossbedded and others thinly laminated and imbricated. The texture and distribution of the lenticular masses is so irregular that along the strike a seemingly continuous stratum of sandstone may change gradually, more often abruptly, in composition from fine, well-rounded grains to coarse, angular grains, and to conglomerate, in which the pebbles of quartz, quartzite, chert, fossil wood and concretionary masses of limestone, ironstone, and clay measure to 1 inch in diameter. Coal is a persistent feature of the Tropic formation, especially of its lowermost 500 feet, where bituminous deposits are associated with earthy lignite, accumulations of macerated plants, and interlaminated with fossiliferous sandy shale.

The stratigraphic boundaries of the Tropic formation are uncertain. In places the contact with the Straight Cliffs sandstone above seems to be indicated by disconformities and accumulations of gravel, which, however, are not persistent with reference to selected horizon markers. The contact with the Dakota (?) sandstone below is so clearly gradational that in mapping the Dakota (?) was treated as basal Tropic.

In composition, topographic expression, general make-up, and relation to adjacent formations, the Tropic formation of eastern Southwest Utah differs considerably from corresponding units elsewhere. It consists predominantly of sandstone in massive beds and shaly beds; the characteristic marine clay shale that in eastern Utah attains thicknesses exceeding 1,000 feet, is rarely as much as 100 feet thick and in some places is lacking; coal beds of commercial value are present in the formation, but absent from the Straight Cliffs sandstone above. Some of these features are duplicated at exposures along the southern face of the Markagunt and Paunsaugunt Plateaus, but as pointed out elsewhere,(1) the Tropic formation, at its type locality (near Tropic, Utah) and generally in the region between Paria River and Glen Canyon of the Colorado River consists predominantly of uniform dark drab, clayey shale [in groups 500 to 650 feet thick] and fine textured sandy shale thinly laminated and soft, which breaks down readily to form slopes and broad gently undulating flats . . . Because this weak formation ranges in thickness from about 600 to 1400 feet it exerts a profound influence on the topography . . . Everywhere it has been stripped back so as to form a broad bench which is underlain by the hard rocks of the Dakota (?) . . . In the upper Paria Valley the lower part of the Tropic consists of fossiliferous shale and sandstone which are somewhat similar to the beds seen elsewhere [in the Kaiparowits region] but which contain alternating beds of coal [thin and discontinuous, generally impure; the workable coal beds are above the Tropic in the Straight Cliffs sandstone.


1. Gregory, H. E. and Moore, H. C., Tbe Kaiparowits region; U. S. Geol. Survey Prof. paper 164, p. 98, 1931.

-from Geology of Eastern Iron County, Utah by Herbert E. Gregory


The fossiliferous marine shales that conformably overlie the Dakota (?) in the San Juan country are undoubtedly the stratigraphic and lithologic, equivalents of the lowermost beds in the Mancos shale prominetly displayed in the Mesa Verde Plateau south of Cortez, Colo. In that region the Mancos has a thickness of 2,000 feet and is overlain by great beds of Mesaverde sandstone. West of the Colorado line the Mancos is poorly represented and the Mesaverde is absent. Generally on Sage Plain the Mancos constitutes small mesas and ridges that rise 10 to 100 feet above the Dakota (?) sandstone. On the lower flanks of the Abajo Mountains, where, protected by piedmont gravel, sections show thicknesses exceeding 200 feet, and near Baker ranger station, about 350 feet of Mancos sedimentary beds are preserved in a shallow syncline. Along the San Juan River below Aneth and west, of Elk Ridge the formation is absent.

The Mancos is predominately steel-gray sandy shale but includes stringers of earthy coal, impure limestones, and many thin beds of fine-grained yellow and brown sandstone, composed chiefly of sub angular and angular quartz grains cemented by lime. Gypsum appears ill some beds, and selenite crystals are common in the talus. In some sections the sandstones are more, numerous and closer together at a horizon 100 to 150 from the base and form the caps of mesas.

-from The San Juan Country - A Geographic and Reconnaissance of Southeastern Utah by Herbert E. Gregory


The Mancos shale includes all the strata between the Dakota sandstone and the Mesaverde formation. It is essentially a thick accumulation of drab sandy clay shales, with a relatively small percentage of sandstone, impure limestone, and coal. The beds are gypsiferous, and they carry many fossils of Benton age. The Mancos strata are conformable with the sediments above and below, but are lithologically unlike them. The contact with the Dakota is established with a fair degree of approximation, for shale carrying Benton fossils lies within 25 to 50 feet of characteristic Dakota beds, and at nearly all localities an "oyster bed" with hundreds of broken shells of Ostrea and Gryphaea is found within the lowest 200 feet of the Mancos. The Mancos-Mesa Verde contact in a few sections measured was located with little difficulty, for a slate-colored shale immediately underlies massive yellow-buff sandstone. At most localities, however, the upper limit of the Mancos must be arbitrarily drawn in the midst of 100 to 300 feet of strata, for the Mancos passes into the Mesa Verde by a series of transitional beds of shale and sandstone possessing features of both formations.


