The Mesa Verde, Straight Cliffs and Wahweap Sandstone Formations
(LATE CRETACEOUS)
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Generally in southern Utah, wherever the recognized subdivisions of the Cretaceous are clearly defined, the Tropic formation is overlain in turn by the Straight Cliffs sandstone and the Wahweap sandstone. In eastern Southwest Utah the Straight Cliffs sandstone is recognizable and the Wahweap sandstone is probably present, but tracing of miles of outcrops resulted in finding no plane of separation marked by persistent physical features or change in fossil faunas. Therefore, in this report the two sandstones are treated as a single unit and shown on the map by one color. In the topography these sandstones are very conspicuous. Over many square miles they form the surface of the Kolob Terrace and wall in its western side. They constitute the rim of the cirque-like South Creek Valley and line the narrow chasms through which run the branches of Coal, Parowan, and Deep Creeks. But though these masses of sandstone are the most prominent part of the Cretaceous and in panoramic views seem to cross miles of country as solidly build uninterrupted walls 500 or more feet high, tracing shows them to consist of groups of beds that as units die out along strike and that the escarpment form is maintained by similar groups at somewhat higher or lower levels.
In addition to dominant sandstones, the combined Straight Cliffs and Wahweap sandstones include beds of conglomerate, shale, and limestone, few of which retain their individuality for more than a few hundred feet. The sandstone beds range in thickness from 3 feet to as much as 40 feet, though beds 5 to 15 feet are the most common. Regardless of thickness, some of the sandstones are massive and some are evenly laminated, but most of them are conspicuously lenticular and in places are merely overlapping wedges of massive, commonly cross-bedded rock. Analyses of selected specimens show that the grains in the coarser sandstones are clear quartz (95+ percent), feldspar, calcite, gypsum, magnetite, fully half of them as much as 3 mm. in diameter. Coarse and fine grains, round and angular grains are in some specimens segregated, in others intermingled. The conglomerates generally displayed as stubby lenses, present three forms: partly rounded and angular pebbles of quartz, quartzite, and chert less than an inch in diameter ; thin slabs of sandstones, vari-colored shale, and limestone, 1 to 5 inches long; and concretionary balls of sandstone, limestone, and ironstone. The usual cement is lime and iron in various proportions. In some hand specimens the grains are tightly bound together by gypsum, in a few by silica. In places where the cement is particularly weak the rock is marked by nits. shallow caves. and "honeycombs."
Like the sandstones, the rocks recorded in measured sections as argillaceous, calcareous, gypsiferous, arenaceous, and carbonaceous shales, vary widely in their physical features and in their stratigraphic position. They generally appear as regularly stratified units a few inches to a few feet thick between the heavy bedded sandstone, but they also form lenses within the sandstone beds. The calcareous shales and associated porous limestones usually less than one foot thick are especially noticeable. They are hard enough to weather as projecting shelves on otherwise fairly uniform slopes and they are richly fossiliferous: some are essentially accumulations of broken oyster shells. The carbonaceous shales, generally segregated as thin lenses and associated with clay shales and sheets of macerated leaves, are lignitic, rarely bituminous.
The features described above relate to the bulk of the undifferentiated Straight Cliffs and Wahweap sandstones, in places to all of it, and constitute what might be called facies of the Straight Cliffs sandstone. However, in Parowan, Fiddler and upper Coal Canyons and to a smaller degree elsewhere, the upper 100 to 400 feet of these sandstones, resemble the Wahweap sandstone at its type locality. This facies of the Wahweap is characterized by thin, fairly regular arkosic beds, colored brown, yellow, and buff; by the rarity of shale and carbonaceous beds ; the scarcity of limestone; and the absence of marine fossils. Some of the fine-grained strongly ferruginous strata contain partly fossilized twigs and leaves and the carbonaceous clay shale contains species of Physa and Vivaparus. In the collection of fossils from beds listed in field note books as Wahweap, Roland W. Brown recognized two plants as "apparently Laramie": cf. Dryophyllum subfalcatum Lesquereux ; cf. Pseudoprotophyllum magnum Hollick.
In gross composition and stratification the facies of the Straight Cliffs sandstone in eastern Southwest Utah is unlike that at its type locality, the Straight Cliffs in Garfield and Kane Counties, Utah, where it constitutes an unscalable wall nearly 1,000 feet high and where its component strata are laminated sandstones 10 to 60 feet thick, sandy shales (6 to 10 percent of the total), and beds of bituminous coal and lignite 3 to 20 feet thick. In tracing the Straight Cliffs sandstone westward from the Kaiparowits plateau it was noted that progressively the number of thick sandstone units decrease and the thin ones increase; the coal beds decrease in number, thickness, and purity; and the limestones and calcareous shales become more abundant. Also, traced westward, marine beds constitute a decreasing part of the formation.
SOURCE
-from Geology of Eastern Iron County, Utah by Herbert E. Gregory
The Mesa Verde of the Navajo country differs in no essential respect from strata of this age at the type locality in Colorado. At its base are one or more heavy beds of sandstone 30 to 50 feet thick; these are succeeded by strata consisting of about equal amounts of thin sandstone and shale with many beds of coal. The top of the formation includes three or more beds of massive sandstone attaining thicknesses of 40 to 100 feet. It is a prominent cliff maker, forming in many places unscalable walls. Mesa Verde strata constitute the cap and the encircling wall of Black Mesa and the precipitous north front of Dutton Plateau and rank with the La Plata sandstone in boldness of topographic expression.
