The Morrison Sandstone
Strata assigned to the Morrison formation in the San Juan country rise . as cliffs on both sides of the San Juan River from Butler Wash eastward to the Colorado State line. Across Sage Plain they are exposed beneath the Dakota (?) rim rock in all canyons tributary to the San Juan and terminate westward in attractive cliffed slopes along upper Butler Wash and middle Cottonwood 'Wash and in Piute Park. Charaeferistic sections are readily accessible on the Bluff-Blanding road and in Big Canyon and Brushy Basin on the Blanding-Kigalia road.In general appearance, style of bedding, mass composition, and age the outcrops at all these places resemble those defined as Morrison or McElmo in western Colorado, New Mexico, and Arizona and as a whole are correlated. with them. Here, as. elsewhere, the upper limit of the formation is the base of the Dakota (?) sandstone, and the lower limit an unconformity that has not been everywhere definitely placed in the stratigraphic column. However, the study of widely separated sections leaves the impression that fuller knowledge of unconformities and conditions of sedimentation will result in the recognition of two or more formations within the Morrison, perhaps even of different ages. The coarse thick-bedded lenticular sandstones, the thick massive sandstones, and the interbedded red shales and white sandstones in the Morrison of the San Juan drainage basin would seem out of place in the Morrison section of eastern Colorado.
In most sections the Morrison in eastern Utah includes two strongly contrasted parts a lower part comprising thin-bedded and massive red sandstone, sandy shale, gray cross-bedded lenticular conglomerate, together with subordinate limestone clay, and gypsum; and an upper part consisting of "variegated shales." In the San Juan country the Morrison formation comprises four rather distinctive lithologic groups or members, which are described below.
BLUFF SANDSTONE MEMBER
The Bluff sandstone member is white, brown-stained commonly cross-bedded, and made up of medium to coarse quartz grains. Typically it is one massive bed 200 to 350 feet thick that here and there includes aggregates of large quartz grains, clay balls, and short thin lenses of red mudstone. In some places it is arranged as long. overlapping sandstone wedges bordered by a little red shale, and in other places as poorly defined beds 20 to 40 feet thick. Traced eastward, the Bluff sandstone that forms the top of Tank Mesa is less persistently massive. Near the, mouth of Montezuma Canyon 10 to 20 feet of bedded white sandstone are incorporated in red sandy shale that thins, thickens, bunches up, or flattens out along the strike.. Traced northward along Butler Wash and Cottonwood Canyon the Bluff sandstone is represented in places by three or more beds. Vertical joints outline square columns and long slabs that weather in place as monuments and furnish huge blocks to the talus. Weathering on cliff faces produces elongated niches, pockets, and "owl holes" suitable for nests of birds and bats, and at the base of the sandstone deeply recessed grooves with flat floors and overhanging roofs, chosen by Cliff Dwellers as building sites. The Bluff sandstone derives its name from the town of Bluff, where, on both sides of the San Juan River, it is the outstanding topographic feature.
RECAPTURE SHALE MEMBER
The interval between the Bluff sandstone member and the lowermost bed characteristic of the Westwater Canyon sandstone member is occupied by a series of strongly colored shales and sandstones 100 to 300 feet thick. They appear in many places as sloping platforms at the base of cliffs and are particularly well displayed near the mouth of Recapture Creek, from which the name is derived'. The shales are prevailingly dark red, but some are variegated-pink, ash, brown, and gray. Many of them include firm, strongly calcareous, beds that break into slabs and friable, imbricated gypsiferous, beds that weather as tiny cliffs. The sandstones are white beds of glistening quartz cemented by lime, few of them more than a foot thick or continuous for more than 1,000 feet. Some are gypsiferous and friable, but most are more resistant than those in the Bluff or the Westwater Canyon member and are represented in the talus as angular blocks. The shales and sandstones combine to form slopes, low mesas, and platforms, and the edges of sandstone beds appear as shelves and small benches. The outcrops are attractively color-banded, but as the shale and sandstone feather out and replace each other along the strike the arrangement of sections 1,000 feet apart is quite different. The amount of sandstone in the Recapture shale member is generally about 15 percent; at the base of Turners Bluff it constitutes so small a part that the shales weather like marls. In gross lithology the strata of the member resemble those observed in the Morrison about 8 miles south of Woodside, Utah, which Gilluly and Reeside place at the base of the Salt Wash sandstone member.
WESTWATER CANYON SANDSTONE MEMBER
In reports on the geology of the region north of Moab and other places in east-central Utah Lupton 18 described a coarse-grained "gray conglomeratic sandstone", in places lenticular and cross-bedded, that forms cliffs about 350 feet from the top of the Morrison-strata sufficiently uniform and persistent to serve as a datum plane for mapping. For this sandstone he proposed the name "Salt Wash member of the McElmo formation." As classified by Gilluly and Reeside the Salt Wash sandstone member lies at the base of the Morrison and includes not only gray conglomeratic sandstones but also clay, limestone, and gypsum. Baker 11 defines this member as "white conglomeratic sandstones interbedded with red sandy mudstones and red shale" that occupy the lower half of the Morrison, south of Moab.
In the San Juan country a sandstone that appears to lie in the position of the Salt Wash member as defined by Lupton plays a large part in developing the topography. It stands as cliffs and forms the base of many canyon walls that continue upward- through variegated shale to a rim rock of Dakota (?) sandstone. Along the San Juan River north and northwest of Bluff, between Brushy Basin and middle Cottonwood Canyon, and in other places where the variegated shales have been removed, this sandstone appears as mesas, ridges, and the tops of broad platforms. Tentatively the local Dame "Westwater Canyon sandstone member", derived from West Water Canyon, is. here given to this sandstone, because its exact equivalency to the typical Salt Wash sandstone has not been satisfactorily established.
