The Moenkopi Formation
(EARLY TRIASSIC)
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In the Moenkopi formation six subdivisions are recognized: stratigraphic units defined in terms of lithic features, origin, and mode of accumulation. In upward sequence these are the Timpoweap member, the lower red member, the Virgin limestone member, the middle red member, the Shnabkaib member, and the upper red member. The features of the Timpoweap member and the lower red beds may be conveniently examined in Spring Creek and Squaw Creek Canyons, and the other subdivisions at most of the places where the Moenkopi is exposed. Except for attitude, thickness, and minor differences in style of bedding, these subdivisions closely resemble the corresponding units at their type localities in the Virgin River Valley. Especially close is the similarity between corresponding units in Southwest Utah and in the adjoining Zion National Monument (1). In brief, the subdivisions of the Moenkopi are characterized as follows:
Shinarump conglomerate: Upper TriassicUnconformity
| Moenkopi formation: Lower Triassic | Feet |
|
250-350 |
|
270-300 |
|
60-120 |
|
50-120 |
|
0-350 |
|
0-200 |
Where the entire Moenkopi is exposed and undisturbed by faulting the upper five subdivisions appear in the landscape as two steep slopes of evenly bedded shales separated by a hogback ridge of gray rock (Virgin limestone member). The higher slope, broken by low steps at irregular intervals, presents the eroded edges of the strata that make up the middle red member, the topographically continuous white Shnabkaib member, and the upper red member: the lower slope marks the position of the lower red member. The lowest subdivision (the Timpoweap member)-the most resistant of the units-appears as a gray ledge of unevenly bedded limestone that, south of Kanarra Creek, constitutes the bulk of the Hurricane Cliffs.
Analyses show that in bedding and composition the lower, middle, and upper red members are substantially alike: they are chiefly thin, lenticular strata composed of gypsiferous and calcareous, predominantly quartzose sand- stones, generally fine grained. They also contain bits of weathered feldspar, mica, iron ore, and rare fossil wood and bones. The Shnabkaib member consists of white and greenish white gypsum beds most of them less than a foot thick interlaminated with red calcareous, gypsiferous, and arena. Virgin shales that vary much in thickness and purity. The Virgin limestone member includes a few resistant massive beds of nearly pure calcite and many beds of friable, earthy, calcareous shales that within short distances along strike gradually or abruptly change from one to another style of bedding, texture, and composition. The Timpoweap member includes not only the predominant gray limestone in thick, massive and thinly foliated beds, but also in places at the base, coarse conglomerate of exotic pebbles, and commonly at the top red and yellow arenaceous shales, in places gypsiferous and slightly petroliferous.
In the stratigraphic column the position of the Moenkopi formation is defined by planes of unconformity which are believed to represent considerable lapses of time. The basal beds of the formation rest on the eroded surface of the Kaibab (Permian) limestone. The topmost beds were worn by streams and some of them stripped away before the overlying formation (Shinarump conglomerate) was deposited.
Fossils from beds near the base of the formation are identified by J. B. Reeside, Jr., as Aviculopecten sp., Meekoceras sp. and Terebratuloid Braichiopoda, representatives of the rich fauna in the Timpoweap member as displayed southward along the Hurricane Cliffs. In beds near the middle of the formation, Reeside noted Pleurophorus (3 species), Myalina sp., Nuculana sp., characteristic parts of the fauna in the Virgin limestone member at its type locality near Virgin City, Utah. Near the southern border of Southwest Utah, in beds of limestone 575 feet above the Aubrey (now regarded as the Kaibab limestone), Lee(2) obtained the following fossils: Pugnax aff. P. osagensis, Dielasma sp., Aviculipecten aff. A. occidentalis, Myalina aff. M. permiana, Bakewellia n. sp., and Schizodus sp. The fossils are fairly abundant, and though most of them are fragments or merely internal molds difficult to identify, they are recognizable as early Triassic forms. Furthermore, the stratigraphic position of the fossiiferous beds as indicated by their meager faunas is confirmed by other evidence. With no significant change in composition or relative position, the subdivisions of the Moenkopi in Southwest Utah may be traced continuously into corresponding units in the Virgin River valley, where their age has been amply demonstrated as Lower Triassic.(3)
1 Gregory, H. E. and Williams, N. C., Zion National Monument, Geol. Soc. Am. Bull., Vol. 58, pp. 211-244, 1947.
2 Lee, W. T., The Southwest Utah Coal field, Utah; U. S. Geol. Survey Bull. 316, p. 362, 1907.
3 Reeside, J. B., Jr. and Bassler, Harvey, Stratigraphic sections in southwestern Utah and northwest Arizona; U. S. Geol. Survey Prof. paper 169-D, 1922. Gregory, H. E., The Zion Park region; U. S. Geol. Survey Prof. paper [awaiting publication].
