The Dakota Sandstone
(EARLY? CRETACEOUS)
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The Dakota sandstone consists chiefly of masses of coarse-grained sandstone and conglomerates fitted together as lenses and wedges, but includes also well-stratified sandstone, clay shale, and earthy coal. The commonest pebbles in the conglomerate are rounded and subangular, smoothly polished, vari-colored quartzite and chert, ironstone, and dense black limestone, 1/8 to 3 inches in diameter. Here and there the rock contains pebbles of igneous rock, slabs of shale and of red sandstone, and fragments of petrified wood. The source of the pebbles is unknown. Obviously few if any of them could have been parts of the underlying Jurassic formations, which contain no quartzite, vein quartz, or metamorphosed limestone. They are substantially like those in the Shinarump conglomerate (Triassic) about 3,000 feet below, which also came from unknown distant sources, and in turn, like the pre-Cambrian rock in Grand Canyon. Some of the chert pebbles contain Paleozoic corals.
SOURCE
-from Geology of Eastern Iron County, Utah by Herbert E. Gregory
LITHOLOGIC FEATURES
As represented in the San Juan country, the Dakota (?) is a group of sedimentary rocks, rather than a bed or a mass of homogeneous material, traversed by bedding planes. Its persistent general features are predominance of sandstone, lenticular arrangement, cross-bedding; and wide range in composition, color, and thickness of the subordinate shale. Its outstanding local feature is coal. The sandstone is prevailingly coarse, much of it conglomeratic. Most of the material called "shale" is in reality thin-bedded-, sandstone, and even the coals and the layers of macerated plants are mingled with sand. The formation is everywhere lenticular: few beds of uniform thickness and texture are continuous for as much as 50 feet. Thick wedges, thin wedges, and curved lenses of conglomerate, coarse sandstone, fine sandstone, and shale overlap or end abruptly and are replaced along the strike by different materials. Some masses of coarse sandstone are completely surrounded by shale, and at a place on Alkali Creek conglomerate borders a coal bed. Most of the individual beds are likewise irregularly lenticular. The sandstones are commonly cross-bedded. In the coarser ones straight laminae 'meet each other at various angles; in the finer ones curved laminae also appear. In places the parting planes between laminae are so evenly spaced as to outline readily worked building blocks.
Plant impressions on sandstone were observed at several localities, also cobbles of petrified wood. Sun cracks, ripple marks, and worm tracks are common in the shales and in the finer-grained, even-bedded sandstone. Local lateral unconformities and features due to channeling are conspicuous. In composition and texture the Dakota (?) varies widely. The coarser parts consist chiefly of sub angular pebbles that average less than half an inch in diameter but include strings of pebbles 1 to 3 inches in diameter. The most abundant pebbles are white, gray, and brown quartzite, and quartz, but black and white chert, feldspar, and dense black limestone are fairly common. The conglomeratic beds include also angular fragments of greenish-white mud shale 2 to 6 inches long, rounded shale balls and sand balls, short slabs of sandstone, ironstone and sand-lime concretions, and tiny chunks of carbonized wood. At a place on middle Recapture Creek angular fragments of chert 'and chalcedony coin stitute as much as one-tenth of a bed 20 feet thick, and in specimens collected in upper Big Canyon the only large grains are kaolinized feldspar.
The cement of the Dakota (?) is calcareous, ferruginous, and siliceous. At outcrops where calcite form, the cement the rock readily crumbles, sand piles replaced talus, and holes bored by bees appear in the soft finer. grained beds. The much more abundant iron cement holds the pebbles firmly together, forms hard black concretionary masses, and gives dark-brown and yellow tones to exposed surfaces. The black color patches on many ledges and the glistening black coating (desert varnish) of boulders on Black Mesa and along the Blanding-Bluff road are doubtless derived from the iron cement. Where the cement is siliceous the rock is especially resistant. It breaks about as readily through the constituent pebbles as around them and forms the least-weathered cliffs in the region. At contacts with igneous rock on the Abajo Mountains all beds, regardless of texture, composition, and cement have been metamorphosed to quartzite. The weathering of the lime cement, the removal of mud pellets and Band balls, and the decomposition of the feldspar and vegetable remains have given the Dakota a porosity which fits it for a ground water reservoir.
STRATIGRAPHY AND CORRELATION
The Dakota (?) of the San Juan country includes the sandstone and conglomerate beds, the shale, and the thin seams of coal that lie between the thick Morrison formation (Jurassic) and the remnant masses of fossiliferous shale (Mancos) of Benton age. The base of the formation in many places is a bed of gray conglomerate that lies unconformably above brightly colored Morrison shales or sandstones, but in sections measured along Butler Wash and Montezuma Creek the lowest beds are white, sandy shale. The top of the formation likewise is in some places a single bed of coarse sandstone, in other places thin-bedded brown sandy shale that grades conformably upward into gray-blue fossiliferous shale, so that the plane of contact of Dakota (?) with Mancos cannot be drawn with assurance. For the region as a whole the total thickness of beds assigned to the Dakota (?) is less than 200 feet. The thinnest section measured is 92 feet.