This member is essentially a series of white sandstones composed of rounded medium to coarse grains of quartz, cemented by calcium carbonate and arranged in lenticular, irregular beds 1 to 30 feet thick. They include conglomeratic bands and stringers composed of quartz aggregates, colored chert, concretionary masses of compact green-white clay, and rare fragments of petrified wood and dinosaur bones. Interbedded with the sandstones are red earthy soft fine-grained sandy shales-perhaps better called "mudstones "-that thin, thicken, or disappear in short distances. With them are associated a few thin short lenses of gray limestone conglomerate. These mudstones, which make up 8 to 20 percent of measured sections, are extremely irregular. A bed in Big Canyon 14 feet thick disappears in a distance of 120 feet, and one less than 2 feet thick in Montezuma Canyon was traced for half a mile. Few beds of shale or of sandstone retain their individuality for as much as a mile. Unconformable contacts at the base of the Westwater Canyon member were observed at several places. Features that indicate exposure of the top beds before the Brushy Basin shale was laid down were noted in McElmo Canyon, but generally the sandstone grades upward through a series of gray sandy shales and merges into the variegated shales at different horizons. The relation is somewhat like that between the Shinarump and the Chinle, and the Westwater Canyon sandstone may prove to mark the beginning of a cycle of sedimentation that continued until interrupted by pre-Dakota (?) erosion. The thickness of eight measured sections of the Westwater Canyon sandstone member ranges from 222 to 295 feet.
BRUSHY BASIN SHALE MEMBER
The upper part of the Morrison of the San Juan country consists of the well known variegated shales (Morrison shales, McElmo shales) that generally in Utah and western Colorado lie immediately below the Dakota (?) sandstone. In fact, they owe their preservation to the resistant Dakota cover. Directly beneath cliffs of Dakota (?) sandstone they stand in almost vertical walls; where the sandstone has been stripped back they form slopes that continue outward into mounds and ridges spread. over a platform of Westwater Canyon sandstone. Their appearance is everywhere the same brightly variegated masses that are exceeded in beauty of coloring only by the Chinle "marls." The dominant beds are white, gray, green, purple, and red sandy shales and sandstones. Subordinate beds are gray, pink, blue, and gray limestones; conglomerates of red, green, and white cherts; and buff hard sandstones. The buff sandstone is more abundant near the base and seems to increase in amount eastward toward the Colorado line. South of the San Juan River much of the bedded shale is replaced by massive white or green-white sandstone, at the "Jump-off" on the highway between Blanding and Bluff some of the shale is evenly stratified, but generally all beds are lenticular. In Brushy Basin, where the variegated shales are admirably displayed for study, the thickness is 450 feet. (See see. 25, p. 77.) Other sections measure 350 to 470 feet. Dinosaur bones from, the lower part of the Brushy Basin shale in McElmo and other canyons of Sage Plain are included in local collections of curios. Parts of a skeleton gathered from the talus in Montezuma Canyon by the late Willis T. Lee were described by C. W. Gilmore as the greater portion of a caracoid, two anterior caudal centra, one medium caudal centrum, and portion of a distal caudal vertebra, all apparently of one individual, a sauropod dinosaur of the Brontosaurus-Apatosaurus group. Associated with this material is the upper articular end of a tibia pertaining to a carnivorous dinosaur. Its size would indicate that it might be tentatively identified as Antrodemus (Allosaurm), the largest carnivorous dinosaur to be found in this country associated with sauropod remains.
MORRISON-DAKOTA (?) CONTACT
In the San Juan country the strata assigned to the Dakota (?) formation are everywhere accompanied by those classed as Morrison, and everywhere the contact between the two formations is a zone of unconformity. But Morrison like sedimentary beds lie above what appears to be the major unconformity, and unconformities are present at horizons not far below., At most places a siliceous conglomerate lies with well expressed unconformity immediately above beds accepted generally as Morrison McElmo of Cross and others). In a few places this conglomerate with some sandy shale and thin-bedded sandstone constitutes most of the section between the Morrison and the Mancos, but * many places a basal conglomerate or its equivalent in sandstones is overlain by or interbedded with a series a few inches to 50 feet thick consisting of gray, red, green, or even variegated shales arranged as lenses or short regular beds. These beds seem out. of place in the Dakota (?). They may represent reworked material brought in from Morrison or post-Morrison areas that were exposed to erosion while the lowest gravel of the Dakota (?) was being laid down in stream channels. It is possible, however, that these shales together with the underlying conglomerate are parts of an undefined formation of pre-Dakota (?) age. In mapping the economic geology of southwestern Colorado Coffin 40 used the terms "Dakota", "pre-Dakota", and post McElmo" for subdivisions of the Cretaceous lying above clearly recognized McElmo (Morrison). His reasons for introducing new terms are given as follows:
Certainly the massive sandstone conglomerate and the unconformity used as the upper limit of the McElmo mark a definite change of conditions of sedimentation. Above this massive stratum are beds of more or less pure limestone, massive beds of chert, and much green shale strata the like of which have never been included in the Dakota. As there are no fossils to aid in the proper placing of these beds, lithologic character and stratigraphic position alone remain as usable evidence of their age. The character of these beds would not admit of their inclusion in the "Dakota" even with the qualified application of this term. Further, to place them in the McElmo formation would require the use of this term in a sense different from that in which it was first applied to strata in McEImo Canyon.
Properly to place the lower limit of the Dakota (?) is but part of the Dakota-Morrison problem. The exact age of these formations and the stratigraphic units that constitute them are alike imperfectly known.
-from The San Juan Country - A Geographic and Reconnaissance of Southeastern Utah by Herbert E. Gregory