In the valley of Simpson Creek, where the Cretaceous section has been greatly reduced by pre-Tertiary erosion, the Mancos is represented by about 200 feet of paper-thin drab shale with thin beds of yellow-gray sandstone and a layer of coal 1 to 1 feet thick. At 220 feet above the Dakota the following Benton fossils were found in abundance: Ostrea sp., Camptonectes, Inoceramus labiatus, Prionotropis Sp., and scales and bones of fishes. The southern wall of Salahkai Mesa includes 550 feet of shale between the typical Dakota and the heavily bedded Mesa Verde cap. About midway up in a section measured at this locality is a 1-inch bed of black calcareous sandstone from which were obtained Lingula sp., Ostrea sp., Prionotropis sp., and shark teeth-an assemblage of Benton forms. The 428 feet of clayey, gypsiferous shale exposed near Jadito Springs contains three thin beds of gray sand shales in which are embedded lnoceramus labiatus and Prionotropis sp. The long mesas of Tusayan, on which are perched the seven Hopi villages, are composed of Mesa Verde sandstone underlain by Mancos shale. The 295 feet of Mancos strata exposed at Shongopovi include many brown-gray sand bands in addition to the normal dark-blue and slate-colored rocks, giving to the cliff a general gray tone. The rocks range from thin-bedded sandstones to sandy limestones and include lenses of coal and much gypsum. Crinkled ammonites, Prionotropis, were collected at one horizon, and in this general locality Newberry11 collected Ammonites percarinatus, Inoceramus crippsii, Pinna, Lingula sp., and Gryphaea pitcheri var. navia.

At Oraibi Butte and Padilla Mesa the Mancos is exposed for vertical distances of 270 and 130 feet respectively; it lies low on the cliffs and extends outward beneath alluvial slopes. The lower boundary of the Mancos is concealed also at Howell Mesa, where 310 feet of gray, drab, and black shales are visible in the Mesa wall. In the section measured at Chilchinbito the upper contact of the Mancos was drawn at the base of the lowest thick bed of sandstone. As thus limited, the gray and drab argillaceous shales with sandy carbonaceous and calcareous beds have a thickness of 565 feet. About 100 feet from the base is a shell heap including hundreds of specimens of Ostrea and lnoceramus labiatus. Beneath the sandstone cap of Lolomai Point is 620 feet of Mancos shale, and at 60 feet from the bottom there is a 3-foot bed of coquina consisting entirely of one species determined by Prof. Charles Schucher as Exogyra laeviuscula, of Colorado age. II the Blue Canyon section 490 feet of the Mancos was measured by Mr. Pogue, who found "oyster beds" at 45, 75, and 95 feet from the base am "numerous crinkled ammonites " at 195 am 230 feet. Vertebrate bones found on the slope justified further search for fossils, am accordingly Mr. Heald returned in 1911 am made a collection of bones which proved to b( the larger part of a dinosaur. On these bone Prof. R. S. Lull 2 reports :

Very largely a plesiosaur skeleton, identified by teeth vertebrae, and the presence of gastroliths, genus not to be determined without extensive development of the material. A marine form. Invertebrates associated with the skeleton are of Benton or Niobrara age, which agree approximately with the level at which the plesiosaur might occur.

In Red Wash, near the Carrizo Mountains, Mr. Emery measured 335 feet of the lower part of the Mancos, in which Ostrea congesta and Gryphaea newberryi were found in abundance. The sandy strata capping the dome of Navajo Mountain have been converted into resistant quartzite by the addition of a siliceous coating to the quartz grains. Only a small part of the lower Mancos is exposed on the mountain summit, but search resulted in finding a few shells, among which Prof. Schuchert recognized Exogyra laeviuscula and Mactra emmonsi, of Colorado age.

The shell bed near the base of the Mancos of Black. Mesa is exposed 4 miles east of Fort Defiance, also in the " Haystacks" region along the road between St. Michaels and Gallup and at other points in Black Creek Valley. It consists essentially of irregularly shaped accumulations of Exogyra, Ostrea, and Gryphaea, closely packed in a sandy calcareous matrix. At the Tuba coalmine Ostrea retina, identified by Prof. Schuchert, constitutes about 90 per cent of the shell heap; on the north slope of Black Mesa Ostrea pellucida And Exogyra are about equally abundant. At Defiance Mesa the only other fossils found among bushels of Ostrea lugubris were fragmentary fish remains, and at Round Top Butte, in the Hopi volcanic field, the whole shell stratum appears to consist of Exogyra columbella.

-from Geology of the Navajo Country, a Reconnaissance of Parts of Arizona, New Mexico and Utah by Herbert E. Gregory