SOURCE
-from Geology of Eastern Iron County, Utah by Herbert E. Gregory
The top of the Moenkopi is an erosion surface over which is spread the Shinarump conglomerate. In the development of this surface parts of the Moenkopi and perhaps higher beds not now anywhere represented were removed from large areas of the plateau province. Erosion continued until both flat-lying beds and tilted beds were worn down to a surface of low relief that marked the end of a physiographic cycle. For beginning a succeeding cycle the conditions were suitable for the deposition of sheets of conglomerate (Shinarump). Where the stratification of the two formations is essentially parallel, shallow channels carved in red-brown shales are filled with gray grits. Where the dip of the Moenkopi beds differs from that of the Shinarump, the grits extend more or less evenly across beveled edges of shale beds. In most places the Moenkopi-Shinarunip contact is not conspicuous, and where the Shinarump is poorly represented and the topmost Moenkopi beds are colored shales it seems gradational.
SHINARUMP CONGLOMERATEThough the thickness of the Shinarump conglomerate in few places exceeds 100 feet, it is a remarkably persistent formation. In southeastern Nevada, southern Utah, and northern Arizona it is an unmistakable horizon marker and generally a maker of prominent cliffs and platforms. Throughout the plateau province its exposures are much alike-massive units of coarse cross-bedded sandstone interbedded with lenses of conglomerate that exhibit the same wide range in thickness, texture, color, bedding, and composition.
In the San Juan country the Shinarlimp is exposed on the eastern flank of the great Monument upwarp, in the bed of Comb Wash and northward along upper Cottonwood Creek and down Mormon Pasture Creek. It is likewise exposed on the western flank of the up warp, in the Red House Cliffs from the San Juan River northward to Armstrong Canyon and westward down White Canyon to the Colorado River. On the crest of the up warp many square miles of Shinarump form the surface of Elk Ridge and its capelike projections. The formation varies widely in thickness and composition. At the, head of Comb Wash the exposed part of the Shinarump, 64 feet thick, is about three-fourths coarse gray sandstone and one-fourth conglomerate with pebbles as much as a quarter of an inch in diameter of black and red quartzite, gray chert, shale lumps, and worn wood. It includes plant impressions and broken logs. The Shinarump that caps Elk Ridge to a thickness of 60 to 120 feet is chiefly gray grit composed of glistening grains of rounded and angular quartz. It appears as huge cross-bedded lenses separated by irregularly deposited yellow-gray fine sandstone. Generally at the base and here and there at higher levels are lenses and stringers of conglomerate, composed chiefly of pebbles of red and black quartzite, white quartz, and white chert an eighth to half an inch in diameter. Near Soldiers Crossing most of the 35 feet of Shinarump is yellow-gray sandstone and ripple-marked brown mud shale with plant impressions, fragments of wood, and big iron concretions that resemble tree trunks. Below Duckett Tanks much of the fossil wood is stained green, and some of it is replaced by copper minerals. At the head of Frey Canyon the Sbinarump is exceptionally coarse, consisting almost entirely of cross-bedded lenticular grits that include concretionary masses of limestone I to 8 inches in diameter, slabs of sandstone 3 to. 8 inches long, big chips of petrified wood, and pebbles of white chert, and gray, black, brown, and white quartz and quartzite more than an inch across.
In and near Clay Hills Pass the Shinarump is a coarse lenticular ash-gray sandstone only 1 to 8 feet thick that consists of aggregates and strings of sub angular quartz, quartzite, and chert pebbles; in places it is preserved only in depressions carved in the underlying Moenkopi. Along White Canyon it is less than 50 feet thick and shows an unusual assemblage of grits, ripple-marked sandy shales, green shales, clay-lime balls as much as I inch in diameter, and widely scattered small pebbles of quartz. In some places it is represented by plasters of quartz, limestone, and chert less than 1 foot thick; in others by lenticular coarse sandstone and limestone conglomerate. It is quite possible that some of these outcrops may have been mistaken for beds in the Chinle. Where the plane of unconformity is inconspicuous and shales lie both above and below the thin conglomerate the sequence appears gradational and the contacts are somewhat. arbitrarily drawn.