In the present report the Mesa Verde is the most recent formation described, as I have not studied Cretaceous beds higher in the strati-graphic column. The Lewis shale and later Cretaceous deposits are known to be present on the Chaco Plateau and along the San Juan,11 and beds younger than the Mesa Verde occupy an undetermined area on the northern part of Manuelito Plateau. Massive sandstones along the Fort Defiance and Tohachi road are overlain by a series of soft sandstones and shales eroded into capped columns and irregularly shaped buttes which carry fossils that may belong to a formation younger than the Mesa Verde. Sphaerium, Planorbis (Bathyomphalus) sp., and Viviparus sp. were obtained in a cliff 5 miles south of Tohachi, and in strata 200 feet below there were found an abundance of plant fossils, including Ficus cf. F. speciosissima, Ficus sp., Juglans, and two undescribed species of Carpites. Near Tohachi a turtle, Baena sp., considered by Prof. Lull 2 as not older than Judith River, was found by Mr. Emery. This locality is worthy of detailed study by those interested in the Mesa Verde. Nearly all of the formation is believed to be present along the north rim of Black Mesa, but elsewhere it has suffered diminution by erosion.
LOCAL FEATURES.
The arrangement of strata shown by the Crown Point section is typical for the lower part of the Mesa Verde formation of northwestern New Mexico. Sections measured at several points on Manuelito, Dutton, and Chaco plateaus exhibit substantially the same features, their differences relating chiefly to the position, number, and thickness of shale and coal beds. As shown by Shaler,3 coal of commercial value occurs in the Gallup region at two horizons separated by about 500 feet of barren strata. The portion of the Mesa Verde involved in the Defiance monocline, on the west side of the Chuska Mountains, is analyzed in the section on page 77.
In the Black Mesa area the thickness of the Mesa Verde and the proportionate amounts of shale, sandstone, and coal vary widely; the formation is nearly twice as thick along the northeast rim of Black Mesa as in the zone extending from Salahkai to Oraibi. This difference is probably the result of erosion, but no definite statement can be made until the strata have been traced across the 50-mile stretch from Lolomai Point to Jadito Wash or Padilla Mesa. At Salahkai Mesa, where 330 feet of the Mesa Verde is exposed, the top 117 feet is coarse cross-bedded sandstone in two massive beds separated by 22 feet of thin-bedded sandstone with many plant impressions. Below these upper sandstones are five groups of strata about equal in thickness consisting of alternating clayey and sandy shales and thin-bedded sandstones. Gypsum is abundant in the clayey shale beds; coal was found only at the bottom.
At Second Mesa the Mesa Verde formation, as determined by Mr. Pogue, is 285 feet thick. The bottom sandstone is 80 feet thick, and the top of the mesa is formed of a massive cross-bedded stratum of coarse-grained sandstone, 55 feet thick, unevenly conglomeratic, with pebbles of quartz and of shale, cemented by lime and by iron oxide. Yellow and dark-purple shales with thin seams of coal constitute most of the remaining portion of the cliff. Sections measured 3 miles northeast of Oraibi Butte and at Padilla Mesa are almost identical. At the top is 95 feet of yellow-gray coarse-grained sandstone, with lenses and strings of cross-bedded conglomerate, consisting of sub-angular fragments of quartz with chert and shale and rare kaolinized feldspar cemented with lime and with yellow iron oxide. Shales and thin sandstones with lenses of impure coal make up the next 100 feet ; then follows 65 feet of thick-bedded to massive yellow-gray sandstone displayed as a cliff face. At Howell Mesa only about 200 feet of the Mesa Verde remains. In Moenkopi Valley 6 miles east of Blue Canyon, where 220 feet of the Mesa Verde is exposed, the section includes three strata of coarse sandstone or grit-two, 45 and 12 feet thick at the bottom and a 25-foot bed at the top. Clayey shale and coal constitute 70 per cent of the remaining 138 feet. Along the northern rim of Black Mesa the Mesa Verde attains a greater thickness than is known at any other place in northern Arizona. In this region the top sandstones are unusually thick and massive and the number of thin coal beds is abnormally large. At Chilchinbito the mesa rim is a vertical wall 320 feet high, of yellow-gray sandstone with inconspicuous lenses of shale, and six coal beds in addition to eight or ten seams of matted plant fragments occur below the sandstones.
The typical arrangement of Mesa Verde strata for the northeast face of Black Mesa is shown in the section on page 78.
TERTIARY FORMATIONS.
PREVIOUS MENTION.
Deposits of "fresh-water Tertiaries, probably of Miocene age, parallel to those of the `Badlands' of Nebraska," were noted by Newberry 1 along his route from Walpi to Ganado, and the Tertiary strata on the Chuska Mountains were mapped by Dutton 2 and by Shaler, 3 both of whom considered them of Eocene age. My reconnaissance surveys show that Tertiary sediments are coextensive with the limits of the Chuska, Tunitcha, and Lukachukai mountains, occupy considerable areas on the Defiance Plateau, and are interbedded with volcanic deposits on the southeastern edge of Black Mesa and within the Hopi Buttes province.
SOURCE
-from The San Juan Country - A Geographic and Reconnaissance of Southeastern Utah by Herbert E. Gregory