In most sections measured three subdivisions are recognizable(1) a basal conglomerate with subordinate shale; (2) thin-bedded sandstones, shales, and coal; (3) sandstone, most of it coarse or conglomeratic. In color, thickness, composition , and continuity these subdivisions show great variation. Their constituent beds interfinger to such a degree that no two sections have the same sequence. The bottom subdivision in 14 sections measured ranges in thickness from 20 to 65 feet. (Thorpe records 84 feet in a well hole 1 1/2 miles east of Monticello.) In six of the sections this subdivision is essentially a massive cross-bedded stratum; in the others it consists of two or more lenticular conglomeratic or sandstone beds with thin sandstones beneath and shale, a little carbonaceous material, and impure limestone here and there on parting planes.
The middle subdivision ranges in thickness from 10 to more than 60 feet. Along lower Recapture, Montezuma, and McElmo Creeks it constitutes 3 to 15 percent of the Dakota (?) present; on northern Sage Plain about 20 to 50 percent. Most of the beds are thin platy sandstones, but this subdivision includes layers of red, brown, green, gray, and black shales, also coal. A little carbonaceous shale but no coal beds were observed in sections studied along Big Canyon, West Water Canyon, and lower Recapture and Montezuma Canyons, but earthy coal is common at Monticello, and farther east these beds become persistent and in places thick enough to mine.
The topmost subdivision, where its surface has not been reduced by erosion, is 30 to 80 feet thick. The highest figures are for outcrops at the northeast base of the Abajo Mountains. Like the sandstone at the base of the formation, those at the top. are displayed as massive beds or thick short lenses. In general they are less coarse but include. conglomerates with pebbles an inch or more in diameter. In places the coarse, thick, strongly cross-bedded strata are replaced by thin, regular sandstone beds or even by shale.
Raving in mind the lithologic differences on which subdivisions of strata classed as Dakota (?) are based and especially the significance of the coal-bearing shales between beds of coarse sandstone, Boyer and Lee argued for the substitution of the term "Dakota formation", as applied by Cross, for the more widely used term "Dakota (?) sandstone."
The stratigraphic and textural features of the Dakota(?) imply topographic and climatic conditions favorable for the deposition of sand and gravel in stream ways and on deltas and for the accumulation of silt in water bodies shallow enough to permit the growth of swamp, vegetation. The formation rests on an erosion surface which in pre-Dakota(?) time approached base level throughout southern Utah and northern Arizona. The mud lumps, sand balls, and fragments of limestone and of highly colored shales in the basal Dakota(?) beds and colored shales higher in the section suggest rework of sediments derived from the underlying Morrison or post-Morrison beds. The dominant pebbles of quartzite and quartz were derived from unknown distant sources. The sequence and composition of beds' and the local unconformities indicate progressive. changes, but with many interruptions, from a forested. landscape marked by streams and fresh water ponds to one that included bodies of brackish water and to a final complete submergence by an advancing sea. As elsewhere. in the plateau province, the elastic lignitic deposits that constitute the Dakota(?) of the San Juan country are believed to represents a basal phase of Colorado deformation; an oscillating progression from near shore to marine sedimentation.
For the San Jaun country the fragmentary invertebrate fossils in the shaly beds and the pieces of wood in the conglomerates are not sufficiently diagnostic to. place the Dakota(?) accurately in the time scale. But fossils, lithologic features, stratigraphic* position, and regional relations prove equivalence with beds pre-mapped as Dakota(?) in western Colorado, northern Arizona, and northwestern New Mexico. It is significant that the beds in the Dakota(?) pass without' known interruption into those marked by a marinefauna of unquestioned Upper Cretaceous age.
SOURCE
-from The San Juan Country - A Geographic and Reconnaissance of Southeastern Utah by Herbert E. Gregory
PREVIOUS DESCRIPTIONS.