Pebbles of quartz, quartzite, chert, limestone, and petrified wood and beds of green-gray shale are features common to all outcrops examined, but in general the Shinarump in the San Juan country appears to be less, thick and less conglomeratic and to contain less fossil wood and quartzite but more clay, limestone, and chert A than in regions south of the San Juan River and Glen Canyon.
The physiographic conditions under which the Shinarump was deposited are difficult to visualize. What conditions could be so persistent and so uniform as to permit the deposition of a thin sheet of material essentially alike over thousands of square miles in Utah, Arizona, and Nevada. The lenticular bedding, the abrupt lateral and vertical change in composition and texture, and the absence of thick deposits point to a land surface of slight relief over which intermittent and ephemeral streams, perhaps also perennial streams of fluctuating volume, all flowing in poorly defined channels, transported and deposited material available for distribution. After the coarse material was exhausted in filling depressions in the Moenkopi surface and building above them, the streams deposited sand in increasingly broader, more regular, and finer-grained beds, until with change in climate the conditions became favorable for the deposition of the marl-like shales and fresh-water limestones of the Chinle formation. The Shinarump marks the beginning of the cycle of deposition that followed the cycle of erosion represented by the Moenkopi-Sbinarump unconformity. It seems to be the basal conglomerate of the Upper Triassic Chinle formation.
The unsolved problem of Shinarump lithology is the source of the pebbles in the conglomerate. The slabs of sandstone and clay balls may have come from underlying beds, the cycads, doubtless grew where their fossils are found, and the petrified trees are probably driftwood. Some of the limestone fragments may represent beds in a once much thicker Moenkopi; others may have come from places like Moab, where pre Shinarump, erosion has locally exposed the Hermosa formation. But no Triassic or Carboniferous formation so far mapped includes material that might supply the varicolored quartz and quartzite. The quartzites differ in no essential from those in the pre-Cambrian of Utah and Arizona.
SOURCE
-from The San Juan Country - A Geographic and Reconnaissance of Southeastern Utah by Herbert E. Gregory
The Moenkopi Formation consists of many thousands of layers of dark, reddish brown shale and siltstone with a total thickness of 200 feet in the vicinity of Lee's Ferry. These rocks rest directly upon the hard, cliff-forming Kaibab Limestone, but because they are soft and easily eroded they have been stripped back away from the canyon rim several tens of miles, leaving a bare surface on the Marble Platform.
The Moenkopi Formation is easily recognized by its chocolate brown color, and in the vicinity of Lee's Ferry it forms the gentle slopes and low cliffs in the vicinity of the campground. One of the most obvious characteristics of the formation is the thin horizontal stratification so prominently displayed wherever a fresh surface is exposed. Close examination of these strata will reveal numerous ripple marks and mud cracks indicating deposition in shallow water. Many tracks and trails of reptiles have also been found in Moenkopi beds. A prominent zone of white gypsum alternates with thin layers of typical reddish brown siltstone to form a very colorful unit in the upper part of the formation.
Geologists consider the Moenkopi to represent deposition in a large tidal flat environment, with the open sea located westward in Nevada. Gypsum in the upper part is the result of evaporation and indicates an and climate in the region during Moenkopi time.
The lack of canyon development on the Colorado River here at Lee's Ferry is largely the result of the soft Moenkopi and Chinle beds brought down to river level on the Echo Cliffs Monocline.
SHINARUMP CONGLOMERATE (TRIASSIC)
A relatively thin resistant formation of coarse sand and gravel known as the Shinahump Conglomerate overlies the soft, chocolate-colored Moenkopi Formation and forms a vertical cliff ranging from 50 to 100 feet high. The formation is well known in the Colorado Plateau area for its uranium and fossil wood. The Moenkopi and Shinarump exposures together are commonly referred to as the Chocolate Cliffs. Excellent exposures are seen near the Arizona-Utah state line south of Kanab, Utah. In the vicinity of Lee's Ferry the Shinarump Conglomerate is inclined 15 degrees to the east and dips below the level of the river a few hundred feet west of the boat landing.
Shinarump beds consist of well-rounded, well-sorted pebbles of quartz and chert averaging less than an inch in diameter. Medium-to coarse-grained sand is also a major constituent and in many places the entire formation is made of sand. Fossil wood is locally abundant and is commonly mineralized with yellow carnotite and other uranium minerals. Much of the Shinarump Conglomerate is thought to have been deposited in stream channels.
SOURCE
-Geology Studies Vol. 15 Part 5 1968