In the area under discussion, the presence of the Dakota sandstone was first recognized by Dutton.The formation was briefly discussed by Schrader and by Shaler. Gardner describes the Dakota sandstone, 200 feet in thickness, as follows:
Light-gray hard sandstone at top and bottom; alternating thin beds of sandstone and shale with bituminous shale and a few irregular coal beds of no commercial importance.In the hogback east of Gallup Darton found 60 to 100 feet of basal Cretaceous, " a hard buff sandstone, in part coarse and massive, lying unconformably on soft white Zuni sandstone." Eight miles southeast of Gallup Darton recognized 250 feet of the Dakota sandstone. In the Navajo country the strata
Near Lohali Mr. Pogue found only 45 feet of the Dakota, about half of which is coal and carbonaceous shale in beds ranging in thickness from 4 to 18 inches. Twelve miles farther north the 80 feet of sandstone and shale representing the Dakota contains only inconspicuous seams of impure coal. The formation is 210 feet thick 6 miles southeast of Marsh Pass and 295 feet at the mouth of Laguna Canyon, and it is believed by Mr. Pogue to attain considerably greater dimensions along the northwest face of Black Mesa, but in Moenkopi Canyon it is represented only by 15 feet of sandstone indurated by iron cement. White Mesa is capped by 10 feet of yellow-gray sandstone, underlain by 5 feet of blue carbonaceous shale and 23 feet of lenticular buff sandstone with plant impressions and iron concretions. In upper Steamboat Wash 40 feet of coarse sandstone in beds 6 to 8 feet thick separated by lenses of shale forms the canyon rim. The lava-capped mesas on the northwest edge of the Hopi Buttes volcanic field include various amounts of the Dakota, and the maximum of 160 feet is shown in an isolated mesa 3 miles north of Comar Spring. Boulders of conglomerate believed to be Dakota were found along the north base of Navajo Mountain. The 170 feet of the Dakota at Carrizo Mountain is predominantly sandstone.
LITHOLOGIC FEATURES.
The Dakota sandstone is highly variable in structure, texture, and composition. It is characterized more by a persistent combination of features than by the persistence of any given bed. The base is commonly but by no means universally marked by conglomerate, and the top is in many places a coarse brown or gray sandstone bed but may be a group of interbedded sandstones and shales or wholly sandy shale of yellow or gray tones. Coal lenses occur prevailingly in the middle of the Dakota but are found in all positions from top to bottom. The formation is everywhere lenticular; lenses and wedges of sandstone, of or a few inches thick overlap, appear, and disappear conglomerate, of shale, and of coal tens of feet appear along the strike and vertically in a most capricious manner. In a branch of Steamboat Wash the Dakota immediately overlying the McElmo sandstones presents four phases within a distance of 1 mile (1) conglomerate with quartz and shale pebbles, half an inch to 1 1/2 inches in diameter; (2) drab mud shale, poorly bedded; (3) brown sandstone, irregularly bedded and cross-bedded, usually with much black carbonized wood; (4) coal with drab shale. At one locality a 15-foot bed of conglomerate is replaced by shale within a distance of 200 feet along the cliff face. The sandstones are commonly cross-bedded, straight laminae meeting one another at high and low angles. In the finer sandstones curved laminae occur.
The material composing the coarser phases of the Dakota varies widely in composition and relative abundance. Along the south side of Todilto Park, the pebbles of white (90 per cent), red, and black quartz, quartzite, chert, and hardened shale (yellow and brown) are one sixty-fourth to one-half inch in diameter, and lenses 6 inches by 10 to 100 feet, made up of sub angular quartz in bunches or strings or individuals, traverse the strata. Kaolinized feldspar and rolled pebbles of shale and sand lumps occur, and at some localities the fragments of shale make up the bulk of the rock. At Chilchinbito pebbles of quartz, quartzite, and chert one thirty-second of an inch in diameter are mingled with broken chunks of sandstone, lumps of mud, and poorly consolidated balls of sand, making a mixture identical with that on the floor of a wash after a sudden shower. On the flanks of Navajo Mountain Mr. Heald found blocks of gray hard cross-bedded sandstone, believed to be Dakota; they contain round to sub angular pebbles one-third to one-half inch in diameter, of which 95 per cent are gray and white and 5 per cent red and black. On the southwest side of Navajo Mountain, at an altitude of 9,000 feet, the Dakota (?) is represented by an arkose breccia consisting of angular fragments of brown, gray, and red quartzitic sandstone, black slate, white quartzite, and abundant feldspar, both fresh and partly decomposed. The sandstone fragments are from 3 inches to 2 feet in diameter. At nearly all localities impressions of leaves, stems, and tree trunks are found, and fossil wood was observed at a few places. Dark-brown ferruginous concretions are abundant in the Dakota. They are commonly small and rounded and weather out as knobs and "boltheads." In places they take the form of pancakes, balls, or lozenges-irregular masses 6 to 10 feet in long diameter.
The cement of the Dakota is calcareous, ferruginous, and siliceous. Where calcareous cement occurs the rock is readily crumbled and is highly porous. Hydrous iron oxide is the most common cementing material and permits the formation of resistant ledges which stand as cliffs. In places the rock is so firmly united as to form a quartzite conglomerate. The removal of cement and of clay pellets and the decomposition of the vegetable remains has produced a porosity which makes the Dakota an excellent water carrier.
SOURCE
-from Geology of the Navajo Country, a Reconnaissance of Parts of Arizona, New Mexico and Utah by Herbert E. Gregory
